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.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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•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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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•2205 DSS shows similar pitting potentials and passive current densities at RT irrespective of processing route.•Cr is depleted adjacent to intergranular Cr2N 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 Cr2N precipitation. Atom probe tomography revealed that no depletion of Cr occurs around intragranular Cr2N. A deduction in Cr was observed adjacent to intergranular Cr2N 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 Cr2N 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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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
′ (L1
2
-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.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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 AlxCoCrFeNi 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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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 ...50s−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.
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•Hot deformation and work hardening behavior of LDX 2101 was studied•Saturated stresses were measured using hyperbolic-sine equation•Work hardening-dynamic recovery regime was modelled by Estrin–Mecking equation•Avrami equation was coupled to the Estrin–Mecking to model the DRX region
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Single phase Al0.3CoCrFeNi (Al0.3) 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 AlNi rich precipitates formed during annealing. The activation energy of static recrystallization (549 kJ/mol) for the as-cast Al0.3 HEA was significantly higher than that of conventional alloys due to the distorted lattice, sluggish diffusion and most importantly the interaction of AlNi rich precipitates with the recrystallization. Homogenization heat treatment on Al0.3 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 Al0.3 HEA mainly due to the pinning effect of AlNi 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 Al0.3 HEA.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
AlxCoCrFeNi 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 Al0.3CoCrFeNi 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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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