Materials and Design

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1 Materials and Design 31 (2010) S36 S43 Contents lists available at ScienceDirect Materials and Design journal homepage: Grain refinement of AA333 aluminium cast alloy by Al Ti granulated flux Bondan T. Sofyan a, *, Daniel J. Kharistal a, Lukfawan Trijati a, Kaspar Purba a, Ragil E. Susanto b a Department of Metallurgy and Materials Engineering, Faculty of Engineering, University of Indonesia, Kampus UI Depok 16424, Indonesia b PT. Astra Honda Motor, Jl. Laksda Yos Sudarso, Sunter I, Jakarta 14350, Indonesia article info abstract Article history: Received 25 August 2009 Accepted 4 February 2010 Available online 13 February 2010 Keywords: AA333 aluminium alloy Low Pressure Die Casting Dendrite arm spacing Growth restriction factor Addition of grain refiner is an option to obtain higher mechanical properties of aluminium cast alloy. Grain refiner will react with molten aluminium and will form nucleant particles that initiate solidification. Therefore, the grain refiner will also be useful to control the solidification processes to reduce shrinkage formation. This study evaluated grain refinement in AA333 aluminium alloys by using Al Ti granulated flux. Experiments were conducted by adding and wt.% Ti granulated flux to the molten aluminium during rotary gas bubble floatation and it was subsequently transferred into the Low Pressure Die Casting (LPDC) holding furnace. The pressure in the LPDC holding furnace was initially 256 kpa and no stirring was applied. The melt was injected to cylinder head moulds with cycle time of 180 s. Samples were cut at the thin and thick sections of the cylinder head to analyze the effects of cooling rate on grain refinement. Fading of grain refiner was studied for the period of 4 h. Hardness testing and microstructural observation were conducted. The results showed that addition of Al Ti granulated flux for and wt.% Ti, increased the hardness and lowered the dendrite arm spacing (DAS). The refinement is more significant in thin samples, because of the assistance by higher cooling rate. The mechanism of grain refinement of Al Ti granulated flux in AA333 alloy is a combination of high growth restriction factor by the large amount of solute and nucleation by Al 3 Ti particles. Fading of grain refiner was detected by the increase in DAS, the lowering of Ti concentration in the melt and the reduction in hardness and strength. The pressure utilized in LPDC was not adequate to give stirring effect to prevent fading. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Al Si alloys are the most important Al-based foundry alloys because of their excellent casting characteristics, weldability and corrosion resistance. AA333 is one of the Al Si alloys that possesses wide applications in the automobile, marine, electrical, and aircraft industries. Cylinder heads, crankcase and other engine body are examples of parts made of this alloy. The AA333 alloy belongs to hypo-eutectic Al Si alloys whose microstructure consists of a large fraction of dendritic primary a-al and secondary phases distributed in the interdendritic area. Silicon eutectic is the first secondary phase to form during eutectic reaction. Copper forms another secondary phase with Al that precipitates during solidification either as block-like Al 2 Cu or in eutectic form as (Al + Al 2 Cu) [1]. Another critical alloying element for AA333 alloy is iron (Fe). During solidification, it forms several intermetallic compounds, such as b- Al 5 FeSi phase, which is hard, brittle and particularly deleterious to the alloy mechanical properties [2]. This intermetallic phase also acts as nucleants for the Al 2 Cu phase [3] and is also found to be * Corresponding author. Tel.: /05; fax: address: bondan@eng.ui.ac.id (B.T. Sofyan). responsible for the occurrence of soldering of aluminium melts in die casting processes [4]. The mechanical properties of most cast alloys strongly depend on dendrite arm spacing (DAS). The tensile strength, ductility and elongation increase as DAS refines. A small DAS also reduces the time required for homogenization heat treatments since the diffusion distances are shorter. Further refinement may also lead to fine equiaxed structure, which yields superior mechanical properties, good castability, improves