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In this study, the influence that porosities play on the mechanical properties of Ti-6Al-4V parts fabricated by laser powder bed fusion (LPBF) was investigated with the aim of improving process efficiency of the LPBF process by establishing the resulting trade-offs to component quality. It was found that build times can be substantially reduced by using higher layer thickness parameters without a significant detriment to mechanical properties. Where porosities formed, spherical pores tend to have a marginal effect on Young's modulus, yield strength, tensile strength, and uniform elongation as these properties can be approximated linearly using the rule of mixtures assuming null strength pores. However, such pores were found to be otherwise more detrimental to total elongation and toughness. Conversely, lack of fusion pores are extremely detrimental and should be avoided. Thus, it is proposed that when building a component, different process parameters can be used at different regions of the component to maximize process efficiency without compromising its overall function. For instance, for surfaces, a highly optimized parameter set with lower layer thickness can be used. At regions with minimal stress distribution during service, a large layer thickness parameter set where the resulting mechanical properties is sufficient can be used. [ABSTRACT FROM AUTHOR]

For SLMed Ti–6Al–4V- x O (x = 0.11, 0.16, 0.21, and 0.25 wt%) alloys, both microstructure and mechanical properties are sensitive to oxygen content. The α′ martensites in as-built state transform to a 'bimodal' microstructure containing α lamellae interspersed with β transformed regions after 2 h annealing at 800 °C, 850 °C, 900 °C, and 950 °C followed by air cooling. The α lamellar width decreases and the β phase volume fraction increases with increasing oxygen content, due to reduced diffusivity of Al and V. The solid solution hardening effect of oxygen was quantified as: σ 0 = 472 MPa + 2159 c 0.5 , where σ 0 is the lattice frictional stress and c is oxygen atomic fraction of the alloy. Increased oxygen content causes reduced fracture toughness and higher fatigue crack growth rate in the unstable growth regime, and the appearance of bilinear growth rate of the Paris regime at high annealing temperatures. The absence of the bilinear behaviour in the low oxygen content alloy and for annealing at low temperatures was interpreted in terms of changes in microstructural characteristics caused by oxygen impeding the diffusion of Al and V. [ABSTRACT FROM AUTHOR]

Coaxial Electron Beam Wire-feed Additive Manufacturing (CAEBWAM), which is a novel additive manufacturing process, can produce fully dense alloy components with equiaxed β grains (EGβ) and isotropical mechanical properties and is considered as a potential manufacturing method for large-scale complex components. However, actual complex components inevitably include bonding regions, which are similar to welded joints, in the wire-feed deposition process, and limited knowledge is available on the microstructures and mechanical properties for this kind of regions. This work thoroughly studied the microstructures and mechanical properties of the Ti-6Al-4V alloy cruciform structure fabricated by CAEBWAM. It was found that the microstructure at the Bonding Zone (BZ) was composed of coarse columnar β grains (CGβ), continuous grain boundary α (α GB) phase, and coarse α laths due to a higher temperature and poor heat dissipation condition. The average width of α lath in the BZ region was larger and the hardness was lower compared with those in the EGβ region. Tensile properties and fracture behaviours of the tensile samples extracted at cruciform structure were examined. The fracture tended to occur at the BZ region and exhibited a mixed fracture mode with trans- and inter-granular fractures. This work will improve the understanding of microstructures and mechanical properties for representative cruciform structure in wire-feed AMed components, which would be conducive to further progress of the actual complex components. [Display omitted] • The microstructure and mechanical properties of the Ti-6Al-4V cruciform structure were investigated. • The formation of inhomogeneous microstructure and the tensile behaviour were elucidated. • Coarse columnar β grains, continuous grain boundary α phase, and coarser α-lath were formed in the Bonding Zone (BZ). • The fracture tends to occur at the BZ region and exhibits a mixed fracture mode with trans- and inter-granular fractures. [ABSTRACT FROM AUTHOR]

