| Friction stir welding has become an important role in addressing the welding challenges associated with aluminum alloy structures,which are increasingly widely used,due to their low-temperature and solid-phase welding characteristics.Friction stir spot welding,including refill friction stir spot welding(RFSSW),is of great significance in overcoming welding issues in thin-plate aluminum alloy lap structures.The unique process characteristics of RFSSW lead to joints with shapes and defects that differ from those found in general spot-welded joints,and the uncertainty of the joint quality increases,thus creating special welding quality testing and evaluation problems.There is less research on nondestructive testing and evaluation methods for RFSSW quality.The lack of effective testing methods and technical devices has resulted in the use of destructive manual testing techniques in engineering,impeding the widespread application of RFSSW.This thesis conducts a comprehensive study of non-destructive testing methods,feature identification,and quantitative analysis of internal connection status,defects,and surface conditions in RFSSW joints based on ultrasonic testing and laser sensing testing for aluminum alloy RFSSW joints.Based on this,this thesis proposes and investigates a method for RFSSW joint quality evaluation based on non-destructive testing of internal and surface state characteristics.Through experiments and finite element numerical simulation analysis,the characteristics and patterns of acoustic wave propagation in ultrasonic testing of RFSSW joint,the characteristics of the ultrasonic echo signal in the time domain and frequency domain,the method of characteristic value characterization,and characterization ability of joint connection state and defect based on these characteristic values are studied.The results indicate that characteristic values from the time-domain signal and detection spectrum effectively characterize the connection state and planar defects in RFSSW joints,with the main frequency characteristic value of detection spectrum being particularly robust.Moreover,the characteristic value of the time-domain signal and the special frequency amplitude of the radio-frequency spectrum have a relatively strong ability to characterize the holes.Based on the comparative study of various ultrasonic characteristic value characterization methods,the main frequency amplitude crossover characterization method of the ultrasonic echo signal is proposed.This method can accurately characterize and identify the boundary of the nugget and planar defect of the RFSSW joint and has good stability.Based on the main frequency amplitude crossover method and the optimized ultrasonic C-scan image method,a quantitative analysis method based on the Hough circle transform method is proposed to realize the automatic quantitative analysis of the effective connection zones of RFSSW joints.For the quantitative analysis of planar defects in RFSSW joints,the thesis proposes a method combining the gray value peak-searching method with the automatic gray value threshold method.Additionally,a variational mode decomposition(VMD)method with correlation selection is proposed to qualitatively analyze annular holes,which solves the problem of identifying annular holes.Based on laser ranging sensing technology,the surface state inspection method of the RFSSW joint is studied to achieve accurate inspection of the RFSSW joint’s surface state and two-dimensional/three-dimensional imaging characterization of the surface topography.A parametric characterization method is proposed to characterize the surface state characteristics of RFSSW joints,and parameters such as net extruded metal volume,flash,dents,and roughness are employed to achieve a comprehensive parameterized characterization of the surface state of RFSSW joints.The correlation analysis reveals that the characteristic parameter of net extruded metal exhibits a high correlation with RFSSW joints that have holes and incomplete refill defects,while the characteristic parameters of the dent have a general correlation with RFSSW joints that have holes and incomplete refill defects.The formation of tensile-shear fracture modes of RFSSW joints and their relationship with tensile-shear load,and the effect of welding process parameters,joint connection state,and the types of defects on tensile-shear fracture mode and tensile-shear load are studied.The results reveal that the plug-type fracture is mainly due to the existence of holes and incomplete refill defects in the nugget boundary,resulting in the lowest tensile-shear load for RFSSW joints.Furthermore,it reveals that welding process parameters,joint forming characteristics,and surface state correlate with the tensile-shear loads of RFSSW joints.Based on the study of the ultrasonic and surface testing methods of the internal and surface characteristics of RFSSW joints,and the research on the relationship between the characteristics of RFSSW joints with the tensile-shear fracture modes and tensile-shear load,a multi-parameter input hierarchical evaluation method of RFSSW joints based on ultrasonic and laser sensor nondestructive testing is proposed,and an evaluation analysis model is established.The evaluation results show that the evaluation model can effectively test the plug-type fracture mode of RFSSW joints,and the accuracy is 95.8%.The evaluation error of the tensile-shear load of RFSSW joints with non-plug-type fracture mode is 3.8%,which achieves the expected target.This thesis focuses on the internal and surface characteristics of RFSSW joints,ultrasonic and laser sensing testing methods,joint forming characterization and quantitative analysis methods,and multi-parameter input hierarchical evaluation method for RFSSW joint quality,which is of great significance in promoting the technological progress and engineering application of intelligent testing and evaluation of RFSSW quality. |
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