Advances on Hot Extrusion and Simulation of Light Alloys

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Authors: Wojciech Z. Misiolek, William R. Van Geertruyden
Abstract: The extrusion process provides conditions for non-uniform metal flow depending on strain, strain rate and temperature of deformation as well as deformation zone geometry. As a result of these conditions significant microstructure gradients are present within the extrudate. The extreme case of the microstructure gradient is a formation of the peripheral coarse grain structure (PCG). This phenomenon is present in many structural aluminum alloys extrudates and its presence should be eliminated or at least significantly reduced because of the mechanical properties and aesthetic points of view. A number of experiments, including physical and numerical simulations, were performed in order to understand and model the origin of the PCG in indirect extrusion of 6xxx alloys. These experiments included different alloy chemistry, various temperatures, extrusion ratios and extrusion speeds allowing analysis of their influence. Parallel to the experimental results the numerical simulation of the metal flow showing the origin of the metal in different extrudate location was performed using software package DEFORM TM. A review of performed research will be followed by the analysis of the results and discussion of future needs.
Authors: Andrew J. den Bakker, Robert J. Werkhoven, W.H. Sillekens, Laurens Katgerman
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.
Authors: Daniel Pietzka, Nooman Ben Khalifa, Lorenzo Donati, Luca Tomesani, A. Erman Tekkaya
Abstract: In this paper experimental investigations aimed at measuring the die deformations during aluminum extrusion process is presented and discussed. A two-holes die generating two U-shape profiles with different supporting legs was produced and tested under strictly monitored conditions. The influence of die deformation on the speed, temperature distribution and distortion of the two profiles is reported and analyzed. AA6082 alloy was used as deforming material while H-13 hot-work tool steel was selected as die material. The experiments were repeated at least three times in the same conditions in order to achieve a statistical distribution of the acquired data: such data are then used as a reference for the 2009 edition of the extrusion benchmark.
Authors: Friedrich Krumphals, Pavel Sherstnev, Stefan Mitsche, S. Randjelovic, Christof Sommitsch
Abstract: Process parameters in aluminium extrusion technology are key points that influence product properties. The precipitation hardening aluminium alloy 6082 is investigated according to different process conditions and the influence onto the final microstructure is simulated as well as experimentally verified. A physical microstructure model based on three dislocation types and three nucleation sites for recrystallization is implemented into the commercial Finite Element package FORGE 2008 to calculate both the microstructure evolution during the extrusion process as well as the recrystallized fraction after the process. The precipitation kinetics during homogenization was investigated using the thermodynamic calculation software MatCalc since the main nucleation mechanism for recrystallization is particle stimulated. The experimental validation was done by miniature extrusion tests and the microstructure was investigated metallographically and by EBSD measurements.
Authors: Annika Foydl, Nooman Ben Khalifa, Alexander Brosius, A. Erman Tekkaya
Abstract: The current investigation is concerned with the grain structure evolution in an Al-Zn alloy (EN AW-7020) during the hot forward extrusion process. In order to analyze that, a miniature hot forward extrusion setup was designed which allows the quenching of the extrusion butt immediately after extrusion. In order to gain a better understanding of the process, the shape of the deformed grains was analyzed and the process was simulated. The shape of these grains was indentified in two directions in the different grain zones, e.g. dead metal zone and shear zone. The FE simulations showing the different grain zones were also illustrated. Simulation results and the micrographs were quite promising to find parameters for simulation models in order to predict grain sizes with the method presented in the current research work.
Authors: Farhad Parvizian, T. Kayser, Bob Svendsen
Abstract: The purpose of this work is to predict the microstructure evolution of aluminum alloys during hot metal forming processes using the Finite Element Method (FEM). Here, the focus will be on the extrusion process of aluminum alloys. Several micromechanical mechanisms such as diffusion, recovery, recrystallization and grain growth are involved in various subsequent stages of the extrusion and the cooling process afterward. The evolution of microstructure parameters is motivated by plastic deformation and temperature. A number of thermomechanical aspects such as plastic deformation, heat transfer between the material and the container, heat generated by friction, and cooling process after the extrusion are involved in the extrusion process and result in changes in temperature and microstructure parameters subsequently. Therefore a thermomechanically coupled modeling and simulation which includes all of these aspects is required for an accurate prediction of the microstructure evolution. A brief explanation of the isotropic thermoelastic viscoplastic material model including some of the simulation results of this model, which is implemented as a user material (UMAT) in the FEM software ABAQUS, will be given. The microstructure variables are thereby modeled as internal state variables. The simulation results are finally compared with some experimental results.
Authors: Ahmet Güzel, Andreas Jäger, Nooman Ben Khalifa, A. Erman Tekkaya
Abstract: A method for the numerical estimation of the final hardness distribution of heat treated aluminum alloys was developed and implemented into a commercial finite element (FE) tool. Jominy end-quench tests were carried out in order to determine the quench sensitivity of the aluminum alloy EN AW-6082. The hardness distribution of the alloy after end-quenching was related to the corresponding cooling rates. The derived relation was tested for an industrial application by investigating the local heat treatment of a prototype crash absorbing structure. Numerical estimations were validated with experimental measurements. Effectiveness of the derived method and possible improvements were discussed.
Authors: Michael Reich, S. Schöne, O. Kessler, M. Nowak, O. Grydin, F. Nürnberger, M. Schaper
Abstract: After the extrusion process most aluminium alloy profiles don´t satisfy the necessary strength requirements. An increase of strength can be obtained by age hardening of hardenable aluminium alloys. Age hardening includes the three steps of solution annealing, quenching and aging and is usually carried out in a separate process after extrusion. The integration of the sub-steps solution annealing and quenching in the extrusion process results in a marked reduction of the complete process chain. The applicability of this integration depends primarily on the quenching power of the cooling module and on the quench sensitivity of the aluminium alloy. Using the finite element method the non-steady-state process of quenching the profiles after leaving the extrusion press has been simulated. The boundary conditions for quenching are varied for a gas nozzle field and a spray cooling using heat transfer coefficients based on experiments. The simulation results support the design of gas nozzle fields or spray cooling for the extrusion process of different aluminium alloys.

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