Quality inspection of micro solder joints in laser spot welding by laser ultrasonic method.

The quality inspection of micro solder joints in laser spot welding (LSW) is a problem of great concern in industrial application. In this paper, a laser ultrasonic technology (LUT) was proposed to inspect the quality of micro solder joints in LSW. Firstly, based on the thermoelastic model of acoustic wave propagation in solid, a theoretical model was built and used to analyze the propagation properties of the Lamb wave in the whole field by finite element method (FEM), the transmitting properties of the excited Lamb wave via solder joint were affected by the effective contacted area of solder joint. Secondly, LUT was used to inspect the 1.2 mm/0.4 mm welding spot of standard/false 304 stainless steel welded components. By comparing the propagating properties of excited ultrasonic wave in different samples with different weld quality, the standard and false welding can be visually distinguished. Finally, a industrial CT was used to check the quality of the samples used in our experiment. Inclusions and pores have been found in the false solder joints, which will reduce the effective contacted area of solder joint, and then affected the propagation of ultrasonic wave. By combining the CT results and the experimental analysis, the experimental results detected by LUT are in good agreement with the simulation results. So, the LUT is a potential method in field of the quality inspection of micro solder joints in LSW.