Directed energy deposition additive manufacturing of a Sc/Zr-modified Al–Mg alloy: Effect of thermal history on microstructural evolution and mechanical properties

Recently, laser additive manufacturing (LAM) of Al–Mg–Sc–Zr alloys has been considered an important method for developing a new generation of high-performance Al alloys. However, one of the key differences between LAM and conventional casting/wrought processes is that it has a unique combination of rapid solidification and in-situ solid-state transformation induced by repeated thermal cycling during deposition. In addition, owing to its near-net shape characteristics, subsequent plastic deformation processing is typically not an option to improve the as-built microstructure. To gain insight into the effect of the complex thermal history during LAM on the microstructural evolution and mechanical properties of the novel alloy systems, an Al–Mg–Sc–Zr alloy was processed via laser-directed energy deposition (L-DED) using a substrate with air cooling (AC) or water cooling (WC). The results show that the solidification microstructure was closely related to the dynamic solidification conditions in the molten pool, which promoted the transition from an equiaxed grain structure at low cooling rate (AC) to a heterogeneous grain structure at high cooling rate (WC). Furthermore, in-situ precipitation occurred during the repeated high-temperature thermal cycling in the AC sample, while it was effectively suppressed in the WC sample by lowering the peak temperature during thermal cycling. After aging, the yield strength of the WC sample was enhanced to ~2 times that of the AC sample, while the uniform elongation was still comparable to that of the AC sample. To achieve good mechanical properties for L-DED-processed Al–Mg–Sc–Zr alloys, it is crucial to conduct integrated control of the thermal history during both rapid solidification and in-situ precipitation.