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Article

Effect of Printing Orientation Angle and Heat Treatment on the Mechanical Properties and Microstructure of Binder-Jetting-Printed Parts in 17-4 PH Stainless Steel

by
Iacopo Bianchi
,
Archimede Forcellese
,
Pietro Forcellese
,
Tommaso Mancia
,
Chiara Mignanelli
,
Michela Simoncini
* and
Tommaso Verdini
Department of Industrial Engineering and Mathematical Sciences, Università Politecnica delle Marche, Via Brecce Bianche 12, 60131 Ancona, Italy
*
Author to whom correspondence should be addressed.
Metals 2024, 14(11), 1220; https://doi.org/10.3390/met14111220
Submission received: 29 September 2024 / Revised: 23 October 2024 / Accepted: 24 October 2024 / Published: 26 October 2024
(This article belongs to the Special Issue Additive Manufacturing for Tooling Applications: Materials, Design, Processes & Impacts)
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Abstract

:
The present work aims to study the effect of printing orientation angle and heat treatment on the mechanical properties and microstructure of 17-4 PH stainless steel 3D-printed parts obtained by the binder jetting process to assess the suitability of the process and material for rapid tooling applications. To this purpose, tensile specimens were printed at different printing orientation angles (0°, 45°, and 90°). Half of the specimens were left in the as-sintered condition after the 3D-printing operation, while the other half of the specimens was subjected to H900 heat treatment. Then, tensile and hardness tests were performed to investigate the macro-mechanical properties as a function of the printing orientation angles and postprocessing thermal treatment. Scanning electron microscopy with energy dispersive X-ray spectroscopy was used to observe the fracture surfaces and microscopical defects on the binder jetting printed parts to evaluate the fracture mechanisms. It was demonstrated that printing orientation angles do not affect the mechanical properties of 3D-printed parts, while a significant improvement in the microstructure and mechanical properties is observed after the H900 heat treatment.

