Modification of the anisotropy and strength differential effect of extruded AZ31 by extrusion-shear

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in AIP Conference Proceedings 1960, 030008 (2018) and may be found at https://doi.org/10.1063/1.5034851.The extrusion of magnesium alloys results in a pronounced fiber texture in which the basal planes are mostly oriented parallel and the c-axes are oriented perpendicular to the extrusion direction. Due to this texture the Strength Differential Effect (SDE), which describes the strength difference between tensile and compression yield strength, and the elastic anisotropy in the sheet plane are obtained during extrusion. The objective of the investigation was to decrease the SDE and anisotropy through specifically influencing the microstructure and texture. To accomplish this objective, the forming processes extrusion (EX) and equal channel angular pressing (ECAP) were combined and integrated into one extrusion die. This combination is called extrusion-shear (ES). With an ES-die, billets of the magnesium alloy AZ31B were formed into a sheet with the thickness of 4 mm and the width of 70 mm. The angles of the used ECAP-applications in the ES-dies were set to 90° and 135°. The results show that the extrusion-shear process is able to decrease the anisotropy and SDE through transformation of the texture compared to conventional extrusion process. Also grain refinement could be observed. However, the outcomes seem to be very sensitive to the process parameters. Only by using the ES-die with an angle of 135° the desired effect could be accomplished

Application of Friction Shear Test for Constitutive Modeling Evaluation of Magnesium Alloy AZ31B at high Temperature

The experimental determination of the flow stress and its mathematical formulation are essential for the numerical simulation of metal forming processes. The hot compression test is widely used to analyze the flow stress evolution as function of temperature, strain and strain rate. The compression test is limited to a relative low strain (ε≤1) which is acceptable when the stress is minor influenced at higher strains. In the case of magnesium alloys the flow stress is strongly influenced by the strain even at high strain (ε>1). In this work the thermo-mechanical behavior of the magnesium alloy AZ31B was investigated to improve the constitutive modeling up to high strains. Experimental stress-strain curves obtained from hot compression tests at different temperatures (450 °C-550 °C) and strain rates (0.01 1/s – 10 1/s) were applied to construct conventional material models such as those proposed by Garofalo (Zener-Hollomon) and Hensel-Spittel. In addition, shear tests under sticking friction conditions were carried out at high temperature (400 °C-500 °C) and different shear speeds (0.1 mm/s - 10 mm/s). During this test, the thin contact subsurface of cylindrical specimens experiences a high plastic shear deformation, while the axial force and stroke are simultaneously measured. Furthermore, a new constitutive modeling approach was proposed, which combine the Zener-Hollomon model and the experimental result of the friction shear test to estimate the flow stress at low and high strain respectively. Numerical simulations of the friction shear test applying the conventional models as well as the new constitutive formulation are presented in this study

Numerical Investigations on Material Flow During Indirect Extrusion of Copper-Clad Aluminum Rods

In the extrusion of clad composite materials with different flow stresses are usually used. This causes an inhomogeneous material flow which can induce sleeve or core fracture. In the present study, the material flow during indirect extrusion of copper-clad aluminum (CCA) rods was analyzed by means of experimental and numerical investigations throughout the process. In order to provide material models for the numerical analysis hot compression tests of the aluminum alloy EN AW-1080A and the copper alloy CW004A were carried out. The indirect extrusion was performed using a conical die with a semi die angle of 45° and an extrusion ratio of 14.8:1. The container was heated to 330°C, while billet, die, and ram were kept at room temperature. The extrusion trial was then modeled with the FEM based software DEFORM 2D. Cross sections were taken from the extruded rod and compared to the corresponding sections of the simulation with regard to the development of the equivalent copper cross section. As a result, the development of extrusion force and equivalent copper cross section could be clarified. The numerical investigations indicated a higher flow velocity for the aluminum core than for the copper sleeve at the bearing channel. Therefore, high tensile stresses and fractures of the copper sleeve were induced. Additionally, the validated numerical analysis made possible to determine the conditions for a successful co-extrusion of the analyzed CCA rod.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität Berli

Fundamental charactarization of the porthole die extrusion process for magnesium hollow profiles : experimental and numerical examination