feeding during solidification, reduces and distributes shrinkage porosity, improves surface finish and provides better dispersion of secondary phases, and other desired properties [5,6]. The most popular grain refiner for hypo-eutectic Al Si alloys is Al 5Ti lb master alloy [5 11]. For a considerable amount of time it was assumed that Al 3 Ti particles must be the heterogeneous nucleation sites as they are a pro-peritectic phase, which it was assumed could nucleate the solid through a peritectic reaction and furthermore had very good lattice matching with Al. When Al B is the grain refiner, they proposed that the AlB 2 phase acts as the nucleant. Mohanty and Gruzleski [9] have recently proposed an alternative mechanism for the grain refinement of Al Si alloys with Al B master alloys. They suggested that the nucleation of Al in hypo-eu /$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.matdes

2 B.T. Sofyan et al. / Materials and Design 31 (2010) S36 S43 S37 tectic Al Si alloys occurs on the pre-existing a-al, which forms by a eutectic reaction from the B-containing Al melt. Zhang et al. [11] concluded that Al 3 Ti (I4/mmm, a = nm, c = nm) is a more powerful nucleating substrate for Al alloy than TiC, TiB 2 and AlB 2 due to better edge-to-edge matching in both close packed directions and close packed planes of the primary a-al phase (m3m, a = nm). On the other hand, some other literature [12,13] agreed on the concept that solute elements segregate and restrict the growth of the solid liquid interface of new grains and therefore cause grain refinement. The term growth restriction factor (GRF) is used to quantify the segregating behaviour of elements upon solidification. It is defined as mc 0 (k 1), where m is the gradient of the liquidus, usually approximated to a straight line, c 0 is the concentration of the solute in the alloy, and k is the partition coefficient between the equilibrium concentrations of the solid and the liquid at the growing interface. When a number of solutes are present in a melt, the GRFs are added, assuming that there is no interaction between the solute phases. Titanium has been found to be a solute with powerful segregating ability [14] at the melt interface and affects the growth of dendrites and also the constitutional undercooling at the solid liquid interface. Most research on grain refinement used Al 5Ti 1B or other master alloys. Other grain refiner available is the Al Ti granulated flux, which is more economical but the effectiveness has not been much studied. Addition of a mixture of KBF 4 K 2 TiF 6 granulated flux to molten aluminium is the most popular way to produce Al Ti B a master alloy [15]. Birol [16] reported that the use of Na 2 B 4 O 7 flux is an effective source of boron to replace KBF 4. However, there is no report whether the granulated flux itself is able to grain refine aluminium cast alloys. This paper reports the grain refining effect and fading characteristics of Al Ti granulated flux in AA333 alloy in Low Pressure Die Casting (LPDC) process. 2. Experimental method Titanium refined alloys were cast by using commercial AA333 as the base alloy. The charge materials consisted of 70% of ingot and 30% of return scrap. These alloys were melted in an industrial reverberatory furnace of 1500 kg capacity at 810 ± 10 C for 3 h. Cover flux of Coveral 111 Ò was added to the molten metal to remove impurities and to form slag at the surface of the melt. The slag was then manually desludged and the melt was removed into a preheated bentone ladle of 500 kg capacity. Grain refiner of Al Ti granulated flux (Coveral GR2815 Ò ) was manually added into the melt before gas bubble floatation process by using argon with the debit of 8 12 l/min. The gas bubble floatation process was conducted for 8 min at 780 ± 5 C with the rotor speed of rpm to degas hydrogen from the melt as well as to stir the granulated grain refiner flux. The SEM micrograph of the Al Ti granulated flux including the microanalysis is provided in Fig. 1. The melt was subsequently transferred to the Low Pressure Die Casting (LPDC) holding furnace (Tounetsu Co., Ltd., Japan) of 500 kg b Element Concentration (wt. %) F Na 0.55 Cl 1.34 K Ti Al Fig. 1. (a) SEM micrograph of Al Ti granulated flux and (b) its microanalysis. Fig. 2. Dimension of LPDC holding furnace.

