The influence of core and ring beam configurations on geometrical and mechanical properties of laser welded Cu hairpins

Laser welding is an industrially established method for the production of hairpin electric drives. High-power active fiber lasers operating in the near-infrared (NIR) range are the most widely used solution for this process. More recently, core/ring beam configurations have emerged as an alternative, offering greater process flexibility to suppress spatter formation and reduce internal porosity. Despite their growing industrial adoption, systematic investigations into the influence of beam shape on mechanical properties remain limited. In this work, the effect of different beam shapes was assessed at a fixed laser power of 5 kW. Four beam configurations were analysed using identical welding speed and trajectory, with the aim of evaluating their impact on process productivity and weld quality. Welds with comparable geometrical features, produced using different beam shapes, were examined in terms of oxidation extent, porosity, and heat-affected zone (HAZ). These characteristics were subsequently correlated with the resulting mechanical and electrical properties. The results indicate that the overall strengthening mechanism is primarily governed by the effective load-bearing area, as well as by grain coarsening and hardness reduction within the HAZ. A beam configuration featuring a moderately intense core, accounting for less than half of the total power and combined with a ring beam, produced the lowest porosity and resulted in large columnar grains elongated along the hairpin length. In contrast, a Gaussian-like beam consisting solely of a core generated a larger effective resistance area and a reduced HAZ, with a microstructure characterised by a mixture of fine equiaxed and elongated grains. Overall, the findings provide guidance on the ideal distribution of core and ring beam intensities and contribute to a deeper understanding of the factors governing weld strength in hairpin welding.