The Rheometal™ slurry preparation process, like other Rheocasting processes, aims to obtain a slurry with small and globular crystals dispersed in the liquid. This slurry displays a thixotropic behavior with liquid-like properties when shear stresses are applied,
e.g., during die-filling, and to cast complex shaped castings with less defects compared to coarser microstructures. The addition of grain refiners to commercial aluminium alloys is a common practice in foundries and a significant number of studies can be found in the literature focused on the Al-5Ti-1B[
7,
34‐
36] and Al-B[
37‐
39] grain refiner systems. However, the effect of grain refiner addition during solidification of an alloy while stirring is applied has not received much attention in the literature. In the Rheometal™ slurry preparation process, solidification occurs while the liquid alloy is stirred by a lower enthalpy material. The cumulative distribution function of the grain sizes was used to evaluate the effect of grain refiners on the different alloys. The results showed slightly greater fraction of
α-Al grains with equivalent circular diameter ≤ 60
µm for the refined alloys compared to the unrefined alloy, as seen in Figure
3. The primary
α-Al grain formation during the Rheometal™ slurry preparation process can be understood considering the solidification and flow conditions while the EEM is immersed and stirred in the liquid. A short time after immersion of the EEM, columnar
α-Al dendrites and equiaxed
α1-Al grains were observed in microstructures of quenched EEM and slurry, respectively.[
23] Therefore, during the initial stages of Rheometal™ slurry preparation, a columnar to equiaxed growth transition occurs. A large thermal gradient is established between the EEM (~ 200 °C) and the liquid (650 °C) after immersion of the EEM, resulting in a thermally undercooled region in the immediate liquid surrounding the EEM. In this region, a large amount of crystal nucleation events can occur, analogous to the “free chill crystal” nucleation mechanism proposed by Chalmers.[
40] Crystals not attached to the EEM surface are likely transported into the bulk liquid by flow caused by the rotation of the EEM. Fragmentation of the columnar
α-Al dendrites in the freeze-on layer may also occur because of the stirring and additional crystals can be mixed into the bulk liquid.[
41] The continuous stirring promotes thermal and compositional homogenization in the liquid and a rapid removal of superheat which increases the survival rate of crystals.[
42] The final grain size is a result of the competition between nucleation rate and growth of crystals.[
43] In the initial stages of the slurry preparation process, the latent heat released by the growing crystals can be rapidly removed by the EEM and nucleation of new crystals can occur at large undercoolings. As a result, nucleation of
α-Al crystals can also occur on less potent substrates existing in the liquid in addition to the nucleant substrates introduced by the grain refiners. At the later stages of the slurry preparation, the heat extraction capacity of the EEM is reduced and the nucleation of new crystals is likely inhibited by the release of latent heat by the growth of existing
α1-Al crystals in the slurry.[
43,
44] That is, inhibition of nucleation occur earlier for an alloy which
α-Al crystals growing faster, resulting in greater release of latent heat and recalescence.[
43,
44] The analysis of Figure
4(b) shows that the area occupied by the
α1-Al crystals increases more for the unrefined alloy compared to the grain refined alloys during solidification in the die-cavity, particularly for the alloy refined with Al-5Ti-1B. That is, the
α1-Al crystal growth during the slurry preparation process is likely greater for the unrefined alloy compared to the grain refined alloys. The growth restriction factor (Q) of the individual elements silicon, iron, magnesium, titanium and boron of the unrefined and refined alloys was calculated by[
43]:
$$ Q = C_{0} m(k - 1) $$
(4)
where C
0 is the initial solute concentration shown in Table
I, m is the liquidus slope and k is the equilibrium partition coefficient of each element. The initial concentration of boron used was the amount added as Al-8B grain refiner. The values of m and k for each element where obtained from the literature.[
45,
46] The growth restriction effect of the various elements in each alloy was obtained by summing the growth restriction value for the different elements in the alloys. The growth restriction values obtained were 59, 56 and 66 for the base, Al-8B and Al-5Ti-1B refined alloys, respectively. The slightly smaller growth restriction values obtained for the unrefined and Al-8B refined alloys suggests a greater growth rate of
α1-Al crystals formed during Rheometal™ slurry preparation compared to the Al-5Ti-1B alloy. That is, the latent heat evolved by the growth of the nucleated crystals can inhibit the nucleation of new crystals earlier for the base alloy and alloy refined with Al-8B compared to the alloy refined with Al-5Ti-1B. Therefore, the greater fraction of small grains obtained in the alloy refined with Al-5Ti-1B compared to the unrefined alloy shown in Figure
3 may be the result of the increased number of potent nucleant substrates dispersed in the liquid and the greater growth restriction factor. The alloy refined with Al-8B showed a slightly larger fraction of grains up to 60
µm compared to the unrefined alloy, most likely resulting from the greater number of potent nucleant substrates provided by the grain refiner. However, lesser fraction of smaller grains was obtained for the alloy refined with Al-8B compared to the alloy refined with Al-5Ti-1B, which may result from the lower growth restriction factor of the alloy refined with Al-8B.
