Cold Spray of Ni-Based Superalloys: A Review on Processing and Residual Stress

Abstract

Cold spray has been extensively applied to deposit a range of materials in many industries. In the recent times, such a method has also shown its potential to deposit nickel-based superalloys, which currently are in demand due to their high tensile strength and corrosion resistance (especially at elevated temperatures); however, cold sprayed nickel super alloy coatings have poor mechanical properties due to the materials’ limited ability to undergo plastic deformation. Regarding this, numerous cold spray process modifications have been experimented, including preheating substrate and feedstock powder, applying laser irradiation, heat treating coatings post deposition, and heat treating feedstock powder, to promote plastic deformation, eliminate porosity and enhance inter particle bonding. Specifically, the important influence of external heat input on the underlying substrate and/or the incoming particles during cold spray deposition was highlighted in multiple studies. These studies indicated that the addition of external heat during cold spray increased the adhesion strength of the coatings due to an increase in the thermal softening effect of the deposited particles. In general, an attempt is made here to systematically review the influence of cold spray process modifications on the microstructure, mechanical properties and residual stresses of nickel super alloy coatings.

Introduction

Cold spray deposition technique has been employed in various industries, including aerospace, energy, automotive, and medical sectors (Ref 1,2,3,4) due to its versatility. In the past, the primary application of this technique was development of metallic, ceramic, and composite coatings (Ref 5,6,7). In recent years, such a technique has demonstrated significant potential as a solid-state additive manufacturing process and as an alternative option for addressing structural repair needs in response to the industrial demands with a rather intricate engineering design. In theory, cold spray process utilises a compressed gas to accelerate feedstock particles through a converging-diverging nozzle, driving the particles to travel at high velocities. Upon the initial particle-substrate impact, the high kinetic energy of the particle developed during expansion process leads to a severe plastic deformation of the feedstock particles, embedding them in the targeted substrate and subsequently forming layers of coating (Ref 5). Cold spray coatings are characterised to possess a strong bonding, which is obtained mechanically and metallurgically, where later exhibits a significant interest among cold spray researchers. In this technique, the deposition process does not involve an external heat source to melt the particles, allowing preservation of feedstock powder phases in the coating, in turn allowing the production of coatings that demonstrate comparable mechanical and/or metallurgical properties to the bulk materials. Due to these beneficial characteristics, cold spray process is extensively employed for depositing different types of materials that are prone to oxidation at high temperatures. Plenty studies have been successfully conducted using cold spray technique to deposit softer materials like aluminium (Ref 6) and copper (Ref 7) with improved coating structures and mechanical properties. Furthermore, such hard-to-deposit materials, in particular nickel-based (Ni-based) superalloys like Inconel 718 (IN718) and Inconel 625 (IN625), have also been successfully deposited via cold spray. Ni-based superalloys are characterised with high tensile strength and corrosion resistance at elevated temperatures, making them desirable as structural materials at high temperatures, particularly aerospace and gas turbines applications. Yet, plenty of studies indicate major defects within the Ni-based superalloys cold sprayed coating structures, such as high porosity and poor metallurgical bonding, which may substantially reduce the mechanical properties, leading to a rapid coating deterioration. Additionally, cold spraying Ni-based superalloys requires higher processing gas pressures and temperatures, which further lead to the introduction of residual stresses. Plenty attempts have been undertaken to address such issues, focusing on modifications in the processing systems, alterations in the feedstock phases, and post-spray treatments.

Sun and co-investigators (Ref 8) have successfully modified their cold spray configuration, where they attached a ceramic heater on the back of the IN718 substrate and found that the IN718 interfacial bonding was significantly improved when the IN718 substrate was pre-heated at 450 °C. This was studied where the pre-heated IN718 substrate enhances the occurrence of thermal softening, hence accommodating an intimate metallurgical bonding in the interface. Ortiz-Fernandez and Jodoin (Ref 9) have designed a feedstock heater of which they pre-heated the IN718 powder inside a heated mixing coil. They reported that the IN718 coatings thickness was increased by 50% when sprayed using pre-heated powder at 700 °C, however exhibited a notable presence of pores. Feedstock heat treatment was explored by Perez et al. (Ref 10) to improve the deformability of the IN718 particles. It was investigated that the presence of $\delta$ phase after solution treatment increased the deposition efficiency and reduced the porosity, which stipulates an improvement in particles deformability through feedstock modification. Recently, a novel cold spray process has been developed, namely laser-assisted cold spray (LACS), where such a hybrid technique utilises laser irradiation to soften the incoming particles and/or substrate, depending on the configuration. Poza et al. (Ref 11) was the first to report LACS on IN625, when they found well-consolidated particles in the coating interface through in-situ laser remelting approach. This technique could remove the requirements of excessive gas pressures and temperatures, and eliminate helium, as it is not viable in an industrial scale. Post-spray heat treatments have been attempted to produce coatings with tensile strength near to bulk materials. As demonstrated by Bagherifard et al. (Ref 12), the tensile strength of a cold sprayed IN718 free-standing sample was increased to around 800 MPa after heat treatment at 950 °C. This was primarily due to the pores elimination and recrystallisation during the heat treatment. Yet, modifying cold spray system to improve deposition efficiency, increase coating thickness, and minimise porosity can result in the development of residual stresses within the cold sprayed Ni-based superalloy coatings. As investigated by Singh et al. (Ref 13), IN718 coatings delaminated at a thickness of 770 µm. This can be sourced from the non-uniform stress distribution as a consequence of a non-continuous peening effect during the intermittent deposition process.

The implementation of these modified cold spray processes has demonstrated substantial improvements and drawbacks in the final coating properties. Consequently, it is crucial to obtain a comprehensive understanding of how the modified processes significantly contribute to the deposited coating. To the best of authors’ knowledge, there is no publication that systematically studies the correlation between the cold spray systems and their influence towards cold sprayed Ni-based superalloys coatings’ mechanical and metallurgical properties, and their effect on the residual stress. Therefore, this comprehensive review provides a fundamental understanding of how factors such as substrate and particle pre-heating, LACS, post-spray heat treatment, and feedstock modification influence the quality of coatings. Additionally, the impact of the modified processes on residual stress in Ni-based superalloys coatings was also discussed as the residual stresses within the cold sprayed Ni-based superalloys coatings play significant role in determining the overall coating quality.

Novel Processing with Cold Spray

The cold spray process has been widely studied to generate high-quality deposits (e.g. low porosity, thick, and dense structures) that are not influenced by high-temperature reactions like oxidation as well as carburisation (Ref 17,18,19). Furthermore, the cold spray process is able to avoid the potential effects of thermally-generated phase transitions during the deposition process (Ref 16). However, the main issue associated with the cold spray process is related to the critical velocity of certain materials, especially hard materials like Ni-based superalloys (Ref 17). If the superalloy particles fail to reach the critical velocity, it may result in a reduction of coating bond strength and density (Ref 18). Such alloys require higher impact velocity to obtain bonding between targeted substrate and particles (Ref 19). The high impact velocity of the particles can be achieved by utilising helium as a process gas and heating the gas at higher temperatures (Ref 24, 25). However, the utilisation of both helium and gas heating can lead to a substantial increase in the operational expenses of the process, which may not be economically feasible for the majority of applications when scaled up to industrial production (Ref 22). Several alternatives have been suggested to enhance the quality of Ni-based superalloy coatings at lower critical velocities. These alternatives involve employing either bulk or localised heating of the substrate or particles. The subsequent sections provide an overview of how modifications in the cold spray process influence the evolution of microstructures and mechanical properties.

Substrate Pre-heating

Bonding quality is determined by the particle-substrate adhesive and particle-particle cohesive strengths. In cold spray process, broadly bonding relies upon the particle impact velocity and temperature, which derive from the feedstock properties and cold spray parameters (Ref 27, 28). Additionally, substrate parameters like surface roughness and temperature have also been investigated to modify the behaviour of both adhesive and cohesive bonding (Ref 29, 30). The bonding characteristics are primarily induced by the deposition of the initial few layers, which may differ based on the conditions of the substrate surface. For instance, higher surface roughness promotes more contact area on softer substrate materials like copper and aluminium, whereas a polished surface has been proven to improve the jet formation in harder substrate materials like Ni-based superalloys, resulting in a prominent adhesion strength and deposition efficiency (Ref 31, 32). Likewise, particle-substrate bonding behaviour is also influenced by the substrate temperature, where higher substrate temperature enhances plastic deformation and metallurgical bonding (Ref 29). Ortiz-Fernandez et al. (Ref 30) investigated the effect of substrate preheating by depositing pure aluminium particles onto Ti-6Al-4 V through a cold spray process where the particle-substrate adhesion strength experienced a 30% enhancement and the area of bonding increased by a factor of six. Moreover, the failure mode of the specimen transformed into a more ductile failure type when Ti-6Al-4 V surface temperature was elevated to 600 °C. The improvements were attributed to the native oxide removal, owing to the additional heat which is generated through the impact between incoming particles and pre-heated substrate. Furthermore, dynamic recovery, as well as static recrystallisation alter the brittle nature of the coating into ductile behaviour, which is sourced from a localised heat distribution within the coating interface. Khamsepour et al. (Ref 31) reported that increasing substrate temperature prior to the cold spray deposition process resulted in oxide layer formations; however, the subsequent thermal softening effect caused the substrate to undergo significant deformation, resulting in the failure of its oxide layers. Tripathy et al. (Ref 32) demonstrated that while depositing nickel particles onto a pre-heated carbon steel substrate improved the deposition efficiency, elevating the substrate temperature diminished the polarisation resistance and raised the corrosion potential. Consequently, the cold sprayed samples exhibited severe corrosion. It was studied that the higher substrate temperatures during deposition process facilitated oxygen diffusion, resulting in coatings with a higher oxygen content (Ref 33, 37, 38).

Similarly, increasing substrate temperature prior to the cold spray deposition process has also been applied to corrosion-resistant alloys, in particular nickel-based superalloys. Various pre-heating methods such as ceramic heaters, heating cartridges, flame torches, and induction heaters have been applied to control substrate temperature, whilst only ceramic and induction heaters are reported for nickel superalloys pre-heating devices (Ref 11, 12, 33, 39). Sun et al. (Ref 8) attached a 16 kW ceramic heater underneath the IN718 substrate to investigate the bonding characteristics of cold sprayed IN718 at elevated substrate temperature. It was discovered that substrate pre-heating exhibited significant improvement in terms of bonding quality. This can be proven when nearly half of IN718 particle is bonded without jetting features presented in non-preheated substrate (Fig. 1a). It can also be examined that IN718 substrate does not undergo substantial deformation, instead demonstrates the presence of gaps and micro-cracks on both sides of the interface. Similarly, raising IN718 substrate to 100 °C prior to the deposition process indicates gaps and microcracks at the interface, even though it presents enhanced particle bonding (Fig. 1b). Substantial improvements are observed when IN718 substrate was pre-heated to 200 °C and 450 °C where jet formations occur which signify a sound particle-substrate bonding (Ref 31, 40, 41). However, one can inspect that discontinuous bonding still can be distinguished when the substrate was pre-heated at 200 °C (Fig. 1c), whereas raising substrate temperature by 56% enhances metal jetting as well as particle-substrate adhesion without a sign of discontinuous bonding with the underlying IN718 substrate (Fig. 1d).

