The Study On The Microstructure And Mechanical Properties Of LC4 Alloy For Squeeze Casting

In the field of equip and manufacture industry, for the sake of economizing material and reducing energy sources depletion, lightening the weight of components is the target that the people pursue over a long period of time, is also one of development trend of modern manufacturing technique The lightweighting can be realized by adopting light metal or higher strength material. The super high strength of Al alloy colligates the two ways of lightweighting. According to the project with Beijing Nonferrous Metal Research Institute, via to form Al-Zn-Mg-Cu alloy castig by squeeze casting, the effects of content of elements, processing parameter and heat treatment on microstructure and mechanical properties of LC4 castig made by squeeze casting are studied.The content of alloy is determined through optimization of alloy composition and modification treatment. The content of elements is as followed(as): Zn: 6, Mg: 2.2～2.5, Cu: 1.4～1.6, Mn: 0.2～0.3, Cr: 0.15～0.20, Ti:0.05～0.10, Al: remains. The typical mechanical properties with the element content after T6 heat treatment are: R_m=520～540MPa, R_p^0.2=495～510MPa, A=2.0～3.5%. (Metal Mould Casting)The study was mainly focused on the effect of squeeze pressure on the microstructure and mechanical properties of casting. The results showed: when its value exceeds 30MPa, deficiency such as shrinkage porosity was reduced. When its value exceeds 110MPa, the squeeze pressure obviously influence the crystal performances of liquid metal and modify the microstructure of casting. The optimum squeeze pressure was 110MPa through testing. The minimum time of pressure holding is 40s through calculation.Squeeze casting can apparently ameliorate the macrostructure of casting. Under squeeze pressure the macrostructure is compacted without serious deficiency such as shrinkage cavity, shrinkage porosity, pore and so on. Squeeze casting also can ameliorate gravity segregation and microsegregation and decrease the segregation proportion of Zn, Al, and Cu. Squeeze casting can also apparently ameliorate the microstructure of casting. The microstructure changes to small equiaxed crystal from bigαcrystal after squeezing. The dendrite arm spacing (DAS) and grain size apparently decrease. Under squeeze pressure the mechanical properties of difFerent parts of the same cast still appears some discrepancy. The mechanical properties of the upper part and lower part are higher than that of the middle part. The mechanical properties are decreased from outboard to inside.The heat treatment process of casting was optimized through orthogonal test. The test result was analyzed with comprehensive scoring. The best heat treatment process is: 475℃×10h+130℃×16h. Under best heat treatment process, the mechanical properties of casting can attain the level: R_m=560～580MPa,R_p^0.2=465～485MPa,A=8.0～10%.