Effect of intermediate heat treatment temperature on microstructure and notch sensitivity of laser solid formed Inconel 718 superalloy

Inconel 718 superalloys deposited by laser solid forming (LSF) were heat treated with solution treatment, intermediate heat treatment (IHT) and two-stage aging treatment in sequence (SITA heat treatment). The effect of IHT temperature on microstructure, tensile property and notch sensitivity of LSFed Inconel 718 superalloy at 500 °C were investigated. As-deposited columnar grains have transformed to equiaxed grains and the grains were refined due to the recrystallization during the SITA heat treatment. It is found that the size and amount of δ phase dispersed at grain boundaries decreased with the increasing of IHT temperature, and δ phase disappeared when the IHT temperature reached 1 020 °C. The ultimate tensile strength (UTS) and yield strength (YS) of smooth samples increased to a maximum when the IHT temperature reached 980 °C and then decreased slightly to a minimum when the IHT temperature was 1 000 °C, and followed by slight increasing again till the IHT temperature reached 1 020 °C, resulted from the competition of precipitation strengthening effect of γ'' and γ' phase and the grain boundary weakening effect caused by the gradual disappearance of δ phase with increasing the IHT temperature. The notch sensitivity factor (q _e) decreased but still greater than 1 as the IHT temperature increased, which is attributed to the decrease of the size and amount of δ precipitation.