Effect of Melt Temperature on the Effectiveness of the Grain Refining in Al-Si Castings

,e results inferred from the present work show that Al3Ti phase has a strong affinity to react with silicon (Si) in the molten alloy leading to formation of (Al,Si)3Ti phase instead. ,is reaction is independent of the grain refiner type. ,e molten liquid temperature would change its morphology from platelets at 750°C into dendritic structure at 950°C. It has also been observed that (Al,Si)3Ti phase platelets precipitate within the α-aluminum dendrites, whereas TiB2 or AlB2 particles are released into the surrounding interdendritic regions. Introduction of the grain refiner, regardless its type, would cause change in the α-aluminum dendrite morphology from an elongated to amore rounded form.,e results also reveal that addition of 100 ppmBwill reduce the initial grain size by ∼85% which is more than the effect of addition of 0.2%Ti in the form of Al-10%Ti (about 65%). Elimination of undercooling is important to obtain the maximum grain refining effect.


Introduction
Many theories have been proposed to explain the mechanism of grain refining.Two among them are the nucleation and phase diagram theories.e nucleation theory is mainly based on the presence of borides in the added grain refiner master alloy (e.g., TiB 2 , AlB 2 , and (Al,Ti)B 2 ).e phase diagram theory (Figure 1) is based on the presence of pro-peritectic TiAl 3 phase particles [1][2][3].When a master alloy containing Al-Ti-B is added to molten metal such that the amount of Ti is less than 0.15%, often boron particles are found at the centers of the grains, with dendrites enriched in titanium growing outside of the grains.In the case of the use of master alloys, AlB particles have been observed to be released towards the grain boundaries, whereas Al 3 Ti particles are located at the centers of the grains [4].Easton and StJohn [5] have confirmed this observation when the master alloy used does not contain Ti.In addition, release of aluminum borides towards the grain boundaries has no significant effect on the grain refining process.
From the thermodynamics point of view, the solubility of Al 3 Ti is independent of the presence of boron (B).
e experimental data obtained by Mondolfo et al. [7], however, show that the presence of B would displace the peritectic reaction towards the Al-rich extremity of the Al-Ti phase diagram, leading to the formation of TiB 2 .
Sigworth [8] and Bäckerud et al. [9] suggested that Al 3 Ti particles are more effective in terms of grain nucleation than TiB 2 phase particles.e authors proposed a hypothesis to explain why the presence of borides would slow down the dissolution rate of Al 3 Ti when the Al-Ti-B master alloy is added to liquid aluminum.According to [8,9], borides would form layers around the aluminide particles resulting in slowing their effectiveness.
Davies et al. [10] and Maxwell and Hellawell [11] observed the presence of Al 3 Ti particles within the α-Al dendrites.e solidification curves obtained by Arnberg et al. [12] revealed the absence of undercooling indicating that the nucleation of grains took place by the peritectic reaction at temperatures higher than the melting temperature of aluminum.Crossley and Mondolfo [13] suggested that the reported improvement of Al-Ti-B master alloy as a grain refiner may be interpreted in terms of the displacement of the peritectic reaction of 0.15% Ti towards the extremity of the aluminum diagram leading to thermodynamically stable Al 3 Ti nucleating sites at low addition, for example, 0.02%. is proposition, however, is incompatible with the Al-Ti-B phase diagram [14,15].
Mohanty et al. [16][17][18][19] injected synthetic TiB 2 particles (about 5 µm in diameter) into liquid aluminum along with di erent amounts of Ti.In the absence of excess of Ti, the e ectiveness of TiB 2 particles for grain re ning was poor, and TiB 2 particles were observed at the grain boundaries.Increasing the concentration of Ti resulted in improving the grain re ning e ectiveness of the borides, and the boride particles were observed at the center of the solidi ed grains.At Ti concentration higher than the peritectic concentration, layers of Al 3 Ti were formed on the surfaces of the TiB 2 particles where layers of α-Al were precipitated.
Jones and Pearson [20] have proposed the concept of hypernucleation at the TiB 2 /liquid metal interface.According to the authors, the addition of excess of Ti causes segregation of Ti atoms at the TiB 2 /liquid metal interface leading to the formation of a stable layer on the surface of the TiB 2 particles.
ese layers were proven to stay stable at temperatures below the fusion temperature of aluminum.e work of Schumacher and Greer [21,22] and Kearns et al. [23] shows that AlB 2 particles would be considered as good nucleates in the absence of Ti.Master alloys of the type Al-B are largely used in the production of ultrapure aluminum to react with transition elements such as V, Cr, and Zr [24,25].
Boron is not considered as an e ective grain re ner when added to pure aluminum [26].Once Si is added to Al, grain re ning is activated together with a change in the α-Al dendritic structure [27].Two types of B compounds exist in the Al-B master alloys: AlB 2 and AlB 12 [28].e AlB 2 compound is stable at room temperature and contains 44.5%B [29].It is inferred from the Al-B binary diagram presented in Figure 1(b) that there is a peritectic reaction at 975 °C: L + AlB 12 → AlB 2 .A eutectic reaction (liquid → α-Al + AlB 2 ) takes place at 660 °C making the maximum solubility of B in Al about 20 ppm, where the α-Al grains would precipitate on the AlB 2 particles.e melting points of AlB 2 and AlB 12 are 1665 ± 50 °C and 2163 ± 50 °C, respectively, resulting in the formation of solid dispersoid particles in the molten liquid [6].Fundamentally speaking, the addition of a grain re ner to the molten metal would result in the nucleation of new grains or its reaction with other elements in the molten metal to form nucleation sites.
e present study was undertaken to investigate the e ect of the following: (1) Type of the applied grain re ner, that is, Al-5%Ti-1% B and Al-4%B in comparison with Al-10%Ti.e latter forms a complex intermetallic with Si, which slows down its e ectiveness as a grain re ner.(2) E ect of superheating (750 °C or 950 °C) prior to casting.

