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

Abstract

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.