A Comparison of Grain Refinement Efficiency by Shearing Above ...

Solid State Phenomena Vols. 217-218 (2015) pp 23-28 © (2015) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/SSP.217-218.23

A Comparison of Grain Refinement Efficiency by Shearing Above and Below the Liquidus Reza Haghayeghi1,a*, P. Kapranos2,b 1

Department of Materials Engineering, Tehran Science and Research Branch, Islamic Azad University, Tehran, Iran.

2

Department of Materials Science &Engineering, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK a

[email protected], [email protected]

Keywords: Electromagnetic Casting, Grain Refinement, Cavitation Mechanism

Abstract There is a debate whether grain refinement is more effective when shearing above or below the liquidus. In this work, examination of the microstructural evolution in AA7075 alloy has been performed, after stirring above & below the liquidus the results suggest that shearing above the liquidus is more effective. The outcomes indicate that at temperatures below the liquidus, the convective forces produced by electromagnetic forces cannot break the dendrites due to low velocities, whilst at shearing above the liquidus through a cavitation-enhanced nucleation mechanism grain refinement is promoted. It appears that by applying magnetic forces above the liquidus temperature, shearing effects result in finer grain sizes and consequent enhancement of properties. Introduction Grain refinement is desirable, especially for high performance applications. By reducing the grain size not only mechanical properties are improved but also casting defects such as hot tearing susceptibility or macro-segregation are limited [1]. There are few means available for grain refinement such as the addition of grain refiners (Chemical refinement) or through the application of external forces on a melt (Physical refinement). However, through the use of chemical refiners some undesired intermetallic compounds are produced, the chemical composition is changed and most of the above particles can turn into impurities adversely affecting the mechanical properties. According to Greer et al. [2] only 1% of a grain refiners act as nucleation sites and the rest settle or transform into impurities. Therefore, the use of physical refinement appears to offer a preferable alternative for producing finer grain size. Physical refinement can be performed either below or above the liquidus. Below the liquidus, breaking of the dendrites could be responsible for grain refinement whilst above, cavitation might be the major refinement mechanism [3]. Although, in both cases several mechanisms have been offered as possible means of refinement no consensus has yet emerged. Moreover, no concrete comparison has been performed to date on the degree of grain refining by shearing above or below the liquidus. Recently, the AA7075 aluminium alloy has attracted attention due to its mechanical properties and various investigators have studied the possibility of processing this alloy in the semisolid state [4, 5, 6]. In this study, an AA7075 aluminium alloy was intensively sheared at the semi-solid region, above the liquidus, and observations and proposals were made in each case on: a) What the main mechanism in each case could be, b) Whether shearing above liquidus is more effective than shearing below or vice versa, c) Researchers such as Easton and St. John [6] or Cao and Campbell [7] have investigated the mechanism of refinement but fewer studies have been reported about the nuclei. Easton and St. John [7] suggested that the growth restriction factor could be the reason of grain refinement but did not offer any ideas about the nature of nuclei. However, they have emphasized All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 5.161.222.52, BCAST, Brunel University, Uxbridge, United Kingdom-12/08/14,13:47:01)

24

Semi-Solid Processing of Alloys and Composites XIII

that the decreasing thermal gradient and concentration could improve the formation of equiaxed grains but these are not enough and the availability of potent nuclei are also paramount important [7]. Similarly, Campbell and Cao [8] suggest that oxide films could act as nucleation sites but did not demonstrate how the wetting of oxides occurred. In fact, oxides have high contact angles with a melt [9] and wetting of oxides seems improbable. Experimental procedure 5 kg of AA7075 wrought aluminium alloy with the composition shown in Table (1) was melted in an electrical resistance tilting furnace. The melt was then transferred to the hot top of a DC caster at two different temperatures, 10 oC below and above liquidus temperature (TM=643 oC). The DC caster consists of a hot top round mould (Radius=80mm), fitted in a water box and a hydraulic movement system with a maximum length of 1800 mm. The water flow rate was set at 120 lit/min and the casting speed at 4 mm/s.

Zn 5.56

Table 1: Chemical composition of the AA7075 (in wt.%) Mg Cu Cr Mn Fe Ti Si 1.95 1.64 0.19 0.10 0.09 0.03 0.01

Al Bal.

