Semi-solid metal (SSM) casting has been widely used in automotive industries to reduce the weight. In RheoMetalTM process which is one of the variations of SSM, the slurry fabrication can be finished within 30 second and can the slurry making process can be integrated into a high pressure die casting (HPDC) route without significant adjustments, making the process a promising alternative for industrial application. However, the application of SSM is still limited due to the semi-solid deformation-induced casting defects, such as macrosegregation and large pores. Due to the short stirring duration, the inefficient stirring is the leading cause of defects formation. Another critical issue in the RheoMetalTM process is the oxidation during the stirring process, which results in the increase of oxides in the castings, reducing the mechanical properties. This study aims to investigate the ductility and the fatigue performance of SSM castings. The main focuses were on the role of the stirring and oxides. The quenched slurry was analyzed to evaluate the effect of the stirring on the particle distribution in the slurry, and its effect on the formation of pores and segregation was discussed. To investigate the oxidation during the slurry making process, two alloys with different Mg content were cast. Scanning electron microscopy (JEOL JSM-7001F SEM) equipped with Energy-dispersive X-ray spectroscopy (EDS) was used to identify the oxides on the fracture surface. Tensile test and fatigue test accompanied with direct current potential drop (DCPD) were performed to investigate the ductility and fatigue performance, respectively. The results suggest that the intensive stirring can avoid the formation of the large clusters, making the particle distribution homogeneous in the slurry. The Mg contents determined the types of the oxides formed in the slurry making process. For alloys with sufficient Mg, the oxides would be MgAl2O4, which exist as small films with numerous cracks, while a large oxides film will be formed in the case of low Mg content. The combined influence of porosity and oxides was concluded. In the 42000 alloy, because of the low Mg content, the ductility was dominated by the large oxide films. In contrast, in the Magsimal 59 alloy, the presence of small oxides (less than 0.2 mm in majority) leads to the influence of oxides on the elongation negligible. However, a good correlation was obtained between the largest pores and ductility. The fatigue test shows that the surface liquid segregation (SLS) determined the fatigue strength under cyclic bend loading, due to its higher hardness. The effect of the inner pores on the fatigue performance was negligible, as the maximum stress was applied on the surface. / Halvsolid metall (SSM) gjutning har använts i stor utsträckning inom bilindustrin för att minska vikten. I RheoMetalTM-processen, som är en av varianterna av SSM, kan slurrytillverkningen avslutas inom 30 sekunder och kan slurryframställningsprocessen integreras i en högtrycksgjutningsrutt (HPDC) utan betydande justeringar, vilket gör processen till ett lovande alternativ för industriell tillämpning. Tillämpningen av SSM är dock fortfarande begränsad på grund av de halvfasta deformationsinducerade gjutdefekterna, såsom makrosegregering och stora porer. På grund av den korta omrörningstiden är den ineffektiva omrörningen den främsta orsaken till att defekter bildas. En annan kritisk fråga i RheoMetalTM-processen är oxidationen under omrörningsprocessen, vilket resulterar i ökningen av oxider i gjutgodset, vilket minskar de mekaniska egenskaperna. Denna studie syftar till att undersöka duktiliteten och utmattningsprestandan hos SSM-gjutgods. Huvudfokus var på omrörningens och oxidernas roll. Den kylda uppslamningen analyserades för att utvärdera effekten av omrörningen på partikelfördelningen i uppslamningen, och dess effekt på bildandet av porer och segregation diskuterades. För att undersöka oxidationen under slurrytillverkningsprocessen göts två legeringar med olika Mg-halt. Svepelektronmikroskopi (JEOL JSM-7001F SEM) utrustad med energidispersiv röntgenspektroskopi (EDS) användes för att identifiera oxiderna på brottytan. Dragtest och utmattningstest tillsammans med likströmspotentialfall (DCPD) utfördes för att undersöka duktiliteten respektive utmattningsprestandan. Resultaten tyder på att den intensiva omrörningen kan undvika bildningen av de stora klustren, vilket gör partikelfördelningen homogen i slammet. Mg-innehållet bestämde vilka typer av oxider som bildades i slurrytillverkningsprocessen. För legeringar med tillräckligt med Mg skulle oxiderna vara MgAl2O4, som finns som små filmer med många sprickor, medan en stor oxidfilm kommer att bildas vid låg Mg-halt. Den kombinerade inverkan av porositet och oxider konstaterades. I 42000-legeringen, på grund av det låga Mg-innehållet, dominerades duktiliteten av de stora oxidfilmerna. Däremot, i Magsimal 59-legeringen, leder närvaron av små oxider (mindre än 0,2 mm i majoritet) till att oxidernas inverkan på förlängningen är försumbar. En god korrelation erhölls dock mellan de största porerna och duktiliteten. Utmattningstestet visar att ytvätskesegregeringen (SLS) bestämde utmattningshållfastheten under cyklisk böjbelastning, på grund av dess högre hårdhet. Effekten av de inre porerna på utmattningsprestandan var försumbar, eftersom den maximala belastningen applicerades på ytan.

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.