Researches On Residual Stress,Microstructures And Properties In Al-Zn-Mg-Cu Alloys Using A Cladding Quenching Process

Al-Zn-Mg-Cu aluminum alloy can obtain excellent comprehensive properties after heat treatment,however the quenching process after solid solution treatment can cause a large temperature variations between the surface and the core of the workpiece,which causes quench residual stress,especially in thick plates.The residual stresses have a great influence on the performance of the materials,for example,it will reduce the stability of the structure,and cause the changes of the shape and size of the workpieces.At the same time,the fatigue strength of the structure and the resistance to stress corrosion are also affected.In the paper,the effects of cladding quenching process on quenching residual stresses and other properties,and the microstructures of Al-Zn-Mg-Cu alloy thick plates were investigated.Besides,the mechanisms of cladding quenching process and the effects of the aging process on the properties of the alloys after the cladding quenching process were discussed.The main research results are as follows.(1)The quenching residual stress in Al-Zn-Mg-Cu alloys can be effectively reduced by utilizing the proposed cladding quenching process.The residual stress was reduced as the cladding layer thickness increased(0.2 mm-0.6 mm),and was almost eliminated under a cladding layer thickness of 0.6 mm,and The aged alloys exhibited superior mechanical properties and improved conductivities after the cladding quenching process.(2)The reduced residual stresses were associated with relieving the unbalanced thermal stresses by the cladding layer,which reduces the residual stresses originally.The properties of the alloy were maintained mainly because the cladded alloys could obtain sufficient quenching cooling rate because of the wrinkles and cracks on the cladding layer generated by the release of thermal stress,and then similar precipitation strengthening effects could be obtained in the subsequent artificial aging treatment.(3)In situ resistivity and hardness measurement showed that the cladding quenching process could affect the supersaturation solid solubility and the subsequent aging kinetics of the alloy.With the increase of cladding thickness(0.2 mm-0.6 mm),the aging precipitation behaviors of the cladded sample were closer to the conventional quenched sample.(4)The cladding quenching alloys required a relatively longer aging time to reach its peak mechanical properties comparing with conventional quenching alloys at a same artificial aging temperature,because the cladding quenching process can significantly reduce the residual stress and the intragranular dislocation density of the alloys,and the precipitation driving force could be also reduced.The hardness of the cladded quenched sample with the cladding thickness of 0.2 mm-0.6 mm could be closer than the conventional quenched sample after peak aging treatment.(5)TEM analysis shows that the cladded quenched alloys had lower dislocation density than the conventional quenched alloys.After the peak aging treatment,the cladded quenched alloy and the conventional quenched alloy had similar nano-precipitation characteristics of grain interiors and more discontinuous precipitates at grain boundaries.