18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika 

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OPTIMIZATION OF CENTRIFUGAL ATOMIZATION PARAMETERS FOR RAPID SOLIDIFICATION OF ALUMINIUM ALLOYS

Filip PRŮŠA, Dalibor VOJTĚCH, Pavel HRYZÁK, Vítězslav KNOTEK, Tomáš POPELA, Pavel NOVÁK

Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Prague, Technická 5, 166 28 Prague 6, Czech Republic, prusaf@vscht.cz

Abstract

This work aims to optimize the parameters of centrifugal atomizer with high-speed rotating graphite disk. For this purpose, AlSi9Cu3 was used as a testing alloy. Optimal rotation speed and diameter of graphite casting nozzle were determined depending on the fraction of obtained powder sizes and their microstructure. Consequently Al-Ni-Mm-Si alloys (Mm represents Mischmetall) with various chemical compositions were prepared. Microstructure and phase composition of these alloys were studied.

Keywords: centrifugal atomization, aluminium, microstructure.

1. INTRODUCTION

Research aimed to describe the structure and properties of materials prepared by rapid solidification began in early 1960’s. First stimulation came from Paul Duwez, who observed rapid solidification by intensive impact of small droplets of molten metal at cold substrate. The materials produced by this way provided new structural properties that have not been observed so far [1]. After that, many researchers discovered, that by using high cooling rate an undercooled state of material can be achieved providing new interesting mechanical and physical properties [2].

Rapid solidification can be defined as rapid removal of thermal energy including both superheat and latent heat during the transition from liquid state at high temperatures to solid material at a room or ambient temperature [3]. Cooling rate greater than 104 K/s is needed to consider a process as rapid solidification, although cooling rates of 103 K/s sometimes generate rapidly solidified microstructures. Time of contact of the melt with cooling medium at high temperatures is limited to milliseconds followed by rapid decrease of materials temperature. Water, salt solution or even liquid nitrogen can be used in order to achieve greater cooling rate to obtain different types of microstructure. This leads to large scale deviations from equilibrium state which can offer some advantages consisting in:

i) formation of non-equilibrium metastable nanocrystalline, quasicrystalline or amorphous phase;

ii) formation of metastable supersaturated solid solutions and their slow decomposition leading to a precipitation hardening;

iii) reduction of microsegregation and creation of homogeneous structure;

iv) change of crystallization mechanism leading to the formation of relatively ductile solid solution instead of brittle primary intermetallic phases. [1][3]

Simply stated, the higher the cooling rate, the finer the microstructure and the better the properties [4]. Rapid solidification processing opens new horizonts for alloy development. This applies to a modification of commercial products and to new compositions which enables to achieve unusual structures and superior properties.

There are many methods for preparing rapidly solidified materials which can be characterized by high efficiency. Main disadvantage of these methods lies in difficult handling but also in high operating costs associated with the use of industrial gases, etc. Solution can be found in centrifugal atomization which can

    18. ‐ 20. 5. 2010, Rožnov pod Radhoštěm, Česká Republika 

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produce large volumes of materials nearly without any additional cost [5]. Low operating costs predispose this technology to more widespread use in the preparation of materials not only for aluminum based materials with interesting properties.

Aluminium alloys belong to prospective materials finding their use in several applications, especially in automotive and aircraft industry [6]. The main advantages of these materials are low density and good mechanical properties. Common aluminium alloys are not suitable for elevated-temperature applications. In this case, aluminium alloys with the addition of transition metals (Ni, Fe..) prepared by rapid solidification method could solve this problem [7]. Metastable phases can be obtained by rapid solidification, which can increase strength and other properties [7]. Thermal stability improvement is caused by the low diffusivity of transitional metals in aluminium. Addition of Ce causes the formation of nanocrystalline phases at lower cooling rates which do not arise without the addition of Ce [8].

2. EXPERIMENT AlSi9Cu3 (DIN 226) alloy was used to optimize the operating parameters of centrifugal atomizer with high speed rotating graphite disc and water-cooled walls (Fig. 1, 2). Materials were melted in electric resistance

furnace at 900°C and then injected through the graphite casting nozzle, preheated to 900°C, to the high-speed rotating graphite disc. From the ratio of obtained powder fractions measured by granulometric analysis, optimum process conditions were determined. Rotating graphite disc speed varied between 7000-20000rpm and diameter of the casting nozzle between 1-3mm. Microstructure of AlSi9Cu3 alloy prepared by the optimized centrifugal atomization process was compared with the microstructure of material prepared using a melt-spinning method and by slow solidification using light microscope Olympus PME-3 and scanning electron microscope (SEM) Hitachi S-450 with attached EDS analyzer. AlNi10, AlNi10Mm10, AlNi10Mm7 and AlNi10Mm7Si2 alloys were prepared by induction melting of appropriate amount of metals with at least 99.9 wt.% purity and of Mm (Mischmetall) containing approx. 45 wt.% Ce, 38 wt.% La, 12 wt.% Nd and other rare earth elements. Composition of Al-Ni-Mm-Si materials was confirmed by XRF (ARL 9400 XP) analysis. Materials were then melted in electric resistance furnace at 1000 °C and then injected through a graphite casting nozzle, preheated to 900 °C, onto a high speed rotating graphite disk. After that, the granulometric analysis was performed to determine the distribution of powder

Fig. 1: Scheme of centrifugal atomizer:

1) casting nozzle; 2) molten metal; 3)

graphite disc; 4) melt droplets; 5) water-

cooled walls; 6) product

Fig. 2: Inner part of centrifugal atomizer

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particle size. In this way, the fractions in the range of powder sizes from 0.25 mm to 2.8 mm were obtained. Metallographic samples were prepared by subsequent grinding on P60-P4000 abrasive papers and then polished with diamond paste. These samples were observed by the above mentioned light and scanning electron microscopes after etching in 0.5 % hydrofluoric acid. Phase composition was determined by X-ray diffraction analysis (XRD) using PANalytical X’Pert PRO x-ray diffractometer.

