Striation Formation and Surface Finish in Laser Cutting of Mild

The mechanisms of melt ejection and striation formation in laser cutting of mild steel are discussed. It is argued that the melt ejection from the cutting front is not a steady state process, but rather shows a cyclic phenomenon. The striation are strongly affected by the unstable characteristic of the thin liquid film on the cutting front during the melt ejection, together with the oxidation and heat transfer process. Dependent on the cutting speed, the liquid film will either rupture or generate waves on the cutting front. Theoretical explanation is given according to the instability theory of a thin liquid film in a high velocity gas jet and the diffusion controlled oxidation theory. The striation frequency and depth can be estimated according to the above theories. Experimental investigations were carried out and the results are consistent with the calculations. The better understanding has shed light on further investigations and optimal process development. NOMENCLATURE A constant b kerf width B constant C oxygen concentration c wave velocity cr friction factor cp heat capacity D diffusion coefficient d the work-piece thickness f wave frequency h liquid film thickness I defined in equation (II) J particle current in oxidation K heat conductivity k wave number ka Boltzmann constant P r nonnal stress exerted at the interface P 8 gas pressure Po the oxygen pressure at the surface of the melt R Reynolds number of liquid, R= Vhlv sm the maximum thickness of the oxide layer T1 tangential stress exerted at the interface To activation temperature t time tp time period per oxidation cycle