By Philipp von Witzendorff, Lorenz Gehrmann, Martin Bielenin, Jean-Pierre Bergmann, Stefan Kaierle and Ludger Overmeyer

Pulsed laser welding is applied for welding of thin aluminum sheets when the heat affected zone has to be minimized. The pulsed laser process enables a low and precise heat input because the heat dissipates away in between the laser pulses. Applications are hermetic sealing of electronics or opto-electronics which are not persistent enough to resist high temperatures. Aluminum has a low absorptivity (~ 5 %) for the laser radiation of industrial established YAG laser sources which restricts the process efficiency. In addition, several aluminum alloys have a high tendency to generate hot cracks during welding which is even more severe in pulsed laser welding because the pulsed mode leads to rapid cooling.

To overcome the poor process efficiency and to suppress the hot cracking when performing pulsed laser welding of aluminum, a pulsed diode laser with a maximum output power of 100 W is superimposed for heating of the process zone. The diode laser is emitting at a wavelength of 808 nm for which aluminum shows an absorption peak (~ 15 %). The diode laser pulse is synchronized with the laser pulse of the YAG laser so that both laser pulses start and end at the same time. The diode laser spot diameter has twice the size of the YAG laser spot to enable heating beside the weld pool. The laser radiation of both lasers is coaxially superimposed by using dichroic mirrors and is focused by one optical lens.

This procedure increased the weld penetration depth because the diode laser irradiation increases the temperature in the welding zone. In addition, the heat input of the diode laser extended the process window for pulsed laser welding of aluminum alloys which are susceptible for hot cracking. In comparison to conventional pulsed laser welding, the new process enabled an increase in penetration depth of 25 % without generating hot cracking. The welding process with the two laser sources has been used to hermetically seal an aluminum housing as shown in Fig. 1. The housing welded with the superposition of a pulsed diode laser and pulsed YAG laser has been positively tested for tightness whereas the conventional pulsed laser welding process led to hot cracking.

Therefore, the new pulsed laser welding process can be used for hermetic sealing applications of aluminum alloys which are susceptible for hot cracking, as Al 6xxx alloys for instance.

Figure 1. Comparison of conventional pulsed laser welding and pulsed laser welding with a superimposed pulsed diode laser

Acknowledgement

We thank the German Federation of Industrial Research Associations (AIF) for project funding and support (IGF-Nr. 17.487 N).