| After a centenary`s development autos have already become more and more popular,at the same time there is a stronger requirement for the safety and durability of autos.So high strength galvanized steel sheets are widely used in auto industry,it is one of the important solutions to meet the requirement,high strength galvanized steel sheets have been widely applied to auto industry for its excellent mechanical property and high corrosion resistance,and its main welding method is spot welding.However,there are two difficult problems for high strength galvanized steel sheets spot welding,first,it is difficult to measure the nugget diameter of the welding spot for the reason of deep dimple that cause the edges of nugget area、corona bond and the dimple merged each other and difficult to identify;secondly,pores often occur during the process of spot welding when the gasified zinc is blended into the nugget.Therefore,an effective technology of quality evaluation for high strength galvanized steel sheets spot welding is very important.The ultrasonic testing technology has significantly progressed with the development of computer technology,the ultrasonic can be stored in the computer as digital signals,the information of time domain and frequency domain contained in those digital signals will be processed by software and demonstrated as oscillograms and two-dimensional diagrams to facilitate further analysis.To evaluate the quality of high strength galvanized steel sheets spot welding,this paper research about ultrasonic testing of the spot welding samples for high strength galvanized steel spot weldingThe spot welding samples were made of designation no.H260YD+Z high strength galvanized steel sheets,in this paper,the research and study include observing of metallographic structures and surface features recording of data and ultrasonic testing.To get accurate ultrasonic testing data,here,a set of specially designed automatic ultrasonic testing device had been developed to realize automatically smearing the coupling agent、automatically scanning with the probe、collecting and storing of data and generating C-scan images,this device provides an ideal platform for the study about ultrasonic testing for high strength galvanized steel sheets spot welding quality.In this paper,by analyzing the metallographic section of high strength galvanized steel sheets welding spot,discover that the starting zone of the dimple in high strength galvanized steel sheets welding spot is corresponding to the corona bond of the welding spot and the slope and bottom of the dimple is corresponding to the nugget zone.With reference to the metallographic section,ultrasonic A signals in time domain and frequency domain of each welding spot area were analyzed and got the measured values of nugget diameter in time domain and frequency domain,and the result of comparing among the measured values in time domain、frequency domain and actual values shows that the measured values in frequency domain have smaller deviation.In this paper,the peaks of first echoes of each point in scanning zone in time domain and the amplitudes of the characteristic frequencies of each welding spot area are taken to generate C-scan images that will be compared with the features of the sample surface.The study and analyses show that the C-scan images generated can effectively reflect the real surface characteristics and internal structures of the welding spot.After processing the C-scan images by using image enhancement and edge detection,we get the two-dimension images that can distinguish the edges of each area within the welding spot clearly.The measured values acquired by the way above matches well with the actual values.In this paper,we had done some study on the ultrasonic testing of high strength galvanized steel sheets welding spot with pore defects,by analyzing ultrasonic A signals and C-scan images of samples with pore defects we get a solution to determine the sizes and positions of the defects. |
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