Fig. 1

Cross-section of IN718 single particle cold sprayed onto IN718 substrate with temperature variables of (a) 25 °C (un-heated), (b) 100 °C, (c) 200 °C, and d) 450 °C. Reprinted from Mater. Sci. Eng., vol 720, W Sun et al., “Deposition characteristics of cold sprayed Inconel 718 particles on Inconel 718 substrates with different surface conditions”, pp. 75-84, Copyright (2017), with permission from Elsevier (Ref 8)

It can be hypothesised that substrate pre-heating provides thermal softening effect that promotes localised plastic deformation at the interfacial region upon particle-substrate impact (Ref 38). Moreover, substrate heating prior to deposition can eliminate the work hardening effect from high kinetic energy where localised plastic deformation results in the adiabatic temperature rise at the interface due to the plastic work dissipation. The combination of thermal softening effect and localised plastic deformation facilitates a robust metal-to-metal bonding between IN718 particles and substrate and, subsequently high quality coating.

The deposition efficiency of the preheating method was examined through a single particle impact test. Generally, the non-pre-heated substrate, as well as surface that pre-heated at 100 °C, exhibit relatively low particle deposition efficiency (Fig. 2a and b). It can be noticed that few IN718 particles are successfully deposited onto IN718 substrate surface, while others fail to adhere, resulting in visible craters formed on the substrate surface. Craters that are present in both substrate temperatures are sourced from impinging particles with an impact velocity lower than critical velocity, providing only a metal particle blasting effect (Ref 36, 43, 44). The particle deposition efficiency was further enhanced as the pre-heating temperature increased. It can be seen that only few carters formed at substrate temperature 200 °C, followed by few tiny white features, which describes the deposition of small IN718 particles (Fig. 2c). Furthermore, raising the pre-heating temperature to 450 °C results in a substantial improvement in the particle deposition efficiency, as proven by considerable amount of smaller IN718 particles attached onto the surface in comparison to the substrate pre-heated at 100 °C and 200 °C (Fig. 2d).

Fig. 2

IN718 splats deposited onto heated IN718 substrates at (a) 25 °C (un-heated), (b) 100 °C, (c) 200 °C, and (d) 450 °C. Reprinted from Mater. Sci. Eng., vol 720, W Sun et al., “Deposition characteristics of cold sprayed Inconel 718 particles on Inconel 718 substrates with different surface conditions”, pp. 75-84, Copyright (2017), with permission from Elsevier (Ref 8)

The 3D profiles of the IN718 surface morphologies also suggest that increasing substrate pre-heating temperature provides obvious jetting. At 25 °C and 450 °C, the maximum particle height above the interface is around 16 and 20 µm, respectively (Fig. 3a and b). Besides, when the substrate is pre-heated to 450 °C, metal jetting is clearly visible. This denotes that as the substrate pre-heating temperature increases, the deformation ratio of the IN718 particles decreases, which can be attributed to the noticeable thermal softening effect. The results correspond well with the analysis provided by Yin et al. (Ref 38) and Khamsepour et al. (Ref 41), where higher substrate pre-heating temperatures are able to provide an oxide-free metal-to metal bonding with the occurrence of adiabatic shear instability, denoted by prominent metal jetting (Ref 46,47,48,49,50).

Fig. 3

Contour heights of IN718 splats deformed onto (a) un-heated (25 °C) and (b) pre-heated at 450 °C IN718 substrates. Reprinted from Mater. Sci. Eng., vol 720, W Sun et al., “Deposition characteristics of cold sprayed Inconel 718 particles on Inconel 718 substrates with different surface conditions”, pp. 75-84, Copyright (2017), with permission from Elsevier (Ref 8)

The surface morphologies provided from the pull-out test are consistent with shear test results where higher substrate pre-heating temperatures improve the interfacial bonding strength. The calculated shear strength data is presented in Fig. 4 where interfacial shear strengths consistently rise as the substrate temperature increases. It is calculated that the interfacial shear strength of IN718 particles deposited onto unheated IN718 substrate is around 66 ± 5 MPa where substantial shear strength improvement of 218 ± 19 MPa is measured on the highest pre-heating temperature, signifying that the substrate pre-heating has pivotal role in the final coating bond strength. The fractured surfaces after the shear test also tell the importance of substrate pre-heating, proven by a few IN718 particles still intact on the unheated substrate surface whilst plenty of IN718 particles are observed to remain on the fractured surface when substrate was pre-heated at 450 °C (Fig. 5a and c).

Fig. 4

IN718 coatings interfacial shear strengths with various substrate pre-heating temperatures. Reprinted from Mater. Sci. Eng., vol 720, W Sun et al., “Deposition characteristics of cold sprayed Inconel 718 particles on Inconel 718 substrates with different surface conditions”, pp. 75-84, Copyright (2017), with permission from Elsevier (Ref 8)

Fig. 5

Interfacial fractographs following a shear test of IN718 coatings deposited onto un-heated (a, b) and heated at 450 °C (c, d) IN718 substrates with red insets indicate the surface characteristics of the craters. Reprinted from Mater. Sci. Eng., vol 720, W Sun et al., “Deposition characteristics of cold sprayed Inconel 718 particles on Inconel 718 substrates with different surface conditions”, pp. 75-84, Copyright (2017), with permission from Elsevier (Ref 8)

Figure 5(b) presents a higher magnification of the surface crater of the unheated substrate, which is denoted by slight dimples at the rim and smooth at the centre. This indicates that the IN718 coatings fracture arises at the coating-substrate interface (Ref 51, 52). Conversely, larger dimples were followed by more retained material from the particle present at the edge of the crater when substrate was pre-heated at 450 °C (Fig. 5d). This implies that fracture happens within the IN718 particles, which corresponds to the bonding strength improvement (Ref 46). Moreover, it also signifies that the coating-substrate bonding strength is far superior than the coating cohesive strength at some regions when substrate pre-heating is applied.

Ortiz-Fernandez and Jodoin (Ref 9) deposited IN718 particles onto pre-heated Ti64 substrate using induction heating equipment. IN718 particles sprayed onto heated Ti64 substrate at 600 °C using nitrogen produced only a thin layer of coating with deposition efficiency was reported less than 1%, which is similar to those sprayed without an induction heater (Fig. 6a and b). However, this contradicts with result provided by Arabgol et al. (Ref 47), where they discovered a significant increase in coating thickness of over 500 µm, accompanied by an improvement in deposition efficiency when utilising an induction heater on titanium substrate. The increase in coating thickness is mainly due to the fact that the fraction of highly strained area correlated to the fraction bonded increased with increasing the coating surface temperature. Furthermore, the low thermal effusivity of the titanium substrate ($e$ = 6.2 kJ/m²/K/s^1/2) facilitates a steady heat exchange process. As a consequence, higher substrate temperature and subsequent thermal softening effect can be attained (Ref 47). A massive disparity in coating thickness and deposition efficiency is primarily attributed to the elevated initial temperature of the substrate, which predominantly influences the first layer of deposited particles (Ref 48). In the case of cold spraying nickel-based superalloys with the assistance of an induction heater, more IN718 particles are deposited at the initial layer since the thermal properties of Ti64 substrate contribute to the efficiency of the induction heater heat transfer. Therefore, the induction heater supplies sufficient temperature to the Ti64 substrate surface, subsequently promoting a notable thermal softening effect for IN718 particles to adhere. Yet, upon particle-particle deposition, the thermal effusivity of the superalloy particle ($e$ = 7.0- 15.7 kJ/m²/K/s^1/2 (Ref 49)) is inadequate to transfer more heat to the upcoming layer; hence the temperature is not high enough to conduct thermal softening effect that can decrease the critical velocity.

Fig. 6

IN718 coatings after five passes deposited onto (a) un-heated and (b) heated at 600 °C IN718 substrates using nitrogen. Used with permission from R. Ortiz-Fernandez and B. Jodoin, “Toward Efficient Cold Spraying of Inconel 718: Understanding the Influence of Coating and Particle Impact Temperatures,” J. Therm. Spray Technol., vol 32(1), pp. 188–207, 2023, Springer Nature (Ref 9)

Correspondingly, increasing particle velocity and substrate temperature reduces the final coating thickness and deposition efficiency. As displayed in Fig. 7(a), the IN718 coating deposited without the use of an induction heater presents a thickness of 461 µm and porosity of less than 1%. However, when the coating is deposited with the aid of an induction heater, a considerable reduction in thickness of 50% is observed, accompanied by the presence of numerous micro-cracks (Fig. 7b). The reduced thickness can be attributed to the different deformation behaviour between the pre-heated substrate and the incoming hard IN718 particles with lower temperature. These findings have been elucidated by Bae et al. (Ref 50), where the thermally softened substrate inhibits the particle deformation by dissipating the kinetic energy into the deformation of the softer counterpart. In addition to lower deposition efficiencies, large cracks develop between layers when the substrate temperature is increased, which is associated with the thermal residual stresses present in the cold sprayed IN718.

Fig. 7

Cold sprayed IN718 layers after five passes deposited with helium onto (a) un-heated and (b) heated at 600 °C IN718 substrates. Used with permission from R. Ortiz-Fernandez and B. Jodoin, “Toward Efficient Cold Spraying of Inconel 718: Understanding the Influence of Coating and Particle Impact Temperatures,” J. Therm. Spray Technol., vol 32(1), pp. 188–207, 2023, Springer Nature (Ref 9)

Many researchers measured cold sprayed IN718 coatings residual stresses without substrate pre-heating where they found that the compressive residual stress values are within the range of 100 MPa (Ref 15, 58, 59), which is lower than the yield strength of the alloy and insignificant in causing cracking or delamination. Yet, Li et al. (Ref 53) and Fardan et al. (Ref 54) investigated that the thermal residual stresses increase significantly when substrate temperature exceeds 400 °C, resulting in crack formations. This finding corresponds to the theoretical estimation presented by Ortiz-Fernandez and Jodoin (Ref 9), where heating the substrate at 600 °C whilst depositing IN718 particles generates a tensile residual stress value around 1 GPa, which surpasses any potential value of coating cohesion strength reported in literature (Ref 62,63,64,65). The integration between theoretical approximation, low thermal conductivity, and high thermal expansion of IN718 leads to the development of tensile residual stresses that exceed the coating cohesion strength. This combination can be the intrinsic reason for the presence of micro-cracks within the IN718 coatings (Fig. 7b).

