Impact of high temperature stress relieving on final properties of Inconel 718 processed by laser powder bed fusion

Abstract

Abstract During manufacture of Inconel 718 (IN718) parts with complex geometry and high dimensional accuracy by laser powder bed fusion (LPBF), effective stress relieving is required before cutting off the parts from the build platform, in order to prevent distortion. The residual stress can be removed completely only by full recrystallization of structure, induced by annealing at the temperature close to that used in hot isostatic pressing (HIP). This work was aimed at investigating the effect of stress-relieving at 1150 °C for 6 h on properties of LPBF-ed IN718, obtained after each of successive thermal processing steps: optional HIP, solution annealing and double aging, in relation to analogous research performed with use of the thermal processing parameters recommended by ASTM standards. In addition, the effect of heating and cooling rates during individual heat-treatment steps on dissolution and precipitation of secondary phases was investigated. It was shown that an advantage of stress relieving at 1150 °C in comparison to the standard annealing at 1065 °C is higher effectiveness of stress elimination, higher resistance to liquid-film formation during heating-up, lack of delta phase, significantly higher elongation (48%) and area reduction (30%) of IN718 in final (without HIP) aged condition. Although the high-temperature stress relieving and HIP treatments resulted in coarse-grained microstructures, the tensile strength values determined at room and elevated temperatures (425 °C and 650 °C) were higher than the nominal values of wrought IN718 with similar grain size and much higher than those of cast and HIP-ed IN718.