Microstructural Evolution of ETIAL 160 Aluminium

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Microstructural Evolution of ETIAL 160 Aluminium Alloy Feedstock. Produced by Cooling Slope Casting. Nurşen Saklakoğlu. 1,a. , Yucel Birol. 2,b and Şefika ...
Solid State Phenomena Vols. 141-143 (2008) pp 575-580 online at http://www.scientific.net © (2008) Trans Tech Publications, Switzerland

Microstructural Evolution of ETIAL 160 Aluminium Alloy Feedstock Produced by Cooling Slope Casting Nurşen Saklakoğlu1,a, Yucel Birol2,b and Şefika Kasman3,c 1

Dept. of Mechanical Engineering, Celal Bayar University, Manisa, Turkey

2

Materials Institute, Marmara Research Center, TUBITAK, Kocaeli, Turkey

3

Izmir Vocational School, Dept. of Technical Programs, Dokuz Eylül University, Đzmir, Turkey [email protected], [email protected], [email protected]

Keywords: Semisolid processing, Cooling slope casting, Aluminium alloys.

Abstract. Owing to its superior flow and mould-filling capability, a fully globular structure is essential for semisolid processing technologies. The present work was undertaken to identify the cooling slope casting process parameters that, upon heating to the semisolid state, gives the required globular structure for the ETIAL 160 alloy. Of the two pouring temperatures investigated, 605 °C and 615 °C, the lower pouring temperature was found to provide more globular grains surrounded by liquid phases. Introduction A number of technologies have been developed over the last three decades, mainly in a laboratory environment, to take advantage of the unique behaviour of semisolid slurries. The progress in the commercialization of many of them seems to indicate the beginning of the large-scale acceptance of semisolid processing by major industries [1]. Semisolid forming combines the advantages of casting to produce complex part geometries with improved mechanical properties. Lower process temperature and laminar flow of the melt lead to less shrinkage porosity and entrapped pores in the finished parts [2]. In Europe, suspension parts, engine brackets and fuel rails for automotives are being produced through semisolid metal (SSM) processing [3]. There have been continuous efforts to develop new SSM processes for aluminium alloys used as automotive parts because of its advantages over traditional casting process. While thixoforming offers significant advantages compared with traditional metal forming methods, the process comes with the requirement for a special feedstock [4]. Among all the techniques employed to produce the thixotropic feedstock for SSP, the cooling slope (CS) casting is particularly attractive as it presents very low equipment and running costs by a simple route. It has thus been investigated by a number of workers [ 4,5,6,7]. In this paper special attention has been focused on the formation of a non-dendritic structure by CS casting process, structural evolution during reheating based on the casting alloy ETIAL 160 (ISOAlSi8Cu3Fe) which is used for manufacturing of clutch-rear case, sigma oil pan, feed bracket etc. for the automotive industry. Experimental The commercial ETIAL 160 aluminium alloy (composition is given in Table 1), of great interest for aluminium foundries in Turkey, was used in this study. The alloy ingot was melted in graphite crucible placed in an electrical resistance furnace. The furnace temperature was set at 750 ºC. Shortly after melting, the molten alloy was transferred to the CS casting unit. The CS casting unit comprises a graphite crucible, a step motor which regulates pouring action, an inclined BN-coated U-profile manufactured from ordinary mild steel, a permanent steel mould with a diameter of 30mm and a depth of 150mm. The inclined steel plate was cooled by water circulation. Once cooled to the pouring temperature, the molten metal was poured over the cooling plate into the permanent mould.

