Influence of SiC Particles On The Grain Refinement of an Mg-Al Alloy

A. Schiffl1,a and M. A. Easton2

1 ARC Light metal competence centre Ranshofen, Post Box 26,

5282 Ranshofen, Austria 2 CAST Cooperative Research Centre, Department of Materials Engineering, Monash University,

Melbourne, Australia, Victoria 3800 a Andreas.Schiffl@arcs.ac.at

Keywords: particle, magnesium, grain refinement, microstructure, SiC

Abstract

SiC particles are effective grain refiners in Mg-Al alloys. Several investigations, from different researchers, into their effect on a range of alloys with different Al contents has been undertaken and it has been found that the greatest reduction in grain size occurs in alloys having low Al contents. Performing grain refinement studies on a range of alloy contents also allows for further investigation into the mechanisms of grain refinement. It was found that the size of the SiC particles is also important in magnesium grain refinement. However, the presence of Mg2Si in the microstructure and the consideration of phase equilibria suggest that SiC can transform into other binary or ternary carbides. If such carbides are formed, they may also act as an effective grain refiner for Mg-Al alloys. In this study, the possibility of formation of new carbides (Al4C3, Al2MgC2, Mn7C3, Mg2C, Mg2C3, Al2CO etc.) and their ability to be good grain refiners for Mg-Al alloys is discussed.

Introduction

Grain refinement is used in magnesium technology when the material needs to fulfill particular requirements. Such demands are for example better feeding, reduction of shrinkage, pore size or heat treatment time, improvement of formability, reduction of hot tearing susceptibility or improvement of the mechanical properties [1]. Fine-grained cast magnesium billets are characterised by improved mechanical strength and ductility. Nowadays, grain refinement plays an important role especially in continuous casting where an increase in billet dimensions is demanded to raise the capacity and thus reduce the price per finished product. A homogeneous microstructure and chemical composition across a thick-walled billet diameter can be achieved by fine grains [2]. In addition, the casting speed can be increased by the use of grain refiners without affecting the billet quality. In a subsequent forming process, such as extrusion or forging, the forming speed can be raised and the temperature decreased, thus also improving the efficiency of the process.

For grain refinement of magnesium two classes of alloys can be distinguished; Mg alloys containing Al and those without. The latter ones can be efficiently grain refined by the addition of Zr master alloys. However, at present the AZ series (Mg-Al alloys) are quantitatively the most used Mg alloys in industry because of the enhanced castability due to the presence of aluminium. There, grain refinement by Zr is not possible as Zr-containing intermetallic phases are formed thus poisoning the grain refining effect of Zr [1]. In the literature different methods are reported for grain refinement of Mg-Al alloys. Such processes include carbon inoculation, superheating of the melt, the Elfinal process, native grain refinement, ultrasonic treatment and shear enhanced nucleation [1, 3-5], but none of these methods are used in industry because of economic or ecological considerations, e.g. formation of polluting side products.

In our study, heterogeneous nucleation by the addition of SiC particles was investigated; it can be regarded as alternative to Zr because cheap and easy available ceramic particles can be used. If

Materials Science Forum Vols. 618-619 (2009) pp 445-448Online available since 2009/Apr/17 at www.scientific.net© (2009) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.618-619.445

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particles are brought into the melt in an simple and loss-free way, they can act as grain refiner in the Mg-Al melt. SiC particles are known to exhibit favorable properties [6, 7]. They exhibit a small lattice misfit (< 4%) and wetting of the Mg-Al melt and the SiC particles is sufficient after performing a simple mechanically forced wetting. So far the exact mechanism of grain refinement by SiC particles is not completely understood. There are assumptions which state that grain refinement is directly achieved by the SiC particles; in other studies the transformation of SiC to Al4C3 is expected [6-8]. However, it has been reported that effective grain refinement by SiC particles can only be achieved at low Al contents (< 6 %). We investigated the grain refining effect of SiC particles on a commercial AZ31 Mg alloy.

Materials and Methods

In this study a commercially available AZ31 Mg alloy (Mg, 3.26%Al, 0.72%Zn, 0.22%Mn, 0.0010%Fe, 0.0005%Be) was used. For each test, approximately 15 kg of melt was heated in a resistance furnace to a temperature of 700°C ± 5°C under a protective gas atmosphere consisting of argon 1 %SF6. Then, the SiC particle content was continuously increased from 0 to 2 wt-% to investigate the minimum amount of SiC particles necessary for effective grain refinement. Additionally, the influence of particle size was analysed; particles were of 1.4µm (denoted as SiC-1) and 0.5 µm (denoted as SiC-2) in average size, respectively. As a reference an Al4C3 grain refiner of approximately 45 µm in particle size was used. After each addition of grain refining particles the melt was intensively stirred for 2 min to achieve a homogeneous distribution of the grain refiner. After a holding time of 5 and 25 min, respectively, conical samples (dimensions: Øbottom 25mm, Øtop 35mm, 40mm in height) were cast. The samples were cut in a distance of 20mm from the bottom and metallographically examined. Grain size analysis was performed according to ASTM E112-96 after etching the samples with picric acid.

