Application of DLC Coated Dies to Variable Shape Extrusion of Aluminum Square Pipes

An aluminum space frame s one of the candidates to be applies to a structure of car bodies in near future because of its lightweight as well as high strength. To omit the secondary forming process in the production of an aluminum space frame, a variable shape extrusion apparatus with either a taper mandrel or a step mandrel and four movable dies was developed, and experiments to investigate the characteristics of hot extrusion were conducted by using the apparatus. Results from the experiments suggest that the step mandrel provides good dimensional accuracy due to its bearings parallel to the extruding direction. On the other hand, it was also found that pick-up occurred easily on the surface of extrusion when the step mandrel was used. Dies were coated with diamond like carbon (DLC) around their bearings to prevent the dies from sticking of aluminum, and it was confirmed that the DLC coating has a significant effect to improve the condition of the surface of extruded pipes.