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Pouring temperatures employed were selected so as to limit the superheat of the melt as much as possible yet to allow complete evacuation of the graphite mould. After several trials and with a consideration of the DSC data, the optimum pouring temperatures were judged to be between 605 o C and 615 oC. Table 1. Chemical composition of the ETIAL 160 alloy. Si 8.62

Fe 0.824

Cu 3.091

Mn 0.2248

Mg 0.2804

The ingots thus obtained were cut into cylindrical samples. These samples were held in a medium frequency induction coil (9.6 kHz, 50 kW) for isothermal heating in the semisolid state for up to 15 minutes. The reheating temperatures were identified and were controlled with a K type thermocouple inserted in the centre of samples. After soaking, the thermocouple was withdrawn from the samples and the sample was quenched in cold water (Fig. 1). The reheated samples were polished using standard metallographic procedures and were finally etched with a 0,5% hydrofluoric acid. The microstructures were examined under the optical microscope. LUCIA Image Analyser Version 4.51 was used to characterize the structural features quantitatively: • Primary α-Al and percentage • Liquid phase percentage • Average grain size of primary α-Al particles • Shape factor (inverse of sphericity)

Zn 0.901

Al 85.79

Fig. 1. The experimental set up used in the reheating experiments.

The shape factor was estimated from: 2

P SF = α 4π Aα

(1)

where Aα and Pα are the area and the perimeter of the primary phase in the microstructure, respectively. The closer the sphericity is to 1, the higher is the globularity of the α-Al particle. Table 2. CS casting parameters employed in the present work. Cooling slope material Mild steel Cooling slope coating BN Cooling slope angle 60º Cooling slope length 300mm Casting temperature range 605-615 ºC Results and Discussion Fig. 2 compares the microstructures of as-received ingots and ingots produced by casting over the cooling plate from pouring temperatures of 615 oC and 605 oC with a cooling length of 300 mm and with a slope of 60°. The original ingot structure is aluminium solid solution dendrites and an interdendritic network of the eutectic phase, typical of conventionally cast aluminium alloys.

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Marked changes are noted in the CS-cast ingots. The primary α-Al dendrites seem to have been replaced by α-Al rosettes and globules. The degeneration of the dendritic structure is promoted further by the limited superheat of the melt. When the molten alloy flows over the cooling plate, the temperature quickly drops below the liquidus temperature and α-Al crystals start to nucleate. These crystals are detached from the cooling plate, trapped in the flowing melt and are collected in the mould at the bottom of the cooling plate before they grow to become ripened dendrites. They act as nucleation centres and cause partial fragments of the dendrites by convection. Final solidification in the mould involves growth of these α -Al crystals into rosettes and occasionally into nearly globular grains [6,7,8]. Shallow temperature gradient due to low superheat removes directional heat extraction from the melt and prevents the formation of dendrites within the slurry [9]. The degeneration of the original dendritic matrix phase seems to be more advanced in the case of casting from 605 ºC, as one would expect with a lower melt superheat. The microstructural features are smaller as well. The evolution of the microstructure in the as-received, original ingot upon heating in the semisolid temperature range, at 565 °C is shown in Fig. 3. After reheating, α-Al dendrites have apparently grown and coalesced with no evidence of globularization. It is thus concluded that it is not possible to get a globular structure in this alloy without first modifying the original dendritic network.

Figure 2. Microstructural features of (a) asreceived alloy and of ingots cast over the cooling plate from (b) 615 ºC and (c) 605 ºC.

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Figure 3. Microstructural features of the asreceived ingot after reheating at 565 ºC for (a) 5, (b) 10 and (c) 15 minutes

Fig. 4 shows a series of micrographs depicting the evolution of microstructure in an ingot cast over the cooling plate from 615 oC after isothermal holding at 565 oC. Thanks to a more favourable starting microstructure (Fig. 2b), the change in the primary phase morphology has advanced as desired. α-Al dendrites fragmented, largely spheriodized, yet failed to transform into a perfectly globular microstructure. There were some fragments which have coalesced trapping a fraction of the liquid inside. There is always a tendency of suspended particles in the liquid for cluster formation and agglomeration. It is thought to be highly dependent on crystallographic orientation and surface energy. Another idea is that boundaries between coalesced particles are low angle and these boundaries may be formed either by sintering after collision of these particles or by twinning growth to form welded joints which are eventually strengthened due to high temperature and easy diffusion [10]. It is fair to conclude that CS casting from 615 oC did not provide the expected globular microstructure even after 15 minutes at 565 oC.

