Birgit Awiszus

Numerical and Experimental Investigations of the Production Processes of Coextruded Al/Mg-Compounds and the Strength of the Interface

Due to a changing environmental awareness and the improved use of resources, the application of light-weight materials, such as aluminium and magnesium, becomes increasingly important and partially substitutes the utilisation of conventional materials such as steel. However, a widespread application demands a better understanding of these materials. This concerns production processes of semi-finished products and the products itself. Coextruded aluminium-magnesium compounds are investigated in the subproject B3 (“Experimental and numerical investigations of the interface behavior of Al-Mg compounds”) which is part of the special research area 692 – HALS at the Chemnitz University of Technology. These compounds are characterized by a very good weight-strength-ratio and allow a wide field of application, for example in automotive industry. The compounds are manufactured in extrusion processes. The interface which is developed during the production is of special interest and the investigation of it is a shared aim of the Department of Experimental Mechanics and the Department of Virtual Production Processes. The two main tasks are the extrusion process optimisation and the microstructural, mechanical and thermal examination of the semi-finished product. The following paper gives an overview of the performed investigations in this subproject.

Formability of Hybrid Aluminum-Magnesium Compounds

In the SFB 692 HALS (High-strength aluminum based lightweight materials for safety components), subproject B-3, the production of an aluminum magnesium compound by a hydrostatic co-extrusion process was investigated. The quality of these semi-finished products, especially the stability and robustness of the interface between the aluminum (AlMgSi1) sleeve and magnesium (AZ31) core, was of particular interest. Previous papers have described the first process optimization steps as the improvement of the die design as well as the numerical methods for identification of important process parameters and the development of a quality model for the interface. This paper describes the formability of such semi-finished products with subsequent forging processes, especially die forging. Therefore, two different die forging strategies were investigated. In the first approach the strand-shaped work piece, with a circular cross-section, was formed along its longitudinal axis with die forging. In the second approach the same geometry was radially formed with die forging. Thereby, the compound was formed in longitudinal direction up to an analytical equivalent strain value of 1.61 and in radial direction up to 1.38. First results showed that the interface of the aluminum magnesium compound is very stable and ductile enough to be forged. Dye penetration tests were performed to prove the stability of the interface in a first step. Then, micro sections were made to investigate the interface metallographically. No cracks or damages were detected with both test methods in the interface of the forged aluminum magnesium compound. Furthermore, numerical simulations were performed to analyze the forging processes in detail. Therefore, a full 3D simulation model was set-up with Forge2011 and the calibration was performed with the press force as well as the geometry aspects. The correlations between experiments and simulations are very well. By means of the calibrated simulation detailed analyses of interface section are performed and the stability of the interface was investigated. This shows that the compound quality reached by the hydrostatic co-extrusion process is very suitable for subsequent forming steps as die forging. The investigations show the potential of such hybrid compounds and clarify their application, especially in the automotive sector.

Numerical simulation of non-circular spinning: a rotationally non-symmetric spinning process

This paper describes the peculiarities encountered in the numerical modeling of non-circular spinning processes using motion-controlled roller tools and applying the Finite Element Method (FEM). This process is suitable for producing non-circular, hollow components in small to medium-sized production lots. Numerical simulation can be used to optimize the process. Therefore, it is necessary to make a realistic sheet thinning and wrinkling calculation by using the FEM. This can be achieved through the definition of the real kinematics, a suitable flow curve and an optimal sheet meshing strategy using solid elements. An optimal sheet meshing strategy is particularly necessary in order to realistically calculate the process within an acceptable computing time. Reference experiments with the rotationally non-symmetric mandrel types, the “Tripode” and “Pagoda”, were carried out to compare simulations and experiments. A comparison of the reference experiments with the “Tripode” mandrel demonstrated that it is possible to simulate non-circular spinning with a deviation of less than 5% with respect to minimum sheet thickness. It is also possible to predict wrinkling in critical, non-circular spinning processes. This has been confirmed by comparing the “Pagoda” reference experiment with the FEM simulation.

The Process of Co-Extrusion – An Analysis

This paper provides an analysis of a co-extrusion process. The compound consists of the sleeve material, aluminum, and the core material, magnesium. It is imperative to produce impeccable compounds without cracks in the interface. Therefore, a simple indicating value for damaging effects during the process is necessary. In the numerical simulation the compound quality is noticeable by the current macro mechanical criterion of axial strain difference. A statistical analysis verifies this criterion as an adequate quality criterion. By means of this criterion it is possible to define a stable process window for the co-extrusion process.

