Research On Microstructure And Mechanical Properties Of Squeeze Cast Al-Mg-Si-Zn Alloys

Due to high specific strength of aluminum alloys,they have become one of the most important materials for the lightweighting of automobiles.Casting process is a main method for forming aluminum alloy components of automobiles.In order to improve the performance of cast aluminum alloys,Al-Mg-Si cast aluminum alloys have been paid great attention by researchers.Al-Mg-Si alloys not only shows good strength and ductility in the as-cast state but also can be further improved in their mechanical properties by adding other alloying elements(such as Zn)and using heat treatment.However,research on squeeze cast Al-Mg-Si-(Zn)alloy has not been reported.In this paper,the microstructure,mechanical properties and heat treatment process of squeeze cast Al-Mg-Si-(Zn)alloys are studied.The multi-component phase diagrams and phase transformation curves of Al-Mg-Si(-Zn)alloy were calculated with Thermo-Calc software,and the influence of Si and Zn element contents on the microstructure was predicted.Based on the calculated results,series of alloys with different compositions were designed and fabricated by squeeze casting process.The microstructure(including nano scale preticipates),mechanical properties and fracture characteristics of Al-Mg-Si(-Zn)alloys were observed,tested and analyzed.The effects of Si and Zn elements were revealed.What's more,the heat treatment process of the alloy was invesitated,and the microstructure,properties and fracture of the alloys after heat treatment were analyzed and discussed.The effect of Si elements on the microstructure and properties of squeeze cast Al-5Mg-xSi alloy was studied.The results have shown that the microstructure of the alloys is composed of primary ?-Al phase and Al3Mg2 intermetallics without Si elements.When the Si content ranges from 0.1 to 1.4 wt%,the alloy microstructure includes primary ?-Al phase,Al-Mg2 Si eutectics and Al3Mg2 intermetallics.When the Si content ranges from 1.5 to 2.3 wt%,the alloy microstructure is composed of primary ?-Al phase and Al-Mg2 Si eutectics.When the content of Si exceeds 2.4 wt%,the needlelike Si particles appear in the morphology.With the increase of Si content,the yield strength,tensile strength and elongation gradually decrease,and the fracture changes from ductile to mixed fracture mode.After solution treatment,the Al-Mg2 Si eutectics and the Al3Mg2 intermetallics are dissolved in the matrix.The yield strength and elongation increase obviously,but the tensile strength doesn't increase obviously.After aging treatment,?''(Mg5Si6)intermetallic precipitates in the matrix,and the number of precipitates increases with Si content,and the tensile strength significantly increases.The effect of Zn elements on the microstructure and mechanical properties of squeeze casting Al-5Mg-Si-xZn alloy was studied as well.The results have showen that the the microstructure of the alloys is composed of Al phase,Mg2 Si phase,Al3Mg2 intermetallic and AlMgZn intermetallic when the content of Zn ranges from 0.5 to 0.8 wt%.When the content of Zn exceeds 0.9 wt%,only Al phase,Mg2 Si phase and AlMgZn intermetallic remains,while Al3Mg2 intermetallic disappear in the microstructure of the alloy.With the increase of Zn elements,the content of AlMgZn intermetallic increases gradually.In the as-cast state,the tensile strength of the alloy increases with the increase of Zn elements.With the increase of Zn content,the tensile strength gradually increase,and the fracture mode is a mixed fracture which have both the ductile fracture and the brittle fracture.And the area of cleavage steps increases,while the area of dimples decreases with the increase of Zn content.After solution treatment,the Al-Mg2 Si eutectic and AlMgZn intermetallic are dissolved into the matrix.The yield strength,tensile strength and elongation of the alloy are enhanced.After aging treatment,needlelike or fibrous ?'(Mg5Si6)intermetallics and granular AlMgZn intermetallics precipitate in the matrix.The amount of AlMgZn intermetallics increases with Zn content.The tensile strength of the alloy is significantly increased,while elongation decrease obviously.After T6 heat treatment,the fracture mode of the alloy is ductile fracture.And the more Zn content,the smaller dimples after aging.