Tribological aspects of aluminium extrusion dies
Sammanfattning: Extrusion is a common method to manufacture long products of aluminium, such as beams, rods and tubes. The aim of this thesis was to improve the material in the die bearing surface, and thereby the service life of extrusion dies through wear studies of actual dies, and experimental simulations. Field tests showed that a die bearing is initially worn through delamination of the surface layer followed by chemical degradation and subsequent mechanical removal of the tool steel by the hot aluminium. An experimental test based on a block-on-ring configuration has been developed with the ability to reproduce the tribological conditions on the die bearing, including wear mechanisms, wear rate and the influence of process parameters typical for extrusion. It was shown by experimental simulation that nitriding has the potential to increase the service life of tool steel with almost one order of magnitude. The iron nitrides protects the steel from chemical degradation, and thereby reduces wear. Especially, the presence of a homogenous compound layer is beneficial for the wear resistance. Furthermore, physically vapour deposited (PVD) coatings of CrN, (Ti,A1)N and TiB2 have the potential to increase the wear resistance significantly, as compared to nitrided steel. They combine a superior mechanical strength with high chemical and thermal resistance against hot aluminum. The wear rate of nitrided steel was found to decrease at lower extrudate temperatures, and at relatively long stop times, associated to the recurrent insertion of new billets during extrusion. Finally, experimental simulation of extrusion was used to develop a model for prediction of the wear resistance of the tool, taking into consideration the individual contributions from the tool steel, nitrided layer and PVD coating.
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