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Tensile Fractography of Artificially Aged Al6061-B4c Composites

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Tensile Fractography of Artificially Aged Al6061-B4c Composites Proceedings of 3rd International Conference on Mechanical and Aeronautical Engineering Held on 18th- 19th January 2017, in Bangkok, Thailand. ISBN: 9788193137390 TENSILE FRACTOGRAPHY OF ARTIFICIALLY AGED AL6061-B4C COMPOSITES Gowri Shankar Achutha Kini Department of Mechanical and Manufacturing Department of Mechanical and Manufacturing Engineering, Manipal UniversityManipal Engineering, Manipal UniversityManipal Institute of Technology,Manipal, Institute of Technology,Manipal Karnataka State, India Karnataka State, India S S Sharma Department of Mechanical and Manufacturing Engineering, Manipal UniversityManipal Institute of Technology,Manipal Karnataka State, India Abstract— Presence of B4C reinforced particles in I. INTRODUCTION Al6061 alloy and various weight percentages of (2, Aluminum- magnesium-silicon alloys (Al-Mg-Si; also 4 and 6) on the hardness and tensile behaviour is denoted as 6xxx series), are medium strength, heat studied in the present work. Under different aging treatable alloys with good formability and corrosion temperatures the influence of artificial aging on the resistance. The 6061Al alloy has been studied mechanical properties was also assessed. Brinell extensively because of their technological importance macro hardness and mechanism of tensile fracture and farther increase in strength obtained by behavior have been discussed. Failure mode of precipitation hardening [1]. Aluminium matrix fracture surface is studied to determine the composites reinforced with ceramic particles such as parameters which influence the crack growth SiC, B4C, Al2O3 and TiC are the most commonly used characteristics. Lower temperature aging and materials in automobile and marine industries, have addition of B4C particles shows improvement in been paid more consideration because of high strength, hardness by 170% and tensile strength by 90% due high modulus and low density. The aluminium MMCs to the precipitation of finer secondary solute rich reinforced with B4C particulates are harder, tougher, phases of alloying elements. Fracture surface more fracture resistant, lighter in weight and possess analysis for composite shows mixed mode fracture higher fatigue strength and reveal substantial and void nucleation growth failure. improvements in properties over other materials[2-5]. Keywords—Metal Matrix Composites (AMMC’s), In the metal matrix composites, the presence of Boron carbide, Stir casting, Aging, Microstructure reinforcement particles in aluminium alloy accelerating the aging process and thus attain to higher 1 Proceedings of 3rd International Conference on Mechanical and Aeronautical Engineering Held on 18th- 19th January 2017, in Bangkok, Thailand. ISBN: 9788193137390 strength. This leads to more nucleation sites for the Carbide India limited, Mumbai. The reinforcement fine precipitates. [6-9]. materials are having irregular shape. Scanning According to the literature, the aging sequence of Electron Microscope (SEM) and X-Ray Diffraction several researchers, the variation of the hardness (XRD) plot of the same shown in Fig.1 confirms the versus aging temperature and time can be correlated to presence of the B4C particles. Al6061- B4C the phase transformations during aging treatment. The composites were fabricated by two stage stir casting increase in the mechanical properties during aging is technique by altering the amount of boron carbide due to the vacancy assisted diffusion mechanism in particles in the range 2-6 % wt. under aged and peak aged conditions [10-13]. The formation of GP zones depends upon the aging temperature, which distorts the matrix lattice planes [14]. This distortion of the lattice planes hinders the dislocation movement as long as the coherency exists in the lattice [15-16]. Several works have been Fig.1(a) SEM micrographs of boron carbide particles reported on the use of two- step (double) stir casting as and (b) XRD plots of B4C particles. a means of improving cast metallic matrix [5]. All Hardness tests were carried out in a Brinell hardness these studies brought out differences in testing machine with steel ball indenter of diameter microstructures of aluminium metal matrix composites 5mm and a load of 250 kgf (SAROJ Brinell Hardness (AMCs) produced through different routes such as, Testing Machine, Model:-B/3000/00, Sl# 13/06/08- (a.) direct casting (no stirring), (b.) manual stirring, India). Tensile properties dictate how the material will and (c.) two step mixing. It was evident from these react to forces being applied in tension. Tensile reported literatures that the two step mixing gives the specimen is prepared according to ASTM-E8M best uniform distribution of the SiC particulates. It