Effect Of Step Quenching And Zr Addition On Microstructure And Mechanical Properties Of Al-Mg-Si Alloy

Al-Mg-Si alloy is a kind of typically age-hardening aluminium alloy. The age-hardening potential of Al-Mg-Si alloy can not be fully exploited through traditional process. Especially after natural aging, the age-hardening response during artificial aging is very small. How to ameliorate the Al-Mg-Si alloy with good formability before stamping, with higher age-hardening response during artificial aging is the main purpose of the research.Through this research, not only the precipitation course of Al-Mg-Si alloy is more definitional in theory, but also the effect of solute atom aggregating on precipitation kinetics ofβ′′metastable phase is more explicit. In practice, Al-Mg-Si alloy with higher strength can be applied to automobile body appropriately and widely to realize lightweight of automotives.In this research, the effect of substituting step quenching process for the traditional water quenching, and also adding zirconium into Al-Mg-Si alloy was investigated. The precipitation process of natural aging and artificial aging under the condition of step quenching was analyzed in detail. The effect of step quenching and addition of zirconium on the precipitation kinetics ofβ′′phse was studied importantly. The effect of step quenching and addition of scandium, zirconium, misch metal on mechanical properties of Al-Mg-Si alloy was also researched.The results reveal that step quenching and additional zirconium influence the microstructure and mechanical properties significantly. Zirconium addition can improve the strength under the precondition of retaining good plasticity. Minor Zr addition can refine the microstructure of as-cast Al-Mg-Si alloy. Meanwhile small and homogeneous equiaxed grains with regular form can be attained. Zioconium can accelerate the speed of precipitation ofβ′′phase, and thus improve the strengthening effect. The step quenching process reduces the activation energy ofβ′′phase and promotes the precipitating ofβ′′phase. Furthermore, the step quenching at 100℃for Al-Mg-Si alloy is the optimal choice. Not only can that acquire the maximum bake hardening response in artificial aging, but also restrain the negative effect of natural aging.The yield strength and elongation of Al-Mg-Si alloy after solution heat treatment is about 110MPa and 29% respectively in the USA and Japan nowadays. After finally artificial aging, the value of yield strength is about 190MPa. In this research, under the condition of step quenching at 100℃, the elongation of Al-Mg-Si alloy in as-quenched state can be 32.3-35.3%. And the yield strength can reach up to 138MPa. After finally artificial aging, the strength is from 220MPa to 230MPa. Under the condition of step quenching at 100℃,the yield strength of Al-Mg-Si alloy with zirconium addition can still reach 200MPa, which has fully demonstrated that the enormously positive effect of step quenching and zirconium addition on the microstructure and mechanical properties.