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Effect of Silicon Oxide Sio2 Reinforced Particles on Ageing Behavior of Al 2024 Alloy

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Effect of Silicon Oxide Sio2 Reinforced Particles on Ageing Behavior of Al 2024 Alloy
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  Proceedings of the 2 nd  International Conference on Current Trends in Engineering and Management ICCTEM -2014 17 – 19, July 2014, Mysore, Karnataka, India   15   EFFECT OF SILICON OXIDE (SIO 2 ) REINFORCED PARTICLES ON AGEING BEHAVIOR OF Al-2024 ALLOY Gowri Shankar M.C 1 , Jayashree P.K 2 , Dr. U.AchuthaKini 3 , Dr. S.S Sharma 4   1, 2, 3, 4 (Mechanical& Manufacturing Engineering Dept, Manipal Institute of Technology, Manipal, Karnataka, India) ABSTRACT Al-2024 alloy can be reinforced with different percentages of Silicon oxide particles using a stir casting method. The solution treatment of the composite sample and the unreinforced alloy was carried out at 550 ᴼ C for 2h followed by aging at 150 ᴼ C for various aging times between 1h and 5 hrs. The existence of SiO 2 particles led to increasing the peak hardness of the alloy. The results revealed that peak hardness of the composite sample took place at shorter times than that of the unreinforced alloy for the samples solution treated for 2 h for both the composite and the unreinforced alloy that led to the fastest aging kinetics and the maximum hardness. Matrix microhardness, tensile strength and wear test measurements were carried out after each step of heat treatment. Keywords: Age-hardening, Al2024, Silicon Oxide (SiO 2 ), Stir casting,  1.   INTRODUCTION In recent years, Aluminium alloys have attracted attention of many researchers, engineers and designers as a promising structural material in different industries like aerospace and automotive. Special 2xxx series of Al alloys have been studied extensively because of their high strength to weight ratio, good formability, age hardenability and other appropriate properties. Among Al alloys, 2024 Al has the highest hardness [1].But some of the mechanical properties such as low wear resistance; have limited application of these materials. Adding SiC reinforcing particles to these materials leads to increase the wear resistance [2]. Aging treatment can significantly increase properties of some of Al alloys and their composites, especially 2xxx and 6xxx series alloys. In investigation of age-hardening kinetics, it has been shown that the addition of ceramic particles to age-hardenable Al alloys has different effects on precipitation of composite compared with unreinforced alloy[3–4]. Considering strict importance and widespread applications of these materials, in this research the effect of heat treatment conditions and also Silicon oxide (SiO 2 ) reinforcing particles on the artificial aging kinetics and behavior of a 2024 Al- SiO 2 composite produced by stir casting technique was studied. Lot of research work were conducted to synthesize aluminium composites especially with wide variety of reinforcement materials. These works has been systematically presented in the chronological order of the work progress. Nieh and Korluk examined the changes in hardness resulting from ageing treatment in a 6061 aluminium alloy reinforced with 23 vol% B 4 C particulate. The peak ageing time for the matrix of the composite was about 3 h, whereas the un-reinforced matrix material exhibited a peak hardness value only after 10 h of ageing at the same ageing temperature of 450 K. The accelerated ageing phenomenon was hypothesized to be a consequence of the short circuit path for pipe diffusion of solutes which is induced by the high dislocation densities and interracial stresses resulting from the reinforcement. [5]. Cottu et al. [6] showed that age-hardening kinetics of Al–Cu–Mg alloy-10 wt.% SiC fiber composite was enhanced bythe presence of the reinforcement during heat treatment. They explained this by the plastic deformation   INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 5, Issue 9, September (2014), pp. 15-21 © IAEME: www.iaeme.com/IJMET.asp Journal Impact Factor (2014): 7.5377 (Calculated by GISI) www.jifactor.com   IJMET   © I A E M E    Proceedings of the 2 nd  International Conference on Current Trends in Engineering and Management ICCTEM -2014 17 – 19, July 2014, Mysore, Karnataka, India   16   induced during heat treatment due to the difference between coefficients of thermal expansion (CTE) of matrix and reinforcement. Considering strict importance and widespread applications of these materials, in this research the effect of heat treatment conditions and also Silicon oxide (SiO 2 ) reinforcing particles on the artificial aging kinetics and behavior of a 2024 Al- SiO 2 composite produced by stir casting technique was studied. 