Powder Pre-heating

In contrast to the thermal spray process, where the particle deposition depends on both thermal and kinetic energy, cold spray deposition process solely relies upon kinetic energy to adhere particles to the targeted surface (Ref 57). As elucidated by Assadi et al. (Ref 55) and MacDonald et al. (Ref 58), the deposition mechanism of cold spray is associated with the critical velocity of specific particles to undergo plastic deformation. It is studied that, depending on the material properties, high-strength materials such as metal superalloys tend to have a higher critical velocity, which makes them difficult to deform, subsequently resulting in low coating adhesion (Ref 17). Schimdt et al. (Ref 59) discovered that critical velocity is also governed by particles temperature, where higher particle temperature reduces the critical velocity of the powder material to experience prominent plastic deformation. Moreover, elevating particles temperature prior to deposition increases the impact-to-critical velocity ratio, which consequently enhances adhesion with the target substrate, cohesive strength among adjacent particles, flattening ratio of the splat, and deposition efficiency (Ref 39, 43). Earlier powder pre-heating configuration utilised the exposure into high-temperature and high-pressure propellant gas prior to expansion in the De Laval nozzle where feedstock material was injected into an extended pre-chamber further upstream (Fig. 8a) (Ref 69,70,71). The extended pre-chamber increases the particle temperature by increasing the dwell time of the particles before they travel towards the divergent section of the nozzle (Ref 24).

Fig. 8

(a) Early method of powder pre-heating utilising an extended chamber at the convergence side, where powder particles experience rapid heating before the expansion process. Reprinted from Surf. Coatings Technol., vol. 167(2-3), C.-J. Li and W.-Y. Li, “Deposition characteristics of titanium coating in cold spraying,” pp. 278–283, Copyright (2003), with permission from Elsevier (Ref 62). (b) Latest model of powder pre-heating method as proposed by Ortiz-Fernandez and Jodoin. Used with permission from R. Ortiz-Fernandez and B. Jodoin, “Toward Efficient Cold Spraying of Inconel 718: Understanding the Influence of Coating and Particle Impact Temperatures,” J. Therm. Spray Technol., vol 32(1), pp. 188–207, 2023, Springer Nature (Ref 9). The powder particles are pre-heated inside the mixing coil and then injected directly into the divergence end of the De Laval nozzle

Cavaliere et al. (Ref 63) were the first to investigate the influence of various gas temperatures and pressures on the deposition of nickel superalloy particles via high-pressure cold spray system equipped with a pre-chamber. They deposited Diamalloy 1005 particles onto carbon steel substrate at different gas pressures and observed that pre-heating particles with higher propellant gas temperatures ranging from 700 °C to 850 °C inside the pre-chamber increased both impact temperature and velocity. They also deposited another type of nickel superalloy with similar compositions, specifically IN625 onto IN718, and found a comparable increasing trend in impact temperature, with a slight decrease in impact velocity (Ref 64). It can be noticed that the cold spray deposition process with powder pre-heating mechanism, utilising extended pre-chamber under similar gas pressures with lower temperatures, results in dense coating structures, indicated by lower porosity in the coatings (Table 1). Furthermore, a substantial porosity reduction of 60% was examined when the pre-chamber temperature was reduced by 175 °C while maintaining equal gas pressure of 40 bar.

Table 1 Influence of processing gas pressure and temperature of multiple Ni-based superalloys powders towards impact temperature and velocity, and porosity

This considerable drop in porosity has been studied and found to be attributed to the variation of particle dimensions and pre-heating temperatures. Larger particles are known to provide a tamping effect, primarily due to their higher mass. Generally, the tamping effect contributes to the formation of a dense coating structure (Ref 65). Such a tamping effect becomes more pronounced as the particle velocity increases (Ref 58). Furthermore, pre-heating particles at higher temperatures may reduce the material’s yield strength and facilitate the incidence of adiabatic shear instability, which enhances the particle-substrate adhesion, consequently, packed morphologies (Ref 76, 77). However, results obtained by Cavaliere et al. (Ref 73, 74) do not implicate the previously mentioned statements. Unlike other materials like aluminium and copper that able to produce coatings with less porosity at a higher pre-heating temperature, Ni-based superalloys coatings show porosity increment at elevated pre-heating temperatures. Pre-heating particles at temperatures as high as 850 °C show a higher percentage of coating porosity compared to those pre-heated in the range of 700 °C to 800 °C. Additionally, Diamalloy 1005 powder, which underwent pre-heating at 700 °C inside the pre-chamber and was subsequently sprayed at 35 bar, presents the highest coating porosity. These distinct porosities have been studied by Zahiri et al. (Ref 68), where such porosities are derived from a lower plastic deformation due to the reduction of impact velocities and temperatures. The impact velocity of particles pre-heated at 700 °C sprayed at 35 bar is equal to 665 m/s, which is lower than those pre-heated at 850 °C and sprayed at 40 bar, providing less deformation. Furthermore, the impact temperature of the Diamalloy 1005 particles at the lowest pre-heating temperature shows a reduction of 16% compared to those pre-heated at 850 °C. On the other hand, pre-heating at lower temperatures (600-650 °C) generates dense coating structures, as also stated by Cavalier et al. (Ref 64), where pre-heating smaller particles at lower temperature ranges contributes to the significant reduction of porosity. This is primarily due to a homogeneous impact temperature and velocity, which is sourced from a smaller particle size range, allowing a more uniform particle pre-heating distribution.

Taherkhani et al. (Ref 69) also investigated the impact of pre-heating IN625 particles through the extended pre-chamber. It was reported that the critical velocity was dropped by 26% when IN625 particles pre-heated and sprayed at 1100 °C and 50 bar. The findings also indicate a similar trend to the results obtained by Cavalier et al. (Ref 73, 74), where increasing pre-heating temperatures lead to an increase in impact temperature. The porosities of the IN625 coatings, however, demonstrate values that are four times higher when compared to those achieved by Cavalier et al. (Ref 64). As summarised in Table 1, the lowest coating porosity was produced when IN625 particles were pre-heated at 1100 °C with porosity around 0.60 %. Furthermore, as the pre-heating temperatures are reduced by 100 °C, the porosity of the IN625 coatings increases by nearly 2.25%. As stated in the previous paragraphs, the rise in porosity is influenced by various parameters, among which the impact temperature and velocity play critical roles in determining the final quality of the coatings. Additionally, they observed that the coating strength was improved dramatically by over 70% when depositing pre-heated IN625 particles of smaller size, in comparison to larger particle sizes. These findings can be correlated with an earlier particle bonding theory in cold spray process as proposed by Schmidt et al. (Ref 70), where the occurrence of adiabatic heating, resulting from heat conduction, particle size, and critical velocity; generated extremely localised shearing, leading to a substantial temperature rise at the interface, which then facilitates prominent bonding.

Cold spraying of pre-heated IN718 particles has also been experimented by Pérez-Andrade et al. (Ref 51) with pre-heating temperatures of 800 °C, 900 °C, and 1000 °C. The coating porosity was reduced by nearly 30% using the highest pre-heating temperature. They also investigated that pre-heating IN718 particles did not massively contribute to the deposition efficiency or the final coating thickness but rather added residual stresses to the coating surface. The low coating porosities may be associated with improved particle flattening due to higher impact temperatures. Furthermore, higher particle velocity upon impact may also increase the particle deformation, leading to a more prominent flattening as suggested by Maev and Leshchynsky (Ref 71), where they discovered copper particles were severely deformed with impact velocity higher than the particles’ critical velocity. The slight coating thickness variations, on the other hand, could be attributed to nozzle clogging in the throat region, which originated from the high processing parameters, as also experienced by Huang et al. (Ref 72).

Further advancement has been made to utilise the beneficial effects of particle pre-heating whilst avoiding particle accumulation in the throat area. In contrast to a built-in pre-chamber where particle temperature cannot be measured independently, the recent modification in the cold spray technique involves pre-heating particles in a separate chamber, providing a fully controlled pre-heating temperature of the particles. Typically, the modified cold spray set-up is equipped with two inlets and pre-heating compartment (Fig. 8b). One inlet is used for powder injection at ambient temperature, while hot gas is supplied from the other inlet. Both powder and hot gas are then directed together into a mixing coil made of stainless steel to undergo heating process to a certain temperature for some time, depending on the coil length, before released into the divergence end of the nozzle. It has been studied that powder injection in the downstream provides a separate particle temperature measurement independent of the primary gas flow originating from the convergence side, allowing more precise control of the particle temperature (Ref 73).

Initial work in cold spray coating deposition using a separate powder pre-heating device was conducted by Kim and Kweon (Ref 74). They investigated the influence of cold spraying pre-heated nickel powder at multiple pre-heating temperatures and concluded substantial deposition rate and coating thickness increments of more than 50%. Furthermore, they reported that the coating porosity was reduced by 3% when coupling pre-heating device with the cold spray apparatus. Powder pre-heating has also been applied for Ni-based superalloys, particularly IN718 particles. Ortiz-Fernandez and Jodoin (Ref 9) have successfully deposited pre-heated IN718 particles onto Ti-64 substrate where they found significant increase in the temperature upon impact. It was reported that increasing IN718 particle temperature enhanced the interfacial impact temperature greater than 40% when sprayed using nitrogen at 34.5 bar. Yet, the impact temperature was reduced by nearly 30% when deposited using helium at 20.6 bar. The considerable impact temperature difference impacted the overall coating formation, where one may observe that all the coating thickness was investigated to be less than 25 µm when sprayed using nitrogen (Fig. 9a, b and c). Furthermore, it was reported that increasing particle pre-heating temperatures did not increase the deposition efficiency, which was measured under 1% (Fig. 9b and c). On the other hand, pre-heating IN718 particles then sprayed using helium enhanced the coating thickness and deposition efficiency by approximately 50% compared to the non-heated particles. Furthermore, the coating porosity was observed to be less than 2% without visual evidence of micro-cracks (Fig. 9d).

Fig. 9

IN718 coatings cold sprayed using mixing coil particle pre-heating configuration with nitrogen (a-c) and helium (d) as carrier gasses. The IN718 powder (Ref 9) was (a) un-heated, and pre-heated at (b, d) 400 °C and (c) 750 °C. Used with permission from R. Ortiz-Fernandez and B. Jodoin, “Toward Efficient Cold Spraying of Inconel 718: Understanding the Influence of Coating and Particle Impact Temperatures,” J. Therm. Spray Technol., vol 32(1), pp. 188–207, 2023, Springer Nature

The cold sprayed IN718 coating formed through powder pre-heating inside a mixing coil has demonstrated significant thickness development yet, further mechanical assessment using micro-hardness test suggested the presence of cracks in the region of the indentation mark (Fig. 10). The micro-hardness of the coatings produced using cold spray process has been well studied by Levasseur et al. (Ref 75) and MacDonald et al. (Ref 76) utilising IN718 and Ti-64 coatings. They concluded the hardness of the cold sprayed coatings is derived from the cracks of poorly bonded particles that are generated from the indentation load. Correspondingly, the IN718 coating micro-hardness obtained by Ortiz-Fernandez and Jodoin (Ref 9) indicated a weak inter-particle bonding as proven by the appearance of cracks. Moreover, the microhardness value of the layer coated with pre-heated IN718 particles was slightly reduced by nearly 2% compared to the non-heated powder, which originated from poorly bonded inter-particles. These findings are not expected since particle pre-heating increases the impact temperature, which subsequently improves the occurrence of adiabatic softening that results in a more pronounced plastic deformation (Ref 60).

