Microstructure Analysis and Rheological Behavior of Magnesium Alloys at Semi-solid Temperature Range
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JMEPEG https://doi.org/10.1007/s11665-018-3571-8
Microstructure Analysis and Rheological Behavior of Magnesium Alloys at Semi-solid Temperature Range Marta S´le˛zak, Piotr Bobrowski, and Łukasz Rogal (Submitted March 7, 2018; in revised form July 9, 2018) Microstructure and rheological properties of AZ91 and Elektron 21 magnesium alloys in semi-solid state were studied. Differential scanning calorimetry revealed significant differences in melting range in analyzed materials. Isothermal annealing for 5, 30 and 80 min, in semi-solid range, corresponding to 50 and 70% of solid fraction, showed slower microstructure coarsening kinetics in Elektron 21 due to the presence of Zrand RE-enriched precipitations. Ostwald ripening and coalescence resulted in decrease in grains density with increasing annealing time and temperature. Hardness after annealing showed overall tendency to diminish with soaking time probably due to the growth of average grain size. Rheological analysis showed tendency to lower the value of dynamic viscosity coefficient as shear rate rise, which confirmed nonNewtonian flow behavior. The highest values of dynamic viscosity coefficient were obtained for alloy AZ91. Keywords
Mg alloys, microstructure, rheological properties, semi-solid state, thixoforming
1. Introduction Magnesium alloys belong to a group of materials, which, due to a low specific gravity and good mechanical properties, may be a successful substitute for steel or aluminum (Ref 1). At present, the studies have been focused on developing new chemical compositions and their further optimizing as well as on new technologies of forming (Ref 1-3). The semi-solid metal processing (SSM) utilizes thixotropic flow in the solidus– liquidus temperature range, which allows obtaining the components with complex shapes and high mechanical properties in a single operation (Ref 2, 3). It is possible due to unique microstructure consisting of the globular grains surrounded by homogeneously distributed eutectic (Ref 4). The rheological studies combined with the microstructural analysis constitute the basis for understanding the mechanisms of flow behavior of the slurry during a semi-solid metal processing (SSM) (Ref 1). The first rheological tests of solidifying alloys were conducted by Flemings and his co-workers. The investigations were performed while stirring the suspension at a specific, assumed temperature. The results of the experiments indicated the influence of the solid-phase content, the shear rate and the ‘‘alloy history,’’ like time of stirring and staying on viscosity. The foregoing effects were related to the mechanism of merging and breaking the grain agglomerates, depending on ‘‘force’’ (Ref 2). The literature contains numerous studies on rheological analysis of aluminum and magnesium alloys (Ref 5-25), whose authors determined the values of dynamic viscosity coefficient (Ref 23, 49-53); however, these data did
Marta S´le˛zak, AGH University of Science and Technology, 30, Mickiewicza Av., 30-059 Krako´w, Poland; Piotr Bobrowski and Łukasz Rogal, I
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