Four-point bending fatigue tests were performed on semi-solid Al–7Si–Mg castings with varying magnesium contents and heat treatment conditions. Additionally, the effect of anodising on the fatigue resistance of semi-solid Al–7Si–Mg castings was evaluated. Fracture surface and microstructure analysis showed that fatigue crack initiation occurred mainly at the periphery of regions of positive macrosegregation at the casting surface, resulting most likely from exudation. The microstructure of these macrosegregation regions was mostly eutectic and was frequently found surrounded by a layer of oxides. This layer of oxides promoted weak bonding between the macrosegregation region and the surrounding material and acted as a crack initiation site. In this study, primary α-Al globule agglomerates at the casting surface and surrounded by a layer of oxides also promoted fatigue crack initiation. Fatigue resistance of semi-solid Al–7Si–Mg castings in the T5 and T6 conditions increased with the increase in the magnesium content of the alloy from 0.3 to 0.45 wt.% due to the higher precipitation hardening response. However, the increase in the magnesium content from 0.45 to 0.6 wt.% resulted in a slight decrease in the fatigue resistance. The oxide layer formed during anodising had no significant effect on the fatigue resistance of the semi-solid Al–7Si– Mg castings in this study due to the dominant effect of the macrosegregation regions on fatigue crack initiation.

In this study, a new tag-and-trace method of α-Al crystals was developed and used to study the dissolution of a rotating Al-3Si-0.15Ti cylinder immersed into a superheated commercial purity aluminum melt. The developed tag-and-trace method consists of tagging the primary crystals of an alloy with the microsegregation of a peritectic forming solute element, e.g., titanium in aluminum alloys. During solidification, the primary crystals form with a high concentration of the peritectic forming solute, decreasing in the adjacent growth regions of the same crystal. After solidification, the solute microsegregation tag in the interior of the primary crystals can be revealed by color etching. In this work, an Al-3Si-0.15Ti cylinder with all the primary α-Al crystals tagged with titanium was immersed into a superheated titanium-free aluminum alloy. The superheat was varied, and all samples were quenched 10s after immersion of the cylinder. The tagged α-Al crystals from the original cylinder could be distinguished from the non-tagged α-Al crystals formed in the thermally undercooled region surrounding the cylinder and during quenching. Indications of liquid penetration were observed in some α-Al crystals, which indicates that disintegration of α-Al crystals may occur during stirring of the alloy cylinder into a superheated alloy.

Primary type polyhedral silicon crystals were observed in the vicinity of oxides located at or near the surface of strontium modified semi-solid Al–7Si–Mg castings. The coarse silicon polyhedrons were observed mainly on the outer side of oxides surrounding the macrosegregation regions. The silicon crystals most likely nucleated on AlP-particles located near the oxides. These AlP particles may nucleate on the wetted surface of oxides, or form at different locations in the interdendritic regions.

Magnesium and silicon concentrations in the interior of primary α-Al of Al-7Si-Mg alloys were studied at temperatures in the liquid-solid range and just after solidification was completed. Analysis of the results showed that the magnesium concentration in the interior of primary α-Al is very low in the temperatures range from the liquidus to the start of the Al-Si eutectic reaction. Formation of silicon-rich phases during eutectic reactions, such as eutectic silicon and β-Al5FeSi, phases trigger a significant increase in the magnesium concentration in the interior of primary α-Al, when sufficient time is allowed for solid-state diffusion to occur. When fast cooling rates are applied during the Al-Si eutectic reaction, most of the magnesium is retained in π-Al8FeMg3Si6 and Mg2Si phases formed during solidification. Semi-solid Al-7Si-Mg castings were produced with varying magnesium contents, and the mechanical properties were evaluated in the as-cast, T5 and T6 conditions. It was found that the 0.2% offset yield strength of the semi-solid Al-7Si-Mg castings in the T5 and T6 conditions increases linearly with the square root of the magnesium concentration in the interior of the α-Al globules formed during the slurry preparation process. 

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.

