Material Flow in Screw Extrusion of Aluminium

Abstract

A new method for the extrusion of aluminium is under development, which has been termed Screw Extrusion of ALuminium (SEAL). In this process, a screw is used as a pressuregenerating device, which is a fundamentally different principle than that of a conventional forward extrusion process. The starting point of this work was a technology whose status was at the proof-of-concept level that was only capable of producing simple profiles of substandard quality. The overall goal of the SEAL project was therefore to prepare the technology for later industrialization and commercialization, thus putting strict requests on the cost, quality and robustness.

The aim of this work has been to create a fundamental knowledge of the process of screw extrusion of aluminium, more specifically, to determine how aluminium behaves under the wide range of thermo-mechanical conditions encountered in the screw extruder. Understanding the fundamental principles of frictional conditions, material flow and the extrusion pressure-generating mechanism was deemed as the most important for achieving this aim.

This thesis presents a compression and rotation (CR) machine capable of reproducing the thermo-mechanical conditions in the screw extruder in a more controllable environment. By using a novel contrast material technique, the thermo-mechanical limit curve for sticking friction between an aluminium alloy AA6060 and a tool steel was found and the material flow within the samples visualized. The use of contrast material was also employed in fullscale experiments using the screw extruder to evaluate the material flow and frictional conditions, and displacement rates were successfully detected for different regions of the extruder, revealing the main material flow paths and the dead metal zones. Furthermore, the use of various contrast material feed schemes led to an understanding of the interaction between newly fed and pre-exiting material inside the extruder. An explanatory model for the extrusion pressure-generating mechanism was created based on measurements of the extrusion velocity and observations made of the material behaviour.

The study has created new knowledge of the most important fundamental phenomena ongoing in screw extrusion of aluminium. New techniques using contrast material have been presented, and their use has been demonstrated on the compression and rotation machine, as well as the screw extruder. The results obtained herein will be useful in the future design of components and process strategies, basing engineering decisions on knowledge rather than trial-and-error. Although this work has substantially increased the understanding of the screw extrusion process, further research is required before the machine will be ready for industrial use, including solving practical challenges such as extrusion capacity, stability and material quality