Enhanced aluminium processing with a high frequency, pulsed CO₂ laser

Plasma formation above the workpiece can seriously degrade the laser process quality of highly reflective and/or refractory materials. Following a combined simulation and experimental research programme on both the laser and the interaction process, a transversely excited, high PRF, CO₂ laser was constructed with characteristics to specifically enhance processing of reflective and/or refractory materials. To enter the highly absorbing interaction regime and avoid plasma formation an incident flux intensity of 10⁷ Wcm² with a pulse duration between 5 to 8 μs required; to avoid the deleterious melt-freeze process of conventional pulse modulated sources, a PRF of 5 to 10 kHz is necessary. Processing of aluminium was investigated using low, mean laser power and different gas mixtures. The laser processing efficiency was found to be critically dependent on the optical pulse shape, in particular the plateau and peak optical power, and less dependent on the PRF. Enhanced processing capabilities were observed with a 1:1:0 (CO₂:N₂:He) gas mixture operating at 1.5 kHz and 60 W mean power. However, drilling was not achieved with a 1:1:4 mixture operating at 2.5 kHz and 100 W. The peak and plateau optical powers ranged from 4 kW to 20 kW, with about 6 μS duration. Methods described to optimise the optical characteristics to enhance the process quality include changing the gas composition.