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The laser-engineered net-shaping (LENSTM) process was employed for directed energy deposition (DED) of Ti-35Nb-7Zr-5Ta (wt.%) or TNZT, a low modulus metastable β-Ti alloy for biomedical applications. The laser power was varied in the range of 400–600 W, while keeping all other parameters constant. A change in the overall grain morphology was noted in the sample with an increase in the laser power. The higher laser power samples showed more columnar grains compared to the equiaxed grains observed in the 400-W condition. The DED-processed TNZT alloys exhibited a combination of high tensile yield strength (~ 500 MPa) and ductility (~ 25%). Moreover, the alloys also exhibited excellent corrosion resistance in Ringer’s solution. Overall, the results indicate that the LENS-deposited TNZT alloy could be a promising candidate for biomedical applications.

The increasing potential for additive manufacturing technology continuously drives the need for printable materials, including novel aluminum alloys. Ce is considered an economically feasible addition that is beneficial to corrosion resistance and high temperature performance of aluminum alloys. In this study, a binary Al–10Ce alloy was additively manufactured by laser powder bed fusion (LPBF) using gas-atomized powders for the first time. Initial investigation was carried out to determine the optimal LPBF parameters for the Al–10Ce alloy, which was found using the laser power of 350 W and scan speed of 1400 mm/s. Alloy samples with nearly full density and outstanding printability were obtained. Room temperature tensile tests of the as-built Al–10Ce alloy yielded 222 ± 2 MPa in yield strength, 319 ± 1 MPa in tensile strength and 10.8 ± 0.1% in elongation. This is superior to the cast counterpart reported in the literature. A uniform microstructure was observed throughout the alloy, and it primarily consisted of extremely fine-scale eutectic Al and Al11Ce3 intermetallic ribbons, arranged in a skeleton pattern. This fine microstructure would have originated from the rapid cooling inherent to the LPBF process, and strongly suggests the presence of the Orowan strengthening mechanism. This study demonstrated that the binary Al–10Ce alloy is a promising base composition with good printability that brings new possibilities for future ternary and higher-order alloy design for LPBF.

We examine the microstructural characteristics of LPBF AlSi10Mg produced by using a wide range of LPBF processing parameters with independently varied laser power, hatch spacing, scan speed, slice thickness, and the normalized energy density. The lower energy density produced lack of fusion flaws from residual interparticle spacing, while the higher energy density produced spherical pores from trapped gas. The highest density (> 99%) samples were produced by using an energy density of 32 to 54 J/mm3. Within this energy density range, use of smaller slice thicknesses increased the processing window that would produce dense AlSi10Mg samples. A cellular structure, consisting of Al–Si eutectic and α-Al (fcc) matrix, within melt pools was quantified in size to determine the cooling rate of 105 to 107 K/s. This sub-grain cellular structure was found to decrease in size with increasing scan speed and increasing slice thickness.

Direct laser deposition of a mixture of titanium (Ti) and boron-carbide (B4C), using the laser engineered net shaping process, results in a complex multi-phase Ti matrix composite due to the incipient interfacial reactions between liquid titanium and solid/partially melted B4C. Detailed microstructural characterization, at multiple length scales, reveals that titanium boride (TiB) precipitates are the initial reaction product which act as heterogeneous nucleation sites for TiC precipitates. Both the TiB and TiC precipitates in turn act as heterogeneous nucleation sites for fine β-Ti grains, within the retained liquid titanium, which eventually transform into fine equiaxed α-Ti grains comprising the matrix.

Additive manufacturing (AM) technology for metallic alloys such as laser powder bed fusion (LPBF) brings tremendous opportunities for development of novel alloys specifically designed for AM that would desensitize the inherent process variability and requires a refined understanding of processing–structure–property relationship that would contribute to future alloy development. In this study, two different alloys, Al-6Zn-2Mg and Al-6Zn-2Mg-0.7Sc-0.3Zr in wt. pct, representing traditional and AM-specific novel aluminum alloys, respectively, were manufactured by LPBF technique using pre-alloyed gas atomized powders. The Al-Zn-Mg ternary alloys exhibited cracks at various LPBF processing parameters, primarily along the grain boundaries of the large columnar grains that extended across multiple melt pools. The severity of cracking in LPBF Al-Zn-Mg alloys was process-dependent and could be correlated to the change in alloy composition due to evaporation of Zn and Mg with high vapor pressure. The Scheil–Gulliver non-equilibrium solidification calculations showed that the Al-Zn-Mg alloys with lower Zn and Mg concentrations had smaller solidification range (i.e., ΔT) and steepness values (i.e., $$ \left| {{\text{d}}T / {\text{d}}f_{\text{s}}^{1/2} } \right| $$ near $$ f_{\text{s}}^{1/2} = 1 $$), which corresponded to a lower cracking severity. On the other hand, no cracks were observed in Al-Zn-Mg-Sc-Zr alloys, although their solidification range and steepness values were similar to the ternary Al-Zn-Mg alloys. The microstructure of Al-Zn-Mg-Sc-Zr alloys exhibited a much refined heterogeneous microstructure containing small equiaxed and columnar grains within each melt pool, owing to the heterogeneous nucleation upon primary Al3(Sc,Zr) particles. Process-dependent microstructure was observed as a result of variation in thermal gradient and cooling rate associated with LPBF parameters. Findings from this study provide guidance for the future design of AM-specific aluminum alloys and insights into the microstructural control by LPBF.

In this review article, the latest applications of machine learning (ML) in the additive manufacturing (AM) field are reviewed. These applications, such as parameter optimization and anomaly detection, are classified into different types of ML tasks, including regression, classification, and clustering. The performance of various ML algorithms in these types of AM tasks are compared and evaluated. Finally, several future research directions are suggested.

