Industrial Production of Microholes by Usage of Ultrashort Pulse Lasers

The drilling of microholes with well-defined geometry is becoming increasingly important in various branches of industry. Applications range from injection nozzles (automotive) over cooling openings in turbine stator vanes (aerospace) and emitter-wrap through perforations (solar) to spinnerets required to adopt more and more complex shapes for the fabrication of functional fibres (textile industry).

Besides electric discharge machining, being very successfully applied in many branches, laser processes are scrutinized and optimized for microhole fabrication due to its inherent, enormous flexibility for many years. It turned out that many parameters must be very precisely controlled in order to enable the generation of holes of just several 10 µm diameter in materials of a few millimetre thickness. For example, copper-vapour lasers, the beam profile of which can be optimized very well to comply with a flat-top profile, can be deployed for the fabrication of holes possessing very good circularity and directrix quality. The manifold capabilities of laser micromachining, however, are not fully capitalized when just stationary laser beams are used (as in the case of percussion drilling).

In the current perspective, the greatest flexibility can be achieved by using the technically very sophisticated concept of drilling with a moved laser beam. Under the latter technology, trepanning as well as helical drilling are subsumed. Upon trepanning, the laser beam is moved along the surface of a cone with customizable opening angle and is in parallel accomplishing a circular movement with defined diameter on the surface of the work piece. Helical drilling distinguishes itself by the fact that in addition to the toggling movement of the laser the laser beam is rotating about its axis of propagation. The latter feature helps eliminate imperfections of the beam profile and provides the capability of drilling holes at highest perfection. Helical drilling requires the utilisation of high-definition drilling heads, the core component of which is a hollow-shaft motor bearing an image rotator, e.g., a Dove prism, as well as adjustment modules for adapting the angle of laser impact and the diameter of the circle scribed on the work piece.

Now that all of these key components can be addressed from the proprietary controls software microMMI, developed by and implemented on all laser micromachining workstations of 3D-Micromac, highly precise micro holes can be machined. Micro bores of just a few 10 µm diameter can be drilled with precisely controllable taper. As an example, in 1 mm thick stainless steel, microholes can be drilled from just one side either with an entrance opening diameter of De = 70 μm and an exit-side opening diameter Da = 140 μm (undercut, Fig. 1), or vice versa, e.g. De = 140 μm versus Da = 70 μm. Every intermediate combination of entrance and exit hole diameter one might think of is feasible, among the latter also exactly cylindrical holes. Deviations from circularity in the order of a few micrometers can be attained on a routine basis. Remarkably, helical drilling is only suitable for thicknesses ranging from a few 100 µm to about 2 mm. This is related to the fact that helical drilling of microholes is not a classical laser-ablation process strictly covered by the rules of ray optics, but rather more resembles a plasma etching process which is excited by the localized application of the laser. Looked at it this way, for too thin material, the erosion plasma does not yet evolve properly while for too thick sheets, the required energy cannot be transferred towards the bottom of the hole.