Microstructure And Mechanical Properties Of Squeeze Casting Al-Cu Alloy

With the rapid development of aerospace and transportation,the demand for high-performance casting aluminum alloy products is also increasing.Al-Cu alloy has attracted extensive attention due to its excellent comprehensive mechanical properties.However,Al-Cu alloy has poor casting performance and serious composition segregation,which makes it difficult for using the traditional casting process to meet requirements of high-performance.It is an effective method to obtain dense microstructure and excellent comprehensive mechanical properties by adding an external field in the casting process of aluminum alloy,so as to improve the alloy casting performance and optimize the solidification structure.Squeeze casting technology,as an advanced casting technology to apply high pressure in the solidification section after punching,maybe a significant method to solve the casting problem of Al-Cu alloy with high-performance,and which makes researchers focus on it.In this paper,the casting Al-Cu alloy was taken as the research object.The influence of Cu content on the microstructure and properties under the conditions of the traditional casting and squeeze casting was clarified firstly.Then,the microstructure,solute distribution and their evolution under different heat treatment systems of Al-Cu alloy fabricated by squeeze casting process were systematically studied.And then the regularity and mechanism of mechanical properties variation were analyzed.It was found that the volume fraction of Al₂Cu phase increased significantly with the increase of Cu content in the metal mold casting process.However,when the Cu content was higher（6.07 wt.%）,although the volume fraction of Al₂Cu phase continued to increase,it was not completely redissolved in the solution treatment process.Therefore,there were a large number of residual Al₂Cu phases on the grain boundary.Thus,the strength and toughness effect cannot continue to be enhanced with the increase of Cu content.As a result,showing the properties trend of Al-Cu alloy increase at first and then decrease.Under the squeeze casting process,the influence of Cu content on the microstructure including Al₂Cu phase is similar as the traditional casting.But due to the scouring and crushing effect of the pressure during the solidification,the size of Al₂Cu phase is significantly refined.For the squeeze casting process,on the one hand,higher extrusion pressure can makethe the melt or solidification part contacting the mold more closely,which promotes the heat transfer efficiency and cooling rate.These improvements can refine the surface structure.At the same time,the continuous pressure can drive the heat movement from the biscuit with high-calorie to the solidified melt.Under the comprehensive influences,the grain structure of squeeze casting Al-Cu alloy shows a typical bimodal structure consisting of the surface fine grain zone and the core coarse grain zone.The grain size of the surface fine grain zone is in the range of 8–12μm,and the grain size of the core coarse grain is in the range of 32–40μm.In addition,the squeeze casting solidification microstructure shows obvious anti-segregation phenomenon,which is due to the combined effect of temperature gradient,negative pressure of solidification shrinkage and solute field caused by pressure.Compared with the metal mold casting process,theα-Al matrix grains and eutectic phases such as Al₂Cu are finer under the squeeze casting process,and the overall solidification structure is significantly refined,so the mechanical properties are more excellent.Al-6.07 wt.%Cu alloy was selected to systematically study the effects of solution and aging processes on the microstructure and mechanical properties of squeeze casting aluminium alloy.The results show that the grains in the surface fine grain region grow up and expand,which increases the proportion of fine equiaxed grains in the whole microstructure.However,heat treatment can not completely eliminate the bimodal characteristics of grain structure,characteristics of the as-cast microstructure behaves similarity.In addition,heat treatment further promotes the diffusion of alloying elements and weakens the abnormal segregation characteristics of as-cast alloy.Under the squeeze casting process conditions of 730°C pouring temperature,150°C mold temperature,130MPa pressure,and 10 h at 500°C solution treatment,13 h at 175°C aging treatment,the mechanical properties at room temperature of the obtained Al-6.07Cu alloy can reach tensile strength,yield strength and elongation of 343 MPa,270 MPa and 3.3%,respectively.