Durch das Strangpressen von Magnesiumwerkstoffen über Kammerwerkzeuge lassen sich komplexere Hohlprofilgeometrien darstellen als durch Verwendung eines Dornes. Jedoch beinhalten so hergestellte Pressprodukte zumeist zwei oder mehrere Längspressnähte, die sich in der Mikrostruktur und daher den mechanischen Eigenschaften lokal vom umgebenden Gefüge unterscheiden können. Daher ist eine Untersuchung der Einflüsse der Prozessparameter beim Strangpressen auf die Ausprägung dieser Längspressnähte notwendig, um die Verwendbarkeit der Hohlprofile bewerten zu können. Der Einfluss sowohl des Strangpressprozesses als auch nachgeschalteter Wärmebehandlungen und Reckprozesse, die für das Erreichen der erforderlichen Geradheit der Profile notwendig sind, wurde unter Verwendung der Magnesiumlegierungen AZ31 und ME21 ausführlich charakterisiert. Die Variation vorrangig des Pressverhältnisses und der Produktgeschwindigkeit führt zu einer deutlichen Veränderung der Hohlprofileigenschaften, wobei das resultierende Eigenschaftsspektrum auch signifikant von der betrachteten Legierung abhängig ist. Grundlegend weisen beide Werkstoffe eine planare Anisotropie in den mechanischen Eigenschaften auf. Diese ist im Fall der Legierung AZ31 auf die richtungsabhängige Aktivierung des {101 ̅2}〈101 ̅1〉 – Zugzwillingssystems zurückzuführen, bei der Legierung ME21 bestimmt maßgeblich die Schmid – Faktor abhängige Aktivierung der basalen – Gleitung die mechanischen Eigenschaften. Die Längspressnähte in AZ31 – Hohlprofilen führen durch größere Korndurchmesser und verstärkte Basaltexturen mit Ausrichtung der c – Achse in transversaler Richtung zu einer lokal erhöhten mechanischen Anisotropie. In den ME21 – Hohlprofilen hingegen führt maßgeblich die charakteristische Textur der Pressnaht zu einer Reduktion der mechanischen Anisotropie. Für beide Legierungen führt jedoch eine Erhöhung des Pressverhältnisses zur Verringerung von Unterschieden des Gefüges und der Textur zwischen Pressnaht und pressnahtfreiem Material und somit auch zu weniger unterschiedlichen mechanischen Eigenschaften. Der an das Strangpressen anschließende Reckprozess bei Raumtemperatur führt durch Kaltverfestigung zu einem deutlichen Anstieg der Rp0,2 – Streckgrenzen durch anfänglich hohe Verfestigungsraten. Durch gezielte Wärmebehandlungen kann die Kaltverfestigung durch statische Erholung und im Fall AZ31 in Verbindung mit moderatem Kornwachstum wieder aus den Profilen abgebaut werden. Durch Variationen des Reckgrades bei einem Profil aus der Legierung ME21 in Verbindung mit anschließenden Wärmebehandlungen wurde an Hand von Kornwachstum und damit verbundener Modifizierung der Seltene Erden - Textur der Gefügeunterschied zwischen Pressnaht und pressnahtfreiem Material abgeschwächt. Mittels Finiter Elemente Methode wurde zudem die Anfälligkeit der ME21 – Legierung, Oberflächenfehler zu produzieren, maßgeblich auf Zugspannungen und lokal erhöhte Umformgrade an der Profiloberfläche im Presskanal zurückgeführt.Extrusion of magnesium alloy using porthole dies enables the production of increasingly complex hollow profile geometries as compared to simple cross sections when using mandrel extrusion. However, press products produced in this way usually contain two or more longitudinal weld seams, which may differ locally from the surrounding material with respect to the microstructure and therefore the mechanical properties. Therefore, an investigation of the influences of the process parameters during extrusion on the formation of longitudinal weld seams is necessary in order to assess the applicability of the hollow sections. The influence of both the extrusion process and the downstream heat treatments and stretching processes necessary to achieve the required straightness of the profiles has been extensively characterized using the magnesium alloys AZ31 and ME21. The variation primarily of the extrusion ratio and the product speed leads to a significant change in the hollow profile properties, the resulting property spectrum also being significantly dependent on the considered alloy. Basically, both materials have a planar anisotropy in the mechanical properties. In the case of the alloy AZ31, this is due to the directional activation of the {101 ̅2}〈101 ̅1〉 – tensile twinning system; in the case of the alloy ME21, the Schmid – factor dependent activation of basal slip is decisive. The longitudinal weld seams in AZ31 hollow profiles lead to a locally increased mechanical anisotropy due to larger grain sizes and enhanced basal textures with alignment of the c – axis towards the transverse direction. In contrast, in the ME21 hollow sections, the characteristic texture of the weld seam leads to a reduction of the mechanical anisotropy. For both alloys, however, an increase in the compression ratio leads to a reduction in differences in microstructure and texture between the weld seam and the seam – free material, and thus also in less different mechanical properties. The stretching process at room temperature following the extrusion leads to a significant increase in yield strength due to strain hardening due to initially high hardening rates. Through selected heat treatments, cold work hardening can be removed again by static recovery and in the case of AZ31 in combination with moderate grain growth. By variation of the degree of stretching of a profile of the alloy ME21 in connection with subsequent heat treatments, the difference in microstructure between the weld seam and the seam – free material was weakened by grain growth and the associated modification of the rare – earth texture. In addition, the susceptibility of the ME21 alloy to produce surface defects was largely attributed to tensile stresses and locally increased degrees of deformation on the profile surface in the bearing channel of the porthole die

