Laser shock processing of low carbon steel

Abstract

The effect of laser shock processing (LSP) on the microstructure and mechanical properties of the low carbon (0.04 wt % C) steel was studied. LSP was performed with a 1.054 ^m wavelength Ndrphosphate laser operating in a pulse mode (600 ps duration and up to 200 J energy with a 3 mm diameter beam) with power densities above 10* * W/cmA Shock waves were generated by volume expansion of the plasma formed when the material was laser irradiated. Maximum shock wave intensities were obtained by not using a plasma-confining overlay. When an energy-absorbing black paint coating was used, the specimen surface yielded LSP-induced deformation without melting. The maximum pressure we were able to generate was approximately 2.5 GPa. The rise-time was about 25 ns and a generally 140 ns decay of the laser-shock wave followed. Mechanical properties of material such as surface hardness and residual stress were improved through modifying the microstructure by shock waves. The microstructure of low carbon steel was shown to be strain rate dependent; high density dislocation arrays (~10Wcm.2) were generated by LSP due to high strain-rate deformation and dislocation cell substructures were produced by the low strain rate deformation of shot peening and cold rolling. Strengthening of low carbon steel was a function of dislocation density regardless of processing method.