Study On Residual Stress Test Of High Strength Aluminum Alloy Variable Polarity Plasma Arc-MIG Hybrid Welding

Variable polarity plasma arc-metal inert gas?VPPA-MIG?hybrid welding is with the advantages of high welding efficiency,wide range of technological parameters and high penetration depth,thus it has obvious advantages to weld thick plate high strength aluminum alloys.The welding method is one kind of hybrid heat source welding.Since the arc energy is concentrated and the welding seam of thick plate is formed at one time,the distribution of residual stress in the welded joint area is complex,which has a certain impact on safety production.However,there are rare reports on the research of residual stress of thick plate high strength aluminum alloy VPPA-MIG hybrid welding.An integrated residual stress testing method of hole-drilling method and modal test method is proposed in the thesis,and the characteristic of residual stress distribution of 7A52 high strength aluminum alloy VPPA-MIG hybrid welding is revealed.The result has important research significance and application value for the thorough analysis of the regularity and mechanism of high energy beam hybrid welding residual stress distribution.In order to meet the needs of the test and analysis for the residual stress of high strength aluminum alloy VPPA-MIG hybrid welding,a residual stress measurement system based on hole-drilling method consists of NI data acquisition card,industrial control computer,virtual instrument Lab VIEW,hole-drilling device and so on.The residual stress measurement system can provide a guarantee for the measurement of residual stress of VPPA-MIG hybrid welded joint of high strength aluminum alloy and the error analysis of welding residual stress measured by hole-drilling method.Besides,a measurement system of modal test consists of INV 931X series force hammer,INV 9824 ICP acceleration sensor,CS3062T0-24 bit cloud intelligent acquisition analyzer,and DASP modal analysis software.The measurement system of modal test provides a strong support for the measurement of welding residual stress by modal test method.Based on the principle of residual stress measurement by hole-drilling method,the error chain affecting the accuracies of final residual principal stress?₁,?₂and angle?has been analyzed by error transfer principle.Through the error experiment and analysis of the residual stress of 7A52 aluminum alloy VPPA-MIG hybrid welding,it is found that the error caused by elasticity modulus,strain gauge pasting and strain reading value time have great influence on the residual principal stress and angle?.The results show that the maximum relative error transferred from elasticity modulus error to?₁and?₂can reach up to 5%,and the elasticity modulus error can be corrected by the curve that is fit to data measured in different zones of VPPA-MIG hybrid welded joint.The relative errors transferred from the error of strain gauge pasting to residual principal stress?₁,?₂and angle?can be ignored after about 24 hours of strain gauge being pasted,while the relative errors generated by strain reading value time error are reduced to 0 after 150 minutes of drilling.After correcting the measurement error of hole-drilling method,the distribution characteristics of residual stress of VPPA-MIG hybrid welded joint of 10 mm thick 7A52aluminum alloy were measured.The results show that the stress in each zone of the welded joint is basically symmetrical distributed with respect to the center line of weld seam,and there is a phenomenon of double peaks.In addition,the peak values of the transverse residual stress and the longitudinal residual stress reach up to 78.2 MPa and 223.3 MPa respectively.The above experimental results can lay a foundation for the measurement of welding residual stress by modal test method.The mechanism of measuring residual stress by modal test was studied using vibration theory,and the influence rule of 7A52 aluminum alloy VPPA-MIG hybrid welding residual stress on natural frequency was obtained.The results show that the residual stress of VPPA-MIG hybrid welding can reduce the natural frequency,and the larger the peak value of residual stress is,the greater the decrement of natural frequency is.Through the modal test,it is found that,due to the existence of the hybrid welding residual stress,the test measured value of the natural frequency will also be reduced.when the orders of natural frequency are 3,4 and 5,the difference between the theoretical calculated value and the measured value of modal test are relatively small,and the natural frequency obtained are relatively reliable,which can be used to establish the mathematical model of the modal test method for measuring the residual stress of hybrid welding.Based on the method of quadratic polynomial fitting,the fitting coefficients of the 3rd,4th and 5th order natural frequencies and hybrid welding residual stress were compared and analyzed.It is found that the fitting precision of 5th order natural frequency and hybrid welding residual stress is the highest.Hence the testing mathematical model of hybrid welding residual stress is established based on modal test according to the 5th order natural frequency.The residual stress distribution of 7A52 aluminum alloy VPPA-MIG hybrid welding plate for 6 mm and10 mm thickness were obtained based on the testing mathematical model.The results show that the distribution characteristics of residual stress measured by modal test method in each zone of the hybrid welded joint are consistent with the results measured by hole-drilling method.Therefore,the established mathematical model can guarantee the reliability of the measurement results of the modal test method.The influence of the ratios of VPPA and MIG arc energy on the distribution of welding residual stress in the process of 7A52 aluminum alloy VPPA-MIG hybrid welding was analyzed by modal test method.The results show that the peak value of hybrid welding residual stress increases with the increase of the ratio of VPPA arc energy.Considering the appearance of weld forming,the parameters of VPPA-MIG hybrid welding of 7A52 aluminum alloy based on the distribution characteristic of residual stress are optimized.When the ratio of VPPA arc energy is selected between 35%and 40%,the peak value of transverse residual stress is no more than 73.3 MPa and 92.0 MPa for 6 mm and10 mm thick plate respectively,and the peak value of longitudinal residual stress is no more than 231.7 MPa and 234.3 MPa for 6 mm and 10 mm thick plate respectively.It shows that the VPPA-MIG hybrid welding with optimized parameters can not only produce weld joint with satisfied macroscopic appearance,but also avoid high residual stress peak.