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Additive manufacturing (AM) is well-known for its capability to produce and repair intricate parts e.g., Ni-based superalloy components used in aero-engine applications. However, a comprehensive understanding of the microstructure evolution and its link to properties remains elusive, with gaps in the through-process evolution of γ grains, γ', γ'', δ, and other phases in response to thermal cycling. The outstanding challenge here is to observe these transformations dynamically during the 3D printing process, as this usually requires complex in-situ measurements. Here we apply a different approach to systematically reveal the impact of thermal cycling on the solid-state microstructure evolution of IN718, via a combination of physical simulations and thermo-kinetic modelling. We replicate the thermal cycles IN718 experiences during laser directed energy deposition. Solid-state thermal cycles are shown to have minor effects on the grain morphology due to short holding times at critical temperatures, however, they do induce plastic strain accumulation. Significantly, we show how thermal cycling influences the evolution of γ', γ", and δ phases. While high initial peak temperatures inhibit the precipitation of γ', γ", and δ phases, prolonged thermal cycling to gradually decreasing peak temperatures promotes their precipitation. The δ phase forms along grain and twin boundaries, while both γ' and γ" predominantly precipitate in Nb-enriched regions, causing heterogeneities in hardness. Our results underpin the suitability of physical simulations for replicating superalloy AM, highlighting its potential as a tool to advance the current limited understanding of the microstructure and property evolution. [Display omitted] • Physical and thermo-kinetic simulation of solid-state thermal cycling in L-DED IN718. • Physical Gleeble experiments show excellent correlation with actual L-DED process. • Variation in γ′, γ′′, and δ precipitates correlates with hardness. • Solid-state thermal cycling affects γ′, γ′′, and δ morphology in individual layers. • Solid-state thermal cycling has negligible influence on grain morphology. [ABSTRACT FROM AUTHOR]

Ti-6Al-4 V is a popular alloy in additive manufacturing (AM) due to its applications in the biomedical implants and aerospace industries where the complex part geometries allowed by AM provide cost and performance benefits. Ti-6Al-4 V goes through a β → α' transformation after solidification which is known to experience variant selection, e.g., through the formation of clusters of variants which, when situated together, partially accommodate the strain of the phase transformation. During electron beam powder bed fusion AM, an in situ decomposition of α' martensite occurs during the cyclic reheating caused by melting successive layers, resulting in α + β microstructures. How variant selection influences the evolution beyond the initial rapid cooling remains an open question. Using 3D electron backscatter diffraction, we provide a clearer understanding without ambiguity from sectioning effects of how α' decomposes into microstructures with distinct morphologies and variant/intervariant distributions. We extract quantitative 3D information on the various intervariant boundaries networks formed in samples printed using three different electron beam scanning strategies. This shows that differing mechanisms during the decomposition result in a shift from self-accommodating clusters in an acicular microstructure, to either the preferred growth of six variants in a basketweave microstructure, or to a colony microstructure where variant selection is determined by prior-β grain boundaries. We propose a new representation of the misorientations arising from the Burgers orientation relationship, which we refer to as intervariant network diagram, to reveal how variant selection during the martensitic transformation and subsequent decomposition leads to the intervariant boundary networks observed. This holistic understanding of the microstructural evolution has the potential to allow tailoring of microstructures and properties for specific applications. [ABSTRACT FROM AUTHOR]

Metal additive manufacturing (AM), also known as 3D printing, is a disruptive manufacturing technology in which complex engineering parts are produced in a layer-by-layer manner, using a high-energy heating source and powder, wire or sheet as feeding material. The current paper aims to review the achievements in AM of steels in its ability to obtain superior properties that cannot be achieved through conventional manufacturing routes, thanks to the unique microstructural evolution in AM. The challenges that AM encounters are also reviewed, and suggestions for overcoming these challenges are provided if applicable. We focus on laser powder bed fusion and directed energy deposition as these two methods are currently the most common AM methods to process steels. The main foci are on austenitic stainless steels and maraging/precipitation-hardened (PH) steels, the two so far most widely used classes of steels in AM, before summarising the state-of-the-art of AM of other classes of steels. Our comprehensive review highlights that a wide range of steels can be processed by AM. The unique microstructural features including hierarchical (sub)grains and fine precipitates induced by AM result in enhancements of strength, wear resistance and corrosion resistance of AM steels when compared to their conventional counterparts. Achieving an acceptable ductility and fatigue performance remains a challenge in AM steels. AM also acts as an intrinsic heat treatment, triggering 'in situ' phase transformations including tempering and other precipitation phenomena in different grades of steels such as PH steels and tool steels. A thorough discussion of the performance of AM steels as a function of these unique microstructural features is presented in this review. [ABSTRACT FROM AUTHOR]

