Laser surface modification of steel

The presented work is an investigation of the laser surface modification of H13 tool steel using pulse laser processing mode. Initial screening experimental designs conducted lead to more optimised detailed designs. A carbon dioxide (CO2) laser system with 10.6 μm wavelength was used. In the experimental designs investigated three different sizes of laser spot used were 0.4, 0.2 and 0.09 mm diameter. The other controlled parameters were laser peak power, pulse repetition frequency and pulse overlap. The laser processing was constantly assisted by in line argon gas at 0.1 MPa pressure. H13 samples were roughened and chemically etched prior processing to improve the surface absorbance at the CO2 laser wavelength. Laser processed samples were prepared for metallographic study and were characterised for physical and mechanical properties. The metallographic study and chemical composition analysis were conducted using scanning electron microscope integrated with energy dispersive x-ray spectroscopy. The crystallinity and phase detection of the modified surface were conducted using an XRD system with Cu Kα radiation and wavelength of 1.54 Å. The surface profile was measured using stylus profilometry measuring systems. The hardness properties of the modified surface were measured by micro- Vickers diamond indentation. A customized thermal fatigue system was used to investigate the effect of surface roughness on the modified surface fatigue properties. A modified surface grain with an ultrafine size of less than 500 nm was observed to be achievable. A modified surface depth which ranged between 35 and 150 μm was developed on the laser processed H13 samples. A reduction of crystallinity was noticeable for the modified H13 surface which was related to the more random distribution of crystallites after laser processing. A minimum modified H13 average surface roughness, Ra, of 1.9 μm was achieved. Another important finding was that at different settings of laser parameters, the modified H13 surface exhibited a range of hardness between 728 and 905 HV0.1. A relationship between thermal simulations findings (heating and cooling rates) and hardness results was established for further understanding of the effects of the laser parameters. These findings are significant to the establishment of surface hardening techniques for wear resistance and thermal barrier coating applications