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Oxide dispersion strengthened (ODS) nickel (Ni)‐based superalloys are advanced materials known for their outstanding tensile and creep performance at temperatures exceeding 1000 °C. Nevertheless, their conventional synthesis presents a longstanding challenge in cost‐effectively producing intricate components for critical applications. In this work, electrostatic self‐assembly (ESA) of powders with the laser powder bed fusion (LPBF) process are successfully combined to produce yttria ODS Inconel 718 (IN718) alloy for the first time. This approach demonstrates a significant contribution of yttria to the strength of IN718 after the solid solution heat treatment, as evidenced by ≈50% improvement in room temperature yield strength with a 0.5 wt.% yttria addition. The addition of yttria by this process leads to a heterogeneous microstructure. This heterogeneous microstructure comprises two distinct grain areas with varying amounts of yttria nanoparticles and dislocation storage. The results show that the yield strength increase can be predicted by the combination of both Y2O3 dispersion strengthening and dislocation strengthening mechanisms. These findings offer an effective approach to tailor heterogeneous microstructures, unlocking new opportunities for cost‐effectively producing high‐performance ODS Ni‐based superalloy products with excellent mechanical properties. [ABSTRACT FROM AUTHOR]

A Laser Powder Bed Fusion Inconel 718 produced with intentionally non‐optimized parameters is investigated to understand the role of defects and microstructure on the high‐cycle fatigue life. Samples from three built directions and three heat treatments were used for fatigue tests at room and 650°C under R=0.1. Printing orientation has a minor effect on fatigue life at both temperatures. Most samples failed from lack of fusion defects, carefully analyzed by scanning electron microscope (SEM) observations. Surface defect leads to transgranular fracture surface, while subsurface initiation leads to crystallographic fracture surface. S−N curves related to hot isostatic pressure condition shown that defects have higher influence at 20°C than at 650°C. Kitagawa diagrams are plotted for both temperatures to quantify the influence of defect size on fatigue limit. Natural cracks are observed at the surface in order to discuss the relative part of initiation life in the whole fatigue life. Highlights: Fatigue is governed by lack of fusion (LoF) ranging from 200 to 600 μmThe fatigue limit is higher at high temperature compared with ambient temperatureHot isostatic pressure (HIP) treatment improves the fatigue resistanceNatural crack initiated from LoF propagates in a very similar way compared with long crack [ABSTRACT FROM AUTHOR]

In this work, the effects of post-LPBF (laser powder bed fusion) heat treatments on microstructure evolution, tensile and Charpy properties of Ti-6Al-4V have been investigated. The results show that the original martensitic microstructure in the LPBF sample has converted to a lamellae mixture of α and β phases when post heat-treatment temperatures are below the β-transus (800°C, 850°C and 900°C) with air cooling. On this basis, a good balance of tensile strength and ductility was obtained from sample post heated at 800°C for 6 h followed by air-cooling, with an increase of ~42% in ductility and a significant toughness enhancement (almost 100% increase) compared to the as-built sample. The relationship betweenα lamellaethickness, β volume fraction and post-LPBF heat treatment has been established. Both α lamellae thickness and β volume fraction increase with the heat-treatment temperature. [ABSTRACT FROM AUTHOR]

• Defects control in SLMed A357 is achieved with optimised processing parameters. • Microstructure homogenisation with Si particle modification is achieved with optimised heat treatment procedures. • Tensile ductility is significantly improved without the presence of large defects. • Fatigue property is substantially improved with defect elimination and Si particle modification. The relationship between defects and mechanical properties, especially fatigue properties, of selective laser melted (SLMed) Al-7Si-0.6Mg alloy built with different processing parameters has been investigated. With relatively high densities, although the tensile strength is not affected, ductility and fatigue properties vitally vary with different defects. Furthermore, an excellent fatigue performance with the run-out stress of 200 MPa has been achieved with optimised processing parameters, which is by far the best fatigue performance in SLMed Al-Si alloys. Microstructure and fractography are characterised by different methods, demonstrating that defects and Si particles are the main controlling factors for the fatigue properties. [ABSTRACT FROM AUTHOR]