Additive manufacturing is a promising alternative method for fabricating components of Ni-based superalloys which are difficult to cast, form and join. However, typical thermal cycles associated with laser powder bed-fusion techniques suppress the formation of desirable microstructures containing γ′ particles, necessitating long-time post-process heat treatments. Here we report in-situ precipitation of γ′ (L12-ordered) particles and carbides during electron-beam powder bed-fusion of Inconel-738. The γ′ particles are homogenously distributed across the build and exhibit a multimodal size distribution. Based on atom-probe microscopy, we propose a eutectic reaction and multiple nucleation, growth, coarsening and dissolution bursts during thermal cycling as formation mechanism. [ABSTRACT FROM AUTHOR]

High entropy alloys (HEAs) show promise as materials for structural applications, even at elevated temperatures. However, their wear behaviour over a wide range of temperatures has not been extensively studied. CoCrFeMnNi and Al x CoCrFeNi HEAs were subjected to pin-on-disc dry sliding wear at temperatures between 25 °C and 900 °C against an alumina ball, and the tribological performance benchmarked against AISI 304 and Inconel 718. A detailed characterisation of the wear tracks using electron microscopy and surface profilometry revealed a transition in wear mechanism from abrasive wear at room-temperature to oxidative and delamination wear above 600 °C. The wear performance of the HEAs, AlCoCrFeNi in particular, is substantially enhanced with increasing temperature, surpassing that of Inconel 718 at 900 °C. The enhanced wear performance of the HEAs above 600 °C is attributed to the formation of a compact oxide scale in the contact region, and relative subsurface strengthening in the form of a fine-grained recrystallised structure containing precipitation hardening phases. • The wear behaviour of various high entropy alloys up to 900 °C is analysed. • Wear rates of high entropy alloys decreases with increasing temperature. • High wear resistance of AlCoCrFeNi due to alumina scale and σ-phase precipitation. [ABSTRACT FROM AUTHOR]

Single phase Al 0.3 CoCrFeNi (Al 0.3) FCC high entropy alloy (HEA) synthesized through arc melting was subjected to cold rolling and subsequent annealing to study its recrystallization and grain growth behaviour. For the as-cast alloy, only partial recrystallization was achieved after cold deformation followed by annealing at 800 and 900 °C, which was attributed to the pining effect of Al Ni rich precipitates formed during annealing. The activation energy of static recrystallization (549 kJ/mol) for the as-cast Al 0.3 HEA was significantly higher than that of conventional alloys due to the distorted lattice, sluggish diffusion and most importantly the interaction of Al Ni rich precipitates with the recrystallization. Homogenization heat treatment on Al 0.3 HEA increased the static recrystallization kinetics and was associated with the lower volume fraction of precipitates formed during annealing. An increased grain growth exponent (n ≈ 5) and activation energy (583 kJ/mol) compared with conventional alloys, particularly at low annealing temperatures, was observed for the Al 0.3 HEA mainly due to the pinning effect of Al Ni rich precipitates. Finally, through investigating the mechanical performance of the alloy after annealing in the precipitation-free (1050 and 1000 °C) and precipitation-containing (900 °C) temperature ranges, the relative contributions for grain boundary and precipitation strengthening were estimated for the annealed Al 0.3 HEA. [ABSTRACT FROM AUTHOR]

