Effect of cooling rates during solidification of Al–5.5%Mg–2.3%Si–0.6%Mn and Al–13%Mg2Si pseudo-binary alloys on their secondary-particle morphology and tear toughness

Abstract

Al–5.5%Mg–2.3%Si–0.6%Mn alloy and Al–13%Mg2Si pseudo-binary alloy were cooled at various cooling rates during solidification. Morphological change of secondary particle was examined. Solidified structure of Al–5.5%Mg–2.3%Si–0.6%Mn alloy consisted of primary α–Al dendrites, Al–Mg2Si eutectic structure and Al6Mn particles. The most significant morphological change was from lamella Al–Mg2Si eutectic structure in the permanent mold cast and the die-cast product to refined and globular Mg2Si phase in the high-speed twin-roll cast product. In contrast, no morphological change was observed for Al6Mn particles. In order to investigate the effect of morphological change of Mg2Si phase on tear toughness, tear toughness tests were performed for Al–13%Mg2Si pseudo-binary alloy cast products produced by permanent mold casting and high-speed twin-roll casting. Unit crack propagation energy (UEp) of the high-speed twin-roll cast products was 5 times higher than that of permanent mold cast products. For permanent mold cast specimens, crack propagation occurred along the interface of plate-like Mg2Si/matrix of the eutectic solidified region and in Al-matrix. But fracture surface of high-speed twin-roll cast specimens was covered with fine dimples. Such difference of fracture mode is due to morphological change of Mg2Si phase from plate-like to globular by rapid cooling.