1. Introduction

In recent years, metal additive manufacturing (M-AM) has gained increasing relevance in many industrial fields due to its design freedom and the ability to manufacture complex metal geometries, which enable optimization of the material distribution, the reducing of mass while maintaining the mechanical and other performance requirements of the part, the combination of multiple components, a reduction in both the risk and cost in manufacturing multiple components, a decrease in potential failure modes across joints, and a reduction in lead time and associated costs [1,2].
For these reasons, M-AM processes are very attractive for companies for rapid tooling (RT), which consists in the use of additive manufacturing technologies to produce tools and molds [3]. Rapid tooling involves both direct and indirect tooling processes. As far as direct tooling is concerned, the mold is made directly in one step using one of the additive manufacturing technologies. In contrast, indirect tooling process allows a mold or tool to be obtained by means of more than one intermediary step by using additive manufacturing model as the first step [4].
Traditional tooling is quite time consuming and expensive. As a matter of fact, conventional mold and tool production depends on machining operations to generate the desired mold cavity and shape, which is becoming less and less competitive. Often, successive iterations, modifications, and redesigns are necessary, and the typical waste of material in subtractive technologies cannot be avoided.
In order to overcome such drawbacks, many industrial and academic researchers are concentrating on the use of additive manufacturing techniques to obtain metallic tools and molds, since additive technologies allow for significant improvements in parts development such as on-demand, low-cost, quick functional prototyping; a simpler supply chain for effective low-volume production; free geometric complexity based on “design for use” rather than “design for manufacturing”; innovation in tool design, allowing the creation of desired cooling channels and other complex features that are difficult or impossible to achieve with conventional tooling processes; the lightweighting of components; parts consolidation, with the consequent removal of the need for assembly operations in many cases; the realization of local manufacturing, parts repair, and refurbishment; and health and humanitarian benefits [5,6].
Technologies that are dominant in the market of metal additive manufacturing include those based on powder consolidation, such as powder bed fusion (PBF-LB/M) [7,8,9], direct energy deposition (DED) [10,11], and binder jetting (BJ) [12].
Among the metal additive manufacturing technologies, BJ involves several steps to produce the first part, typically called a “green” part, which is made from powder particles that are bonded together by a binder. Production of the green part includes printing, curing, and depowdering. BJ is a powder-bed-based AM technology in which the powdered material is spread into a thin layer and selectively joined into the desired layer shape with a binder, which is typically a polymeric liquid [13]. As the build progresses layer by layer, the layers are bonded together, resulting in a box of powder with binder arranged in the 3D shape of the desired part geometry. Then, the printed green parts are removed from the powder bed by means of a depowdering process. The green part is then postprocessed using heat treatments that involve debonding and sintering to obtain the final dense metal part with the desired properties [5,12,14,15,16].
Compared with metal additive manufacturing processes that use a powerful source of energy, such as a laser or electron beam (i.e., PBF-LB/M and DED), binder-jetting-printed parts do not exhibit thermally induced stress and distortion due to large thermal gradients. Therefore, BJ technology allows for more complex geometries to be obtained, which are also characterized by thin walls, without the use of support structures, compared with both metal AM processes that use thermal energy to melt raw materials [10] and metal injection molding (MIM) processes [13,14,15,16,17,18,19,20,21]. Furthermore, binder-jetting-printed components have significantly lower costs—up to 60% lower—compared with PBF technologies, with strong potential for high-speed production [22].
The main technologies used in rapid tooling are those that are laser-based [23,24] due to their higher part density and strength, wider material selection, better surface quality, and higher precision and accuracy. Then, in a direct rapid tooling approach, additive manufacturing processes are directly used to print real tools and molds in the end material [3].
In this context, the binder jetting process could be used in rapid tooling, since it ensures high-speed and low-cost manufacturing with respect to the other metal technologies based on the thermal energy selectively used to fuse regions of a powder bed. Based on the results obtained by scientific research analyzing the dimensional accuracy and surface quality of binder-jetting-printed parts [21,25,26], the BJ technology might not only produce tools and molds with mechanical properties comparable to those obtained through traditional methods but could also improve the efficiency of the production process. Furthermore, BJ could offer several advantages in terms of tool productivity, as it has both a higher efficiency and faster build speeds than additive manufacturing processes based on powerful sources of energy. It is a more cost-effective process, avoids postprocessing operations to remove support structures, allows a high level of recyclability of the unused powder, and has a large build volume [27,28,29]. The effectiveness of this process depends on the selection of appropriate parameters, such as the control and qualification of powder characteristics, binder selection, the binder deposition method and its compatibility with printing, the powder–binder interaction, stability, and burnout characteristics; also, the printing process parameters (such as printing speed, printing orientation angle, layer thickness, drying time, roller speed, and rate of debinding) and specifications, the correlation of densification kinetics with complex geometry, and the required post-processing procedures (such as sintering, infiltration, post-heat treatment and surface finish) play a key role in the properties of the final product [12,30]. Radhakrishnan et al. [31] and Emanuelli et al. [32] studied the effect of H900 and H1150 heat treatment on the mechanical properties and microstructure of 17-4 PH printed by binder jetting. Nezhadfar et al. [33], Kaletsch et al. [34], and Huber et al. [35] investigated the effect of hot isostatic pressing in combination with heat treatment on the mechanical tensile behavior of 17-4 PH binder-jet-printed parts and its microstructure. Favi et al. [36] and Mueller et al. [37] studied the 3D-printing technology based on bed fusion and its applications on the rapid production of tooling.
In this framework, the present work aims to study the applicability of the BJ technology for the rapid production of tools and molds in 17-4 PH stainless steel. To this purpose, the effect of printing orientation angle and postprocessing heat treatment on the mechanical properties of binder-jetting-printed parts was investigated. Tensile specimens were obtained at different printing orientation angles (0°, 45°, and 90°). Half of the specimens were left in the as-sintered condition after the 3D-printing operation, while the other half of the specimens was subjected to postprocessing H900 heat treatment. Experimental tensile tests, hardness tests, and metallographic analysis were carried out to evaluate the mechanical properties and microstructure of as-sintered and thermal-treated specimens. Finally, the fracture surfaces and microscopical defects of the binder-jetting-printed parts as a function of the different printing orientation angles and postprocessing thermal treatments were observed by means of scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis.
The paper is organized as follows: after the Introduction, Section 2 describes the materials and the experimental procedures used in the present research; Section 3 reports the results and the discussion, considering the scientific literature outcomes; then the main conclusions are drawn.