Highlights • Two types of austenite morphologies, equiaxed (EQ) and Widmanstätten (W), were produced. • W-A showed a larger degree of coherency at δ/γ interfaces relative to the EQ-A. • W-A showed unexpected propensity to the formation of secondary precipitates. • Higher extent of interface area and unstable δ were found to contribute to this behaviour. Abstract Two types of austenite morphologies, equiaxed and Widmanstätten, were produced through different phase transformation routes to evaluate the critical factors affecting the intergranular corrosion susceptibility in a 2205 duplex stainless steel. These distinct austenite morphologies behaved quite differently in secondary phase precipitation on exposure to sensitization temperature. Although the Widmanstätten microstructure was found to have a larger degree of coherent ferrite/austenite interface area compared with the equiaxed one, it showed a higher degree of sensitization. It was clarified that, in addition to the ferrite/austenite interface coherency, the extent of an interface area and presence of un-stable ferrite also play prominent roles in intergranular corrosion susceptibility. [ABSTRACT FROM AUTHOR]

Abstract Al x CoCrFeNi high entropy alloys (HEAs) with Al molar ratios of 0.3, 0.6 and 0.9 were subjected to hot plane strain compression testing at varying strain rates and strains in the temperature range of 930–1030 °C. A detailed analysis of the flow curves and microstructural evolution of these alloys was conducted under different hot working conditions. Very slow dynamic recrystallization kinetics and a relatively high activation energy of hot deformation (Q=549 kJ/mol) were found for the Al 0.3 CoCrFeNi FCC HEA. The main restoration mechanism for this alloy was the formation of new grains through strain-induced migration of the grain boundaries. Increasing the Al content to 0.6 and 0.9 molar ratio changed the microstructure to a duplex one comprising of FCC and BCC phases. A complex restoration behaviour was observed for the FCC phase in the duplex structures consisting of discontinuous dynamic recrystallization at the vicinity of the interphases, and a gradual evolution of substructures resembling continuous dynamic recrystallization within the grains in regions far from interphases. The main softening mechanism within the BCC phase in the duplex microstructures was continuous dynamic recrystallization, characterised by a progressive conversion of low-misoriented subgrains into grains bounded by a mixture of low and high angle grain boundaries. [ABSTRACT FROM AUTHOR]

A detailed understanding of the hot deformation and work hardening behavior of LDX 2101 dual phase steel has been obtained through a wide range of hot compression tests with strain rates from 0.01 to 50 s − 1 and temperatures from 900 to 1250 °C. In most of the cases, the material showed typical dynamic recrystallization (DRX) behavior i.e., a peak followed by a gradual decrease to a steady state stress. The work hardening rate showed a two stage behavior i.e., a transient sharp drop at low stress values followed by a gradual decrease at higher stresses. Using the work hardening rate behavior at the latter stage, the saturation stress was calculated for different hot working conditions. Regression methods were used to develop a hyperbolic-sine equation linking the saturated stress to the deformation conditions. A physically-based Estrin–Mecking (EM) constitutive equation was then employed to model the flow behavior in the work hardening (WH)-dynamic recovery (DRV) regime. Finally, the Avrami equation to describe the evolution of the softening fraction was coupled to the EM model to extend the model to the dynamic recrystallization region. The results show that the model which is based on the stress-strain and work hardening behavior accurately predicts the flow behavior of this microstructurally complex steel. [ABSTRACT FROM AUTHOR]

[Display omitted] • The heterogeneities in the microstructure and mechanical properties of AM Ni-based superalloys are reviewed. • The origins of heterogeneities are linked to the variations in thermal conditions throughout the build. • A short case study is presented. • Strategies to minimize microstructure heterogeneity are discussed. The adaptation of additive manufacturing (AM) for Ni-based superalloys has gained significance in aerospace and power-generation industries due to the ability to fabricate complex, near- net -shape components on-demand and with minimal material waste. Besides its advantages, challenges remain in metal AM, especially for printing complex alloys such as superalloys. These challenges are often linked to heterogeneity in the as-fabricated parts and continue to limit the practical applications of AM products. A thorough understanding of the relationship between the complex AM process and the resulting microstructure heterogeneity needs to be established before mitigation strategies can be developed. The ability to fabricate more homogeneous Ni-based superalloy parts is expected to unlock not only better mechanical properties but also additional fields of applications. This review aims to summarize the current understanding of heterogeneities in the microstructure and mechanical properties of AM Ni-based superalloys. Microstructure heterogeneities discussed include heterogeneity in the chemical composition, phase constitution, porosity, grain and dendrite morphology, and solid-state precipitates. Related heterogeneities in hardness, tensile, creep, fatigue, and residual stress are discussed to represent mechanical properties, and mitigation strategies are summarized. The origins of heterogeneity in the as-fabricated parts are linked to the variations in AM thermal conditions caused by the complex thermal histories. [ABSTRACT FROM AUTHOR]

