Microstructural refinement of aluminium-zinc-silicon coated steels

- The microstructure of 55Al-Zn-Si coating on steel sheet was refined by laser processing at scan rates up to 0.33 m/s.
- Cooling rates (105 C/s) achieved were much higher than the typical cooling rates (100 °C/s) encountered in continuous galvanising process.
- Dendrite arm spacing was significantly reduced from 10 µm in the untreated coating to 0.9 µm.
- The coating hardness was increased from 78 HV to 135 HV.

This paper presents the analysis of microstructural refinement of a 20 μm thin alloy coating, containing (in weight percentage) 55% Al, 43.4% Zn and 1.6% Si, on a 0.42 mm thick steel sheet. Refinement of the 55Al-Zn-Si coating microstructures was achieved without affecting the steel substrate, by rapid thermal processing using a high power laser. A quantitative analysis was conducted based on real-time surface temperatures data obtained using a fast response optical pyrometer during laser processing, and measurements of dendrite arm spacing and microhardness. For refined coatings, it was found that the peak surface temperatures ranged from 800 °C to 1500 °C, and cooling rates on the order of 105 °C/s were achieved. The refined microstructures had very small dendrite arm spacing of less than 2 μm compared to 10 μm in untreated coatings. The spacing was reduced from 2.2 μm to 0.9 μm when the processing speed was increased from 0.083 m/s to 0.33 m/s. The rapid processing also increased the hardness of the coatings from about 78 HV up to 135 HV. Such refinement in microstructure can be expected to enhance the corrosion resistance of aluminium-zinc-silicon coated steel sheets.

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