Co-based and Ni-based superalloys have been widely used in aerospace, with much attention given to additive manufactured Ni-based superalloys. However, the equally important Co-based superalloys have received less attention. In this work, a Co-based Haynes 188 superalloy is fabricated using Laser Powder Bed Fusion (LPBF). This research shows the impressive mechanical properties of Haynes 188 superalloy before and after heat treatment at both room and high temperatures for the first time. In the as-built specimen, arrayed stacking faults, Lomer–Cottrell locks, dislocations, and M 6 C/M 23 C 6 eutectic carbides are found. The yield strength, ultimate tensile strength, and elongation of the as-built specimen are 642 MPa, 928 MPa, and 57.3 %, respectively. For the heat-treated specimen, the grain size increases, dislocation density reduces and the eutectic carbides dissolve into the matrix, which reduces the yield strength to 488 MPa. However, annealing twins are found in the heat-treated specimen, and the twin-twin interaction networks further enhance the pile-up of dislocations and work hardening ability during the deformation, which leads to an increase in the ultimate tensile strength and elongation to 1002 MPa and 68.1 %, respectively. This work provides an effective method to improve the mechanical properties of Co-based superalloys by additive manufacturing, which is essential for the development of high-performance superalloys and their manufacturing methods. [ABSTRACT FROM AUTHOR]

• The influence of the GB-α on the fatigue behavior of LPBF Ti–64 was investigated. • HCF property is improved due to low aspect ratio α lath and discontinuous GB-α. • Discontinuous GB-α could improve the crack propagation resistance. • α lath with a low aspect ratio inhibits the crack initiation at the interface. High cycle fatigue (HCF) performance of additive manufactured (AM) titanium products is of great significance for its structural and functional applications. However, tying fatigue performance to the complex microstructures in AM titanium alloys is challenging. The work here carried out a thorough investigation into the influence of microstructures (particularly the grain boundary α-phase (GB-α) with varied morphologies) on the fatigue performance of laser powder bed fusion (LPBF) Ti–6Al–4V (Ti-64). Results showed that the improvement in the high-cycle fatigue life of LPBF Ti-64 could be achieved by the formation of low aspect ratio α lath and discontinuous GB-α via optimized post-fabrication heat treatment. Discontinuous GB-α could fully accommodate the deformation, improving the fatigue crack propagation resistance. Moreover, α lath with a low aspect ratio could lead to less strain accumulation on the interface between adjacent α lath, and thereby inhibit the crack initiation at these interfaces. This study enhances the understanding of how LPBF-induced complex microstructures influence fatigue behavior, and provides a pathway for the improvement of fatigue performance of additive manufactured titanium alloy. [ABSTRACT FROM AUTHOR]

Additive manufacturing, or 3D printing, has brought tremendous opportunities for the design and making of metallic components with high geometrical complexity. However, in order to maximize the performance and functionality of a specific part, the additive manufacturing industry currently still faces fundamental issues of material processability and limited mechanical properties. Here, we report a high strength in-process and post-process friendly Al alloy specifically developed for the selective laser melting (SLM) process, one of the most commonly used additive manufacturing techniques. We found that the introduction of Mn and Sc as major strengthening elements significantly improved the processability of the Al alloy and its corresponding mechanical properties due to the rapid solidification nature of the SLM process. The developed alloy demonstrates an exceptional high thermal stability, which enables the utilization of a very simple post heat treatment to relieve the residual stresses generated during the SLM process, maintain a high solid solution strengthening effect from Mn and simultaneously achieve exceptional precipitation strengthening from a high density of nano-sized Al 3 Sc precipitates. We created a new high strength Al alloy with a yield strength of up to 560 MPa and a ductility of about 18% after a simple industrially desirable post heat treatment of 5 h at 300 °C. The highly enhanced alloy properties, good processability and simple post heat treatment of the developed alloy offers tremendous benefits for the fabrication of complex high performance lightweight engineering components made by SLM. Image 1 [ABSTRACT FROM AUTHOR]

