Invited Paper Overview Of Flow Dynamics In Gas-Assisted Laser Cutting

Material cutting techniques with a focussed c/w CO2 laser beam use a gas jet from a nozzle to assist ejection of material from the cut kerf. Traditional techniques use a circular tipped nozzle mounted coaxial with the focussing beam. It is usual for these nozzles to be operated at low pressures (<100 kPa-gauge). Higher pressures can give higher cutting speeds but this can be associated with lack of reproducibility of cut quality. One of the reasons for the unreliable behaviour is the complex nature of the shock fronts and associated phenomena that can occur in a supersonic gas jet impinging on a workpiece. Shocks can result in a reduction of the stagnation pressure on the surface of the workpiece. Experiments are described that demonstrate a strong correlation between the local stagnation pressure and the maximum laser cutting speed. Measurements of this cutting pressure and flow visualisation techniques have been used to investigate a number of nozzle and workpiece geometries. The formation of a strong normal shock (the Mach shock disk, MSD) in underexpanded cutting jets is found to impair cutting performance. Not only does a MSD cause a significant reduction in the cutting pressure but it can also encourage the formation of a stable stagnation bubble on the surface of the workpiece. It is suggested that the adverse flow conditions in the bubble could impede cut debris removal. Two new nozzle designs are described whereby the formation of a MSD and stagnation bubble is prevented at normal operating pressures. Examples are given of nozzles which can exert cutting pressures in excess of 500 kPa-gauge at damage resistant stand-off distances of 4 to 7mm. The flow characteristics of nozzle designs used by other investigators is reported. These include the flat-tip nozzle, the tri-jet off-axis array and the ring or annular nozzle.