laserdrilling

Percussion drilling uses several laser pulses that are applied to the same point in space to achieve greater drilling depths than are otherwise possible with single-pulse drilling. Lasers with stable resonators that deliver high energies and pulse strengths along with good beam qualities at frequencies of up to 10 Hz are usually used for holes with diameters greater than 0.1 mm. Pulse strengths of up to 50 kW are employed in percussion drilling. The beam quality decays significantly at higher frequencies, which causes the thermal load on the laser rod to produce a considerable drop in depth of field. Higher frequencies are usually not required, however, because the laser-induced plasma generated by the power input almost entirely absorbs the laser beam and prevents drilling from being speeded up. The fact is that both the laser beam's intensity on the work piece and the shape and position of the beam caustic determine the quality and reproducibility of the results achieved with the process. Only stable resonators in a pump range in which the thermal lens' effects are minimised will produce optimum beam caustics and beam diameters appropriate to the desired hole geometry. Special resonator optimisation in conjunction with the optimisation of the optic system's design considerably reduce the effect of the thermal lens even at greater pump performances. The benefit to drilling applications is that the drilling process can be carried out even at higher frequencies. It may be assumed that high working frequencies will enable the laser-beam-induced plasma to be maintained from the point of entry to the base of the hole – thus making it possible to improve how the laser-beam energy is introduced into the metallic material. The tap hole at the drilling base can be maintained with the aid of the steam pressure if a sufficiently high repetition rate can be achieved. The multiple reflections thrown back from the hole walls' metallic surfaces ensure that the laser beam reaches the point of application. The low vaporisation rates in turn result in some of the metallic reflection properties being maintained at the surface so that it is possible to focus the beam in the centre of the area of work. Taking these conditions into account, pulsed rod lasers may be used to achieve aspect ratios in excess of 1:200. In contrast to conventional percussion-drilled holes, these deep holes are characterised by fewer deposits, fewer heat-exposure effects and relatively short drilling times. Holes with diameters ranging between 20 and 200 µm may be produced by applying this drilling strategy. The maximum depths that can be achieved in steel exceed 20 mm. Drilling depths in excess of 40 mm can be achieved with conventional drill resonators, but such small hole diameters, however, cannot be realised with the corresponding high aspect ratios.