MECHANICAL PROPERTIES AND THERMAL STABILITY OF ALSI-X BASED ALLOYS PREPARED BY CENTRIFUGAL ATOMIZATION. Filip PRŮŠA*, Dalibor VOJTĚCH

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1 MECHANICAL PROPERTIES AND THERMAL STABILITY OF ALSI-X BASED 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 based alloys are well known for their low density and convenient mechanical properties filling the imaginary gap between titanium alloys and common aluminium alloys. Aluminium alloys prepared via centrifugal atomization technique (CA) can contain higher amount of transition metals (Fe, Cr, Mn), characterized by low diffusivity in solid aluminium, enhancing their properties, especially thermal stability. In this work, AlSi23Fe8Cr1 and AlSi23Fe8Mn5 (in wt. %) alloys were prepared via centrifugal atomization. Prepared powders were compactized by uni-axial pressing at 6 GPa and 450 C for 60 min. The microstructure of prepared powders and compact samples was studied by light (LM) and electron scanning microscopy (SEM). Phase composition was determined by X-ray diffraction (XRD). The thermal stability of the alloys was studied by mechanical testing after long-term annealing at C and by creep testing at 300 C and compressive stress of 250 MPa. Obtained results were compared to the "thermally stable" casting AlSi12Cu1Mg1Ni1 alloy that is commonly utilized in the automotive industry for engine parts manufacturing. Keywords: aluminium alloys, thermal stability, creep, centrifugal atomization, microstructure. 1. INTRODUCTION Al-Si based alloys are well known for their good strength-to-weight ratio, high thermal conductivity and excellent castability. These materials are usually used for fabrication of various lightweight components such as pistons or whole engine blocks. For more wide spread use, there is a necessity to improve their thermal stability that can be enhanced by addition of appropriate amount of transition metals (TM) such as Fe, Cr and Mn. These elements are known to be slow diffusers in Al [1-3]. However, their concentrations are limited to only a few percent due to their low solubility in Al, otherwise a relative excessive fractions of hard and brittle intermetallic phases are produced. Presence of these brittle particles is generally unwanted and almost always negatively influences mechanical properties. Increasing production of aluminium alloys offer every year many tons of aluminium scraps that needs to be recycled. Typically, aluminium scraps contain high fractions of iron that needs to be removed e.q. by magnetic separation or by melt dilution by pure Al. All of these steps increase the cost of recycled aluminium. Another option how to partially reduce the negative influence of Fe on Al-Si based alloys can be found in addition of Mn. Centrifugal atomization, as one of many rapid solidification techniques, offers way how to process aluminium wastes with higher concentrations of above mentioned elements. Centrifugal force used in this type of device allows to produce flake-like powders that can be easy compacted into bulk products. Almost immediate contact of molten metal with water-cooled walls during rapid solidification refines the structure, reduces the volume fractions of intermetallic phases and forms nano-, quasi-crystalline and/or amorphous structures [4-5]. All of these features are beneficial to achieve required combinations of strength, ductility and thermal stability. Further processing of prepared powders can be done by simply compression and sintering or by HIP (hot isostatic pressure) that operates at moderate pressures of approximatelly 100 MPa, that are due to

