Investigations in instable process conditions during laser beam microwelding of copper

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The main objective is to reveal the fundamental effects during laser microwelding of thin copper foils using NIR and mixed NIR/VIS laser pulses. From experimental observation as well as from literature it is known that laser welding of copper exhibits a very low reproducibility and, moreover, the process is quite unstable. Within the project the working hypothesis should be verified that these instabilities are (1) due to a positive feedback loop originating from the temperature dependence of specific material numbers like absorption and heat diffusion coefficient, and (2) modifications of the surface topography and the resulting absorption mechanisms can be the origin of this behavior. Therefore, comparative studies with NIR/VIS pulsed lasers as well as with aluminum foils are expected to deliver more details about the process behavior. The experiments will be accompanied by dynamic FE simulations. Based on the experimental and theoretical investigations a process window will be located where reliable joints between copper foils can be realized. Using adequate sensors and closed-loop control the process can basically be stabilized also with single NIR wavelength lasers. In the midterm timescale stable and reproducible aluminum-copper-hybrid joints have to be realized because there is a growing demand in electromobile applications.