Fig. 10

Microhardness indentation profiles of the IN718 coatings fabricated using mixing coil set-up. The IN718 particles were (a) un-heated and (b) heated at 400 °C, then sprayed using helium. Used with permission from R. Ortiz-Fernandez and B. Jodoin, “Toward Efficient Cold Spraying of Inconel 718: Understanding the Influence of Coating and Particle Impact Temperatures,” J. Therm. Spray Technol., vol 32(1), pp. 188–207, 2023, Springer Nature (Ref 9)

Thus, it is worth analysing the fundamental of cold spray deposition process, which is the kinetic energy, to correlate the inter-particle bonding within the coating. As suggested by Hussain et al. (Ref 37), the kinetic energy in the cold spray process is governed by the particle velocity, where an improvement of particle velocity is desirable to enhance the kinetic energy upon impact. The higher kinetic energy increases the incidence of adiabatic shearing, subsequently facilitating an extensive formation of metal jet. The metal jet arrangement has been widely accepted to be a sound indicator of intense plastic deformation in cold spray process, which also enhances the inter-particle bonding within the coating (Ref 77). The low impact velocity of the pre-heated IN718 particles measured by Ortiz-Fernandez and Jodoin (Ref 9) could be the reason of the low deposition efficiency. This then stipulates that although an increase in impact temperature may lower the critical velocity for particle adhesion, relying solely on higher impact temperatures is inadequate to develop coating with dense structures. Instead, achieving higher impact velocities is also necessary. As also suggested by Assadi et al. (Ref 55), coatings formation can be significantly improved when the particles impact velocity is larger than their critical velocity by promoting the particles flattening, subsequently enhancing deposition efficiency. In the case of pre-heated IN718 powder inside a separate mixing coil, the particles were propelled in the diverging section of the nozzle, which increased the propellant gas velocity and reduced the gas temperature due to expansion. The lower gas temperature in the expansion region simultaneously decreased the pre-heated IN718 particles’ temperature through convection, causing heat dissipation.

Laser-Assisted Cold Spray

Laser-assisted cold spray (LACS) process employs the benefits of cold spray and laser irradiation (Ref 99). Various materials that are difficult to be deformed through a conventional cold spray route, such as, stellite−6 (Ref 92, 93), WC-SS316L (Ref 102), Al7075 (Ref 103), Mo (Ref 104), and Ni-based superalloys (Ref 11) have been successfully deposited with the assistance of laser irradiation. In general, LACS arrangement comprises a generic cold spray system with the addition of a laser and pyrometer (Fig. 11). In LACS, a pyrometer is directed to the illumination region where surface temperature reading is then fed through a closed-loop feedback, which adjusts the laser power accordingly to maintain the targeted surface temperature.

Fig. 11

Reproduced from https://doi.org/10.3390/met13101640 under the MDPI Open Access policy: (Ref 105)

LACS configuration developed by TWI.

Generally, the process can be categorised according to the laser functions, which are determined by the relative positioning of the laser with respect to cold spray nozzle. For instance, several researchers (Ref 97,98,99,100) have conducted LACS with cold spray nozzle trailed laser beam. In this configuration, laser can be applied to eliminate native oxides of the substrate and pre-heat the deposition area prior to particle deposition. A fair amount of work (Ref 101,102,103) has also been demonstrated using a co-axial configuration where the laser spot coincides with the cold spray stream. The co-axial setup allows substrate heating and particle pre-heating within the same period, which subsequently elevates the impact temperature due to the friction between particle-particle and substrate-particle, plastic deformation, and laser irradiation. Hence, promoting a thermal softening effect which is beneficial to cause a prominent adiabatic shearing. Additionally, an in-situ post-spray treatment can be performed with laser irradiating the coating structure at certain temperatures, providing an annealing effect and melting the deposited particles (Ref 104, 105).

In the context of Ni-based superalloys, lasers are primarily employed to pre-heat the substrate and to remelt the coating layers. Poza et al. (Ref 11) were the first to study the microstructures and mechanical properties of the cold sprayed IN625 coatings after the laser remelting process. They observed that the IN625 coatings exhibited a pores reduction of 10% with an increase in the heat input, which is also commonly found in a laser treatment process (Ref 107, 108). The laser remelting technique also leads to a microstructural change where IN625 coatings splat microstructures have undergone a complete transformation. As depicted in Fig. 12, the as-deposited IN625 particles are characterised by a relatively dense structure with coating thickness and porosity of 480 µm and 3%, respectively. Moreover, one may examine that a significant proportion of unbonded IN625 particles in the coating interface, which may be attributed to the weak bonding between each particle (Fig. 12a). Further observation of the as-deposited IN625 coatings suggests a typical splat formation in cold spray coatings with elongated particle interior, which is attributed to severe plastic deformation during the deposition process (Fig. 12b). Additionally, the splats microstructures remain unchanged, showing original dendrites that are formed by rapid solidification through atomisation method.

Fig. 12

SEM BSE micrographs at (a) low and (b) high magnifications of the as-sprayed IN625 coating produced by Poza et al. (Ref 11), exhibiting highly-deformed IN625 splats derived from repetitive cold work during deposition process. Reprinted from Surf. Coatings Technol., vol 243, P. Poza et al., “Mechanical properties of Inconel 625 cold-sprayed coatings after laser remelting. Depth sensing indentation analysis,” pp. 51–57, Copyright (2014), with permission from Elsevier

In contrast, the introduction of laser to the IN625 coatings induces a notable microstructural change. Using a lower heat input (Fig. 13a and b), microstructural changes become evident, in particular in the upper-to-mid layer of the coatings, where fine columnar dendritic structures are generated from the rapid solidification during the melting process. However, the coating interface did not experience a melting process, where the unbonded particles and deformed splats retain their original state, similar to the initial cold sprayed IN625 coatings. Furthermore, few distinct pores can still be noticed in the middle region of the coatings. In addition to the laser irradiation, increasing thermal input resulted in a more pronounced effect in the coating microstructures (Fig. 13c and d). The interparticle boundaries in the middle layer are completely welded followed by considerable pores reduction. Moreover, higher heat input improves the metallurgical bonding in the coating interface where the weakly bonded particles are fused with the adjacent particles.

Fig. 13

IN625 coatings after laser remelting process at 700 W with heat input of 15.5 J/mm (a & b) and 28.0 J/mm (c & d), demonstrating columnar dendritic structures (Ref 11). Reprinted from Surf. Coatings Technol., vol 243, P. Poza et al., “Mechanical properties of Inconel 625 cold-sprayed coatings after laser remelting. Depth sensing indentation analysis,” pp. 51–57, Copyright (2014), with permission from Elsevier.

Mechanical properties of the laser-melted IN625 coatings have also been reported by Poza et al. (Ref 11) utilising a depth sensing indentation technique. As demonstrated in Fig. 14(a), the as-sprayed IN625 coating Young’s modulus is comparable to that of the bulk IN625 alloys, with values around E ≈ 214 GPa. Yet, the coatings present higher hardness values compared to the bulk form of the alloys. This is a characteristic mechanical property often observed in coatings fabricated through the cold spray technique, attributed to the work-hardening effect during particle deposition. The hardness values then gradually decrease with increasing penetration depth until they reach an asymptotic value, which has been studied due to the indentation size effect in FCC metals (Ref 114).

Fig. 14

Elastic modulus (E) and hardness (H) of the (a) as-sprayed and (b) laser-remelted at 15.5 J/mm IN625 coatings, procured from nanoindentation test (Ref 11). Reprinted from Surf. Coatings Technol., vol 243, P. Poza et al., “Mechanical properties of Inconel 625 cold-sprayed coatings after laser remelting. Depth sensing indentation analysis,” pp. 51–57, Copyright (2014), with permission from Elsevier

Following laser treatment, the hardness values of the coatings are reported to be lower than those untreated, with an average value of 5.7 GPa (Fig. 14b). Although laser treatment reduces the coating’s hardness, a slight increment of approximately 6% in the Young’s modulus compared to the as-sprayed IN625 coatings can be identified. The hardness reduction and the increase in Young’s modulus after laser remelting have been identified to be strongly correlated to microstructural modifications. Pore reduction, along with splats consolidation after laser treatment promotes the coating’s resistance to elastic deformation, as also observed in several metal processing techniques through laser assistance (Ref 110, 111). Besides, the microstructural transformation into columnar dendrites stimulates stress relaxation within the coatings since an intense particle deformation develops immense residual stresses, hence decreasing the hardness of the coating structures.

The tribological property of Ni-based superalloy processed through the LACS approach has also been reported by Dey et al. (Ref 117), where they deposited a solid-lubricious coating of IN625-WS₂ onto stainless steel substrate under multiple laser power. It was reported that a thicker coating was developed when the deposition temperature was elevated by gradually increasing the laser power (Fig. 15). In the case of coatings deposited through LACS process, the introduction of laser irradiation is able to cause a major plastic deformation by decreasing the yield strength of the particles and/or targeted substrate through thermal softening effect, hence promoting the incidence of thick layer of coatings without the need for higher critical velocity (Ref 118). As presented by Fig. 16(a), trace amount of IN625-WS₂ particles adhered to the stainless steel substrate when deposited with the lowest laser power. Furthermore, using a laser power of 300 W did not effectively remove the residual polishing colloidal alumina solution, which is still detected in the coating structures (Fig. 16b). Increasing laser power by 40% demonstrates significant coating thickness improvement of more than 90% without any signs of alumina inclusion (Fig. 16c). However, some cracks and pores still occur in the coating interface which may be due to the insufficient thermal softening effect of both substrate and the incoming particles (Fig. 16d).

