MICROSTRUCTURE AND PROPERTIES OD RAPID SOLIDIFIED AL-SI-FE AND AL-SI-FE-CR ALLOYS PREPARED BY CENTRIFUGAL ATOMIZATION. Filip PRŮŠA*, Dalibor VOJTĚCH

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1 MICROSTRUCTURE AND PROPERTIES OD RAPID SOLIDIFIED AL-SI-FE AND AL-SI-FE-CR ALLOYS PREPARED BY CENTRIFUGAL ATOMIZATION Filip PRŮŠA*, Dalibor VOJTĚCH Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Prague, Technická 5, , Czech Republic, Abstract Aluminium alloys belong to perspective materials characterized by low density and convenient mechanical properties. Combination of rapid solidification technique and addition of transition metals, which can be characterized by low diffusivity in aluminium, can produce alloys with increased thermal stability and excellent mechanical properties. It fills imaginary gap between aluminium and titanium alloys prepared by conventional methods. These alloys are used especially in automotive and aerospace industry. This work aims to prepare AlSi14Fe8 and AlS14Fe8Cr1 (in wt.%) alloys by centrifugical atomizer with highspeed rotating graphite disc. So prepared powders were uniaxially pressed by ultra-high pressure of 6 GPa at 450 C for 1 hour. Microstructure of powders and pressed samples was observed by light and electron scanning microscope. Thermal stability of pressed samples was evaluated like hardness change during thermal treatment at various temperatures for 100 hours. Consequently, so prepared samples were compressive tested. Strcutures, mechanical properties and thermal stability were compared to AlSi12Cu1Ni1Mg1 (in wt.%) casting alloy, which is generally considered to be thermally stable material. Keywords: aluminium, rapid solidification, transition metals. 1. INTRODUCTION Rapid solidification (RS) can be defined as rapid extraction of thermal energy including both superheat and latent heat during liquid-solid state transition [1]. Generally to consider process as rapid solidification, cooling rates have to be greater than 10 4 K.s -1 although sometimes cooling rates of 10 3 K.s -1 generate rapidly solidified microstructures [1, 2]. Aluminium alloys prepared via RS technique offers extraordinary properties comparing to the conventional cast alloys. There are at least three possible ways to upgrade the properties of aluminium alloys consisting of possibility to increase the content of alloying elements, improve homogeneity of the alloy and creation of micro-, nano- or quasi-crystalline structures [3, 4]. Increasing the cooling rate of RS technique will cause finer microstructure and better properties but often means higher economical costs [5]. RS can be realized by several procedures like melt spinning, gas atomization or by centrifugal atomization. Centrifugal atomization (CA) is simple cost-efficient method using high-speed rotating disc ensuring almost immediately transport of molten metal to water-cooled walls with cooling rates up to 10 5 K.s -1 [1]. Typical product of CA process is powder consisting of flake-like particles (Fig. 3) which can be more efficiently pressed into bulk samples than ribbons prepared by melt spinning technology [5]. These facts predispose CA to be more often used in production aluminium alloys with increased mechanical properties while maintaining low costs. Aluminium alloys and its use has been restricted due to low degree of strength and poor corrosion resistance. Now, booth properties can be enhanced by adding of appropriate amount of alloying elements. Transition metals improve thermal stability due to their low diffusivity in aluminium [5]. Figure 1 shows that transition metals like Fe and Cr are very slow diffusers in Al when compared to commonly used elements like Cu or Mg. However, its concentrations in casting and wrought aluminium alloys are limited.

2 Fig. 1: Arrhenius plots of diffusion coefficients of various metals [6] CA method may solve this problem and together with ultra-high pressure compaction allowing production of pore-free samples was used in this work. 2. EXPERIMENT AlSi14Fe8 and AlSi14Fe8Cr1 (in wt.%) alloys were prepared by induction melting of appropriate amount of master alloys (AlFe11, AlCr11), aluminium and silicon with at least 99.99% purity in a graphite crucible under argon atmosphere. Composition of prepared materials was confirmed by XRF (ARL 9400 XP) analysis. These materials were melted in an electric arc furnace at 1000 C and injected through graphite casting nozzle with diameter of 2 mm at high speed rotating (15000 rpm) graphite disc (Fig. 2). Injected melt was by centrifugal force immediately carried to the water-cooled walls ensuring almost immediately solidification of melt forming a fine flake-like product. The operating parameters of centrifugal atomizer were determined in previous work by using AlSi9Cu3 alloy like testing material [7]. Fig. 2: Schematic drawing of centrifugal atomization Fig. 3: Typical morphology of prepared powders Prepared powders were than subjected to granulometric analysis in order to determine the distribution of powder particle size. In this way, the fractions in the range of powders sizes of 2.8 mm and smaller were obtained. Powders were then uniaxially hot-pressed (450 C for 1h) by ultra-high pressure of 6 GPa into small cylindrical specimens with 28 mm in diameter and approximately 10 mm in height. Pressure applied in our experiment was one order of magnitude greater than the common pressures used for pressing and HIP. In the following text, the alloy prepared by the procedure will be denoted as as-compacted. So prepared samples were long-term (100 h) thermally treated in air at temperatures ranged from 300 C to 500 C, than compressive tested by universal materials testing machine LabTest 5.200SP1 with strain speed of