In each case, an electromagnetic field of 30 Hz and 80 A was applied for 60 s and in the case of semisolid condition, the stirring occurred at a solid fraction of 22%. The variable magnetic field was generated by a 100 turn induction coil supplied with an alternating current adjusting parallel to the vertical axis of the billet. The experiments were carried out with a magnetic induction system composed of two coil systems that allow for the generation of both rotating and traveling magnetic fields. A schematic view of the experimental setup is depicted in Figure 1. The bore diameter of the magnetic system is 200 mm, wherein the fluid vessel was placed concentrically. In order to preclude flow artefacts arising from symmetry deviations of the experimental setup, conformity of both the cylinder, and magnetic-field axis, special care was necessary to ensure precise positioning of the cylinder inside the magnetic system. The Magnetic field was generated by an arrangement of six air-cooled copper coils, fed with a three-phase electric current. The magnetic system is able to provide field intensities of at most 25 mT and a frequency in the domain between 10 and 400 Hz. The homogeneity of the magnetic field was checked using a three-axis Gauss meter (Lakeshore model 560, sensor type MMZ2560-UH, Lakeshore). Within a radius of 30 mm, selected as the radial dimension of the container, the variance of the magnetic-field strength was found to be smaller than 5 pct. The 50-Hz frequency in the magnetic field has been modulated with sequences of rectangular pulses. The modulation has been carried out by switching the power supply with an external wave form generator. The modulation frequencies (typically below 1 Hz) are considerably lower than the magnetic field frequency of 50 Hz. The as-cast grain structures of transverse crosssections were observed after grinding and polishing. To reveal microstructural features, the samples were anodized at 3% HBF4 at 20V DC. Grain sizes were measured according to ASTM E112-96 [10].

Solid State Phenomena Vols. 217-218

25

Fig.1: The used electromagnetic casting apparatus. Results and discussion The grain structures of the samples at both temperatures are illustrated in Figure (2). As observed, the structure of the sample that has been sheared above the liquidus has smaller grain size in comparison with the one sheared in the semi-solid region. Particularly in the semi-solid sheared microstructure, semi-equiaxed grains dominate the structure, although some non-broken dendrites are also observed. a

b

Centre

500 µm Edge

Fig. 2: Microstructure of the samples; a) treated at above liquidus, b) treated at below liquidus. The average grain size achieved when treated below the liquidus was around 102µm as compared to an average grain size of 43µm when the melt was treated above the liquidus, challenging the idea of achieving a fine grain size by implementing below liquidus shearing i.e. in the semi-solid region. It has been proposed in the past that treating melts in the semi-solid region is one of the best ways for obtaining fine and equiaxed grain size but no concrete research or comparison has been performed on intensive shearing above the liquidus. Above the liquidus, every melt contains exogenous particles such as oxides and impurities. By applying intensive shearing, a pressure pulse is generated leading to formation of bubbles at micro slits of these particles. However, these bubbles could not stand permanently and collapse breaking the oxide layer through fatigue, thus converting them to active substrates [3], figure (3). These particles wet and contribute to nucleation [3]. By releasing the pressure initiated by the collapse of bubbles, the melting point alters according to

[[

26


Clapeyron’s equation [11]. An increase in the melting point is produced by releasing the pressure is equivalent to increased undercooling, therefore enhancing nucleation [12, 13]. The above mechanism, known as cavitation, induced heterogeneous nucleation [12], see figure (4). This includes improved wetting of exogenous particles, local undercooling upon the collapse of cavitation bubbles and pre-solidification inside fine capillaries.

Fig. 3: The mechanism of Bubble creation and collapse, (1) Bubble growth, (2) maximum bubble size, (3) bubble collapse, (4) creation of new bubbles. Cavitation not only can provide a uniform temperature gradient but also can flatten the solidification front. Moreover, the strong streaming due to the collapse of the bubbles provides a suitable condition for the formation of a more equiaxed microstructure. The induced flow through the magnetic field also causes intensive shearing further encouraging the formation of fine, equiaxed microstructures. Cavitation

Melt overheating

Flattening of solidification front

Reducing thermal gradient and contraction

Activation of non-metallic Particles by wetting through fatigue

Providing additional nuclei

Grain refinement

Formation of fine equiaxed microstructure with improved plasticity and reduced defects (e.g. hot tearing susceptibility)

Fig.4: The effects of cavitation.

Solid State Phenomena Vols. 217-218

27

When treating the melt below the liquidus temperature, dendrite fragmentation occurs, supposedly through convection, dispersing across the entire bulk. [14]. There is another point of view that forced convection cannot break the dendrites as the maximum speed of 0.2-0.3 m/s is not strong enough to do so [15]. The other possible way is through re-melting of the dendrite arm roots [16]. Nevertheless, fragmentation by re-melting dendrite arms might be difficult as a steady state situation is implemented by shearing. Nastac [17] suggests that below the liquidus temperature, cavitation is the main mechanism for grain refinement, playing a more important role than the dendrite fragmentation. When applying shear in the semi-solid region, cavitation occurs on oxides, impurities and intermetallics, causing refinement. However, as the caviation threshold increases with decreasing temperature, only some particles could contribute to nucleation. According to the free growth model proposed by Greer et al [2], those particles with bigger size have priority. Although, the availability of oxides may decrease the caviation threshold to some extent, it could not make significant changes, thus nucleation would not effectively improve. Consequently, the grain size with shearing below the liquidus temperature would be larger than that with shearing above the liquidus. Regarding the separation theory [18], the nucleation on the mould wall and their detachment may improve the nucleation rate but its contribution is not considerable. Similar results on various alloys through the application of ultrasonic disturbances [19] and melt conditioning techniques [20] at below and above the liquidus further support the above results. Summary

