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Direct Laser Grain Writing in Steels
D. Gortat, M. Sparkel, S.B. Fairchild and W. O’Neill

Laser melting the surface of 304 stainless steel allows controlled grain growth in the direction of the laser scan [1]. We demonstrate the application of laser surface melting with a Yb fibre laser as a technique for single crystal grain refinement and security marking in polycrystalline metals via localized grain nucleation. Single crystals were achieved after three consecutive passes with constant laser parameters throughout the length of the laser raster at 19.17 kJ/cm2 average energy at 300K, 0.1% O2 environment. The depth of localized grain nucleation was measured to be approximately 20 mm for a single pass, making hidden messages in the bulk of the material possible after mechanically removing the immediate surface melt. The patterns are undetectable by conventional optical microscopy but can be viewed with interferometric microscopy due to fine height differences between untreated and laser treated surface regions, this way establishing a metal security marking technique.

Keywords: Yb fibre laser, 304 stainless steel, 316 stainless steel, laser surface melting, microstructure, single crystal, security marking

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