Emanuela Cerri

Post-processing heat treatments of Ti-6Al-4V parts produced by additive manufacturing are essential for restoring the peculiar martensitic structure that originates from the extremely high cooling rates typical of this technology. In this study, the influence of a 1050 °C annealing on a Ti-6Al-4V alloy, produced by additive manufacturing, on the minimum creep rate dependence on applied stress and temperature, was investigated at 650 °C. Experimental data obtained after two different subcritical annealings were also considered for comparison purposes. The analysis of the experimental creep data demonstrated that the alloy annealed at the highest temperature exhibited lower creep rates. The improved creep response was attributed to the combined effect of the presence of extended α-β interfaces and of a small volume fraction of Ti3Al particles.

Laser powder bed fusion is an additive manufacturing process characterized by different advantages like the manufacture of samples with complex geometry without the use of tools and/or molds. Generally, the manufactured samples are characterized by high tensile strengths which, however, can be affected by the presence of defects due to the unoptimized process parameters. In a large applications field, a low density of the as-built AlSi10Mg samples is a very important parameter to considered, e.g., due to both the loss of the tensile strengths correlated with a premature failure of the samples and the increase in time and costs associated with the manufacturing process. In addition, different post-process heat treatments can increase these effects leading to an ineffective manufacturing process. In this scenario, the present work shows the analysis of spherical and lack-of-fusion pores induced by the laser powder bed fusion process on the AlSi10Mg samples and their variations after d...

The present study analyzed the microstructure and the mechanical properties of AlSi10Mg SLMed bars (10 × 10 × 300 mm) and billets (10 × 100 × 300 mm) before and after the direct aging at 200 °C for 4 h and the T6 heat treatment. The discussed results are compared to those obtained by the AlSi10Mg samples manufactured with the same geometry but using different process parameters (layer thickness higher than 40 μm and a hatch spacing lower than 100 μm) and also through the Quality Index (QI). These work conditions allow the obtaining of a microstructural variation and different tensile properties in as-built top samples. In both batches, the cycle time was 45 h and together with the preheated build platform at 150 °C, induced an increase of UTS (Ultimate Tensile Strength) and yield strength on the bottom rather than the top samples due to the aging phenomena. Upon completion of the direct aging heat treatment, the effects induced by the platform were cancelled, keeping a full cellular...

Laser powder bed fusion (L-PBF) is an additive manufacturing technology that is gaining increasing interest in aerospace, automotive and biomedical applications due to the possibility of processing lightweight alloys such as AlSi10Mg and Ti6Al4V. Both these alloys have microstructures and mechanical properties that are strictly related to the type of heat treatment applied after the L-PBF process. The present review aimed to summarize the state of the art in terms of the microstructural morphology and consequent mechanical performance of these materials after different heat treatments. While optimization of the post-process heat treatment is key to obtaining excellent mechanical properties, the first requirement is to manufacture high quality and fully dense samples. Therefore, effects induced by the L-PBF process parameters and build platform temperatures were also summarized. In addition, effects induced by stress relief, annealing, solution, artificial and direct aging, hot isost...

Friction stir processing (FSP) has confirmed its valuable contribution to refining microstructures and the improvement of mechanical properties for Mg-Al alloys. Reference papers illustrate that post-processing aging treatments enhance hardness, but with the application of a solution heat treatment prior to FSP. In this work, aging was performed at two different temperatures (170 and 300 °C) directly on friction stir processed samples of AZ91 produced using high pressure die casting (HPDC). High rotational speeds of the tools (2500 and 3000 rpm) increases the heat input and the temperature of the plates during the process up to 270 °C. Vickers microhardness (HV) increased by 15–20% in the nugget, compared to the as FSPed condition; moreover, a greater homogeneity of hardness values was found at the higher aging temperature used. The β-Mg17Al12 precipitates are randomly distributed inside grains of the stirred area, while in the thermomechanical affected zone (TMAZ) they have grown i...

