Influence of choked angle of bearing channel on profile grain structure during multi-hole extrusion of aluminum alloy

Direct extrusion of aluminum alloy EN AW-6060 was carried out applying a four-hole die with pair-wise parallel and choked long channels. Due to the dissimilar friction inside parallel and choked channels profiles with different length were extruded simultaneously. In order to investigate the grain structure evolution along the whole extrusion process, multiple sections from the beginning to the end of the products were analyzed. Macroetch tests revealed unrecrystallized fibrous, fully recrystallized as well as partially recrystallized grains. The results also showed an axial and radial grain structure variation. At the beginning of the extrudates unrecrystallized fibrous microstructure was observed, while a fully recrystallized structure characterized the end of the products. Additionally, finer grains were present at the surface, whereas coarser grains were found in the center of the extrudates. Finally, numerical simulations allowed estimating the temperature, strain and strain rate evolution along the whole product length. Thus, a correlation between the extrusion parameters, deformation conditions and the grain structure was obtained

Influence of temperature and sliding speed on the subsurface microstructure evolution of EN AW-6060 under sticking friction conditions

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 1896, 140012 (2017) and may be found at https://doi.org/10.1063/1.5008168.The microstructure evolution of the friction boundary layer of the aluminum alloy EN AW-6060 was investigated. Sticking friction tests at different temperatures and sliding speeds were carried out. A severe deformation below the friction surface was observed by means of LOM and EBSD mapping. Thus, the thickness variation and the grain structure of the high deformation zone could be described. Fibrous structure was observed at 300 °C and 400 °C, while equiaxed grains with high misorientation angle (>15°) were generated at higher temperatures. Additionally, abnormal grain growth and coarse grains were detected at high sliding speeds (10 mm/s, 42 mm/s) at 450°C and 500 °C respectively

Robust a priori and a posteriori error analysis for the approximation of Allen–Cahn and Ginzburg–Landau equations past topological changes

A priori and a posteriori error estimates are derived for the numerical approximation of scalar and complex valued phase field models. Particular attention is devoted to the dependence of the estimates on a small parameter and to the validity of the estimates in the presence of topological changes in the solution that represents singular points in the evolution. For typical singularities the estimates depend on the inverse of the parameter in a polynomial as opposed to exponential dependence of estimates resulting from a straightforward error analysis. The estimates naturally lead to adaptive mesh refinement and coarsening algorithms. Numerical experiments illustrate the reliability and efficiency of this approach for the evolution of interfaces and vortices that undergo topological changes

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

Microstructure Evolution of Friction Boundary Layer during Extrusion of AA 6060

The tribological behavior between the aluminium alloy AA 6060 and the hot working steel 1.2344 has been recently studied using a new axial friction test for extrusion processes. With this new test, the friction forces as well as the specimens’ plastic deformations generated during sliding tests can be investigated. Selected specimens tested at high temperature (300-500 °C), normalized normal stress (σn/kfo=1.5) and high relative speed (0.1, 50 mm/s) have been sectioned and the microstructure on their mid-planes has been investigated. Light optical microscope analysis of the friction boundary layer revealed highly stretched grains and a thickness variation of the shear layer from 750 to 1600 μm depending on testing conditions. Moreover, EBSD analysis showed a grain refinement in the high shear zone especially at 300 °C with a grain size about 4 μm. Dynamic recrystallization was observed at 400 °C and an abnormal grain growth at 500 °C. Hardness measurements revealed a light hardening effect of 8% at 300 °C as well as a softening effect of 6% at 400 °C and 500 °C

Extrusion of Aluminum Tubes with Axially Graded Wall Thickness and Mechanical Characterization

In this study the indirect extrusion of seamless aluminum tubes with variable wall thickness was investigated. Therefore, an axially moveable stepped mandrel was applied. Investigations revealed that wall thickness transitions can either be graded over the tube length or very sharp. The microstructures in thin-walled and thick-walled tube sections were investigated. The local variation of the extrusion ratio and with that the tube wall thickness, product velocity and product temperature during the process lead to significantly different local microstructures at TB=400 °C. At TB=500 °C the microstructure was homogeneously recrystallized with similar grain size in all different tube sections. Furthermore, the mechanical tube properties were characterized by three point bending tests

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

Analysis of the flow imbalance on the profile shape during the extrusion of thin magnesium sheets

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 1567, 1098 (2013) and may be found at https://doi.org/10.1063/1.4850162.The extrusion process facilitates the production of magnesium sheets featuring a very thin thickness as well as excellent surface properties by using a single process step only. However, the extrusion of the magnesium sheets applying not optimized process parameters, e.g. low billet temperature or/ and poorly deformable magnesium alloy, produce pronounced buckling and waving of the extruded sheets as well as a variation of accuracy in profile shape along the cross section. The present investigation focuses on the FEM-simulation of the extrusion of magnesium sheets in order to clarify the origin of the mentioned effects. The simulations identify the flow imbalance during extrusion as the main critical factor. Due to the flow imbalance after passing the die a large compression stress zone is formed causing the buckling and waving of the thin sheets. Furthermore, the simulations of the magnesium sheet extrusion reveal that the interaction of the material flow gradients along the width and along the thickness direction near the die orifice lead to the variation of the accuracy in profile shape