was standards. The specimen prepared for above test is also suggested that production of AMCs without the subjected to age hardening heat treatment. Specimens use of two step stirring results in less dispersion of the are soaked at 558°C for duration of 2h, then particulates and higher porosity levels which might be immediately quenched in water at room temperature. in excess of the acceptable limits. Thus the present The quenched specimens were artificially aged in the work emphases on investigation of the effect of furnace at 100, 150 and 200°C for various durations of reinforcement and artificial aging on microstructure time. According to the Al-Mg-Si phase diagram and fracture behavior of Al6061-B4C composites. melting of ternary eutectic Mg2Si-(Al)-(Mg) phase II. MATERIALS AND METHODS takes place at 558ᴼC. The presence of Mg2Si is a The base matrix chosen in the present study is the strengthening intermetallic phase and dissolves aluminium 6061 (0.52% Si, 0.95% Mg, 0.55% Fe, completely at 558°C during solutionizing and 0.24% Cu, 0.14% Mn and 0.25% Cr) because it is one precipitates during age hardening to maximize the of the most extensively used 6000 series aluminium strengthening effect. These secondary precipitated alloys. The boron carbide reinforcement particles used phases result in particle strengthening and coherency for preparation of composite is brought from Boron of the crystal structure of the particle and the matrix. It 2 Proceedings of 3rd International Conference on Mechanical and Aeronautical Engineering Held on 18th- 19th January 2017, in Bangkok, Thailand. ISBN: 9788193137390 is reported that the samples of Al6061 composite, with in weight percentage of B4C in the composites gives the solution heat-treated at 558°C, exhibit better positive effect on hardness value. Lower aging strength [9]. temperature shows increase in hardness of base alloy III. RESULTS AND DISCUSSION as well as composites as compared to higher A. HARDNESS MEASUREMENT temperature aging. Lower temperature aging Peak hardness values obtained in as cast and different contributes to the increased hardness by increasing the aging temperatures at 100, 150 and 200°C as shown in number of intermediate zones during precipitation, Table 1. Increase in hardness values were observed in increase in the number of finer inter-metallic’s & as cast condition with the addition of born carbide decreased interparticle distances. Higher the aging particles when compared to the unreinforced alloy. temperature, lower is the time required to attain peak The hardness value increases with increase in weight hardness [9]. From the above results it can be percentage of boron carbide particles. Increased concluded that heat treatment has a profound influence content of boron carbide particles (2, 4 & 6% wt.) in on the hardness of matrix alloy as well as composites. the matrix alloy leads to higher dislocation densities B. TENSILE STRENGTH during solidification due to the thermal mismatch of Tensile test is carried out on as cast and peak aged the matrix alloy and the reinforcement. The matrix specimens. The average value of the three readings in deforms plastically to accommodate the smaller as cast and peak aged conditions is tabulated in Table volume expansion of the reinforcement particles 2. There is a marginal increase in the ultimate tensile leading to increased dislocation density. Enhancement strength (UTS) with the addition of reinforcements in dislocation densities results in higher resistance to when compared to unreinforced alloy in as cast plastic deformation and responsible for additional condition. The reinforcement particles control the increase in hardness of composites [17]. mechanical properties of the composites due to the Similar to base alloy, the Al6061 matrix composite strong interface, which transfers and distributes the is very sensitive to age hardening irrespective of lower load from the matrix to the reinforcement exhibiting or higher aging temperature. It is evident that increased elastic modulus and strength. The UTS in composites exhibit accelerated rate of aging kinetics as composite or base alloy is very sensitive towards age compared to unreinforced matrix alloy. Aging kinetics hardening. There is a minimum of 60% additional gets accelerated in the composites with increase in wt. increase in the UTS by age hardening over untreated % of reinforcements. Aging is accelerated due to the specimen. From Table 2, it is clear that higher the presence of areas with a high concentration of weight percentage of reinforcement in the composite dislocation close to Al6061 matrix & B4C and lower the aging temperature better is the ultimate reinforcements interface. These high density locations tensile strength. The increase in strength is due to the provide heterogeneous nucleation sites for the combined effect
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