Fig. 15

Maximum deposition temperatures during LACS process and final coating thicknesses produced on multiple laser power with constant laser scanning speed of 60 mm/min. L1 = 300 W, L2 = 500 W, L3 = 700 W, L4 = 900 W (Ref 117). Reprinted from J. Alloys Compd., vol. 983, D. Dey et al., “Microstructural and tribological properties of high-performance Inconel 625 alloy-WS2 self-lubricious composite coating by laser assisted cold spray,” p. 173840, Copyright (2023), with permission from Elsevier

Fig. 16

IN625-WS₂ coating profiles developed through LACS technique with varying laser power. (a, b) 300 W, (c, d) 500 W, (e, f) 700 W, and (g-i) 900W

Intensifying the laser power further to 700 W elevates the deposition temperature to 1120 °C, generating the thickest IN625-WS₂ coating in their study. The coatings are characterised as crack-free structures without pores, and interfacial cracks exist within the microstructures (Fig. 16e and f), which is proven by the influence of on the particles and substrate softening. The interaction between high laser input and particle deformation enhances the temperature of the deposition region, which a typical thermodynamic process in LACS, consequently improving the adhesion of the particles and deposition efficiency. However, coatings did not form when the highest laser input was applied in the process, which can be attributed to a considerable strength reduction of the substrate, causing harder particles to erode the softer surface (Fig. 16g, h and i). A similar phenomenon has also been experienced by Luo et al. (Ref 119) when Stellite 6 particles eroded the carbon steel substrate during LACS process with the site temperature adjusted to 1200 °C. Furthermore, in some LACS cases (Ref 93, 96, 105), deposition temperature that is higher than the substrate or particle solvus temperature may result in material deterioration, including oxidation, porosity, and the formation of detrimental phases. This further emphasises the importance of taking into account the intrinsic mechanical and metallurgical properties of each material in LACS process to obtain the optimum laser-cold spray interplay.

The hardness profile of the thickest IN625-WS₂ coating was then measured where results varied from 360 HV_50gf to 520 HV_50gf, then dropped massively in the interface (Fig. 17). Moreover, they reported that the average microhardness of the IN625 splats was around 381 HV, which is comparable to the hardness values of induction and furnace heat-treated cold sprayed IN625 coatings (321-340 HV₀₅) (Ref 120). This can be attributed to the fact that the IN625-WS₂ particles showed minimal dislocations during impact, resulting in a reduced level of deformation and work hardening effect (Fig. 18). The low degree of dislocations is highly influenced by the laser interaction during the deposition process as laser induces thermal softening of both particles and substrate, which has been discovered to decrease the particle’s critical velocity. In contrast, the inclusion of WS₂ matrix contributes to an increase in the hardness value of the coating, where it was reported that the decomposition of WS₂ powder particles through high particle impact produces nano-size tungsten fragments. These fragments have a high hardness value (~430 HV (Ref 121)) and fill the interspace matrix region, enhancing the coating hardness via the Orowan strengthening effect (Ref 122). This validates that the laser irradiation during the cold spray process did not significantly influence the WS₂ matrix that has a higher melting temperature than IN625, hence retaining its microstructures subsequently hardness value (Fig. 17b).

Fig. 17

Microhardness profile of the LACSed IN625-WS₂ coating produced with laser power of 700 W (Ref 117). Reprinted from J. Alloys Compd., vol. 983, D. Dey et al., “Microstructural and tribological properties of high-performance Inconel 625 alloy-WS2 self-lubricious composite coating by laser assisted cold spray,” p. 173840, Copyright (2023), with permission from Elsevier

Fig. 18

(a) SEM BSE micrographs of IN625-WS₂ coating produced by Dey et al. (Ref 117) using LACS technique with laser power of 700 W. Reprinted from J. Alloys Compd., vol. 983, D. Dey et al., “Microstructural and tribological properties of high-performance Inconel 625 alloy-WS2 self-lubricious composite coating by laser assisted cold spray,” p. 173840, Copyright (2023), with permission from Elsevier. (b) Yellow inset indicates larger magnification in the middle of the coating, exposing retained WS₂ particles (dark contrast) and heavily-deformed IN625 particles (bright contrast)

Dey et al. (Ref 117) also studied the tribological properties of the laser cold sprayed IN625-WS₂ coatings utilising ball-on-disk method. They found that both coefficient of friction (COF) and wear rate were significantly lower than the steel substrate, which is expected since steel has lower hardness than the IN625-WS₂ particles (Fig. 19a). The COF and wear rate of the deposited LACS IN625-WS₂ coatings were further reduced by 86% and 94% when compared to the cold sprayed coatings of pure IN625 produced by Wu et al. (Ref 123). Furthermore, the COF profile after 1800 s of wear test shows constant values in the range of 0.55-0.60, signifying the coating's ability to retain its wear property (Fig. 19b). This is primarily due to the substantial amount of the lubricious WS₂ particles distributed across the coating microstructure. During the LACS process, the laser irradiation is only sufficient to cause thermal softening of the IN625 and the steel substrate, whereas the WS₂ particles with notably higher melting points do not undergo a softening effect, hence conserving the structure of the tungsten particles.

Fig. 19

(a) Coefficient of friction (COF) and wear rate of the LACSed IN625-WS₂ coating deposited with laser power of 700 W. (b) COF profile of the IN625-WS₂ coating and steel substrate after 0.5 h of wear test (Ref 117). Reprinted from J. Alloys Compd., vol. 983, D. Dey et al., “Microstructural and tribological properties of high-performance Inconel 625 alloy-WS2 self-lubricious composite coating by laser assisted cold spray,” p. 173840, Copyright (2023), with permission from Elsevier

Cold spraying Ni-based superalloys with the addition of laser irradiation has demonstrated a significant bonding improvement. This was studied where the presence of laser as an external heat source promotes the metallurgical bonding in cold sprayed coating. Depending on the laser configuration, the post deposit remelting has displayed a substantial microstructural change, followed by pores reduction. Varying laser power has also revealed that higher metallurgical bonding can be achieved when tuning the laser power to 700 W, whereas increasing laser power to 900 W resulted in major degradation due to massive strength reduction of the underlying substrate, causing an intensive erosion effect. Hence, optimising the cold spray deposition of Ni-based superalloys through laser assistance involves precise control of laser power to achieve the desired surface temperature. In the case of remelting, increased laser power effectively decreases coating porosity and induces microstructural changes. Moreover, depending on the substrate material properties, it is essential to carefully maintain a certain surface temperature to avoid erosion effect during laser irradiation process.

Heat Treatment

As previously mentioned, depositing Ni-based superalloys through a cold spray process introduces several issues in the final coatings, such as porosity (Ref 124), micro-cracks (Ref 125), reduction in tensile strength (Ref 126), and loss of ductility (Ref 127), which are mainly caused by the high strength particles that have limited plastic deformability as well as the work hardening effect from the processing standpoint. The post-spray heat treatments are commonly applied to mitigate pores and brittleness of the coatings. The heat treatment processes have been proven to enhance the tensile strength of the Ni-based coatings near to bulk (Ref 78). Furthermore, feedstock heat treatment has also been experimented on to study the feasibility of increasing the plastic deformation of the particles, subsequently improving the deposition efficiency with significant pores reduction. Various post-spray and feedstock powder heat treatment methods have been attempted, which predominantly revolve around solution heat process. This section elucidates the correlation between the heat treatment process and the coatings’ properties of the cold sprayed superalloys.

Coating Heat Treatment

Cold spray of IN718 particles followed by post-spray heat treatment was first performed by Levasseur et al. (Ref 75), where they demonstrated heat treatment of free-standing IN718 coatings on several temperatures. They found that the porosity was reduced significantly by 92% when exposed to heat treatment temperature of 1250 °C. The flexural strength value of the heat-treated sample was improved substantially compared to the as-sprayed coating. This improvement can be correlated with the sintering phenomenon within each IN718 particle, which enhances the interparticle bonding of the sample (Ref 75). In addition, the presence of numerous dimples on the fractured surface serves as evidence of interparticle fractures, demonstrating the sample’s ductility (Fig. 20).

Fig. 20

Fractographs of IN718 coatings following pressureless sintering at 1250 °C for 3 h. (a) Black arrows represent intersplat boundaries; and (b) presence of dimples (Ref 75). Reprinted from Mater. Sci. Eng. A, vol. 556, D. Levasseur, S. Yue, and M. Brochu, “Pressureless sintering of cold sprayed Inconel 718 deposit,” pp. 343–350, Copyright (2012), with permission from Elsevier

Correspondingly, Wong et al. (Ref 128) fabricated IN718 coatings through cold spray process using both nitrogen and helium, then heat-treated under similar temperatures to investigate the coatings properties. They investigated that the IN718 coating porosity was slightly decreased when free-standing samples were exposed to heat treatment temperatures of 950 °C, 1010 °C, and 1250 °C, with the last showing a noticeable microstructural feature. It was reported that the splat boundaries were nearly invisible when heat-treated at 1250 °C for both spray conditions. The tensile strength of both samples subjected to solution treatment at 1250 °C indicated different behaviours. It was reported that the IN718 coating sprayed using nitrogen at higher pressure showed a higher strain compared to the sample fabricated with helium at low pressure after the heat treatment process, with an average strain value of 24.7% and 2.2%, respectively.

The pores reduction and strengths improvement of the cold sprayed IN718 after heat treatment can be derived from the metallurgical change point of view. As stated by Sinclair-Adamson et al. (Ref 129), in the cold spray process, sintering is beneficial to reduce porosity and coalesce splats, consequently promoting a large metallurgically bonded region. Sintering occurred in the heat treatment temperature of 1250 °C applied by Levasseur at al. (Ref 75) and Wong et al. (Ref 128), which was also sufficient to initiate pore spheroidisation and effectively eliminate any remaining pores existing in the interparticle boundaries. The post-spray heat treatment process also has a significant impact on the microstructural evolutions of the cold sprayed IN718 coatings. The exposure to high temperatures beyond the solvus temperatures leads to some distinct microstructural changes, which could bring both beneficial and detrimental influences to the mechanical properties of the coatings.

Microstructural evolutions of the cold sprayed IN718 coating were observed by Sun et al. (Ref 83) through solution followed by double aging heat treatments on multiple temperatures. The solution treatment at 900 °C was sufficient to dissolve $\gamma ^{\prime\prime}$-phase into $\gamma$ matrix with subsequent transformation of dendrite into equiaxed grains structures. After the double aging process, numerous amounts of MC carbides and $\delta$ phase developed in the grain boundaries and particle interiors (Fig. 21b). As suggested by Gao et al. (Ref 130), in the additive manufacturing process of IN718 that requires high processing temperatures, $\delta$ phase appears in as needle-like form, which increases material’s tensile strength to nearly 1400 MPa since this secondary phase is able to hinder dislocation slip at the grain boundaries. A further study conducted by Sun et al. (Ref 83) showed that higher solution treatment temperatures resulted in a complete dissolution of $\delta$ phase and a grain enlargement, and the following double ageing steps promoted the precipitation of $\gamma ^{\prime\prime}$ and $\gamma ^{\prime}$ Phases (Fig. 21c, d and e). They then correlated the microstructural changes of the heat-treated samples with hardness and tensile tests when they discovered massive improvements in both mechanical properties compared to the as-sprayed samples. The hardness values of all the heat-treated samples showed no variation, which may be attributed to the residual stress relaxation and the work hardening effect reduction through heat treatment, as also experienced by Bagherifard et al. (Ref 12). Furthermore, the cold sprayed IN718 samples that were heat-treated at 950 °C exhibited tensile strength around 800 MPa, which was 75% higher than the as-sprayed specimens, which is due to the smaller equiaxed grain sizes and sufficient amount of $\delta$ phase and MC carbides within the particles and grain boundaries. Solution heat treatment was also performed on the cold sprayed IN625 coatings by Shrestha et al. (Ref 131) and Devi et al. (Ref 94) at 800 °C and 1250 °C, respectively. Similar to the heat-treated IN718 coatings, the solution-treated IN625 indicated pores spheroidisation and reduction. Shrestha et al. (Ref 131) noticed that IN625 coating porosity and hardness were reduced by 42% and 24%, respectively.