The remaining melt containing 2wt-% SiC was slowly solidified in the crucible. As the SiC particles and the possibly formed new phases have a higher density than the liquid Mg-Al melt, particle agglomerations and particles have settled. Thereby enough particle density was provided to be analysed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). SEM was performed on an EVO 40SEM with an acceleration voltage of 15kV and a working distance of 9.5mm. XRD was done on a PANalytical. The samples were measured with a speed of 0.0045°/min on a rotating spinner.

Results and Discussion

Figure. 1 demonstrates a drastic decrease in grain size by the step-wise addition of SiC particles at contents < 0.5wt-% for SiC-1 and at contents < 0.25wt-% for SiC-2, respectively. For larger SiC contents the change in grain size was only marginal as the critical amount of nucleation sites was achieved. The smaller the SiC particles the faster the grain size decreased because there existed more potential nucleation sites for the same particle content (in wt-%). However, these small particles can agglomerate, as shown in Figure 2 and 3. As consequence, they are inhomogeneously distributed within the melt and not anymore available as single particles. This leads to a reduction of grain refining ability. The slight difference in SiC particle size has a significant influence on the tendency to agglomerate and thus the grain size. The reference Al4C3 particles performed in a similar way to the SiC particles. A reason therefore could be that the lattice misfit between magnesium and SiC or Al4C3 varies only slightly [6]. The difference in grain size can be ascribed to the particle size as there are less potential nucleation sites available if the particles are bigger. For high Al4C3 contents (> 2 wt-%) this effect is equal due to achievement of the critical amount of particles.

Optical microscopy revealed that the amount of Mg2Si phase increased with the amount of added SiC. This indicates that small SiC particles are not stable for the long-term in Mg; SiC may dissociate into Si and C and react with Mg to form Mg2Si during solidification. In addition, the free

446 Light Metals Technology 2009

carbon can react with the aluminium from the melt to form aluminium carbides which can also act as nucleation sites. According to Barin et al. [9] Mg2Si and Al4C3 are more stable than SiC due to their higher formation enthalpy. If aluminium carbides are formed from SiC particle agglomerates, they look like bridges between the agglomerates (Fig. 2).

Figure. 1. Gain size in dependence of added amount of SiC particles.

Figure 2. Optical micrograph of reacted SiC-1 particle agglomerations in the sump. The bridged particles are shown in the

two scopes.

To analyse the actual active grain refiner in Mg-Al alloys, the particles were allowed to settle; so the nuclei and the particle agglomerates were accumulated on the bottom of the crucible in the sump, as shown in Figure. 2 and 3. In Figure. 3, the original SiC particles are hardly recognisable; a high fraction of the SiC particles reacted into needle- or big plate-like carbides, which have a similar shape and morphology as the carbides formed in carbon fibre-Al metal matrix composites [10, 11]. Sometimes, in the centre of the carbides SiC particles could be detected.

Figure 3. SEM micrograph of the reacted SiC-1 particle (needles and big plates)

agglomeration in the sump.

Figure 4. XRD plot of the analysed SiC-1 sump.

XRD analysis confirmed the presence of carbides (Fig. 4). However, the expected Al4C3 could not be detected. Instead, the ternary phase Al2MgC2 was observed, which probably formed according to the following reaction (1).

2SiC +2Al+ 5Mg ↔ 2Mg2Si + Al2MgC2 (1)

No other carbides such as Mn7C3, Mg2C, Mg2C3 or Al2CO were found by XRD; for Mg2C and Mg2C3 the formation enthalpy is even lower than the one of SiC [9]. For reference, samples containing Al4C3, the grain refining particles did not change significantly after addition to the melt. On the surface of the large carbides which were added as grain refiner, new and smaller carbides were formed. However, at the moment these phases have not been identified.

needle

plate SiC-partcle

Materials Science Forum Vols. 618-619 447

Conclusions

It was shown that additions of SiC particles in the range of approximately 0.2wt-% resulted in effective grain refinement. The smaller the particles the more effective is their performance. However, if there are too many particles added, agglomerations can form, reducing the amount of active nucleation sites and thus the grain refinement ability. Analysis of the sump revealed that the SiC particles dissolve and ternary carbides Al2MgC2 are formed, independent of the SiC particle size. These may act as new potent grain refiner. No other grain refining carbides were detected.

References

[1] StJohn, D.H., Qian, M., Easton, M.A., Cao, P. and Hildebrand, Z. (2005) Grain Refinement of Magnesium Alloys, Metallurgical and Materials Transactions A, 1669-1679.

[2] Schiffl, A., Easton, M.A. and Kettner, M. (2008) Influence of Grain Refinement with SiCP and Casting Speed on Grain Size of AZ31 During Vertical Direct Chill Casting, in Second International Conference on Advances in Solidification Processes Graz/Seggau, Austria, p. 77.

