Design Of Al-Si Hypoeutectic Alloy Suitable For Combined Casting And Forging Process

Aluminum alloy has the characteristics of low density,good casting performance and high specific strength.It is often used in the production of automobile industry.The structure of automobile steering knuckle is complex,the performance of forged parts is good,but the process is complex.The casting process can form parts with complex shape at one time,but it is prone to various casting defects.Therefore,the combination of casting and forging can simplify the forming process,improve the product performance and reduce the production cost.It can be used in practical application and has a market prospect.At present,the existing alloy system is mostly used for the aluminum alloy materials of the casting forging combined process of prefabricated billet + forging at home and abroad,and there is no detailed research report due to the confidentiality of the manufacturer's technical details.Therefore,developing a new type of aluminum alloy material suitable for casting forging combined process,obtaining the influence law of various elements on alloy properties,and studying its microstructure and property evolution law in the process of casting forging,not only has the value of theoretical research,but also can provide reference basis for the actual production of aluminum alloy casting forging combined process.In this paper,the Al-Si-Cu-Mg alloy system is preliminarily selected.Through the orthogonal table design composition of four factors and three levels,the optimal composition of Al-Si-Cu-Mg alloy is optimized.After comparing and different content of Mn,the effects of Si,Cu,Mg and Mn elements on the mechanical properties and microhardness of the alloy are studied.For the optimized Al-7Si-0.4Cu-0.6Mg-0.2Mn alloy,the combined process of casting and forging was studied by rolling instead of forging.The effects of homogenization treatment and rolling process with different reduction on its microstructure and mechanical properties were studied.In addition,the effects of solution treatment and aging treatment on the microstructure and mechanical properties of the above alloys were also studied.On this basis,Al-7Si-0.4Cu-0.6Mg-0.2Mn alloy was used for the casting and forging process of automobile steering knuckle.Low pressure casting billet ? homogenization treatment ?forging ? T6 treatment were adopted.Samples were taken at different positions of steering knuckle to analyze the relationship between mechanical properties and microstructure at different positions of steering knuckle.The automobile steering knuckle produced with A356 alloy was also compared and analyzed.The main research results are as follows:(1)According to the orthogonal experimental analysis,the content of Mg is the most significant factor affecting the yield strength,tensile strength and elongation.The content of Si is the most significant factor affecting the microhardness.The composition of Al-7Si-0.6Mg-0.4Cu was comprehensive optimizated.The addition of Mn significantly improved the yield strength and tensile strength.The Al-7Si-0.4Cu-0.6Mg-0.2Mn alloy system was comprehensively optimized through the properties of different Mn contents.(2)For the Al-Si-Mg-Cu alloy,the microhardness is not significantly affected by the reduction.And the yield strength and tensile strength fluctuate when the reduction is small.When the reduction reaches more than 30%,the yield strength and tensile strength both increase with the increase of the reduction,and the elongation after fracture decreases slightly and then increases significantly with the increase of the reduction.(3)Homogenization treatment can change the morphology of eutectic Si of Al-7Si-0.4Cu-0.6Mg-0.2Mn alloy from fibrous and short rod-like to nearly spherical.The average shape factor S increases from 0.56 to 0.72.At the same time,the average size becomes larger.The homogenization also improves the segregation of other alloying elements except Si and Mn in the alloy.The strength and elongation are also improved.Hot rolling with different reductions improve the overall mechanical properties of the Al-7Si-0.4Cu-0.6Mg-0.2Mn alloy by changing the internal lattice arrangement of the primary ?-Al and fragmenting the Al Si Mn Fe phase.With the increase of the reduction,the strength of the alloy increases while the elongation decreases.No obvious dynamic recrystallization is observed during the hot rolling process.(4)The grain size with a reduction of more than 30% of alloys can be reduced significantly by solution treatment.And the proportion of recrystallized grains was increased;for solid solution alloys with different reductions,the difference in strength is not obvious,and the elongation varies with the reduction.The reduction increases with the increase.The peak value of microhardness after aging at 170? appears at 6h.Compared with the solid solution state,the strength of the T6-treated alloy is generally increased by more than100 MPa.And the elongation is decreased by more than 10%.Increasing the reduction of the T6-treated alloy can improve the strength and plasticity at the same time,T6 alloy under 50%reduction can comprehensively improve the strength and toughness of Al-7Si-0.4Cu-0.6Mg-0.2Mn alloy.The properties are 321 MPa of yield strength,384 MPa of tensile strength,13.7% of elongation.(5)Different sampling positions on the steering knuckle have obvious effects on the microstructure and mechanical properties of the alloy.The properties of the branch structures of different sizes and the thicker parts of the body are different.There are still unimproved casting defects,and the forging process needs to be further optimized.The comprehensive properties of the alloy after forging are significantly improved,and the performance improvement of the branch structure is not as significant as that of the thicker part of the body.Compared with A356,the yield strength and tensile strength of Al-7Si-0.4Cu-0.6Mg-0.2Mn are increased by about 24?58MPa.And the elongation is only reduced by 0.1?0.8%.The Al-7Si-0.4Cu-0.6Mg-0.2Mn alloy designed in this paper is not only fully suitable for the production of combined casting and forging process,but also has better comprehensive mechanical properties than the commercial A356 alloy used in the combined casting and forging process.