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 irrad iated by a moving laser source, and the changing shape of single or ov erlapping grooves carved into it by evaporation of material. The laser may operate in CW or in pulsed mode (with arbitrary temporal intensit y distribution) and may have an arbitrary spatial intensity profile. T he governing equations are solved, for both constant and variable ther mophysical properties, using a finite-difference method on a boundary- fitted coordinate system. Results are presented for ablative groove de velopment, including the effects of laser entry and exit (laser scanni ng across the edge of the material), single and overlapped groove shap es and temperature distributions in the solid at different traverse sp eeds, pulsing conditions, and power levels. Experimental results were obtained for groove shapes of single and overlapped grooves, using gra phite as the ablating material and employing a CW CO2 laser (10.6 mu m ) 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 cm s(-1). Comparison between experimental and theoretical results indicates good qualitati ve agreement between theory and experiment within the limits of the (r ather large) uncertainty with which material properties are known to d ate.