2 the formation of oxide layers on aluminium powders, preventing diffusion bonding, not suitable. Solution can be found in compaction at high pressures, typically greater than 1 GPa, that can produce sufficient compact and pore-free materials [6]. In this work, we investigated the influence of ultra-high pressure applied during compaction on mechanical properties of rapidly solidified aluminium alloys containing high concentrations of TM's. Additions of Cr and Mn were chosen on the basis to modify the morphology of intermetallic phases and their ability to increase the thermal stability. For comparison, Al12SiCu1Mg1Ni1 alloy, that is generally considered to be thermally stable, was studied as reference material. This alloy is foremost utilized for engine parts manufacturing. 2. EXPERIMENT In this work, two alloys with nominal chemical compositions of AlSi23Fe8Cr1 and AlSI23Fe8Mn5 (in wt.%) were studied. These alloys were prepared by melting the pure elements and master alloys in a vacuum induction furnace under argon protective atmosphere. After a sufficient homogenization, the melt was poured into a cast iron mould with diameter of 20 mm and 150 mm in length. Composition of so prepared materials was confirmed by X-ray fluorescence analysis (XRF, ARL 9400 XP). Ingots were than remelted under argon atmosphere and ejected through graphite casting nozzle onto a high speed rotating (30000 rpm) graphite disc (Fig. 1). The product, flake-like particles, were sieved to obtain fractions with dimensions ranging from 0,1 to 2 mm with a thickness of approximately 50 µm (Fig. 2). Prepared powders were then placed into a tungsten carbide mould and compactized by uni-axial pressing at 450 C by an ultra-high pressure of 6 GPa for 60 min to prepare compact samples. The casting Al12SiCu1Mg1Ni1 alloy, used in this work as a reference material, was provided by external supplier in the form of an ingot. Fig. 1 Schematic drawing of centrifugal atomizer Fig. 2 AlSi23Fe8Mn5 flake-like morphology The microstructures of prepared alloys were examined by light microscope (LM, Olympus PME-3), scanning electron microscope (SEM, TESCAN VEGA 3-LMU) and energy dispersive spectrometry (EDS, Oxford Instruments Inca 350). Chemical and phase composition was confirmed by XRF and X-ray diffraction analysis (XRD, PANalytical X Pert PRO), respectively. Mechanical properties, represented by room temperature Vickers hardness measurements with 5 kg load and by compressive stress-strain tests performed on LabTest 5.250SP1-VM machine at a deformation speed of 1mm/min, were examined. Thermal stability of tested materials was determined by hardness change and by compressive tests after long-term annealing at temperatures ranging from C. To gain full image about the thermal stability of investigated materials, creep tests were done at the temperature of 300 C with compressive load of 250 MPa.

3 3. RESULTS 3.1 Microstructure The microstructure of slowly solidified AlSi23Fe8Cr1 alloy (Fig. 3a) was composed of three structural components: Si particles (dark), α-al+si eutectic mixture and dendritic intermetallic phases (light grey). X-ray maps performed on SEM with EDS analyzer confirmed presence of Fe, Si and Cr in sharp-edged dendritic intermetallic phases. This phase was identified by point and X-ray diffraction analysis as β-al 5 SiFe, phase that has in aluminium alloys its typical needle-like morphology, therefore, it is possible to presume that dissolved chromium can modify the morphology from needle-like to dendritic [7]. The microstructure of slowly solidified AlSi23Fe8Mn5 alloy (Fig. 3b) was almost identical to the previous one, consisted of Si particles, α-al+si eutectic mixture and dendritic intermetallic phases containing Fe, Si and Mn. Point analysis performed on SEM with EDS analyzer and X-ray diffraction analysis indentified this phase as α-alfemnsi phase, generally with dendritic and/or script like morphology. The chemical compositions of this phase varied depending on the actual Fe, Mn and Si concentrations. The X-ray analysis of slowly solidified alloy denoted this non-stoichiometric phase by chemical formula of Al 17 (Fe 3,2 Mn 0,8 )Si 2. The casting Al12SiCu1Mg1Ni1 alloy (Fig. 3c), generally considered to be thermally stable material, used in this work as a reference material, was composed of α(al) dendrites, α-al+si eutectic mixture and Mg 2 Si, Al 3 Ni and Al 6 Cu 3 Ni intermetallic phases. Fig. 3 Microstructures of slowly solidified: a) AlSi23Fe8Cr1, b) AlSi23Fe8Mn5, c) AlSi12Cu1Mg1Ni1 After compaction, significant structural refinement, probably caused by rapid solidification during the powder preparation, was observed (Fig. 4). The materials showed almost no porosity with good particle-to-particle contact that was obviously caused by ultra high pressure of 6 GPa. The fine intermetallic phases and silicon particles were homogenously dispersed in the α-al matrix forming composite-like structures. This morphology predetermined these materials to exhibit good mechanical properties such as high strength and acceptable plasticity as will be shown on next pages. Fig. 4 Microstructure after compaction: a) AlSi23Fe8Cr1, b) AlSi23Fe8Mn5

