Study On The Relationship Between Mushy Zone Strength And Hot Cracking Of Mg-Zn Based Magnesium Alloys

Magnesium alloy is a light material applicated widely,which has superior comprehensive properties.However,the wide solidification zone,rapid volume and linear contraction,slow fluidity of magnesium alloy make it easy to produce shrinkage and hot cracking defect.Hot cracking has already turned into a bottleneck in the development of magnesium alloy,while structural design becomes lighter,thinner and more complex.Obviously,the hot cracking of magnesium alloy is related to the strength of alloy during solidification,but there are few studies on this aspect now.In this paper,the relationship between strength and hot cracking of magnesium alloy with different compositions is studied by means of a self-designed high temperature tensile tester.Apart from this,the effects of Zn,Y and Cu elements on the hot cracking mechanism are analyzed.The main findings are as follows:(1)In the alloys studied in this paper,with the increase of the solid fraction,the strength of the alloy generally exhibits a growing trend,and the formation of strength is mainly at the final stage of solidification due to the more interdendritic bridging phenomenon.The strength of the alloy,fracture extension value,formation rate of strength and microstructure at the end of the solidification process(under 0.9-1.0 solid fraction)are the important factor of hot cracking.(2)In the Mg-Zn alloy studied in this paper,when the Zn content increases from 2%to 6%,the strength of the alloy increases,the fracture extension value does not change obviously.When the Zn content increases from 6%to 10%,the fracture extension value increases,which means plasticity becomes better,the strength does not change obviously.Increasing Zn content can not only reduce formation rate of strength,but also refine the grain in the solidification process.Increasing Zn content can reduce hot cracking of the alloy.(3)In the Mg-Zn-1Y alloys studied in this paper,increasing Zn content will raise hot cracking of the alloy.When the Zn content increases from 1%to 3%,the strength of the alloy decreases,and the fracture extension value decreases,which means plasticity deteriorates,the formation rate of strength is slightly larger and it appears at a higher solid fraction.When the Zn content increases from 3%to 5%,the strength of the alloy increases,but the fracture extension value near the end of solidification decreases,which means plasticity deteriorates,the formation rate of strength becomes significantly faster.The increase of Zn content leads to the precipitation of more of the second phase in the liquid phase,resulting in a deterioration of the liquid phase flow and a decrease in the feeding effect.Although increasing the Zn content can enhance the strength at the end of the solidification,it has a promoting effect on the thermal cracking resistance,but is smaller than the negative effect of fracture extension value,formation rate of strength and microstructure.(4)In the Mg-Zn-Cu alloys studied in this paper,increasing Zn content mainly increases the strength of Mg-Zn-Cu alloys at the final stage of solidification and refine the grain in the solidification process.Increasing Cu content mainly increases the fracture extension value of Mg-Zn-Cu alloys at the final stage of solidification and enhance filling effect of liquid film at the end of the solidification process.Increasing Zn or Cu content can reduce formation rate of strength.All above will reduce hot cracking of the alloy.