Si Foundry Research Articles

Distribution of trace elements in a modified and grain refined aluminium–silicon hypoeutectic alloy

The influence of modifier and grain refiner on the nucleation process of a commercial hypoeutectic Al–Si foundry alloy (A356) was investigated using optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis technique (EPMA). Filtering was used to improve the casting quality; however, it compromised the modification of silicon. Effect of filtering on strontium loss was also studied using the afore-mentioned techniques.EPMA was used to trace the modifying and grain refining agents inside matrix and eutectic Si. This was to help understanding mechanisms of nucleation and modification in this alloy. Using EPMA, the negative interaction of Sr and Al3TiB was closely examined. In modified structure, it was found that the maximum point of Sr concentration was in line with peak of silicon; however, in case of just 0.1wt% added Ti, the peak of Ti concentration was not in line with aluminium, (but it was close to Si peak). Furthermore, EPMA results showed that using filter during casting process lowered the strontium content, although produced a cleaner melt.

In-situ observation of the nucleation kinetics and the mechanism of grain refinement in Al–Si alloys (Part I)

The nucleation behavior of primary aluminium phase in a hypoeutectic Al–Si foundry alloy is studied using the 3DXRD microscope during the liquid–solid phase transformation for continuous cooling. Grain nucleation and grain growth for few different casting conditions of a commercial aluminium alloy (A356: Al-7Si-0.4 Mg-0.1Fe-0.1Ti wt.%) were investigated using three dimensional X-ray diffraction microscope (3DXRD) located at ID11 at European Synchrotron Radiation Facility ( www.ESRF.eu). To conduct the study a monochromatic hard X-ray beam (energy of 70 keV) with a beam size of 200×200 µm 2 was used and using a special furnace the microstructure evolution during solidification of a commercial Al–Si foundry alloy (A356) was monitored in-situ. Results gathered from solid fraction information showed adding 0.1 wt.% Ti (as Al-3Ti-B) changes the primary aluminium nucleation temperature and aluminium grain size. Furthermore, it showed that at slower cooling rate (0.04–0.1 K/s) grain refiner can alter the primary aluminium nucleation temperature by 20 °C, whereas at higher rates (2 K/s) this figure was reduced down to 5 °C.

A novel Al–Ti–B alloy for grain refining Al–Si foundry alloys

Al–Ti–B refiners with excess-Ti (Ti:B > 2.2) perform adequately for wrought aluminium alloys but they are not as efficient in the case of foundry alloys. Silicon, which is abundant in the latter, forms silicides with Ti and severely impairs the potency of TiB 2 and Al 3Ti particles. Hence, Al–Ti–B alloys with excess-B (Ti:B < 2.2) and binary Al–B alloys are favored to grain refine hypoeutectic Al–Si alloys. These grain refiners rely on the insoluble (Al,Ti)B 2 or AlB 2 particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. It would be very attractive to produce excess-B Al–Ti–B alloys which additionally contain Al 3Ti particles to maximize their grain refining efficiency for aluminium foundry alloys. A powder metallurgy process was employed to produce an experimental Al–3Ti–3B grain refiner which contains both the insoluble AlB 2 and the soluble Al 3Ti particles. Inoculation of a hypoeutectic Al–Si foundry alloy with this grain refiner has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for holding times up to 15 min.

The effect of dissolved titanium on the primary α-Al grain and globule size in the conventional and semi-solid casting of 356 Al–Si Alloy

The chemistry of molten alloy plays an important role on grain refining. Small addition of alloying elements reduces the grain size of the as-solidified structures to some degree depending on the alloy efficiency. The effect of dissolved Ti has been studied on the microstructure of Al–Si foundry alloys cast conventionally and by semi-solid casting routes. It is shown that Ti in solution could restrict the grains and globules size in both processes. A parallel plate compression test machine was employed to study the effect of dissolved Ti on the rheological behavior of the billets. It was confirmed billets with higher dissolved Ti-content have superior flow.