A high pressure die cast (HPDC) magnesium alloy was friction stir processed (FSP) at high rotation rates with different advancing speeds. The AZ91 plate was 3 mm thick and the pin had a truncated cone tip. The friction stir processing induced the disappearance of porosity (typical of the HPDC process) in the nugget zone but some tunnel defects were introduced. The presence of characteristic FSP zones is not obvious due to the low plastic deformation of this alloy. The stirred zone is very narrow around the tool and this makes the FSP difficult to occur in the whole thickness of the plate. Microhardness values and electrical conductivity are sensitive to stirred zone and differences arise according to FSP parameters. The grain size is refined and homogenized by FSP due to partial solubilisation and disappearance of the eutectic phase surrounding Mg-alpha. X-rays diffractometry was performed on FSP samples to index phases and calculate peak shifts of Mg-alpha.

A high pressure die cast (HPDC) magnesium alloy was friction stir processed (FSP) at high rotation rates with different advancing speeds. The friction stir process induced the disappearance of porosity, typical of the HPDC process, and formation of very fine grain boundary phases. Nanoindentation was used to mechanically characterize the different welded zones of interest, the thermomechanical heat affected zone (TMAZ), the stirred zone (SZ), in the advancing and the retreating side, at different sheet section depths. Aging at high temperature showed precipitates uniformly distributed inside the magnesium grains, due to the increased level of Al taken into solid solution.

In the present study, AlSi10Mg samples produced by selective laser melting (SLM) were studied. Samples were machined from two types of bars obtained through different methods: either single laser (SL) or multiple laser (ML) machine setup. The bars were built perpendicular to the platform, which was pre-heated at 150 °C (working temperature), up to a height of 300 mm. The effect of the distance from the platform on the mechanical properties was investigated through tensile samples in as-built condition and after unconventional heat treatments (U-HT). Tensile strength changed by 80 MPa along the Z-axis (build direction) for SL case and by 100 MPa for ML case in the as-built samples. Vickers microhardness revealed an analogous gradient. This was correlated to a gradient in intra-granular precipitates' distribution along the Z-axis, as revealed by scanning electron microscopy (SEM). An unconventional heat treatment at 175 °C for 6h slightly improves the mechanical strength; higher t...

The effects of postprocessing annealing at 225 °C for 2 h on the creep properties of AlSi10Mg alloy were investigated through constant load experiments carried out at 150 °C, 175 °C and 225 °C. In the range of the experimental conditions here considered, the annealing treatment resulted in an increase in minimum creep rate for a given stress. The reduction in creep strength was higher at the lowest temperature, while the effect progressively vanished as temperature increased and/or applied stress decreased. The minimum creep rate dependence on applied stress was modeled using a physically-based model which took into account the ripening of Si particles at high temperature and which had been previously applied to the as-deposited alloy. The model was successfully validated, since it gave an excellent description of the experimental data.

The present study aims at investigating the effect of the peculiar microstructure of additive manufactured samples on the creep behavior of an AlSi10Mg alloy. Constant load creep experiments were carried out between 150 and 205 ℃ on samples produced by powder bed fusion additive manufacturing (AM). The specimens were mostly strained up to rupture, although in some cases the tests were interrupted at the early onset of the tertiary region. By analyzing the time to rupture, in the different load and temperature conditions, as a function of the applied stress, it can be clearly seen that the alloy produced by AM is substantially comparable, in terms of time to rupture, with an alloy of similar composition, tested in the die-cast state. The high values of the stress exponent suggest that the creep behavior is strongly affected by the presence of secondary-phase particles.

