Effect of process parameters and heat treatments on delta-phase precipitation in directed energy deposited alloy 718

Abstract

AbstractThis article outlines a detailed study of solution treatments and delta precipitation treatments carried out on laser-directed energy deposited (DED) alloy 718 specimens. Two different sets of DED process parameters were used in high and low energy conditions that yield different microstructural features to study the effect of process parameters on delta precipitation. These two conditions were subjected to solution treatment at 1010 °C and 1050 °C each for 1 h, which improved homogeneity and altered grain texture with introduction of annealing twins. The as-built and solution-treated specimens served as the initial reference condition for subsequent delta processing treatments (DPT) performed at three temperatures of 850 °C, 900 °C, and 950 °C to study the effect of short- and long-term exposures ranging from 1 to 48 h. When as-built specimens were subjected to DPT, interdendritic delta precipitates were observed at Nb-rich regions. In contrast, solution-treated specimens under short-term exposure to DPT resulted in intergranular delta phase precipitates whereas under long-term exposures to DPT yielded predominantly intragranular delta precipitates, which grew denser and longer with increased time of treatment. For longer exposure times of 24 and 48 h, a continuous film of intergranular delta phase was noticed. The morphology, location, and volume fraction of delta phase precipitates studied in this research are imperative for designing the performance of alloy 718 built by DED process.