Morphology Control Of Primary Mg₂Si In Complex Modified Al-20wt.% Mg₂Si Alloys

Intermetallic compound Mg₂ Si exhibits low density,high melting temperature,high elastic modulus as well as low thermal expansion coefficient.Owing to the outstanding combination of physical and mechanical properties,Mg₂ Si has been regarded as a reinforced phase in Al-Mg-Si alloy.However,under equilibrium solidification conditions,in Al-high Mg₂ Si alloys（the content of Mg₂ Si is more than 13.9 wt%）,primary Mg₂ Si,the first phase precipitating from the melt,is usually dendritic.The formation of dendritic primary Mg₂ Si can deteriorate the mechanical properties of Al-Mg-Si alloys and limit their applications.Therefore,controlling the morphology of primary Mg₂ Si is essential to the design of more feasible lightweight Al-Mg-Si alloys.Modification treatment and inoculation nucleation are effective methods to control the morphology of primary Mg₂ Si.Recently,modification treatment and inoculation nucleation of primary Mg₂ Si by single element modification has been widely studied.However,large modifiers usage in single element modification process leads to the high preparation cost of Al-Mg-Si alloy.Moreover,owing to the high precipation temperature of primary Mg₂ Si,it is diffcult to develop a perfect nucleating agent of primary Mg₂ Si by single element modification.Complex modification regarded as a new modification method has been gradually focused.However,influence of kinetic factor on complex modified primary Mg₂ Si morphology evolution under non-equilibrium solidification condition is still unknown.Moreover,both crystal nucleation kinetics factors and crystal growth kineticsfactors play an important role in deciding the final morphology of a crystal.Morphology of primary Mg₂ Si can be controlled by modifiers under non-equilibrium solidification condition while equilibrium solidification process will be obtained by decreasing solidification cooling rate and hence new microstructure will also appear.However,influence of kinetics factors on primary Mg₂ Si morphology evolution in the process of transformation from non-equilibrium solidification to equilibrium solidification is still unknown.Effect of the nucleus on morphology evolution and growth process of primary Mg₂ Si is investigated by complex modification（Ca-Sb,Li-Sb,La-P）.Meanwhile,influence of kinetics factors on modified primary Mg₂ Si morphology evolution in the process of transformation from non-equilibrium solidification to equilibrium solidification was investigated by decreasing the cooling rates of Al-Mg-Si alloy.Moreover,hot-working is conducted to complex modified Al-Mg-Si alloy to obtain spherical primary Mg₂ Si.The results are as follows:1)It is found that CaSb₂ and Li2 Sb can act as the nucleus of primary Mg₂ Si,the morphologies of primary Mg₂ Si transformed from coarse dentrite to truncated octahedron.2)The atomic ratio of Ca/Sb plays an important role in controlling the morphology of primary Mg₂ Si in Al-20Mg₂Si-0.5（Ca-Sb）alloy;the morphology evolution of primary Mg₂ Si in the alloy with Ca/Sb atomic ratio of 1:3,1:2,1:1,2:1,3:1 is as follows: coarse dentrite → equiaxed-dentrite → truncated octahedron → flat truncated octahedron→truncated cube.3)In Al-20Mg₂Si-0.2Ca alloy,with the solidification cooling rates decreasing from 8.8 to 6.3 and then to 4.7 and finally to 0.6 oC/s,morphology evolution of primary Mg₂ Si is as follow: cube→truncated cube→truncated octahedron and dodecahedron→equiaxed-dendrite and octahedron;influence of kinetics factors（the adsorption of Ca）on primary Mg₂ Si morphology was weakened with the cooling rates decreasing（solidification process is close to equilibrium）.4)Growth process of truncated cubic,flat truncated octahedral and jewel-like truncated octahedral primary Mg₂ Si crystal was investigated.5)Hot-extrusion and T6 heat treatment was conducted to Al-20Mg₂Si-4.5Cu-0.5（Ca-Sb）alloy and spherical primary Mg₂ Si particle was prepared;moreover,the existence of fine spherical primary Mg₂ Si played an important role in strengthening the alloy,the ultimate tensile strength（UTS）increased from ²27 to ³03 MPa,elongation to failure increases from ².4 to ³.4 %;also,the micro-hardness of the matrix increases from ¹42.4 to ¹57.1 Hv.The results achieved here will help to guide the spheroidization of reinforcement in light-weight alloy.