| Magnesium alloys, as well known for their low density, high strength to weight ratio and high stiffness to density ratio, have drawn wide attention in the application in the aerospace, automotive and electronic industries. In recent years, the research and development of heat-resistant magnesium alloys has become a hot research topic in the field of magnesium alloys technology. The AS series of silicon-contained heat-resistant magnesium alloys has the advantages of excellent high temperature performance, high liquidity and low cost, however, its strength needs to be further improved due to its application limited to the thin-walled castings with faster cooling rates.In this thesis, the AS31 magnesium alloys is studied. To develop novel low-cost silicon-contained heat-resistant magnesium alloys, investigations onSr micro-alloying, optimization of solidification process, optimization of alloy composition and the strength of heat treatment are carried out, based on the theory of microstructure refinement, modification and solid solution-aging treatment.The results show that the grain size can be refined and the eutectic Mg2Si be modified significantly with Sr micro-alloying. The average grain size decreases gradually and the shape of the eutectic Mg2Si is modified from Chinese script to fine fibers or polyhedral. It is shown that optimal grain refinement and phases modification can be achieved with addition of 0.5wt.%Sr, while an enhanced AS31 alloy with optimum tensile properties can be obtained, where the ultimate strength, yield strength and elongation are 185.9MPa,112.2MPa and 5.7% at ambient temperature and 135.4MPa,104.6MPa and 7.3% at 150℃ respectively, with increase of 31.2%,39.2%,72.7% and 43.7%,65.0%,58.7% respectively.The casting temperature and melt holding time are of great influence to the microstructure and tensile properties of Sr micro-alloyed AS31 alloy. It is found that the best effect of Sr micro-alloying on Mg2Si can be obtained when the alloy is casted at 700℃ with melting holding for 30min, where the ultimate strength, yield strength and elongation can reach 177.5MPa,105.5MPa and 5.4% at ambient temperature and 113.2MPa,94.6MPa and 6.0% at 150℃ respectively.The effects of component content of Al and Si on the microstructure and tensile properties of Mg-Al-Si-Sr alloy are studied. The average grain size increases and then decreases with the increasing of Al additions (3wt.%,5wt.%,7wt.%,9wt.%). The minimum average grain size can be obtained with 7wt.% of Al addition. There are polyhedral shape, fine fibers shape, a few Chinese script Mg2Si particles and β-Mg17Al12 divorced eutectic phase appeared in the interdendritic area in Mg-3Al-1Si-0.5Sr alloys. With the increase of Al content, more and more β-Mg17Al12 divorced eutectic phase appear in semi-continuous distribution. The lamellar-like shape β-Mg17Al12 secondary-precipitated phase emerge with Al addition of 5wt.%,7wt.% and 9wt.%. The optimum tensile properties of Mg-7Al-1Si-0.5Sr alloy can be obtained, where the ultimate strength, yield strength and elongation are 201.1MPa,160.7MPa,4.8% at ambient temperature and 164.5MPa, 128.0MPa,7.4% at 150℃ respectively.The Chinese script Mg2Si particles are found disappeared and the volume of primary block Mg2Si phase increase with increasing Si content from lwt.% to 3wt.%, which lead the tensile properties of the alloy to deteriorate.The effect of solution treated and aging treated on the microstructure and tensile properties of Mg-7Al-1Si-0.5Sr alloy are also investigated. After multi-stage solid solution treatment,both the β-Mg17Al12 divorced eutectic phase in the interdendritic area and the lamellar-like shape β-Mg17Al12 secondary-precipitated phase dissolve in the matrix. The Mg2Si phase are refined, while the balling effect occur.. The ultimate strength and elongation are improved to 221.1MPa,5.7% at ambient temperature and 179.2MPa,8.7% at at 150℃ respectively. However, the yield strength decrease. The lamellar-like β-Mg17Al12 secondary-precipitated phase appears again after aging treatment. The ultimate strength the yield strength and elongation are 235.9MPa,169.0MPa,3.8% at ambient temperature and 192.5MPa,136.2MPa,6.4% at 150℃ respectively. The ultimate strength the yield strength increase respectively 17.3%,5.2% at ambient temperature and 17.0%,6.4% at 150℃ after aging treatment. |
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