Quantification of Melt Ejection Phenomena During Laser Drilling
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Quantification of Melt Ejection Phenomena during Laser Drilling K.T.Voisey, C.F.Cheng & T.W.Clyne* Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, England Tel: +44 (0)1223 334332 Fax: +44 (0)1223 334567 *Email: twc l [email protected] ABSTRACT During laser drilling, material removal in general occurs both by vaporisation and by the expulsion of molten material. The latter commonly arises as a result of the rapid build-up of gas pressure within the growing cavity as evaporation takes place, but the precise mechanisms responsible for the phenomenon are still unclear. The current work is aimed at gaining an insight into these mechanisms via measurements of the amount of material ejected from cavities during laser drilling under different conditions. Attention is first devoted to the issues which need to be considered when making experimental measurements of the fraction of material removed by melt ejection. These include the collection efficiency and the possibility of chemical changes occurring during the process. Results are then presented from work with a range of metallic substrates (mild steel, tungsten, copper, titanium, aluminium and nickel), drilled with a JK701 Nd-YAG laser under different conditions. Observed variations in the melt ejection levels have been studied for mild steel and aluminium and these are briefly considered in terms of the expected effects of certain material property values and the mechanisms of melt ejection. Results from an existing finite difference heat flow model are used to investigate the significance of melt ejection. INTRODUCTION Laser drilling is of considerable interest, since narrow, intense laser beams can be used to drill relatively deep, fine diameter holes, with little thermal or mechanical damage to the surrounding material. Industrial applications include drilling of cooling holes in turbine blades, to allow higher engine operating temperatures. There is also interest in drilling arrays of fine holes in aerofoil surfaces to reduce turbulence and associated drag[ I]. Material removal can take
place via two main routes during laser drilling; vaporisation and melt ejection. Melt ejection means removal of material in the condensed (usually molten) state during laser drilling. It is not well understood and the mechanisms involved appear to be complex. Since the (relatively large) latent heat of vaporisation does not need to be absorbed when material is removed by melt ejection, it could be regarded as an efficient drilling mechanism. In general, the energy required to remove material by melt ejection, even significantly superheated, is about one quarter of that which would be needed to vaporise the same volume. For example, to remove lm of iron by vaporisation 65.8 GJ are needed whereas only 12.3 GJ are required for the removal of the same volume in the molten state[2]. On the other hand, melt ejection can result in irregular and poorly-controlled hole dimensions, particularly due to incomplete expulsion of melt[3], so t
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