3 S38 B.T. Sofyan et al. / Materials and Design 31 (2010) S36 S43 Table 1 Nominal composition (wt.%) of alloys in this study. Alloys code Si Cu Mg Fe Mn Ti Zn Sn Ni Pb Cr Al A (AA333) rem B rem C rem a Thin area Thick area Hardness (BHN) Fig. 3. Cylinder head samples showing thick and thin areas, where samples for hardness and microstructural observation were taken capacity, that was kept at 705 ± 5 C. Fig. 2 illustrates the dimension of the holding furnace, including the position of the melt supply door and the down tube. Remaining slag was manually skimmed and samples for compositional analysis were taken by a steel plunger through the supply door for optical spectroscopic measurement. The nominal composition of alloys used in this study is presented in Table 1. As shown in Table 1, the initial AA333 alloy (alloy A) had wt.% Ti, while alloys B and C contained and wt.% Ti, respectively. Therefore, the addition of Ti was and wt.% in alloys B and C. Molten metal was also taken and poured into a dog-bone shape mould in accordance with JIS Z2201, test piece no. 4 for tensile testing. Dies of the LPDC were preheated at 375 C to avoid thermal shock, and the shape of the cast was cylinder head as shown in Fig. 3. Injection temperature was kept at 705 ± 5 C and two cylinder heads were produced in a 180 s cycle time. During injection, the holding furnace was kept closed to avoid air penetration. The pressure in the holding furnace was initially 256 kpa and gradually increased by the diminishing melt volume. No stirring was conducted on the melt. The injection process was continued for 4 h until the remaining melt in the holding furnace was 250 kg. Fading phenomenon was followed by examining the hardness and microstructures of cylinder head samples produced at 1 h intervals up to 4 h. After 4 h, melt was taken and poured for compositional analysis and tensile testing. Samples were taken at thick and thin areas of the cylinder head, representing area with low and high cooling rate, respectively (Fig. 3). Samples were cut into mm blocks for microstructural and microanalysis. Hardness measurements were performed by Brinell method, using steel ball indenter of 3.15 mm diameter and kg of load. Seven indentations were taken for each hardness measurement. Modification of microstructure was observed by means of a light microscope and LEO 420 SEM/EDXS. Samples for microstructural analysis were prepared by Tucker etchant (45 ml HCl + 15 ml HNO ml HF (48%) + 25 ml H 2 O). 3. Results and discussion 3.1. Effects of Ti granulated flux addition Effect of Ti granulated flux addition on hardness and tensile strength of AA333 alloys is presented in Fig. 4. The addition of b Tensile strength (MPa) Ti addition (wt. %) Ti addition (wt. %) Fig. 4. Effect of Ti addition on (a) hardness and (b) tensile strength of AA333 through LPDC process and wt.% Ti increased hardness of 2.8% and 7.17%, respectively in thick area, in comparison with that of the initial alloy. This is linear with the significant increase in tensile strength, of 28.7% and 33.4%. The increase in hardness and strength was due to refinement of dendrite arm spacing (DAS), as can be seen in Figs. 5 and 6. Fig. 5 shows microstructures of thick and thin areas in alloys A (Fig. 5a and b), B (Fig. 5c and d), and C (Fig. 5e and f). Dendritic structures were found dominant, because of high solute content of the alloys, including significant amount of Si of 9 wt.%. The quantitative measurement of DAS is provided in Fig. 6, in which it is clear that the DAS was in the range of lm. The figure also reveals that greater addition of Ti decreased the dendrite arm spacing (DAS). Lee et al. [17] reported that increasing Si content of Al Si hypo-eutectic alloy led to well developed dendritic structures, although they did not measure the DAS. The reduction of DAS found in this work was in agreement with previous study [5 8,10], however the size was significantly lower. Most studies reported the grain size, not the DAS, such as the study by Jaradeh and Carlberg [5], which reported grain size of 160 lm by addition of wt.% Ti, while grain size of lm was obtained by alloys with wt.% Ti in other study [6]. Greer et al. [7]

