Multi-scale microstructural development and mechanical properties of a selectively laser melted beta titanium alloy

A beta titanium alloy, Ti-10V-2Fe-3Al, was selectively laser melted under a modulated pulsed laser mode with different processing conditions. The as-fabricated samples were examined using a range of characterization techniques and properties evaluated through tensile testing. It is shown that with a small powder layer thickness (30 mu m), a low laser power and a short exposure time (i.e., low energy density) led to development of fine beta columnar grains and widespread cell structures whereas increased laser power and exposure time (i.e., high energy density) resulted in pronounced grain growth, increased texture and significantly decreased cell structures. Increasing powder layer thickness effectively promoted the columnar-to-equiaxed grain transition (CET), leading to a greatly reduced texture and a hybrid microstructure which consists of small and chunky equiaxed grains together with a small number of large columnar grains. Athermal omega precipitates were observed in all the as-fabricated samples. In the samples made with high energy densities, alpha laths which tend to constitute a grid-like structure were observed. The samples with the finest columnar grains show both high strengths and good ductility thanks to full plastic deformation through both slipping and twinning. The samples with the hybrid grain structure, however, exhibits a highly limited or no ductility due to intergranular fracturing. The alpha-containing samples which also have coarse grains all failed in a cleavage fracture mode and exhibited almost no ductility. Transmission electron microscopy study reveals that the alpha-demarcated grid structure tended to confine plastic deformation within the beta matrix and suppress the macroscopic plastic deformation throughout the samples.