| Compared with other welding technology, spot welding has shorter heating time, smaller welding deformation, etc. It has been applied to automotive industry and Aerospace widely. Aluminum alloy has low density and high corrosion resistance. It is a kind of important structure materials for Aerospace. However, it is hard to weld and liable to generate flaw. Some important parts in the area of Aerospace are strict with the quality of welding spot of aluminum alloy. For the purpose of being up to standard, they must ensure diameter of nugget big enough besides high strength of joint. But welding spot doesn't have convenient and reliable nondestructive examination methods.This article studies ultrasonic echo features of different area of welding spot of aluminum alloy, finding that ultrasonic echo height of joint line of welding spot varies with different area of welding spot. We can choose the first ultrasonic echo of top shelf's bottom of spot welding sample to be the label point of diameter of nugget. We can measure diameter of nugget of welding spot by calculating coordinate of label point. B-scan image interrelates with A-scan signal and B-scan image is more visual. This article studies acquiring diameter of nugget by image processing and designing relevant program to measure diameter of nugget on the basis of analyzing B-scan image features.Phase Array can do electronic scanning. It can realize B-scan by controlling delay of motivation of crystals. This article studies the feasibility of measuring diameter of nugget by Phase Array.Finally, we use metallography photos to validate the accuracy of measuring value of diameter of nugget by ultrasound method. The results show that the measuring values by A-scan method and B-scan image method are more accurate than the others. Although Phase Array has high scan efficiency, its test accuracy is subject to structure and frequency of probe. It can become an convenient and practical test method for measuring diameter of nugget of welding spot.
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