The sliding behavior of two titanium alloys processed by laser powder bed fusion, with and without heat treatment, was investigated alongside two as-received conventionally-processed versions of these alloys. These samples presented a wide range of microstructures often encountered in Ti alloys. Oxidative, abrasive and adhesive wear always occurred when sliding against steel. However, wear rates were similar regardless of the friction and hardness if material removal mainly occurred through abrasion and/or adhesion. Martensitic phases decomposed during sliding while nano-crystalline grains enhanced oxidative wear which reduced friction. With a stable nanocrystalline microstructure that confers ultrahigh hardness, wear rate and friction can be substantially reduced by promoting oxidative over abrasive or adhesive wear. • Dry sliding of two conventionally- and LPBF-processed Ti alloys was investigated. • A nano-crystalline microstructure enhances oxide formation which reduces friction. • α′ or α″ is unstable in the presence of frictional heat and oxygen diffusion. • A stable ultrahard nanocrystalline microstructure can reduce both wear and friction. • This microstructure promotes oxidative wear and minimizes adhesion/abrasion. [ABSTRACT FROM AUTHOR]

[Display omitted] • Powder-fed laser-based directed energy deposition of Beta-C was successful. • Due to a high dislocation density, the Beta-C alloy had a fast-ageing response. • Higher strength was achieved compared to conventionally-processed aged Beta-C. • The ageing response was further accelerated by carbon additions. • With added carbon, the alloy was harder, less ductile but strength was unchanged. This work investigates the viability of fabricating Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-CTM) using laser-based and powder-fed directed energy deposition. To determine an appropriate ageing strategy, heat treatments were conducted at 440 °C and 480 °C over different periods of time and their effectiveness were determined through hardness testing. After identifying an optimal ageing treatment, tensile properties were investigated on the samples in the as-built and aged conditions. The microstructure, hardness and tensile properties of the as-built sample was found to resemble that of a conventionally-manufactured and solution-treated alloy. However, as compared to conventionally-manufactured Ti-3Al-8V-6Cr-4Mo-4Zr subjected to the same ageing temperature, the ageing process was much quicker with the DED-processed alloy because of its high dislocation density. Furthermore, the latter can also achieve much higher strength while also having comparable ductility. To achieve a similar ageing response and tensile behavior, conventionally-manufactured Ti-3Al-8V-6Cr-4Mo-4Zr would require cold-working prior to ageing. Carbon additions were also investigated to further accelerate the ageing response and to refine the β grain size. While the ageing response was much quicker and a much harder alloy can be produced, the carbon amount added was insufficient for β grain refinement, which resulted in reduced ductility without a change in strength. [ABSTRACT FROM AUTHOR]

Heat treatment (HT) is an effective way to attain the desired microstructure and mechanical properties in laser powder bed fusion (LPBF) Ti–6.5Al–2Zr–1Mo–1V (TA15). However, previous studies only obtained the high tensile ductility of LPBF TA15 but were accompanied by a large loss of strength, which was unable to achieve balanced mechanical properties. Here, we evaluated the tensile properties of LPBF TA15 at their usage temperatures (500 °C), and a superior combination of high-temperature strength and ductility in LPBF TA15 was successfully achieved via a single-step heat treatment. After the heat treatment close to the β transus temperature (950 °C as used in this study), the tensile strength of the sample was ∼30% higher than that of as-cast TA15, accompanied by the obvious enhancement of ∼67% in elongation. The high strength originated mainly from the secondary α-phase and discontinuous grain boundary α-phase (GB-α), whereas the large ductility correlates to the complete decomposition of brittle α' martensite. Discontinuous GB-α could inhibit crack initiation at the grain boundary and the fine secondary α-phase impeded the dislocation motion, which could enhance the tensile strength. Meanwhile, the α lath directions after heat treatments were far away from the 45° of the load direction, which also avoided the localized deformation within α laths and thus improved the fracture resistance of LPBF TA15. [ABSTRACT FROM AUTHOR]

Numerous studies have been conducted in the recent past to assess different nickel-based superalloys fabricated using laser-based powder bed fusion (PBF-LB). Most of these works focused on the processability, microstructural evolution, post-fabrication treatments and the mechanical behaviour of components manufactured by PBF-LB under static loading. However, limited efforts have been made to investigate the fatigue crack growth (FCG) characteristics of PBF-LB built alloys, thereby imposing a barrier to their use as critical components. As such, the aim of the present study was to evaluate and understand the effect of build orientations (0° and 90°) and post-fabrication treatments (solution heat treatment and hot isostatic pressing) on the FCG behaviour of Hastelloy X (HX) fabricated by PBF-LB process. The results from the FCG tests showed that the build orientations had no discernible influence on the fatigue threshold and FCG rate within near-threshold and Paris regions. Nevertheless, the post-fabrication treatments led to an increase in the fatigue threshold and crack growth resistance within the near-threshold region because of the alleviation of detrimental tensile residual stress alongside the crack growth direction. In conjunction with FCG tests, scanning electron microscopy and electron backscattering diffraction were used to investigate the crack path and fracture surfaces within near-threshold and Paris regions. • FCG tests were performed on as-built, SHT and HIP compact tension specimens with crack growth parallel and perpendicular to build direction. • The effect of anisotropic microstructure and residual stress on fatigue crack threshold and fatigue crack growth rate is investigated. • Transgranular mode of crack propagation was observed in near-threshold and Paris regions of crack growth in as-built and post-treated PBF-LB HX. [ABSTRACT FROM AUTHOR]

