Laser machining and its thermal effects on silicon nitride and steel

Abstract

<p>An investigation was undertaken to examine the physical and thermal processes involved in laser machining of ceramics and metals. Two types laser machining viz. milling and cutting were studied. The objective is to provide a better understanding of laser machining that in turn facilitate achieving tighter tolerances, excellent surface integrity and improved properties of the parts;A 1500 W continuous wave CO2 laser was used for this purpose to groove and mill reaction bonded silicon nitride (RBSN) and hot-isostically pressed silicon nitride (HIPSN). Experimental results showed that the material removal mechanism was essentially chemical decomposition at 2150 K leading to the formation of silicon and gaseous nitrogen. Deeper cuts (≥0.5 mm) were obtained but at the expense of formation of high density cracks, rough surfaces and recast layer. In milling, nitrogen produced deeper cuts than oxygen because of the presence of recast layer is minimum. Deeper cuts and less cracks were observed in the laser milling of RBSN as compared to HIPSN mostly due to the large amount of porosity present in RBSN that enabled better absorption of laser energy and reduction of thermal stresses;Two key variables that promote excessive recast layer are the number of successive, overlapping passes and the assist gas flow condition. Experimental measures such as use of defocused beam, chemical etching, use of nitrogen and high gas flow rate were attempted to reduce the size of recast layer;Laser cutting, the most widely used laser machining process in the industry today, of mild steel plates was conducted to determine the dimensional accuracy of holes which depends on two factors: the accuracy and repeatability of the positioning system, and the thermal effect of the laser beam on the material. A physical model was formulated to estimate the dimensional tolerances due to thermal shrinkage of the steel during laser cutting of holes. The model was used to calculate the size the hole and cut-out disk of varying radii in steel plates with thicknesses of 3.2 mm and 6.4 mm. The model data was in excellent agreement with the experimental data especially for smaller diameter holes.</p>