Papers by Author: Henry Sigvart Valberg

Abstract: The relative balance between the metal flow in two portholes in extrusion has been investigated by experiments and FE-analysis. The investigation deals with asymmetric extrusion, i.e., the billet is extruded through a die with portholes of unequal size. Metal flow has been characterized by an experimental grid pattern technique. An optimized FEM-model of the experiment has been built and the experimental metal flow is found to be mimicked accurately by this model. The velocity conditions in the two differently sized ports feeding material into the weld chamber, and further from here into the extrudate, have been investigated to see if the balance between the flow through the two channels changes as extrusion proceeds. Increasing asymmetri between the two portholes has been realized in the analysis by displacement of the die bridge laterally in relation to the direction of extrusion.

Abstract: In extrusion of aluminium alloys, using porthole dies, the billet metal is split into separate metal streams during flow into the ports. After that, the streams join by seam pressure welding in the weld chamber of the die and finally flow through the die exit emerging as the desired extrusion. Because of re-joining of the metal streams the extrusion will contain as many extrusion seam welds as there are bridges in the die. Dependent on the die design the metal will flow in such a way that a gas pocket may form behind the die bridge and remain stable here throughout the course of the extrusion process. If, at this stage of the process, the feed of metal into the region behind the bridge is increased the space behind the bridge may fill up completely and the gas pocket will disappear. In this work we have investigated how finite element modeling can be used (for a simple idealized case of 2-D extrusion welding) to understand the extrusion seam welding process, and to characterize what extrusion conditions will provide stable gas pocket formation behind a bridge, and when the pocket will disappear. The geometry of the welding chamber and the die opening was varied in a number of simulations to study this phenomenon using the FEM-code Deform 2D®. Based on these simulations we have been able to propose an “extrusion seam weld limit diagram (ESWLD)”. The ESWLD shows when extrusion will occur with a gas pocket present behind the bridge and when there is transfer to a state where the pocket disappears and the die fills up completely with metal. By curve-fitting the results can be presented by a mathematical model, i.e. by an equation allowing the reduction in extrusion at the point of transfer from a state of presence of the pocket to a state of complete die filling, to be computed from the dimensions of the porthole and the weld chamber, for a given billet size and a given bridge geometry. The model also shows which geometrical extrusion parameters of the die will favor complete filling of the space behind a die bridge instead of gas pocket formation there.

Abstract: Hollow and semi-hollow profiles are commonly produced by extrusion using porthole dies. The main characteristics of such dies are the presence of a mandrel (core) to shape the inner contour of hollow profile and bridges or legs to carry the mandrel. The bridges split the billet material into multiple metal streams that flow through the porthole channels and meet in the welding chamber behind the bridge where they are joined by pressure welding. When hollow profiles with different wall thickness are made the size of two adjacent portholes may be different. The material then flows through the two portholes with different flow velocity so that there is more feed through the bigger porthole into the weld chamber behind the bridge. Experiments have been performed and are reported here in which a grid pattern technique was used to characterize the metal flow through a 2D-die with porthole channels of unequal size. The design of the laboratory die has been modified in relation to the symmetric case to get different sizes of the two portholes. Since the metal flow through such a die is asymmetric the grid pattern technique was also modified to characterize the experimental flow. The results of an experimental metal flow study performed for a short billet was presented in a previous article [1]. Corresponding experiments performed with longer billets are now reported; so that two stages of the extrusion process is analysed here. The grid pattern technique has successfully mapped the non-symmetric material flow as in industrial extrusion when using different wall thickness over the section. The lateral movement of metal during extrusion is obtained from one set of experiments; the vertical movement from the other set. Finite element analysis of the extrusion process has been performed using Deform 3D. The encountering of the two metal streams behind the die bridge and the deformation characteristics within the welding chamber has been studied this way. Extrusion weld formation and deformations around the die bridge are considered here with the help of experimental results and simulation models. The nature of the metal flow achieved from the FE-model is compared with the experimental results. As regards the short billet some results are presented in [1], however improvement to the previous model gives a more perfect match. The model also provides information about the boundary conditions in real extrusion.

Abstract: The mechanics of metal flow through long choked die channels have been investigated in unlubricated hot aluminum extrusion. Experiments were performed in a laboratory press at an earlier occasion by letting a grid pattern introduced into the billet flow down into the choked die channel to appear adjacent to the channel wall. The grid pattern was then revealed to characterize the metal flow in the channel. A 2D-model of the extrusion process was made. The model was applied to study the conditions in the extrusion experiments and in this model good similarity was obtained with the experiment. New knowledge regarding the metal flow through a choked die channel have been obtained this way, such as; contact conditions, presence of sticking and sliding zones, friction conditions in the sliding contact zone and the velocity profile over the cross-section of the channel.

Abstract: The article presents an outline of a scientific approach for testing constitutive relations for the aluminum extrusion process. By comparing ram force, container friction, die face pressure, outlet temperature measurement during rod extrusion with corresponding simulated data, inferences can in principle be drawn with respect to the validity models. The paper indicates that simulation results from the 2D ALMA2π program are in fair agreement with measurements during extrusion of AA6060, but more work needs to be done to control thermal conditions during extrusion.

Abstract: A review is given of experimental work done at the author’s university during the last two decades, to investigate metal flow in aluminum extrusion. Partially extruded billets with internal grid patterns are difficult to remove from the container without post-deforming the internal pattern during the removal operation. A technique was therefore developed by which such billets can be removed from the container without any damage. In addition to this, a special grid pattern technique was developed. This technique applies contrast material stripes in the symmetry plane of the billet, and is advantageous because the pattern obtained remains clearly visible after extrusion, even in shear zones subjected to very heavy deformations. Traditional scratched patterns become invisible in such regions, and do not provide metal flow information in shear zones. When the two techniques, i.e. the new removal technique and the new grid pattern technique, were used concurrently, “perfect” type of metal flow experiments were conducted. A three-dimensional grid pattern technique was also developed. It is well suited for characterization of metal flow in complex shape extrusion, when there is no symmetry plane in which to conduct traditional grid pattern analysis. Applications of the new techniques for metal flow studies in various cases of extrusion are reported. It is shown that precise metal flow information indeed is a necessary requirement to get metal flow correct in computer simulation.