High-speed infrared monitoring and simulations of bulk metallic glass casting
- Doctoral Thesis
Rights / licenseIn Copyright - Non-Commercial Use Permitted
Bulk metallic glasses (BMGs) are exceptional candidates for small, highly precise, load-bearing parts in the mm- to cm-range. Their high strength and elasticity make them superior to their crystalline counterparts. In order to obtain the amorphous atomic structure of this special group of alloys, the molten material must be quenched sufficiently quickly to below their glass transition. Die casting into permanent molds made of copper has frequently been applied to vitrify glass-forming melts at the laboratory scale. However, a transition into industrially available, economical casting techniques has not yet been realized. The comparably high cooling rates which occur during BMG casting hamper the applicability of existing casting techniques. Systematic studies dedicated to BMG casting are scarce, and therefore improvements via casting technology are limited due to the absence of applicable knowledge. In this thesis, (BMG) die-casting experiments were monitored using a high-speed infrared camera for the first time. To successfully deploy this technique, the main requirements were identified and intrinsic phenomena reported. A casting apparatus was tailored which included means for shielding unwanted emission, anti-reflection coating, alignment, and a new cold-crucible induction melter. A suitable mold was designed using the commercial casting simulation software ProCAST. The infrared (IR) emission properties were thoroughly investigated, revealing distinct behavior for each individual alloy studied (a Au-based BMG, a Zr-based BMG, and AlSi7Mg0.3 as a crystalline reference). The data were applied to (re)calibrate the IR camera. Die-casting experiments using Cu molds covered with an IR-transparent sapphire wafer at the front, i.e. the side facing the camera, allowed observation of the melt during mold filling and subsequent cooling. Immediate insights were obtained concerning how the flow patterns depend on the flow direction with respect to gravity. The mold-filling dynamics for horizontally flowing melt was assessed by comparing the volumetric flow rate determined from experiments and state-of-the-art casting simulations. These casting simulations seem to overestimate the mold-filling capabilities compared with results from high-speed thermography. The mode of visualization also helped to identify a high degree of turbulence which spreads from the sprue through the gating, disturbs the melt front, and obscures the initial casting conditions during horizontal casting. In contrast, counter-gravity casting generated particularly reproducible casting quality with reduced turbulence. The mold geometry designed may be deployed for future studies on heat transfer, on the importance of skin formation during casting, and on local cooling. The latter was described thoroughly using a low-melting Au-based alloy. Because of its weak interaction with the sapphire wafer, IR calibration generated good agreement with complementary fast differential scanning calorimetry (FDSC) experiments, which produced a detailed picture of the crystallization kinetics of the alloy. It was obvious that the cooling rates in these experiments vary greatly. It is not just the cooling rates that count: the actual thermal history, especially directly in front of the “crystallization nose”, played a bigger role in crystallization (or successful vitrification). Combining the two techniques – high-speed IR monitoring and FDSC – offers great potential for studying processes with high cooling or heating rates. The knowledge about casting modeling and heat transfer gained through the high-speed IR monitoring technique was applied to investigate a continuous casting process for precious BMGs on the site of the industrial partner PX Holding SA. Bulk glassy rods with diameters of 10 mm and lengths > 500 mm were processed. Comparing temperature data recorded during continuous casting and setting up a computational model revealed that precise knowledge of the heat transfer coefficient is indispensable for sound casting simulations. Again, the results from experiments and simulation were complemented by data of the crystallization kinetics determined by FDSC. Show more
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ContributorsExaminer: Löffler, Jörg F.
Examiner: Battezzati, Livio
Examiner: Blatter, Andreas
Examiner: Champion, Yannick
SubjectBulk metallic glasses (BMGs); CASTING, POURING OPERATIONS (FOUNDRY TECHNOLOGY); Casting; Metallurgy; Infrared thermography; Simulation
Organisational unit03661 - Löffler, Jörg F. / Löffler, Jörg F.
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