In this work, strength and microstructural evolution of superalloy Inconel 718 (IN718) are characterized as a function of the initial microstructure created via direct metal laser melting (DMLM) additive manufacturing (AM) technology along with subsequent hot isostatic pressing (HIP) and heat treatments as well as wrought processing. Stress-strain curves are measured in tension and compression from room temperature to 550 °C and crystallographic texture is characterized using neutron diffraction. Furthermore, a recently developed crystal plasticity model incorporating the effects of precipitates is extended to interpret the temperature dependent deformation behavior of the alloy. The model accounts for solid solution, precipitate shearing, and grain size and shape contributions to initial slip resistance, which evolves with a dislocation density-based hardening law considering latent hardening, while non-Schmid effects are taken into account in the activation stress. Part of the experimental data is used for calibration of the model, while the rest is used for experimental validation of the model. It is shown that the model is capable of modeling the data with accuracy. Based on the comparison of the data and model predictions, it is inferred that the grain structure and texture give rise to plastic anisotropy of the alloy, while its tension-compression asymmetry results from non-Schmid effects and latent hardening.

This paper deals with the physics of high-pressure gas atomization in metal powder production. To gain understanding of the effect of gas pressure on droplet size distribution, a numerical two-phase flow study is performed using Eulerian-Eulerian Volume of Fluid (VOF) interface tracking method. Annular-slit, close-coupled gas atomizer is considered to atomize molten aluminum using nitrogen as the atomizing gas. Four cases with different gas pressures are considered, while geometry and other operational parameters are fixed. Characteristics of several interfacial instabilities have been identified at different stages of the atomization process. Despite the increment in the rate of the atomization with the increasing gas pressure, deformation characteristics and the breakup mechanisms remain unchanged. Droplet size and the cumulative volume distributions indicate that the effectiveness of the atomization process increases with the elevating gas pressure. Cumulative volume obtained from the numerical simulations at low gas pressures display similar trends to the experimental results.

This work adapts a recently developed multi-level constitutive model for polycrystalline metals that deform by a combination of elasticity, crystallographic slip, and deformation twinning to interpret the deformation behavior of alloy WE43 as a function of strain rate. The model involves a two-level homogenization scheme. First, to relate the grain level to the level of a polycrystalline aggregate, a Taylor-type model is used. Second, to relate the aggregate level response at each finite element (FE) integration point to the macro-level, an implicit FE approach is employed. The model features a dislocation-based hardening law governing the activation stress at the slip and twin system level, taking into account the effects of temperature and strain rate through thermally-activated recovery, dislocation debris formation, and slip-twin interactions. The twinning model employs a composite grain approach for multiple twin variants and considers double twinning. The alloy is tested in simple compression and tension at a quasi-static deformation rate and in compression under high strain rates at room temperature. Microstructure evolution of the alloy is characterized using electron backscattered diffraction and neutron diffraction. Taking the measured initial texture as inputs, it is shown that the model successfully captures mechanical responses, twinning, and texture evolution using a single set of hardening parameters, which are associated with the thermally activated rate law for dislocation density across strain rates. The model internally adjusts relative amounts of active deformation modes based on evolution of slip and twin resistances during the imposed loadings to predict the deformation characteristics. We observe that WE43 exhibits much higher strain-hardening rates under high strain rate deformation than under quasi-static deformation. The observation is rationalized as primarily originating from the pronounced activation of twins and especially contraction and double twins during high strain rate deformation. These twins are effective in strain hardening of the alloy through the texture and barrier hardening effects.

This study takes a benchmarking approach to the processing of 17-4 PH using selective laser melting by having two facilities that use their own best practices to process materials. Hot isostatic pressing (HIP) is used by both facilities as part of the thermomechanical processing following printing to explore whether it can improve the consistency of mechanical properties. Results revealed that HIP reduced average porosity of 17-4 parts and that the yield strength of parts following solutionization and aging met wrought material property targets. Strain to failure of one of the facilities parts was less than 5% compared to greater than 9% for the other facility. Inspection of failure surfaces revealed this discrepancy was caused by pores (2-4% area fraction) on the failure surfaces of the low ductility parts. These results are viewed with respect to the intended application of this material as a structural element for wind tunnel testing.

High-strength materials lack a synergistic combination of mechanical properties and corrosion resistance desired for most structural applications. Phase and grain boundary architecturing is done in the material to attain balance in these properties. A friction stir processed Fe38.5Mn20Co20Cr15Si5Cu1.5 (Cu-HEA) exhibited excellent mechanical properties and high corrosion resistance in synergy. The excellent mechanical properties were attributed to the controlled transformation of ultrafine γ matrix and e twinning; whereas good corrosion resistance was due to homogenized fine grained γ microstructure. In short, Cu-HEA design opens a new pathway towards realizing strong and ductile alloys with tunable corrosion resistance by engineering matrix metastability.

IN718 alloy was fabricated by laser powder bed fusion (PBF) for examination of microstructure, precipitates and mechanical properties in the as-built state and after different heat treatments. The as-built alloy had a characteristic fine cellular-dendritic microstructure with Nb, Mo and Ti segregated along the interdendritic region and cellular boundary. The as-built alloys were then subjected to solution heat treatment (SHT) at 980 °C or 1065 °C for 1 h. SHT at 980 °C led to the formation of δ-phase in the interdendritic region or cellular boundary. The segregation was completely removed by the SHT at 1065 °C, but recrystallization was observed, and the carbides decorated along the grain boundaries. The as-built alloy and alloys with SHT at 980 °C and 1065 °C were two-step aged, which consisted of annealing at 720 °C for 8 h followed by annealing at 620 °C for 8 h. Transmission electron microscopy revealed the precipitation of γ' and γ” in all alloys after two-step aging, but the amount and uniformity of distribution varied. The Vickers hardness of the PBF IN718 alloy increased from 296 HV to 467 HV after direct aging. The hardness decreased to 267 HV and 235 HV after SHT at 980 °C and 1065 °C, respectively, but increased to 458 HV and 477 HV followed by aging. The evolution of Young’s modulus after heat treatment exhibited similar trend to that of hardness. The highest hardness was observed for IN718 after SHT at 1065 °C and two-step aging due to precipitation with greater amount and uniform distribution.

