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The β→α transformation is a crucial and fundamental phase transformation in titanium and its alloys. This work investigated the effect of elastic compressive stress (ECS) on the microstructure and mechanical property during cooling from β phase region to α + β phase region in Ti-6Al-4V alloy. Results indicated that ECS in this process results in significant refinement and increased aspect ratios of α lamellae (α L) and α colonies (α C). These changes are attributed to the enhanced phase transformation driving force to promote nucleation of α lamellae. The ECS-cooling induced alloy exhibits higher hardness than the stress-free counterpart, mainly resulting from the refined microstructure. This work provides a potential method to tailor the microstructure and improve the mechanical properties of Ti alloys. [Display omitted] • The phenomena of α L refinement are for the first time identified under elastic stress, and the formation mechanism of such phenomena was elucidated. • The elastic compressive stress influences Gibbs free energy are the main reasons for the refinement and variant selection of α lamellae and α colony in Ti-6Al-4V alloys. [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]

The as-Selective Laser Melted (as-SLMed) Ti-6Al-4V generally has a martensitic microstructure. In the present study, martensite decomposition was investigated in various annealed samples by XRD, SEM and TEM. It has been found that annealing at 700 °C or 800 °C for 2 h was not sufficient to obtain a fully decomposed microstructure. The amount of martensite did reduce greatly and volume fraction of β phase increased significantly when the annealing temperature increased from 700 to 800 °C. However, the best combination of mechanical property was not obtained. It is to be noted that annealing at 800 °C for 2 h is widely used for SLMed Ti-6Al-4V as reported in literature. Detailed SEM and TEM analysis revealed the presence of residual martensite containing twins and a high density of dislocations even in the 800 °C/2 h annealed sample. The twin was determined as { 10 1 ¯ 1 } < 10 1 ¯ 2 > type, which are retained twin formed in martensitic transformation during the cooling in SLM. Annealing at 800 °C for 6 h or more provided a fine equilibrium α + β microstructure, which provided a strong and ductile SLMed Ti-6Al-4V. [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]

• Complex intermetallic phases in the LPBF Al-Mn-Sc based alloy has been intensively studied by 3DAP. • Three different phases are identified in fine grain region in terms of their distribution and chemistry. • Phases in coarse grain region are revealed to exhibit distinct chemistry and distribution from those in fine grain region. The recently-developed Al-Mn-Sc based alloys specific for laser powder bed fusion (LPBF) have shown excellent mechanical performance. However, the complicated intermetallic particles in the microstructure remain to be identified, hindering the deep understanding of their effects on mechanical properties and further property improvement. In this work, a range of phases in a LPBF-built Al-Mn-Sc based alloy have been systematically studied by atom probe tomography. The results clarify characteristic intermetallic phases in two different grain-size regions in the microstructure. In the fine grain (FG) region, three distinct phases have been identified. A Sc-rich phase having an average composition of Al 3.3 (Sc 0.8 Zr 0.2) is observed at both the grain boundary (GB) and grain interior, but they have different morphologies. The Mn-rich phase with an average composition of Al 4.3 (Mn 0.9 Fe 0.1) is only observed along GBs. In addition, Mg-rich oxide has been observed either with a separate distribution or attached to Sc-rich particles. In the coarse grain (CG) region, the GB particles exhibit different size and composition from FG region. The Sc-rich GB particles contain Mg enrichment rather than Zr and the composition is determined to be Al 3.4 (Sc 0.75 Mg 0.25). The Mn-rich GB particles in the CG region, with higher Fe contents, are Al 4.3 (Mn 0.8 Fe 0.2) along the GBs and Al 4.5 (Mn 0.85 Fe 0.15) inside the grains. Many smaller Mg-rich oxides and Sc-rich particles are also observed in the internal grains of the CG region. [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]

The hemming process is one of the most commonly used forming processes for aluminium alloys in the automotive industry and requires bending the aluminium alloy sheet to 180° without generating cracks or surface cosmetic defects. However, existing commercial Al–Mg–Si alloys (AA6XXX aluminium alloys) used in the automotive industry could easily fail during the hemming process. Textures in sheet aluminium alloys are commonly known as influential in the deformation behaviour and surface defect formation, while a comprehensive correlation of textures and hemming performance of aluminium alloy sheets is not well established. In this work, the influence of different texture components on the hemming behaviour of an AA6XXX aluminium alloy sheet was thoroughly investigated. The hemming testing was carried out using samples taken along rolling and transverse directions and a quantitative approach via profilometer was used to measure the hemming performance. The detailed grain orientation spread analysis, Taylor factor and Schmid factor analysis with different texture components from EBSD characterizations show that the dominance of Cube and Cube ND texture component contributes to the hemming performance. Cube RD and Goss texture components could only benefit the hemming performance at one of the bending directions. In contrast, Brass and Goss texture components in Bent_RD and P component in Bent_TD may not be preferred for the hemming performance. Image 1 • Good hemming performance with the nominal factor of 0.5 was achieved in an AA6XXX aluminium alloy. • Surface profile measurements by a profilometer provided more quantitative characterization of the hemming performance. • Cube and Cube ND texture components are the most preferable for hemming performance. • Cube RD and Goss texture components benefit the hemming with loading normal to the transverse direction. [ABSTRACT FROM AUTHOR]