Madlen Ullmann

Magnesium semi-finished products for vehicle construction

At the Institute of Metal Forming, basic research-oriented and application oriented, two research foci on magnesium are pursued. One focus is on the production of semi-finished products for magnesium sheets and strip production by twin-roll-casting, the other focus is on the production of long products as wire, intended for the fabrication of joining elements. Magnesium sheets produced by twin-roll-casting and subsequent rolling offer very good processing properties and are partly superior to conventionally produced sheets. Advantages result from a shortened process chain whose technology leads to a favourable initial condition for strip production. Additionally, economical advantages due to the few process steps clearly exist. Wire production for the production of joining elements is based on a specifically developed calibration which causes advantageous stress and deformation conditions during caliber rolling. Properties of both product groups of magnesium alloy AZ21, AZ31, AZ91 and WE43 are described comprehensively. Their application for the production of parts and components is shown on examples.

Properties of Magnesium Strips Produced by Twin-Roll-Casting and Hot Rolling

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.

Effect of Different Finish-Rolling Parameters on the Microstructure and Mechanical Properties of Twin-Roll-Cast (TRC) AZ31 Strips

The microstructure and the resulting mechanical properties of Twin Roll-Cast (TRC) AZ31 strips have been investigated after rolled from thickness of 5.2 mm to 1.00 mm and 1.25 mm, respectively. Twin-Roll-Casting (TRC) was used to produce AZ31 strips with a near-net final thickness directly from the liquefied material. The two-stage rolling experiments were carried out on the four high reversing mill at the Institute of Metal Forming in Freiberg. The first stage was two roughing passes followed by intermediate annealing. The second stage was subsequent finish rolling with 1 to 3 rolling passes. The influence of the finish-rolling on the properties of the final strip was investigated, including the variations of rolling pass and pass reduction. The TRC strip exhibits a heterogeneous microstructure with random texture due to a deformed structure combined with partial cast columnar and equiaxed grains. Significant grain refinement was achieved using high deformation degree per pass (> 30 %). Increasing rolling passes to 3 during finish-rolling reduces the strain per pass and also leads to a temperature drop so that incomplete dynamic recrystallization after the final rolling pass occured, leading to a more coarse and heterogeneous microstructure. It was found, that a 2-pass finish rolling provides the optimum strategy for the material properties as well as the process stability. Due to homogeneous fine grained microstructure, the 2-pass rolled strip showes high mechanical properties with low anisotropy. This includes yield point of 234 MPa, tensile strength of 285 MPa and total elongations of 25 %.

Dynamic Recrystallization Behaviour of Twin Roll Cast AZ31 Strips during Hot Deformation

The recrystallization behaviour of a Twin Roll Cast AZ31 alloy, under deformation conditions corresponding to the initial stands of a magnesium hot rolling process, has been investigated. A set of strain levels, beginning at the critical strain for the onset of dynamic recrystallization has been studied. The dynamic recrystallization, taking place during hot deformation of magnesium alloy AZ31 is analyzed with help of plane strain compression tests conducted together with microscopic examination. Dynamically recrystallized volume fraction and size of dynamically recrystallized grains increase with increasing strain. When the strain is constant, the dynamically recrystallized volume fraction is higher at higher deformation temperatures or lower strain rates. Furthermore, it has been found out that deformation temperature plays a major role, rather than strain rates, in activating the dynamic recrystallization process during deformation. The results were used to identify the coefficients for the JMAK-model approach on the dynamic recrystallization of an AZ31 TRC-sheet.

Procedure to Analyze the Formation of Segregations Using the PLS-SEM Approach

Decreasing optimization potential concerning the weight reduction in aircraft and automotive components boost the demand for lightweight materials and new technologies. An opportunity to achieve considerable weight-savings is the application of magnesium strips produced by the novel Twin-roll casting and hot rolling technology. During the production the quality of magnesium strips can be reduced by segregations, which also influence the distribution of the mechanical properties. Exploring the formation of segregations is one research objective of this paper using the partial least squares structural equation modeling (PLS-SEM).

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.

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...

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.