| With the rapid development of advanced manufacturing industry,resistance spot welding technology,as a major technique of sheet metal welding,has been widely used in the fields of vehicles and ships,electronic science and technology,power industry and so on,which has greatly promoted the industrial progress.Meanwhile,welding quality is directly related to the safety of the people.At present,most of the spot welding quality detection methods at domestic and abroad are based on the time frequency analysis of A-scan signal or the establishment of various welding parameters(such as voltage,time,electrode pressure,etc.)and the numerical model of spot welding quality.However,there are many factors in the process of welding,so there are some disadvantages in the methods mentioned above,which affect the accuracy of quality evaluation of resistance spot welding and threaten the life and property.Ultrasonic imaging technology,as one of the most potential non-destructive testing methods,has been applied to evaluate the quality of various products,and is gradually developing towards high-quality and rich-performance characterization.Under this context,this paper presents a study on the quality detection of resistance spot welding using ultrasonic imaging technology,which is based on synthetic aperture focusing technology to obtain three-dimensional high-resolution focused ultrasonic images inside spot welding.In addition,combined with quantitative research,the parameters of nugget diameter and indentation depth can be measured finally with high precision.The main research contents are as below:1.The influence of the directivity of sound beam on conventional imaging is analyzed,and the synthetic aperture focusing technique is introduced;Based on the imaging principle of explosive reflection and back propagation,two imaging algorithms,signal delay stacking and spectrum phase shift migration,are described and derived,the case of homogeneous multi-layer medium is also discussed;In addition,the resolution of defect imaging is investigated by simulation,the results proved the superiority of the synthetic aperture imaging technique proposed in this paper.2.A resistance spot welding model of stainless steel was built based on COMSOL finite element simulation platform,and the propagation characteristics of ultrasonic wave in spot welding components were analyzed;Through simulation the comparison between B-scan image and synthetic aperture focused image is made.It is shown that B-scan image is affected by the diffraction phenomenon of sound wave,which results in the serious disadvantage of ARC flinging.However,synthetic aperture focused image technology can overcome this shortcoming,which verified the ability of synthetic aperture imaging technology to detect the defect of molten core;In addition,the resolution of defect imaging is investigated by simulation.The results showed that the imaging results in frequency domain are better than in time domain when the two adjacent defects are separated by half the aperture of the array element,this lays a foundation for the construction and design of the experimental system.3.Aiming at the practical application of ultrasonic imaging testing in resistance spot welding,an experimental system is set up to verify the accuracy of simulation results when there are defects such as too small nugget,too deep indentation and so on;On the basis of simulation,-6 d B method is used to measure and analyze the nugget diameter.The results showed that the average error of phase shift migration imaging in frequency domain is nearly twice as low as that in time domain;In addition,the surface contour of spot welding indentation is reconstructed by pulse-echo migration,it is proved that the synthetic aperture imaging detection technology can better restore the real shape of indentation by comparing with the result of imaging quantitative detection,and the internal high quality image representation of spot welding is realized effectively.4.Based on the above research,a 3D-PSM detection technique is proposed.The feasibility of this method is proved by COMSOL 3D finite element simulation and experimental plane scanning;Experiments on standard test blocks and spot-welded specimens in different directions and positions showed that the technique can effectively characterize the characteristics of the detected objects in three-dimensional form,which is compared with traditional ultrasonic C-scan imaging,three dimensional frequencydomain imaging technology can achieve physical focus in both directions,and further improve the image quality and accuracy of detection results;The average errors of nugget diameter and indentation depth in two directions are 0.92% and 1.98%respectively by quantitative measurement. |
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