Fabrication and microstucture of spray formed powder metallurgy superalloy FGH4095M

Abstract

Spray forming is a kind of near-net-shaped rapid solidification process based on powder metallurgy gas atomization technology. In this work, the FGH4095M is fabricated by spray forming. The pre-alloy is prepared by vacuum induction melting and vacuum arc remelting techniques. Then the alloy is sprayed by SK2 facility with atomization gas nitrogen at University of Bremen in Germany. In this paper we study the density and microstructure of the spray-formed billet, especially the special morphology of γ’ phase. The results show that density is associated with different parts of the deposited billet. The relative density of the bottom part is higher (99.63%) than those in the other parts. The relative density of top part (98.91%) is lowest. After hot isostatic pressing, the relative density can be up to 100%. Uniform and fine equiaxed grains are the remarkable morphology of spray-formed alloy without prior particle boundary. The sizes of grains are in a range of about l0-40 μm and the grains at bottom part of billet are finest. The grain sizes of primary γ’ phase are in a range of about 0.6-0.8 μm, and the grain sizes of secondary γ’ phase in a range of about 0.1-0.5 μm as well as dispersed spherical tertiary γ’ particles with the sizes of 10-20 nm. The special morphology of secondary γ’ phase occurs with the splitting of γ’ particle, which is related to the low cooling rate of the depositing process. The splitting behavior reduces the total energy of γ’ particle. Total energy of γ’ particle includes elastic interaction energy, elastic strain energy and surface energy, among which the elastic strain energy is invariable. The surface energy increases with the splitting process and the elastic interaction energy reduces. The effect of elastic interaction energy on particles is the major reason why the total energy is reduced. The trend of splitting behavior is analyzed by calculating the equivalent diameter of splitting γ’ particle. It indicates that when the equivalent diameter is over 0.40 μm, there is the possibility to split. Subsequently, spray-formed FGH4095M billet is treated by hot isostatic pressing, isothermal forging and heat treatment process to obtain the FGH4095M alloy turbine disk. The research of tensile property of FGH4095M alloy turbine disk shows an excellent property either at room temperature or at high temperature for the optimized alloy. The relationship between special morphology of γ’ phase and excellent property needs further investigating.