Research On The Effect Of Welding Thermal Cycle And Natural Ageing On Properties Of A7N01-T4 Alloy

A7N01 alloy is an Al-Mg-Zn-Cu alloy with high specific strength,which has been widely used in the body manufacturing of high-speed train industry at home and abroad.Since the car body structure must be joined using a welding process,the influence of the welding process on the material properties is an important consideration.Because of the influence of welding,the microstructure and properties of A7N01-T4 aluminum alloy welded joints will be seriously deteriorated compared with the base metal.The most serious degraded area is usually the heat affected zone in aluminum alloy welded joints,its mechanical properties and corrosion resistance are reduced.Because the distance between the deteriorated area and the center line of weld is close to the peak temperature of welding thermal cycle,the property of the degraded zone has a great relationship with the distance of weld center line.At the same time,the property of the heat-affected zone is gradually recovered during the natural aging period.It is necessary to study the effect of the peak temperature and natural aging time on the microstructure and property of the heat affected zone.This paper's study objective is A7N01-T4 aluminum alloy.The welding thermal cycle curve of different distance was tested by single-pass laser-MG hybrid welding method.The welding thermal cycle were simulated by the Gleeble-3500 Thermal Simulation Tester.The thermal cycle curve was used to obtain the microstructure of different peak temperatures of HAZ.The chemical composition and crystal structure of the precipitated phase were obtained by transmission electron microscopy(TEM).The precipitation ruler and thermodynamic behavior of the phase was analyzed by Differential Scanning Calorimeter(DSC);the mechanical properties such as tensile test and Vickers hardness test of the initial T4 and the thermal simulation specimens at different natural aging times were analyzed,and the laws of mechanical properties change with natural aging were obtained.The conductivity test?intergranular corrosion test and electrochemical test were carried out on them,and the corrosion behavior changed with natural aging were obtained.At last,the support vector machine method was used to perform regression analysis on the hardness change of the sample subjected by two thermal cycles during the natural aging period.Microstructure analysis results show that,the activation energy of ? in the A7N01 alloy was 164.17 kJ/mol.Based on DSC experiments,it is estimated that the solid solution temperature of ?' in the condition of thermal cycle with high-speed heating rate is below 300 °C,the formation and coarsening temperature of ? is between 300 °C and 400 °C,the solution temperature of ? is between 400 °C and 500 °C..At the same time,the orientation relationship between the ? precipitate and the matrix in the A7N01 alloy was clarified,and a ? ariant which orientation relationship is different with the conventional ? was observed.Mechanical properties results show that,quantitative data can be obtained by analyzing the distribution of precipitates in the TEM images of different peak temperature samples.Combining these indicators with hardness and strength results can analyze the change of mechanical properties of different thermal cycling samples during natural aging period.The hardness and strength decreases after thermal cycling and then increases with the natural aging time.When the value tends to be stable,the higher peak temperature leads to lower hardness and strength.Corrosion property analysis results show that,the peak temperatue 400 °C samples have better corrosion resistance in stress corrosion,intergranular corrosion and electrochemical corrosion than 300 °C and 500 °C at the first week of natural aging,it may be attributed to 400?sample with more coarse ? phase was formed during the thermal cycle.At the same time,the corrosion resistance of 300??400? and 500?sample was significantly improved with the natural aging time increased,recovery to the initial T4 sample.The SVM regression results show that,the c&g parameters are obtained by crossvalidation and used to train the model.The regression results have small MSE and MAE,and the R value is high,which indicates that the deviation between the predicted value and the real value is very small and the correlation is good.At the same time,the hardness predict value with the real value at the 7th week is very consistent,indicating that the model is well correlated and there is no sign of overfitting.