Effects of on-line solution and off-line heat treatment on microstructure and hardness of die-cast AZ91D alloy
(1. State Key Laboratory for Fabrication and Processing of Nonferrous Metals,
General Research Institute for Nonferrous Metals, Beijing 100088, China;
2. School of Materials and Metallurgy, Inner Mongolia University of Science and Technology,
Baotou 014010, China;
3. Material Technology Institute of Commercial Vehicle Technical Center,
Dongfeng Motor Co., Ltd., Shiyan 442001, China;
4. School of Materials Science and Engineering, Shenyang University of Technology,
Shenyang 110870, China)
General Research Institute for Nonferrous Metals, Beijing 100088, China;
2. School of Materials and Metallurgy, Inner Mongolia University of Science and Technology,
Baotou 014010, China;
3. Material Technology Institute of Commercial Vehicle Technical Center,
Dongfeng Motor Co., Ltd., Shiyan 442001, China;
4. School of Materials Science and Engineering, Shenyang University of Technology,
Shenyang 110870, China)
Abstract: The effects of on-line solution, off-line solution and aging heat treatment on the microstructure and hardness of the die-cast AZ91D alloys were investigated. Brinell hardness of die-cast AZ91D alloy increases through on-line solution and off-line aging treatment but decreases after off-line solution treatment. By X-ray diffractometry, optical microscopy, differential thermal analysis, scanning electron microscopy and X-ray energy dispersive spectroscopy, it is found that the microstructures of the die-cast AZ91D magnesium alloy before and after on-line solution and off-line aging are similar, consisting of α-Mg and β-Al12Mg17. The precipitation of Al element is prevented by on-line solution so that the effect of solid solution strengthening is enhanced. The β-Al12Mg17 phases precipitate from supersaturated Mg solid solution after off-line aging treatment, and lead to microstructure refinement of AZ91D alloy, so the effect of precipitation hardening is enhanced. The β-Al12Mg17 phases dissolve in the substructure after off-line solution treatment, which leads to that the grain boundary strengthening phase is reduced significantly and the hardness of die cast AZ91D is reduced.
Key words: die-cast magnesium alloy; AZ91D alloy; on-line solution; off-line solution treatment; aging treatment; β-Al12Mg17 phases; microstructure refinement; precipitation hardening