The study of microstructure evolution in additive manufacturing of metals would be aided by knowing the thermal history. Since temperature measurements beneath the surface are difficult, estimates are obtained from computational thermo-mechanical models calibrated against traces left in the sample revealed after etching, such as the trace of the melt pool boundary. Here we examine the question of how reliable thermal histories computed from a model that reproduces the melt pool trace are. To this end, we perform experiments in which one of two different laser beams moves with constant velocity and power over a substrate of 17-4PH SS or Ti-6Al-4V, with low enough power to avoid generating a keyhole. We find that thermal histories appear to be reliably computed provided that (a) the power density distribution of the laser beam over the substrate is well characterized, and (b) convective heat transport effects are accounted for. Poor control of the laser beam leads to potentially multiple three-dimensional melt pool shapes compatible with the melt pool trace, and therefore to multiple potential thermal histories. Ignoring convective effects leads to results that are inconsistent with experiments, even for the mild melt pools here.

An unmodified, non-spherical, hydride-dehydride (HDH) Ti-6Al-4V powder having a substantial economic advantage over spherical, atomized Ti-6Al-4V alloy powder was used to fabricate a range of test components and aerospace-related products utilizing laser beam powder-bed fusion processing. The as-built products, utilizing optimized processing parameters, had a Rockwell-C scale (HRC) hardness of 44.6. Following heat treatments which included annealing at 704 °C, HIP at ~926 °C (average), and HIP + anneal, the HRC hardnesses were observed to be 43.9, 40.7, and 40.4, respectively. The corresponding tensile yield stress, UTS, and elongation for these heat treatments averaged 1.19 GPa, 1.22 GPa, 8.7%, 1.03 GPa, 1.08 GPa, 16.7%, 1.04 GPa, 1.09 GPa, 16.1%, respectively. The HIP yield strength and elongation of 1.03 GPa and 16.7% are comparable to the best commercial, wrought Ti-6Al-4V products. The corresponding HIP component microstructures consisted of elongated small grains (~125 microns diameter) containing fine, alpha/beta lamellae.

Quality inconsistency due to uncertainty hinders the extensive applications of a laser powder bed fusion (L-PBF) additive manufacturing process. To address this issue, this study proposes a new and efficient probabilistic method for the reliability analysis and design of the L-PBF process. The method determines a feasible region of the design space for given design requirements at specified reliability levels. If a design point falls into the feasible region, the design requirement will be satisfied with a probability higher or equal to the specified reliability. Since the problem involves the inverse reliability analysis that requires calling the direct reliability analysis repeatedly, directly using Monte Carlo simulation (MCS) is computationally intractable, especially for a high reliability requirement. In this work, a new algorithm is developed to combine MCS and the first-order reliability method (FORM). The algorithm finds the initial feasible region quickly by FORM and then updates it with higher accuracy by MCS. The method is applied to several case studies, where the normalized enthalpy criterion is used as a design requirement. The feasible regions of the normalized enthalpy criterion are obtained as contours with respect to the laser power and laser scan speed at different reliability levels, accounting for uncertainty in seven processing and material parameters. The results show that the proposed method dramatically alleviates the computational cost while maintaining high accuracy. This work provides a guidance for the process design with required reliability.

This paper examines monotonic and cyclic behavior of WE43 alloy in several conditions including direct-chill as-cast and rolled heat-treated with an emphasis on the comparison of microstructural features governing the difference in their behavior. To facilitate the evaluation of the effect of hot working conditions on the material behavior, the as-cast-WE43 was converted into two rolled plates with one (P2) being rolled at a temperature for 16.5 °C lower than the other plate (P1). The plates were further processed into T5 and T6 conditions. The plate P2 in the T5 condition was found to have the highest strength and elongation to fracture in tension along both the rolling direction (RD) and the transverse direction (TD). The other plate, P1, showed a higher ductility in compression, while exhibiting slightly lower strength and elongation in tension. The trend of lower strength and elongation in tension with higher ductility in compression continued to the material in the T6 condition and finally to the as-cast material. Strain to fracture in compression was the largest for the as-cast material. In addition to quasi-static strength, P2 was found to exhibit superior low cyclic fatigue (LCF) and high cyclic fatigue (HCF) behavior. Consistent with the quasi-static strength, the rolled plates in the T5 conditions exhibited slightly longer lives in LCF and HCF along TD than along RD. The interesting deformation behavior characteristics of the alloy in different conditions are rationalized in terms of their microstructures. The results reveal that it is possible to further optimize the hot working conditions to obtain the alloy WE43 exhibiting better performance characteristics.

Dense and crack-free Al-6Zn-2Mg (wt%) alloys with 1 wt% (Sc + Zr) addition were additively manufactured by laser powder bed fusion (LPBF) using gas atomized powders. As-built microstructure consisted of small equiaxed grains near the melt pool boundary and columnar grains between adjacent melt pools. Alloying of Sc + Zr promoted the formation of Al3(Sc,Zr) particles, which contributed to the grain refinement. The alloy exhibited outstanding tensile properties (i.e., 418 ± 3 MPa yield strength, 436 ± 3 MPa tensile strength and 11 ± 1% elongation) after heat treatment. The results demonstrate that high strength aluminum alloy can be fabricated by LPBF through alloy design and microstructural control.

