Effect Of Solidification Cooling Rate On Microstructure Evolution And Mechanical Properties Of Rolled Mg-4.5Al-1.5Sn-xCa Alloy

As the lightest structural metal material,magnesium and its alloys have received extensive attention in light weight due to their low density,electromagnetic shielding,and easy recycling,and have been used in 3C,communications,military,and other fields.However,magnesium has a close-packed hexagonal?HCP?structure and few slip systems,so bottleneck problems such as difficulty in deformation at room temperature and difficulty in improving strength and plasticity at the same time severely limit its further industrial applications.As we all know,alloying can significantly improve the mechanical properties of magnesium alloys,but the addition of alloying elements also leads to the formation of various coarse eutectic phases in the as-cast structure,forming a severe macroscopic deviation.Due to the different solid solubility of the alloying elements in magnesium and the nature of the eutectic phase,some eutectic phases are difficult to eliminate once they appear,which leading to cracks during deformation processing,which seriously limits their production and application.Therefore,how to eliminate or refine these harmful eutectic phases and make them fully play the role in improving the mechanical properties of magnesium alloys is particularly important.The Mg-Al-Sn-Ca alloy easily forms coarse CaMgSn phase during solidification,which seriously damages its rolling behavior and mechanical properties.Existing methods to eliminate large-size,insoluble eutectic phases mainly include large-strain crushing or pre-deformation combined with heat treatment.These methods are relatively cumbersome,and for high melting point eutectic phases,such as CaMgSn,the refinement effect is not obvious.In this work,based on the new idea of liquid-solid synergistic regulation and control,sub-rapid solidification combined with rolling of the rotating is used to achieve the effective refinement and spheroidization of the coarse CaMgSn eutectic phase in the Mg-Al-Sn-Ca alloy,which significantly improves the rolling performance of the alloy,finally obtains the desired fine-grained structure and dispersed second phase,while improving the alloy's strength and plasticity.This work explores the influence of Ca content on the composition,size,morphology and quantity of eutectic phases in the as-cast structure of Mg-4.5Al-1.5Sn alloy;through sub-rapid solidification technology combined with heat treatment,the coarse CaMgSn phase is effectively refined and spheroidized;Using the rotary rolling process,the fine-grained structure?³?m?and the dispersion of the second phase are obtained,which promotes the synchronous improvement of the alloy's strength and plasticity.The main conclusions are as follows:?1?The cooling rate of solidification has a significant effect on the size,morphology,quantity and matrix grain size of the as-cast eutectic phase.As the cooling rate increases,the grain size decreases significantly,and the size of the CaMgSn phase decreases from³0-40?m to¹0-20?m.Defects occur inside the long rod-like and needle-like eutectic phases.In addition,increasing the cooling rate will increase the amount of Mg₁₇Al₁₂ and decrease the amount of Mg₂Sn.This is because increasing the cooling rate promotes the solid solution of the Sn element,but has no effect on the Al element,resulting in the size and number of Sn-containing eutectic phases reduced;?2?Solution treatment can further improve the eutectic phase distribution.After heat treatment at 500?/24 h,both Mg₁₇Al₁₂ and Mg₂Sn were dissolved back into the magnesium matrix.Although the CaMgSn phase cannot be eliminated by heat treatment,the CaMgSn phase in the sub-rapidly solidified sample is completely globalized,and there is no obvious change in the morphology of the CaMgSn phase in the conventionally solidified sample.This phenomenon is because the size of the CaMgSn phase in the sub-rapidly solidified sample is significantly refined,the curvature of the phase surface is large,and the CaMgSn phase is preferentially decomposed from the position of large curvature;?3?The Ca content has a significant effect on the microstructure of the as-cast and rolled Mg-4.5Al-1.5Sn alloys.With the increase of Ca content,the number of CaMgSn phases increased significantly,the degree of aggregation also increased,at the same time,the grain size decreased.During the heat treatment,the CaMgSn phase pinned the grain boundaries to suppress grain growth.The size of the precipitated phase of Ca-containing alloy is significantly reduced?¹6 nm?.The increase of Ca content consumes more Sn elements during the formation of CaMgSn,thereby increasing the ratio of the amount of precipitated phases Mg₁₇Al₁₂ and Mg₂Sn.When the Ca content is 0.1 wt.%,The rolling alloy texture weakening effect is most obvious?4?The mechanical properties of sub-rapidly solidified-rolled samples are significantly higher than those of conventional solidified-rolled samples,and the tensile properties of the alloy are significantly improved with the increase of Ca content,of which Mg-4.5Al-1.5Sn-0.5Ca is tensile strength³30 MPa,elongation²0%.The mechanical properties is improved.On the one hand,due to the fragmentation and spheroidization of the coarse eutectic phase,which reduces the cleavage effect on the matrix during deformation and inhibits early fracture,on the other hand,the refined CaMgSn eutectic phase promotes the PSN mechanism.The grain of the matrix is refined,which is beneficial to improve the strength and plasticity.