Correlations between hole properties and pulse parameters in single pulse Nd:YAG laser drilling of aluminium and titanium Available to Purchase

In a free-running Nd:YAG laser there is a variation between pulses which is often not within the control of the laser operator. Not only do the energy and spatial profile of the pulses fluctuate, but the temporal pulse length and temporal structure within the pulse also vary randomly due to relaxation oscillations within the gain medium. This paper investigates the effects of these pulse-to-pulse variations on the properties of holes produced by the single pulse drilling of titanium and aluminium. The temporal profile of the pulse is described by its total energy, length and its first and second moments in the time domain. The spatial profile of the pulse is characterised by the diameter of the focussed spot, as determined by the second moment method of defining the diameter of a laser beam, and the symmetry of the focussed spot, defined as the ratio of the shortest and longest dimensions of the slightly elliptical spot. The statistical method of simple linear correlation analysis is used to investigate degrees of correlation between the pulse characteristics and the size and shape of the holes produced. Also presented are entrance and exit diameter size distribution curves of arrays of holes, along with correlations between entrance and exit hole diameters as the material thickness is varied. These results are discussed with reference to the material removal process in which the ejection of molten material is known to be important.