In this work the effect of manufacturing strategy and post process treatment on the high strain rate (HSR) compressive deformation behavior of additively manufactured powder bed fusion 17-4PH stainless steel is studied. Specimens were fabricated using three different laser vector path strategies to impart different thermal histories and resulting microstructures in the material. The effect of post processing in the form of hot isostatic pressing and heat treatment and their effect on HSR compressive deformation response of the material was studied. Defect characteristics were quantified using x-ray micro computed tomography. HSR behavior was characterized using split Hokinson bar testing at rates from 1000–4000 s−1. It was found that the laser vector strategy had a strong influence on the development of microstructure and defect characteristics and spatial distribution in the materials which strongly influence the HSR response and the HSR compressive flow stresses of the materials varied by as much as 43% in the regimes tested.

The design of non-equiatomic high-entropy alloys (HEAs) opens huge compositional space to develop new materials with exceptional properties. Among them, HEAs with flexible microstructures showed an adaptive phase stability that enhanced the work hardening (WH) ability of the material drastically. With the same motive, here we present a new friction stir processed Fe39Mn20Co20Cr15Si5Al1 HEA that demonstrated an unexpected strength–ductility response just upon annealing. The inter-competing precipitation and grain/twin formation events during low-temperature annealing resulted in an unexpected f.c.c. (γ) → h.c.p. (ɛ) transformation. This unusual phase evolution triggered development of refined, ɛ-dominated microstructure coupled with a uniformly dispersed fine γ phase. The controlled 〈c+a〉 slip and twinning in the ɛ phase along with the transformation of a refined γ matrix resulted in higher elongation of 52% with enhanced ultimate tensile strength of 1.12 GPa during deformation. Thus, the metastability-assisted design of ɛ-martensite-dominant HEAs by annealing opens a new path to obtain strong yet ductile alloys, which is otherwise not feasible in conventional steel/HEA designs.

Recent analytic criteria for isotropic porous materials developed by Cazacu et al. (2013) revealed the importance of considering specificities of plastic behavior in the matrix. On one hand it was shown that if the matrix material is governed by the von Mises criterion, the yield surface of the porous material should be centrosymmetric and, with the exception of hydrostatic and purely deviatoric loadings, there are combined effects of the mean stress and third-invariant of the stress deviator on void growth or collapse; but on the other hand if the matrix plastic deformation displays strength differential (SD) effects, the response is also sensitive to third-invariant and there is a lack of symmetry of the yield surface of the porous material. In this paper, we use a unit cell modeling approach in conjunction with a crystal plasticity finite element model to verify these theoretical predictions. It is assumed that each porous polycrystal contains a regular array of initially spherical voids and a random initial texture. At the grain-level, we consider that plastic deformation is governed by Schmid law and a recent non-Schmid formulation by Savage et al. (2017a) that intrinsically accounts for tension–compression asymmetry in a physical sense. Unit cell FE calculations are performed for axisymmetric tensile and compressive loadings corresponding to a fixed value of the stress triaxiality and the two possible values of the Lode parameter. The resulting numerical points representing the homogenized yield surfaces and void growth/collapse curves are found to be in agreement with the analytical model's predictions.

Two high entropy alloy (HEA) compositions were compared to explore the individual effects of Co and Ni on phase stability and resultant deformation response in Fe-Mn-Cr-Si-containing HEAs. It was observed that Co-Si-containing HEA depicted responsive phase evolution upon friction stir processing owing to decreased γ (f.c.c.) matrix stability as against Ni-Si-containing HEA. As a result, the Co-Si HEA showed the presence of dual-phase microstructure with the dominance of e (h.c.p.) phase (52 pct), whereas the Ni-Si HEA showed single-phase γ microstructure under similar processing condition. Also, the dominant deformation mechanisms were different in the two alloys. Co-Si HEA showed uniform strain partitioning between the f.c.c. and h.c.p. phases. Conversely, single-phase f.c.c. microstructure in Ni-Si HEA accommodated strain by twinning-induced plasticity.

The microstructure and precipitates within selectively laser melted (SLM) AlSi10Mg alloys in the as-built state and after T6 heat treatment were examined and correlated to the hardness evolution. The as-built alloy exhibited columnar patterns perpendicular to the build-direction and fish-scale pattern along the build-direction originating from the melt pool. The microstructure consisted of fine Al cells and eutectic structure with up to 4 μm in size. Tiny needle-like Si particles with highly faulted structure were identified within the Al cells and formed semi-coherent interface with the matrix. Nanoscale Si particles and π-Al8Si6Mg3Fe phase were observed to segregate along the cell and grain boundary. Solution heat treatment (SHT) at 520 °C resulted in the dissolution of Al cellular microstructure. Precipitation and coarsening of the Si particles occurred, while the aspect ratio of Si particles remained nearly constant. The π-Al8Si6Mg3Fe phase decomposed into plate-shaped β-Al5SiFe. The artificial aging (AA) at 160 °C did not alter the microstructure but led to the formation of metastable Mg2Si precipitates. The GP zones and β″ were identified through transmission electron microscopy after AA of 6 h, and β″ existed up to AA of 24 h. The hardness decreased after SHT due to the coarsening of the microstructure and reduced solid solution hardening. Hardness reached maximum magnitude after AA of 6–10 h due to the precipitation hardening, and remained relatively unchanged up to AA of 24 h. Microstructure and/or hardness of gas atomized powders and bulk cast AlSi10Mg alloys were also examined as references.

Conventional high strength alloys lack ductility due to lower work hardening and early onset of damage nucleation. To overcome this deficiency, high entropy alloys (HEAs) enjoy the benefit of metastability of phases to tune the deformation mechanisms for engineering strength and ductility. Inspired by this, here we present friction stir processed Fe40Mn20Co20Cr15Si5 HEA with ultrafine face centered cubic (f.c.c.) γ grains embedded in a refined hexagonal closed packed (h.c.p) e matrix. Transformation of γ grains and twinning in e matrix triggered uniform strain partitioning among these phases and sustained work hardening during deformation thereby reversing the conventional strength-ductility trade-off.

