Laser Shock Peening of Ni-Ti Bulk Structures Developed by Laser Additive Manufacturing

Abstract

Three different compositions of bulk Ni-Ti samples fabricated by laser additive manufacturing technique are subjected to laser shock peening with multiple shots of 1 GW/cm2 irradiance. A numerical simulation is implemented to predict the amount of residual stress to be generated on the sample surface to optimize the processing parameters. The experimental results reveal the number of shots to be three, in order to achieve the desired amount of residual stress from the chosen power density for processing. LSP yields a total increase in the residual stress by 40-45% on the surface of the samples. To investigate the nature of samples, after LSP several characterizations are done to study the mechanical, microstructure, phase transformation and crystalline properties. The scanning electron microscopy images reveal the surface morphology of the samples, to be roughened in nature by the effect of LSP. Also, the enhancement in the microhardness by 20% vividly shows improvement in the mechanical properties. X-ray diffraction shows a substantial fluctuation in peak intensity and peak positioning post LSP. The peak diminishing is observed after LSP in the XRD graph, which confirms the amorphization induced in the samples. Differential scanning calorimetry shows an increase in the phase transformation temperature by 20-30 °C for the NiTi50 sample. The study reveals the composition of NiTi50 fabricated by LAM was able to sustain the pressure generated by LSP by withholding its physical and chemical nature.