| Large-size Al-Zn-Mg-Cu alloy is a key structural material in modern aerospace industry.The supersaturated solid solution obtained by rapid quenching can precipitate a large number of dispersed phases during aging process and strengthen the alloy,but large residual stress,which seriously affects its subsequent machining and final performance,also produced on the rapid quenching process.In order to solve the contradiction between residual stress and mechanical properties,a novel cladding quenching method was proposed in this paper.The influence of cladding quenching method on the quenching cooling rate,residual stress,and properties and microstructure of Al-Zn-Mg-Cu alloy thick plates was studied,and the mechanism of the cladding layer on the quenching cooling rate was determined.A finite element model for cladding quenching was further established.Combined with the changes of temperature field,stress/strain field and time-temperature-performance curve,the mechanism of cladding quenching method to reduce residual stress and maintain performance was explored.By changing the thickness of the Al-Zn-Mg-Cu alloy thick plate and studying the cladding quenching residual stress of the stepped components,the effective application range of the cladding quenching process was expanded,and the corresponding optimal cladding quenching process was determined.Based on the ability of cladding quenching to control the quenching residual stress,the effect of residual stress on the properties and microstructure of Al-Zn-Mg-Cu alloy after aging process was studied.The mechanism of residual stress on the precipitation behavior of Al-Zn-Mg-Cu alloy was explored by combining stress aging experiment and in-situ resistivity test.The main results of this paper are as follows:(1)Based on the traditional and cladding quenching experiments and cooling curve test,the influence of cladding quenching on the quenching cooling curve of AlZn-Mg-Cu alloy was studied,and the mechanism of cladding layer on quenching cooling rate during cladding quenching was clarified.The results show that the cladding quenching method can significantly reduce the average quenching cooling rate of Al-Zn-Mg-Cu alloy.According to change of the cladding layer and the cooling curve,the cladding quenching process was divided into three stages:the complete stage,the cracking and shedding stage and the stable stage of the cladding layer.The complete stage of the cladding layer could significantly reduce the cooling rate of the alloy,and the larger the thickness of the cladding layer,the lower the cooling rate;the cracking and shedding stage of the cladding layer would lead to an increase in the cooling rate,and its cracking point and duration were also affected by the thickness of the cladding layer.The stable stage of the cladding layer is in the later stage of quenching,and its cooling rate could decrease slightly.(2)Based on the residual stress test results of hole-drilling method and finite element simulation,the influence of cladding quenching method on the quenching residual stress of Al-Zn-Mg-Cu alloy thick plate was studied.The results show that cladding quenching can significantly reduce the quenching residual stress,and with the increase of cladding thickness,the elimination effect of residual stress was better.At the same time,as the thickness of the Al-Zn-Mg-Cu alloy thick plate increased,the optimal cladding thickness required to reduce the residual stress(a decrease of about 90%)also increased.When the plate thickness was 10 mm,the optimal cladding thickness was 0.6 mm,and the residual stress could be reduced by more than 90%.When the plate thickness was 40 mm,the appropriate cladding thickness range was 1-1.4 mm,and the residual stress could be eliminated by 88-93%.When the thickness of the plate increased to 160 mm,the simulation predicts that the cladding quenching process with a cladding thickness of 1.4 mm can still eliminate more than 60%of the residual stress,and the optimal cladding thickness should be above 1.4 mm.(3)Based on the micro structure characterization and mechanical properti e s measurements,the effect of cladding quenching on the micro structure and properties of Al-Zn-Mg-Cu alloy was studied.Combined with the alloy time-temperatureperformance curve and finite element simulation results,the mechanism of cladding quenching method significantly eliminating residual stress and maintaining alloy properties was proved.The results show that in the early stage of quenching,the cladding quenching could reduce the temperature difference between the inside and outside of the alloy and delay the time of the maximum temperature difference,and then the thermal stress caused by the temperature gradient between the inside and outside of the alloy was significantly reduced,resulting in only very little or even uneven plastic deformation would be produced in the quenching process.Therefore,the residual stress after quenching was significantly eliminated.In the sensitive temperature range of alloy precipitation in the middle of quenching,the cracking and shedding of the cladding layer would lead to the increase of quenching cooling rate.The solute atoms in the supersaturated solid solution were too late to precipitate or the degree of precipitation was low,which would not affect the precipitation behavior in the subsequent aging process,thus the alloy performance could maintain.(4)The complex shaped Al-Zn-Mg-Cu alloy components were simplified by three-stage stepped parts with different section thicknesses,and the correlation between section thickness and quenching residual stress was determined.The mechanism of section thickness on quenching residual stress was revealed by finite element simulation.The results show that the maximum residual tensile or compressive stress after traditional quenching was positively correlated with the thickness of the section,that is,with the increase of the thickness of the section,the maximum residual tensile or compressive stress of different step samples increased.When the section thickness was 10 mm,20 mm and 40 mm,the maximum residual compressive stress in the corresponding rolling direction was-40.8 MPa,-177.7 MPa and-339.4 MPa,respectively.The maximum residual tensile stresses were 66.7 MPa,235.4 MPa and 381.8 MPa,respectively.The maximum transverse residual compressive stresses were-57.3 MPa,-214.7 MPa and-312.3 MPa,respectively.The maximum residual tensile stresses were 57.0 MPa,191.9 MPa and 304.0 MPa,respectively.In addition,the cladding quenching method can also significantly reduce the quenching residual stress of the stepped parts.When the thickness of the cladding layer was 1.4 mm,the residual stress of the stepped parts can be reduced by more than 80%,and the mechanical properties of the alloy can be guaranteed after aging.(5)Combined with the regulation of cladding quenching on residual stress and reasonable design experiments,the evolution of microstructure and mechanical properties of Al-Zn-Mg-Cu alloy with residual stress during aging process was revealed,and the influence mechanism of residual stress on precipitation behavior was further explored.The results show that the elastic lattice distortion caused by residual stress can promote the nucleation of precipitated phases during aging,and promote the formation of a large number of fine and dispersed GPII zones and η’precipitated phases.Therefore,the existence of residual tensile stress in the alloy core would increase the hardness of the alloy core after aging process.However,in the early stage of aging process,the partial release of residual stress in the alloy surface after heating would lead to the generation and movement of dislocations,which could promote the local generation of coarse η phase,resulting in the decrease of alloy properties.Therefore,the hardness of the alloy surface with quenching residual compressive stress decreased after aging process.In addition,this effect was more obvious with the residual stress increase. |
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