Material flow in extrusion of aluminium hollow profiles
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The process of aluminium extrusion has great importance in the metal forming industry because of its ability to produce profiles of various complicated geometries, shapes and sizes. The flow of metal within the extrusion die influences the mechanical properties of extruded products. Metal flow being the core of process, attracts academia and industries to develop better understanding how does metal flow occur during extrusion. Extrusion welding is another prominent topic which has attained attention of industry that is keen to produce hollow profiles with sound extrusion welds. The aim of my research work has been to gain increased understanding about metal flow, extrusion welding and deformation characteristics of aluminium when extruding through a porthole die. I have focused to encircle the topic of flow of metal and to characterize welding conditions during production of aluminium hollow profiles. The art to achieve real metal flow from extrusion process has been developing for some decades. Initially it was the intention to expand the study to the end of the work by including 3Dsituations of extrusion welding corresponding to that occurring in industrial extrusion. But as the work proceeded it was realized that the conditions are so complex in extrusion welding, that it was sufficient challenge to map what takes place in the simplified 2D-extrusion process. A state of the art grid pattern technique has been used to achieve metal flow from extrusion experiments where the conditions are close to perfect 2D-flow creating an extruded strip with an interior extrusion seam weld. The work comprises of concept development to the final results both for symmetric and non-symmetric metal flow during extrusion. The FE-models have been made to mimic the experimental results and obtain information about different aspects of extrusion process and their influence on metal flow, deformation characteristics and weld formation. The work has substantially increased the knowledge about the non-symmetric flow of metal around the die bridge for hollow profile production. It is found that there is more volume of metal flow and at higher velocity through the bigger porthole of the die. The extrusion weld line is therefore pushed towards the big porthole side at the rear end of the bridge and displaced towards the side of small porthole over the length of extruded profile. It has been shown that by FEA it is straight forward to investigate the local metal flow at the rear end of the bridge where the extrusion weld is created. The analysis shows that a dead (or stagnant) zone of metal will appear at the rear end of the die bridge (where welding occurs) when both pointed and butt-ended rear end of the bridge are used. Because of slow flow velocity in the dead zone, and much increased velocity at the tail end behind it, the material here will become severely stretched. The term stretching zone has therefore been attributed to this region where the material elongates strongly. It is clear that the presence of this stretching zone at this location and the phenomenon of the material becoming heavily stretched contribute to give favourable seam welding conditions here. A comprehensive picture of the topic is made by creating new knowledge both with the help of FEA and experimental results of metal flow. Grid patterns made from contrast pins combined with finite element analysis for study of metal flow of metal forming has proved to be very useful for understanding the complex mechanics of Al extrusion.