The vehicles industry is facing increasing demands for fuel efficiency and cost reduction due to environmental legislation, sustainability and customer demands. Therefore, there is a great need to develop and produce lightweight components by using materials and processes that offer higher specific strength and/or design optimization. Semi‐solid aluminium casting offers design freedom and castings with lower shrinkage and gas entrapment defects compared to high pressure die castings. The lack of understanding of microstructure and defect formation, and design data, for semi‐solid castings is a barrier for foundries and designers in the vehicles industry to use semi‐solid castings. In this study, the effect of two grain refiners on slurry formation and surface segregation of semi‐solid Al‐7Si‐0.3Mg castings produced by the Rheometal™ process was evaluated. The influence of grain refinement on primary α‐Al grain size, shape factor and solid fraction was analysed in addition to the solute content of the surface segregation layer. The influence of magnesium on the formation of intermetallic phases during solidification and the heat treatment response of Al‐7Si‐Mg semi‐solid castings was investigated. The magnesium content was varied from 0.3 to 0.6wt.% and the semi-solid castings were analysed in the T5 and T6 conditions. Energy dispersive spectroscopy was used to identify the intermetallic phases formed during solidification. Tensile testing was performed and the results were correlated to the magnesium and silicon concentration measured in the interior of the α‐Al globules formed during slurry preparation. The results suggest that the addition of grain refiner decreases the solid fraction obtained in the Rheometal™ process. However, no significant effect was observed on the α‐Al grain size and shape factor. A good correlation was obtained between the magnesium concentration in the interior of the α‐Al globules formed during slurry preparation and the offset yield strength for all alloys. The low magnesium solubility in α‐Al at temperatures in the solidification range of the Al‐7Si‐Mg alloys is suggested to be the reason for the low hardening response for the T5 heat treatment compared to the T6 condition.

This thesis work is an experimental study of one type of Semi-Solid Metal casting (SSM) process which is called RheoMetalTM. This method is an efficient type of Rheocasting that creates a semi-solid slurry within 30 seconds that is used for a high pressure die casting machine. The purpose of using a slurry in a high pressure die casting machine is that the slurry has a higher viscosity due to its solid fraction. This makes the filling of the die cavity more laminar which reduces air entrapment in the casting. The difficulty with this type of casting is to control the process parameters to be able to insure a casting with desired properties. A few studies within RheoMetalTM has already been made but there is still a lack of knowledge of to what extent the process parameters affect the slurry. The goal in this work is to study how the different RheoMetalTM process parameters influence the primary α-Al solid fraction, shape and size. The process parameters that were studied in this work was the stirring rate, superheat and EEM amount. In this study, the so called growth layer has been removed to make more precise calculations of the primary α-Al. This work also aims to study how grain refinement affect the primary α-Al which is commonly used to improve the quality of castings. To be able to perform this study, both practical and theoretical work has been implemented. The casting process involved making of ladles and preparation of various equipment. The cast samples were then prepared by standard metallurgy procedure for optical analyse of the microstructure. A special etching reagent was used to analyse the microstructure in a microscope. The etching is called Weck’s reagent and its purpose is to differentiate the growth layer from the primary α-Al. The growth layer is formed during quenching and by excluding it, the calculation of the slurry’s primary α-Al becomes more precise. This is because the slurry is not quenched before it is inserted into the high pressure die casting machine, therefore no growth layer is formed. To analyse the cast samples, a special program was used to identify and to calculate the solid fraction, shape and size of the primary α-Al. The result from the calculations made by the program gave different tendencies when changing the EEM amount. The stirring rate showed a tendency to decrease the solid fraction and increase the shape factor. The superheat decreased the solid fraction and increased the shape factor. The grain refinement also decreased the solid fraction and increased the shape factor. There was no clear tendency showing that the equivalent circular diameter of the primary α-Al was affected by any of the parameters.