Franz Berge

Magnesiumbleche und -bänder mit besseren Eigenschaften

Am Institut für Metallformung der TU Bergakademie Freiberg erfolgt die Herstellung von Magnesiumflachprodukten durch kombiniertes Gießund Bandwalzen. Diese Herstellungsroute ermöglicht durch die Einsparung von Prozessschritten eine Reduktion der Herstellungskosten sowie eine Verbesserung des Umformvermögens und der mechanischen Eigenschaften der erzeugten Bandhalbzeuge. Nachfolgend werden die Technologie und die wesentlichen Resultate des Gießund Bandwalzens vorgestellt.

Influence of Temperature and Loading Rate on the Forming Limit Behaviour of Twin-Roll Cast, Rolled and Heat-Treated AZ31 as a Function of the Stress State

Magnesium and its alloys have high potential for lightweight applications in the automotive and aerospace industries. In order to design parts for new applications with optimized mechanical properties and higher, more economic production rates, the forming limit behaviour of thin sheets (t < 1.0 mm) has to be known for different temperatures and loading rates. In this study, forming limit curves of 0.8 mm thick AZ31 sheet were measured for deformation at 200 °C and 250 °C and at loading rates of 1 mm/s and 10 mm/s with the Nakajima test. The investigations showed that an increase in temperature from 200 °C to 250 °C tends toward higher forming limit values for all stress states. In contrast, an increase in the loading rate from 1 mm/s to 10 mm/s induces a reduction in formability. It can be seen that the temperature, loading rate, and stress state influence the force-distance curves, the distribution of the local major strains, and the sheet thickness reduction.

Influence of Heat-Treatment and Rolling Conditions on the Mechanical Warm Forming Properties of Twin-Roll Cast AZ31

Magnesium sheets are used for wide-ranging applications in the automotive sector. In contrast to conventional magnesium processing routes for strips, twin-roll casting (TRC) and hot rolling is a cost-efficient production process for magnesium strips and sheets. As part of previous research strategies, the optimization of the thermomechanical treatment of magnesium alloys has resulted in excellent mechanical properties for the component design. However, the previously determined results at room temperature cannot be correlated with the warm forming behaviour during the component production. This is due to different deformation mechanisms, which are active at various temperatures. For TRC material, there is a lack of knowledge about the influence of heat-treatment and rolling on the final mechanical properties at hot working temperatures. This article depicts the investigations done on the influence of heat-treatment and hot rolling conditions on the mechanical properties of AZ31 strips at tensile deformation temperatures of 20 °C and 300 °C.

Microstructure Investigations of Inverse Segregations in Twin-Roll Cast AZ31 Strips

During twin-roll casting (TRC) of magnesium alloy AZ31 specific process conditions may promote the formation of inverse segregations. This phenomenon is characterized by pressing remaining melt, which is enriched with alloying elements, into small cavities within the rapidly solidified material at the chill surface. The occurrence of inverse segregations, which remain in the microstructure after an annealing treatment, may have an adverse effect on the mechanical properties. Under mechanical load, the inverse segregations in AZ31 strips in annealed conditions act as a crack initiator and lead to material failure below the maximum deformation limits. In order to clarify the mechanisms, which are responsible for the development of inverse segregations, fundamental investigations regarding composition, influencing process parameters and effect on material properties were carried out. For determining the qualitative chemical composition of the inverse segregations, SEM investigations using the EDX analysis were carried out. In order to specify the individual phases of the segregations, the qualitative X-ray phase analysis was applied. Potential generation mechanisms of inverse segregations during TRC, based on the present results, were discussed. Via tensile tests at room temperature, the influence of the inverse segregations on the elongation to failure A80 in heat-treated AZ31 TRC sheets was proved. Magnesium sheets with and without inverse segregations were compared. Interventions for prevention of inverse segregations were discussed.

Influence of Deformation Controlled Strain Rate on Tensile and Compression Behaviour of Magnesium and Steel Wire

This article demonstrates the difference in the flow curves of an AZ31 magnesium alloy and S235JR structural steel wire caused by non-linear strain rates during uniaxial tensile and compression testing at elevated temperatures. Throughout tensile deformation, the traverse velocity of the testing machine has to be adapted according to the current elongation of the specimen, thus accelerating, to ensure a constant strain rate during the admission of the stress-strain curve. The equivalent is necessary during compression testing, where the traverse velocity of the testing machine needs to decelerate ensuring a constant strain rate. Nevertheless, tensile and compression tests are performed with constant traverse velocity, which lead to divergent flow curves in comparison to deformation controlled traverse velocities. The results of the research show the difference in flow behaviour of magnesium and steel wire, when the temperature and strain rate are varied in conjunction with constant and deformation controlled traverse velocities.

Influence of the Sheet Manufacturing Process on the Forming Limit Behaviour of Twin-Roll Cast, Rolled and Heat-Treated AZ31

In the age of lightweight design, magnesium alloys play an increasing role in weight reduction of transport vehicles. The specific strength compared to aluminium alloys and steel grades is superior, giving the material great potential in lightweight application. The automobile and aeronautic industry use sheet metals with minimum thicknesses, making research in this field very important. Successful sheet metal forming depends on various process parameters and material characteristics. Thus, the influence of sheet thickness on the forming limit behaviour of twin-roll cast, rolled and heat-treated AZ31 was investigated. Nakajima tests were performed on a hydraulic sheet metal testing device at elevated temperatures with various sheet thicknesses from 0.6 mm to 2.0 mm. The results show an increase in formability with rising temperatures for all sheets. Furthermore, changes in formability among the sheet thicknesses were linked to their divergent microstructures, which result from the different sheet manufacturing parameters.

The Effect of Sheet Thickness, Loading Rate and Punch Diameter on the Deformation Behaviour of AZ31 during 3-Point Bending

In this study, the influence of sheet thickness, loading rate, and punch diameter on the bending behaviour of twin-roll cast, rolled and heat-treated AZ31 magnesium alloy was investigated. Therefore, the 3-point bending test was performed at room temperature using an electromechanical testing machine (v = 0.1−10 mm/s) with different punch diameters (D = 2 mm, 8 mm, 16 mm). The initial material has a recrystallized microstructure with grain sizes of 6−9 µm. It is shown by the mechanical investigations that the bending force increases with the sheet thickness. In contrast to this, the bending angle is independent of the sheet thickness. In addition, the punch diameter and the loading rate do not influence the maximum force and the bending angle significantly.

Influence of Temperature, Strain Rate, and Sheet Thickness on the Deformation Behaviour of Twin-Roll Cast, Rolled and Heat-Treated AZ31 under Uniaxial Loading

The influence of temperature, strain rate, and sheet thickness on the mechanical properties of twin-roll cast, rolled and heat-treated AZ31 was investigated under tensile loading from different directions (0°, 90°). To assess the forming behaviour of different sheet thicknesses (0.6 mm, 1 mm), tensile tests were performed with an electromechanical testing device between 20 °C and 300 °C at strain rates of 10−4 s−1 and 10−2 s−1. With rising temperature, the flow stress decreased while the elongation to failure (A80) increased, which may be related to the enhanced dislocation motion and the activation of additional nonbasal slip systems at T > 200 °C. It can be seen that the anisotropy of flow stress and elongation to failure was influenced by the temperature, the strain rate, and the sheet thickness.