Hybrid laser powder bed fusion and hot isostatic pressing of Ti-6Al-4V parts without degassing for process efficiency

Abstract

Despite gaining widespread adoption, the slow production speed of Laser Powder Bed Fusion (LPBF) remains a challenge. To address this issue, some studies have proposed a hybrid approach where the outer region (“shell”) of a part is processed by LPBF and powder-filled internal region (“cavity”) is consolidated via hot isostatic pressing (HIP). Typically, the cavity is designed to be externally accessible to degas trapped Ar gas. This, however, restricts design freedom and adds an additional processing step. Therefore, this study investigates the viability of using HIP to consolidate non-accessible powder-filled cavities in an LPBF Ti-6Al-4V part through microstructures analysis and tensile behavior. For comparison, samples were also fabricated via conventional HIP using the same powder. When HIP was performed at 850 °C or 930 °C, globularisation of the α phase occurred in the powder particles resulting in an equiaxed microstructure as opposed to the lamellar microstructure found in the shell. At 1010 °C, however, rapid grain growth in the β phase field resulted in a similar coarsened microstructure across the entire material upon cooling. Despite trapped Ar in the cavities, only ~0.5 % porosity remained regardless of the HIP temperature. Tensile failure always occurred within the cavity region which highlights the excellent bond across the shell/cavity interface. Strengths were similar between the hybrid and the conventional HIP samples, though the former had poorer total elongation (~10 %) due to remnant porosity. Higher HIP temperatures resulted in poorer strength while HIP at 930 °C, displayed the best elongation.