3. RESULTS AND DISCUSSION Microstructure of AlSi9Cu3 prepared by conventional method using slow solidification containing silicon (dark) and CuAl2 (light grey) particles is presented in Fig. 3a. In comparison with the microstructure of

materials prepared by rapid solidification techniques (Fig 3b, c), the silicon particles size is significantly higher. Fig. 3b shows the microstructure of a material prepared by centrifugal atomization. This structure is very similar to the material prepared by melt-spinning technology (Fig. 3c). Materials with fine structure prepared by rapid solidification can be considered as materials with promising mechanical properties e.g. hardness, abrasive resistance and other. Optimization of centrifugal atomizer parameters was based on the weight fraction of powders with size less than 2.8 mm which can provide materials with fine microstructure. Thickness of powders depending on the selected parameters ranged from 30 to 50 µm which allowed efficient dissipation of heat from molten metal and almost immediate solidification and fine microstructure creation (Fig. 3b).

Amount of powders, which size was smaller or equal to 2.8 mm, prepared by using 15000rpm and 1 mm graphite casting nozzle was the largest (Fig. 4). To keep the device operable, 15000rpm with diameter of 2 mm graphite casting nozzle was chosen, providing powders with the same microstructure but smaller percentage of the required weight fraction.

Fig. 3: Microstructure of AlSi9Cu3 prepared by: a) slow solidification; b) centrifugal atomization

(15000 rpm, 2mm graphite nozzle); c) melt spinning

Fig. 4: Particle size distribution

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Microstructure of AlNi10Mm10 (Fig. 6 a, b) consists of Al, Al3Ni and Al11Ce3 phases which were also detected in AlNi10Mm7. Microstructure of AlNi10Mm7Si2 consists of Al, Al3Ni and Ce2Ni0,8Si1,2. Particle coarsening was observed in these materials depending on fraction size. It can be caused by not immediate contact of small molten metal droplets with water-cooled walls.

Fig. 6: Microstructure of AlNi10Mm10: a) 2,8 - 1,4 mm; b) <0,25 mm

Explanation can be found in fact, that this small drop of molten metal may fly inside the centrifugal atomizer what can provide sufficient time to create coarser microstructure.

Fig. 5: Materials prepared by: a), b), c) centrifugal atomization

(different fraction); d) melt spinning

Morphology of prepared powders (Fig. 5a, b, c) considerably

differs from the materials prepared by using melt-spinning

technology (Fig. 5d). Centrifugally atomized materials can be

pressed directly because they consist of fine flake-like particles.

On the other hand, ribbons produced by melt spinning have to be

milled to obtain a powder for further consolidation. Special milling

techniques, such as cryogenic milling, should be used to prevent

the microstructure changes.

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Fig. 7: X-ray elemental map in AlNi10Mm10 (2,8 – 1,4 mm fraction)

Point analysis and X-ray elemental map (Fig.7) performed on SEM with EDS analyzer was applied to recognize present phases. Small white particles can be presumed as Al3Ni phase and the large smooth particles containing certain amount of Mischmetall as Al11Ce3 phase.

4. CONCLUSION Centrifugal atomization is a simple rapid solidification method using high-speed rotating graphite disc. Optimal parameters were determined by the observation of product microstructure as following: rotation speed of the graphite disc of 15000rpm and graphite casting nozzle diameter of 2 mm. Using this parameters, fine microstructure of Al-Ni-Mm-Si materials were prepared. Microstructure coarsening depending on decreasing powder fraction size was probably caused by not-immediate contact of small molten metal droplets with cooled wall, so there was enough time to produce coarser microstructure.

AKNOWLEDGEMENT This research was financially supported by internal grant of ICT Prague titled: Light materials for

elevated temperature application. Authors wish to thank the Ministry of Education, Youth and Sports of the Czech Republic (project no. MSM6046137302) and Czech Academy of Sciences (project

no. KAN300100801) for financial support. LITERATURE

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[2] LIANXI, H. et al. Microstructure and mechanical properties of 2024 aluminium alloy consolidated from rapidly solidified alloy powders. Mater. Sci. Eng., A, 2002, vol. 323, p. 213–217.

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[3] LAVERNIA, E. et al. The rapid solidification processing of materials: science, principles, technology, advances and applications. J. Mater. Sci., 2010, vol. 45, p. 287–325.

[4] KATGERMAN, L. et al. Rapidly solidified aluminium alloys by meltspinning. Mater. Sci. Eng., A, 2004, vol. 375-377, s. 1212–1216.

[5] SATOH, T. et al. High-temperature deformation behavior of aluminium alloys produced from centrifucally-atomized powders. J. Mat. Proc. Technol., 1997, vol. 68, p. 221-228.

[6] CHIRITA, G. et al. Advantages of the centrifugal casting technique for the production of structural components with Al-Si alloys. Materials and Design. 2008, vol. 29, p. 20-27.

[7] ZHANG, Z. et al. Microstructure selection map for rapidly solidified Al-rich Al-Ce alloys. J. Cryst. Growth., 2004, vol. 260, p. 557–565.

[8] MICHALCOVÁ, A. aj. Structure and properties of Al-TM-Ce alloys, In sborník z conference METAL 2009. Ostrava: TANGER, 2009, sborník na CD.