Cavitation is the major mechanism in shearing at below or above liquidus. However, due to the higher cavitation threshold in the semi-solid region, the exogenous particles could not be activated as much as when above the liquidus temperature and as a result finer grain size is obtained by shearing above the liquidus than shearing from below. The nuclei in both cases are exogenous particles activated by cavitation and these particles are wetted through the collapse of caviation bubbles. The suggested grain refinement mechanism includes improved wetting of solid particles, local undercooling upon the collapse of cavitation bubbles and pre-solidification inside fine capillaries. Nucleation and its rate depend on the collapse and growth rate of cavitation bubbles occurring easier at higher temperatures where the cavitation strength is lower.

Acknowledgements The authors express their gratitude to all contributor companies for their extensive and profound support. References [1] B.S. Murty, S.A. Kori, M. Chakraborty, Grain refinement of Al and its alloys by heterogeneous nucleation and alloying, Int. Mater Rev, 47 ( 2002) 2–29. [2] A.L. Greer, Grain refinement of alloys by inoculation of melts, Phil Trans Math Phys Eng. Sci, 361(2003) 479–95. [3] R. Haghayeghi, E. Ezzatneshan, H. Bahai, L. Nastac, Numerical and experimental investigation of the grain refinement of liquid metals through cavitation processing, Metals & Mater Int., 19 (2013) 959-967. [4]. S. Chayong, H V Atkinson and P Kapranos, “Thixoforming 7075 aluminium alloys”, Materials Science and Engineering, A 390, 2005, 3-12. [5] G. Vaneetveld, Ahmed Rassili, Jacqueline Lecomte-Beckers, H.V. Atkinson, Thixoforging of 7075 Aluminium Alloys at High Solid Fraction, Solid state phenomena, 116-117 (2006) 762765. [6] S.L. George, R.D. Knutsen, Composition segregation in semi-solid metal cast AA7075 aluminium alloy, J. Mater Sci., 47 (2012) 4716-4725.

28


[7] M.A. Easton, D.H. St.John, An analysis of the relationship between grain size, solute Content and the potency and number density of nucleant particles, Metall Mater Trans A, 36 (2005) 1911-20. [8] X. Cao, J. Campbel, The nucleation of Fe-rich phases on oxide films in Al–11.5Si–0.4Mg cast alloys, Metall Mater Trans A, 34 (2003) 1409-1420. [9] N. Eustathopoulos, B. Drevet, Determination of the nature of metal–oxide interfacial interactions from sessile drop data, Mater. Sci. Eng. A, 249 (1998) 176–183. [10] ASTM, Standard test for determining average grain size, ASTM Int, PA, USA 14 (2002) 256– 281. [11] D.G. Eskin, Effect of melt overheating on primary solidification in aluminium alloys, Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques, 87(1996) 295-299. [12] I.G. Brodova, P.S. Popel, G.I. Eskin, Liquid metal processing, Taylor and Francis, London, 2002. [13] G.I. Eskin, Ultrasonic treatment of light alloy melts, Gordon & Breach Science publisher; 1998. [14] Z. Yan, X. Li, Z. Cao, X. Zhang, T. Li, Grain refinement of horizontal continuous casting of the CuNi10Fe1Mn alloy hollow billets by Rotating Magnetic Field (RMF),Mater letter, 62 (2008) 4389-4392. [15] J. Pilling, A. Hellawell, Mechanical deformation of dendrites by fluid flow, Metall Mater Trans A, 27 (2005) 229-232. [16] D. Ruvalcaba, R.H. Mathiesen, D.G. Eskin, L. Arnberg, L. Katgerman, In situ observations of dendritic fragmentation due to local solute-enrichment during directional solidification of an aluminum alloy, Acta Mater, 55(2007) 4287-4292 [17] L. Nastac, Multiscale modeling of the solidification microstructure evolution in the presence of ultrasonic stirring, 2012 IOP Conf. Ser.: Mater. Sci. Eng., 33, 012079(2012). [18] Ohno A., Solidification: the separation theory and its practical applications, Springer, Verlag, New York, 1987. [19] R. Haghayeghi, P. Kapranos, Direct-chill casting of wrought Al alloy under electromagnetic and ultrasonic combined fields, Mater letter, 2013; 105: 213-215. [20] R. Haghayeghi, L. Nastac, On microstructural refinement of an AA7449 aluminium alloy through shearing above liquidus temperature, Mater letter, 2011; 65: 3230-3233.