2. EXPERIMENTAL PROCEDURE The properties of materials adopted and methods followed for the fabrication and testing of MMCs in the present studies are presented in the following sections.  2.1 Aluminium Alloy Matrix The matrix for the present studies selected was Al2024 alloy and were procured from Fenfee Metallurgicals, Bangalore, in the form of ingots. The chemical composition of Al2024 alloy is given in Table 1. It is extensively used in applications like aircraft structural components, aircraft fittings, hardware, truck wheels and parts for the transportation industry. This alloy is reinforced by particles of SiO 2  (3 and 6 wt.% ) with an average diameter of 150 µ m. Table 1:  Chemical composition of AA2024  2.2   Preparation of Composites The Al2024-SiO 2 composites were prepared by the vortex method [7]. The SiO 2 contents used for the preparation of the composites were 0%, 3%, and 6%. This is because SiO 2 compositions of 9% and above would lead to rejection from the melt. Addition of SiO 2 reinforcements into the molten aluminium alloy melt above its liquidus temperature of 700 °C was carried out by creating a vortex in the melt using a mechanical stainless steel stirrer coated with aluminite (to prevent migration of ferrous ions from the stirrer material into the aluminium alloy melt). The melt was rotated at a speed of 500 rpm in order to create the necessary vortex. The SiO 2 particles were preheated to 200 °C to improve wettability and added to the melt through the vortex. The SiO 2 reinforcement particles were added, and the melt was thoroughly stirred and subsequently degassed by passing hexachloroethane (C 2 Cl 6 ) solid degasser. The molten metal was then poured into permanent moulds for casting. The preheated particles were introduced into the vortex and stirred for eight minutes at a speed of 500 rpm. A pouring temperature of 720 °C was adopted and the molten composite was poured into prepared moulds. The cast aluminium- SiO 2 reinforced composite specimens were obtained after solidification in air for about 2 hours. The solidified stir cast composite are shown in Fig.1. The microstructure of the composite after casting was examined to study the effect of reinforcements particles. Photographs were taken for a polished section of each sample. Figure. 1:  As cast specimens solidified in the crucible  Proceedings of the 2 nd  International Conference on Current Trends in Engineering and Management ICCTEM -2014 17 – 19, July 2014, Mysore, Karnataka, India   17   2.3 Hardness Test Vickers Hardness test were conducted and average of at least six indentations are taken at room temperature. Hardness measurements were performed using a Vickers hardness tester (MATZUSAWA MICRO VICKERS HARDNESS TESTER, MODEL-MMT X 7A) with a load of 100 gmf and dwell time of 15 seconds was given. The cylindrical specimens of 12mm diameter and length 15mm were used for the test. Before testing the specimen surface was polished in metcobainpol polishing machine using diamond paste abrasives. 2.4 Tensile Test The tensile strength tests were performed on Electronic tensometer (model-ER3) as per ASTM-E8M standard for as-cast, as quenched and peak hardened condition. Circular cross section specimen was prepared with diameter 6 mm and gauge length 24mm. The load cell value was kept to 20.5 kN and test mode was selected as break. The cross head speed was kept constant at 12 mm/ min, with length increment value of 0.01 mm. 2.5 Wear Test The Wear resistance tests were performed on pin-on-disc tribo-meter under dry sliding conditions (WEAR AND FRICTION MONITOR TR- 201CL). The test were conducted on 8 mm diameter, 25 mm long cylindrical specimens (ASTM G-99) against a rotating EN-32 steel disc (count face) having hardness 63Rc. Care should be taken to note that the test sample’s end surfaces were flat and polished metallo-graphically prior to testing. Conventional aluminium alloy polishing techniques were used to get ready the contact surfaces of the monolithic aluminium specimen for wear test. The procedure involves grinding of aluminium surfaces manually by 240, 320, 400, and 600 grit silicon carbide papers and Fine polishing was done in a rotating disc polishing machine with velvet cloth impregnated with diamond paste. The track diameter was kept constant at 60mm. The test was conducted for three different loads (10 N, 20 N and 30 N) with a constant speed of 300 rpm. Initial weight of the sample was noted down. For a particular load at a particular speed the test was run for 1 hr. At an interval of 15 minutes the weight loss was noted by weighing the sample. 