Microalloying Elements Controlling The Residual Crystal Phase And Mechanical Properties Of Al-Cu-Si Alloys

Al-Cu casting aluminum alloy has a wide application prospect in industrial production because of its high strength,high toughness and light weight.However,due to the wide solidification temperature range,large tendency of hot cracking,and it is easy to produce defects such as composition segregation and shrinkage porosity,the application of this kind of alloys is limited.It is found that Si element can improve the casting properties of Al-Cu casting alloy and reduce the hot cracking tendency.This promotes the exploration and development of new Al-Cu-Si series casting aluminum alloy.In addition,the introduction of microalloying elements is an important mean to improve the properties of aluminum alloys and develop new aluminum alloys.However,only few studies have been made on the effect of adding Er or Y on Al-Cu-Si alloys on the microstructures and mechanical properties of Al-Cu-Si series casting aluminum alloys.In the present thesis,Al-5Cu-1.5Si alloys with different contents of Er or Y were prepared by tilting casting.The effects of microalloying elements Er or Y on the microstructures and mechanical properties of Al-5Cu-1.5Si alloys were studied by means of DSC,SEM,EDS,TEM,deep corrosion and tensile test.The research findings are shown as follows:（1）The solidification crystalline phases of Al-5Cu-1.5Si casting alloy（original composition alloy）are mainlyα-Al matrix,bone-like Al₂Cu and lath-like Si.The elongation of the alloy is 4.6%and the tensile strength is 159.6 MPa.（2）With the addition of Er,the Al₂Cu and Si phases are significantly modified from wide bone-like and lath-like to coral-like,thus the mechanical properties of the alloy are significantly improved.At the same time,Er can promote the precipitation of Er-rich phase.When the Er content is about 0.1%,the short rod-like Al₃Er are precipitated.When the Er content is more than 0.1%,a large number of long needle-like Al₈Cu₄Er are precipitated.The suitable addition of Er is 0.1%.At this time,the elongation of the alloy is 9.1%and the tensile strength is 201.1 MPa.For alloy with Er content of 0.5%,the precipitation sequence of solidified crystalline phases isα-Al→Al₈Cu₄Er→Al₃Er→Binary eutectic（Al₂Cu+Si）.（3）With the addition of Y,not only Al₂Cu and Si phases are significantly modified, but also the grains are obviously refined.Y can promote the precipitation of Y-rich phase.When the content of Y is less than 0.3%,most of the Y-rich phase is in short rod-like Al₃Y,and the mechanical properties are good.When the content of Y is more than 0.3%,the Y-rich phase becomes long needle-like Al₆Cu₆Y,which makes the mechanical properties worse.The suitable addition amount of Y is 0.2-0.3%.When the Y content is 0.2%,the elongation can reach 9.3%,and when the Y content is 0.3%,the tensile strength can reach 205.7 MPa.For alloy with Y content of 0.5%,the precipitation sequence of solidified crystalline phases isα-Al→Al₆Cu₆Y→Al₃Y→Binary eutectic（Al₂Cu+Si）.（4）In the solid solution treatment,the modified Al₂Cu can be dissolved into the matrix more quickly.However,the addition of Er or Y will reduce the copper content in the solid solution alloy matrix.The morphology evolution of bone-like Al₂Cu and lath-like Si,coral-like Al₂Cu and Si,and needle-like Er/Y-rich phases during solid solution are shown as follows:The bone-like Al₂Cu is broken and transformed into massive particles,then gradually spheroidized until dissolved.The lath-like Si gradually dissolves and transforms into small near-spherical Si（with good roundness）,and then tends to aggregate and grow into massive Si.Coral-like Al₂Cu tends to spheroidize directly until it dissolves.Coral-like Si will first dissolve and transform into finer near-spherical Si,and then tend to aggregate and grow into massive Si.The needle-like Er/Y-rich phase will gradually dissolve and transform into many fine particles,and then remain basically unchanged.Under the experimental conditions,the better solid solution treatment parameter is 515°C×8 h.（5）The residual crystalline phases in the original composition alloy are mainly Si and Al₂Cu.The residual crystalline phase in the alloy modified by Er/Y are mainly rich in Er/Y phases,Si and a small amount of Al₂Cu.For the original composition alloy,the residual crystalline phases Si and Al₂Cu have great influence on the mechanical properties of the alloy.For 0.1%Er alloy and 0.2%Y alloy,the residual crystalline phase Si has a great influence on the mechanical properties of the alloy.For 0.5%Er alloy and 0.5%Y alloy,the residual crystalline phase rich in Er/Y has a great influence on the mechanical properties of the alloy.Under the condition of solid solution at 515°C×8 h and aging at 140°C×12 h,the tensile strength of the original composition alloy is 280.9 MPa,and the elongation is 11.1%.The tensile strength of 0.1%Er alloy is 310.1 MPa,and the elongation is 11.5%.The tensile strength of 0.2%Y alloy is 305.9 MPa,and the elongation is 11.3%.