Laser machining of ablating materials — overlapped grooves and entrance/exit effects

A three-dimensional conduction model has been developed to predict the transient temperature distribution inside a thick solid that is irradiated by a moving laser source, and the changing shape of single or overlapping grooves carved into it by evaporation of material. The laser may operate in CW or in pulsed mode (with arbitrary temporal intensity distribution) and may have an arbitrary spatial intensity profile. The governing equations are solved, for both constant and variable thermophysical properties, using a finite-difference method on a boundary-fitted coordinate system. Results are presented for ablative groove development, including the effects of laser entry and exit (laser scanning across the edge of the material), single and overlapped groove shapes and temperature distributions in the solid at different traverse speeds, pulsing conditions, and power levels. Experimental results were obtained for groove shapes of single and overlapped grooves, using graphite as the ablating material and employing a CW CO2 laser (10.6 μm) focused with a 5-inch (12.7 cm) lens for powers ranging from 400 to 1200 W and scanning speeds ranging from 2.5 to 10 cms−1. Comparison between experimental and theoretical results indicates good qualitative agreement between theory and experiment within the limits of the (rather large) uncertainty with which material properties are known to date.