Effect of sample thickness and heat treatment on microstructure and mechanical properties of Ti-6Al-4V fabricated using laser powder bed fusion

While many studies investigate the effects of post-processing heat treatments on bulk material properties of Ti6Al-4V fabricated using laser powder bed fusion additive manufacturing (PBF-LB AM), there is limited data on how thermal post-processing impacts the microstructure and mechanical properties of thin-walled Ti-6Al-4V made by AM. In the present work, the effects of sample thickness and post-processing heat treatments on the microstructure and mechanical properties of Ti-6Al-4V fabricated using PBF-LB AM were investigated. All samples were stress relieved (SR), with subsets of samples subjected to super-transus annealing (SR1000a), subtransus annealing (SR800a), or sub-transus hot isostatic pressing (HIP, SRHIP). Samples that were further heat treated (SR800a, SR1000a, and SRHIP) and a subset of SR samples were also subjected to a chemical surface finish to investigate the effects of surface roughness on mechanical properties. Internal porosity was evaluated using X-ray computed tomography, and prior-beta grains and alpha-laths were characterized using optical and scanning electron microscope imaging to correlate microstructural features with tensile strength and ductility and microhardness. Thinner samples had thicker alpha-laths, resulting in lower tensile strength, but a stronger <0001> texture along the build direction, leading to higher microhardness compared to thicker samples. Surface roughness positively correlated with scan speed and was more detrimental to the ductility of thinner unfinished samples than thicker unfinished samples due to the larger ratio of surface notch depth to thickness. Although heat treatments did not impact texture, they increased alpha-lath width, resulting in increased strain to failure for 56% of the processing parameters and decreased strength and hardness for all processing parameters compared to same thickness SR samples after surface finishing. The wide range of measured tensile strengths, which was due to differences in as-fabricated microstructures, was successfully narrowed by performing SR1000a and SRHIP.