The Study On Microstructures And Properties Of Squeeze Casting Al-Zn-Mg-Cu Alloy

Al-Zn-Mg-Cu alloy is a kind of the low density,good hot working,ultra-high strength and good welding performance materials,which become one of the most important structural materials.Al-Zn-Mg-Cu alloy is widely used for structural components in critical aerospace,transportation applications.Al-Zn-Mg-Cu alloys are usually forged which result in high strength.However,the cost of forging is very high compared to the alternative casting route.Nevertheless,disadvantages of conventional casting are found in the material microstructure with existence of casting defects such as pores and shrinkage cavities,including lower strength.Squeeze casting is an advanced and efficient near net forming technology which combines forging and casting in a single operation.Squeeze casting Al-Zn-Mg-Cu alloy has the properties close to the forging level,while the squeeze casting process is simple,reliable,low cost,having good economic benefits.Hot cracks defects are easy to occur in the casting process which has become an urgent problem to be solved in the casting process of Al-Zn-Mg-Cu alloy with poor casting performance.In this thesis,the effect of RE on the microstructures and properties of cast Al-6Zn-2.5Mg-1.8Cu alloy is studied.The effects of squeeze casting process and heat treatment on the microstructures and properties of Al-6Zn-2.5Mg-1.8Cu alloy are studied.RE content has a significant effect on the microstructure and properties of Al-6Zn-2.5Mg-1.8Cu alloy.The addition of RE can refine the grain size,increase the number of second phases and improve the mechanical properties of the alloy.The addition of RE increases the number of liquid films on the grain surface with the crack propagation of the hot tearing bar propagate along the grain boundary.When the alloy contents 0.1wt.% RE,the alloy gets the finest microstructure with grain size of about 30.34?m.And the alloy gets the best mechanical properties with the tensile strength is 180 MPa,the elongation is 3% and the Brinell hardness is118.8HB.And the alloy has the lowest hot cracking tendency and the most number of liquid film.The critical fracture diameter of the hot tearing bar is 6 mm,and hot tearing sensitivity is1.03.The grain structure is refined,the shape is rounded and the mechanical properties are improved of the alloy by squeeze casting.The mechanical properties of the alloy are the best with the tensile strength,elongation and hardness are 315 MPa,4.8% and 133.8HB,respectively when the squeeze pressure is 500 MPa.Through an optimization of both the solution heat treatment and the artificial aging conditions applied to the alloy to improve the mechanical properties of the alloy.The optimum T6 heat treatment condition for squeeze casting Al-6Zn-2.5Mg-1.8Cu alloy is 470°C for 12 h and 120°C for 24 h.The coarse second phase dissolves into the matrix after solution treatment.The mechanical properties of the alloy are greatly enhenced after aging treatment with the tensile strength,elongation and hardness are 585 MPa,5.75% and 190.7HB,respectively.