Formation Mechanism of Dilute Region and Microstructure Evolution in Laser Solid Forming TA15/Ti₂AlNb Dual Alloy

TA15/Ti₂AlNb multiple-layer samples and a dual-alloy sample were fabricated by laser solid forming (LSF) in this study. The formation mechanism of the dilute region and microstructure evolution of the dual alloy were analyzed. The results confirmed a "step" distribution of the composition among several initial layers in the multiple-layer samples, which can be explained by calculating the ratio of the remelted zone to the deposited Ti₂AlNb zone in each deposited layer. However, the "step" compositional distribution disappears, and the compositional variation tends to be more continuous and smooth in the TA15/Ti₂AlNb dual-alloy sample, which is attributed to alloy elements' diffusion at the subsequent multiple re-melting and the longer thermal cycle. The macrostructure of the TA15/Ti₂AlNb dual-alloy sample consists of epitaxially grown columnar prior β grains at the TA15 side and equiaxed grains at the Ti₂AlNb side, while the microstructure shows a transition of α+β→α+α₂+β/B2→α₂+β/B2→α₂+B2+O with increasing amounts of Ti₂AlNb, leading to the microhardness also changing significantly.