Microstructural evolution and high temperature resistance of functionally graded material Ti-6Al-4V/Inconel 718 coated by directed energy deposition-laser

This work focuses on the additive manufacturing of Ti-6Al-4V/Inconel 718 functionally graded material coated by directed energy deposition-laser (DED-L). The microstructural evolution, phase characteristics, microhardness change, and high-temperature resistance were investigated. With Inconel 718 proportion increase gradually, more alloying elements (Ni, Cr, Fe etc.) were added and the secondary phase (Ti₂Ni, TiNi, etc.) precipitated. The microstructure was found to transform from columnar grain to equiaxial grain. A series of phase transformation with the increase of Inconel 718 occurred in the following sequence: α+β→α+β+Ti₂Ni→β+TiNi→γ+Laves which were evidenced by experimental analysis and computational results. The microstructure is transformed and a Ti-Ni diffusion zone formed between the Ti-6Al-4V and transition layers after 800 °C exposure. The microhardness of the functionally graded material coating increases with the percentage of Inconel 718 increase. When the 100% of Inconel 718 deposited at the end, the microhardness of the layer reaches the highest value as 1030 HV₁, which is attributed to the formation and precipitation of Ti₂Ni, TiNi etc. The microhardness, microstructure, and phase transformation results are also generally consistent with the phase diagrams obtained from computational results. The results provide an innovative method which combines experimental analysis with computational phase diagram to research functionally graded material coating by DED-L.