タンデム式縦型高速双ロールキャスト法で作製したAl-Mn/Al-Si合金クラッド材皮材の凝固挙動

抄録

  A vertical-type tandem twin-roll caster is able to produce three-layer clad strips from molten metal directly. Three Al-Si alloys (10%Si, 12.6%Si, 14%Si) were used as the overlay alloy to clad the A3003 base alloy. The effects of Si composition on the solidification manner of the overlay and bonding conditions of the base/overlay interface were investigated to examine whether high Si composition brazing sheets can be fabricated by subsequent cold rolling. Regardless of the Si composition, the microstructure of the overlay consisted of an α-Al phase and eutectic phase from the surface to the mid-central region in the thickness direction. This is due to the high cooling rates, which promotes the shift of the coupled zone. On the other hand, the microstructure of the overlay close to the interface of the base strip was not uniform along the casting direction. An α-Al rich region and eutectic-rich region was observed intermittently. The local difference in α-Al ratio was due to the compressive and shear forces generated by the roll rotation. At the base/overlay interface, no un-bonded area was observed in 10%Si, but local un-bonded areas were observed for 12.6%Si and 14%Si. From the EBSD analysis, the orientations of the base and overlay crystal were equivalent at the bonded area. This indicates that heterogeneous nucleation of the overlay alloy took place on the base alloy strip surface. Thus, solidification of the overlay on the base strip surface is the one of the important factors promoting strong base/overlay bonding. A local un-bonded area at the base/overlay interface and dispersion of fine primary Si particles were observed for 12.6 and 14%Si. Regardless of this, cold rolling of these clad strips was successful without any fracture. This implies that the Si composition range applicable for the overlay of the brazing sheet can be expanded by vertical-type tandem twin-roll casting.