Design of multi-phase high entropy alloys uses metastability of phases to tune the strain accommodation by favoring transformation and/or twinning during deformation. Inspired by this, here we present Si containing dual phase Fe42Mn28Co10Cr15Si5 high entropy alloy (DP-5Si-HEA) exhibiting very high strength (1.15 GPa) and work hardening (WH) ability. The addition of Si in DP-5Si-HEA decreased the stability of f.c.c. (γ) matrix thereby promoting pronounced transformation induced plastic deformation in both as-cast and grain refined DP-5Si-HEAs. Higher yet sustained WH ability in fine grained DP-5Si-HEA is associated with the uniform strain partitioning among the metastable γ phase and resultant h.c.p. (ε) phase thereby resulting in total elongation of 12%. Hence, design of dual phase HEAs for improved strength and work hardenability can be attained by tuning the metastability of γ matrix through proper choice of alloy chemistry from the abundant compositional space of HEAs.

The sintering of a metallic alloy powder into bulk form using only direct current (DC) electric energy input at ambient room temperature is presented for the first time. It was found that a flash sintering phenomena is achievable in aluminum alloy powder, with no addition of external thermal energy, at applied DC electric fields in the range of 175–330 V/cm in which the formation of interparticle necks occurs rapidly and is characterized by a near-instant change in the physical properties of the compact from electrically non-conductive to electrically conducting in a time period on the order of seconds. It was found that the kinetics of this effect have a logarithmic dependence on the magnitude of the applied electric field. Above approximately 330 V/cm, the critical field strength is reached at which an incubation time for flash sintering is not required and sintering occurs instantly with the application of the DC field. This technique has promise to greatly reduce processing time and costs associated with sintering of powder metallurgy products as well as consolidation of nanostructured metals by limiting exposure to high temperatures which result in excessive grain growth.

This paper describes the main results from an experimental investigation into tailoring the phase content and grain structure for high strength of a microstructurally flexible high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at%), using rolling, friction stir processing (FSP), and compression. Optical microscopy, neutron diffraction, and electron backscatter diffraction were employed to characterize microstructure and texture evolution. The material upon rolling was found to have triplex structure consisting of metastable gamma austenite (γ), stable sigma (σ), and stable epsilon martensite (e) phases. The adaptive phase stability exhibited by the selected HEA of very low stacking fault energy with strain, strain rate, and temperature was used along with grain refinement to enhance the strength. To this end, the complex structure was refined by FSP at 350 revolutions per minute (RPM) tool rotation rate, while increasing the fraction of γ and decreasing the σ and e content. The strength was further enhanced by FSP at 150 RPM by further refinement of the grain structure and increasing the fraction of e phase. The peak ultimate tensile strength of ~1850 MPa was achieved by double pass FSP (350 RPM followed by 150 RPM), the sequence which even more refined the microstructure and increased the fraction of σ phase. While the content of diffusion created σ phase remains constant during subsequent compression, the fraction of e increases due to the diffusionless strain induced γ→e phase transformation. The transformation facilitates plastic strain accommodation and rapid strain hardening, which has been attributed to the increase in highly dislocated e phase fraction and transformation induced dynamic Hall-Petch-type barrier effect. Interestingly, a great deal of hardening ability was exhibited by the HEA even at very high strength. Roles of texture, grain size, and phase content on the transformation during compression have been rationalized and discussed in this paper.

Laser powder bed fusion (LBPF) was employed to fabricate a strain-transformable β-Ti alloy, Ti-10V-2Fe-3Al (wt%). While the alloy is known to exhibit transformation induced plasticity (TRIP), the as-fabricated alloy, under tensile loading, did not show the same TRIP effects, even though it exhibits the same β + ω microstructure. The repeated heating-cooling cycles experienced during the LBPF process leads to the early stages of rejection of solute elements (Fe, V, and Al), forming isothermal omega (ω) precipitates, which were captured via detailed investigations coupling transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). While these homogeneously distributed isothermal ω precipitates lead to a higher yield strength, the TRIP/TWIP effects within the β matrix were suppressed, leading to very low ductility and virtually no strain-hardenability. Interestingly, after a simple β-solution heat treatment followed by quenching, leading to a β + ω (athermal) microstructure, the TRIP/TWIP effects were reactivated in the same LPBF Ti-10 V-2Fe-3Al alloy. The alloy exhibited substantial recovery of tensile ductility and a very large strain hardening (tensile strength minus yield strength ~500 MPa), with a high average strain hardening rate ~15000. Such a very high strain hardening rate in case of LBPF processed Ti-10 V-2Fe-3Al, appears to arise from a rapid strain-induced transformation from β to α" at the early stages of plastic deformation, leading to a high-volume fraction of the martensitic phase, coupled with hierarchical twinning within the martensite plates.

In this work, we study the deformation behavior of a direct chill cast WE43 Mg alloy. This material initially has equiaxed grains approximately 40 µm in diameter and a random texture. The room temperature, quasi-static response exhibits little plastic anisotropy when evaluated parallel to and normal to the solidification direction and no initial yield tension-compression asymmetry. The deformation at room temperature is accompanied by significant basal texture development and formation of three types of deformation twins: { 10 1 2 } 〈 1 011 〉 , { 10 1 1 } 〈 10 12 〉 , and { 11 2 1 } 〈 1 1 26 〉 as well as double twins { 10 1 1 } 〈 10 12 〉 - { 10 1 2 } 〈 1 011 〉 , although each in small amounts 1.0 true strain) without fracturing.