2. Materials and Methods

2.1. Materials

The material used in the present research is Type 630 stainless steel, commonly designated as 17-4 PH stainless steel. The chemical composition of the investigated material, obtained by means of the Spark Analyzer Spectrolab (AMETEK Inc., Berwyn, PA, USA), is shown in Table 1; this analysis shows that the element composition (wt%) of 17-4 PH, after the sintering phase, is in accordance with the technical datasheet (15–17.5% chromium, 3–5% nickel, and 3–5% copper).
This alloy belongs to the class of martensitic precipitation-hardening stainless steels and is renowned for its superior tensile strength, excellent corrosion resistance, and reliable mechanical performance at temperatures up to 320 °C. The main mechanical properties of 17-4 PH stainless steel, as defined by MPIF Standard 35 [38], are summarized in Table 2. These properties make the material particularly well suited for a wide range of industrial applications, including those with mildly corrosive environments and high-strength requirements, such as in components for oilfield valves, pump shafts, gears, aerospace structures, chemical processing equipment, and manufactured tools for automotive applications.
The material investigated in the present work was supplied by the manufacturer of the 3D printer Desktop Metal in the form of a powder characterized by a mean value of the particle diameter of under 25 μm.

2.2. Metal Binder Jetting Process

The commercial 3D printer Desktop Metal Shop System (Desktop Metal, Inc., Burlington, MA, USA) was used. This apparatus is composed of four different machines (Figure 1): the binder jetting printer, the depowdering station, the curing station, and the sintering furnace.
The BJ process involves spreading a thin layer of dry powder, typically 40–100 μm thick, onto a build platform, and then compacting it to improve the part density. Afterwards, a print head jets droplets of liquid binding agent onto the powder bed in order to realize the 2D pattern for the layer. The curing process is activated using infrared heating to solidify the binder.
After each layer, the build platform is lowered to make room for the next layer, and the process is repeated until the 3D-printed green part is completed, supported by the surrounding powder. After the printing step, the powder bed is subjected to a heat treatment, performed at a temperature ranging from 80 °C to 250 °C, in order to harden the binder, enable depowdering, and to remove the excess powder; the final green part revealed is composed of the metal powder and hardened binder. Then, the green parts are debinded in an oven at temperatures of up to 600 °C, eliminating the binder and turning the green part into a “brown” part. This step is fundamental for maintaining the part shape until sintering, by which metallurgical bonds are formed. The final step of the BJ is the sintering process, performed by means of a sintering furnace, where the parts are heated to near-melting temperatures of up to 1400 °C for about 40 h in order to consolidate the metal powders and to remove any residual binder, resulting in the desired additively manufactured parts. This process is carried out under vacuum conditions inside the furnace using an inert gas mixture (97.1% argon and 2.9% hydrogen) introduced between the different racks. At the end of the thermal process, the 3D-printed parts can be removed from the furnace.

2.3. 3D Printing and Heat Treatment Processes of Tensile Specimens

In order to investigate the effect of the printing orientation angle on the mechanical properties of binder jetting 3D-printed parts using 17-4 PH, tensile specimens, designed according to the ASTM E8 standard [39] using the commercial CAD software Rhinoceros 7 (Robert McNeel & Associates, Seattle, WA, USA), were manufactured (Figure 2).
The tensile specimens were binder jetting printed at different printing orientation angles (0°, 45°, and 90°), as shown in Figure 3. Specifically, in the D0° condition, the direction of the longitudinal axis of the specimen is the same as that of the binder deposition one (coincident with the X axis) in Figure 3, whilst the D90° and D45° conditions are characterized by samples printed with the axes at 90° and 45°, respectively, with respect to the direction of the binder deposition. Ten specimens per condition, for a total of 30 specimens, were realized. The process parameters relating to the deposition rate and layer height, respectively equal to 100 cc per hour and 0.075 mm, were kept constant.
After the sintering process (as-sintered condition), half of the tensile specimens were subjected to a further hardening treatment (aging H900 condition), as reported in Table 3. A FALC Muffle furnace 8.2 Lt—FM 8.2 (Bergamo, 24047 Treviglio, Italy) with a maximum temperature of 1200 °C was used. According to the ASTM B883 standard [40], the aging H900 condition requires preheating of the chamber furnace to up to 480 °C, a holding time of 1 h, and then cooling in air (Figure 4). The specimens were placed in the furnace after the preheating phase.

