Investigations on the formation of pores during laser beam welding of hairpin windings using a high-speed x-ray imaging system

Abstract

Electric drives continue to grow in importance for future mobility. In recent years, hairpin winding has become established for stator production. For connecting rectangular hairpin ends, a laser beam welding process is usually implemented. For sufficient vibration resistance and current carrying capacity, pores must be avoided. This work investigates the pore formation by using a high-speed x-ray imaging system during the welding process of hairpins. This allows us to observe the formation of pores during the welding process using x-ray videos. Specifically, the use of different intensity distributions with static beam shaping (ring and core shape) is analyzed. In addition, the welding speed and the use of a protective gas (argon and helium) are taken into account. The welding results are evaluated with an x-ray CT-analysis. It is found that the formation of pores most likely occurs at the transition area of one pin end to the other. The results also show that with different intensity profiles of the laser beam, the number of process pores occurred can be influenced and reduced. An optimum welding speed and the use of a protective gas also have positive effects on pore formation.