Simultaneously improved strength and ductility of selective electron beam melted Ti6Al4V using ultrasonic surface rolling process

In this study, surface ultrasonic rolling processing (USRP) was used to strengthen the surface of Ti6Al4V via selective electron beam melting (SEBM), and the effect of different loads (200 N, 400 N, 600 N, 800 N, 1000 N) during USRP on the microstructure and mechanical properties was assessed systematically. The results showed that a gradient microstructure at surface of Ti6Al4V was formed during USRP, and the depth of the gradient microstructure increased with the increase of loads. Specifically, when the load was 1000 N, more than 500 μm gradient microstructure was induced on the surface of Ti6Al4V, which consisted of equiaxed nano-sized α-Ti grains (Region 1), ultrafine and nearly equiaxed α-Ti grains, nano-lamellar structures, and tangled dislocations (Region 2), nearly equiaxed α-Ti grains and ultrafine strip-shaped structures (Region 3), strip-shaped structures and few β-Ti (Region 4), some layered α-Ti destroyed grain boundaries and a small amount of dislocation entanglement (Region 5), from the outermost layer to the interior of the Ti6Al4V sample. The formed dislocations and stacking faults during USRP played an important role in the refinement of microstructure, and the ultimate tensile strength and elongation of Ti6Al4V after USRP at 1000 N reached 1089 MPa and 15.6 % compared with 1001 MPa and 14.3 % for as-built Ti6Al4V. The enhanced strength and plasticity for the Ti6Al4V was attributed to the coordinated deformation of the core coarse-grained region and the surface gradient microstructure region. These results provide a new surface enhancement method for the mechanical properties of metallic parts by SEBM. • USRP treatment could significantly enhance the surface quality of SEBM-built Ti6Al4V. • USRP treatment could enhance microhardness and introduce large residual compressive stress on the surface of Ti6Al4V. • More than 500 μm gradient microstructure could form on the surface of Ti6Al4V sample after USRP at 1000 N. • The UTS and elongation of Ti6Al4V after USRP at 1000 N were 1089 MPa and 15.6% respectively, both improved compared with the as-built Ti6Al4V.