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

A three-dimensional conduction model has been developed to predict the temperature distribution inside the solid, and the shape of one or several overlapping grooves formed by partial evaporation of a thick, rectangular body, if the body is irradiated by a moving laser source. The governing equations are solved, for both constant and variable thermophysical properties, using a finite-difference method on a boundary-fitted coordinate system. The laser may operate in CW or pulsed mode (with arbitrary temporal intensity distribution) and may have an arbitrary spatial intensity profile. This has application in laser machining where material is removed by repeated scanning of a focused beam on the workpiece surface. 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 lens for powers ranging from 400 to 1200 W and scanning speeds ranging from 2.5 to 10cm/s. 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.