The knowledge of the rheological behaviour of metallic suspensions is an important issue when simulating die filling in thixoforming processes. Therefore rheological experiments like step-change in shear rate tests, shear stress ramps and oscillation experiments have been carried out to characterise the flow behaviour of semi-solid alloys exemplarily for A356 and Sn-15%Pb. The material exhibits a yield stress and shows shear-thinning and thixotropic flow behaviour, which is strongly influenced by the solid fraction, the particle shape and size and the degree of particle agglomeration. The instantaneous response on a change in shear rate was observed and pretends shear-thickening properties. All experiments were carried out in a Couette rheometer. The experimentally gained data are well fitted to a model approach, which consists of a modified Herschel-Bulkley law and accounts for the time-dependent effects by introducing a structural parameter K.
Oscillation experiments, creep tests and shear stress ramps have been performed to analyze the yield stress and its time dependency. It has to be distinguished between iso-structural, dynamic and static yield stress. The iso-structural yield stress occurs immediately after shearing. Since the slurry structure remains unchanged, it is equivalent to the structure during shearing. At rest an internal structure builds up, this leads to an increase of the yield stress, which is referred to as the dynamic yield stress. It increases until its maximum value, the static yield stress, is reached.
investigates intensively on thixoforming. This work is supported by Deutsche Forschungsgemeinschaft (DFG) within a collaborative research center. Thixocasting and -forging are modern ªsoft formingº processes in which semisolid slurriesÐcom-parable to the stiffness of ªbutterº±can be formed at substantially lower pressure and temperature than used for conventional forming processes. The process gains an increasing interest. The automotive industry is focused on process technologies which combine complex parts with improved mechanical properties. A significant potential of energy-and costs-savings can be expected.Although the process basics were already developed in 1972 by the MIT (Massachusetts Institute of Technology, Cambridge); the required improvement of process stability and the introduction of a suitable quality management system are still to be enhanced. The technology is applied especially for near net shape forming of complex structures with good mechanical properties.This paper evaluates macro-segregation using computer modeling. Macro-segregation takes place during the forming step of thixoforming and has to be avoided, because the component quality can be reduced by this defect. Due to this macro-segregation simulation was developed and confirmed by metallurgical tests.The chemical segregation is calculated for a prototype component by means of a diphasic model for the liquid and solid phase of a slurry. The calculation is confirmed by chemical analysis of prototype components and thixoforming defects can be minimized.Thixoforming is an innovative process for semisolid alloys, which have to be shaped into complex geometries with improved mechanical properties. The near net shape processing leads to a significant reduced number of process steps. Especially automobile companies, demanding numerous components with high quality, are strongly interested in this technology. Components with high-quality mechanical properties can be produced. Using thixoforming technology new applications for commercial alloys are expected. It opens new perspectives for processing critical alloys by conventional casting and forging. [1] Although the process fundamentals have been investigated in 1972 by the MIT) a strong demand on semisold processing know-how exists still today. In order to improve the component quality and process stability an intense study of defect avoidance is needed.The different departments of the collaborative research center in Aachen, are focusing on raw material development, [2,3] thixoforming processing by casting and forging, determination of typical thixoforming defects (e.g., chemical segregation), quantifying the influence on mechanical properties [4] and development of recycling strategies. [5] The aim of this investigation is to determine the driving force for chemical segregation during the process in the semisolid state. The results are used to define the optimal process parameters. Chemical segregation was made predictable and a practical guideline was developed in order to reduce the infl...
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