Multiscale mechanical performance evaluation of L-DED Ti6Al4V by novel ultrasonic burnishing (UB)

Abstract

Laser-directed energy deposition (L-DED), as a novel technology to produce titanium (Ti) alloys (e.g., Ti6Al4V), has potential impactful applications. However, Ti alloys by L-DED is largely compromised by defects (porosity, cracks, etc.) at or near surfaces, which prevents strength-ductility synergy. In this study, a new method for Ti6Al4V sample preparation using ultrasonic burnishing coupled with heat treatment (UB/HT) after L-DED has been suggested. The results show that UB/HT has a more positive effect than mere UB to overcome turning's disadvantages, when the roughness, porosity, microstructures, and multiscale mechanical performance of L-DED Ti6Al4V are considered. UB/HT transforms residual tensile stress into compressive stress and causes severe plastic deformation (SPD) within a depth of approximately 10 μm on the surface layer. With this, UB/HT improves multiscale mechanical properties, e.g., hardness to 428 HV and strength to 976.7 MPa, and the related failure modes and fracture features induced by the UB/HT crack inhibition effect are confirmed with the fatigue results. With these observations, this research provides a good theoretical and practical reference for multi-scale mechanical analysis and understanding of L-DED Ti6Al4V, which expand the UB/HT applications.