[Display omitted] • There is a demand for more highly-printable high-strength aluminium alloys for industrial laser powder bed fusion. • Research efforts are making progress in finding ways to combine high strength with good processability. • A growing number of industrially available alloys can now achieve a yield strength of >400 MPa. Laser powder bed fusion (LPBF) is one of the major additive manufacturing techniques that industries have adopted to produce complex metal components. The scientific and industrial literature from the past few years reveals that there is a growing demand for the development of high-strength aluminium alloys for LPBF. However, some major challenges remain for high-strength aluminium alloys, especially in relation to printability and the control of defects. Possible strategies that have been identified to achieve high strength with printability include the adaptation of existing high-strength cast and wrought alloys to LPBF, the design of new alloys specifically for LPBF, and the development of aluminium-based composites to achieve unique combinations of properties and processability. Whilst review papers exist for aluminium alloys in general for the related work up to 2019, the purpose of this paper is to review the latest developments related to high-strength aluminium alloys for LPBF up to early 2022, including alloy and process design strategies to achieve high strength without cracking. It aims to provide fresh insights into the current state-of-the-art based on a review of extensive yield strength data for a wide spectrum of aluminium alloys and tempers that have been studied and/or commercialised for LPBF. [ABSTRACT FROM AUTHOR]

In this study, the tribological properties of an AlSi10Mg alloy that was fabricated using laser powder bed fusion, an additive manufacturing technique, was investigated in both the as-built (but stress-relieved) and T6-treated condition. The stress-relieved condition consists of elongated Al grains and a dendritically-arranged nano-crystalline Si-phase. T6 treatment, a conventional heat treatment for cast AlSi10Mg alloys, caused this Si phase to dissolve, re-precipitate and coarsen, which resulted in a loss of hardness, toughness and strength. Dry sliding wear tests were conducted against both a bearing stainless steel and a much harder SiC, and with the sliding direction either perpendicular or parallel to the building direction. In almost all conditions it was found that the T6 treatment either had no statistically significant effect on the wear rate or was detrimental. • In the stress-relieved condition, the wear performance is isotropic. • Wear anisotropy occurred in the T6 condition. • The wear rate against steel is more severe than against SiC. • Frictional response is not necessarily indicative of wear performance. • T6 treatment is generally not recommended for sliding wear performance. [ABSTRACT FROM AUTHOR]

A series of complex post-build heat treatment procedures are usually inevitable for selective laser melted (SLMed) Al-Si-Mg alloys, aiming to remove significant process-induced residual strains. In this work, we propose a new method to avoid the post-build heat treatment without compromising the mechanical properties in SLMed Al alloy A357. By heating up the build platform (up to 90 °C) during SLM fabrication or applying post heat treatment after SLM, the evolution of microstructure and mechanical properties for Al-7Si-0.6Mg alloy are compared at different conditions. Characterisations using scanning electron microscopy (SEM), X-ray Diffraction (XRD), transmission electron microscopy (TEM), and hardness tests suggest heating up the build platform is more effective in alleviating residual strain and resulting in uniform precipitates in the Al matrix. Tensile properties imply that SLMed A357 fabricated with a heated substrate possesses improved mechanical properties compared to SLMed A357 build on room temperature platforms and even with post-build heat treatment. Unlabelled Image • Heating build platform during selective laser melting avoids further heat treatment. • The new method results in better mechanical properties than its counterpart. • Residual strain after selective laser melting results in super solid solubility of Si. • Precipitation of Si due to heating build platform relieves residual strain. [ABSTRACT FROM AUTHOR]