Influence of post-heat treatments on the microstructure and mechanical properties of Inconel 738 produced by laser directed energy deposition

In this paper, the non-weldable precipitation-hardened superalloy Inconel 738 thin-wall parts were fabricated by laser directed energy deposition (LDED). The influence of post heat treatments including solution and aging heat treatments on the microstructures and mechanical properties of LDED-manufactured IN738 parts were investigated. The research results show that the phase composition of as-built and heat-treated IN738 alloys consisted of γ/γ’ and MC carbide. The microstructure of as-built IN738 alloy included fine columnar dendrites with a strong <100> fibre texture parallel to the build direction. The smallest size of γ′ precipitates in the matrix led to the highest hardness value. By applying the post-heat treatment of 1120°C-2h+850°C-24 h, intense dislocation lines formed the network around the cuboidal γ′ precipitates. Moreover, the elongation of transverse and longitudinal tensile specimens reached the highest value of 26% and 17.4% respectively. With the increase of solution temperature, the fine columnar dendrites gradually enlarged along the direction vertical to the building direction. When the solution temperature increased to 1270 °C, the elongation of samples decreased obviously due to the enlarged columnar grains and the disappearance of dislocation network.