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Ullmann M.,TU Bergakademie Freiberg | Kawalla R.,TU Bergakademie Freiberg | Vogt H.-P.,MgF Magnesium Flachprodukte GmbH
Metallurgia Italiana | Year: 2014

The new energy efficient method of producing magnesium strips down to 1,0 mm thickness based on Twin Roll Casting (TRC) and Strip Rolling in industrial scale developed at the Institute of Metal Forming at the TU Bergakademie Freiberg in cooperation with the MgF Magnesium Flachprodukte GmbH Freiberg (Germany) is described. The design of rolling schedule was analyzed, based on the control of the recrystallization behaviour, to achieve fine grain in the strip rolling process of the magnesium alloy AZ31. The dynamic recrystallization behaviour of this alloy during hot deformation was determined with help of plane strain compression tests and microscopic examination. The influence of temperature, strain and strain rate on the activation of dynamic recrystallization process during deformation was analyzed. The deformation behaviour was also simulated with semi-empirical models including specific coefficients related to processing parameters (strain, strain rate and temperature). The results show that the grain size of this alloy refine steadily with increasing rolling passes by dynamic recrystallization. The tensile strength and ductility were improved correspondingly. Source


Saleh H.,TU Bergakademie Freiberg | Weling T.,TU Bergakademie Freiberg | Seidel J.,TU Bergakademie Freiberg | Schmidtchen M.,TU Bergakademie Freiberg | And 3 more authors.
Oxidation of Metals | Year: 2014

Magnesium alloys are very important for lightweight applications. Industrially, these alloys cannot be used without some necessary processing to improve their corrosion properties. The most widely used methods include coating, surface treatments and cladding. In these processes, the magnesium oxide scale plays an important role in the bonding mechanism between the substrate and the coating or the cladding materials. The aim of this study is to understand the spontaneous oxide formation and the initial oxidation kinetics of the TRC AZ31 magnesium alloy. The results are important for the understanding of the subsequent surface treatment processes of that alloy. Therefore, the study comprises: first, the analysis of the native oxide which forms spontaneously after Twin Roll Casting of an AZ31 magnesium sheet, and second, the investigation of the oxidation behavior of the AZ31 magnesium alloy heated in air at 300°C with different exposure times ranging from 1 to 180 min. The results showed that the thickness of the native oxide is 5 nm and the oxide surface mainly comprises of magnesium, oxygen, and carbon compounds. The oxide film contains magnesium oxide in the form of MgO as the main oxide compound with a very thin layer of MgCO3 and Mg(OH)2. The X-ray photoelectron spectroscopy results revealed two stages of oxidation kinetics during exposure to 300°C. Rapid growth represents the first stage, which lasts for about 30 min. After that period, the oxide growth slowed, indicating a steady state character, where the oxide film growth approaches a limit value. This slow growth is due to the lack of diffusion of oxygen into a dense oxide layer possessing a low concentration of defects. The oxidation kinetics follows an inverse logarithmic law. © 2013 Springer Science+Business Media New York. Source


Kawalla R.,TU Bergakademie Freiberg | Ullmann M.,TU Bergakademie Freiberg | Schmidt C.,TU Bergakademie Freiberg | Dembinska J.,TU Bergakademie Freiberg | Vogt H.-P.,MgF Magnesium Flachprodukte GmbH
Materials Science Forum | Year: 2011

The combination of Twin-Roll-Casting (TRC) and subsequent rolling constitutes the most promising process chain for producing magnesium strips economically. Fast solidification (10 times faster than continuous casting) combined with partial deformation lead to a fine primary microstructure as well as less shrinking holes, pores, segregations and brittle precipitations which all together has a very positive effect on forming behavior of the initial material and quality of the final product. The paper elaborates on metallurgical processes in consideration of microstructure and texture results and on the obtained mechanical properties of TRC magnesium strips and finished strips. In addition, the influence of twin-roll-cast and rolling conditions on the mechanical properties will be discussed. The investigation has also been expanded to possible heat treatments and their influence. Concluding remarks will be made on results of rolling trail which were carried out on an industrial scale rolling mill, revealing that the production of hot rolled thin sheets of magnesium alloy AZ31 is possible with a very promising combination of strength and ductility. © (2011) Trans Tech Publications. Source


Ullmann M.,TU Bergakademie Freiberg | Saleh H.,TU Bergakademie Freiberg | Schmitdchen M.,TU Bergakademie Freiberg | Kawalla R.,TU Bergakademie Freiberg | Vogt H.-P.,MgF Magnesium Flachprodukte GmbH
Archives of Civil and Mechanical Engineering | Year: 2013

Magnesium alloys have great potential in the area of lightweight production especially in the automotive industry due to their favorable combination of mechanical properties and low density. The effects of various rolling parameters on ductility were investigated using the Taguchi method. The optimum combination of process parameters has been found through analysis of main effects of ductility and signal-to-noise ratio, and the significant parameter were identified depending on analysis of variance. The results demonstrate that reduction is the most important factor for improving the ductility of the final strips, followed by rolling temperature and rolling speed. The optimal levels for the controllable factors were rolling temperature of 375. °C, rolling speed of 175. m/min and reduction of 70% for higher ductility. © 2012 Politechnika WrocŁawska. Source

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