An Al-Mn-Mg-Sc-Zr alloy was additively manufactured and subsequently deformed to investigate the effect of high defect densities on the precipitation behavior, work hardening capability and ductility. For this, the LPBF-fabricated alloy was deformed by rotary swaging up to a true strain of 2.5 following a laser powder bed fusion (LPBF) process. Compared to the LPBF condition, swaging results in a refinement of the microstructure by one order of magnitude and an increased hardness and ultimate tensile strength (UTS) which is mainly attributed to the finer microstructure of the swaged alloy. By annealing, a higher peak-aging hardness of (209 ± 2) HV0.1 and UTS of (717 ± 2) MPa of the swaged alloy at a lower peak-aging temperature of 300 °C (1 h) was obtained. Significant improvement of uniform elongation by enhanced work hardening capability of the swaged and annealed alloy is obtained for annealing temperatures above 300 °C while strength is only moderately affected. The significant improvement of aging kinetics is discussed alongside a profound microstructural characterization of the heterogeneous grain structure and precipitate distribution. • Work hardening of high strength Al alloys is improved by deformation prior ageing. • Enhanced uniform ductility is obtained. • UTS of (612 ± 9) MPa at a uniform strain of (1.4 ± 0.2) % is found. • Heterogeneous microstructure by additive manufacturing resists cold work. [ABSTRACT FROM AUTHOR]

Additive manufacturing of 25Cr super duplex stainless steel (SDSS) has been successfully demonstrated using laser powder bed fusion (L-PBF) in recent years. The as-built L-PBF 25Cr SDSS exhibits a microstructure that is close to 100% ferritic (due to the fast cooling in L-PBF) and requires a high temperature thermal treatment to obtain the desired duplex structure. Directed laser deposition (DLD), as a variant of the additive manufacturing methods, is characterized by a much slower cooling rate than L-PBF and is potentially beneficial to achieve the duplex phase fraction in the as-deposited condition. DLD of 25Cr SDSS is new, the microstructure development and the mechanical properties remain unclear. The microstructure and mechanical properties of additively manufactured 25Cr SDSS produced by metal powder based DLD are reported in this work. The as-deposited microstructure revealed a balanced duplex structure, with a near 50/50 ferrite-austenite phase ratio, differences to the previously reported microstructure of SDSS produced by L-PBF are discussed. The mechanical properties of DLD 25Cr SDSS were investigated using uniaxial tensile testing and compared with the commercial wrought alloy. DLD 25Cr SDSS showed superior tensile strength to the commercial wrought alloy, the origin of the strength and strain hardening is discussed, and the flow strength for DLD and wrought 25Cr SDSS are described using a simple rule of mixtures model. The modelling supports the notion that the main difference between the mechanical response of the DLD 25Cr SDSS and the commercial alloy arises from the finer scale of the phases in the DLD SDSS. The findings demonstrate the feasibility of additive manufacturing using DLD as applied to 25Cr SDSS. • Ferrite/austenite duplex structure is formed in-situ using directed laser deposition. • Dislocations and oxide particles are beneficial to the austenite nucleation. • The high tensile strength is mainly contributed by dislocations and the fine austenite phase. • The strain hardening behaviour is understood by a flow strength modelling based on the dislocation storage. [ABSTRACT FROM AUTHOR]

Understanding the microstructure and mechanical properties associated with different metal additive manufacturing techniques is crucial for its wide-scale acceptance in industries. In this work, we aim to understand the microstructural variations and mechanical properties of Hastelloy X (HX) manufactured by electron beam powder bed fusion (PBF-EB) and laser-based powder bed fusion (PBF-LB) process. The size and shape of melt pool and grains were analysed and correlated to final microstructure. The elevated powder bed temperature during PBF-EB was found to promote the formation of grain-boundary precipitates. The higher thermal gradient and cooling rate in PBF-LB resulted in higher tensile residual stress (σ max = 447 ± 10 MPa) within the parts in comparison to PBF-EB parts (σ max = 16 ± 13 MPa). The as-fabricated PBF-EB HX specimens showed a lower tensile strength of 590 MPa but a higher elongation of 60%, whereas its PBF-LB counterparts demonstrated a significantly higher tensile strength of 825 MPa but a lower elongation of 38%. This notable difference in the mechanical behaviour of PBF-EB and PBF-LB built HX was attributed to the columnar microstructure,<100> crystallographic texture and underlying strengthening mechanisms. Furthermore, the mechanical properties of PBF-EB and PBF-LB built HX specimens were predicted using multiple strengthening mechanisms, which demonstrated a good agreement with that of experimentally measured. ga1 • Microstructure and mechanical properties of PBF-EB HX were investigated for first time and compared with PBF-LB HX. • Significant differences in the microstructure and crystallographic texture was observed in PBF-EB and PBF-LB built HX. • Difference in powder bed temperature had a notable effect on precipitation and residual stress with PBF-EB and PBF-LB HX. • The mechanical properties of PBF-EB and PBF-LB built HX were predicted using multiple strengthening mechanisms. [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]