Effects Of Al,Ca And Heat Treatment On Microstructure And Properties Of Mg-Si Series Alloys

As light alloys,Mg-Si series magnesium alloys have good development prospects in the aerospace and automotive industries.Mg-Si alloys are more cost-effective to form under gravity casting,if Al and Ca elements were added to the gravity casting Mg-Si alloy,the mechanical properties and corrosion resistance of the alloy can be improved,but the addition is the key factor,too much will reduce the properties of the alloy.In Mg-Si series alloys,Si content will directly affect the properties of Mg-Si series alloys,so the determination of Si content is also very important.In addition,the properties of Mg-Si alloy can be further improved by proper heat treatment after alloying.In this experiment,Al and Ca elements were added to Mg-Si alloy to form Mg-xAl-y Si-zCa alloy,in which x,y,z represented content,in order to explore the alloyed Mg-Si alloy with the best composition.The microstructure and properties of the alloy were studied by means of OM,SEM,EDS,XRD,DSC,microhardness tester,universal mechanical stretcher,electrochemical workstation and heat treatment furnace,so as to determine the appropriate Si content of Mg-Si series alloys and the optimum addition of Al and Ca.Finally,the effect of heat treatment process on the microstructure and properties of alloyed Mg-Si alloy with optimal composition was studied.The experimental results showed that the Si content mainly affected the size of the Chinese script type Mg₂Si phase,the more the Si content,the larger the Chinese script type Mg₂Si phase;the Al content affected the morphology and size of both the Mg₂Si phase and the Mg₁₇Al₁₂phase,the more the Al content can restrain the size of the Chinese script type Mg₂Si phase,and it can promote the transformation from the Chinese script type Mg₂Si phase to the blocky Mg₂Si phase in the case of containing Ca.With the increase of Al content,the Mg₁₇Al₁₂phase changed from granular to continuous strip,and when the Al content was 6%,the alloys were more likely to reach the eutectic composition of Mg₁₇Al₁₂phase.when the content of Ca element was changed,if the content of Al and Si was less,it only affected the number of strip CaMgSi phase.When the content of Al and Si were high and the value of Al/Si was greater than 0.7,the morphology of Mg₂Si phase will be transformed from Chinese script type to block.At the same time,the growth of Chinese script type Mg₂Si phase to block Mg₂Si phase will be affected by four factors,that is,the content of Si should reach a certain extent,the solid-liquid range should be narrow,the liquidus should be low,and there are heterogeneous nucleation points in the alloy solution.When the Mg₂Si phase in the alloy showed a fine Chinese script type shape,the mechanical properties of the alloy were the best,and the strength of the alloy containing block Mg₂Si phase was higher than that of the alloy containing coarse Chinese script type Mg₂Si phase.The results showed that Mg-4Al-0.25Si-0.3Ca alloy has the best comprehensive properties,the tensile strength was 174.3MPa and the elongation was 10.38%.If the mismatch between the second phase and the matrix is greater,the alloy was more likely to fracture on the second phase.In addition,the orientation of the slip system of the phases on both sides of the phase boundary was different,which will lead to the fracture in the second phase.With the addition of Ca element in the alloy,the Ca-containing phase in the alloy was first corroded to form a Ca₂SiO₄protective layer,thus improving the corrosion resistance of the alloy.After solid solution of Mg-4Al-0.25Si-0.3Ca alloy at 420℃for 10 h,solution at 500℃for 3 h and aging at 220℃for 7.5 h,Mg₂Si phase and CaMgSi phase were spheroidized,and when Mg₁₇Al₁₂was the same,the alloy precipitated continuously in the grain and discontinuously at the grain boundary.After heat treatment,the tensile strength of the alloy increased to 194 MPa,the corrosion current density decreased by 54%,and the elongation decreased slightly to 9.1%.