Effects of Different Si Content and Thermal Stories on the Secondary Phase Formation, Hot Ductility, and Stress Rupture Properties of Alloy 718 Investment Castings

Abstract

Herein, the effects of microstructure on high-temperature properties (hot ductility and stress rupture life) were investigated in alloy 718 investment castings manufactured with different Si contents (0.051, 0.11, and 0.17 wt pct) and different cooling rates (0.52 °C/s and 1.65 °C/s). For the casting with a low cooling rate, an additional solution treatment step was added to the standard pre-weld heat treatment. Even small amounts of residual Laves phase (0.14 to 0.35 wt pct) adversely affected both the hot ductility and stress rupture properties. Hot ductility tests indicated that the presence of Laves phase led to an earlier decrease in hot ductility in on-heating tests and retarded ductility recovery in on-cooling tests. Both effects are usually associated with poorer weldability and greater hot cracking susceptibility. Ductility recovery was also affected by grain size and aspect ratio: columnar and smaller grain sizes showed better ductility recovery behavior. Stress rupture tests indicated that stress rupture life and elongation decreased with increasing Si content, due to the presence of a semicontinuous network of Laves phase at grain boundaries instead of intergranular δ phases. This effect was much more pronounced in stress rupture than in hot ductility tests. The presence of residual Laves phase was associated with higher Si content and lower cooling rates. The additional solution treatment step of 2 hours at 1052 °C did not eliminate the Laves phase completely.