Influence of nanosecond prepulsing on the stability of copper spot welding with IR laser radiation

Spot welding with laser radiation strongly depends on the material of the joining partners. While most iron based metals can be welded with high process stability, copper alloys and especially pure copper for electronics manufacturing cannot be welded with a satisfactory yield. Here, the lack of process stability so far has prevented the establishment of laser spot welding. While this problem is known, the reason for it and an effective measure against it are missing. In this work, a theoretical expression for laser spot welding stability is developed and the control of the laser absorption on the copper surface is identified as the key for improving copper spot welding stability. With a laser pulse in the nanosecond pulse duration regime (prepulse), the copper surface is oxidized prior to each spot welding laser pulse. The local oxidation leads to increased absorption of the welding pulse radiation. Thereby, the stability of copper spot welding with IR laser radiation is significantly increased. In a wide parameter range, reliable spot welding with consistent spot weld diameters is achieved. Experimental results and process monitoring data are used to confirm the theoretical stability model.