Evaluating technology of spot weld quality for coated high strength steel sheet based on ultrasonic guide wave

Resistance spot welding of the zinc-coated high strength steel sheet is widely used with increasing demands for higher fuel efficiency and the safety requirement in automobiles. However, tiny fluctuation in welding parameters will lead to quality deficient such as stick weld, deficient diameter and spatter due to the poor weldability of the high strength steel and the adverse influences of the coated zinc. Ultrasonic testing (UT) is recognized as a promising technology in estimating the quality of the spot welds. Usually, diameter of the weld spot may be estimated through examining the ultrasonic sequence echo, the diameter of the probe should be the same as that of the regular weld spot and the tester must possess special knowledge on the ultrasonic test. Thus, it is difficult to increase the testing efficiency, decrease testing cost and ensure the testing reliability. In the research, the Lamb wave was adopted to test the spot weld. The wave mode transformation is simplified and the dispersion phenomenon is obviously decreased by using an ultrasonic oblique incidence method. Moreover, the attenuation, sound velocity and the sound components are investigated in theory and experiment. Thereafter, the Lamb wave is adopted in the evaluation on spot weld quality of zinc-coated high strength steel. The interactions between the Lamb wave and nugget characters such as diameter, indentation, columnar crystals are investigated and the relation between the maximum tension load and ultrasonic characteristic amplitude is obtained. The technology proposed in the present work avoids distinguish the complex wave sequence of the spot weld and has higher sensitivity, especially for stick weld compared to the traditional method. Moreover, the testing cost and requirements of the testing system must be decreased by using of the low frequency probe. Meanwhile, the inaccessible area with ultrasonic impulse echo method can be investigated due to the arrangement of the probes. Thus, the method proposed in this research is an effective nondestructive evaluation method for spot welds of coated high strength steel sheet.