2.4. Uniaxial Tensile Tests and Hardness Tests

The mechanical properties of binder-jetting-printed specimens in 17-4 PH stainless steel were analyzed by means of uniaxial tensile tests carried out at room temperature in accordance with the ASTM E8/E8M standard, using a servo-hydraulic testing machine (MTS 810®, MTS Systems Corporation, Eden Prairie, MN, USA) equipped with a 250 kN load cell. During the tests, the crosshead speed of 0.1 mm/s was kept constant. An extensometer clamped onto the sample was used to measure the instantaneous strain along the loading direction. The experimental results were plotted as engineering tensile stress versus engineering tensile strain curves, from which the elastic modulus (E), yield strength (YS), ultimate tensile strength (UTS), the percentage elongation to the onset of necking (EN), the percentage elongation to failure (EF), and the strain-hardening coefficient (n) were evaluated. To ensure the repeatability of the experimental data, five tensile specimens were printed and tested for each condition investigated.
To evaluate the hardness of the binder-jetting-printed material pre- and post-H900 heat treatment, Rockwell hardness (HRC) tests were carried out due to their suitability for harder materials and the ability to reflect the effect of heat treatment on material properties. To perform the HRC tests, a ZwickRoell ZHR8150CLK durometer was used (Zwick GmbH & Co. KG., Ulm, Germany). The HRC hardness was evaluated by measuring the penetration depth of a cone diamond penetrator, characterized by an opening angle of 120°, on which a load of 1470 N was applied. Five replications were considered for each specimen (pre- and post-H900 treatment). The mean values and standard deviations were measured.

2.5. Microstructural Analysis

Light optical microscopy was carried out on the 17-4 PH samples, both in the as-sintered and heat-treated conditions, before the tensile tests, using the Leica DM2700 M (Leica MicroSystems, Wetzlar, Germania).
Specifically, microstructural analysis was performed in order to evaluate the microstructure as a function of the H900 hardening treatment.

2.6. Scanning Electron Mocroscopy and Energy-Dispersive X-Ray Spectroscopy

The FESEM ZEISS SUPRA™ 40 (Zeiss, Oberkochen, Germany), equipped with a compact GEMINI® objective lens (Zeiss, Oberkochen, Germany), was used to achieve high-resolution imaging of the fractured surfaces of the 3D-printed specimens after the tensile tests. Energy-dispersive X-ray spectroscopy (EDXS) analyses were also carried out during SEM observation with the Spark Analyzer Spectrolab. Specifically, sections parallel to the external surfaces of the specimens obtained at the three different orientation angles investigated were analyzed.