Fluctuations in energy distribution during additive manufacturing (AM) can result in spatial and temporal thermal transients. These transients can lead to complexities, most significantly when alloys with multi phases are subjected to AM. Here we unveil such complexities in a duplex stainless steel, where we report an unanticipated formation of a Ni-Mn-Si rich phase at grain boundaries and a local fluctuation in Cr and Fe concentrations in regions close to grain boundaries, providing Cr-rich precursors for Cr 2 N formation after laser powder bed fusion (LPBF). The formation of these phases is believed to be due to severe thermal gyrations and thermal stresses associated with LPBF resulting in a high-volume fraction of ferrite supersaturated with N and Ni, and a high density of dislocations accelerating diffusion and phase transformations. [Display omitted] [ABSTRACT FROM AUTHOR]

Duplex stainless steel (DSS) 2205 produced by laser powder bed fusion (LPBF) additive manufacturing was found to possess poor repassivation ability and weak resistance to localised corrosion. This was due to the nonequilibrium phase composition and elemental distribution in the as-built LPBF DSS affected by the rapid solidification inherent to LPBF processing. A short-time post-processing heat treatment at 1000 °C for 10 min followed by water quenching to room temperature was shown to be able to enhance the repassivation ability and localised corrosion resistance by retrieving the equilibrium dual-phase microstructure. A discussion is given on the mechanism behind such behaviour. • LPBF DSS showed a nonequilibrium microstructure consisting of 98% ferrite and 2% austenite with numerous Cr 2 N precipitates. • As-built LPBF DSS exhibited a weak repassivation ability and localised corrosion resistance. • Post-LPBF heat treatment enhanced the repassivation ability and resistance to localised corrosion. • Microstructural changes induced by post-LPBF heat treatment were identified as the reasons for repassivation enhancement. [ABSTRACT FROM AUTHOR]

Metal additive manufacturing (AM) offers exceptional design freedom, but its high thermal gradients often generate non-equilibrium microstructures with chemical and interfacial instabilities. Steels that solidify as δ-ferrite often experience a further solid-state phase transformation to austenite during AM. The detailed nature of this phase transformation during AM is yet to be fully understood. Duplex stainless steel, which is known for its unique combination of high corrosion resistance and mechanical properties, is a suitable alloy to further study this phase transformation. The current study aims to gain novel insights into solid-state phase transformations and mechanical properties of duplex stainless steels during laser powder-bed fusion (LPBF). As-printed microstructures exhibit significant deviations when compared to conventionally manufactured counterparts in terms of phase balance and morphology, elemental partitioning, and interface character distribution. During LPBF, only a small fraction of austenite forms, mostly at the ferrite-ferrite grain boundaries, via a phase transformation accompanied by diffusion of interstitials. Austenite/ferrite boundaries are shown to terminate on {100} F //{111} A planes. This is due to the character of parent ferrite-ferrite boundaries which is dictated by the sharp <100> texture and geometry of austenite grains induced by directional solidification and epitaxial growth of ferrite. Benchmarking mechanical properties against a wrought counterpart demonstrates that AM offers high strength but relatively low ductility and impact toughness. A short heat treatment reverts the microstructure back to its equilibrium state resulting in balanced tensile and toughness properties, comparable to or even better than those of wrought counterparts. [ABSTRACT FROM AUTHOR]

