Study On The Effect Of Trace Rare Earth Elements On Microstructure And Properties Of Aluminum-Zinc Alloy

In this paper,aluminum-zinc binary alloy was prepared by ingot metallurgy method.On the basis of this alloy,five different compositions of Al-32%Zn-0.35% Zr-0.15%Cr-0.12%Ti-XCe alloys(X=0,0.05,0.1,0.2,0.3)and five different compositions of Al-32%Zn-0.35%Zr-0.15%Cr-0.12%Ti-XEr alloys(X=0.05,0.1,0.2,0.3,0.4)were prepared through the same process.The effects of rare earth elements Ce and Er and heat treatment on the microstructure and properties of aluminum-zinc alloy were respectively investigated by means of using optical microscope,differential scanning calorimetry,X-ray diffraction,Vicker's hardness,scanning electron microscope and energy dispersive spectroscopy,as well as tensile and damping properties testing.The following main conclusions are obtained.(1)The addition of trace alloying elements Zr,Cr and Ti in Al-32%Zn alloy made the as-cast microstructure of the alloy significantly refined.The shape of coarse dendritic disappeared and was transformed into fine and uniform equiaxed crystal grain.The average size of the grains was reduced from 550?m to 90?m.The tensile strength,hardness and ductility of the alloy achieved 251 MPa,111HV and 6.1%,which increased by 6.8%,8.8% and 10.9% compared with those of without adding Zr,Cr and Ti alloy.The hardness of Al-32%Zn-0.35%Zr-0.15%Cr-0.12%Ti alloy arrived at 144 HV after heat treatment at 420?×3h + 110?×2h,which was 29.7% higher than that of the untreated alloy.(2)After the addition of rare earth element Ce to Al-32%Zn-0.35%Zr-0.15% Cr-0.12%Ti cast alloy,the as-cast grain of the alloy was obviously refined.The equiaxed grain size of the alloy has a tendency to decrease first and then increase with the increase of Ce content,and Ce is present in the form of a block containing Ce intermetallic compound in the alloy.When the addition of 0.1%Ce,the average grain size of the alloy is about 35~45?m,and its tensile strength,ductility,damping performance and hardness respectively arrived at 280 MPa,6.7%,0.043(A=800×10-6)and 121 HV.Comparing with those of without adding Ce alloy,its properties increased by nearly 11.6%,9.8%,34.4% and 9.0%.After heat treatment at 420 ? ×3h+110 ? ×2h,the peak hardness of Al-32%Zn-0.35%Zr-0.15% Cr-0.12%Ti-0.1%Ce alloy reached 148 HV.(3)After the addition of rare earth element Er to Al-32%Zn-0.35%Zr-0.15% Cr-0.12%Ti as-cast alloy,the as-cast grain of the alloy was clearly refined.The equiaxed grain size of the alloy shows a tendency to decrease first and then increase with the increase of Er content,and Er partially exists in the form of a block containing Er intermetallic compound in the alloy and is partially dissolved in the ? matrix.When the addition of 0.2%Er,the grain size of the alloy is the smallest and about 25~35?m.its strength of extension,ductility,damping performance and hardness arrived at 333 MPa,8.0%,0.061(A=800×10-6)and 127 HV,compared with the those of without containing Er alloy,respectively,which increased by nearly 32.6%,31.2%,90.6% and 12.7%.After heat treatment at 420 ? ×3h+110 ? ×4h,the hardness of Al-32%Zn-0.35%Zr-0.15% Cr-0.12%Ti-0.2%Er alloy reached 153 HV.The addition of rare earth element Er slows down the time at which the alloy reaches the peak of aging hardness.(4)The effect of rare earth element Er on the improvement of as-cast microstructure and mechanical properties of Al-32%Zn-0.35%Zr-0.15%Cr-0.12%Ti alloy is better than that of rare earth element Ce.