3. Results and Discussion

In order to evaluate the effect of the printing orientation angle and heat treatment on the mechanical properties of binder-jetting-printed parts in 17-4 PH and to assess the optimal conditions for rapid tooling, tensile specimens printed at different printing orientation angles were tested at room temperature. Furthermore, the effect of H900 heat treatment was investigated. Figure 5 shows as-sintered specimens before and after tensile tests.
Figure 6 shows the mean stress–strain curves of the printed tensile specimens in 17-4 PH, obtained using the binder jetting process at different printing orientation angles, in the as-sintered condition. Table 4 summarizes the mechanical properties of the as-sintered specimens and their standard deviation values at the different printing orientation angles investigated. Irrespective of the printing orientation angle investigated, the tensile stress linearly increases with strain in the elastic region until yielding. Then, the stress rises with a non-linear behavior up to a peak value corresponding to the onset of necking. Finally, the stress decreases with increasing strain until failure.
As far as the effect of the printing orientation angle is concerned, it can be observed that the stress–strain curves obtained by testing the specimens in the D0°, D45°, and D90° conditions exhibit negligible discrepancies; in particular, in terms of the E, UTS, and strain at the necking values. Such results can be attributed to the low influence of the binder deposition direction with respect to the subsequent sintering process on the microstructure of the specimens.
However, the specimen in the D0° condition is characterized by the lowest E, YS, and EF values. On the contrary, similar ductility levels were obtained by testing tensile specimens in the D45° and D90° conditions. These results are reflected in the different elongations to failure achieved by the tensile specimens, as can be seen in Figure 5.
Figure 7 shows the mean stress–strain curves obtained after tensile tests of the H900 heat-treated specimens. Table 5 reports the values of the mechanical properties resulting from the tensile test after the H900 heat treatment, as well as their standard deviations.
Linear growth of the stress with strain can be observed until the YS is reached. In the plastic region, tensile stress increases with strain until necking, with a non-linear trend. After necking, a decrease in stress appears, and failure rapidly occurs.
The comparison between the stress–strain curves obtained by the as-sintered (Figure 6) and H900 heat-treated tensile tests (Figure 7) highlights that the H900 heat treatment allows significant improvement of the tensile strength levels of 17-4 PH stainless steel, regardless of the printing orientation angle, both in the elastic and plastic regions. To this purpose, Figure 8 shows the effect of the H900 heat treatment on the mechanical properties obtained after tensile tests on the binder-jetting-printed specimens. Irrespective of the printing orientation angle investigated, the H900 treatment leads to an increase in the elastic modulus, with an average increase of 7% (Figure 8a). Furthermore, the H900 heat-treated specimens exhibit a significant improvement in terms of the YS and UTS values as compared with the as-sintered ones (Figure 8b,c); in particular, the YS and UTS tend to rise, with an average increase, respectively, of about 40% and 22%. In addition, an increase in the strain-hardening coefficient (n) was observed after the heat treatment (Figure 8d). This leads to a delay in the onset of necking. As a matter of fact, while necking occurs at approximately 50% of the total deformation in the as-sintered specimens, it occurred at approximately 80% of the total deformation in the H900 heat-treated specimens; this result can explain the low stress reduction after the necking in the H900 heat-treated specimens.
Unfortunately, the H900-treated specimens are characterized by a poor post-necking deformation as compared with the as-sintered ones, in which the heat treatment causes a reduced ductility. Specifically, the ductility of specimens at the D90° and D45° conditions is reduced by about 25% as compared with the as-sintered specimens, whilst a negligible effect on the ductility of the D0° specimens can be observed.
As far as the hardness measurement is concerned, Rockwell tests were performed on the binder-jetting-printed specimens at different printing orientation angles, both in the as-sintered condition and the H900 heat-treatment one. To this purpose, Table 6 summarizes the results obtained by the hardness tests.
Irrespective of the heat treatment investigated, in the as-sintered condition, a negligible effect of the orientation printing angle on hardness can be observed. A mean hardness value of about 30 HRC was measured. This level of hardness is typical of a 17-4 PH steel that has not been heat treated, since the material is predominantly austenitic or slightly martensitic after sintering.
As the H900 heat treatment is performed, a significant increase in hardness can be observed (Figure 9).
Such results can be attributed to the aging treatment, which changes the microstructure. The microstructure of the as-sintered specimens before the tensile tests, observed by optical microscopy, is a combination of ferrite and martensite (Figure 10a). This is a typical microstructure of martensitic steels with a high chromium content, such as 17-4 PH. Delta ferrite is formed due to carbon diffusion and depends on the sintering temperature and dwell time. Such a result is consistent with what has already been observed by other researchers [41,42,43]. After heat treatment (Figure 10b), the predominant phase is lath-type martensite, while a decrease in delta ferrite can be observed. Additionally, the SEM-EDSX analysis revealed the presence of Cu precipitates located at the grain boundaries (Figure 11), which was also observed by Di Pompeo et al. in [44].
These precipitates reinforce the martensitic matrix, increasing the hardness and mechanical strength of the material. The printing orientation angle affects the hardness increase; in particular, the highest rise in hardness occurs as the D0° specimen condition is considered. This could be due to the different defects’ distribution or to the density of hardening particles in the different orientations [45,46,47].
In order to assess the fracture mechanisms, scanning electron microscopy (SEM) analysis was conducted on the fracture surfaces of binder-jetting-printed specimens at different printing orientation angles, both in the as-sintering and H900 heat-treated conditions. High-resolution SEM images at various magnifications were obtained to examine the morphology and to identify potential defects or features, such as dimples, voids, or cracks, that provide insights into the failure behavior.
As far as the as-sintered specimens are concerned (Figure 12A–C), it can be observed that the fracture surfaces are characterized by a porous and rough texture with a noticeable presence of microvoids and regions with incomplete sintering. The lack of a well-consolidated grain structure after the sintering process leads to a low material density, which contributes to reducing the mechanical strength and stiffness. On the contrary, the H900 heat-treated specimens (Figure 12D–F) exhibit fracture surfaces characterized by a more uniform and consolidated structure. As can be observed by the SEM images realized at 1000× magnification in Figure 13D–F, a significant reduction in porosity appears with respect to the as-sintered specimens (Figure 13A–C); furthermore, the microstructural features are smoother than in the as-sintered condition. Irrespective of the printing orientation angle, the heat treatment enhances the bonding between particles and densification, leading to improved mechanical behavior, particularly in terms of toughness and strength. The results obtained by the SEM analysis explain the different mechanical behavior of the as-sintered (Figure 6) and H900 heat-treated specimens (Figure 7).
Considering the results in the previous literature review, it is possible to compare those with the results obtained in this study. In particular, these results demonstrate that the printing orientation does not influence the tensile behavior of parts obtained by binder jetting technology. The scanning electron microscopy and mechanical properties obtained from tensile tests indicate that BJ technology can supply similar performances for parts produced in different binder deposition directions, as compared with other metal additive manufacturing processes, such as selective laser melting (PBF-LB/M), which employ intense heat sources to fuse raw materials, generating anisotropic components. As a matter of fact, as demonstrated by Challis et al. in [48], selective laser melting induces anisotropic properties in the investigated material due to the preferential grain orientations that form during the process. Consequently, different mechanical properties, such as the Young’s modulus, are obtained as a function of the build orientation.
It is therefore possible to use binder jetting to rapidly produce tools and molds with advantages over the PBF-LB/M technology. Specifically, compared with PBF-LB/M, binder jetting allows the attainment of lower residual stresses, lower energy consumption, and higher material utilization with, consequently, more efficient recycling of unused powder and the realization of complex geometries with fewer constraints related to thermal management [19,31,32,33,34,35,49,50].

