The process of wetting of Al2O3p/60611Al substrate by Zn-Al filler metal in air was investigated. The filler was found to wet the substrate by undermining the oxide layer on the substrate. Dynamic observations Of Substrate oxide undermining, conducted using conventional illumination and recording, were correlated with post-cooling microscopy and analysis. The undermining mechanisms were discussed and the influences of temperature, surface roughness and gas evolution on the propagation of undermining were examined. The Zn-Al filler droplet exhibited excellent wettability to the Al2O3p/6061Al substrate via undermining and it completely spread on the Substrate in the end. Spreading of the filler was found to be rapid before the filler droplet was depleted, wherein the increase of the undermining region radius was a linear function of time. However, after the depletion of filler droplet, the spreading became significantly sluggish and negligible further undermining was observed. The current wetting system was non-reactive and the linear spreading was believed to be associated with the presence of a precursor film showing up ahead of the macroscopically observable spreading front. The undermining velocity at the 'linear' spreading stage was independent of the volume of the filler droplet, but increased linearly with temperature. The propagation of undermining was Much rapider on the substrate with higher toughness and was encouraged by the escaping of gaseous products from the substrate. (C) 2008 Elsevier B.V. All rights reserved.