Al-Si strips of various silicon contents were produced by the unequal diameter twin-roller strip casting process, to study the solidification structure. Both the roller speed and the contact angle were varied in the process. The results indicate four zones of solidification structure, from the bottom surface, near the main roller to the top surface: 1) columnar dendrite zone, 2) equi-axed cell zone, 3) mixed zone, and 4) upper columnar dendrite zone. Increasing roller speed and silicon content can decrease the thickness of equiaxed cell zone. The latter also has the effect of increasing the thickness of columnar dendrite zone. Increasing contact angle can decrease the thickness of equiaxed cell zone and increase those of the other three zones. The solidification rate, which increases with increasing roller speed and silicon content, is an important factor leading to the various microstructures of the Al-Si strips. Estimated from the secondary dendrite arm spacing and equiaxed cell size, the solidification rate is high in columnar dendrite zone (10~2℃/ sec-10~3℃/sec) and low in equiaxed cell zone (10℃/sec-20℃/ sec). Columnar dendrite zone, mixed zone and upper columnar dendrite zone of higher solidification rate contain rod-like eutectic silicon particles. Contrarily, the eutectic silicon particles in equiaxed cell zone of lower solidification rate are acicular.
展开▼