Laser-powder bed fusion (L-PBF) additive manufacturing offers unprecedented microstructural fine-tuning capabilities. Naturally, benefitting from such capability requires alloys that are amenable to microstructural heterogeneity and hierarchy (MHH) and that exhibit a low hot-cracking susceptibility (HCS). However, columnar growth, which is characterized by capillary effects and poor strain accommodation capabilities, is prevalent in L-PBF and increases the HCS of the processed alloys. Further, while solute segregation is prominent in cellular and dendritic growth modes during L-PBF, the effects of solute segregation on the alloy HCS and L-PBF processing window remain widely unexplored. Here, we demonstrate that solute segregation affects columnar growth, grain coalescence behavior during solidification, MHH and mechanical properties of a metastable Fe40Mn20Co20Cr15Si5 (at.%) high entropy alloy (CS-HEA) doped with 0.5 wt.% B4C (termed CS-BC). A theoretical framework is proposed, which reveals that a boundary-strengthening segregant may reduce the alloy HCS during L-PBF. In as-built CS-BC, boron, a boundary strengthener, segregated to the solidification cell boundaries, whereas carbon remained in the solid solution. The as-built CS-BC exhibited suppressed columnar growth, more random texture, smaller cell size and higher strength as compared to the as-built CS-HEA. Further, a wide crack-free L-PBF processing window of CS-BC allowed fine-tuning of its MHH and thus the mechanical properties. Upon annealing, as carbon-containing precipitates formed, CS-BC exhibited a metastable microstructure and transformation induced plasticity effect, which led to high synergistic strength-ductility. These findings will foster design of alloys that facilitate application-specific manufacture with L-PBF and thus, an extended outreach of L-PBF for structural applications.

Laser engineered net shaping (LENS) processing of an in-situ Ti-B4C composite, results in the natural formation of a novel periodically layered structure, when a mixture of Ti powders and B4C powders are used as a feedstock. One of the layers predominantly consisted of TiB2 and inter-dendritic TiC phases, while the other alternating layer exhibited a rather complex microstructure, comprising of TiB, TiC, partially melted-B4C and α-Ti phases. Increasing the laser power (300–700 W) results in an increase in the height/thickness of these layers, as well as the number density of in-situ formed ceramic precipitates (TiB, TiC) in the TiB + TiC + α-Ti layer. Additionally, the Heipel-Roper theory of weld pool dynamics was employed to rationalize the unconventional microstructural evolution in these multi-layered LENS processed Ti-B4C composites. Microhardness and wear properties revealed that among the three powers, the 700 W condition exhibited the best combined wear and hardness which can be attributed to reduced porosity, and an increase in hardness of both layered regions due to an increase in number density of precipitates in the TiB + TiC + α-Ti layer. Such AM process induced naturally layered composites open up a new avenue for design and development of hybrid materials for future engineering applications.

Fatigue failure is ubiquitous in structural components. In additively manufactured (AM) components, the processing induced defects limit the fatigue performance. Further, the stochastic nature of defects in laser-powder bed fusion (L-PBF) make it difficult to predict the fatigue life in these components. In this work, we explored exceptional work hardening (WH) of a metastable Fe40Mn20Co20Cr15Si5 high entropy alloy (CS-HEA) to obtain high fatigue-resistance with L-PBF. Further, a fatigue life estimation tool based on the statistical size distribution of microstructural entities such as grains, pores and solid-state inclusions and, their mutual interaction was used to estimate the fatigue life of as-printed material. Upon deformation, CS-HEA exhibited γ (f.c.c.) → e (h.c.p.) martensitic transformation and subsequent twinning in e (h.c.p.) phase. Such deformation behavior resulted in sustained WH and is specifically beneficial in the vicinity of critical pores. A high normalized fatigue strength of 0.65 with respect to the yield strength was thus obtained in as-printed condition. Further, the model accurately predicted extended crack initiation life for CS-HEA. The current work therefore provides guidance towards developing defect-tolerant alloys for L-PBF and presents a tool for estimation of fatigue life of AM alloys with unconventional WH behavior.

Additive manufacturing (AM) of metallic parts is generating significant interest due to the ability to produce complex parts in a short period of time with minimal finishing required. However, the effect of laser scan strategy on the properties of finished parts is not well understood. In this paper the effects of laser scan line strategy on the microstructure and mechanical properties of stainless steel produced using metal Powder Bed Fusion (PBF) AM were characterized. Microstructure and phase identification were measured using X-ray diffraction and quantitative optical microscopy which found that all samples had a dual phase austenite-ferrite composition. Shorter scan lines perpendicular to the load direction resulted in 25% retained austenite, while elongated scan lines parallel to the load direction more than doubled the amount of austenite retained. A change of direction within the scan line path resulted in increased delamination porosity along the melt pool boundary and changes in volume fraction of retained austenite. Fractography, revealed cracks that propagated along melt pool boundaries. Understanding the effect of strategy on the microstructure and mechanical properties allows the producer of AM parts to implement materials by design strategies. Keywords: Additive manufacturing, Powder Bed Fusion, Stainless steel, Processing-microstructure-property relations, Failure analysis

Aluminum 5083 powder, both as atomized and cryomilled, was consolidated via spark plasma sintering (SPS). This study quantified and compared the effects of heating an aluminum alloy powder directly through Joule heating vs indirectly through thermal conduction from the die during SPS processing. When consolidated under the same processing conditions, the cryomilled powders showed faster heating rates and densification than the as atomized powder. It was also possible to process the cryomilled powder in a non-conductive die but not the as atomized powder. This could be ascribed to an improvement in electrical conductivity of the powder due to the break up and redistribution of surface oxides after cryomilling. The changes in behavior as a result of cryomilling and/or changing die material led to samples with different fracture morphologies and increased hardness values.

