Journal of Materials Processing Technology 172 (2006) 22–29 Laser cutting of steel sheets: Influence of workpiece thickness and beam waist position on kerf size and stria formation Cihan Karatas a , Omer Keles b,∗ , Ibrahim Uslan b , Yusuf Usta b a Department of Nuclear Engineering, Hacettepe University, Ankara, Turkey b Department of Mechanical Engineering, Gazi University, Muhendislik Mimarlik Fakultesi, Maltepe, 06570 Ankara, Turkey Received 17 February 2005; received in revised form 3 August 2005; accepted 18 August 2005 Abstract CO 2 laser cutting of steel sheets is considered and influence of beam waist position relative to the workpiece surface and the workpiece thickness on the striation formation is examined. The kerf width is modeled using a lump parameter analysis. The measurements are composed with the experimental findings. SEM and optical microscopy are conducted to examine the cutting surfaces. It is found that beam waist position has significant effect on the kerf size and as the workpiece thickness reduces, the relative location of beam waist position varies for the minimum kerf width. The microcracks are found in the resolidified material at the cutting surface, which is due to non-uniform and rapid cooling of the resolidified material at the surface. © 2005 Elsevier B.V. All rights reserved. Keywords: Laser; Cutting; Beam waist; Stria 1. Introduction Laser cutting is one of the important applications of laser machining process. In laser gas assisted cutting operation, assist- ing gas can be reactive or inert depending on the process require- ments. In metal gas assisting cutting process, in general, oxygen (reactive gas) is used while argon or helium (inert gasses) is used for wood and plastic cutting. Use of oxygen provides exothermic reaction in the cutting section enhancing the energy available for the cutting process. On the other hand, argon or helium pre- vents high temperature exothermic reactions and avoids the side ways burnings and excessive mass removal from the cutting kerf. Moreover, regulation of gas pressure and proper selection of nozzle are necessary for controlled cutting process. In laser cutting, the end product quality is that matters. Due to oscillation in laser output power, assisting gas mass flow rate, material impurities and workpiece scanning speed result in insta- bilities associated with molten layer in kerf. This, in turn, causes striation formation and out of flatness at the cutting edges. The striation can be characterized geometrically by dross height and stria frequency [1]. The pulsation of laser output power at cer- ∗ Corresponding author. Tel.: +90 312 2320427; fax: +90 312 2320427. E-mail address: omer@gazi.edu.tr (O. Keles). tain frequencies may reduce the dross height of stria. However, the combination of parameters reducing the dross height has not been established yet. Consequently, investigation into stria characteristics is necessary for improved laser cutting process. Considerable research studies were carried out to examine laser cutting process. The influence of surface plasma on laser cutting was studied earlier [2–7] and penetration time during laser cutting process was examined by Yilbas et al. [8]. The qual- ity improvements for laser machining operation were considered by Pietro et al. [9]. They indicated that the process manipulation could lead to significant quality improvements. Laser cutting of metallic coated steel sheets was carried out by Wang [10]. He employed a statistical analysis when optimizing the laser cutting process. Laser cutting of non-metallic materials was examined by Zhou and Mahdavian [11]. They introduced two correction parameters to improve the mathematical model. Laser cutting of fibre reinforced plastic composite materials was considered by Cenna [12]. He compared the predictions with experimen- tal results and indicated that they were in good agreement. The parameters affecting the laser cutting process were investigated by Yilbas [13]. He developed a method for pattern classification using an artificial neural network and identified the striation pat- terns successfully. Laser machining operation was investigated by Meijer [14]. He indicated that the trends in miniaturization in laser machining were expected to continue due to short and 0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.08.017