Fig. 21

Reproduced from 10.3390/coatings12030347, licensed under a Creative Commons 4.0 license: https://creativecommons.org/licenses/by/4.0/

Microstructural variations of the cold sprayed IN718 coatings after a subsequent process of solution treatment and double aging at multiple temperatures (Ref 83).

The solution treatment process induces stress relaxation, consequently softening the material, decreasing the hardness value. Likewise, results produced by Devi et al. (Ref 94) demonstrated pores reduction and stress relaxation. They argued that the heat treatment process of cold sprayed IN625 coatings was not solely influenced by the alloy’s properties but also governed by the cold spray process itself. The higher gas temperatures in cold spray process increase the dynamic recrystallisation (DRX) mechanism, which also diffuses the interparticle boundaries during heat treatment as also proposed by Ma et al. (Ref 79) on the cold sprayed Ni-based superalloys. The ultrafine grains at the contact area between each particle generated from the severe plastic deformation are easier to sinter when exposed to heat treatment, leading to a highly inter-splat diffusion into the deposit matrix. Whilst plenty have performed post-spray heat treatment at high temperatures with the primary aim of pore elimination, low temperatures post-spray heat treatment of cold sprayed IN718 coatings was investigated by Kim et al. (Ref 88) to study the stress relaxation within the substrate-coating system, where further analysis is discussed in the residual stress section.

Powder Heat Treatment

In cold spray of high strength materials, the deformability relies upon the microstructure and properties of the material which are originated from the atomisation process. The rapid solidification during the atomisation process causes heavier elements to segregate, resulting in an interdendritic structure with a higher hardness value (Ref 132, 133). Powder heat treatment has been widely applied for materials with limited deformability to reduce their strength, consequently providing a prominent plastic deformation during cold spray. Commonly, solution heat treatment is used to dissolve the intermetallic network into the primary matrix to further decrease the hardness of the feedstock powder. As previously experimented by Sabard et al. (Ref 134) and Story et al. (Ref 135) on the high strength aluminium, a complete dissolution of the intermetallic phase after feedstock solution treatment increases cold spray deposition efficiency by 25% compared to the as-atomised powder, followed by large dislocations within the splat and outside the particle. To date, there is only one work on the powder heat treatment of the Ni-based superalloy, which was developed by Perez et al. (Ref 10). They conducted a solution heat treatment of IN718 powder at multiple temperatures using a fluidised bed method to investigate the influence of powder microstructural changes towards the coatings’ properties. The solution treatment of IN718 powder at 700 °C revealed no discernible changes in material microstructures, with no observable signs of intermetallic dissolution (Fig. 22a and d).

Fig. 22

IN718 particles heat-treated at (a, d) 700 °C, (b, e) 800 °C, and (c, f) 900 °C. The bright contrast corresponds to secondary phase whilst dark contrast indicates primary phase with lighter atomic weight elements (Ref 10). Reprinted with permission of ASM International. All rights reserved

Higher heat treatment temperatures at 800 °C and 900 °C exhibited a cellular network of the intermetallic phase that started to precipitate into the $\upgamma$ matrix. Furthermore, the heat treatment process at a temperature of 900 °C converted $\gamma ^{\prime\prime}$ into a stable ${\updelta }$ phase (Fig. 22b and e; c and f), which can also be observed in the heat treatment of wrought IN718 (Ref 136). They then deposited the solution-treated IN718 particles using helium and observed a gradual porosity reduction with increasing heat treatment temperatures, where the porosity was greatly reduced by 60% when 900 °C was applied as the heat treatment temperature. Furthermore, the coating thickness of the heat-treated IN718 particles was increased by approximately 48% compared to the as-produced powder (Fig. 23). Although the heat treatment temperatures applied by Perez et al. (Ref 10) did not dissolve the intermetallics completely, the current solution treatment temperatures were sufficient to enhance the quality of the coating and could be used as reference temperatures for IN718 powder heat treatment, stipulating that a complete interdendritic dissolution is not always necessary to reduce material’s strength.

Fig. 23

Cold spray deposits of heat-treated IN718 powder at (b) 700 °C, (c) 800 °C, and (d) 900 °C. Bottom micrographs show higher magnifications (Ref 10). Reprinted with permission of ASM International. All rights reserved

Depositing Ni-based superalloys via cold spray process presents challenges such as porosity, micro-cracks, reduced tensile strength, and loss of ductility, primarily stemming from limited plastic deformability of high-strength particles and work hardening effects. Utilising post-spray heat treatments is a prevalent strategy to address these issues, leading to enhancements in tensile strength and reductions in porosity. Studies underscore the pivotal role of heat treatments in improving coating properties through mechanisms like particle sintering, pore reduction, and impactful microstructural alterations. An alternative heat treatment has been demonstrated by solution treating Ni-based superalloy feedstock. The particle heat treatment process plays a crucial role in enhancing material deformability, reducing hardness, and optimizing deposition efficiency within cold spray processes. These treatments highlight the importance of microstructural modifications in elevating coating quality without necessitating complete intermetallic dissolution.

Residual Stress

Residual stress also develops in the coating deposited in thermal spray through the impact of molten or solid particles (Ref 137,138,139,140,141,142), heat transfer during rapid cooling of the deposited particles (Ref 143,144,145,146,147,148) and due to mismatch in the coefficient of thermal expansion (CTE) (Ref 141). In cold spray process, the formation of coatings in the solid-state can be achieved through cold gas spraying process where solid metal particles are accelerated onto targeted substrate to a supersonic velocity (500-1000 m/s) (Ref 142). Constant impact of high-velocity metal particles directed to the substrate leads to the creation of localised strain in both coating and the surrounding interface, ultimately forming significant amounts of compressive residual stress within the coating and substrate. Similar to other thermal spray techniques, residual stresses produced by cold spray process are also effected by thermal attributes like temperature gradients, cooling rates, and thermal mismatch in dissimilar coatings (Ref 143). Depending on the metal particle and substrate materials, as well as spraying parameters, the interplay between mechanical and thermal loads during coating deposition process can either relieve residual stress or convert compressive to tensile residual stresses (Ref 144). The presence of residual stresses does not affect the state of equilibrium between the material and its surrounding environment, yet residual stresses within the coating deposited via cold spray process might exhibit destructive impacts on the coating integrity like peeling and delamination (Ref 152, 153), which cause premature fatigue crack initiation. For this reason, residual stress measurement should be addressed to study the process-performance relationship of the deposited coating. A full summary of residual stress of Ni-based coatings comparison from multiple spray conditions is presented in Table 2.

Table 2 Residual stress comparison from multiple cold sprayed Ni-based superalloys coatings

Influence of Post-spray Heat Treatment on Coating Residual Stress Distributions

Bagherifard et al. (Ref 12) quantified the as-sprayed IN718 coating residual stresses focusing on in-depth distribution utilising x-ray diffractometer. The as-sprayed IN718 coatings exhibited slight deviations in residual stresses, with the highest values of both tensile and compressive residual stress measuring 50 MPa. In the cold spray deposition method, compressive residual stress is induced by a shot peening effect caused by the impact of the incoming metal particles. Conversely, tensile stress is generated through quenching stresses by the cooling and contraction of each splat as it rapidly cools to match the substrate temperature (Ref 147). They then studied the residual stress behaviours of the IN718 coatings after post-deposition machining and heat treatment processes where it was calculated that all samples demonstrated compressive residual stresses. As-machined samples displayed substantial compressive residual stresses with maximum value nearly 500 MPa close to coating top surface. These results signify that the formation of compressive residual stresses in IN718 coatings is primarily due to the machining process rather than the cold spray coating deposition itself. This is consistent with the residual stress characteristics of parts produced through conventional machining techniques (Ref 154,155,156). IN718 coatings were heat-treated at 1050 °C/3 h and 1200 °C/1 h where compressive residual stresses relaxed after heat treatments. IN718 coatings heat-treated under 1050 °C/3 h exhibited highest compressive residual stresses of 450 MPa and relaxed to 300 MPa between the depth of 0.06-0.08 mm from coating top surface. Similarly, compressive residual stress profiles within IN718 coating heat-treated at 1200 °C/1 h similar were investigated to follow the stress profiles of the previous heat treatment condition. Considerable stress relaxation with highest compressive residual stress value was 500 MPa and relaxed to 450 MPa within the depth of 0.06-0.08 mm from coating top surface. The slight variations in compressive residual stress of IN718 coatings at both heat treatment conditions may be sourced from a thin layer of oxide on the coating’s surface after heat treatment at 1200 °C/1 h. Surface oxidation at high temperature increases volume at the atomic level whilst the original layers are restricted by the IN718 bulk material that has the potential to generate compressive residual stresses at the surface. Therefore, the annealing treatment at 1050 °C/3 h and oxide films formation at 1200 °C/1 h provide counteracting effects that minimise compressive residual stress variations in IN718 coatings heat treated at 1200 °C/1 h. Bagherifard et al. (Ref 90) again conducted residual stress measurement of IN718 coatings using x-ray diffractometer. Under the same conditions discussed in their previous study (Ref 12), they added that the as-sprayed IN718 coatings lost their work hardening effects from shot peening phenomenon that led to IN718 coatings were not in compressive stress. In cold metal spray, the solid metal particles are accelerated at supersonic velocity where corresponding peening effects result in high compressive residual stresses in the coatings (Ref 52). However, in this case, the processing gas temperature of 1000 °C greatly dominated the beneficial effect of impacts with high kinetic energy for generating compressive residual stresses which resulted in stress relaxation of the as-sprayed IN718 coatings (Ref 159, 160).

Pérez-Andrade et al. (Ref 51) reported the residual stress profiles of IN718 coatings deposited on various processing gas temperatures (Fig. 24). The residual stress variations observed in all specimens suggested that both the IN718 deposits and substrate were under compressive residual stress across the entire coating thickness of the as-sprayed samples, down to a depth of 700 µm into the substrate. Stress relaxation occurred within the substrate near 1.5 mm from coating top layer where continuous slope ascended to tensile region with maximum value of 300 MPa at 1.6 mm away from coating outer layer. The as-sprayed samples were then heat-treated at 1163 °C where significant compressive residual stress relaxation observed. The heat-treated samples sprayed using gas temperatures of 800 and 900 °C presented compressive residual stress reduction of 53% compared to those without heat treatment process. Likewise, compressive residual stresses of IN718 coatings cold sprayed at 1000 °C decreased by 33% after heat treatment procedure.

