The effect of hydrogen on the grain refinement and mechanisms for Ti6Al4V alloys during laser melting deposition

Titanium alloys have been widely used for AM (additive manufacturing) to manufacture complex parts duo to its excellent comprehensive properties. As one of the most important additive manufacturing methods, laser melting deposition (LMD) have been extensively used during the last few years. However, coarse columnar grains (CG) cannot be ignored generated in the LMD which will reduce the end-use performance of the alloy. In order to refine the microstructure, a well-designed THP is adopted. The powders with six kinds hydrogen content are used in a single-track experiment, and the optimum process parameters are obtained through deposit height, width, penetration depth as well as defects. Multi-layer deposition is used to analyze the evolution of the microstructure after forming. The results manifest that there is a number of acicular α′/α after solid solution treatment. With the increase of hydrogen concentration, the size of coarse CG gets smaller and finally turns to equiaxed grains at the bottom and middle area when hydrogen content is 1.0 wt%. Besides, the phase transformation point of deposition sample declines with the increase of hydrogen content and tends to be stable 900 °C when hydrogen content beyond 0.65 wt%.