The high strength metastable β-Ti alloy, Ti–1Al–8V–5Fe (wt%), also referred to as Ti-185, has been successfully processed using the directed energy deposition (DED) based laser engineered net shaping (LENS) process, obviating the beta fleck problem associated with Fe micro-segregation that has been reported in conventionally processed counterparts. The large solidification range for this alloy resulted in finer scale equiaxed β grains in the as deposited condition for a range of process parameters, unlike the large columnar grains observed in case of AM of other titanium alloys such as Ti–6Al–4V. Furthermore, based on the process parameters, a homogeneous distribution of fine scale ω or α precipitates form within the β grains, which has been rationalized based on quantitative thermo-kinetic modelling of a multi-layered deposition process. Atom probe tomography results indicate early stages of β/ω compositional partitioning, leading to a higher tensile yield strength, close to 1000 MPa, as compared to the solution treated/quenched condition of conventionally processed Ti-185. Homogeneous fine scale α precipitation, with a more pronounced compositional partitioning, resulted in an exceptional yield strength exceeding 1200 MPa in the as-processed condition.

WE43 is a high strength, high creep resistant Mg-alloy containing Y, Nd, and Zr, and has potential for many lightweight structural applications in the automotive, aerospace, and biomedical industries. Additive manufacturing technology such as laser powder bed fusion (LPBF) brings an opportunity to produce complex geometries such as lattice structures. In this study, fabrication, compressive behavior, and fracture modes of 24 different microlattice structures were investigated by varying unit cell type, strut diameter, and number of unit cells. These complex lattice structures were produced by LPBF using the parameter set: laser power = 200 W, scan speed = 1100 mm/sec, slice thickness = 0.04 mm, which was optimized in our previous study to build fully dense (> 99 %) WE43 alloy. Overall, the lattice structures exhibited oscillations in stress, showing many local maxima and minima, with a global maximum in stress at or near 5 % strain. The highest compressive strength, and the corresponding specific strength found in this study were 71.48 MPa and 38.85 MPa·g−1·cm3, respectively, from the cubic fluorite lattice structure with a strut diameter of 0.75 mm and an unit cell number of 10. During compression testing, two different failure modes were observed: 45° shear fracture and crushing. Due to the inherent low ductility of WE43, brittle crushing was predominant after elastic yielding, which resulted in similar strength-density relationships for each lattice type along with similar failure modes.

Consistent manufacturing of volumetrically dense engineering components, free of solidification cracks by laser powder bed fusion (LPBF), has been demonstrated for Al-Si alloys such as AlSi10Mg and Al12Si. The success in LPBF of these alloys is attributed to the near eutectic composition with a small freezing range. To illuminate this observation, cracking susceptibility was examined from Scheil-Gulliver solidification modeling by calculating the hot cracking susceptibility, |dT/dfS1/2|. To validate the findings from hot cracking susceptibility calculations, six binary Al-Si alloys, whose compositions were strategically chosen at hypo-, near-, and hyper-eutectic compositions, were gas atomized into alloy powders, and processed by LPBF. Only Al-Si alloys with 1.0 and 2.0 wt.% Si were found to exhibit cracking, which was predicted by relatively large magnitudes of |dT/dfS1/2|. Either as particles or with a eutectic structure, Si segregation at the intercellular boundaries was observed to define the sub-grain cellular structure. For selected compositions, measurement of the cellular structure allowed for estimation of the cooling rate to be 106 to 107 K•s−1. Excluding the alloys with solidification cracking, an increase in tensile strength and the corresponding decrease in ductility were observed with an increase in Si concentration, which were attributed to the formation of a cellular structure and the amount of Al-Si eutectic found at the intercellular boundaries.

Direct current (DC) electric currents were applied during sintering of aluminum alloy (AA5083) green powder compacts and it was found that the kinetics of sintering were greatly enhanced compared to samples processed without a field. In situ sintering kinetics during pressure-less sintering employing electric field strengths and amperages ranging from 0 to 56 V/cm and 0 to 3 A were quantified using digital image correlation. It was found that the application of a DC field during sintering results in a discontinuous change in volume at a critical temperature along with a transition in electrical properties of the compact from insulating to conductive. This effect is similar to the phenomena observed in the flash sintering process currently being actively researched for ceramic powder processing. The temperature at which the flash event occurs was found to be field strength dependent and doubling the field strength was found to decrease the flash temperature by 25 pct. Joule heating of the specimen was measured using thermal imaging and it was found to not contribute enough additional thermal energy to account for the substantially increased sintering rates observed in specimens processed using electric fields.

Spark plasma sintering has been a well-studied processing technique primarily for its very high cooling and heating rates. However, the underlying phenomenon driving the sintering behavior of powders under an electric field is still poorly understood. In this study, we look at the effect of changing current pathways through the powder bed by changing die materials, from conductive graphite to insulating boron nitride for sintering aluminum alloy 5083 powder. We found that the aluminum powder itself was insulating and that by changing the current paths, we had to find alternate processing methods to initiate sintering. Altering the current pathways led to faster temperature raises and faster melting (and potentially densification) of the aluminum powder. A flash sintering effect in metallic powders is observed in which the powder compact undergoes a rapid transition from electrically insulating to conducting at a temperature of 583 K (310 °C).

A finite-element based plasticity model is developed for polycrystals deformed to high-strain-rates. The model is multiscale, covering from thermally activated dislocation motion on a specific crystallographic slip system ( nm ), to single crystal plasticity ( μm ), to polycrystalline aggregate plasticity ( mm ), and ultimately heterogeneous deformation of the macroscale test sample ( m ). Within the model, the rate dependence in macroscale response arises solely from the microscopic characteristic stress to activate dislocation motion. This is accomplished by introduction of a novel methodology, used at the intermediate length scales, to relax the extraneous rate dependencies occurring as a result of the visco-plastic rate sensitive flow rule commonly associated with single crystal plasticity formulations. The multi-scale model developed here also permits simulations to be carried out in stress-imposed, strain-rate imposed, and mixed stress/strain-rate-imposed boundary conditions, another advancement over previous techniques. Simulation results are presented for the deformation of high-purity Zr in a Taylor impact cylinder test. The variation in sample shape changes, texture evolution, and deformation twin fraction after the test are experimentally examined. These same quantities are calculated with the model and good agreement is achieved in all aspects. We show that without adjustment of material parameters that the thermally activated hardening model applies to much higher strain-rates (10 4 /s–10 5 /s) than the strain-rates used previously to characterize it. This model can be broadly applied to understanding microstructure-property relationships in high-strain-rate deformation processes that generate spatially and temporarily heterogeneous mechanical fields.

