Strengthening effect in laser metal deposited Ti6Al4V alloy via layer-by-layer ultrasonic impact treatment

The present work investigates the influence of incorporation of the layer-by-layer ultrasonic impact treatment (UIT) into the laser metal deposition (LMD) processing of Ti6Al4V alloy. The microstructure and mechanical properties of the samples with/without the assistance of UIT are comparatively studied. The curved shape boundaries of alpha/alpha ' phase with inconsistent misorientation angles and the ultra-fine alpha phase are observed in the UIT-assisted LMD (ULMD) sample. The alpha/alpha ' phase in the ULMD sample is characterized by high-density dislocations, larger lattice strain and distortion in comparison with that in the LMD sample. An ultimate tensile strength of 1386 MPa is obtained in the ULMD sample that exceeds that of the LMD sample by 373 MPa and is superior to the values reported in majority of the previous works. Crystallographic orientation change, increased dislocation density and alpha/alpha ' phase lattice distortion, and formation of ultra-fine alpha grains are found to influence the properties. The present work provides an in-depth understanding of the mechanisms/factors influencing the mechanical strengths of additive-manufactured titanium alloys.