Laser glazing – a new process for production and control of rapidly chilled metallurgical microstructures

A new process for producing rapidly chilled metallurgical microstructures under reproducible cooling-rate conditions has been developed. Processing is accomplished by rapidly traversing a continuous, high energy-density laser beam over the material to produce a very thin molten layer at the material surface. The high specific rate of energy delivery facilitates surface-localized melting at very high melting efficiencies, i.e. the major portion of the absorbed energy is used for melting, with only a very small fraction going into heating of the solid, subsurface material. This ability to maintain a cold substrate while melting a thin surface layer of material results in rapid quenching of the molten layer once the energy source is removed. Calculations made for pure nickel indicate that cooling rates of 5 × 10⁸ Ks⁻¹ are attainable in layers that are 2·5 × 10⁻³ mm thick, while 2·5 × 10⁻² mm thick layers cool at rates of up to 5 × 10⁶Ks⁻¹. The importance of rapid quenching is based on the fact that the unsurpassed homogeneity of the liquid can be preserved (or nearly preserved) in the solid, which can then be utilized in that form, or these structures can be further modified in order to produce other desired properties. The subject process, termed’ laser glazing’, has been shown to be capable of producing a wide variety of novel and interesting metallurgical microstructures on the surfaces of bulk metals. Amorphous surface layers have been produced on bulk substrates by laser glazing compositions at or near deep eutectic troughs. In other compositions, extended solid-solution phases have been obtained. These can be decomposed into finely divided multiphase microstructures in the solid state. Such compositions are candidates for new dispersion-strengthened alloys. At higher second-phase contents, extremely fine filamentary eutectic microstructures have been obtained. A strong tendency for oriented overgrowth (epitaxial growth), in various orientations, has been demonstrated for a variety of mono crystalline substrate materials. Laser glazing (in conjunction with surface alloying, post-glaze heat treatment, and/or deformation) has great flexibility as a means to generate a broad range of novel surface microstructures and properties. Furthermore, although currently being employed primarily for the surface treatment of materials, it also has the potential for processing bulk materials by the sequential build-up of one glazed layer upon another.