The Ultrasonic Inspection Research On The Quality Of Spot Welding Joint Of Stainless Steel

Resistance spot welding is widely used in industry, the stainless steel body ofurban rail vehicle, which has ten thousand spot-welded joints, is mainly welded usingresistance spot welding process, so the quality of stainless steel spot-welded joints hasimportant influence on manufacturing quality of stainless steel bodies. Traditionalinspection method of spot welding quality is physical teardown, during which thespot welding joint of test piece or product component is peeled, spot welding qualityis evaluated by testing the mechanical strength of the specimens or measuring thediameter of nugget size of spot welding. The method has low inspection efficiencyand causes great waste of materials, thus nondestructive inspection methods andtechniques which are convenient and reliable to test the quality of spot welding jointare necessary. Ultrasonic inspection technology, with its simple inspection principle,convenient operation and advantage of realizing nondestructive testing easily, hasimportant significance and value in actual production of spot welding.This paper researches on ultrasonic inspection system of stainless steel spotwelding which is automatic and intelligent. Ultrasonic inspection system can realizestep-by-step scanning detection using the ultrasonic probe in preset plane area. Byanalyzing testing data using the intelligent analysis software, numerical analysisresult of nugget size and C-scan image of spot welding can be obtained. Theinspection data are stored in a database at the same time.Time-domain feature of ultrasonic A-scan signal of stainless steel spot welding isanalyzed. The waveform of A-scan signal regularly varies with scanning area ontesting surface of spot welding specimen. Extract datum point of A-scan signal, thenset threshold of40%,50%and60%according to datum point. The nugget size of spotwelding can be get.Ultrasonic measuring method of FFT information of stainless steel spot weldingis studied. Fast Fourier transform is carried out to transform ultrasonic A-scan signalfrom time domain to frequency domain. Change law of A-scan signal is tried to findin the frequency domain. Experimental study shows that there is a strong correlationbetween the critical point at which master frequency changes on amplitude-frequencycurve and the nugget size of spot welding. Processing method of ultrasonic C-scan image of stainless steel spot welding isstudied in this paper. N*N mean filtering, Krisch edge detection and windowgray-scale transformation are carried out to process original C-scan image of stainlesssteel spot welding so that noise can be removed and edge and outline of spot weldingnugget can be highlighted. C-scan image will be displayed more clearly. Theseprocessing methods make measurement of nugget diameter of spot welding easier.Research on accuracy and stability of measurement methods of ultrasonicnondestructive inspection based on A-scan signal, FFT information and C-scan imageof stainless steel spot welding have been carried out. The testing results show thataccuracy and stability of measurement method of A-scan signal are higher whenthreshold value is40%; the accuracy of measurement method of FFT information ishigher, but stability is not good enough; the accuracy of measurement method ofC-scan image is higher, and the stability is the best.