In this work, we develop a multi-level constitutive model for polycrystalline metals that deform by a combination of elasticity, slip and deformation twinning. It involves a two-level homogenization scheme, where the first level uses an upper bound Taylor-type crystal plasticity (T-CP) theory to relate the single-crystal scale to the polycrystal meso-scale and the second level employs an implicit finite elements (FE) approach to relate the meso-scale to the macro-scale. The latter relaxes the iso-strain constraints imposed by the Taylor model. As such, we name the model T-CPFE. At the single crystal level, the model features a dislocation-based hardening law providing the activation stresses that governs slip activity within the single crystals. For deformation twinning, it contains an advancement of a composite grain model that retains the total Lagrangian formulation. Here we use the T-CPFE model to analyze the mechanical response and microstructure evolution of extruded AZ31 Mg alloy samples in simple compression, tension, and torsion under strain rates ranging from 10−4 s−1 to 3000 s−1 and temperatures ranging from 77 K to 423 K reported in Kabirian et al. (2015). Taking the experimentally measured initial texture and average grain size as inputs, the model successfully captures stress-strain responses, deformation texture evolution and twin volume fraction using a single set of material parameters associated with the thermally activated rate laws for dislocation density. The distinctions in flow stress evolution among the loading conditions result from differing relative amounts of slip and twinning activity, which the model internally adjusts based on evolution of slip and twin resistances in the response to the imposed loading conditions. Finally, we show that the T-CPFE model predictions of geometrical changes during compression compare favorably with corresponding geometry of samples deformed experimentally. For this application, it predicts the anisotropy and asymmetry of the material flow resulting from crystallographically soft-to-deform extension twinning and basal slip and hard-to-deform contraction twinning and pyramidal slip. The formulation developed is sufficiently general that the T-CPFE model can be applied to other materials that slip and twin.

This paper presents a multi-level crystal plasticity-based simulation framework for modeling mechanical response and microstructure evolution of Ti-6Al-4V with α-lath/lamellar microstructures. The model is a correlated structure visco-plastic self-consistent (CS-VPSC) formulation linking three scales: a single crystals micro-scale, a lath/lamellar colony meso-scale, and a lath/lamellar aggregate macro-scale. A selected hardening law for the evolution of critical resolved shear stress per slip system used in CS-VPSC is phenomenological. However, it adjusts the resistances of basal and prismatic slip systems based on the geometry of slip transfer between adjacent lamellae. Consistent with experimental evidences, the resolved shear stress on the pyramidal slip planes is dependent not only on the stress in the direction of slip but also on the two orthogonal shear stress components and the three normal stress components (non-Schmid effects). Electron backscatter diffraction (EBSD) data in conjunction with a procedure relying on α→β phase transformation is used to construct paired variants of α-laths/lamellae satisfying their local crystallographic correlations. The procedure fits volume fractions of individual laths/lamellae with the experimental EBSD data and selects a distribution of habit planes between adjacent variants with respect to the loading direction. The simulation framework is applied to interpret the deformation behavior in tension and compression along two sample directions of Ti-6Al-4V fabricated via laser powder bed fusion. Moreover, the model is used to simulate texture evolution during rolling of the material to large strains. It is demonstrated that the model is capable of predicting plastic anisotropy/asymmetry and the concomitant texture evolution. While the model reveals a significant effect of habit plane inclination with respect to the loading direction on yield stress, the comparison of the data and model predictions shows that a random distribution of habit planes fits the flow response. It is further inferred that the tension-compression asymmetry arises from the non-Schmid effects.

Use of laser powder bed fusion (LPBF) additive manufacturing (AM) in structural applications requires development of a) damage-tolerant alloys (alloys that exhibit high strength and ductility despite the presence of pores and/or microcracks), and b) corrosion resistant alloys. High entropy alloys (HEAs) offer abundant alloy design space that can be used to tune deformation mechanisms and address both these challenges effectively. In line with that, transformation induced plasticity (TRIP) assisted Fe38.5Mn20Co20Cr15Si5Cu1.5 HEA (Cu-HEA) was printed with LPBF-AM. Despite the presence of 1.5 vol. % of microcracks and pores, as-built Cu-HEA exhibits tensile strength of ∼1235 MPa (highest among as-built HEAs) and ductility of ∼17.2 %, thus displaying damage-tolerant behavior. Additionally, as-built Cu-HEA exhibits a steeper polarization slope as compared to SS 17-4 PH and as-cast Cu-HEA, thus exhibiting higher passivation tendency. These findings demonstrate an effective strategy for developing damage-tolerant anticorrosive materials for LPBF-AM.

This work investigates the effects of test temperature and sample pre-straining on the high cycle fatigue behavior of a magnesium alloy, WE43. Fatigue strength is found to deteriorate with environmental temperature from room to 100℃. In contrast to the samples pre-strained in tension, fatigue strength improves for the samples pre-strained in compression. The peak improvements of over 2x is determined for the samples pre-compressed to 6% strain. Significantly, the alloy sufficiently pre-strained in compression begins to exhibit the endurance limit. The interesting characteristics of the alloy behavior in fatigue are rationalized in terms of the microstructural evolution and deformation mechanisms.