On Vertical Drag Defects Formation During Direct Chill (DC) Casting of Aluminum Billets
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TRODUCTION
A problem in the production of aluminum billets is the surface defects, which are of large importance for the subsequent extrusion of the billets to profiles. The productivity of the extrusion process is influenced by the billet surface properties, and it sets demands on the size of the segregation zone, which is the governing factor for the appearance of different phases in the surface region. Billet surface quality, for example, is evaluated by measuring surface appearance, segregate zone thickness, large Mg2Si and b-particles, and area fraction of pores near the surface. A strong improvement of billet surface quality has followed from the introduction of the air-bearing mold technology. However, the surface quality can still be a problem and certain types of defects can occur. One of the most common defects found at DC casting by the air-slip technology is vertical drag (VD) defects. This article is focused on the VD defects and is partly based on an earlier paper[1] in which a comparison between segregation to smooth surfaces and to VD defects was made. The VD defects were shown to increase the segregation zone and change the type and amount of particles precipitated at the surface, but it was shown also that different alloys were influenced differently by the defects. It is only in a limited number of papers[1–4] that the cause and nature of the vertical drag defect has been discussed, and a clear understanding of the defect has not evolved. It has been recognized that the VD defects are connected to an increased surface segregation, and the occurrence of such a defect TORBJO¨RN CARLBERG, Professor, is with the Mid Sweden University, 851 70 Sundsvall, Sweden. Contact e-mail: torbjorn.carlberg@ miun.se ANDERS E.W. JARFORS, Professor, is with the School of Engineering, Jo¨nko¨ping University, 55111 Jo¨nko¨ping, Sweden. Manuscript submitted September 3, 2013. Article published online December 17, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
will therefore embody the most sensitive point on the billet for formation of extrusion defects and be the limiting factor for the productivity in the subsequent processing. It is thus of importance to understand the mechanisms behind the formation of the VD defects and the thereto-coupled segregation. The current article is a study to understand such segregation based on recently published data on dilatancy in semisolid slurries.[5–7]
II.
BACKGROUND KNOWLEDGE CONCERNING VD FORMATION
A possible cause for formation of VDs at air-slip hot top casting is that, due to uneven and/or low air pressure in the graphite ring, metal sticks to the graphite ring in the DC casting mold. It is well known from casting practice[4] that old graphite rings and rings, where some sections have low gas permeability, cause surface defects on the billets. In Reference 2, the meniscus and the gap between the graphite ring and meniscus was observed in situ, and it was actually seen that bridges were formed over the gap. Those bridges were associated with an immobilization of the meniscus su
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