2.6 Precipitation hardening/ Age hardening treatment An aging study was conducted on specimens taken from the matrix alloy and composites in order to determine the time required to attain peak hardness. The aging process essentially involves a solution treatment of the sample for 2 h at 550 ᴼ C, then quenching in water followed by artificial aging at 150 ᴼ C for various aging times between 1hr to 5 hrs. 3. RESULTS AND DISCUSSION In the following sections, the Microstructure structural analysis, Hardness, Tensile strength, Wear resistance strength in both untreated and heat treated behavior of the materials examined are presented. 3.1   Microstructure Structural Features Fig.2, shows the optical micrographs of Al2024 alloy reinforced with 0 %, 3% and 6 wt.% of SiO 2 composites. Micrographs reveal that there is fairly uniform distribution of SiO 2 particulates throughout the matrix alloy and the porosity is lower. It is reported that higher hardness is always associated with lower porosity of the MMCs [8]. Also, it can be observed that there is good bonding between the matrix and the reinforcement particulates resulting in better load transfer from the matrix to reinforcement material. Figure 2:  Microstructure of Al2024 alloy with different percentage of SiO 2  (a) 0 wt.%; (b) 3 wt.%; (c) 6 wt.% reinforced composite (Magnification 100X)  Proceedings of the 2 nd  International Conference on Current Trends in Engineering and Management ICCTEM -2014 17 – 19, July 2014, Mysore, Karnataka, India   18   3.2Vickers hardness test The resistance to indentation or scratch is termed as hardness. It is a function of the stress required to produce some specific types of surface deformation. Figure 3 is a graph showing the effect of SiO 2 reinforcement on the hardness of cast Al2024-SiO 2 particulate composites. Each value represented is an average of six measurements shown in Table 2. For the 6% Wt. composition, there was an increase in the hardness value until the peak hardness was achieved after 2 hours. For the 3% Wt. composition the peak hardness was achieved after 2 hrs 30 mins. But the peak hardness of unreinforced Al2024 alloy achieved after 3 hrs 45 mins. The hardness values decreased after the maximum limit was achieved. The hardness of the composite sample and the unreinforced alloy that both were solution treated for 2 hrs with no aging treatment, decreased due to more dissolution of the precipitates and will accelerate aging kinetics. Table 2:  Hardness of composites (Before and after age hardening) Figure 3:  Variation of hardness values with time (peak hardened condition) One can notice that an increase in the hardness of the composite materials occurred after the aging treatment as shown in Figure 3. However, it is of interest to note that peak hardness was observed at lower aging times for the reinforced composites compared to the Aluminium base alloy (3 hrs 45 min for the Al alloy and 2 hrsfor the composite). These results indicate that the addition of reinforcement to the aluminum matrix accelerates the aging kinetic. This behavior can be related to the high matrix dislocation density induced by the mismatch between the matrix and the reinforcement [9-10]. It is well known that high dislocation density in the metal matrix promotes dislocation-assisted diffusion of the aging elements. On the other hand, the influence of the reinforcement on the aging behavior can be attributed to the heterogeneous nucleation capability of metastable phases on the SiO 2  particles. It can, therefore, be concluded that both nucleation and growth of the Guinier-Preston zones (GP zones) are influenced by the ceramic particles. 3.3Tensile test Figure 4 is a graph showing the effect of reinforcement content on the Ultimate Tensile Strength (UTS) of cast Al2024, Al2024- SiO 2 under various conditions. It can be seen that as the SiO 2  weight percentage increased from 3% to 6 %, the UTS of the composite material increases monotonically by significant amounts. In fact, as the SiO 2 content is increased from 0% to 6%, the UTS increases by about 20 to 40% as shown in Table 3. This increase in UTS may be due to the reinforcement particulates acting as barriers to dislocations in the microstructure [10]. Thermal mismatch between the metal matrix and the reinforcement is a major mechanism for increasing the dislocation density of the matrix and, therefore, increasing the composite strength. Type of specimen VHN (Vickers Hardness Number) pre-heat treatment As quenched (Solution treated) Peak hardened Al2024+0 % Wt. 136.00 123.05 176.10 (3 hrs. 45 mins) AA2024+3 % Wt. 140.85 130.43 184.46 (2 hrs 30 mins) AA2024+6 % Wt. 145.75 134.37 191.2.4 (2 hrs)
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