Papers by Author: Robert J. Werkhoven

Abstract: The surface quality of aluminium extrusion products can be hampered by undesired surface features like die lines and pickups. In particular the presence of pickups is considered as undesirable. Surface pickups appear as intermittent torn marks on the aluminium extrusion products, often terminated with a protruding lump rising above the surface up to hundreds of microns in height. Using a model calculating initiation, growth as well as detachment of the lumps on the die bearing surface, the surface quality of aluminium extrusion products can be predicted. The results of the model can be presented in terms of surface quality diagrams, where contour lines of the (normalized) calculated number of lumps are presented in terms of exit speed and extrudate surface temperature. These diagrams are unique for a certain combination of geometry of the extrudate and the aluminium alloy and can be used to optimise process conditions with respect to surface quality within a certain process window. In order to validate the model, the size and number of pickups on the surface of a labscale extruded strip of AA6063 have been measured. The results of the model are being compared to the experiments and show good agreement.

Abstract: Longitudinal weld seams are an intrinsic feature in hollow extrusions produced with porthole dies. The formation of longitudinal weld seams is a solid bonding process, controlled by the local conditions in the extrusion die. Being the weakest areas within the extrusion cross section, it is desirable to achieve adequate properties of these weld seams. In our research, the concept of a weld seam integrity indicator as a means of quantifying bonding efficiency is introduced. The value of this indicator depends on a number of factors: the material flow within the die weld chambers, an adequate pressure level acting on the weld planes and finally the evolution of the metal microstructure. Optimisation of the welding conditions leads to a higher value of the weld seam integrity indicator and thus to improved weld seam properties. The objective of the research presented in this paper is to assess the feasibility of this concept. In lab-scale experiments, AA6060 and AA6082 aluminium alloy billets were formed into strips by means of the direct hot extrusion process. By utilising porthole dies a central longitudinal weld seam is formed. The effect of different geometries of the weld chamber and the processing conditions on the quality of the weld seam are investigated. Characterisation of these weld seams through mechanical testing, focusing on the ability of the weld seam area to accommodate plastic deformation following the onset of plastic instability, and microstructural analysis provides insight into bonding performance. The outcome of this characterisation provides a basis for an estimation of the weld seam indicator. Through computer modelling, the particular process conditions related to weld seam formation are calculated and correlated with the experimental results. The experimental results clearly demonstrate that weld seam formation is controlled by a combination of factors as described above. Inadequate fulfilment of these conditions, verified by the FE-simulations, is the cause of inferior weld seams, associated with low values of the weld seam integrity indicator. Through further elaboration of the concepts presented in this work, the weld seam integrity indicator is to be developed, with the future aim of predicting the weld seam performance through finite element simulations.