Laser drilling has been carried out with a pulsed Nd:YAG laser. The laser has been applied to the drilling of APC-2 (PEEK-61%C fibre), with specimens up to 1 mm in thickness and using both moving and stationary beams. Hole shapes and heat-affected zone (HAZ) microstructures were studied using SEM, optical microscopy of polished sections and optical interferometric surface profilometry. Two prominent microstructural features were noted in the HAZ of the composite. These were (a) swelling of the ends of the carbon fibres (from about 8 microns to about 12 microns in diameter) and (b) an extensive zone around the hole in which the matrix had clearly undergone melting and some flow. The fibre swelling was such that neighbouring fibres were brought into close contact. The degree of swelling was found to remain constant as the laser pulse energy was increased, although the morphology of the fibre ends became more wrinkled and porous. The phenomenon is thought to be caused by internal gas pressure generation and distortion of the turbostratic structure within the fibre.
A heat flow model has been developed to simulate the drilling process, based on an explicit finite difference formulation and a Cartesian (3-D) mesh. The post-iterative heat accumulation method was used to simulate the effect of latent heat absorption. The explicit approach was found superior to implicit schemes, which tend to exhibit poor stability when handling the phase change algorithm in regions of extremely high thermal gradient. The total solution time was kept within reasonable limits by using a graded mesh, which was fine only in the regions close to the beam axis. Comparisons between predicted and observed hole dimensions showed good agreement over a range of conditions, including variations in beam energy and beam motion. This suggests that effects omitted from the model, such as melt ejection, were not playing a significant role.