レーザ粉末床溶融結合法により作製されたγ-Fe／Fe₂Nb２相共晶合金の造形性と組織形態

抄録

An attempt was made to apply the laser powder bed fusion (L-PBF) process to Fe–Ni–Nb ternary alloys as a major composition of heat-resistant Fe-based alloys strengthened by the Fe₂Nb intermetallic phase. This study focused on a composition of Fe–40Ni–15Nb (at%) near a eutectic composition of austenite (γ-Fe) and Fe₂Nb in the Fe–Ni–Nb ternary system for the pre-alloy powder with an average particle size of approximately 17 μm, which was produced by the gas-atomizing process. The Fe–40Ni–15Nb pre-alloy powder exhibited insufficient L-PBF processability for fabricating fully dense centimeter-sized samples, whereas several alloy samples with relative densities above 90 % were manufactured. The L-PBF manufactured alloy samples exhibited a representative melt-pool structure in which regions had locally melted and rapidly solidified by scanning laser irradiation in the L-PBF process. The L-PBF sample exhibited a high hardness of approximately 900 HV compared to the slowly solidified alloy sample (about 550 HV). The high hardness could be attributed to the formation of nanoscale γ-Fe/Fe₂Nb eutectic microstructure by the L-PBF process. Intriguingly, the observed melt-pool boundary had several tens of micrometers in width, which significantly varied depending on the applied laser condition. Nanoindentation tests demonstrated the melt-pool boundary region exhibited a relatively lower hardness than inside the melt pools. It was assumed that the unique melt-pool boundary region would consist of a relatively coarsened solidification microstructure formed at the interface of liquid with the solid and a locally coarsened microstructure affected by laser local heating, which can be controlled by manipulating the laser conditions.