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Surface Pick-up Formation: New Limits in the Limit Diagram

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Surface Pick-up Formation: New Limits in the Limit Diagram

A.J. den Bakker, Nedal Aluminum, Utrecht, The Netherlands
X. Ma, M2i Materials Innovation Institute, Delft, The Netherlands
R.W. Werkhoven, TNO Science & Industry, The Netherlands
M.B. de Rooij, University of Twente, Enschede, The Netherlands


ABSTRACT—Surface quality of aluminum extrusions can be hampered by undesirable
features, such as pick-up. The key area for pick-up formation is the die bearing, where a
sticking zone and a slipping zone can be distinguished. The length of the slipping zone can be
calculated based on the pressure distribution and the frictional behavior in the bearing area.
Within the slipping zone, material from the extrudate can be transferred to the tool surface.
Based on a pick-up formation mechanism, a model describing surface quality of aluminum
extrusion products has been developed. The model calculates initiation, growth, as well as
detachment of the lumps on the bearing surface. Important parameters are the micro-geometry
of the bearing, the constitutive behavior of the aluminum alloy, the pressure distribution in the
bearing area, and extrusion parameters such as exit speed and extrudate surface temperature.
Results from AA6063 lab scale tests show good agreement with the model results. The
calculations can be presented in terms of surface quality diagrams, where contour lines of the
(normalized) calculated number of pick-ups are presented. Combined with conventional limit
diagrams, these surface quality diagrams provide an enhanced view of the true operational
process window.

© Extrusion Technology for Aluminum Profiles Foundation (ET Foundation). All rights reserved. No part of The Proceedings may be reproduced in any form without the express written permission of the ET Foundation.