4 B.T. Sofyan et al. / Materials and Design 31 (2010) S36 S43 S39 Fig. 5. Microstructure of AA333 alloys on thick and thin areas with addition of (a and b) 0 wt.% Ti, (c and d) wt.% Ti, and (e and f) wt.% Ti. 35 Dendrite arm spacing (µm) Thick area Thin area Ti addition (wt. %) Fig. 6. Effect of Ti addition on DAS of thick and thin areas. added 0.43 wt.% Ti into pure aluminium and resulted in grain size of 310 lm. Much larger grain size was described by Kashyap and Chandrashekar [8], that was 1900 lm by a very small addition of Ti of wt.%. While smaller grain size of lm was stated by Kori et al. [10] with addition of wt.% Ti in Al 7Si. Reports from Bonollo et al. [18] and Zhang et al. [19] showed that their LPDC products by using similar alloy produced DAS of lm, which is quite comparable with the DAS in this study, but they did not use Ti grain refiner. The small DAS observed in this research was also caused by the complex alloy system which contributed to higher growth restricting factor (GRF). The alloys consisted of at least 11 solute elements. Calculation by using the mc 0 (k 1) values compiled by Easton and St. John [13] showed that the GRF of alloys A, B and C were 66, 85 and 91, respectively. These values were significantly higher than those reported before [8,13,20]. Aside from that, the Low Pressure Die Casting process also report contributes to higher cooling rate [18]. Fig. 7. SEM micrographs of AA333 alloys with addition of (a) wt.% Ti (alloy B) and (b) wt.% Ti (alloy C). Points of EDX microanalysis are labeled, and the results are available in Tables 2 and 3. In alloy B, the DAS decreased by 46.3% and 20.2% in thin and thick areas, respectively, whereas further addition of Ti in alloy C reduced DAS by 7.5% and 7.9% for thin and thick areas, respectively. Finer DAS found in thin area was due to higher cooling rate in the mould, although the rate was not quantitatively measured. Be-

5 S40 B.T. Sofyan et al. / Materials and Design 31 (2010) S36 S43 Table 2 EDX microanalysis of alloy B (with wt.% Ti), at points shown in Fig. 7a. No. Composition (wt.%) Colour Phase Si Cu Fe Mn Ti Al Light grey Al 15 (Fe,Mn) 3 Si 2 [4] Dark grey Al Si eutectic [1] White Al 2 Cu [4] Grey a-al White Al 2 Cu with Ti segregation Table 3 EDX microanalysis of alloy C (with wt.% Ti), at points shown in Fig. 7b. No. Composition (wt.%) Colour Phase Si Cu Fe Mn Ti Al Light grey Al 15 (Fe,Mn) 3 Si 2 [4] Dark grey Al Si eutectic [1] White Al 2 Cu [4] Grey a-al Table 4 Nominal composition (wt.%) of alloys after 4 h in LPDC holding furnace. Alloys code Si Cu Mg Fe Mn Ti Zn Sn Ni Pb Cr Al A rem B rem C rem Tensile strength (MPa) hour 4 hours Ti addition (wt. % ) Hardness (BHN) Thin area (hardness) Thick area (hardness) Thick area (DAS) Thin area (DAS) Holding time (h) Fig. 9. Change of hardness and DAS of alloy C (added with wt.% Ti) in 4 h- period Dendrite arm spacing (µm) Fig. 8. Change of tensile strength of alloys A (0 wt.% Ti), B (added with wt.% Ti) and C (added with wt.% Ti) after 4 h. cause of higher cooling rate, the refinement occurred in thin area was more significant of 50.4%, compared to 26.6% in thick area. The combination of high cooling rate and Ti refinement led to significant DAS reduction. Fig. 7a and b shows SEM micrographs of Ti-containing alloys, and results of microanalysis are provided in Tables 2 and 3. Secondary phases are spreading in between a-al dendrites. The needle-like light grey phase was Al 15 (FeMn) 3 Si 2 (position 1), as reported by Dash and Makhlouf [4], and this phase was mainly responsible for the brittle characteristics of the alloys. No chinese-script Fe-rich phase was detected. The eutectic Al Si (position 2) took the form of acicular and flaky [1], and they were well distributed in the matrix. White Al 2 Cu phase (position 3) was found in irregular or rounded form and many was observed attached to needle Al 15 (Fe,Mn) 3 Si 2 confirming previous study [4]. It is interesting to note that titanium was hardly revealed in the alloys. Only a small amount of Ti was detected near Al 2 Cu phase (position 5), and this could not be concluded as Al 3 Ti, which was believed to be the nucleant of the solidification process [8,21]. This result may support the theory of solute segregation in restricting growth of new grain [12,13], instead of the nucleant theory. The titanium granulated flux grain refiner used in this study might be easy to dissolve in aluminium matrix, and did not form Al 3 Ti nucleant particles Fading of Ti granulated flux grain refiner To observe the fading phenomenon, melt was taken from the holding furnace before the first injection and after 4 h of injection, and afterwards poured for compositional analysis and tensile test-

