There are a number of imperfections which laser-drilled holes in metals can exhibit. The main ones associated with pyrolytic drilling (i.e. via melting and/or vaporisation) are discussed below, together with possible means for preventing or minimising them.
During drilling, a recast layer can form on the inside walls of the hole. This layer is formed by re-solidification of vaporised or molten material on the walls of the hole, and it is in general not evenly distributed along the hole circumference and depth. Detrimental effects of the recast layer are decreased accuracy and repeatability of the hole size and geometry, changes in flow characteristics (important in for example spray nozzles and cooling holes for aero-engine components), and increased heat affected zone (HAZ) width and thermal stresses, which can lead to micro-cracking (see below). The occurence of coarse recast is particularly associated with melt expulsion during percussion drilling.
To minimise the recast layer thickness, parameters should be chosen that allow the material to be expelled from the hole effectively. This in general means short pulses at high intensity, which changes the material removal mechanism from molten ejection to an evaporation dominated process. The use of a suitable process gas may also be beneficial.
Large HAZ width
The HAZ results from heat conduction into the material directly adjacent to the hole. As the HAZ will generally have properties worse than those of the parent material (e.g. because of grain growth or precipitation/segregation), it should be kept as small as possible. The HAZ can be minimised by using short pulses and low repetition rates (limiting the heating of the component), and by evaporation dominated material removal rather than molten ejection.
Micro-cracks result from excessive thermal stresses, and are more likely to occur when a thick recast layer is present. Micro-cracks may be confined to the recast layer, but can also extend well into the parent material. Means of minimising the likelihood of micro-cracks are similar to those for minimising the recast layer and HAZ thickness (see above).
Taper (variation in hole diameter, along its depth) results from the choice of focus position, as well as erosion by ejected material, and therefore tends to be more prevalent in deeper holes. Taper can be present at both the entrance and exit of the hole. Although for some applications taper can be tolerated or even beneficial, it is unwanted in most cases. Taper is, in general, less when long focal length lenses (and thus longer focal depths) and shorter pulses are used. The position of the focal point is of paramount importance: with the focal point at the workpiece surface, cylindrical holes are drilled, whereas with the focal point above or below the surface, tapered and conical holes are drilled.
Material can build up at the entrance and exit of the hole, reducing repeatability and flow characteristics and roughening the surface. It generally results from the same mechanisms as recast, and thus the means of prevention are similar.