A smaller average grain size would be expected for the refined alloys compared to the unrefined alloy resulting from the larger fraction of small grains with equivalent circular diameter ≤ 60
µm obtained for the refined alloys. However, the average grain size obtained was similar for all alloys, as seen in Table
II. The expected smaller grain sizes for the refined alloys are not observed in the average grain sizes values shown in Table
II, most likely due to the large scatter obtained in the measurements. The solidification conditions obtained during the different stages such as slurry preparation, holding in the shot sleeve and inside the die-cavity resulted in the formation of grains with significant variation in size.[
17] Therefore, the large scatter obtained in the measurements of grain sizes of the SSM castings. In HPDC casting, most of the solidification occurs in the die-cavity at very high cooling rates which results in a fine grain microstructure.[
22] In SSM casting, a significant fraction of crystals is formed during slurry preparation that can grow large compared to the in-cavity solidified crystals. Consequently, the greater fraction of small grains shown in Figure
3 and the smaller average grain size of 44 ± 20
µm obtained for the HPDC casting compared to the SSM castings, as seen in Table
II. The slight effect of the grain refiners obtained in this study can be understood considering the solidification conditions and the chemistry of the commercial base alloy used in this study. The initial thermally undercooled region near the EEM and dendrite fragments originating from the freeze-on layer may decrease the effect of grain refinement as observed in Reference
5 for an electromagnetically stirred liquid alloy. In the New Rheocasting process the grain refiner effect decreased as the pouring temperature was reduced because of the large thermal undercooling obtained when the liquid contacts the cold wall of the cup.[
7] The disintegration of the freeze-on layer and EEM result in additional crystals introduced into the liquid as observed by Payandeh
et al.,[
23] which are unlikely affected by the grain refiner particles in the liquid. The addition of Al-5Ti-1B grain refiner introduces soluble TiAl
3 and insoluble TiB
2 particles into the liquid.[
47,
48] Many studies reported the poisoning effect of silicon on the TiB
2 particles that reduced the grain refinement efficiency.[
34,
47] Therefore, the poisoning effect of silicon on TiB
2 can explain the low effect of Al-5Ti-1B grain refiner in this study. The AlB
2 particles introduced by Al-B master alloys into liquid aluminium showed enhanced grain refining effect in hypoeutectic Al-Si foundry alloys compared to Al-5Ti-1B master alloy in different studies.[
38,
39,
49] Chen
et al.[
39] proposed that the formation of a SiB
6 layer at the interface between AlB
2 and Al can significantly improve the nucleation potency of the AlB
2 particles. Alamdari
et al.[
37] studied the heterogeneous nucleation mechanism of boron particles in aluminium by addition of ultrafine boron particles in pure aluminium and Al-Ti melts. It was found that boron dissolves very rapidly in pure aluminium. However, in aluminium liquid with solute titanium the boron dissolution is inhibited by the formation of a thin layer of TiB
2 on boron particles which can act as nucleants for
α-Al. Birol[
47] found that the grain refinement effect of Al-B is poor in commercial Al-7Si-Mg alloys containing 0.04 to 0.1 wt pct titanium due to the formation of TiB
2 particles which are poisoned by silicon.[
47] In the present work a commercial Al-7Si-0.3Mg base alloy was used that contained a significant amount of titanium, as is typical for commercial aluminium alloys.[
47] This titanium could interact with the boron introduced into the liquid by the Al-8B grain refiner forming TiB
2 particles. Consequently, the grain refinement effectiveness of Al-8B inoculant was most likely influenced by the formation of TiB
2 particles which are poisoned by silicon. Additionally, the interaction of strontium in the base alloy with boron could also negatively affect the grain refinement of the primary
