Quality improvement of copper welds by laser microwelding with the usage of spatial power modulation

Abstract

Laser welding of copper is gaining more and more importance to create electrical contacts due to the high temperature resistance and high achievable mechanical loads. However, laser welding of copper in the past was always associated with low process stability. By using high brightness fiber laser sources, small laser spots can be generated which allows keyhole-based deep penetration welding even in copper with high quality. Unfortunately, a small spot size and keyhole welding lead to a small connection area between the two joining partners. To avoid this, spatial power modulation—a linear feed with superposed circular motion—has been introduced. By using an additional parameter, oscillation frequency, and amplitude, in conjunction with spatial power modulation, we can increase not only the connection area but also the stability of the laser welding process and quality characteristics of the welds. In addition to the conformity of the weld depth, the surface roughness of the weld is a measure of the quality. Especially in interconnection of lithium ion cells and high power electronics, the consistency of the weld depth influences the connection area, which is essential for the current-carrying capacity. The quantity of weld defects and irregularities has a direct impact on the surface roughness of the weld. This paper presents recent results on the influence of the spatial power modulation on quality characteristics during laser microwelding by analyzing the weld via three-dimensional longitudinal cross sections and via laser measuring of the weld surface. Experimental investigations are supplemented by finite element simulations which enable a detailed analysis of the influence of the laser scanning strategy on the geometry of the fusion zone.