Influence of platform preheating on in situ precipitation in an FeCoMo alloy during laser powder bed fusion

The nanostructure of a laser powder bed fused (LPBF) FeCoMo maraging alloy was investigated with atom probe tomography (APT). Two as-built conditions were examined: without platform preheating and with a nominal platform preheating temperature T _P=500°C. The results showed that the martensite start temperature of the FeCoMo alloy is lower than T _P and the intermetallic strengthening μ-phase precipitates from the martensite phase and not from the austenite phase. By comparing the μ-phase precipitation stages in the specimens, a model for the temperature distribution during LPBF with T _P=500°C was proposed. It was concluded that the platform surface did not attain the nominal preheating temperature and that the temperature in the built part decreased with increasing distance from the platform. Intrinsic heat treatment (IHT) during LPBF alone did not provide sufficient energy (temperature and/or time) for μ-phase precipitation.