Henry Valberg

Extrusion welding in aluminium extrusion

Pressure welds occur in extruded aluminium profiles either as longitudinal welds (seam welds) or as transverse welds (charge welds). While longitudinal welds only occur in hollow profiles, transverse welds always occur when the billet is extruded on top of a metal rest left in the die from the previous extrusion, i.e. in extrusion of hollows as well as in billet-to-billet extrusion, and in pre-chamber extrusion of solid profiles. Longitudinal welds are formed as the metal streams of different die ports gather behind each die-bridge and join. They extend longitudinally through the profile in the location behind each die web. Transverse welds form as the front end face of each billet pressure welds to the sheared-off back-end face of the extrusion residue left inside the die from the previous extrusion. Transverse welds commonly have a tongue-like shape inside the resulting extrusion and often extend over a large length of the extrusion. Since extrusion welds are formed by solid pressure-welding, they are produced with mechanical properties corresponding to the rest of the profile. Unlike fusion welding, there are no filler metal or soft heat-affected zones. But in spite of this, problems in terms of poor weld quality sometimes occur in industrial production of aluminium profiles. Extrusion welds of insufficient quality contain no voids or cavities and could, therefore, be difficult to detect by non-destructive tests as ultrasonic or x-ray inspection. Since extrusion welds defects are often unfavourable conditions in the extrusion process, they can to a large extent be eliminated through corrective measures taken by the extruder. This paper reviews the existing knowledge, as regards formation of extrusion welds, and defects in such welds. Preventive measures that can be made by the extruder to avoid formation of such defects are also discussed.

Experimental Techniques to Characterize Large Plastic Deformations in Unlubricated Hot Aluminum Extrusion

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.

Studies of Porthole Extrusion through Die with Different Sizes of Portholes

Aluminium extrusion industry produces hollow profiles that have different wall thickness in the cross section. Production of such profiles requires billet extrusion through porthole die with different sizes of the portholes and this result in variation of material flow velocity through the different portholes. Experiments are reported here in which a grid pattern technique was used to characterize the metal flow through two porthole channels of unequal size. The design of laboratory die that represents the industrial case of porthole extrusion has been modified to get variation in size of portholes. The technique has successfully mapped the non‐symmetric material flow i.e. similar to the extrusion of industrial profiles having different wall thickness across the section. One of the channels delivers more material into the extrusion weld zone behind the die bridge than does the other channel. The applied technique is described and the mapped flow of metal are provided as new information regarding flow velo...

Comparison of the Thermo-Mechanical Conditions in an Industrial Sized Hot Aluminium Extrusion Process in Relation to those in Corresponding Small Sized Laboratory Processes

It is common to use scaled down laboratory extrusion processes in order to physicalmodel the industrial big-sized aluminum profile extrusion process. In industrial extrusion use ofbillets with a diameter size of ~210 mm, or above, is common. In the scaled-down laboratoryprocesses half this size is often used, and in mini-extrusion also 1/7th of this size. An investigationhas been undertaken in order to study what are the thermo-mechanical conditions in extrusionprocesses of such different sizes of processes, and to what extent a small-sized process is able tophysical model accurately the conditions in an industrial large sized process.

Mechanical Properties in Hot Forged Steel Parts in Relation to Properties in Corresponding Parts Made by Casting Solely

It is a common understanding that hot forging will improve the properties of a steel part in relation to when the same part is made by casting solely. A study has been performed where two crank pin disks of a particular steel alloy, one hot forged and the other cast, both in quenched and annealed condition, have been tested using a new innovative “eye”-specimen bending test. The used test procedure is described, and it is shown that the forged and the cast material will collapse and beak down in very different way in this test.

Metal Flow in Two-Hole Extrusion of Al-Alloys Studied by FEA with Experiments

Forward two-hole extrusion of Al has been investigated with the purpose of studying how metal flow inside the billet is influenced by the location of the holes in the dies, i.e. whether they are position near to or far apart from each other. The study has been conducted by means of finite element analysis (FEA) using the software DEFORM 3D® and validation of simulation results are done by comparison with grid pattern experiments performed long time ago by one of the authors. The analysis shows that the experimental conditions are well reproduced by FEA. New insight into the metal flow phenomena in two-hole extrusion is also gained thanks to the analysis. It is shown, for instance, that moving the holes far apart from each other brings about a distinct shift in the metal flow. The deformations subjected to the peripheral outer shear zones of the billet material then become much more localized than when the two holes are close.

Aluminium Extrusion Weld Formation and Metal Flow Analysis in Hollow Profile Extrusions of Different Section Thickness

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.

Use of Axisymmetric Shearing as Technological Test Method to gather Flow Stress Data for Metals

Cutting by shearing creates heavy shear deformation in a layer extending between the two applied shearing edges. Prediction of FEM‐simulation is that effective strain and strain rates in the shear zone would reach very high levels even at mode rate shearing velocity. In this article experiments coupled with FEM‐analysis are used to evaluate the potential of using a xisymmetric shearing for collecting flow stress data for metal forming purposes. It is shown that if accurate flow stress data are to be collected this way it is important to know how shearing occurs inside the shear zone.

Analysis of Metal Flow of Aluminum through Long Choked Die Channels

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.

Analysis of the influence of back-pull during drawing of aluminium wires

A laboratory wire-drawing process was investigated with respect to friction conditions in the die-wire interface by a combined experimental and numerical analysis using the finite element method. Back-pull on the wire was provided during the experiments by placing two dies in sequence. The drawing force of the first die then provided back-pull into the second die. Experiments without back-pull were also performed as reference by pulling the wire through a single die. Dies of different geometry and different reductions were used in the investigation. A fluid lubricant was used, and the wire material consisted of an AA 6082 aluminium alloy heat-treated to a hard and a soft condition, respectively. The Bridgman-corrected flow curve for different wire material variants was determined by tensile testing. A power-law stress-strain relationship was then obtained by curve-fitting. In the experiments, the drawing conditions, and thus also the contact conditions in the die-wire interface, were varied within broad limits. Moreover, the drawing forces were measured in each experiment. All experiments were reproduced by FE-simulation in order to determine the friction conditions in this indirect way. The investigation showed that there was good correspondence with respect to drawing forces between experiments and simulations, when friction was modelled as Coulomb friction with an approximately constant coefficient of friction. The influence of back-pull on the drawing process was studied by performing simulations of two corresponding drawing operations, one with, and the other without back-pull. The simulations showed that the main effect of back-pull was to alter the stress conditions inside the plastic zone in such a manner that the contact pressure between die and wire was reduced.