Research On Propagation Characteristics And Defect Detection Technology Of Weld-guided Waves In Friction Stir Welding

Friction stir welding(FSW)is an important technology for manufacturing large aluminum alloy tanks.Due to the stress corrosion and periodic load in the long-term service,FSW weld becomes the weak area in the service of rocket tank.Therefore,online monitoring of FSW weld is the key step to ensure the structural integrity and service safety of rocket tank.There are many researches on the off-line detection technology of FSW weld defect detection,but few on-line detection technologies.In this thesis,ultrasonic guided waves technology is used to realize the on-line monitoring of FSW weld,because it can detect the surface and internal defects well,and can realize the long-distance and large-scale detection.However,the traditional ultrasonic guided waves detection of weld damage does not consider the characteristics of the weld itself.Most of them still adopt the pitch-catch mode with high-density sensor network layout arranged on both sides of the weld,and the damage feature information is extracted by the interaction between the damage and ultrasonic guided wave passing through the weld.In this thesis,6 mm thick FSW plate of 2219 aluminum alloy is taken as the research object,and the propagation characteristics of ultrasonic waves in FSW weld of the same material and its related influence parameters are studied in order to provide technical support for FSW weld online monitoring.In this thesis,the microstructure difference between FSW plate and fusion plate is analyzed in detail by means of metallographic analysis and relevant material parameters measurement.Then semianalytical finite element(SAFE)method is used to analyze the weld-guided waves of the actual FSW model and its characteristics,and the parametric analysis of the structural geometric parameters and material parameters is carried out.Finally,piezoelectric sensors are arranged on the actual FSW test piece,and the defect in the weld is detected by the Pulse-echo mode.The main research contents are as follows:(1)The microstructure of FSW plate is observed by metallographic analysis,and the grain size of weld zone and base metal zone is compared.It is found that the grain size in the weld zone is obviously finer than that in the base metal zone.Through testing the micro-hardness,Young’s modulus and density of the weld zone and the base metal zone,it is found that the micro-hardness and Young’s modulus of the weld zone and the base metal zone are significantly different.Therefore,it is necessary to distinguish the difference between the weld zone and the base metal zone of FSW plate of the same material in the later study.(2)The model of FSW plate is built in COMSOL software.The cross-sectional energy distribution and displacement mode shape of each mode in FSW plate are calculated by SAFE method.It is found that there is AO-like weld-guided wave whose energy is mainly concentrated in and around the weld when the center frequency range is 100kHz-210kHz,and its dispersion curve at the center frequency of 120 kHz is calculated.The A0-like weld-guided wave is very suitable for long-distance detection.The piezoelectric sensors are arranged on the actual FSW plate,and the pitch-catch mode is used to realize one actuator and multiple receivers.The signal amplitude of receiving sensors in the weld zone and the base metal zone is compared,and it is verified that the energy of A0-like mode is mainly concentrated in and around the weld,that is,the ultrasonic guided waves have an energy trapped effect in the FSW weld.(3)In order to explore the influence of geometric parameters of weld cross-section and material parameters of welded plate on the propagation characteristics of weldguided waves,the differences of weld width,weld cross-section shape and Young’s modulus between weld zone and base metal zone are analyzed by parameterization.The analysis results show that the above parameters have influence on the propagation characteristics of weld-guided waves,and the width of the weld and the young’s modulus of the welded plate are important factors for the formation of weld-guided waves.The influence of weld width and weld cross-section shape on the energy distribution of weld-guided waves is small.When the three modes exist,A0-like mode has the highest energy concentration in and around the weld compared with SH0-like mode and S0-like mode.The difference of the young’s modulus between the weld zone and the base metal zone has a significant effect on the propagation characteristics of weld-guided waves.The difference of Young’s modulus between the weld zone and the base metal zone has a significant effect on the propagation characteristics of the weldguided waves.When the young’s modulus of the base metal zone is less than the weld zone,there is no weld-guided waves.When the young’s modulus of the base metal zone is greater than the weld zone,there are different situations of the weld-guided waves.(4)In order to detect the defect in the weld zone of FSW by using the weld-guided waves,the defect detection principle of the weld-guided waves with pulse-echo mode is analyzed firstly.By monitoring and analyzing the echo signal,the information of defect and structure is extracted to realize the defect location.To further verify that the energy of the weld-guided waves is less attenuated than that of the Lamb waves in the same thickness plate,defects of the same size are set at the same position in the same size FSW plate and aluminum plate,and the piezoelectric sensors are arranged in the same position.It is found that the reflectivity of the A0-like weld-guided wave in the FSW plate at the defect and at the end face of the weld is higher than that of the A0 mode guided wave in the aluminum plate of the same thickness.Piezoelectric sensors are arranged in the weld zone,and the defects in the weld are effectively located by use Pulse-echo mode.The relative error of damage location is 0.015%for the size of 10 mm × 4.5 mm × 3 mm,and 5.084%for the size of 6 mm×2 mm×2 mm.