α-Al.[
48,
50]
Another potential negative influence on grain refiner efficiency could be the stirring effect on agglomeration of nucleant substrates. Wang
et al.[
38] found that mechanical stirring applied during Al-B master alloys production resulted in large AlB
2 agglomerates with increased settling tendency in molten aluminium. That is, a loss of the inoculant refinement potency can occur by agglomeration and settling of substrates as a result of increased nucleant particles interactions due to stirring. Schaffer and Dahle[
51] suggested that agglomeration has a significant effect in determining the rate at which the loss of refinement occurs due to nucleant substrates settling, which was much faster than predicted by Stokes law. The much shorter stirring time applied in this work (18 seconds) compared to other studies[
38] could suggest that stirring would have a smaller effect on agglomeration of substrates. However, in this study stirring was applied under non-isothermal conditions during solidification of refined alloys, while most other studies focus on isothermal stirring of a liquid with nucleant substrates.[
38,
51,
52] In a semi-solid material, the volume of liquid is comparatively less than in the fully liquid condition. Therefore, the probability of particle interactions (
e.g., agglomeration) can be expected to be higher and gradually increase with decreased liquid volume. Consequently, further studies are required to understand the effect of stirring on the refinement efficiency of nucleant substrates.
After the Rheometal™ slurry preparation process is complete, the mixture of
α1-Al crystals and solute enriched liquid obtained is poured into the shot sleeve. During pouring of the mixture into the shot sleeve, the formation of new
α-Al crystals is restricted to the thermal undercooled regions near the shot sleeve wall and plunger.[
22] The addition of grain refiner to HPDC titanium free A356 alloy resulted in larger fraction of crystals solidified in the shot sleeve and finer grain size compared to the unrefined alloy.[
22] However, there is significant differences between HPDC and SSM casting that are important to analyze. First, in HPDC a superheated liquid is poured in the shot sleeve while in SSM casting a slurry is poured. As a result, the latent heat released by the growth of the slurry
α1-Al crystals in the shot sleeve is likely more significant than that released by the growth of the lower fraction of crystals formed in the shot sleeve in HPDC. Therefore, the nucleation of new crystals between the pre-existing crystals during slurry holding in the shot sleeve may be inhibited for SSM castings. Secondly, the silicon content of the existing liquid of the slurry poured into the shot sleeve is larger in SSM casting compared to HPDC of the same alloy. Consequently, the poisoning effect of silicon of the TiB
2 particles dispersed in the liquid can be increased and reduce the number of active substrates dispersed in the liquid of the slurry.
In the die-cavity, the solidification occurs at very large cooling rates and for this reason the addition of grain refiner is considered unnecessary to obtain a fine microstructure.[
22] Additionally, increased silicon content of the existing liquid of the slurry, as seen in Table
IV, can reduce the effect of TiB
2 substrates dispersed in the refined alloys. The growth of
α1-Al crystals in the die-cavity was smaller for the refined alloys compared to the unrefined alloy, as shown in Figure
4(b), particularly for the alloy refined with Al-5Ti-1B. The lesser growth and solid fraction obtained for the alloy refined with Al-5Ti-1B can result in larger fraction of in-cavity solidified crystals compared to the other alloys and consequently, finer microstructure obtained, as shown in Figure
5(c). The potent substrates dispersed in the liquid in the grain refined alloys can assist the nucleation of in-cavity solidified crystals regardless of the poisoning effect of silicon enriched liquid.