Effects Of Casting And Heat Treatment Processes On Microstructure And Properties Of Al-Zn-Si-Cu Alloy

Aluminum（Al）alloys are widely used in aerospace,transportation,electronic communications and other fields due to their advantages such as light weight,high specific strength,high thermal conductivity,and recyclability.For the application as new energy vehicles and microcomputer cooling shells in emerging fields,the die-casting A380 alloy（Al-Si-Cu alloy）for the thin-walled precision castings still faces some challenges included the low heat transfer and insufficient mechanical properties.In general,there are two methods to solve these problems:（1）alloy composition optimization,and（2）preparation processing adjustment.Among the alloying elements,Zn can improve the mechanical properties and deformability of alloy.On the other hand,increasing the cooling rate to could increase the undercooling of melt,and thus achieve the structural refinement.Therefore,the effects of different Zn contents and casting processes on the microstructure and properties of casting Al-Zn-Si-Cu alloy were studied in this paper.In addition,the properties of alloys were further optimized by the follow-up heat treatment processing.Firstly,the microstructure results of the as-cast alloys showed that there exist three phases in the gravity-cast specimen including the Al matrix,bulk and lath Si phases,and a small amount of Al₂Cu phase distributed along the grain boundaries.The addition of Zn has no obvious effect on the type,morphology and distribution of the second phases in the microstructure.For the rapidly solidified alloy specimen,the microstructure was obviously refined and the distribution of the second phases become more uniform.Moreover,the Si phase transforms into cluster structure,while the Al₂Cu phase shows a discontinuous network distribution.The effects of alloy composition and cooling rate on the change of microstructure were studied.Secondly,the results of mechanical and thermal conductivity of as-cast alloys showed that the addition of Zn improves the strength and plasticity,however,has little effect on the fluidity and thermal conductivity for the gravity-cast specimen.For the rapidly solidified specimen,the texture type,tensile strength and plasticity,hardness,the fluidity increase,while the texture strength,thermal conductivity decrease.Combined with the microstructure and fracture characteristics of the alloy,the strengthening mechanism and the changes of fluidity and thermal conductivity were analyzed.Finally,the microstructure and properties of the heat-treated alloys showed that the Si phase transforms into the spherical structure,while the discontinuous network Al₂Cu phase become much finer and distributes at the grain boundaries.The long-strip grains change to the equiaxed ones,while the spherical and strip Al₂Cu phases with an average size of 40 nm-100 nm precipitate.The texture strength increases,which lead to the decrease of the tensile strength and the incrase of the tensile plasticity.The fracture mode gradually transforms from brittle fracture to ductile fracture.The fraction of the small-angle grain boundary as well as the dislocation density decrease,while the mcirostructure of alloy becoms more uniform.This results in the decrease in the heat transfer barrier,and thus an increase in the thermal conductivity.