The influence of cooling rate on the formation of iron-rich intermetallic phases during solidification of unmodified and strontium-modified Al-7Si-0.3Mg alloys has been investigated. The effect of strontium on the intermetallic phases was evaluated in unquenched and quenched samples. Samples were quenched before the start of the Al-Si eutectic reaction, along the Al-Si eutectic reaction and just after the end of solidification. The results show that the addition of strontium increased the size of both β-Al5FeSi and π-Al8FeMg3Si6 at low cooling rates. For unmodified and strontium-modified alloys, an increase of cooling rate resulted in a decrease in size of the intermetallic phases, particularly in the strontium modified alloy. In the strontium modified alloy quenched before the start of the Al-Si eutectic reaction, π-Al8FeMg3Si6 appeared as thin platelets at the eutectic cell boundaries. Chinese script-like π-Al8FeMg3Si6 and platelet-like β-Al5FeSi intermetallic phases were observed uniformly distributed in the eutectic regions in the unmodified alloy quenched before the start of the eutectic reaction. Strontium modified semi-solid Al-7Si-0.3Mg castings were produced and the type of intermetallic phase, morphology, size, area fraction and distribution were similar to that observed in the strontium modified alloy quenched before the start of the Al-Si eutectic reaction.

In semi-solid casting, a slurry consisting of primary α-Al crystals and liquid is injected into the die cavity. The solidification in the die-cavity occurs by the growth of the primary α-Al crystals formed during slurry preparation and in the shot sleeve, nucleation and growth of in-cavity solidified crystals and ends with the eutectic reaction. During solidification in the die cavity, the cooling rate near the die wall is higher in comparison to the centre of the casting, particularly for thick-walled castings. The solidification conditions for the slurry α-Al crystals that are closer to the die wall can be very different compared to the slurry α-Al crystals located at the casting centre. This can result in different solute concentration in the interior of the α-Al globules in different regions of the semi-solid casting cross-section and consequently, different response to heat treatament. The RheoMetal™ process was used to produce thick-walled semi-solid castings. Semi-solid castings in the as-cast and T6 conditions were investigated. Indentation tests for hardness measurements in the nano-range were performed in the interior of α-Al globules near the surface and at the casting cross-section centre. The hardness variation across the casting cross-section was evaluated by low-force Vickers hardness. The castings in the as-cast condition showed more uniform properties in the cross-section compared to the T6 condition. Additionally, the results suggest that microsegregation in the interior of α-Al globules is very low in castings in the as-cast and T6 conditions. 

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 study aims to evaluate the effect of grain refinement on slurry formation and surface segregation in semi-solid castings produced by the Rheometal™ process. The effect of two grain refiners, Al-8B and Al-5Ti-1B, on the slurry α-Al grain size, shape factor and solid fraction was evaluated. The results suggest that the addition of a grain refiner can affect the solid fraction obtained in the RheometalTM process and, consequently, reduce the solute content near the casting surface. Grain refiner addition resulted in a larger fraction of α-Al grains ≤ 60 µm for the refined alloys compared with the unrefined alloy. Additionally, the growth of α-Al slurry globules was greater for the unrefined alloy compared with the refined alloy during solidification in the die-cavity. A more homogeneous and finer microstructure was observed near the surface in the grain-refined castings compared with the unrefined castings. Evidence of significant liquid penetration was identified in some α-Al globules, indicating that disintegration of α-Al globules may occur during the Rheometal™ casting process.

Decreasing the weight of heavy-duty vehicles is an ongoing concern. However, the need to deal with high temperatures in components such as manifolds imposes, by itself, some restrictions regarding material selection, being further limited when other required properties (e.g., functional, manufacturing or cost requirements) are taken into account. Cast austenitic stainless steels may represent a good choice in this context but the existence of concentrated chromium carbides can generate undesirable results. A good combination of heat treatments can be applied to cast heat-resistant austenitic stainless steels, in an effort to achieve the dispersion of fine carbides, consequently improving their microstructure, mechanical properties and creep resistance. In this work, an austenitic stainless steel usually used in high temperature applications was characterized and subjected to solution annealing and aging heat treatments. The material analyzed was the austenitic cast stainless steel HK30 and the goals of the work were to evaluate the effects of solution annealing heat treatments on the dissolution of grain boundary chromium carbides and the effects of aging treatments on creep resistance. The results show that the elimination of grain boundary chromium carbides is possible by applying a solution annealing heat treatment. Additionally, the precipitation of fine dispersed carbides is obtained after the aging treatment with an increase of hardness and, consequently, an expected improvement of creep resistance. Thus, the novelty presented by this work consists of selecting the best heat treatment combination in order to promote dispersion of carbides, thus avoiding further crack nucleation phenomena when parts are cyclically subjected to load and unload; this work also found the most adequate mechanical properties and achieved corrosion resistance regarding the application in heavy-duty vehicle components subjected to mechanical and thermal fatigue. By discovering methods of improving the properties of cast materials, large savings can be made both in terms of production costs as well as in the overall weight of the components.