Flow behavior of the billet surface layer in porthole die extrusion of aluminum
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I. INTRODUCTION
THERE are many casting defects, such as an inverse segregation layer, ripple, etc., in the surface layer of a cast billet. In addition, impurities such as oxides or lubricants are likely to adhere to the surface of the billet during various stages: homogenizing, cutting, and the heating process for extrusion. When the billet is extruded, such surface defects of the billet contribute to various extrusion defects, such as pickup, tearing, blur after anodizing treatment, etc. For these reasons, it is desired in extrusion of aluminum and its alloys to avoid the flow of the billet surface into the product. As a matter of fact, understanding of how the material close to the billet surface flows and how the flow of the billet surface layer is influenced by extrusion parameters is of great importance because it lets us know the soundness of extruded products, the appropriate discard length, etc. Most research[1–8] on the flow behavior of the surface layer has been made principally about axisymmetric extrusion, that is, the simple rod extrusion, while little has been known about the flow behavior of the surface layer in porthole die extrusion. The metal flow in porthole die extrusion is much more complicated than simple rod extrusion; in addition, seam welds exist. It has been a special concern whether the billet surface layer flows into the welding region. In this study, the flow behavior of the billet surface layer was investigated by changing some extrusion parameters with respect to porthole die extrusion of aluminum. II. EXPERIMENTAL The indicator method in which pins or rings were inserted into the billet surface has been used to investigate the flow behavior of the billet surface layer in extrusion. Regarding aluminum extrusion, AA4xxx or AA2xxx alloys were usually used as a flow indicator, while the base metal was
YOUNG-TAE KIM, Graduate Student, and KEISUKE IKEDA, Professor, are with the Department of Materials Processing, Tohoku University, Sendai 980-8579, Japan. Manuscript submitted May 24, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
different from these alloys for discrimination. Accordingly, it would be liable that there was some discrepancy between the experimental result and the actual flow of the surface layer due to different mechanical properties between them. To get rid of this plausible discrepancy, two alloys, AA1050 and AA1100, between which mechanical properties were almost identical, were selected in this experiment. The billet for experiment consisted of these two kinds of aluminum alloys, which showed different etching characteristics. After etching treatment, the AA1100 alloy turned dark and the AA1050 alloy remained white. The chemical composition of the two alloys is listed in Table I. The sheet (0.5 mm in thickness) and wire (0.5 mm in diameter) of AA1100 alloy were prepared as the flow indicators. Longitudinal sections of two types of billets for experiments are shown in Figure 1. The prepared sheet was wound tightly around the circumference of the billet, and both end
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