| The object of this research is a new type,high strength,weldable Al-Zn-Mg-Zr-Sc aluminum alloy.The alloy were studied under homogenization heat treatment system,solution and aging treatment,in which the aging processing was carried out with three different treatments of single-stage aging,two-stage aging and retrogression and reaging treatment(RRA),to explore the effects of different heat treatment on the microstructure and properties of alloys,eventully to find out the optimum heat treatment process of the alloy.Semi-continuous casting Al-Zn-Mg-Zr-Sc aluminum ingots to small equiaxed grain,but the grain size is not homogeneous,generally the grain gradually increases from the outside to the center part.There is also dendritic segregation,ingot residue stress,which would leading to the crack in the subsequent cutting process.The alloy ingot cast's overburnt temperature determined by DSC analysis is 476.3?.After the homogenization process for the alloy,organizational segregation was substantially eliminated,while the non-equilibrium phase solubility and conductivity gradually increased along the homogenization time.Eventually,this research found the best single-stage homogenization system of the alloy was 460?/20h.The conductivity and hardness of the alloy gradually increased along the aging time,while the "double peak" phenomenon for the tensile strength also appeared.The best single-stage aging treatment for the alloy was 120?/48h.The tensile strength under this treatment was 540.4MPa,and the stretch was 12.9%,while the electrical conductivity was 39.9%IACS,and hardness is 165.4HV.This research also found that relative importantce of four major factors in determinating the properties and conductivity of Al-Zn-Mg-Zr-Sc in two-stage aging treatment though a series of orthogonal experiment.The order was the final aging temperature T2>the final aging time t2>the pre-aging temperature T1>the pre-aging time t1.However,the order turns into the final aging temperature T2>pre-aging temperature Tl>pre-aging time t1>final aging time t2 when it comes to the contribution to the alloy hardness.This series experiment showed that the best two-stage aging treatment was 95?/6h+160?/8h.The tensile strength of the alloy under the treatment was 531.8MPa,and the elongation was 13.5%while conductivity was 41.3%IACS and hardness was 151.5HV.The comprehensive properties of the alloy bar were enhanced with increasing temperature in lower regression temperature condition for the RRA process.The strengthening alloy phase were mainly ?' phase and fine ? phase.However,the recrystallization of the alloy tended to take place,while the precipitation phase prone to shift from ?'phase to coarse ? phase under the higher regression temperature.This would reduce the hardness of the alloy.The tensile strength and hardness of alloy decreased when the regression temperature kept increasing when the regression temperature was too high.Eventually,this research found the optimum RRA treatment for alloy bar was 120?/24h+220?/15min+120?/24h.The alloy strength was 542.7 MPa and elongation was 12.9%,while the conductivity was 40.4%IACS and hardness was 140.2HV under this treatment.The resistance to intergranular corrosion performance of the alloy gradually increased with the aging tim in a single-stage aging process.The resistance of the alloy crystal corrosion performed almost the same for the alloy optimized two-stage aging state and optimal RRA state(corrosion grade 3),and both of them was better than the single-stage aging peak state(T6,corrosion grade 4). |
PDF Full Text Request