4. Conclusions

In the present paper, the effect of printing orientation angle and heat treatment on the mechanical properties of binder-jetting-printed parts in 17-4 PH stainless steel was investigated. In particular, the study aimed to evaluate the suitability of binder-jetting-printed 17-4 PH for rapid tooling applications. To this purpose, binder jetting technology was used to realize tensile specimens at printing orientation angles equal to 0°, 45°, and 90°. Half of the specimens were left in the as-sintered condition after the 3D-printing operation, while the other half of the specimens was subjected to H900 heat treatment. Then, tensile and hardness tests were performed to investigate the macro-mechanical properties as a function of the printing orientation angles. Finally, scanning electron microscopy was carried out on the fracture surfaces of binder-jetting-printed specimens at different printing orientation angles and thermal treatments to assess the fracture mechanisms.
The main outcomes can be summarized as follows:
  • The stress–strain curves obtained by testing as-sintered specimens in the D0°, D45°, and D90° conditions exhibit negligible discrepancies, in particular in terms of the elastic modulus, ultimate tensile strength and strain at the onset of necking. The specimen in the D0° condition is characterized by the lowest ductility.
  • Irrespective of the printing orientation angle investigated, the H900 treatment leads to an increase in the elastic modulus, yield strength, and ultimate tensile strength of, on average, 7%, 40% and 22%, respectively, compared with the as-sintered condition, even if a reduction in ductility appears, in particular for the D90° and D45° conditions.
  • A significant increase in hardness can be observed as H900 heat treatment is performed; this is due to the aging treatment, which changes the microstructure by increasing the martensitic phase, and the formation of Cu precipitates.
  • The thermal treatment strongly affects the microstructure of the binder-jetting-printed specimens and the fracture mechanisms: the H900 heat treatment allows the attainment of a more uniform and consolidated structure, a significant reduction in porosity, and a high densification compared with the as-sintering condition.
  • Binder-jetting-printed parts in 17-4 PH stainless steel, subjected to H900 heat treatment, exhibit mechanical properties suitable for the rapid production of tools and molds.
  • Compared with metal additive manufacturing processes that use heat sources to melt raw powders, binder jetting technology can be used to rapidly produce tools, since it allows the attainment of lower residual stresses, lower energy consumption, and higher material utilization with consequently more efficient recycling of unused powder as well as the realization of complex geometries with fewer constraints related to thermal management.
In future developments, X-ray computerized tomography analyses can be added to evaluate the void content and distribution; furthermore, a life cycle analysis can be included to assess the environmental impact of parts printed by binder jetting and subjected to thermal treatments to improve their mechanical properties.

