Basic Study On Ultrasonic Measurment Of Welding Residual Stress Based On Acoustic Elasticity Theory

The quality of welding structure runs through every link of the welding process.It is not only the key to ensure the safety and high efficiency of welding products into the market cycle,but also the key to promote the sustainable development of welding technology in the industrial field.Welding residual stress is the key factor that affects the quality of welding structure,so the measurment of welding residual stress is a feasible method to achieve the quality assurance of welding structure.However,there are some problems in the measuring methods of welding residual stress at present,so it is urgent to explore a suitable method to measure the welding residual stress.In this paper,the welding method of stainless steel,carbon steel and aluminum alloy is taken as the object of study.Based on the acoustic elasticity theory of critical refraction longitudinal wave(L_cr wave),the technical method and mechanism of L_cr wave to evaluate its residual stress are discussed.The influence of grain size on the acoustic elasticity coefficient of L_crwave is prepared and discussed by means of heat treatment.The influence of grain size on the elastic modulus of L_cr wave is obtained by the heat treatment process.And the elasticity coefficient of L_crwave,and the mechanism of the grain size on the acoustic elasticity coefficient of L_cr wave is clarified by the theory of elastic-plastic deformation.The welding joint with different structure forms is obtained by different welding methods,A modified method of evaluating the residual stress of welding by L_cr wave is proposed,and the residual stress of welding is evaluated by L_cr wave.Based on the acoustic elasticity theory of L_cr wave,the propagation time indifference stress theory formula for L_cr wave evaluation of welding residual stress is deduced and the optimization study of cross-correlation length is realized.The results show that the cross-correlation step is a key factor affecting the calculation of ultrasonic time difference.With the increase of the cross-correlation step length,the accuracy of the L_cr wave propagation time difference is gradually improved.When the cross-correlation step reaches 1 cycle,The L_cr wave propagation time difference is basically the same.It is concluded that the length of a cycle is the optimal cross-correlation step.The L_cr wave is used to measure the stress of 304 L stainless steel,carbon steel?Q235 steel and 45 steel?and 6061 aluminum alloy by breaking the L_cr wave acoustic elasticity coefficient test.The grain elasticity coefficient of L_cr wave is established.With the corresponding relationship,revealing its impact mechanism.The results show that the propagation velocity of L_cr wave in 304 L stainless steel,carbon steel?Q235 steel and 45 steel?and 6061 aluminum alloy decreases with the increase of stress.When the stress reaches the stress turning point,the L_cr wave increases with the stress.In which the propagation velocity varies irregularly with the stress.It is concluded that the non-uniform elasto-plastic deformation caused by the anisotropic microstructure of 304 L stainless steel,carbon steel?Q235 steel and 45 steel?and 6061 aluminum alloy is the root cause of the above results.With the increase of grain size,304 L stainless steel,Steel?Q235 steel and 45 steel?,6061 aluminum alloy L_cr wave acoustic elastic coefficient are gradually reduced,the rate of change has gradually become smaller.The L_cr wave was used to measure the residual stress of 304 L stainless steel with T-type joint,carbon steel laser welding with butt joint,6061 aluminum alloy with double-axis friction stir welding joint?SR-FSW?by the correspondence between grain size and acoustic elastic coefficient of L_cr wave.The correction method is clarified and the correction result is verified by the hole-driling method.The results show that the L_cr wave measurment of 304 L stainless steel T-type joint,carbon steel laser welding butt joint,6061 aluminum alloy SR-FSW residual stress and welding residual stress theory is consistent with the distribution of hole-driling method,the maximum error of 31,29 and 25 MPa,within the allowable range of error.This also indicates that the L_cr wave measurment of the welding residual stress is feasible in this study.