[Display omitted] • 2205 DSS shows similar pitting potentials and passive current densities at RT irrespective of processing route. • Cr is depleted adjacent to intergranular Cr 2 N particles but remains above the critical value for passivation. • As-built LPBF 2205 DSS shows a lower CPT compared to its hot-rolled counterpart. • Post-AM heat treatment enhances the critical pitting temperature to the level of the hot-rolled DSS. The effects of additive manufacturing (AM) and post-AM heat treatment on microstructural characteristics and pitting corrosion of 2205 duplex stainless steel were studied and benchmarked against its conventionally hot-rolled counterpart. The rapid solidification and possible loss of N associated with AM resulted in a non-equilibrium microstructure dominated by δ-ferrite with a minor fraction of austenite and abundant Cr 2 N precipitation. Atom probe tomography revealed that no depletion of Cr occurs around intragranular Cr 2 N. A deduction in Cr was observed adjacent to intergranular Cr 2 N particles, however, Cr content in these regions remained above the critical value of 13 wt%. Post-AM heat treatment was effective in restoring the duplex microstructure while dissolving the Cr 2 N precipitates. Although the pitting resistance in the as-built AM specimen was lower than that of its hot-rolled counterpart, it was fully recovered after post-AM heat treatment. [ABSTRACT FROM AUTHOR]

• Large volume, high resolution method for collecting 3D crystallographic data. • Collection, reconstruction, and analysis methods were developed for Ti alloys. • Several phenomena related to variant selection and interconnectivity are quantified. • A tutorial style description of these techniques is presented. • Raw data and analysis algorithms are made publicly available. 3D electron backscatter diffraction (3D-EBSD) is a method of obtaining 3-dimensional crystallographic data through serial sectioning. The recent advancement of using a Xe+ plasma focused ion beam for sectioning along with a complementary metal-oxide semiconductor based EBSD detector allows for an improvement in the trade-off between volume analyzed and spatial resolution over most other 3D characterization techniques. Recent publications from our team have focused on applying 3D-EBSD to understand microstructural phenomena in Ti-6Al-4V microstructures as a function of electron beam scanning strategies in electron beam powder bed fusion additive manufacturing. The microstructures resulting from this process have fine features, with α laths as small as 1 μm interwoven in a highly complex fashion, presenting a significant challenge to characterize. Over the course of these fundamental works, we have developed best-practice 3D-EBSD collection protocols and advanced methods for 3D data reconstruction and analysis of such microstructures which remain unpublished. These methods may be of interest to the 3D materials characterization community, especially considering the lack of standard commercial software tools. Thus, the current paper elaborates on the methods and analysis used to characterize fine titanium microstructures using 3D-EBSD and presents a detailed description of the new algorithms developed for probing the unique features therein. The new analyses include algorithms for identifying intervariant boundary types, classifying three-variant clusters, assigning grains to variants, and quantifying interconnectivity of branched α platelets. [ABSTRACT FROM AUTHOR]

Metal additive manufacturing (AM) has unlocked unique opportunities for making complex Ni-based superalloy parts with reduced material waste, development costs, and production lead times. Considering the available AM methods, powder bed fusion (PBF) processes, using either laser or electron beams as high energy sources, have the potential to print complex geometries with a high level of microstructural control. PBF is highly suited for the development of next generation components for the defense, aerospace, and automotive industries. A better understanding of the as-built microstructure evolution during PBF of Ni-based superalloys is important to both industry and academia because of its impacts on mechanical, corrosion, and other technological properties, and, because it determines post-processing heat treatment requirements. The primary focus of this review is to outline the individual phase formations and morphologies in Ni-based superalloys, and their correlation to PBF printing parameters. Given the hierarchal nature of the microstructures formed during PBF, detailed descriptions of the evolution of each microstructural constituent are required to enable microstructure control. Ni-based superalloys microstructures commonly include γ, γ′, γ′′, δ , TCP, carbides, nitrides, oxides, and borides, dependent on their composition. A thorough characterization of these phases remains challenging due to the multi-scale microstructural hierarchy alongside with experimental challenges related to imaging secondary phases that are often nanoscale and (semi)-coherent. Hence, a detailed discussion of advanced characterization techniques is the second focus of this review, to enable a more complete understanding of the microstructural evolution in Ni-based superalloys printed using PBF. This is with an expressed goal of directing the research community toward the tools necessary for a thorough investigation of the processing-microstructure-property relationships in PBF Ni-based superalloy parts to enable microstructural engineering. [ABSTRACT FROM AUTHOR]

