Heat treating additive-manufactured alloys: A comprehensive review

The high cooling rates and thermal gradients inherent in additive manufacturing can produce out-of-equilibrium microstructures with refined grains and metastable phases, including supersaturated solid solutions, and may also suppress the formation of complex intermetallics. The thermal cycles involved during layer-by-layer build-up affect the microstructure during processing and may cause variations along the different regions of the material. Directional heat extraction may also favor an anisotropic structure formation, with the presence of columnar grains, and high residual stresses may cause defect formation. A heterogeneous structure is also formed due to its layer-by-layer and track-by-track construction. All these characteristics affect the phase formation and microstructure of additive-manufactured alloys, making them typically very different from conventionally processed counterparts. Consequently, the procedure adopted for heat treatments is also normally very different than standardized procedures described in handbooks and other heat treatment guides. This review summarizes the heat treatments carried out for additive-manufactured alloys and compares them with standard procedures. The differences are discussed in terms of phase formation, microstructure, and mechanical properties. Some key topics requiring attention for further development are highlighted and discussed.