Grain Refinement and Ductility Improvement by Hot Extrusion Using a Heteromorphic Die with Small Holes

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It has been shown using an Al–4.0mass%Cu alloy as a demonstration material that hot extrusion with the use of a specially–designed die leads to grain refinement and improvement in ductility. The specially–designed die is called in this study the “heteromorphic die” and it consists of two plates and one spacer. One plate is called the “strain–giving die” and has six small round holes of 5 mm in diameter, while the other plate is called the “shape–giving die” and has one big rectangular hole of 9 mm × 15 mm. The spacer separates these two plates at a distance of 10 mm and provides a space between them. The Al–Cu alloy billet was hot–extruded through the six small round holes of the strain–giving die to produce six thin round rods, and after filling the space between the two dies, the six rods passed together through the shape–giving die to form a thick rectangular bar or plate. Tensile tests of the plate were conducted at room temperature, and the results were compared with those of the un–extruded billet and the extruded plate produced by conventional hot extrusion method, that uses only the shape–giving die. The elongation values of the three different tensile specimens produced from (1) the un–extruded billet, (2) the plate extruded by the conventional method and (3) the plate extruded with the heteromorphic die were 21, 35 and 57%, respectively. Most significant elongation was found in the sample extruded with the heteromorphic die, which is most probably due to the finest grain structure observed in this sample. It is suggested that the recrystallization is induced by a large strain generated in the heteromorphic die due to the friction on the large contact area between the extruded metal and the die orifice wall of the “strain–giving die.”

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