6 B.T. Sofyan et al. / Materials and Design 31 (2010) S36 S43 S41 Fig. 10. Microstructures of alloy C (with wt.% addition of Ti) in thick and thin areas, after (a and b) 0 h, (c and d) 1 h, (e and f) 2 h, (g and h) 3 h, and (i and j) 4 h. ing samples. Composition of alloys after 4 h in holding furnace is presented in Table 4, and at this stage, alloys A, B and C contained 0.020, 0.047, and Alloy C underwent a significant decrease in Ti concentration, and its final Ti concentration was even lower than that of alloy B. Results of tensile testing are provided in Fig. 8. No change in strength was detected in alloy A after 4 h, while alloys B and C had a decrease of 11.6% and 1.7%, respectively. The reduction in strength in each alloy was not comparable with the de-

7 S42 B.T. Sofyan et al. / Materials and Design 31 (2010) S36 S43 Fig. 11. SEM micrograph of alloy C (with wt.% Ti) after 4 h. Table 5 EDX microanalysis of alloy C (with wt.% Ti) after 4 h, at points shown in Fig. 11. No. Composition (wt.%) Colour Phase Si Cu Fe Mn Ti Al Light grey Al 15 (Fe,Mn) 3 Si 2 [4] Dark grey Al Si eutectic [1] White Al 2 Cu [4] Grey a-al crease in Ti concentration. Further examination on the fracture surface of tensile test samples revealed the presence of impurities that initiated breakage. This might contribute error to the results of tensile test. The overall decrease in Ti concentration and strength in 4 h indicated fading that might be due to settlement of Al 3 Ti nucleant particles. To confirm this, the trend in hardness and DAS of the cylinder head samples of alloy C, is shown in Fig. 9. Alloy B showed similar trend, therefore it is not presented here. The DAS was measured from the microstructures as seen in Fig. 10. In general, thin area possessed higher hardness than the thick area, due to faster cooling rate. It was supported by finer DAS as shown in Fig. 10. The hardness decreased throughout the period of 4 h, while the DAS raised. Some fluctuation of DAS was noted in thick area, however, it finally increased after 4 h. A SEM micrograph was taken from alloy C after 4 h, and is displayed in Fig. 11. The microanalysis is presented in Table 5. It was confirmed that the types of secondary phases stayed the same. The Al 15 (FeMn) 3 Si 2 phase (position 1) remained present with slightly larger size. The Al Si eutectic (position 2) and white Al 2 Cu phase (position 3) were also obvious. However, it is worth noting that no titanium was detected in the microstructures. Settling of Al 3 Ti and TiB 2 particles has been described as the mechanism predominantly responsible for the fade when melts are held for longer time [22,23]. This mechanism seems to operate in the current work, although no such particles were found. The construction of the holding furnace (Fig. 2) should be considered. The position where the melt was taken, was 15 cm above the bottom. Therefore, at this position not many nucleant particles were expected because most of them would have settled down. The down tube was also positioned 15 cm above the bottom of the furnace in area with larger melt volume. Therefore, the melt, which flew through the down tube, might have less Ti concentration by extended period of time. Therefore, after 4 h, no Ti was detected in the cylinder head samples that led to coarsening of DAS. The results also suggest that the pressure and mechanism of melt flow in LPDC process, do not give sufficient stirring effects to prevent fading. Poisoning effect from high Si concentration in the alloy may also occur that leads to ineffectiveness of Ti as grain refiner [24]. The fading mechanism is mostly described for Ti master alloy grain refiner [22 24]. Similar mechanism that operates in this work suggests that the Al Ti granulated flux behaves alike. The salt contained in the granulated flux (Fig. 1) might burn during the process, leaving the Ti in the melt, and producing slag. Some Ti may dissolve in the matrix and some may react with Al to form Al 3 Ti, while the slag floats at the surface of the melt and then manually desludged before the injection process was started. No indication that Ti went further into the slag during the process, since the melt was almost free from slag after 4 h. However, Fig. 7 only reveals the segregation of Ti near Al 2 Cu phase, not the nucleant particles, which suggests that growth restriction factor by solute elements also works. Therefore, it seems that combination of grain refinement by nucleant particles and solute growth restriction factor work in the system of AA333 alloy with LPDC process. Further studies required to confirm the presence of Al 3 Ti at the bottom of the furnace. 4. Conclusions The major conclusions and suggestions drawn from the results of this work are as follows: 1. The addition of and wt.