The present work investigates a narrow range of secondary dendrite arm spacing (SDAS), in an as-cast A356 alloy with and without copper (Cu) additions. Cu was added to the base A356 alloy melt to reach the target concentration of 0.5 and 1 wt.%. Samples were selected from 3 different positions within the cast plate, offering 30, 35, and 40 μm SDAS variants. Tensile curves revealed a strong influence between the specimen cutting position and strength, with a pronounced effect in the Cu-containing alloys. Hardness measurements did not confirm the tensile response; hence, to understand the phenomenon, microstructural features have been investigated in detail. Eutectic silicon (Si) particle equivalent diameter (ED) size decreased from the top (T) to the bottom (B) position of the cast. Eutectic Si particle surface area (A%) was found to be denser at the B as compared to the T and simultaneously in the Cu-containing alloy as compared to the Cu-free reference alloy. Backscattered electron...

A high-pressure die-cast magnesium alloy plate was friction stir processed at high rotation rates with different advancing speeds. The stirred zone was very narrow around the tool and this made the friction stir process difficult to occur in the whole thickness of the plate. Intermetallic-phase network at grain boundaries was refined due to partial dissolution and fragmentation of Mg17Al12 β-phase during the friction stir process; the likely increment of solute content in solid solution was exploited for aging to improve hardness. The ductility of friction stir processed samples deformed at 300° and 350°C substantially increased compared to the base material and to room temperature strained samples.

ABSTRACT The high-temperature behaviour of the AS21X magnesium alloy was tested by means of tensile and compression constant strain rate and creep tests in the temperature range between 70 and 210°C. Strain rate vs. stress data, representing the maximum resistance condition obtained by constant load and constant strain rate tensile data, lay on the same curves. The strain rate dependence on stress between 70 and 180°C was described by a conventional power law with stress exponent close to 13 and an activation energy equivalent to that for self-diffusion in Mg. Compression tests indicated that in this mode of deformation maximum resistance was slightly greater than in tension.

This paper analyzes hot torsion flow curves [1-5], microstructures [1,2,6-9], constitutive equations and extrusion finite element modeling (FEM) [10-13] of aluminum composites. Those results come mainly from previous studies of prof. H.McQueen et alii. Metal-matrix composites (MMC) of 6061, 7075, 2618 and A356 alloys with Al2O3 or SiC particles ( 15-30 μm) were produced by liquid metal mixing. Aluminum alloy matrices reinforced with particles of Al2O3 or SiC possess higher strength and stiffness as well as greater wear resistance and improved high temperature properties [14-17]. MMC produced by liquid metal mixing are secondarily fabricated by traditional mechanical forming (extrusion, forging or rolling) [18-21]. Materials were deformed over the temperature range 300 to 500°C and strain rates 0.1 to 4/5 s-1. At 400°C (and lower T) the strength of composites is higher than that of the alloys. With exception of 6061 and 2618, there is almost no difference in strength at 450°C while at 500°C composites appear to be softer than the alloys. 2618 MMC exhibit lower ductility then A356 and 6061 MMC that exhibit similar ductility. 7075 alloy and MMC decline from good ductility at 400° to very low at higher T because of GB precipitation [1-5]. The softening of the alloys with increasing T (and with decreasing strain rate) is due to improved DRV. The softening of composites depends on more complicated changes in microstructure: DRX occurs to a limited extent along with dynamic recovery. Furthermore, the composites retain heterogeneous substructures in both quenched and air cooled torsion specimens since no static recrystallization occurred after torsion [1,2,6-9]. Constitutive analysis was developed according to Garofalo hyperbolic stress equation and showed that the MMC increase in strength and in activation energy QHW as alloying element and particles contents rise. The extrusion was modeled using the finite element software DEFORMtm. This program uses a flow formulation approach and an updated Lagrangian procedure; it possesses an automatic remeshing scheme to allow the modeling of large or localized deformations. Extrusion was modeled for a billet with diameter 178 mm and height 305 mm, an extrusion ratio R = 31 and ram speed VR = 2.6 or 5 mm/s in similarity to previous modeling [10-13]. The constitutive laws determined by the torsion tests have been used in the model to calculate flow stresses. Models were developed for the initial billet temperatures TB from 300 to 500°C. From modeling temperature T, strain e, strain rate έ and stress σ distribution together with TMax and PMax were determined. The results were validated by comparing to actual extrusions. The grid distortions and distributions of e and έ are independent of material properties. As TB increases (from 300°C to 500°C) the composite extrusion pressure decreases towards that of the bulk alloys. This applies: from 300 °C to 400°C for A356 and 7075 above which the composite pressure is equal or lower than that of alloys and from 300°C to 500°C for 6061 and 2618. The maximum load increases in order of matrix alloy 6061, A356, 7075 and 2618. The temperature increases in the same order. Because of incipient melting in 2618 is near to 500°C, TB must be limited for this alloy. Since constitutive analysis for a new alloy, on its adoption for a previous extrusion production, is often available, correlation of maximum extrusion pressure PMax with activation energies QHW was made [22].