Experimental Procedure
e composition of the A356 alloy used in this study is shown in Table 1.
Samplings from the di erent melts prepared were poured into a preheated (600 °C) cylindrical graphite mold to achieve near-equilibrium solidi cation conditions  e grain re ner was introduced in the form of Al-10%Ti, Al-5%Ti-1%B, or Al-4% B cut rods.Prior to pouring, the molten metal was mechanically stirred to prevent sedimentation of the added grain re ner at the bottom of the crucible.Prior to casting, the melt surface was skimmed to remove oxide lms.e temperature-time data were obtained using a K-type (chromel-alumel) thermocouple inserted through a hole at the bottom of the graphite mold along the centerline, its tip reaching up to one-third the height of the mold from the bottom.
e cooling curves were recorded using a data acquisition system attached to the thermocouple (Figure 1(c)).Chemical analysis of all melts/castings was determined using a Spectrolab Jr CCD Spark Analyzer (average of three burns per sample).
e solidi ed castings were sectioned perpendicular to the centerline axis of the cylinder, at the level of the thermocouple tip, and polished for metallographic examination, using standard polishing procedures.e polished samples were examined using an optical microscope and an electron probe microanalyzer (JEOL JXA-8900L operating at 20 kV), equipped with energy dispersive X-ray spectroscopic (EDS) and wavelength-dispersive spectroscopic (WDS) facilities.
For measurements of the grain size, the polished samples were chemically etched using Keller's reagent (66 vol.%HNO 3 , 33 vol.%HCl, and 1 vol.%HF), diluted with water using a plastic bottle with a ne nozzle to slow down the etching process.Once etching was completed, the sample surfaces were rinsed, dried, and then photographed using a setup consisting of four projectors emitting red, blue, green, and yellow light.e grain sizes were measured from the digital photographs using Sigma Scan Pro 4.0 software, employing a method similar to the line intercept method.

