Microstructural evolution and strengthening mechanisms of additively-manufactured Al-Mg-Sc-Zr alloys: Laser directed energy deposition versus laser powder bed fusion

Al-Mg-Sc-Zr is a high-performance Al alloy developed for laser additive manufacturing (LAM). The addition of Sc/Zr can stimulate both grain-boundary and precipitation strengthening, which are closely related to the precipitation behavior of the primary and secondary Al₃(Sc,Zr) phases, respectively. For LAM, laser direct energy deposition (L-DED) and laser powder bed fusion (L-PBF) are two typical processes. However, little is known about the disparities in the microstructural evolution and strengthening behavior of Al-Mg-Sc-Zr alloys under the L-PBF and L-DED processes. In this study, a comparative analysis of the microstructure and strengthening behavior of L-DED- and L-PBF-processed Al-Mg-Sc-Zr alloys was conducted. Owing to the higher cooling rate during L-PBF, fewer primary Al₃(Sc,Zr) phases precipitated during solidification, and more secondary Al₃(Sc,Zr) phases formed during aging, resulting in higher precipitation strengthening. More importantly, the ultrafine grains at the submicron scale in the L-PBF sample not only enhanced grain-boundary strengthening but also provided additional strengthening owing to the lack of mobile dislocations in the microstructure. This work is helpful for further optimizing the mechanical properties of LAM-processed Al-Mg-Sc-Zr alloys.