Microstructure and mechanical properties of LMD-SLM hybrid forming Ti6Al4V alloy

Laser melting deposition (LMD) and selective laser melting (SLM) are two major metal additive manufacturing technologies that explore the near-net shaping of large components and net shaping of small complex structures. In order to achieve subscale complex structures, researchers proposed LMD-SLM hybrid manufacturing processes. Ti6Al4V is an α-β dual-phase, moderate strength titanium alloy that is widely used in the fields of medicine, aeronautics, and astronautics. In this study, thin (1.5-2.5 mm) horizontal, vertical SLM plate and rolled plate are used as substrate materials for the LMD process to analyze tensile properties, microhardness, microstructure, and internal defects. The results show that the LMD process forms a hybrid with the aforementioned plates. The relative density of hybrid-forming area can reach 99.5%, because of the existence of the pores with diameter <20 μm. Tensile strength and elongation of the hybrid thus produced can reach respectively 918 MPa and 11%, and fractures are located in the LMD zone. Internal layer fracture of the LMD zone increases elongation, whereas layer interface fracture decreases it. The laser deposition process epitaxially generates coarse columnar crystals, and laser remelting reduces the microhardness of the SLM substrate in the 2- to 3-mm-thick grain-increased heat-affected zone (HAZ). The microhardness distributions of the LMD zone, HAZ, and substrate material are found to be 344, 343, and 375 (horizontal SLM); 346, 334, and 386 (vertical SLM); and 351, 328, and 340 HV (rolled plate), respectively.