Dynamic Reactive Wetting and its Role in Hot Dip Coating

摘要

The effect of substrate preheat temperature on the dynamic wetting of 55Al-43.4Zn-l.6Si hot dip coating melts on low carbon steel substrates has been investigated. An experimental apparatus based on the sessile drop technique was developed which allowed the substrate to be preheated to a different temperature than the bath. The initial wetting and spreading of a molten metal droplet on a substrate was recorded at 1000 frames per second using a high speed digital camera. Wetting was improved (θ decreased from 120°to 25°) as the substrate preheat temperature was increased and approached the bath temperature, beyond which the improvement in wetting was negligible. Immersion experiments using a thermocouple instrumented substrate dipped into a coating bath were performed for varying substrate preheat temperatures. Interfacial heat fluxes and interfacial resistances were calculated from the temperature responses. The "minimum" interfacial resistance was decreased by an order of magnitude (1x10~(-4) to 2x10~(-5) m~2K/W) as the substrate preheat temperature was increased from room to bath temperature. The reduction in interfacial resistance was related to the improvement of the initial wetting and the increase in mass transfer of iron atoms from the substrate across the interface. There was an apparent increase in the "minimum" interfacial resistance for substrate temperatures greater than the bath temperature. This was due to the increased rate of alloy layer formation and the exothermic nature of the reactions. The significance of these findings was discussed with respect to the mechanism of alloy layer formation at the interface during the initial stages of solid-liquid contact.