Results and Discussion
Figure 2 shows the size and distribution of Al 3 Ti, TiB 2 , and AlB 2 in the master alloys used in the present investigation.
e average Al 3 Ti particle length varies between 10 and 50 µm, whereas the average length of TiB 2 is less than 1 µm.In order to arrive at a clearer understanding of the grain re ning mechanism when Al-10%Ti master alloy is used, di erent concentrations of Ti, varying between 0.1 and 0.6 wt.%, were added.e molten metal was poured at 750 °C or 950 °C (depending on the amount of added Ti).In each case, the liquid metal was held for 30, 60, and 90 minutes prior to pouring.During the holding time, the liquid metal was continuously stirred using a graphite impeller.Figure 3 exhibits the variation in the nucleation temperature as a function of the added Ti content and melting conditions.

Advances in Materials Science and Engineering
Based on this gure, the following observations may be made: (1) e nucleation temperature is strongly in uenced by the Ti concentration regardless the melting conditions.At 750 °C, 0.3%Ti (broken line in Figure 3) results in depressing the nucleation temperature regardless the holding time.(2) At 950 °C, adding 0.1%Ti produces the same e ect as that observed for 0.3%Ti at 750 °C.In this case, increasing the Ti content results in a continuous decrease in the nucleation temperature.(3) Holding time seems to have no signi cant e ect on the nucleation temperature.In contrast, the melting temperature separates the curves into two distinct series.
e variation in the alloy grain size as a function of Ti and melting conditions is illustrated in Figure 4. e initial average grain size of the as-received alloy was about 1854 ± 20 µm when the molten metal was poured at 750 °C.Although addition of 0.15-0.2%Tiproved to be su cient to reduce the grain size to approximately 600 ± 10 µm (∼70%), there was a tendency for slight increase in the grain size at the holding time of 90 min.is increase may be interpreted in terms of either particle sedimentation or loss in the e ectiveness of grain re ning.
ese results are in good agreement with those reported by other researchers [30][31][32].As mentioned previously [1,2], the Al 3 Ti particles have the tendency to react with Si from the surrounding matrix to form a complex compound, mainly (Al,Si) 3 Ti which contains 10%Si, as shown in Figure 5 and Table 2.    Advances in Materials Science and Engineering e morphology of Al 3 Ti depends on the pouring temperature: platelets (750 °C; Figure 5) or dendritic (950 °C; Figure 6).In all cases, however, the (Al,Si) 3 Ti/TiB 2 phase particles are seen to be localized within the α-Al dendrites (Figure 7) suggesting that these particles are acting as nucleation sites.Advances in Materials Science and Engineering in the interdendritic regions.It is inferred from Figure 9 that, in contrast to Al 3 Ti addition, introduction of TiB 2 particles leads to a continuous increase in the alloy nucleation temperature with the increase in the added amount of Ti. us, the increase in the concentration of TiB 2 particles would result in the nucleation of new equiaxed grains at rates higher than their growth rates, causing grain re ning at a lower level of Ti (0.1%) compared to that obtained from the addition of Al-10%Ti master alloy (Figure 4).In general, the effectiveness of a nucleating agent is determined by several factors, for example, its chemical stability, density, size, and the amount of constitutional undercooling which is a function of the alloy chemical composition.Kori et al. [33] and Nafisi and Ghomashchi [34] studied the effectiveness of Al-4%B master alloy as a grain refiner of A356 alloy.e results show that a small addition of boron shifts up the cooling curve causing the recalescence to disappear entirely.e dendritic structure transforms from a columnar to equiaxed morphology.For master alloys containing mainly AlB 12 particles, it is proposed that the dissolved boron, the AlB 2 particles present in the master alloy, and some transformation of AlB 12 to AlB 2 are responsible for the enhanced refining.
In the present study, B was added to the A356.2 alloy melts in different concentrations, as shown in Figures 10 and  11.As may be observed, the nucleation temperature increases relatively slowly compared to that seen in the case when Al-5%Ti-1%B master alloy was used. is observation is more clearly noted when the molten metal was poured at 950 °C.Evidence of reduction in undercooling with the increase in B concentration is observed in Figure 11.From the thermal point of view, an ideal grain refiner would remove the recalescence entirely [34][35][36].
Figure 12 demonstrates the variation in the alloy grain size as a function of the added B and pouring temperature.It is evident that addition of 200 ppm B to the melt at 750 °C is sufficient to produce finer grains (∼250 µm) than those obtained from using the Al-10%Ti master alloy (∼850 µm) or the as-received alloy (∼1854 µm) poured at 750 °C, as presented in Figure 4.In this case, Figure 13    pure aluminum as con rmed by the associated WDS analysis (Table 3), whereas Figure 14 shows the distribution of B and Si phases in Al-7%Si alloy.As can be seen, both elements precipitate away from each other (marked zones A and B) in Figures 14 and 15. is observation is con rmed by the X-ray elemental distribution for each zone (Figure 16), indicating that there is no a nity for the reaction between these two elements.In contrast, when Ti was added with B (as Al-5%Ti-1%B master alloy) to the 1050 commercial aluminum, a clear interaction between Ti and B took place, leading to the formation of TiB 2 .us, it could be concluded that B has no grain re ning capacity when added to ultrapure aluminum, which is not the case when the metal contains traces of Ti, leading to the possible formation of di erent nucleation sites that coexist, such as AlB 12 , AlB 2 , and TiB 2 in Al-Si alloys.Evolution of the grain size of A356.2 alloy using di erent grain re ners is exhibited in Figure 16, whereas Figure 17 displays the variation in the grain size of A356.2 alloy as a function of the added type of the grain re ner.

