Effect of heat treatment on the microstructure and tensile properties of a new superalloy designed for additive manufacturing

Abstract

The microstructures and tensile properties of a novel superalloy especially designed for additive manufacturing (AM) were investigated under four states: (1) AD (as-deposited), the state of part after LMD (laser metal deposition); (2) HT1, heat treated at 870 °C for 16 h + air cooling; (3) HT2, heat treated at 1080 °C for 4 h + air cooling; (4) HT3, heat treated at 1080 °C for 4 h + air cooling, then heat treated at 870 °C for 16 h + air cooling (HT2+HT1). Each of them were extracted from the vertical (V) and horizontal (H) section of the part, respectively. Methods such as OM, SEM, TEM, XRD and unidirectional tensile tests were used for the investigation. The AD sample displays cellular dendrite microstructures with <001> crystal direction parallel to the building direction. The major phases are γ/γ'; while the minor phases are fine MC and Laves with no more than 1% in volume fraction. The size of primary γ′ ranges from 100 nm to 150 nm, the secondary γ′ particles are no more than 30 nm. With the aid of calculation, the processing HT1 and HT2 are confirmed as ageing and sub-solution treatment, respectively. The former raised the volume fraction of γ′ from 30.93% (AD) to 48.57% (HT1), and raised the size of γ′ at the expense of dissolving the secondary γ′ particles. The latter didn't change the volume fraction of γ′ strikingly, but refined the γ′ with the exception of a small amount of extraordinary large particles in the interdendritic region (IR). HT3 is a two-stage processing (HT2+HT1) which increased the volume fraction of γ′ slightly but raised the size apparently. The value of ultimate tensile strength (UTS) and yield strength (YS) of vertical samples are superior to that of the horizontal. The HT1-V shows the best UTS and poor Elongation (UTS, 1082 MPa; EL, 35.28%), while the AD-V has excellent strength and ductility match (UTS, 1054 MPa; YS, 744 MPa; EL, 50.45%); The best ductility is found among samples of HT3 (EL, 62.1% for HT3-H). The samples of HT2-H are inferior in both strength and ductility, and the obtained properties are scattered. These investigation results provide important fundamental support for optimizing the procedure, microstructure and properties of the new alloy in the future application research.