Research On Microstructure,mechanical Properties And Corrosion Behavior Of Cast AlSiMgCr Alloys

A356?Al Si7Mg0.4?is one of the most representative kinds of Al-Si-Mg cast aluminum alloy,which is widely used in the automobile industry to manufacture wheel hub,steering knuckle,engine cylinder head,etc.Recent studies have shown that reducing the Si content in Al-Si-Mg alloys can increase the amount of?-Al matrix.Through the adjustment of various nano-precipitated phases in the matrix,the mechanical properties of cast Al-Si-Mg alloys can be improved.Microalloying is an important way to achieve a variety of nano-precipitated phases,microalloying also has a certain effect on the corrosion resistance of aluminum alloys.Therefore,for the commonly used A356 aluminum alloy,this paper develops a new Al-3Si-0.5Mg-x Cr alloy by reducing the Si content,increasing the Mg content,and adding an appropriate amount of Cr element.Alloys were prepared by gravity casting and squeeze casting.The influence of Cr element on the microstructure,mechanical properties and corrosion resistance of Al-Si-Mg alloy was studied.The phase diagram and non-equilibrium solidification process of Al-3Si-0.5Mg-x Cr alloy were calculated by Thermo-Calc phase diagram software.Four kinds of Al-3Si-0.5Mg-x Cr alloys with different Cr contents were prepared by gravity casting.The heat treatment process of the alloy was established by DSC analysis and age hardening curve.The effect of Cr content on the microstructure,mechanical properties and fracture morphology of as cast and heat treated Al-3Si-0.5Mg-x Cr alloy was analyzed.The results show that when Cr content?0.1 wt.%,Cr element has no effect on the phase composition of the alloy,and when Cr content?0.2 wt.%,Al₁₃?Cr,Mn,Fe?₄Si₄ phase is formed in the alloy;after adding Cr element,the alloy formed Cr-containing nano-precipitates during solution treatment,which improved the strength of the alloy,and when the Cr content changed,the change law of the alloy strength after heat treatment is the same as the change law of the volume fraction of the Cr-containing nano-precipitates;when Cr content?0.1 wt.%,Cr element has no obvious effect on the fracture morphology of the alloy,and when Cr content?0.2 wt.%,Al₁₃?Cr,Mn,Fe?₄Si₄ phase leads to more secondary cracks.The alloy was prepared by squeeze casting,and the effects of squeeze casting on the microstructure,mechanical properties and fracture morphology of the Al-3Si-0.5Mg-x Cr alloy as-cast and after heat treatment were analyzed.The results show that,compared with gravity casting,squeeze casting greatly reduces the shrinkage defects in the alloy and improves the plasticity of the alloy;the fracture mode of as cast alloy changes from quasi cleavage fracture under gravity casting to mixed fracture with both brittle fracture and ductile fracture under squeeze casting;squeeze casting increases the number of dimples in the fracture after heat treatment.The effect of different content of Cr element on the corrosion resistance of Al-3Si-0.5Mg-x Cr alloy was analyzed by neutral salt spray corrosion test.The effect of Al₁₃?Cr,Mn,Fe?₄Si₄on the protective layer of surface corrosion products and on the corrosion resistance of the alloy were analyzed.The results show that when Cr content?0.1 wt.%,Cr element has no obvious effect on the corrosion resistance of the alloy,and when Cr content?0.2 wt.%,Al₁₃?Cr,Mn,Fe?₄Si₄ phase makes the corrosion resistance of the alloy decrease.