Effects of combined additions of Sr and AlTiB grain refiners in hypoeutectic Al–Si foundry alloys

Strontium is the most widely used and a very effective element for modifying the morphology of eutectic silicon, while Ti and B are commonly present in the commercial grain refiners used for Al–Si alloys. Systematic studies on the effects of combined additions of Sr and different AlTiB grain refiners on the Al + Si eutectic and primary aluminium solidification have been performed. While slight coarsening of both eutectic Si and primary aluminium grains occurs during holding, no obvious interactions are observed between Sr and AlTiB grain refiners when the addition level of grain refiners is low. As a result, a well-modified and grain refined structure was obtained. However, strong negative interactions between Sr and Al1.5Ti1.5B were observed as the addition level of the grain refiner increases. It was found that these interactions have a much more profound impact on the eutectic solidification than the primary Al solidification. The melt treated with combined additions of Sr and Al1.5Ti1.5B still shows good grain refinement efficiency even after losing its modification completely. The mechanism responsible for such negative interactions is further discussed.

As-cast morphology of iron-intermetallics in Al–Si foundry alloys

The as-cast three-dimensional morphologies of alpha-Al-15(Fe,Mn)(3)Si-2 and beta-Al5FeSi intermetallics were investigated by serial sectioning. Large beta-Al5FeSi intermetallics were observed to grow around pre-existing dendrite arms. The alpha-Al-15(Fe,Mn)(3)Si-2 intermetallic particle was observed to have a central polyhedral particle and an external highly convoluted three-dimensional structure. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Combining Sr and Na additions in hypoeutectic Al–Si foundry alloys

From recent published data, it is still unclear whether combining additions of Na and Sr have synergistic effects or deleterious interactions. This paper clarifies the interactions between these two modifiers and investigates the effects of such interactions on alloy solidification and castability. It was found that combined additions of Sr and Na do not appear to cause improvement of the modification of the eutectic microstructure even after only a short period after addition. Na addition may promote Sr vaporization and/or oxidation kinetically, leading to a quicker loss of both modifiers, which is blamed for the rapid loss of the modification effect during melt holding. Quenching trials during the eutectic arrest indicate that addition of Sr into Na-modified melts does not alter the eutectic solidification behaviour. The effect of Na on eutectic solidification dominates, and the eutectic is observed to evolve with a significant dependency on the thermal gradient. Combining Sr and Na additions produced no beneficial effects on porosity and casting defects.

Thermodynamic aspects of grain refinement of Al–Si alloys using Ti and B

A thermodynamic assessment of ternary Al–Si–Ti phases was performed. Published datasets for the other subsystems were checked and adapted. Based on that, a consistent thermodynamic description of quaternary Al–Si–Ti–B alloys was generated. This was applied in a calculation of Al–Si–Ti–B phase diagram sections for practically relevant temperatures and compositions of Al–Si alloys from Al-rich to typical Al–Si foundry alloys. These stable and metastable phase diagrams could be correlated to many detailed aspects of possible reactions observed or suggested in experimental studies of grain refining. Understanding the mechanisms of grain refining of Al wrought alloys and Al–Si foundry alloys using titanium and boron requires a fundamental knowledge of both thermodynamic and kinetic aspects of this complex process. This work focuses exclusively on the thermodynamic aspects and the phase diagrams, which were not available for the quaternary alloys and partly incomplete and inconsistent for the ternary subsystems.

Eutectic solidification in hypoeutectic Al–Si alloys: electron backscatter diffraction analysis

Nucleation and growth of the eutectic in hypoeutectic Al–Si foundry alloys has been investigated by the electron backscatter diffraction (EBSD) mapping technique using a scanning electron microscope (SEM). Sample preparation procedures for optimizing mapping have been developed. To obtain a sufficiently smooth surface from a cast Al–Si eutectic microstructure for EBSD mapping, an appropriate preparation technique by ion milling was developed and applied instead of conventional electropolishing. By comparing the orientation of the aluminum in the eutectic to that of the surrounding primary aluminum dendrites, the growth mechanism of the eutectic can be determined. Two different results were found, in isolation or sometimes together, but distinct for different strontium contents: (1) crystallographic orientations of aluminum in eutectic and surrounding primary dendrites are identical, and (2) wide variation in orientations of the aluminum in the eutectic.

Determination of Eutectic Solidification Mode in Sr-modified Hypoeutectic Al-Si Alloys by EBSD

The effect of eutectic modification by strontium on nucleation and growth of the eutectic in hypoeutectic Al–Si foundry alloys has been investigated by electron back-scattering diffraction (EBSD) mapping. Specimens were prepared from three hypoeutectic Al–Si base alloys with 5, 7 and 10 mass%Si and with different strontium contents up to 740 ppm for modification of eutectic silicon. By comparing the orientation of the aluminium in the eutectic to that of the surrounding primary aluminium dendrites, the growth mode of the eutectic could be determined. The mapping results indicate that the eutectic grew from the primary phase in unmodified alloys. When the eutectic was modified by strontium, eutectic grains nucleated separately from the primary dendrites. However, in alloys with high strontium levels, the eutectic again grew from the primary phase. These observed effects of strontium additions on the eutectic solidification mode are independent of silicon content in the range between 5 and 10 mass%Si.