Electron-beam powder bed fusion (EPBF) has been demonstrated to enable crack-free additive manufacturing of the traditionally non-weldable IN738 Ni-based superalloy. This is related to grain boundary (GB) segregation and precipitation phenomena during EPBF thermal cycling. We investigate such GB microstructures around typical interphase boundaries in IN738. Cyclic reheating results in γʹ dissolution, reprecipitation, and formation of layers enriched in refractory elements at γʹ-γʹ interfaces. Interfacial excess shows that >10 atomic layers of Cr and 3.5 of Co at the GB suppress the segregation of W, B, and C. GBs around heterogeneously nucleated γ grains are decorated with less Cr and Co. This is linked to microsegregation of carbide and boride-forming elements, facilitating diffusion of minor elements during cooling. A heterogeneous interfacial excess profile at a γʹ-γʹ interface is reported. These findings improve the current understanding of interphase boundaries and segregation in EPBF-manufactured IN738, possibly contributing to crack-free additive manufacturing. [Display omitted] [ABSTRACT FROM AUTHOR]

Thermal gyrations inherent to metal additive manufacturing (AM) result in complex microstructural heterogeneities that are linked to the process parameters, geometry, and alloy composition. AM 17–4 precipitation hardened (PH) stainless steel displays heterogeneities due to its multi-phase microstructure where different as-built phases have been reported. These phases include austenite, martensite, and ferrite, as well as varying degrees of AM induced Cu clustering and/or precipitation. To enable a systematic study of the evolution of these strengthening phases, we utilize a non-standard concentric scan strategy to achieve variations in thermal conditions across a build. Using atom probe microscopy and thermal modeling, we show that the response of Cu varies across the build from no clustering at the outer regions to an increased tendency for Cu precipitation towards the center. Our results will enable the tuning of processing parameters to achieve homogenous Cu precipitation, and thus improve mechanical properties of AM 17–4 PH. [Display omitted] [ABSTRACT FROM AUTHOR]

The grain boundary character distribution (GBCD) in an austenitic stainless steel produced by additive manufacturing (AM) in both as-built and annealed conditions was studied. Relatively fine grains and a non-fibre texture was achieved by AM, and as-built structure showed a high population of Σ3 boundaries. A five-parameter GBCD analysis revealed that the microstructure is mostly dominated by highly incoherent Σ3 boundaries. The grain boundary network also consisted of random high angle, coherent Σ3s terminating on (111) planes with a pure twist character, and tilt Σ9 boundaries. The findings show prospects for the possibility of engineering the grain boundary network of materials in-situ , via the stress and heat induced by the thermal cycles during AM. Graphical Abstracts Image, graphical abstract [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) of alloys has attracted much attention in recent years for making geometrically complex engineering parts owing to its unique benefits, such as high flexibility and low waste. The in-service performance of AM parts is dependent on the microstructures and grain boundary networks formed during AM, which are often significantly different from their wrought counterparts. Characteristics such as grain size and morphology, texture, and the detailed grain boundary network are known to control various mechanical and corrosion properties. Advanced understanding on how AM parameters affect the formation of these microstructural characteristics is hence critical for optimising processing parameters to unlock superior properties. In this study, the difficult-to-weld nickel-based superalloy Inconel 738 was fabricated via electron-beam powder bed fusion (EPBF) following linear and random scanning strategies. Random scanning resulted in finer, less elongated, and crystallographically more random grains compared to the linear strategy. However, both scanning strategies achieve unique high grain structure stability up to 1250 ℃ due to the presence of carbides pinning the grain boundaries. Despite significant difference in texture and morphology, majority of grains terminated on {100} habit planes in both linear and random built samples. The results show potential for controlling grain boundary networks during EPBF by tuning scan strategies. • Electron powder bed fusion Inconel 738 builds possess thermally stable grain structures up to 1250 ℃. • Change of scanning strategy from linear to random results in texture randomization and grain refinement. • The unique thermal stability can be due to the pinning of grain boundaries by carbides. • Grain boundaries mostly terminate on {100} habit planes irrespective of the scanning strategy. [ABSTRACT FROM AUTHOR]