ABSTRACT The isothermal forming of AA2618 + 20% Al2O3 was studied by employing hot torsion and hot compression tests in the temperature and strain rate ranges of 350–500 °C and 10-3-1 s-1 strain rate, respectively. The processing and stability maps of the material were calculated following the dynamic material model leading to identification of the best forming conditions. The constitutive equations were calculated for the studied material and the damage produced by forming operation was calculated in all the test conditions. Both optical and scanning electron microscopy were used to evaluate the microstructural evolution of the material.

The electrospark deposition (ESD) technique has been studied as a potential method to repair locally damaged 2024 rolled sheets supplied in natural-aged (T4) and artificial-aged (T6) conditions. The 2024-T4 and 2024-T6 tensile samples were first notched, and then the notches were filled (repaired) by ESD with the same aluminum alloy. The effect of process parameters on the microstructure of the filling material and the substrate properties was studied by optical and scanning electron microscopy. Tensile and hardness tests were performed. The tensile test showed that T4 and T6 as-repaired specimens had low tensile properties, which was due to defectiveness and residual stress caused by high cooling rate during reparation. However, the as-repaired specimens were heat-treated at either 135°C or 190°C to improve the mechanical properties. A better yield strength was observed for the T4 heat-treated alloy. The ductility and ultimate tensile strength did not change, being mainly affected ...

In questo lavoro si è studiato l'effetto dei trattamenti termici sulla microstruttura, sulle proprietà meccaniche e sui meccanismi di danneggiamento microstrutturale, indotti mediante deformazione plastica, delle leghe di alluminio A356 e 319 thixocolate. Il processo di thixocolata conferisce ai getti proprietà meccaniche notevolmente superiori a quelli ottenibili secondo i metodi di colata tradizionali poiché riduce le difettosità ed utilizza una materia prima dalla microstruttura globulare fine ed omogenea. Le caratteristiche meccaniche dei getti thixocolati sono suscettibili poi di ulteriore miglioramento mediante comuni trattamenti termici di invecchiamento artificiale (T5) e solubilizzazione ed invecchiamento artificiale (T6). Caratteristica comune alle leghe basate sul sistema Al-Si è che il decadimento delle proprietà meccaniche in seguito all'applicazione di uno sforzo è imputabile principalmente alla frattura o decoesione delle particelle di silicio (1). E' stat...

Una lega di alluminio della serie 6060 è stata caratterizzata dal punto di vista microstrutturale e meccanico sia nello stato come ricevuto che in quello solubilizzato. In particolare l'effetto del trattamento di solubilizzazione a 587°C in funzione del tempo di mantenimento in temperatura (fino a 25h) è stato analizzato mediante indagine microstrutturale e prove di durezza e conducibilità elettrica. La deformabilità a caldo del materiale è stata studiata mediante prove di trazione a caldo per temperature comprese tra 400 e 490°C e velocità di deformazione tra 10 -2 e 10 -5 s -1 su campioni sia nello stato as-cast che in quello solubilizzato (585°C-7h). L'analisi dei dati tramite equazioni costitutive ha permesso di valutare l'energia di attivazione del processo che è risultata conforme a quanto riportato in letteratura. Per entrambi gli stati del materiale il comportamento a caldo è caratterizzato da una duttilità che cresce con la temperatura di prova, da un picco di t...