Fig. 24

In-depth residual stress profile of the cold sprayed IN718 coatings at different spraying temperatures (Ref 51). Reprinted from Surf. Coatings Technol., vol. 378, L. I. Pérez-Andrade et al., “Optimization of Inconel 718 thick deposits by cold spray processing and annealing,” p. 124997, Copyright (2019), with permission from Elsevier

Sundaram and Raghupatruni (Ref 89) studied the residual stress of self-mated cold sprayed IN718 utilising x-ray diffractometer. The produced samples were heat-treated at 954 °C for one hour, followed by thorough thickness residual stress calculation for up to 2000 µm depth from the coating top surface, where residual stress profiles are plotted in Fig. 25. The IN718 coating and substrate were measured compressive residual stresses with a maximum value of ~ 100 MPa. Stress relaxation occurred at around 500 µm from coating top surface with compressive residual stresses of 10 MPa, mainly due to the heat treatment. Furthermore, the low compressive residual stresses across the coating thickness and in the substrate can be instigated by three different factors: processing gas temperature, coating thickness, and heat treatment. As experimented by Bagherifard et al. (Ref 90), coating deposition via a cold spray process utilising processing gas temperature of 1000 °C decreased peening and work hardening effects, subsequently reducing the formation of compressive residual stresses within the IN718 coating. Coating thickness also decreases the compressive residual stress values of the deposited coatings, as proven by Singh et al. (Ref 13). Where they discovered compressive residual stresses were greatly reduced by 92% with an increase in coating thickness, mainly due to dynamic recovery and recrystallisation during or after deposition.

Fig. 25

Through depth residual stress measurement of IN718 coating performed by Sundaram and Raghupatruni (Ref 89). H. Sundaram and P. Raghupatruni, Residual Stress, Microstructure, and Characterization of Self-Mated Additive Repair of Inconel 718 Alloy Using Cold Spray Process, pp. 225–237, 2022, Springer Nature

Shrestha et al. (Ref 85) examined the residual stress of IN718 and IN625 coatings produced by cold spray process exercising x-ray diffractometer. The IN718 and IN625 coatings were separated from the aluminium substrate to construct free-standing coating samples where all coating specimens indicated compressive residual stresses in every measurement direction. In the transverse direction, the as-deposited IN718 and IN625 coatings demonstrated the greatest compressive residual stresses at the sub-surface or coating core, measuring 225 MPa and 229 MPa, respectively (Fig. 26). After post-spray heat treatment at 800 °C for one hour, compressive residual stresses of IN718 and IN625 samples were drastically reduced more than 79%, particularly in 45° and longitudinal directions, respectively. It was reported that the transformation of compressive to tensile residual stress was due to porosity reduction during post-spray heat treatment, which caused stress relaxation within the free-standing samples. The as-sprayed IN718 and IN625 coatings exhibited relatively compact structures, with porosity levels of 4.39 and 1.67%, respectively. Following the heat treatment process, the porosity levels were reduced by 48 and 42%, respectively. Higher pore reduction rate within IN718 samples resulted in higher tensile residual stress formations, as a consequence, the compressive residual stress that was initially generated during cold spray process was eliminated. The results obtained by Shrestha et al. (Ref 85) correspond well with IN718 coatings residual stress measured by Kim et al. (Ref 88) where pore elimination forms tensile residual stress within the coatings after heat treatment process. A similar explanation can also be applied to IN625, where relaxation occurs due to pore elimination.

Fig. 26

Residual stress profiles of (a) IN718 and (b) IN625 coatings after heat treatment process calculated inside the coating at 45 °, longitudinal, and transverse directions (Ref 85). Reprinted with permission from D. Shrestha, F. Azarmi, and X. W. Tangpong, “Effect of Heat Treatment on Residual Stress of Cold Sprayed Nickel-based Superalloys,” J. Therm. Spray Technol., vol. 31, no. 1–2, 2022, Springer Nature

Srinivasan et al. (Ref 152) deposited IN625 particles onto a low-alloy AISI 4130 steel using high pressure cold spray and they calculated through thickness residual stress of the IN625 coating via x-ray diffractometer. The residual stress profiles of the as-sprayed IN625 coatings were compared to those of NiCr coatings, and the results are presented in Fig. 27. The residual stress distributions of both coatings were compressive in nature where IN625 coating displayed compressive residual stresses within the range of 100-500 MPa. In contrast, the NiCr coating exhibited a greater magnitude of compressive residual stresses, ranging from 200-800 MPa, possibly reaching very close to the material’s yield strength, in some pockets (Ref 152). The high difference between IN625 and NiCr coatings residual stresses is due to materials composition. IN625 is composed of several alloying elements, while NiCr, as its name implies, consists only of Ni and Cr. The presence of Nb which is a refractory metal in IN625 provides additional strength at high temperature to the alloy, which also reduces the deformability of the alloy (Ref 153). In contrast, NiCr exhibits greater plastic deformability, which results in the generation of higher levels of compressive residual stresses in the coatings. The residual stresses developed within the cold sprayed Ni-based superalloys coatings have presented that coatings are compressive in nature. The compressive residual stress is developed through shot peening and work hardening effects, which is derived from the cold spray parameters and mechanical properties of the alloy. The subsequent post-deposit heat treatment in multiple temperatures converts compressive residual stress into tensile residual stress, which is developed during pores elimination. Moreover, careful consideration should be taken into account when selecting cold spray parameters since changing one parameter may result in a different residual stress development.

Fig. 27

Residual stress distribution of IN625 and NiCr coatings with respect to the coating thickness before and after heat treatment (Ref 152). “Characterization of Cold-Sprayed IN625 and NiCr Coatings,” D. Srinivasan et al., J. Therm. Spray Technol., vol. 25, no. 4, Springer Nature, 2016, reproduced with permission from SNCSC

Impact of Coating Thickness on Residual Stress Profiles

Singh et al. (Ref 13) investigated the influence of coating thickness upon residual stresses of the IN718 coatings residual stress deposited using high-pressure CS system, exercising substrate bending and incremental hole-drilling methods. Residual stress calculation through the substrate bending method is derived from the curvature profile within the substrate before and after deposition. They then measured the first set of curvature on different coating thickness ranging from 216 µm to 1173 µm where it corresponded with the coating thickness (Fig. 28).

Fig. 28

Residual stress values on multiple IN718 coating thicknesses (Ref 13). Reprinted from Surf. Coat. Technol., vol. 350, R. Singh et al., “Influence of coating thickness on residual stress and adhesion-strength of cold-sprayed Inconel 718 coatings,” pp. 64-73, Copyright (2017), with permission from Elsevier

The residual stresses of the IN718 coatings suggested a high value of compressive residual stresses where thinnest coatings indicated highest compressive residual stresses of 1300 MPa. Compressive residual stresses relaxed with an increase in coating thickness, denoted that coating thickness massively influenced the formation of residual stresses within the coatings. They then quantified the IN718 coatings residual stresses by modifying spraying process where the deposition was paused after each step followed by curvature calculation. Residual stresses formed withing the coatings with four different coating thicknesses are presented in Fig. 29.

Fig. 29

Residual stress variations with increasing coating thickness estimated through bending method, where a total delamination occurred at the highest thickness (Ref 13). Reprinted from Surf. Coat. Technol., vol. 350, R. Singh et al., “Influence of coating thickness on residual stress and adhesion-strength of cold-sprayed Inconel 718 coatings,” pp. 64-73, Copyright (2017), with permission from Elsevier

The residual stress measurements showed that the IN718 coatings were compressive in nature and continuously decreased with increase in coating thickness which corresponded to the residual stress profiles of the first set. However, coating delamination was observed when the coating thickness reached 770 µm. The delamination corresponded with the deposition process where the peening effect was interrupted during pass/raster change, and relaxation of inherent residual stress occurred within the coating by disintegrating the entire IN718 coating from the substrate.

It can be noticed that thin coatings generate high curvature in the substrate and subsequently form significant amount of compressive residual stresses. The high curvature in the thin coatings is sourced from the combined effect of both stresses in the substrate and coating itself, where high kinetic energy particles bombardment increases substrate’s curvature, whereas peening effect in the coating results in the accumulation of coating residual stress (Ref 154, 155). The peening effect caused by the incoming solid metal particles accumulates with recurrent impacts, which stipulates that higher coating thickness deposited via the cold spray process causes the prominent peening effect. This then leads to an increase in compressive residual stress (Ref 156). However, impact stress relaxation might take place either during or after the deposition process through dynamic recovery and recrystallisation (Ref 13) which could be the intrinsic reason for the decrease in compressive residual stresses of thicker IN718 coatings.

Singh et al. (Ref 13) then calculated residual stresses depth profile via incremental hole-drilling mechanism in both coating and substrate under equal spray parameters as samples coated for curvature quantification. The residual stresses were measured in two measuring directions, noted as ${\sigma }_{1}$ and ${\sigma }_{2}$ where residual stresses in both directions were nearly identical, which stipulated the equi-biaxial residual stress nature of the coatings (Fig. 30). It can be interpreted that residual stresses of IN718 coatings displayed as compressive nature and continuously transformed into tensile residual stresses towards the coating-substrate interface where the average compressive residual stresses for both coating thicknesses were reported to be around 500 MPa. The incremental hole-drilling method quantified residual stress profiles in a coating/substrate with certain coating thickness, whereas bending method identified a mean coating residual stress on different coating/substrate systems with coating thickness variations (Ref 157). Nevertheless, both measuring techniques presented similar trend where compressive residual stresses were formed within the IN718 coatings.