Numerical Investigation of Knurled Shaft-Hub Connections and Especially of the Joining Process

Faced with increasing cost pressures, manufacturers need to find new solutions for shaft-hub connections. One example is the knurled interference fit, where a shaft with knurls and interference is pressed in a soft hub with a cylindrical bore. At present the potential of the knurled interference fit is not fully used, because there are no general standards available for the design and for the joining process. But the joining process in particular has an important impact on the join strength. A predominantly forming joining process offers some advantages compared to cutting. Forming allows a clearly higher contact pressure over the groove because of the displacement of the hub material, which leads to greater axial reliability. Also, the natural material structure is preserved by the forming, and this, in combination with the hardening of the hub, should allow a higher transmittable torque. The Finite Element Method is used for detailed investigation of the joining process and the transmission behavior in the axial direction. With this method, the local loads, for example the hardening of the hub over the groove due to the formed join, are investigated as well as further parameter variations (e.g., chamfer angle, interference). Hence, an analytical approach for the determination of the necessary joining force was derived.

Modelling the influence of carbon content on material behavior during forging

Nowadays the design of single process steps and even of whole process chains is realized by the use of numerical simulation, in particular finite element (FE) based methods. A detailed numerical simulation of hot forging processes requires realistic models, which consider the relevant material-specific parameters to characterize the material behavior, the surface phenomena, the dies as well as models for the machine kinematic.This data exists partial for several materials, but general information on steel groups depending on alloying elements are not available. In order to generate the scientific input data regarding to material modelling, it is necessary to take into account the mathematical functions for deformation behavior as well as recrystallization kinetic, which depends alloying elements, initial microstructure and reheating mode. Besides the material flow characterization, a detailed description of surface changes caused by oxide scale is gaining in importance, as these phenomena affect the material flow and the component quality. Experiments to investigate the influence of only one chemical element on the oxide scale kinetic and the inner structure at high temperatures are still not available. Most data concerning these characteristics is provided for the steel grade C45, so this steel will be used as basis for the tests. In order to identify the effect of the carbon content on the material and oxidation behavior, the steel grades C15 and C60 will be investigated. This paper gives first approaches with regard to the influence of the carbon content on the oxide scale kinetic and the flow stresses combined with the initial microstructure.Nowadays the design of single process steps and even of whole process chains is realized by the use of numerical simulation, in particular finite element (FE) based methods. A detailed numerical simulation of hot forging processes requires realistic models, which consider the relevant material-specific parameters to characterize the material behavior, the surface phenomena, the dies as well as models for the machine kinematic.This data exists partial for several materials, but general information on steel groups depending on alloying elements are not available. In order to generate the scientific input data regarding to material modelling, it is necessary to take into account the mathematical functions for deformation behavior as well as recrystallization kinetic, which depends alloying elements, initial microstructure and reheating mode. Besides the material flow characterization, a detailed description of surface changes caused by oxide scale is gaining in importance, as these phenomena affect the mater...

Property Oriented Wire Rolling Technology for Mg-Al Alloys

Magnesium and magnesium alloys offer high potential as lightweight materials. Current works are mainly focused on the metal forming technologies and material development for sheet and strips to provide magnesium flat products for industrial applications. However, the technology for the production of magnesium long products for fasteners or other connecting elements is exclusive the extrusion process. A cost-efficient alternative can be the caliber rolling technology for magnesium rods and wire with regard to refined microstructure and specific required properties. But this whole process is rarely applied up to now and all material-specific as well as deformation relevant basics must be developed and additionally validated under industrial conditions. This paper gives the overview for a magnesium-specific wire rolling technology under consideration of chemical composition (AZ31, AZ61, AZ80) and their influence to final mechanical properties in correlation with the microstructure evolution along the whole process line. Therefore, the process-and material-dependent microstructural evolution during rolling process was investigated. The structural constitution is detailed by the grain size and the precipitation conditions. For the determination of the mechanical properties hardness measurement as well as tensile testing was carried out. To preliminary design and determine the material flow, the temperature distribution, and the logarithmic strain, a commercial numerical simulation tool was applied on base of the implemented material-specific deformation and recrystallization behavior. Hence, it was possible to design a magnesium specific caliber sequence for the production of fine-grained magnesium wires with Ø 8 mm and excellent mechanical properties.