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Figure 4. Microstructural features of ingots cast over the cooling plate from 615 ºC after reheating at 565 ºC for (a) 5, (b) 10 and (c) 15 minutes.

Figure 5. Microstructural features of ingots cast over the cooling plate from 605 ºC after reheating at 565ºC for (a) 5 and (b) 10 minutes. 10 minutes at 565 oC sufficed to achieve individual fine globular solid particles uniformly suspended in a liquid matrix with the ingot cast over the cooling plate from a pouring temperature of 605 oC (Fig. 5). Some agglomerations were pointed out in Fig. 5-b. It is thus concluded that a casting temperature of 605 oC provides the microstructural features required for semisolid processing of the present alloy. Table 3 compares the liquid and solid phase fractions, average grain sizes, shape factors and sphericities for samples cast over the cooling plate from 605 °C and subsequently held at 565 °C 5 and 10 mins. The samples cast over the cooling plate from 615 ºC were not measured, due to the fact that globulazation was not sufficient.

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As holding time increased, the grain size increased about 10% as expected. At the same time, the sphericity of primary α-Al grains increased from 0.597 to 0.701. Therefore, it is fair to conclude that 10 min. soaking yielded more globular structure. The liquid phase surrounding the grains results in the decrease of apparent viscosity with higher formability. It is well known that 30–50% liquid is needed in the feedstock for thixoforming [4]. With higher liquid phase content, 10 min. soaking time presented good morphology for semisolid processing. Table 3. Effect of reheating time at 565 °C on the ETIAL 160 alloy morphology Heat treatment Fraction of Fraction of Average grain Shape holding time at primary phase, liquid phase, size of primary factor (SF) 565 oC, min. % % phase, µm. 5 72.7 27.3 49.076 1.673 10 64.9 35.1 53.955 1.426

Sphericity (1/SF) 0.597 0.701

Summary The semisolid structure is successfully produced by the cooling slope casting and reheating process for the ETIAL 160 aluminium alloy. The results show that casting temperature is an important parameter with a great impact on the evolution of primary particles during CS casting. Pouring at 605 °C over a cooling length of 300 mm at a slope of 60º yielded more globular grains than that obtained with CS casting at 615 oC. Acknowledgement It is a pleasure to thank O. Çakır and F. Alageyik for their help in the experimental part of this work. Financial support of TUBITAK and the State Planning Organization of Turkey is gratefully acknowledged. References [1] F. Czerwinski: JOM, Vol. 58, No.6 (2006), p. 17 [2] G. Hirt, H. Shimahara, I. Seidl, F. Küthe, D. Abel, A. Schönbohm and R. Kopp: CIRP Annals – Manufacturing Technology, Vol. 54, Issue 1 (2005), p. 257 [3] T. Li, X.Lin, W.Huang: J Mater Sci. Vol. 42 (2007), p.2669 [4] Y. Birol, J. Mater. Process. Technol.: Vol. 186 (2007), p. 94 [5] G. Hirt, R. Cremer, T. Witulski and H.C. Tinius: Materials and Design, Vol.18, Issues 4-6 (1997), p. 315 [6] T. Haga, S. Suzuki: J. Mater. Process. Technol. Vol. 118 (2001), p .169 [7] Q.D. Qin, Y.G. Zhao, P.J. Cong, W. Zhou, B. Xu: Materials Science and Engineering A 444 (2007), p. 99 [8] Y. Birol, Int. J. Mat. Res. (formerly Z. Metallkd.): Vol. 98 (2007), p. 10 [9] O. Lashkari, R. Ghomashchi: J. Mater. Process. Technol.: Vol. 182 (2007), p. 229 [10] I. Diewwanit: Thesis, Doctor of Science in Metallurgy, MIT, (1996)