In-situ development of a sandwich microstructure with enhanced ductility by laser reheating of a laser melted titanium alloy

Metallic additive manufacturing, particularly selective laser melting (SLM), usually involves rapid heating and cooling and steep thermal gradients within melt pools, making it extremely difficult to achieve effective control over microstructure. In this study, we propose a new in-situ approach which involves laser reheating/re-melting of SLM-processed layers to engineer metallic materials. The approach involves alternate laser melting of a powder layer at a high laser power and laser reheating of the newly formed solidified layer at a low or medium laser power. This strategy was applied to Ti-6Al-4V with a range of laser powers being used to reheat/re-melt solidified layers. It was found that the SLM-processed sample without undergoing laser reheating consist of a pure martensitic needle structure whereas those that were subjected to laser reheating/re-melting all consist of horizontal (alpha + beta) bands embedded in martensitic alpha' matrix, leading to development of a sandwich microstructure in these samples. Within the (alpha + beta) bands, beta exist as nano-sized precipitates or laths and have a Burgers orientation relationship with a matrix, i.e., {0001}alpha//{110}beta and < 11 (2) over bar0 >alpha//< 111 >beta. The width of (alpha + beta) banded structure increased first with increased laser power to a highest value and then decreased with further increased laser power. With the presence of these banded structures, both high strengths and enhanced ductility have been achieved in the SLM-processed samples. The current findings pave the way for the novel laser reheating approach for in-situ microstructural engineering and control during metallic additive manufacturing.