Author Contributions

Conceptualization, A.F. and M.S.; methodology, I.B., C.M., T.M. and T.V.; software, I.B., C.M., T.M. and T.V.; validation, I.B., C.M., T.M. and T.V.; formal analysis, I.B., P.F., C.M., T.M. and T.V.; investigation, I.B., P.F., C.M., T.M. and T.V.; resources, A.F. and M.S.; data curation, I.B., C.M., T.M. and T.V.; writing—original draft preparation, I.B., C.M., T.M. and T.V.; writing—review and editing, A.F. and M.S.; visualization, I.B., C.M., T.M. and T.V.; supervision, A.F. and M.S.; project administration, A.F. and M.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank ELMEC Spa and Desktop Metal, particularly Eng. Ferrari, for their contribution.

Conflicts of Interest

The authors declare no competing interests.

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Figure 1. Commercial Desktop Metal Shop System apparatus.
Figure 1. Commercial Desktop Metal Shop System apparatus.
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Figure 2. Tensile specimen designed according to the ASTM E8 standard (unit: mm).
Figure 2. Tensile specimen designed according to the ASTM E8 standard (unit: mm).
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Figure 3. Schematic representation of the different orientations of the longitudinal section of the 3D-printed tensile specimen with respect to the binder deposition direction.
Figure 3. Schematic representation of the different orientations of the longitudinal section of the 3D-printed tensile specimen with respect to the binder deposition direction.
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Figure 4. Temperature versus time curve during H900 heat treatment.
Figure 4. Temperature versus time curve during H900 heat treatment.
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Figure 5. As-sintered specimens (a) before and (b–d) after tensile tests: (b) D0°, (c) D45°, and (d) D90° conditions.
Figure 5. As-sintered specimens (a) before and (b–d) after tensile tests: (b) D0°, (c) D45°, and (d) D90° conditions.
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Figure 6. Effect of the printing orientation angle on the mean stress–strain curves obtained by as-sintered specimens after tensile tests.
Figure 6. Effect of the printing orientation angle on the mean stress–strain curves obtained by as-sintered specimens after tensile tests.
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Metals 14 01220 g007
Figure 7. Effect of printing orientation angle on the mean stress–strain curves obtained by H900 heat-treated specimens after tensile tests.
Figure 7. Effect of printing orientation angle on the mean stress–strain curves obtained by H900 heat-treated specimens after tensile tests.
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Figure 8. Comparison between the as-sintered and H900 heat-treated conditions in terms of the (a) elastic modulus, (b) yield strength, (c) ultimate tensile strength, and (d) strain-hardening coefficient. The percentage values represent the increase in the mechanical property after the H900 heat treatment.
Figure 8. Comparison between the as-sintered and H900 heat-treated conditions in terms of the (a) elastic modulus, (b) yield strength, (c) ultimate tensile strength, and (d) strain-hardening coefficient. The percentage values represent the increase in the mechanical property after the H900 heat treatment.
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Figure 9. Comparison between the hardness values measured in as-sintered and H900 heat-treated specimens obtained at different printing orientation angles.
Figure 9. Comparison between the hardness values measured in as-sintered and H900 heat-treated specimens obtained at different printing orientation angles.
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Figure 10. Light optical microstructure of (a) the as-sintered and (b) H900 heat-treated specimens before tensile tests.
Figure 10. Light optical microstructure of (a) the as-sintered and (b) H900 heat-treated specimens before tensile tests.
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Figure 11. (a) SEM image of Cu precipitates; (b) SEM-EDSX observation of Cu precipitates.
Figure 11. (a) SEM image of Cu precipitates; (b) SEM-EDSX observation of Cu precipitates.