Conclusions
Based on the obtained experimental results reported in this study, the following conclusions may be made: (1) Addition of grain re ner master alloys in large amounts increases the nucleation temperature of the formation of α-aluminum and eliminates the undercooling.(2) e Al-4%B master alloy is more effective as a grain refiner compared to other types of master alloys.e presence of traces of Ti in the base A356.2 alloy would react with B leading to the formation of TiB 2 , which, in turn, would improve the effectiveness of the master alloy used.(3) Since B has no affinity to react with Si in the base alloy, AlB 2 particles are more effective as nucleation sites than the Al 3 Ti particles.(4) Addition of a grain refiner also changes the morphology of the α-Al dendrites from elongated to a rosette or more rounded form.(5) Maintaining the liquid metal for a long period prior to pouring may lead to agglomeration of the intermetallic particles which would reduce the refining effectiveness of the master alloy used.(6) Addition of 0.2 wt.% Ti is sufficient to obtain good grain refining.Increasing the Ti content beyond that would lead to more precipitation of intermetallics with no further improvement in grain refining.

2
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Figure 7 :
Figure 7: Morphology of α-Al as a function of the added grain re ner: (a) no melt treatment, (b) localization of (Al,Si) 3 Ti phase particles within the α-Al dendrites in the A356.2 alloy treated with 0.3%Ti and poured at 750 °C, and (c) 0.1%Ti, 200 ppm B. Note the rosette morphology of the α-Al dendrites (open arrows).(d) 0.1%B.

Figure 13 :
Figure 13: (a) Presence of AlB 12 in ultrapure aluminum (possible decomposition of AlB 12 to AlB 2 ) and (b) X-ray image of B distribution in AlB 12 phase particles in (a) (arrow).

Figure 14 :Figure 15 :
Figure 14: Presence of B and Si in Al-7%Si alloy treated with 800 ppm B. Note the geometric shape of the particles in Zone A.

Table 2 :
Identi cation of the phase rich in Ti.