Research On On-line Ultrasonic Detection Technology For Resistance Spot Welding Of Stainless Steel

With the gradual development of transportation integration,rail transit has gradually become the first choice to cure the public traffic headache.The manufacturing technology of stainless steel urban rail car which is one of the mainstream car body structures,develops rapidly.The joining of the stainless steel car body mostly adopts thin plate welding structures to achieve design goal of lightweight,and resistance spot welding is the main welding method.Due to the large current and short welding time,defects like spatter and incomplete weld appear easily during the welding process.Simultaneously,the spot welding nugget grows in a closed space,which cannot be observed directly and cause great difficulty for welding quality control.Therefore,the technology of the quality evaluation in the spot welding which is efficient and accurate is of great significance for obtaining qualified welding joints and ensuring the safety of railway vehicles.There are two commonly used quality evaluation methods for resistance spot welding,that is,the destructive test after welding and the online monitor based on welding process parameters,which both have some shortages.Ultrasonic detection is a non-destructive testing technology with high sensitivity and fast test speed,which is often used for quality evaluation after resistance spot welding.In order to achieve the goal of effective quality evaluation for all resistance spot welding joints on stainless steel car body,this paper studies the on-line ultrasonic detection technology for resistance spot welding of stainless steel.This paper independently designed a test device to achieve the close contact of stainless steel specimens under static pressure and an online ultrasonic detection system in spot welding process.The test device and system used ultrasonic probes with frequencies of 5MHz,10 MHz,and 15 MHz,respectively,to study the influence of the stainless steel specimen interfaces on the characteristics of ultrasonic A-scan echo signals under different ultrasonic frequencies and different static pressure conditions.When the electrode pressure met the requirement of the spot welding process,the study found that the ultrasonic probe with 5MHz cannot distinguish the interfaces of stainless steel specimens whose thickness is 2mm.However,the ultrasonic probe with 15 MHz has a resolution of 0.7mm.When using 15 MHz ultrasonic probe,the test results showed that as the static pressure,that is,the electrode pressure increased,ultrasonic signal traveled through the spherical electrode and entered the specimens.The ultrasonic probe received the reflected echo generated at each interface of specimens,achieving the contact coupling.The coupling effect of the contact under pressure was small different from that of the conventional coupling agent,which provided test basis for on-line ultrasonic testing of spot welding quality.The continuous ultrasonic A-scan signals of welding joints in different spot welding processes were obtained by changing welding parameters such as welding current and welding time.Data processing and feature extraction were performed on the detection signals to obtain the B-scan image,which reflected the growth status of nugget during spot welding.Finally,the metallographic analysis was used to compare with the actual connection status inside the joints.The results showed that the B-scan image of each welding process can accurately reflect the change of the nugget's growth state caused by welding parameters,and keep the same with the actual connection state inside the joint.Therefore,this timing diagram can be used as a reliable basis for quality evaluation,which is of great significance to the research on on-line ultrasonic detection for resistance spot welding of stainless steel.