Forging of eccentric co-extruded Al-Mg compounds and analysis of the interface strength

Within the subproject B3 of the Collaborative Research Center 692 it has been shown that Al-Mg compounds with a good bonding quality can be produced by hydrostatic coextrusion. During processing by forging, the aluminum sleeve is thinned in areas of high strains depending on the component geometry. To solve this problem an eccentric core arrangement during co-extrusion was investigated. Based on the results of FE-simulations, the experimental validation is presented in this work. Rods with an offset of 0.25, 0.5 and 0.75 mm were produced by eccentric hydrostatic co-extrusion. Ultrasonic testing was used to evaluate the bonding quality across the entire rods. For the forging investigations the basic process Rising was chosen. The still good bonding quality after forging was examined by dye penetrant testing and optical microscopy. For an optimal stress transfer between the materials across the entire component, a sufficient bonding between the materials is essential. To evaluate the interface strength, a special bending test was developed. For the conception of the bending specimens it was required to analyze the Rising specimens geometry. These analyses were performed using a reconstruction of the geometrical data based on computer tomography (CT) investigations. The comparison with the numerically deter-mined Rising specimen geometry shows good correlation. Parametric Finite Element Analyses of the bending test were used to develop the load case and the specimen geometry. By means of iterative adaption of load application, bearing and specimen geometry parameters, an advantageous stress state and experimentally applicable configuration were found. Based on this conception, the experimental setup was configured and bending tests were performed. The interface strength was deter-mined by the calculation of the maximum interlaminar interfacial tension stress using the experimental interface failure force and the bending FE model.

Experimental and numerical investigation on cold flat rolling processes of DC04 sheets with special focus on residual stresses

The process of cold flat rolling is a widespread industrial technique to manufacture semi-finished products, e.g., for the automotive or homewares industry. Basic knowledge of the process regarding dimensioning and adjustment of defined characteristics is already state of the art. However, a detailed consideration and analysis with respect to local inhomogeneous residual stresses in several process steps mostly remains disregarded. A broad understanding of the process due to the distribution of residual stresses in the workpiece and the direction of the stress tensors allows for a definition of the characteristics of the workpiece even before the actual manufacturing process. For that purpose, it is necessary to perform numerical investigations by means of the finite element analysis (FEA) of cold flat rolling processes. Within this contribution, several approaches for the calibration of the FEA with the real flat rolling process will be addressed and discussed. To ensure that the numerical consideration provides realistic results, this calibration is indispensable. General parameters such as geometry, height reduction, rolling temperature, process time, and the rolling speed are considered as well as a photogrammetric survey, and calculated residual stresses with results of X-ray diffraction (XRD) will be compared. In the course of the experiments, a good agreement between the stress results of the FEA and the XRD was found in the center of the specimen. In combination with the allocation of the stress orientations, the agreement close to the edges is also fine. Some issues that cause differences between the FEA and the experiment are dis-cussed.

Numerical simulation of metallic wire arc additive manufacturing (WAAM)

Additive-manufacturing technologies have been gaining tremendously in popularity for some years in the production of single-part series with complex, close-to-final-contour geometries and the processing of special or hybrid materials. In principle, the processes can be subdivided into wire-based and powder-based processes in accordance with the Association of German Engineers (VDI) Guideline 3405. A further subdivision is made with respect to the smelting technology. In all of the processes, the base material is applied in layers at the points where it is needed in accordance with the final contour.The process that was investigated was wire-based, multi-pass welding by means of gas–metal arc welding. This was accomplished in the present study by determining the material parameters (thermo-mechanical and thermo-physical characteristics) of the welding filler G3Si1 (material number: 1.5125) that were necessary for the numerical simulation and implementing them in a commercial FE program (MSC Marc Mentat). The focus of this paper was on simulation and validation with respect to geometry and microstructural development in the welding passes. The resulting minimal deviation between reality and simulation was a result of the measurement inertia of the thermocouples. In general, however, the FE model can be used to make a very good predetermination of the cooling behaviour, which affects the microstructural development and thus the mechanical properties of the joining zone, as well as the geometric design of the component (distortion, etc.).