[3] Fan, Z., Wang, Y., Zhang, Z.F., Xia, M., Li, H.T., Xu, J., Granasy, L. and Scamans, G.M. (2009) Shear Enhanced Heterogeneous Nucleation in Some Mg- and Al-Alloys, International Journal of Cast Metals Research. Vol. 22

[4] Ramirez, A. and Qian , M. (2007) Grain Refinement of Magnesium Alloys by Ultrasonic Vibration, in Magnesium Technology 2007, Olrandeo, Florida, 127-132.

[5] Nelson, C.E. Grain Size Behavior in Magnesium Casting Alloys, in Charles Edgar Hoyt Annual Lecture,

[6] Easton, M.A., Schiffl, A., Yao, J.-Y. and Kaufmann, H. (2006) Grain refinement of Mg-Al(-Mn) alloys by SiC additions, Scripta Materialia, 379-382.

[7] Gunther, R., Hartig, C. and Bormann, R. (2006) Grain refinement of AZ31 by (SiC)P: Theoretical calculation and experiment, Acta Materialia, 5591-5597.

[8] Yanhui, L., Xiangfa, L. and Xiufang, B. (2004) Grain refinement of Mg-Al alloys with Al4C3-SiC/Al master alloy, Materials Letters, 1282-1287.

[9] Barin, I. and Knacke, O. (1973) Thermochemical properties of inorganic substances, Springer-Verlag Berlin Heidelberg New York, Verlag Stahleisen m.b.H. Düsseldorf, 921.

[10] Lacom, W., Degischer, H.P. and Schulz, P. (1996) Assessment and Control of Surface Reactions of Carbon Fibres in Light Weight Metal Matrix Composites Proc. CMMC 686-694.

[11] Etter, T., Schulz, P., Weber, M., Metz, J., Wimmler, M., Löffler, J.F. and Uggowitzer, P.J. (2007) Aluminium carbide formation in interpenetrating graphite/aluminium composites, Materials Science and Engineering: A, 448 (1-2), p. 1-6.

448 Light Metals Technology 2009

Light Metals Technology 2009 doi:10.4028/www.scientific.net/MSF.618-619 Influence of SiC Particles on the Grain Refinement of an Mg-Al Alloy doi:10.4028/www.scientific.net/MSF.618-619.445 References[1] StJohn, D.H., Qian, M., Easton, M.A., Cao, P. and Hildebrand, Z. (2005) GrainRefinement of Magnesium Alloys, Metallurgical and Materials Transactions A, 1669-1679. [2] Schiffl, A., Easton, M.A. and Kettner, M. (2008) Influence of Grain Refinement with SiCPand Casting Speed on Grain Size of AZ31 During Vertical Direct Chill Casting, in SecondInternational Conference on Advances in Solidification Processes Graz/Seggau, Austria, p.77. [3] Fan, Z., Wang, Y., Zhang, Z.F., Xia, M., Li, H.T., Xu, J., Granasy, L. and Scamans, G.M.(2009) Shear Enhanced Heterogeneous Nucleation in Some Mg- and Al-Alloys,International Journal of Cast Metals Research. Vol. 22 [4] Ramirez, A. and Qian, M. (2007) Grain Refinement of Magnesium Alloys by UltrasonicVibration, in Magnesium Technology 2007, Olrandeo, Florida, 127-132. [5] Nelson, C.E. Grain Size Behavior in Magnesium Casting Alloys, in Charles Edgar HoytAnnual Lecture, [6] Easton, M.A., Schiffl, A., Yao, J.-Y. and Kaufmann, H. (2006) Grain refinement of Mg-Al(-Mn) alloys by SiC additions, Scripta Materialia, 379-382.doi:10.1016/j.scriptamat.2006.04.014Can't connect to PubMed [7] Gunther, R., Hartig, C. and Bormann, R. (2006) Grain refinement of AZ31 by (SiC)P:Theoretical calculation and experiment, Acta Materialia, 5591-5597.doi:10.1016/j.actamat.2006.07.035Can't connect to PubMed [8] Yanhui, L., Xiangfa, L. and Xiufang, B. (2004) Grain refinement of Mg-Al alloys withAl4C3-SiC/Al master alloy, Materials Letters, 1282-1287. [9] Barin, I. and Knacke, O. (1973) Thermochemical properties of inorganic substances,Springer-Verlag Berlin Heidelberg New York, Verlag Stahleisen m.b.H. Düsseldorf, 921. [10] Lacom, W., Degischer, H.P. and Schulz, P. (1996) Assessment and Control of SurfaceReactions of Carbon Fibres in Light Weight Metal Matrix Composites Proc. CMMC 686-694. [11] Etter, T., Schulz, P., Weber, M., Metz, J., Wimmler, M., Löffler, J.F. and Uggowitzer,

P.J. (2007) Aluminium carbide formation in interpenetrating graphite/aluminiumcomposites, Materials Science and Engineering: A, 448 (1-2), p. 1-6.doi:10.1016/j.msea.2006.11.088Can't connect to PubMed