Key Engineering Materials Vol. 367

Abstract: The experimental conditions chosen as a reference for the 2007 edition of the extrusion benchmark and the corresponding main results are summarized in this work. The die design stage is first explained in order to address the main features of the experiment and its objectives. The die is a flat one with multiple holes; four angular profiles were produced with different pocket geometries, the experimental plan being entirely described. The initial temperatures for the billet and the die set, together with the temperature development during the process strokes are also reported. The results are shown, for each profile, in terms of final profile length, mean exit speed, global process load, profile exit temperature.

Abstract: In the course of the increasing discussions about a reduction of the CO2 emissions magnesium has gained importance since it is the lightest metal for structural applications. Currently magnesium alloys are almost exclusively used as cast parts in the automotive industry because due to their microstructure extruded magnesium profiles exhibit a strong asymmetry in the mechanical properties under tensile and compressive loading (strength differential effect). In order to improve the mechanical properties a detailed knowledge about the influence of the different extrusion parameters on the microstructure of the extrudates is necessary. Therefore, the parameters extrusion method, billet temperature, product speed, extrusion ratio and cooling condition were varied for the extrusion of the magnesium alloys AZ31, AZ61 and AZ80. Subsequently the microstructure was analyzed and the mechanical properties determined. With an additional analysis of the deformation modes of the extruded and cold deformed products it could be discovered that an improvement of the mechanical properties can be achieved by a modification of the extrusion process. Since the strength differential effect in caused by twinning which due to the texture of the extrudates is only active under a compressive loading along the extrusion direction the modification of the extrusion process aims at a suppression of this twinning. Because on the one hand compared to that for dislocation glide the Hall-Petch-Constant for twinning is bigger a grain refinement of the extruded products could be achieved by a predeformation using ECAE similar processes. On the other hand a process has been developed where the profiles are extruded into a hydrostatic counter pressure in order to alter the texture during the extrusion. Thereby the twinning is already activated during the extrusion. Both modifications of the extrusion process result in an increase of the critical resolved shear stress for twinning during the subsequent cold deformation and thus in improved mechanical properties.

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.

Abstract: Materials which form the surface and subcutaneous layers of an extrudate experience large deformations when they traverse the die land. This, when added to the inhomogeneity caused by the dead metal zone, leads to considerable modifications to the deformation parameters when compared to the remainder of the extrusion. The distribution of structure is therefore greatly inhomogeneous. Reference to both empirical and physical models of the recrystallisation process indicates that nucleation and growth will differ at these locations in those aluminium alloys that are usually solution treated and aged subsequent to the deformation process. Since static recrystallisation has a significant influence on many of the properties of the extrudate, it is therefore essential to provide the methodology to predict these variations. In the work presented, a physical model, for AA2024, based on dislocation density, subgrain size and misorientation is modified and integrated into the commercial finite element method (FEM) code, FORGE, to study the microstructure changes. Axi-symmetrical and shape extrusion are presented as examples. The evolution of the substructure influencing static recrystallisation is studied. The predicted results show an agreement with the experimental measurement. The distribution of equivalent strain, temperature compensated strain rate and temperatures are also presented to aid interpretation. Importantly the properties of hard alloys improve as the temperature of the extrusion is raised. This phenomenon is discussed and theoretically justified. This paper also presents some innovative work where the physically based models, and the Cellular Automata (CA) method, are combined to simulate the static recrystallisation process. The FEM is adopted to provide the initial morphology and state variables for the structure models, such as the equivalent strain, the temperature and the equivalent strain rate. The subgrain size, and dislocation densities are calculated from physically based models and are transferred to CA models to construct the data required to define the initial state for recrystallisation. Simulation results are compared with experimental measurements. It is demonstrated that CA integrated with the physically based models is effective in predicting the structural changes by selecting a suitable neighbourhood and reasonable transition rules.

Abstract: Even though Extrusion is often regarded as a semi stationary process, the defor- mations of the die at the beginning of the process can have great influence on the process later on. During filling of the die, the deformation of the die depends on the location of the flow front up to a point where parts of the profile will be opened or closed, especially in porthole dies. In this paper we present an accurate 2D method to simulate the filling of extrusion dies. The method is based on the pseudo concentration technique. We compare different options to model the pseudo material and choose the best.

Abstract: The decrease of the bearing length in extrusion processes results in increasing of the material flow and offers, through this, the possibility for manipulation and optimization. This paper presents a simulation based optimization technique which uses this effect for optimizing the material flow in direct extrusion processes. Firstly, the method is used in a multi-extrusion process with equal pitch circle profiles, then in an extrusion process of an asymmetric profile. Furthermore, a composite extrusion process is analyzed where endless wires of high strength steel are embedded in a base material of aluminum. The insertion of reinforcement elements into the base material flow, especially within the small ratio between profile thickness and the reinforcement diameter, can lead to significant local disturbances inside the die, which result in undesirable profile defects. Hence, the simulation-based optimization method is especially used to optimize inhomogeneous wall thicknesses in composite profiles.

Abstract: New innovative direct extrusion process variants, curved profile extrusion (CPE), twisted profile extrusion (TPE), and hollow profile extrusion (HPE), which increase the flexibility of aluminum profile manufacturing processes, are presented in this paper. These processes are characterized by influencing the material flow inside the die so that the forming process is completed when exiting the die. On the one hand, three-dimensionally curved profiles are produced and analyzed by CPE regarding the accuracy, the influencing parameters, and the compensation strategies. On the other hand, TPE and HPE make it possible to manufacture helical profiles usable, for example, as screw rotors in fluid machinery.

Abstract: In the present case study, finite element (FE) simulation was performed to evaluate the design of a spreading pocket die by analysing the metal flow during the extrusion of the 6061 alloy to produce a thin-walled wide profile for ground transportation applications. The results obtained from the FE simulation were in good agreement with those from industrial extrusion trials. The velocity and temperature non-uniformities on the profile cross section, revealed from the FE simulation, suggested the die bearing area for die correction. The FE simulation also showed that ram speed had little influence on the velocity non-uniformity but a marked effect on the temperature and temperature distribution of the profile. In the case of extrusion through the spreading pocket die, more heat dissipation from the hotter billet to the die took place, especially when ram speed was low. Therefore, to reach a temperature sufficient for the dissolution of Mg and Si, ram speed must be raised. The FE simulation in the transient state of the extrusion process could give an indicative ram speed for trial extrusion to reach a sufficiently high temperature for the solution treatment on the one hand and to avoid hot shortness on the other hand. It also showed that ram speed had a moderate effect on the breakthrough pressure. Therefore, in the selection of ram speed, attention should be paid to its effect on the maximum profile temperature and temperature distribution.