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Metals 14 01220 g012
Figure 12. SEM observations at 100× magnification of fracture surfaces of binder-jetting-printed specimens at different printing orientation angles, both in the as-sintered (A–C) and H900 heat-treated conditions (D–F).
Figure 12. SEM observations at 100× magnification of fracture surfaces of binder-jetting-printed specimens at different printing orientation angles, both in the as-sintered (A–C) and H900 heat-treated conditions (D–F).
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Figure 13. SEM observations at 1000× magnification of fracture surfaces of binder-jetting-printed specimens at different printing orientation angles, both in the as-sintered (A–C) and H900 heat-treated conditions (D–F).
Figure 13. SEM observations at 1000× magnification of fracture surfaces of binder-jetting-printed specimens at different printing orientation angles, both in the as-sintered (A–C) and H900 heat-treated conditions (D–F).
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Table 1. Chemical composition of 17-4 PH stainless steel analyzed by the Spark Analyzer Spectrolab.
Table 1. Chemical composition of 17-4 PH stainless steel analyzed by the Spark Analyzer Spectrolab.
%Fe%C%Si%Mn%Cr%Ni%Cu
wt%Bal.0.030.350.8017.732.983.19
Table 2. Typical tensile properties of 17-4 PH stainless steel, according to MPIF Standard 35.
Table 2. Typical tensile properties of 17-4 PH stainless steel, according to MPIF Standard 35.
PropertyValue
Yield strength (MPa)650
Ultimate tensile strength (MPa)790
Elongation at break (%)4
Young’s Modulus (GPa)196
Hardness (HRC)27
Density (g/cm3)7.5
Table 3. Number of specimens per orientation condition and thermal process.
Table 3. Number of specimens per orientation condition and thermal process.
Orientation ConditionNumber of Specimens per Treatments
As SinteredH900 (480 °C for 1 h)
0°55
45°55
90°55
Table 4. Mechanical properties of as-sintered specimens obtained at different printing orientation angles.
Table 4. Mechanical properties of as-sintered specimens obtained at different printing orientation angles.
E [GPa]YS [MPa]UTS [MPa]EN [%]EF [%]n
0°155.3 ± 0.6612.7 ± 11.9974.9 ± 37.35.3 ± 0.38.0 ± 0.10.3
45°163.32 ± 0.9656.0 ± 23.7974.25 ± 49.15.0 ± 0.710.7 ± 0.80.31
90°158.26 ± 1.2667.8 ± 21.5957.60 ± 43.85.9 ± 0.911.0 ± 1.10.31
Table 5. Mechanical properties of H900 heat-treated specimens obtained at different printing orientation angles.
Table 5. Mechanical properties of H900 heat-treated specimens obtained at different printing orientation angles.
E [GPa]YS [MPa]UTS [MPa]EN [%]EF [%]n
0°168.3 ± 2.6890 ± 23.71164.8 ± 16.16.8 ± 0.28.6% ± 0.20.36
45°169.9 ± 2.1922.5 ± 45.21228.1 ± 50.78.1 ± 0.59.4% ± 0.70.4
90°173.03 ± 1.8903.2 ± 42.91155.8 ± 67.36.9 ± 0.88.3% ± 1.30.35
Table 6. Hardness values measured in as-sintered and H900 heat-treated specimens obtained at different printing orientation angles.
Table 6. Hardness values measured in as-sintered and H900 heat-treated specimens obtained at different printing orientation angles.
Hardness [HRC]
As-SinteredH900
0°28 ± 1.239 ± 0.6
45°33 ± 2.038 ± 3.5
90°32 ± 1.537 ± 0.6
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MDPI and ACS Style

Bianchi, I.; Forcellese, A.; Forcellese, P.; Mancia, T.; Mignanelli, C.; Simoncini, M.; Verdini, T. Effect of Printing Orientation Angle and Heat Treatment on the Mechanical Properties and Microstructure of Binder-Jetting-Printed Parts in 17-4 PH Stainless Steel. Metals 2024, 14, 1220. https://doi.org/10.3390/met14111220

AMA Style

Bianchi I, Forcellese A, Forcellese P, Mancia T, Mignanelli C, Simoncini M, Verdini T. Effect of Printing Orientation Angle and Heat Treatment on the Mechanical Properties and Microstructure of Binder-Jetting-Printed Parts in 17-4 PH Stainless Steel. Metals. 2024; 14(11):1220. https://doi.org/10.3390/met14111220

Chicago/Turabian Style

Bianchi, Iacopo, Archimede Forcellese, Pietro Forcellese, Tommaso Mancia, Chiara Mignanelli, Michela Simoncini, and Tommaso Verdini. 2024. "Effect of Printing Orientation Angle and Heat Treatment on the Mechanical Properties and Microstructure of Binder-Jetting-Printed Parts in 17-4 PH Stainless Steel" Metals 14, no. 11: 1220. https://doi.org/10.3390/met14111220

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