An experimental method is discussed to analyse the influence of the various drilling parameters on the hole forming process of through holes of 15 to 100μm diameter in stainless steel X2 CrNiMo 18 14 3 of 0.08 to 1.5mm thickness. Nonreacting nitrogen was used as an assist gas to expel the melt. The experiments were carried out using a TEM00 fundamental mode slab laser. To investigate the laser drilling process, a HeNe laser beam and the Nd:YAG laser are focused on the metal surface. The larger HeNe spot lies on top of the Nd:YAG focus. The HeNe light, which shines through the hole during the drilling process, was detected by a photo diode and recorded in function of time. The HeNe signal is a measure for the momentarily open area of the hole.
The obtained curve depends on the material thickness and on the drilling parameters, such as the pulse duration, the pulse energy and the assist gas pressure. These different curves classify into seven main groups. Every group corresponds to a specific combination of drilling parameters. This classification allows the prediction of the roundness, taper and surface condition of the holes.
A characteristic time, the penetration time, can be measured from the beginning of the laser pulse to the first reaction of the photo diode to the HeNe light. This is the time which the Nd:YAG laser beam needs to melt through the metal sheet so as to produce the initial, small hole. The influence on this penetration time of the pulse energy, the pulse duration and the material thickness are discussed. The hole diameter grows in stages up to its final diameter. The experiments were carried out with and without a beam expander to investigate the relevance of the Rayleigh length and of the beam intensity on the penetration time and on the hole diameter.