Solidification Structure and Secondary Particles in Vertical-Type High-Speed Twin-Roll Cast 3003 Aluminum Alloy Strip

The effect of cooling rate on microstructure of vertical-type high-speed twin-roll cast 3003 aluminum alloy strip was investigated. The solidification structure was characterized in terms of grain size and kinds, size, morphology and chemical composition of secondary particles. The 3003 aluminum alloy strip consisted of cell structure, dendritic structure, globular grains and eutectic structure along the strip thickness direction. From the relationship between the cooling rates and as-cast grain size, the cooling rate of high-speed twin-roll cast strip surface area was estimated as 3.1 x 10(3)K/s. Significant differences in the formation of secondary particles were found between direct chill (DC)-casting and the high-speed twin-roll casting as a result of the different cooling rates; Al-6(Mn, Fe) and alpha-Al(Mn, Fe) Si phase were identified in the DC-cast sample, whereas only alpha-Al(Mn, Fe) Si was predominant in the high-speed twin-roll cast strip. The Al-6(Mn, Fe) particles in DC-cast sample were script-like morphology with high aspect-ratio. In contrast, alpha-Al(Mn, Fe) Si particles in the high-speed twin-roll cast strip was spore-like morphology. Most alpha-Al(Mn, Fe) Si particles in the strip were considered to be formed as one of the eutectic components from the liquid droplets trapped in the inter-dendrite regions. In particular, Fe-rich alpha-Al(Mn, Fe) Si phase was formed at strip central area due to the increase in Fe segregation at the growth front.