Influence of die surface topography and lubrication on the product quality during indirect extrusion of copper-clad aluminum rods

Copper-clad aluminum rods are usually fabricated using hydrostatic extrusion, since during direct and indirect extrusion fracture of the copper sleeve is difficult to avoid. In this study, different die surface topographies and lubrication conditions were applied to improve the material flow during indirect extrusion of copper-clad aluminum rods. Thus, conic dies with different roughness (polished and sandblasted) and surfaces shapes (fine and coarse grooves) were tested. Additionally, the effects of a wax-graphite-based lubricant as well as a graphite-like carbon (GLC) coating of the die conic surfaces were investigated. The composite billets were made of aluminum EN AW-1080A cores and copper CW004A sleeves with an equivalent copper cross section of 0.24 of the total billet cross section. For all trials, an extrusion ratio of 14.8:1 and a conic die angle of 2α = 90° were chosen. Non-isothermal extrusion trials were carried out using a container at 330 °C and billet and tools at room temperature to reduce the flow stress ratio σCu/σAl. The extruded composite rods’ integrity, surface quality, interface integrity, and equivalent copper cross section were analyzed. In addition, a visual inspection of the sleeve-die contact surface was performed. The results showed that the GLC coating proved to be unsuitable due to a lack of lubrication, which causes accumulated sleeve fractures and longitudinal grooves on the extruded rods. The best results were achieved with the combination of the sandblasted die surface and the wax-graphite-based lubricant, observing a uniform material flow without sleeve fractures.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität Berli

Optimization of the Indirect Extrusion Process of Copper-Clad Aluminum Rods by Methods of Statistical Experimental Designs and Numerical Analyses

The success of composite extrusion is influenced by multiple process parameters. In order to investigate the significance of specific parameters during indirect extrusion of copper-clad aluminum (CCA) rods, statistical methods were applied and a central composite experimental design was implemented. The runs of the experimental design were modeled with the finite element method based software DEFORM 2D and evaluated with respect to product quality, described by four response variables. Using variance and regression analyses, as well significant linear and quadratic effects of the five investigated process parameters as interactions between them were identified. Based on a statistical model, an overall optimum setting for the process parameters was predicted utilizing the response surface methodology with a desirability approach. By applying the output of the statistical analysis to an extrusion trial, the extrusion of a high quality CCA rod was achieved. Moreover, the results of the statistical analysis could be verified by comparing predicted and experimentally determined values of the investigated quality characteristics.DFG, 414044773, Open Access Publizieren 2021 - 2022 / Technische Universität Berli