Fig. 30

Through thickness residual stress profile of IN718 coatings with thickness of 613 µm and 730 µm measured using hole-drilling method. The measurements were taken in ${\upsigma }_{1}$ and ${\upsigma }_{2}$ directions (Ref 13). Reprinted from Surf. Coat. Technol., vol. 350, R. Singh et al., “Influence of coating thickness on residual stress and adhesion-strength of cold-sprayed Inconel 718 coatings,” pp. 64-73, Copyright (2017), with permission from Elsevier

Zhang et al. (Ref 80) deposited IN718 coatings via high-pressure cold spray and atmospheric plasma spray (APS) deposition techniques and compared both coatings residual stresses using x-ray diffractometer. The x-ray stress measurement exposed that IN718 coatings deposited by cold spray process showed compressive residual stresses with an average value of 30.5 MPa. Conversely, IN718 coatings fabricated using the APS technique exhibited tensile residual stress variations, with an average residual stress of 254.4 MPa. It was reported that the significant contrast in residual stresses was attributed to the structural characteristics of the coatings produced by each deposition technique where the cold spray process generated coatings with thickness of 683.9 µm and maximum porosity of 0.5%. In comparison, the coatings produced through APS were 65% thinner and had a maximum porosity of 1.5%. Variations of coatings characteristics can be sourced from deposition techniques where cold spray process utilises plastic deformation originated from high kinetic particle energy whereas APS relies on heat transfer mechanism between particles and plasma to solidify molten particles into a coating layer (Ref 158). As a consequence, coatings produced via the cold spray process exhibit thick, dense, and oxide-free structures while coatings deposited through APS present thin, pores, unbonded interface, as well as oxide layers on their structures (Ref 80). The compressive residual stress value in the cold sprayed IN718 coatings produced by the authors is correlated with results presented by Singh et al. (Ref 13), where compressive residual stresses decreased with an increase in coating thickness. APS, however, involves high temperature (12000-16000 °C) for the deposition process (Ref 159) where tensile residual stresses are developed through splats formations which involve solidification process from molten particles (within the region of melting temperature) to the temperature of the underlying substrate (Ref 160).

Seng et al. (Ref 161) evaluated the influence of multiple spray angles towards residual stresses of IN718 coatings using x-ray diffractometer. They deposited IN718 particles onto Al6061-T6 substrate with varying nozzle spray angle in relation to the surface plane where they discovered all coatings were under the influence of compressive residual stresses. It was observed that a decrease in spray angle resulted in an increase in the magnitude of compressive residual stresses whilst changes in processing pressure and gas temperature did not significantly alter the residual stress variations (Fig. 31). Substantial compressive residual stress reduction of 50% was measured when impact angle was below 60° for highest temperature and pressure with highest particle velocity. However, it was investigated that the effect of particle velocity on the compressive residual stresses was less pronounced when cold spraying was conducted at higher spray angles above 70°.

Fig. 31

Coating thickness of the IN718 coatings for different deposition parameters developed at various spray angles (Ref 161). Reprinted from Surf. Coatings Technol., vol 466, D. H. L. Seng et al., “Impact of spray angle and particle velocity in cold sprayed IN718 coatings,” Copyright (2023), with permission from Elsevier

The sharp rise in compressive residual stresses of the samples when decreasing the spray angle can be investigated from total coating accumulated on the Al6061-T6 substrate. As presented in Fig. 32 that cold spraying at oblique spray angle of 50° with highest particle velocity provides the least amount of IN718 coating thickness around 200 µm. The findings correspond to results provided by Singh et al. (Ref 13) and Zhang et al. (Ref 80) where thicker coatings possess lower compressive residual stresses compared to those which have less coating thickness. As previously noted, compressive residual stresses are primarily generated in cold metal spraying through the peening effect of continuous bombardment of solid metal particles onto the surface, which results in work hardening and compression of the coating. However, impact stress relaxation may occur during or after the deposition process through dynamic recovery and recrystallisation which could be a contributing factor to the decrease in compressive residual stresses in thicker IN718 coatings.

Fig. 32

Reproduced from https://doi.org/10.1007/s11666-020-00988-w, licensed under a Creative Commons CC BY 4.0 license: https://creativecommons.org/licenses/by/4.0/

Residual stress variations of (a) thin (73 µm), (b) medium (211 µm), and (c) thick (350 µm) IN718 deposits (Ref 52).

Vaßen et al. (Ref 52) also examined the effect of different coating thickness towards residual stress of IN718 coating using incremental hole drilling technique. They found that the residual stress depth distribution within the IN718 coatings in multiple thicknesses indicated compressive residual stress profiles (Fig. 33). The compressive residual stress profiles of thin IN718 coatings were approximately 100 MPa in both stress components, where stress intensification happened within coating-substrate interface. Stress relaxation occurred within the substrate, roughly 0.1 mm away from coating interface and transformed completely into tensile state at the depth of 0.3 mm from coating-substrate interface. The residual stress profiles of medium and thick IN718 coating layers were also examined, and the stress distributions were consistent with those obtained from the thin layer. One may notice that the highest compressive residual stresses for all samples were located near coating-substrate interface. It is investigated that high compressive residual stresses near coating-substrate interface are originated from grit blasting process of the substrate prior to cold spray deposition, which induces higher compressive stress level at the surface (Ref 162, 163). The combined effect of grit blasting and tamping effect during cold spray process results in accumulation of coating compressive residual stress, in particular for thin layer. In thicker layer, IN718 coatings may demonstrate dynamic recovery and recrystallization which subsequently cause relaxation.

Fig. 33

Residual stress value of the LACSed IN625/WS₂ (Ref 117). Reprinted from J. Alloys Compd., vol. 983, D. Dey et al., “Microstructural and tribological properties of high-performance Inconel 625 alloy-WS2 self-lubricious composite coating by laser assisted cold spray,” p. 173840, Copyright (2023), with permission from Elsevier

A study investigated how coating thickness affects residual stresses in IN718 coatings deposited using a high-pressure cold spray system. Thinner coatings exhibited higher compressive stresses that decreased with increasing thickness, resulting in delamination at 770 µm. Another research compared cold spray and atmospheric plasma spray methods, showing contrasting residual stress types. Lower spray angles were found to amplify compressive stresses in IN718 coatings. Residual stress depth profiles revealed compressive stresses near the coating-substrate interface, influenced by grit blasting of the substrate and the cold spray process.

Residual Stress in LACSed Ni-based Superalloy Coating

To this date, there is only one publication that studies the influence of laser irradiation towards the formation of residual stress in the Ni-based alloy coatings. The investigation was performed by Dey et al. (Ref 117), who laser cold sprayed IN625-WS₂ particles with laser input equal to 700 W, where stress measurement was measured through XRD method. The residual stress calculated was lower than those deposited without the laser irradiation process. As mentioned previously in most cases, the cold sprayed coatings’ residual stress is compressive in nature due to the peening effect. Likewise, the presence of residual stress in LACS Ni-based coatings also displays a similar characteristic where compressive residual stress occurs within the IN625-WS₂ coating (Fig. 34). The average compressive residual stress value was quantified to be around 218 MPa, which correlates to the values reported by Srinivasan et al. (Ref 152) and Silvello et al. (Ref 164) when also measured using XRD technique. This can be derived from the laser interplay during the deposition process, where the deposited layer is contracted due to the subsequent heating and cooling in short period of time. Upon heating, the in-flight IN625-WS₂ particles impacted the substrate or previously deposited layer at high temperature then contracted during cooling, which places the coating under compression.

Fig. 34

Residual stress value of the LACSed IN625/WS2 (Ref 117)

Summary and Concluding Remarks

Cold spray has emerged as a novel technique for depositing Ni-based superalloys, with an increasing number of research studies in the past decade. Many discovered that the high-strength characteristic derived from the complex physical metallurgy of these alloys, leads to the lack of plastic deformation, subsequently generating coating structures with poor mechanical and metallurgical properties. Some attempts have been initiated, including modifying the cold spray configuration as well as the feedstock powder and conducting a post-spray treatment process. The correlations between the modified processes with the mechanical and metallurgical properties, as well as residual stress, were investigated to provide a comprehensive understanding of how these processes influence the coating structures.

First, it was studied that cold spraying Ni-based superalloys particles onto a pre-heated substrate provided a localised thermal softening, which led to a substantial plastic deformation. As a result, the coating adhesion was improved to nearly 70% by increasing substrate temperature compared to those sprayed onto the unheated substrates. However, the pre-heated substrate did not supply sufficient thermal softening to the subsequent layers, indicated by pores and cracks within the coatings. This has been thoroughly explained where the thermal effusivity of the Ni-based superalloy particles was inadequate to perform heat transfer to the following layers, leading to a rather large non-uniform thermal gradient between the higher temperature of the initial layer and the subsequently incoming cold particles. Whilst cold spraying of Ni-based superalloys particles onto pre-heated substrate was heavily relied upon the temperatures of the initial layer, it is worth noting that cold spray process is also governed by the particles’ kinetic energy, which is often correlated with the critical velocity of the particles. It was investigated that depositing pre-heated Ni-based superalloys particles could increase the deposition rate and coating thickness of more than 50%. Yet, it was found that the improvements were not solely derived from the hot particles but were also attributed to the gas parameters employed in the process. The higher impact velocities can be achieved from a higher gas pressure and temperature, whilst higher impact temperatures can be enhanced through increasing particle pre-heating temperatures. Such a combination could deposit a coating with a porosity as low as 0.18%, which can be determined as two crucial factors throughout this configuration. A novel cold spray method using laser irradiation technique has been examined with a large proportion of metallurgical change within the IN625 coating. The in-situ remelting mechanism through laser irradiation caused the cold sprayed IN625 coatings to possess a columnar dendrite structure, which is not a typical cold spray intersplat formations. It was reported that increasing laser input may decrease the hardness values of the coating followed by a potential Young’s modulus increment. Yet, the correlation between the improved mechanical properties and the influence of laser irradiation remain unclear, leaving an open question of whether laser irradiation can be applied to improve coatings’ mechanical properties through microstructural changes or obtained through reduced critical velocity.

Heat treatment processes, including coating and powder, have also been covered, concluding substantial metallurgical changes within the coatings. The beneficial effects of the heat treatment processes of both coatings and feedstock powder relied upon the presence of the precipitates, primarily $\delta$ phase. The presence of the intermetallics could elevate the tensile strength of the cold sprayed IN718 coating as high as 75% compared to the as-sprayed sample, whereas the presence of $\delta$ phase in feedstock may increase deposition efficiency. Although the feedstock heat treatment had successfully demonstrated improved coating structures, there are many gaps within the metallurgical perspectives, such as what are the primary phases that govern the feedstock ductility, the influence of the precipitates towards the degree of plastic deformation, and the effect of heat treating feedstock on the tensile strength. Such gaps should be addressed to gain a fundamental understanding to correlate feedstock solution treatment with cold spray process. The residual stress of the cold sprayed Ni-based superalloys coatings demonstrated similar trends with compressive residual stress predominantly measured within the coating layer regardless of the process modification methods. This was mainly due to the impingement process, causing severe plastic deformation within the layer. With an increasing number of research in this particular processing technique in the past decade, the challenges are mainly revolved around porosity and delamination, which should be addressed through several modified techniques. Yet, each modified method still demonstrates various drawback, which we think that a set of parameter like impact temperature and velocity should be established. The two parameters will assist many researchers to generate a cold sprayed Ni-based superalloys coatings with consistent coating properties regardless the modification routes applied. We are certain that these modified cold spray processes can be pursued depending on the targeted purposes, whether served as a protective layer, additive manufacturing, or structural repair, to generate coating structures with desired mechanical and metallurgical properties, while minimising waste of resource.