% Ti granulated flux grain refiner in LPDC, increases the hardness of AA333 aluminium alloy by refining the DAS. 2. The DAS reduction in thin area was much more significant because of combination of higher cooling rate and Ti refinement. 3. The grain refinement mechanism operated by Al Ti granulated flux in AA333 alloy was the combination of nucleation by Al 3 Ti particles and high growth restriction factor due to complex solute elements. The salt contained in the Al Ti granulated flux might burn during the process, leaving the Ti in the melt, and producing slag that floats on the surface of the melt. 4. The fading phenomenon of the Al Ti granulated flux grain refinement was confirmed. This indicates that the pressure in LPDC holding furnace does not give sufficient stirring effects to prevent fading. Acknowledgement This research is funded through the Competence Grant (Hibah Kompetensi) 2009 from the Directorate General of Higher Education, Ministry of National Education, Indonesia. References [1] Moustafa MA, Samuel FH, Dotty HW, Valtierra S. Effect of Mg and Cu on the microstructural characteristics and tensile properties of Sr-modified Al Si eutectic alloys. Int J Cast Metals Res 2002;14: [2] Couture A. Iron in aluminium casting alloys a literature survey. AFS Int Cast Metals J 1981;10:9 17. [3] Crepeau N. Effect of iron in Al Si casting alloys: a critical review. AFS Trans 1995;103: [4] Dash M, Makhlouf M. Effect of key alloying elements on the feeding characteristics of aluminum silicon casting alloys. J Light Metals 2001;1(4): [5] Jaradeh M, Carlberg T. Effect of titanium additions on the microstructure of DCcast aluminium alloys. Mater Sci Eng A 2005; : [6] Sritharan T, Li H. Influence of titanium to boron ratio on the ability to grain refine aluminium silicon alloys. J Mater Process Technol 1997;63: [7] Greer AL, Bunn AM, Tronche A, Evans PV, Bristow DJ. Modelling of inoculation of metallic melts: application to grain refinement of aluminium by Al Ti B. Acta Mater 2000;48(11): [8] Kashyap KT, Chandrashekar T. Effects and mechanisms of grain refinement in aluminium alloys. Bull Mater Sci 2001;24(4): [9] Mohanty PS, Gruzleski JE. Grain refinement mechanisms of hypoeutectic Al Si alloys. Acta Mater 1996;44(9):

8 B.T. Sofyan et al. / Materials and Design 31 (2010) S36 S43 S43 [10] Kori SA, Murty BS, Chakraborty M. Development of an efficient grain refiner for Al 7Si alloy and its modification with strontium. Mater Sci Eng A 2000;283(1): [11] Zhang M-X, Kelly PM, Easton MA, Taylor JA. Crystallographic study of grain refinement in aluminum alloys using the edge-to-edge matching model. Acta Mater 2005;53: [12] Spittle JA, Sadli S. Effect of alloy variables on grain refinement of binary aluminum-alloys with Al Ti B. Mater Sci Technol 1995;11: [13] Easton M, StJohn D. Grain refinement of aluminium alloys: Part I. The nucleant and solute paradigms a review of the literature. Metall Mater Trans A 1999;30A: [14] Li H, Sritharan T, Lam YM, Leng NY. Effects of processing parameters on the performance of Al grain refinement master alloys Al Ti and Al B in small ingots. J Mater Sci Technol 1997;29:10 8. [15] Davies P, Kellie JLF, Patron DP, Wood JV. Metal matrix alloys, US Patent No. 6228, vol. 185; [16] Birol Y. Production of Al Ti B grain refining master alloys from Na 2 B 4 O 7 and K 2 TiF 6. J Alloys Compd 2008;458: [17] Lee YC, Dahle AK, StJohn DH, Hutt JEC. The effect of grain refinement and silicon content on grain formation in hypoeutectic Al Si alloys. Mater Sci Eng A 1999;259: [18] Bonollo F, Urban J, Bonatto B, Botter M. Gravity and low pressure die casting of aluminium alloys: a technical and economical benchmark. La Metallurgia Italiana 2005;6: [19] Zhang B, Garro M, Tagliano C. Dendrite arm spacing in aluminium alloy cylinder heads produced by gravity semi-permanent mold. Mater Sci Technol 2003;21(1):3 9. [20] Easton M, StJohn D. Grain refinement of aluminium alloys: Part II. Confirmation of, and a mechanism for the solute paradigm. Metall Mater Trans A 1999;30A: [21] Qiu D, Taylor JA, Zhang M-X, Kelly PM. A mechanism for the poisoning effect of silicon on the grain refinement of Al Si alloys. Acta Mater 2007;55: [22] Limmaneevichitr C, Eidhed W. Fading mechanism of grain refinement of aluminium silicon alloy with Al Ti B grain refiners. Mater Sci Eng A 2003;A349: [23] Schaffer PL, Dahle AK. Settling behavior of different grain refiners in aluminium. Mater Sci Eng A 2005; : [24] Asenio-Lozano J, Suarez-Pena B. Effect of the addition of refiners and/or modifiers on the microstructure of die cast Al 12Si alloys. Scripta Mater 2006;54:943 7.