Application of a three-dimensional heat flow model to treat laser drilling of carbon fibre composites

Abstract

Fine holes have been laser drilled in thin sheets of metal and of carbon fibre-reinforced thermoplastic. A numerical finite difference heat flow model has been developed, allowing prediction of the hole size and shape, for given irradiation conditions. Good general agreement is observed between measured and predicted hole dimensions, for a range of cases. The major source of error is thought to originate in the physical ejection of molten material from the vicinity of the hole, under the influence of the pressure pulse generated by the laser beam. This apparently resulted in slight underestimates of the hole size for the metals, the effect being more pronounced when the metal is of lower density and higher thermal conductivity (leading to wider molten zones). There was apparently little or no melt ejection with the composite specimens. It was also noted that the carbon fibres in the heat-affected zone around the hole exhibited swellings of up to 50% in diameter. This has been attributed to irreversible changes in the arrangements of the basal planes, caused by the large and rapid thermal expansion. This effect may have been accentuated by the rapid pressurization of fine pores within the structure of the fibres.