The origin of ‘anomalous’ microsegregation in Al–Si foundry alloys—modelling and experimental verification

Abstract Slowly cooled Al–7 wt.% Si foundry alloys show ‘anomalous’ segregation of Si in the dendrite arms, with higher concentrations in the centre of the dendrites than near the eutectic. Calculations and experiments show that this is caused by growth of particles in the eutectic during the cooling period after solidification is completed.

Measurements of oxide films in Al-(6–17) wt%Si foundry alloys using the Qualiflash filtration technique

The Qualiflash technique was used to assess the melt cleanliness of Al-Si foundry alloys. This is a simple filtration technique where the melt cleanliness is gauged by the number of steps covered by the liquid metal that filters through into a step-like ingot mould placed underneath the filter. The present work was performed on A356 and A390 aluminum foundry alloys. The main objectives were to validate the reliability of the Qualiflash technique in measuring the oxide content, and to determine the role of grain-refining and Si-eutectic modification agents in relation to oxide formation. The results indicate that Qualiflash is a quick and simple technique to determine the concentration of aluminum oxides (films and massive particles), as well as other oxides, i.e. MgO, MgAl2O4, in terms of the number of ingot steps of filtered metal obtained. Grain-refining or modification agents themselves do not create oxides. However, the method of their introduction into the molten metal contributes to oxide formation. The number of ingot steps varies linearly with the alloy melt temperature. The slope is strongly related to the amount of oxide films that exist in the liquid metal prior to filtration. Such a relationship is expressed in terms of the Quality Temperature Index (QTI). The effect of oxide concentration on the QTI was investigated in detail for the two alloys studied. The results show that the relationship between the amount of oxides and the number of ingot steps is exponential, with a fitness coefficient of the order of 0.87.

Equiaxed solidification of Al–Si alloys

An experimental programme has been undertaken to determine which of the grain formation mechanisms of equiaxed crystals are dominant in the solidification of Al–Si foundry alloys. Small ingots were cast from alloys of varying silicon concentration with and without gauze barriers, using different types of mould materials and different mould preheats. The results show that two mechanisms of grain nucleation are operating. The first is a wall mechanism where crystals are nucleated either on or near the mould wall owing to thermal undercooling. The second is a constitutional supercooling mechanism where nucleants are activated in the constitutionally undercooled zone ahead of the advancing interface. As a consequence, the grain size decreases with increasing silicon content. However, a transition in the growth mode occurs once a critical degree of constitutional undercooling is exceeded. This change in growth is accompanied by an increase in grain size. The transition point can be shifted with respect to solute content by changing the casting conditions, and a mechanism is proposed to explain this effect.

Anomalous microsegregation in Al–Si foundry alloys

Measurements of Si and Mg concentrations across dendrites in a commercial Al–Si–7Mg alloy have shown a higher Si concentration in dendrite centers decreasing towards the dendrite boundaries, which is opposite to what would be expected from the phase diagram.

Grain refinement mechanisms of hypoeutectic AlSi alloys

The general concept of the grain refinement mechanism in hypoeutectic AlSi alloys has been adopted from that in wrought alloys without consideration of the influence of the silicon content. In the present investigation the commercial grain refinement practice in AlSi foundry alloys has been experimentally simulated by introducing synthetic TiB2 crystals directly into AlSi melts. To explore the possible use of a new generation of AlTiC master alloys in these alloys, the behaviour of synthetic TiC has also been studied. Experimental findings indicate that in the presence of Si, TiC crystals are highly unstable in aluminium melts. TiB2 crystals alone do not nucleate the α-phase, whereas in the presence of dissolved Ti a ternary AlTiSi interfacial layer is formed on the TiB2 which subsequently nucleates the α-Al, via a peritectic reaction. However, due to a drastic decrease in the peritectic temperature, the efficiency of grain refinement is greatly reduced. A theoretical analysis indicates that when boron is added in excess of TiB2 stoichiometry, or simply as AlB2, grain refinement occurs by a eutectic reaction.