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Abstract Electron beam melting (EBM) has been applied to fabricate a high Nb-TiAl alloy with a fully dense microstructure and good tensile properties at both room and high temperatures. The effects of preheating and melting parameters on melting, solidification, phase transformation and resulting microstructure formation in as-EBM high Nb-TiAl alloy were investigated by performing a design-of-experiments. Results show that the limited EBM processing window can be broadened to produce different characteristic microstructures ranging from nearly fully lamellar γ/α 2 to equiaxed γ grains. Such a broadened processing window has been achieved by using stronger preheating beam current. A numerical simulation was performed to understand temperature evolution at a fixed point of interest where electron beam passed several times with a certain line offset within one build layer. Both the preheating and melting stages were considered in the model. Modelling results show that a higher preheating beam current resulted in a longer hold time within the temperature range between 1300 and 1380 °C (i.e. single α-phase region). This helped to produce fine lamellar microstructure in the high Nb-TiAl alloy. Fundamental principles are thus proposed in terms of controlling microstructure formation and fabricating fully dense high Nb-TiAl alloy in as-EBM condition. Graphical abstract Unlabelled Image Highlights • A fundamental principle is proposed to control the microstructure of Nb-TiAl alloy in EBM with competitive tensile properties. [ABSTRACT FROM AUTHOR]

In the present work, we report a novel bimodal and multi-layered grain structure in pure titanium produced via powder metallurgy. It was found that the hot-extruded pure Ti consists of multiple grain layers, which exhibited substantially different mean grain sizes. The microstructural development during hot extrusion was then investigated for the pure Ti via an interrupted extrusion experiment. The influence of this unique structure on the mechanical properties of the material was also studied under uniaxial quasi-static tension. The experimental results showed that the samples with different arrangement of the grain layers exhibited very different mechanical behavior. Namely, with combining a small part of fine grain layers, the material showed significantly increased yield strength and slightly decreased uniform plastic strain. Yet, the elongation-to-failure was decreased markedly for the multilayered material. Postmortem examinations indicated that this may attribute to the absence of deformation twins in the fine grains that leads to formation of microvoids, which finally develop into large cracks. [ABSTRACT FROM AUTHOR]

Abstract The influences of EBM processing and post-processing on microstructure, mechanical properties and cracking behaviour in a γ′ precipitation-strengthened nickel-base superalloy DZ125 are critically assessed. Results show that a solution treating and ageing (STA) is required to obtain γ′ precipitates with a cuboidal shape. The columnar grain width was found to gradually increase from the bottom to the top of the as-EBM sample having a total build height of 80 mm, whereas there was little gradient in γ′ size. The presence of EBM induced intergranular cracks can be closed by hot isostatic pressing (HIP), however reappearance of intergranular cracks with a much wider crack opening width was observed after STA treatment. EBM induced cracks are classified as liquid-state cracking, as the classic dendritic morphology were found on the cracked surfaces. The post-processing induced cracks can be attributed to the effect of γ′ dissolution and re-precipitation of fine γ′ during the cooling stage of solution treatment. The results suggest that HIP is not an effective approach in healing liquid-state cracks in EBM fabricated γ′ precipitation-strengthened superalloys. Graphical abstract Unlabelled Image Highlights • γ′ precipitation-strengthened Ni-base superalloys are gaining increasing attention for electron beam melting (EBM). • This work represents the first study to reveal the crack reappearance in hot-isostatic-pressing (HIP) post-processed EBM Ni-base superalloy. • EBM induced cracks are classified as liquid-state cracking. • The reappearance of such cracks after post-processing suggests that HIP is not an effective approach in healing liquid-state cracks. [ABSTRACT FROM AUTHOR]