Effect of defocus on blue laser spot welding of electrical-steel-laminations

We first developed a spot welding method that utilizes a flat-topped blue laser to join electrical steel laminations, specifically designed for high-performance motor applications, which boasts a high laser absorption rate and low heat input. This paper focuses on the impact of defocus distance on the welding quality. The results show that under negative defocus, the laser beam irradiates on the substrate in a convergent state, which improves the efficiency of laser penetration, intensifies the evaporation of silicon steel coating and melting metal, forming a "single hump" molten pool with a plume above it. As the defocus decreases from 0 mm, the penetration depth first increases (maximum value at -2 mm) and then decreases. Positive defocus leads to a wider and shallower molten pool without generating a plume. The welding melting mode can be changed by the defocus distance, but it can't be quantified by conventional laser power density due to the characteristics of beam convergence or divergence. Furthermore, the grain size decreases as the focus position moves up, and the shape of columnar grains changes from slender to coarse. The grain type at the molten pool boundary is quantified by the G/R value calculated via the mathematical model of the thermal profile. Finally, it is recommended to select the -2 mm defocus distance to obtain a large weld area and high surface quality.