3 1 mm/min. Thermal stability of tested materials was evaluated measuring Vickers hardness performed at room temperature with a loading of 5 kg. Conventional cast AlSi12Cu1Ni1Mg1 alloy, generally considered as thermal stable material for manufacturing pistons, cylinder heads and other parts of combustions engines, was used for comparison of mechanical properties and thermal stability. Metallographic samples of materials were prepared by subsequently grinding on P60-P4000 abrasive papers, polished with diamond paste and etched in solution containing HF, HNO 3 and HCl acid. Prepared samples were observed using Olympus PME-3 light microscope (LM) and TESCAN VEGA 3 LMU scanning electron microscope (SEM). Elements distribution in each sample was determined by X-ray elemental map and point analysis performed at above mentioned SEM with EDS analyzer. 3. RESULTS 3.1 Microstructure Microstructure of AlSi14Fe8 (Fig. 4a) alloy prepared by uniaxially hot-pressing was consisted of α-al, fine eutectic α-al+si and needle-like phase enriched of Fe and Si. Phase composition of AlSi14Fe8Cr1 (Fig. 4b) was practically identical except presence of spherical phases. It is also important to notice, that the ascompacted materials showed almost no porosity and very good contact between particles. Fig. 4: Microstructure of: a) AlSi14Fe8, b) AlSi14Fe8Cr1 as-prepared (6 GPa, 450 C, 1h), (SEM) Microstructure of materials subjected to annealing at various temperatures (Fig. 5a, b) showed almost no change. Enhanced mechanical properties of as-prepared materials can be thus attributed to the fine microstructure and high internal stress in Al lattice obtained during ultra-high pressure compaction. Fig. 5: Microstructure of: a) AlSi14Fe8; b) AlSi14Fe8Cr1 after annealing (500 C/100h), (SEM)

4 Fig. 6: X-ray elemental map of annealed (500 C/100h) compact AlSi14Fe8, (SEM) After annealing (Fig. 6, 7), coarsening of eutectic silicon particles which precipitates from the supersaturated Al matrix at temperatures ranged of K was observed [8]. Fig. 7: X-ray elemental map of annealed (500 C/100h) compact AlSi14Fe8Cr1, (SEM) It s coarsening can be explained by both a tendency to reduce the α(al)/si interface area and a relatively high diffusion coefficient of silicon in solid Al (Fig. 1). Iron and chromium are characterized by a lower diffusion coefficient than silicon in aluminium, and therefore coarsening of particles containing these elements even after 100 hours of annealing was not observed. 3.2 Thermal stability Thermal stability was expressed like hardness change of tested material depending on time of annealing. As is shown on micrographs (Fig. 6), representing change of Vickers hardness, tested materials (AlSi14Fe8 and AlSi14Fe8Cr1) exhibited better results at each temperature in comparison to AlSi12Cu1Ni1Mg1. It should be noticed, that those temperatures considerably exceed typical applications temperatures of most aluminium components.

5 Fig. 6: Thermal stability of tested materials compared to AlSi12Cu1Ni1Mg1 Annealing up to approximately 25 h leads to a progressive reduction of hardness, which remain almost constant up to 100h of annealing. Even after 100 hours of annealing at 500 C exhibited materials prepared via CA method better hardness results in comparison to AlSi12Cu1Ni1Mg1 annealed at 300 C. Decrease of hardness can be attributed to the relaxation of internal strain in Al lattice. Fig. 7: Compressive stress-strain diagrams of as-prepared materials Figure 7 shows that as-compacted materials achieved excellent compressive and yield strength which confirmed the excellent diffusion bonding between powder particles due to high pressure used to compaction. Fig. 8: Compressive stress-strain diagrams of annealed materials compared to AlSi12Cu1Ni1Mg1 (300 C, 100h)

6 Both the hardness and the compressive strength of as-compacted materials decrease during annealing at C, but these decrease occur much more slowly than in the case of the casting AlSi12Cu1Ni1Mg1 alloy (Fig. 6, 7, 8). As a result of annealing, increased plastic deformation of both materials (Fig. 8), probably caused by decrease of stress in the matrix due to depletion of alloying elements and by silicon coarsening, was observed. 4. CONCLUSION It has been showed in this work that materials, when prepared by a combination of CA method and highpressure compaction, had fine microstructure, which positive impact on strength and plasticity. Rapidly solidified materials prepared via hot-pressing showed superior mechanical properties compared to conventionally cast AlSi12Cu1Ni1Mg1 alloy commonly used for the production of combustion engine parts. Centrifugal atomization is therefore prospective cost-efficient method for preparation of alloys containing high amount of transition metals. ACKNOWLEDGEMENTS Authors wish to thank to the Czech Science Foundation (project no. P108/12/G043) for its financial support. REFERENCES [1] LAVERNIA, E., SRIVATSAN, T. The rapid solidification processing of materials: science, principles, technology, advances and applications. Journal of Materials Science, 2010, vol. 45, p [2] KARAKOSE, E., MUSTAFA, K. Structural investigations of mechanical properties of Al based rapidly solidified alloys. Materials and Design, 2011, vol. 32, p [3] BARTOVA, B., VOJTECH, D. et. al. Structure and properties of rapidly solidified Al-Cr-Fe-Ti-Si powder alloys. Journal of Alloys and Compounds, 2005, vol. 387, p [4] NAYAK, S., CHANG, H. et. al. Formation of metastable phases and nanocomposite structures in rapidly solidified Al-Fe alloys. Materials Science and Engineering A, 2011, vol. 528, p [5] KATGERMAN, L., DOM, F. Rapidly solidified aluminium alloys by meltspinning. Materials Science and Engineering A, 2004, vol , p [6] DU, Y., CHANG, Y. Et. Al. Diffusion coefficients of some solutes in fcc and liquid Al: critical evaluation and correlation. Materials Science and Engineering A, 2003, vol. 363, p [7] PRUSA, F. et al. Optimization of centrifugical atomizer parameters for rapid solidification of aluminium alloys, Sborník z konference Metal 2010, Ostrava: TANGER, 2010, 6s. [8] BIROL, Y. Microstructural evolution during annealing of rapidly solidified Al-12Si alloy. Journal of Alloys and Compounds, 2007, vol. 439, p