The effects of process parameters and heat treatment on the surface and mechanical properties of laser powder bed fusion Ti-6Al-4V parts

The study investigates the effects of pulsed laser-based powder bed fusion (LPBF) process parameters and post-processing heat treatment on the density, surface roughness, microhardness, microstructure and tensile properties of Ti-6Al-4V parts. Four laser parameters, viz. power, point distance, exposure time and hatch distance were varied to achieve three energy densities: 36, 43 and 51 J/mm³. A parametric combination of 200 W laser power, 60 µm point distance, 61 µs exposure time and 0.095 mm hatch distance rendered a 98.5% relative density and 4 µm top face average roughness (Sa) and were subsequently employed to fabricate tensile test pieces. As-built tensile specimens had yield strengths of 1256-1328 MPa and failure strains of 5.3-7.9% which decreased to 1064-1156 MPa and 4.8-5.5%, respectively, following heat treatment (HT) below the β-transus temperature. The yield strength and failure strain further decreased to 449-524 MPa and 1.9-2.2% when HT above the β-transus. In terms of the microstructures, an HT below the β-transus showed equiaxed α+β grains, with needle-like α lamellae, while that above the β-transus exhibited coarse β grains, with α-phases along the grain boundaries. The microhardness of the LPBF parts varied between 393-445HV 0.1 , regardless of the HT conditions.