Improvement of the ultrasonic testing accuracy of laser welding fusion width

Due to the tiny dimensions of lap laser welding joints, there is significant error in weld width detection when using the traditional 6 dB method. In order to improve the method’s detection accuracy and study its source of error, the finite element analysis method is used to analyze the propagation law of incident ultrasonic waves and reflected ultrasonic echo characteristics inside a laser-welded joint. Based on a modified 6 dB method, a laser welding joint melt width evaluation model is constructed and verified through physical experiments. The experimental results show that the primary echo amplitude of the bottom surface of the upper plate can be used as a characteristic value that reflects the internal structure of the joint. When the center of the probe corresponds to the edge of the weld fusion line inside the joint, the attenuation of the primary echo amplitude varies with the thickness of the upper plate, and the traditional 6 dB method can be modified according to the attenuation degree which related to the upper plate’s thickness. Based on this, the effective weld width at the contact surface of the inner plate of the joint can be calculated quantitatively. The ultrasonic testing results of the actual laser welding joints confirmed that the melt width of the laser welding joints obtained by the modified 6 dB method agree with the results of the physical experiments, which means that this provides a very practical method for accurate ultrasonic testing of laser welding joints in real-world production.