4 After annealing at different temperatures, Si particles coarsening was observed in both materials (Fig. 5). The silicon particles grew quickly from original sub-micrometre size to several micrometres. Fig. 5 SEM element distribution maps of: a) AlSi23Fe8Cr1 as compacted, b) annealed at 400 C/100h, c) AlSi23Fe8Mn5 as compacted, d) annealed at 400 C/100h This phenomenon can be explained by both the tendency to reduce the α-al/si interface area and by relatively high diffusion coefficient of Si in solid Al. On the other hand, Fe, Cr and Mn have much lower diffusion coefficients and therefore, coarsening of intermetallic phases during annealing was much more slower. 3.2 Mechanical properties, thermal stability The thermal stability, expressed as hardness change during annealing, showed significantly higher initial hardness of materials prepared by centrifugal atomization than the casting counterpart. After first five hours of annealing, significant hardness decrease was observed, and then, retained almost constant for the rest of the test (Fig. 6). This decrease, when compared to the casting counterpart, occured much more slowly in the centrifugally atomized alloys. The best results were achieved in the case of AlSi23Fe8Mn5 material.

5 Fig. 6 Hardness decrease during annealing at various temperatures (300 and 400 C) The compressive stress-strain tests performed after long-term annealing revealed better mechanical properties such as strength and yield strength (both compressive) on the site of centrifugally atomized alloys (Fig. 7). Among the tested materials, AlSi23Fe8Mn5 exhibited the best results followed by the AlSi23Fe8Cr1 and the casting AlSi12Cu1Mg1Ni1 alloys. Fig. 7 Compressive stress-strain curves after annealing at different temperatures To complete the thermal stability investigations, creep tests at a temperature of 300 C and compressive stress of 250 MPa were performed (Fig. 8). Creep curves of tested materials contained only the primary creep stage, characterized by a high initial creep rate then a progressive decrease of the creep rate. The main difference between materials was in compressive strains achieved before the creep rate drops to zero. On the basis of the total compressive strain decrease, the casting AlSI12Cu1Mg1Ni1 alloy (70 % of total compressive strain) showed the worst creep resistance among tested materials. The AlSi23Fe8Mn5 alloy exhibited the best creep resistance with total compressive strain of 35 %. Fig. 8 Compressive creep curves of investigated materials (300 C/250MPa) The best mechanical properties and thermal stability observed in the case of rapidly solidified alloys can be attributed to following explanations. The lower diffusivity of Fe, Cr and Mn in solid Al influenced the microstructure coarsening and therefore, intermetallic phases containing these elements coarsed much slower. The presence of hard particles effectively hindered the growth of α-al grains positively affecting the mechanical properties.

6 4. CONCLUSION Alloys containing higher concetrations of transition elements can be successfully prepared by centrifugal atomization as one of many rapid solidification techniques. The AlSi23Fe8Mn5 alloy studied in this work shoved the best results among tested materials, exceeding the mechanical properties of casting AlSI12Cu1Mg1Ni1 alloy in every way. Microstructures were refined with large volume fractions of homogenously dispersed intermetallic particles, identified as β-al 5 SiFe and α-alfemnsi phases in the AlSi23Fe8Cr1 and AlSi23Fe8Mn5 alloy, respectively. Ultra-high pressure of 6 GPa used for compaction produced materials with good particle-to-particle contact showing almost no porosity. The presence of homogeneously distributed thermally stable intermetallic particles, containing elements such as Fe, Cr and Mn, positively improved the thermal stability of centrifugally atomized materials. ACKNOWLEDGEMENTS The authors wish to thank the Czech Science Foundation (project no. P108/12/G043) and to Specific University Research (MSMT No. 20/2013) for their financial support. REFERENCES [1] 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 [2] CAVOJSKY, M., BALOG, M. Et. Al. Microstructure and properties of extruded rapidly solidified AlCr 4,7Fe 1,1Si 0,3 (at. %) alloys. Materials Science and Engineering: A, 2012, vol. 549, p [3] KATGERMAN, L., DOM, F. Rapidly solidified aluminium alloys by meltspinning. Materials Science and Engineering: A, 2004, vol , p [4] 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 [5] 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 [6] VOJTECH, D., MICHALCOVA, A. Et. Al. Properties of the thermally stable Al 95Cr 3,1Fe 1,1Ti 0,8 alloy prepared by cold-compression at ultra-high pressure and by hot-extrusion. Materials Characterization, 2012, vol. 66, p [7] CAI, Y., LIANG, R., Et. Al. Effect of Cr and Mn on the microstructure of spray-formed Al-25Si-5Fe-3Cu alloy, Materials Science and Engineering: A, 2011, vol. 528, p