Effects Of Solidification Cooling Rate On Microstructure Evolution And Mechanical Properties Of Rolled Mg-1Al-1Zn-0.2Sn-xCa Alloy

Magnesium and its alloys show huge application potential in numerous industries because of low density,high strength,good casting performance and anti-damping ability.However,conventional Mg alloys exhibit low strength and inferior ductility,which are less used in key structural components.Alloying is an effective way to improve mechanical properties of Mg alloys.Among various Mg alloys,Mg-Al-Zn series alloys have received widespread attention due to their good formability and mechanical properties.However,the research on high strength and superplasticity of Mg alloys mainly focuses on the alloys with medium and high alloying elements which improve the strength by solid solution strengthening and precipitation strengthening.In addition,high density dispersed secondary phases can pin grain boundaries,inhibit the growth of grains,and improve the high-temperature ductility of Mg alloys.Nevertheless,the deformation process of high-alloyed alloys is extremely hard because of the existence of coarse eutectic phases along grain boundaries,which can act as the source of cracks during the deformation process.Therefore,lowering the amount of alloying elements in Mg alloys has been suggested to be effective in improving the formability and ductility.However,low-alloyed Mg-Al-Zn series alloys such as AZ31 are very difficult to prepare high-strength magnesium alloy by conventional manufacturing methods owing to the lack of secondary phases.Therefore,adding trace alloying elements to promote the formation of secondary phases is considered an effective way to improve the strength of low-alloyed alloys.It has been found that Sn can reduce the stacking fault energy(SFE)of Mg alloys and facilitate the activation of non-basal slip systems,resulting in enhanced ductility of Mg alloys.In addition,the relatively inexpensive alkaline-earth Ca element has been proved to be efficient in refining the solidification microstructure of Mg alloys and improving the mechanical properties at room and high temperatures.Even though the co-addition of Ca and Sn seems to be promising in the achievement of high-performance Mg alloys,coarse Ca Mg Sn phase is easy to form,which will damage mechanical properties and formability.Based on the idea of liquid-solid synergy,this work uses sub-rapid solidification technique combined with the one-pass rolling with a large reduction to prepare low-alloyed Mg-Al-Zn-Ca-Sn alloys,which has excellent mechanical properties at room temperature and low temperature superplasticity by short-pass process.This paper systematically studies the influence of trace Ca addition and solidification cooling rates on the microstructure and mechanical properties of cast and rolled alloy.In addition,analyzing the formation mechanism of bimodal grain structure and the influence of annealing time on microstructure evolution have been analyzed.The main conclusions are as follows:(1)The solidification cooling rate has a significant effect on the grain size of cast alloy and the amount of eutectic phases.With the increasing of cooling rates,the grain size of cast Mg-1Al-1Zn-0.2Sn-0.1Ca alloy is reduced from?199?m to?88?m,and the volume fraction of eutectic phase is reduced from?2.6%to?1.3%,which could attribute to the high cooling rates that favor for capturing more solute atoms in the matrix to reduce the formation of eutectic phases.(2)The solidification cooling rate has a significant effect on the types and size of precipitates of deformed alloy;compared to the conventional solidification,the precipitates of sub-rapid solidified alloy have smaller size which can pin grain boundaries obviously,and improves the thermal stability.The grain size during the process has been restrained and the mechanical properties has been improved.This is because sub-rapid solidification process enables more solute atoms in the matrix,which are more likely to precipitate during the subsequent plastic deformation.(3)The Ca content will significantly affect the species and quantity of eutectic phase in cast alloys;during sub-rapid solidification process,Ca atom is preferentially forming the Ca Mg Sn phase with the Sn atom,then forming the Mg Al Zn Ca phase with Mg,Al and Zn atoms.Finally,as the Ca content increases,the remaining Ca atoms and Al elements form Al₂Ca phase.The eutectic phase volume fraction increases from?1.3%to?1.9%with the increasing of Ca content.(4)The Ca content will significantly affect the thermal stability of the deformed alloys,which in turn affects the mechanical properties;as the Ca content is increased,the volume fraction of nano-size precipitates has increased from 0.2%to 0.7%.The nano-size Ca Mg Sn precipitates stabilize the grain boundary,inhibiting the grain growth and static recrystallization process to improve the thermal stability of the alloy.Thus,a bimodal size structure can be maintained,which helps to increase the ductility at room temperature and high temperature.(5)Based on the idea of liquid-solid synergisy(the synergistic control of solidification cooling rates and rolling deformation),short-pass process was designed to prepare low-alloyed magnesium with high performance.New Mg-1Al-1Zn-0.2Sn-xCa alloys with high performance at room temperature and low temperature superplasticity have been successfully prepared by combination of sub-rapid solidification and one-pass rolling with a large reduction.The high strength at room temperature is proved to be as a result of the synergistic effect of grain boundary strengthening,dislocation strengthening and second phase strengthening.Superplasticity is a result of the CDRX in the coarse grains at initial stage and grain boundary sliding in the continuous dynamic recrystallized grains.