Metal additive manufacturing (AM) refers to any process of making 3D metal parts layer-upon-layer via the interaction between a heating source and feeding material from a digital design model. The rapid heating and cooling attributes inherent to such an AM process result in heterogeneous microstructures and the accumulation of internal stresses. Post-processing heat treatment is often needed to modify the microstructure and/or alleviate residual stresses to achieve properties comparable or superior to those of the conventionally manufactured (CM) counterparts. However, the optimal heat treatment conditions remain to be defined for the majority of AM alloys and are becoming another topical issue of AM research due to its industrial importance. Existing heat treatment standards for CM metals and alloys are not specifically designed for AM parts and may differ in many cases depending on the initial microstructures and desired properties for specific applications. The purpose of this paper is to critically review current knowledge and discuss the influence of post-AM heat treatment on microstructure, mechanical properties, and corrosion behavior of the major categories of AM metals including steel, Ni-based superalloys, Al alloys, Ti alloys, and high entropy alloys. This review clarifies significant differences between heat treating AM metals and their CM counterparts. The major sources of differences include microstructural heterogeneity, internal defects, and residual stresses. Understanding the influence of such differences will benefit industry by achieving AM metals with consistent and superior balanced performance compared to as-built AM and CM metals. [ABSTRACT FROM AUTHOR]

316L stainless steel produced by additive manufacturing (AM) offers superior mechanical and corrosion properties compared to parts of the same steel processed by conventional methods. However, thorough understanding of the detailed microstructural evolution during various additive manufacturing techniques is lacking when compared to conventionally produced counterparts. As such, the present work contributes to filling this gap in knowledge by using advanced microscopy techniques to analyse the microstructure of 316L stainless steel builds produced by laser powder bed fusion. The results show that changes in processing parameters lead to variations in grain structures, texture, and hardness. For instance, the dominant texture and grain structure is heavily influenced by changes to the laser scanning rotation between individual layers, while increasing laser power reduces hardness. The current study also provides additional insights into how grains in AM 316L steel accommodate strain. Due to the complex strain fields in austenite grains, they are usually split into complex-shaped regions separated by dislocation boundaries with characters similar to deformation- and micro-band boundaries in conventionally deformed austenite. Through electron probe micro-analysis over a large area, the current study reveals the tendency of Mo and Si concentrations to fluctuate around the cell and melt pool boundaries. • Dominant texture and grain structure is heavily influenced by changes to the laser scanning rotation between layers. • Selected austenite grains split into fragments which cumulatively accommodate significant misorientations. • Some austenite grains consist of dislocation boundaries with systematically oscillating misorientations. • EPMA shows fluctuations of Mo and Si at cell and melt pool boundaries. [ABSTRACT FROM AUTHOR]

• SLM-produced 316L SS has shown markedly high intergranular corrosion (IGC) resistance. • This unusual IGC behaviour was explained by the avoidance of localised Cr depletion. • The IGC was affected by grain boundary character of the SLM-produced 316L SS. The intergranular corrosion (IGC) resistance of 316L stainless steel (316L SS) produced by selective laser melting (SLM) was investigated using microscopy analysis and electrochemical measurements. The IGC resistance of SLM-produced 316L SS, determined using a double-loop electrochemical potentiokinetic reactivation test, was found to be substantially higher than that of conventional 316L SS. This unusual behaviour was explained by the fact that no Cr-rich precipitates were detected for SLM-produced specimens after long-term sensitisation heat-treatment and those SLM-produced specimens exhibited a high frequency of twin boundaries and low-angle grain boundaries along with fine grains, leading to the avoidance of localised Cr depletion. [ABSTRACT FROM AUTHOR]