Microstructure, precipitates and mechanical properties of powder bed fused inconel 718 before and after heat treatment

摘要

IN718 alloy was fabricated by laser powder bed fusion(PBF) for examination of microstructure, precipitates and mechanical properties in the as-built state and after different heat treatments. The as-built alloy had a characteristic fine cellular-dendritic microstructure with Nb, Mo and Ti segregated along the interdendritic region and cellular boundary. The as-built alloys were then subjected to solution heat treatment(SHT) at 980°C or 1065°C for 1 h. SHT at 980°C led to the formation of δ-phase in the interdendritic region or cellular boundary. The segregation was completely removed by the SHT at 1065°C, but recrystallization was observed, and the carbides decorated along the grain boundaries. The as-built alloy and alloys with SHT at 980°C and 1065°C were two-step aged, which consisted of annealing at 720°C for8 h followed by annealing at 620°C for 8 h. Transmission electron microscopy revealed the precipitation of λ’ and λ" in all alloys after two-step aging, but the amount and uniformity of distribution varied. The Vickers hardness of the PBF IN718 alloy increased from 296 HV to 467 HV after direct aging. The hardness decreased to 267 HV and 235 HV after SHT at 980°C and 1065°C, respectively, but increased to 458 HV and 477 HV followed by aging. The evolution of Young’s modulus after heat treatment exhibited similar trend to that of hardness. The highest hardness was observed for IN718 after SHT at 1065°C and two-step aging due to precipitation with greater amount and uniform distribution.