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Characterisation of casting defects in DC cast magnesium alloys
[Thesis]. Manchester, UK: The University of Manchester; 2014.
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Abstract
The continued interest in the use of magnesium alloys for new applications demandthe successful production of high quality wrought alloys. Magnesium Elektron seekto reliably produce high quality alloy billets by the DC casting method combinedwith ultrasonic inspection. The main objectives of this study are to characterize thedefects which are currently found in the material and to understand the ability ofthe ultrasonic inspection technique currently employed to detect the defects.This study began by locating defects using the ultrasonic inspection method whichwere then characterised using X-ray Computed Tomography (XCT) 3D imagingtechnique. Attempts were then made to understand and simulate the mechanismsby which the defects form during the casting process. The simulations were used toinvestigate the flow patterns during casting and the growth kinetics of theintermetallic phase.The initial phase of this research established that the defects found comprised of anentrained oxide film entangled with an abundance of intermetallic phase particles.These defects were found to be present in the size range of 0.5 – 5 mm, and weredeleterious to the materials mechanical properties. Greater understanding of theultrasonic inspection process was achieved and informed improvements to assistingthe production of high quality feedstock.Simulation of the formation of the defects indicated that there was a region inwhich the oxide films could form and be free to enter into the final cast product.Simulation of the growth of the intermetallic particles demonstrated thatprecipitation from the liquid occurs in the mould during which particles are carriedby the melt flow and experiences a complex thermal history. The combination ofthe two phases was established to be due to entanglement of the oxide andparticles which when combined will settle out of the melt as a single defect.Improved filtering and melt handling methods were recommended to eliminate thedefects and reliably produce high quality alloys.