Study On Microstructure And Properties Of ZL702A Alloys Modified By Er And Sc

In recent years,modification and refinement of cast aluminum alloy through rare earth has become a research focus due to the high efficiency and convenience of improving properties of aluminum alloy through rare earth.Although Al-Si alloy has excellent casting properties,its limited mechanical properties seriously restricts its application in complex castings.On the basis of the traditional modification and refinement methods of Al-Si alloy,research on the modification and refinement of aluminum alloy through new rare earth elements to improve the properties of Al-Si alloy so as to broaden the application of high-performance Al-Si alloy in large and complex castings become popular.Based on ZL702A alloy,the effects of Er and Sc on the microstructure and properties of ZL702A alloy were systematically studied.The properties of ZL702A alloy were further improved through T6 heat treatment.Meanwhile,the mechanisms of refinement and strengthening were discussed.The microstructure of as-cast and conventional heat treated ZL702A alloy modified by Er and Sc were investigated.The effect and mechanism of Er and Sc on ZL702A alloy were discussed quantitatively by means of phase analysis,microstructure observation and composition determination.The results showed that Er and Sc could change the eutectic phase from a coarse and slender needle to a fine dendrite and a fiber in as-cast state.The grain refinement effect of Er was more obvious than that of Sc.The dendrite spacing were reduced to 27?m?44?m after modification with mixed addition of Er and Sc.The optimized additions of Er,Sc,mixed Er and Sc were 0.3wt%,0.35wt%and 0.25wt%Sc+0.2wt%Er,respectively.During T6 heat treatment,the silicon phase of ZL702A alloy could be changed from needle rod with round edge to point rod with smooth edge by adding Er and Sc alone or in combination,which had a strong effect on modification and refinement of the alloy.The optimized additions of Er,Sc,mixed Er and Sc were 0.3wt%,0.25wt%and 0.25wt%Sc+0.2wt%Er,respectively for the effect of metamorphism refinement after T6 heat treatment.The tensile strength of ZL702A alloy was improved significantly by Sc and the elongation was improved significantly by Er after conventional T6 heat treatment.The maximum tensile strength was 323MPa and the maximum elongation was 3.6%with the mixed adding contents were 0.25wt%Sc+0.2wt%Er.The tensile fracture surface presented a mixed fracture mechanism with both brittle and ductile characters accompanied with cleavage surface and dimple.The mixed additions of 0.25wt%Sc and 0.2wt%Er were the optimized modification proposal.The heat treatment scheme of ZL702A alloy modified by 0.25wt%Sc+0.2wt%Er was determined by DTA analysis and aging strengthening experiment.The optimized heat treatment schemes were 510? for 4 hours+525? for 8 hours+175? for 12 hours.Al3Sc/Al3Er particles with.the size of 0.05?m and 0.03?m dispersed and pinned in ?-Al matrix and took effects of strengthening obviously.The hardness of modified ZL702A alloy by 0.25wt%Sc+0.2wt%Er after optimized heat treatment was 136 HBW.