High-Temperature Properties of Alloy 718 Made by Laser Powder-Bed Fusion

Abstract

The properties of alloy 718Alloy 718 made by laser powder-bed fusion (LPBF) have been widely reported, and while their room-temperature static properties are often similar to wrought material, elevated temperature properties have proven inferior, especially at slow strain rates. Stress ruptureStress-rupture tests performed on LBPF 718 material after Hot isostatic Pressing (HIP) and heat treating in conformance with AMS 5662 have persistently led to brittle notch failures with limited elongationElongation. Creep testingCreep testing at 650 °C and 690 MPa shows a similar tendency, with samples showing capability of sustaining a load but with limited elongationElongation and abrupt, intergranular failure. Alternative heat treatmentsHeat treatment performed to enhance high-temperature ductility have shown success in adjusting delta phaseDelta phase population but without benefit for stress ruptureStress-rupture. Alternative HIP schedules have also been performed that led to changes in room-temperature and elevated temperature static strength but no benefit in stress ruptureStress-rupture behavior. The root cause of this behavior is attributed to the dispersion of NbC that is a consequence of the LPBF process that results in fine-scale segregation of Nb and C during rapid solidification. Deliberately lowering the carbon content of the powder feedstock led to a greater number of smaller Y” particles and smaller size NbC particles which resulted in an increase in static strength at room and elevated temperatures but no improvement in stress ruptureStress-rupture. The presence of a large number of NbC particles leads to environmental sensitivity of LPBF 718 that is most apparent at elevated temperatures and slow strain rates.