In the hot extrusion process, the working material is forced to flow through a die with the desired profile. Depending on the billet material and the die geometry, extrusion is performed at the room temperature or at the elevated temperature enabling manufacture of parts with various and complicated cross sections. According to the cross-sectional shape of extruded parts, extrusion can be divided into two classes: solid shape extrusion and hollow shape extrusion. For extrusion of hollow shapes, as in tube extrusion, dies with complex structure are employed, such as bridge dies, spider dies, and porthole dies, which have stub mandrels as an integral part of them. Parts with hollow cross sections made by any of the above dies have inevitably one or more seams or longitudinal weld lines, because the billet is divided into several sections to flow around the core supports and subsequently welds together in the welding chamber of the die. Thus, the failure of hollow extruded products mostly occurs along one or more weld lines when the products are subject to severe internal pressure or expansion in the practical use. In general, the bonding strength of solid-state welding of metals is affected considerably by the following factors: welding temperature, welding pressure, and surface extension of the welded interface. In most cases, the welding temperature is a given external condition: it is not related to the die structure. Meanwhile, the design change of a die is very closely related to the change of welding pressure and surface extension of the welded interface. In particular, welding pressure is more sensitive to the modification of a die than surface extension of the welded interface, which means welding pressure is more amenable to control.
In this work, a modified porthole die for tube extrusion has been developed in order to obtain larger welding pressure than that of conventional porthole dies, and the effect of the improved porthole die on welding pressure has been investigated by performing the finite element analysis on aluminum tube extrusion. Traditional design of extrusion dies has been usually carried out based on human experience and knowledge, i.e., trial and error, which is time- and cost-consuming. In order to reduce time and cost required for the die modification, the rigid-plastic finite element method has been adopted to calculate the pressure in the welding chamber. The results of two- and three-dimensional analyses of the welding chamber have shown that the pressure of the chamber of conventional porthole dies is insufficient to ensure high bonding strength of the welded interface. Moreover, they have found that the observation of pressure distribution of the chamber for varied chamber height and tip radii of the webs has proposed the fact that the pressure increases sufficiently as the welding chamber becomes longer and narrower, and the webs sharper. The comparison of welding strength of tubes extruded by the modified porthole die with that of tubes made by a conventional porthole die on an artificial expanding test has shown that the tubes from the modified die have improved welding strength.
