Effect of Si Content on Strip Thickness and Solidified Structure in High-Speed Twin-Roll Cast Al-Si Alloy Strips

Abstract

Al-Si alloy strips with various Si compositions were cast by using high-speed twin-roll caster. The strip thickness measurement and microstructure observation were carried out. The strip thickness changed prominently with varying Si composition. The strip thickness change was considered to result from change in solidification manner (skin or mushy-type solidification) depending on freezing temperature range of the alloy. The solidified structure of the mid-thickness region is also greatly influenced by the freezing temperature range. Internal crack appeared at the mid-thickness region of 2 mass% Si strip, in which the central band consists mostly of globular grains and equiaxed dendrites. The internal cracking is considered to be related with residual liquid in the central band region and its solidification.

Solidification Structure and Casting Defects in High-Speed Twin-Roll Cast Al–2 mass% Si Alloy Strip

“…11) It was already shown in the previous study 14) that a thick band structure consisting of fine globular grains formed in the alloys with wide freezing temperature range, and the internal cracking always occurred in the central band region. The fractography 14) found that the crack surface was covered with smooth round tips of dendrite branches and globular grains.…”

“…Fractography of the crack surface revealed that a shrinkage cavity was the primary reason for the cracking. 11,14) This suggested that distribution of residual liquid in the mid-thickness region when the strip goes through the roll nip was a dominant factor controlling the internal cracking.…”

“…1518) However, a severe problem arose in the high-speed twinroll cast alloy with a wide freezing temperature range. 11,12,14) Large-scale cracking occurred in the mid-thickness region during the strip casting. Fractography of the crack surface revealed that a shrinkage cavity was the primary reason for the cracking.…”

“…14) For this purpose, hypo-eutectic binary AlSi alloys with various Si contents were cast under the same casting conditions. The strip thickness changed distinctly with changing the Si compositions, and the largest strip thickness was obtained in Al2 mass% Si strip.…”

“…A feeding nozzle was installed over the rolls to introduce a large hydrostatic pressure. Various kinds of aluminum alloy strips such as AlMgSi based, 1013) and AlSi alloys 14) were fabricated by using the caster. The cast strip with refined solidification structure due to the high cooling rate improved toughness and formability.…”