Study on the Effect of Focal Point Location and Incidence Angle of Laser on Cut Quality of Thick Stainless Steel Sheet by Yb-Fiber Laser

Abstract

Fiber laser owing to its higher efficiency, beam quality, reliability and beam deliverability through optical fiber has a potential to outperform the more traditionally used CO₂ lasers in sheet metal cutting applications. However, it is found that the high focusability and shorter wavelength of this laser is advantageous for cutting thin metal sheets up to 2.0 mm only, while CO₂ laser still performs better in cutting thicker sheets. It has been reported that this has an unfavourable angle of incidence, resulting in poor absorption at metal surface. The narrow kerf produced by fiber laser may cause an earlier flow separation resulting in poor cut in thicker sheets. Therefore, the effect of focal point position and incidence angle, along with other process parameters, viz. laser power, cutting speed and O₂ gas pressure on cut quality of 4 mm thick steel sheet has been investigated. It has been found that cutting with the focal point located at the bottom surface with fixed stand-off distance makes the kerf-width wider, resulting in a better laminar gas flow through kerf for a longer distance, which reduces uncontrolled burning, producing a better quality cut. The change in laser incidence angle towards and away from the cut front, both improved the cut quality. Incidence angle influences the absorption of laser light, gas flow characteristics, laser intensity distribution and shear force at the surface. Results of the detailed parametric study are presented.