Investigations on the Formation of Defect Bands in Semi-Solid High Pressure Die Cast Aluminium-Silicon Alloys
Time: Fri 2020-05-08 09.00
Subject area: Metallurgical process science
Doctoral student: Madeleine Law , Materialvetenskap
Opponent: Docent Jessica Elfsberg, Scania
Supervisor: Prof. Pär Jönsson, Materialvetenskap; assist. Prof Christopher Hulme-Smith, Materialvetenskap; Assoc. Prof. Taishi Matsushita, Jönköping University
High-pressure die casting of semi-solid aluminium-silicon alloys is used in the automotive industry to manufacture components, like housings, brackets, and bars. It is commonly known that during high-pressure die casting, defect bands may be created that follow the contour of the component surface. These bands consist mainly of a eutectic phase. This phenomenon is also observed in semi-solid metal slurry high-pressure die casting. These bands could lead to premature failure of the component in service. The origin of these bands is not fully understood and so this research focuses on investigating these bands and their origins further. A series of casting trials were conducted with varying plunger velocity. Subsequent investigation using optical and scanning electron microscopy showed that a change of the plunger velocity alters the number of bands present in the samples. Energy dispersive X-ray spectroscopy revealed that a measurable difference in aluminium quantity across the band was noticed and it was postulated that aluminium migrates towards the component centre. Therefore, different mechanisms responsible for particle migrations found in literature were investigated and assessed quantitatively using experimental data and information from published literature. It was found that the Saffman lift force and the Mukai-Lin-Laplace effect were the mechanisms that were most likely to cause such a migration of aluminium. Further experimental investigation is recommended to identify which of the two mechanisms is ultimately responsible for the migration and to optimise the high-pressure die casting procedure to minimise defect band formation.