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.

The use of lightweight parts by the vehicle industry produces immediate benefits on the reduction of emissions because less energy is consumed during the production, service, and recycle stages of a product life cycle. Therefore, the development of processes that allow high design freedom for topology optimisation and materials with high specific strength is a great need. Semi-solid Al-7Si-Mg castings provide great potential for weight reduction, particularly in critical applications where materials such as steel and cast iron are typically used. However, critical applications have higher requirements in mechanical and fatigue properties compared to conventional aluminium castings applications. Therefore, the control of microstructure and defect formation in all steps of the semi-solidcasting process is essential to produce lightweight, reliable castings for future demands.

In semi-solid aluminium casting, a slurry consisting of primary α-Al crystals dispersed in the liquid is injected into the die-cavity. In this study, the slurry preparation involved the immersion of a cylinder (so-called EEM) while rotating into a superheated alloy. This investigation showed that the α-Al crystals in the slurry are a combination of equiaxed α-Al crystals that nucleate in the thermal undercooled liquid surrounding the EEM, crystal fragments from the columnar dendrites solidified on the EEM surface and undissolved crystals from the original EEM. The addition of grain refiners has no significant effect on the size and shape of the α-Al crystals in the slurry. The dissolution of the EEM duringslurry preparation was studied using a new tag-and-trace method of α-Al crystals. Whenthe EEM disintegrates into large α-Al crystal agglomerates during slurry preparation can result in detrimental effects on the fatigue properties of SSM castings.

Alloy composition, cooling rate, strontium modification, and heat treatment affect the type, size, and shape of the intermetallic phases formed in the Al-7Si-Mg castings. This study showed that high cooling rates and strontium modification are beneficial for the formation of smaller and less detrimental iron-rich intermetallic phases to mechanical and fatigue properties.

The precipitation hardening response of the SSM Al-7Si-Mg castings strongly affects mechanical and fatigue properties. The results in this study showed that the 0.2% offset yield strength increases linearly with the increase of the magnesium concentration in the interior of the α-Al crystals formed during slurry preparation of SSM Al-7Si-Mg castings in the T5 and T6 conditions. Macrosegregation regions surrounded by an oxide layer were preferential sites for fatigue crack initiation in the SSM Al-7Si-Mg castings tested in this study.

Viktminskningen av detaljer som ska användas inom fordonsindustrin ger omedelbara fördelar för reduktion av utsläpp. Detta beror på att det går åt mindre energi att producera, flytta och återvinna dessa lätta komponenter. Utvecklingen av processer som möjliggör hög designfrihet för topologioptimering och material med hög specifik styrka har stor efterfrågan och är därför viktig. Gjutgods framtagning genom semisolidgjutning av Al-7Si-Mg ger stora möjligheter till viktminskning, och då särskilt i kritiska applikationer. Här har traditionella material, som stål och gjutjärn samt processer som smide använts, med större designbegränsningar. Emellertid har kritiska applikationer strängare krav påmekaniska egenskaper och utmattningsegenskaper jämfört med konventionellt gjutgods av aluminium. Därför är styrningen av mikrostruktur och defekter i alla steg i semisolidgjutningsprocessen avgörande för att producera lätt, pålitligt gjutgods för framtida behov.

I semisolidgjutning av aluminium pressas en slurry bestående av primära α-Al-kristaller dispergerade i smältan in i formhåligheten. Slurryberedningen i den studerade processen innebär att cylinder, vid låg temperatur (så kallad EEM), förs ner i en överhettad smälta under rotation. Den fasta fasen i slurryn är en kombination av de likaxliga α-Alkristallerna som kärnbildas i den underkylda smältan närmast EEMet, och kristallfragment från de dendriterna som bildar på EEM cylinderns yta och från EEMet. Tillsats av ympmedel har ingen signifikant effekt på storlek eller formen på α-Alkristallerna i slurryn. När EEMet faller sönder i mer eller mindre stora α-Al-kristaller kan kluster i slurryn resultera i defekter som kan skada utmattningsegenskaperna hos semisolidgjutgods.

Legeringssammansättning, kylhastighet, strontiummodifiering och värmebehandling påverkar typ, storlek och form på de intermetalliska faserna som bildas i Al-7Si-Mg-gjutgods. Höga kylhastigheter och strontiummodifiering är fördelaktiga för bildandet av mindre stora järnrika intermetaller som dessutom är mindre skadliga för mekaniska egenskaper och utmattningsprestandan hos gjutgodset.

Härdningsresponsen för semisolidgjutet Al-7Si-Mg-gods påverkar mekaniska egenskaper och utmattningsegenskaper kraftigt. Resultaten i denna studie visade att den sträckgränsen ökar linjärt med öknande magnesiumhalt i det inre av α-Al-fasen bildad under slurrytillverkningen av semisolitt Al-7Si-Mg-gjutgods i T5- och T6-tillstånd. Makrosegregeringregioner omgivna av ett oxidskikt var primärorsaker för initiering av utmattningssprickor av semisolitt Al-7Si-Mg gjutgods.