ABSTRACT The creep behaviour of two novel Al-Si alloys produced by rapid solidification was studied at temperatures ranging from 493 to 573K. The creep strength of the Al-Si-Ni-Cr alloy (alloy A) was found to be strongly higher than the Al-Si-Cu-Mg (alloy B) one. The creep fracture in the alloy A proceeded along the numerous particle-matrix interfaces, while in the alloy B the rupture was accompanied by extensive necking and had the classical appearance of the ductile "dimples" fracture. The minimum creep rate dependence on applied stress and temperature was estimated for both the studied materials; furthermore, a universal law for transient, secondary and early tertiary creep was proposed in order to give an acceptable simulation of the strain-time experimental curves.

ABSTRACT The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M23 C6 (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.

Among the numerous aluminium alloys, the 6xxx family (AlSiMg) is appreciated for its good formability and extrudability, good mechanical properties, resistance to corrosion and easy weldability. In this series. the 6082 is particularly interesting for the good combination of mechanical properties and corrosion resistance which turns out to be fundamental in the automotive field. Table I reports the mechanical properties of 6082 and other 6xxx alloys as a function of the heat treatments [1-5]. It is shown that its strength is comparable with the others; only the recent 6069 is more stronger. This study aims to be a contribution about the problem of abnormal grain growth that has been encountered during the forming of this alloy. In fact, some automotive components made by 6082 evidentiated the abnormal structure after forming and T6 heat treatment (necessary for the hardening of the alloy). In this framework, the authors wish to correlate the strain to the microstructure evolution in...

The casting of metal matrix composites in the semisolid state is especially advantageous compared with conventional casting, as the presence of the primary solidified phase prevents the ceramic particles from agglomeration. Bars of a particle reinforced thixo-cast 357 aluminium alloy composite were heat treated in T5 and T6 conditions. Electrical conductivity and hardness measurements were performed on aged samples to follow the precipitation process. T6 condition produces slightly higher hardness peaks and enhancing kinetics of precipitation due to super-saturation. After ageing, a set of samples were prepared for tensile testing to study the effect of T6 heat treatments on mechanical properties. Yield strength and ultimate tensile strength show a similar trend as a function of time at constant temperature. Ultimate tensile strength decreases with increasing elongation. A comparison with mechanical data of the matrix is also reported.

Hot deformation of some alloys can lead to cavitation (fissures) phenomenon. Cavitation is due to microstructural features and/or process parameter. The phenomenon is particularly critical because it limits the hot workability of materials, can cause premature fracture and serious problems on mechanical properties of finished products. The interest in understanding and controlling the microstructural and process factors that cause cavitation has led to this study. The paper analyzes the cavitation exhibited by two Al-Zn-Mg alloys deformed by both hot tensile test and creep at different temperatures (T) and strain rate (έ). The study has allowed of identify and justify the fundamental relationship between cavitation, microstructure and process parameters. Furthermore, the influence of different heat treatments (aging T6 and solutionizing) on cavitation has been evaluated. Microstructural and mechanical characterization of these alloys may also provide information for identifying thei...

This paper studies the high temperature mechanical properties of as-cast Al–Zn–Mg using tensile and creep testing. Hot tensile tests were performed in the temperature and strain rate ranges of 350–400 °C and 10−5 to 10−3 s−1 respectively. Creep tests were carried out under constant load with initial stress ranging from 5 to 22 MPa, at 350 and 400 °C. The alloy exhibits class A creep behavior, due to the viscous glide of dislocations. The tensile flow curves, corrected to account for the effect of sample elongation on strain rate, also show a decline in stress commonly observed in Al–Mg alloys. The highest ductility and strain rate sensitivity were both observed for strain rates ranging from 10−5 to 10−4 s−1. During hot deformation the alloy exhibits cavitation which increases with temperature. The analysis of these phenomena has shown that cavity growth is mainly controlled by plastic strain.