縦型高速双ロールキャストしたAl-Mg合金板の表面模様に及ぼすMg含有量の影響

抄録

  Vertical-type high-speed twin-roll casting can continuously produce aluminum alloy strips, and have excellent cooling capacity and high productivity. Al-Mg alloy is a typical non-heat treatment type aluminum alloy, and its main strengthening mechanism is solid solution strengthening by magnesium. Periodic patterns consisting of shiny region and un-shiny region with cracks are observed in the casting direction on the surface of the AC7A aluminum alloy fabricated by this method. Deterioration of surface quality, that is, increase in un-shiny region, is anticipated when magnesium content is increased for higher solid solution strengthening effect. In this study, Al-Mg alloy strips with magnesium content varied from 1 % to 12 % was fabricated, and the effect of magnesium composition on the periodic patterns was investigated. On the surface of the strips with the magnesium content of 2 % or more, periodic patterns were observed. As the magnesium content increased, the contrast of the patterns became clear. Magnesium segregation along the grain boundaries was observed in the vicinity of the strip surface in the un-shiny region. That residual liquid in the inter-dendrites and grain boundaries is thought to have been squeezed out to the surface by the solidification shrinkage at the region with locally lowered cooling rate. This resulted in the un-shiny region. The amount of magnesium squeezed out to the surface also increased with the increase in the magnesium content, resulting in, the contrast of the patterns became clear. The spacing of periodic patterns decreased with increasing magnesium content. Since the viscosity of the molten alloy decreases as the magnesium content increases, the vibration of the molten metal meniscus in the vicinity of the nozzle tip may occur more easily. This may be why the spacing of periodic patterns becomes small.