Existing form of Sc in metal-inert gas welded Al-0.60Mg-0.75Si alloy and its role in welding strength

The existing form of Sc in Si-containing 6xxx-series Al alloys and its role in strength has always been controversial. In this study, isothermal section phase diagram of the Al-0.60 wt%Mg-Si-Sc system at 350 °C was calculated by CALPHAD method and Pandat software, and the results show that Sc exists in Al3Sc or V (AlSc2Si2) phases, depending on the contents of Si and Sc. Subsequently, we designed a Al-0.60 Mg-0.75Si-0.05Sc (wt%) Melt Inert Gas (MIG) welded alloy, and investigated its microstructure and mechanical property by tensile tests and electron microscopy. HRTEM and STEM-EDS results show that, Sc exists in the form of core(Sc)-shell (Si) structured (Al, Si)3Sc transition phases with the size range of 60–80 nm in the fractured zone (heat affected zone, HAZ) of the welded joints, deriving from equilibrium phase diagram, V (AlSc2Si2) theoretically. The HAZ of the Al-Mg-Si-Sc joint consists of partly recrystallized grains and strong rolling texture, different from the completely recrystallized and coarsened grains in the Al-Mg-Si joint. In addition, the (Al, Si)3Sc phases increase the density of β'′ aging strengthening phases and prevent them from coarsening under welding heat cycle. The above positive effects of (Al, Si)3Sc particles increase the yield strength, ultimate tensile strength and elongation of Al-Mg-Si welded joints by 31 MPa (20.6%), 41 MPa (21.2%) and 0.8% (12.9%), respectively.