Synthesis and Characterisation of Al 7075 reinforced with SiC and B 4 C nano particles fabricated by ultrasonic cavitation method

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1 Journal of Scientific & Industrial Research Vol 74, May 2015, pp Synthesis and Characterisation of Al 7075 reinforced with SiC and B 4 C nano particles fabricated by ultrasonic cavitation method S Gopalakannan 1 * and T Senthilvelan 2 *1 Department of Mechanical Engineering, Adhiparasakthi Engineering College, Melmaruvathur, Tamil Nadu, India Department of Mechanical Engineering, Pondicherry Engineering College, Puducherry, India Received 30 July 2012; revised 24 December 2013; accepted 2 January 2015 The application of ultrasonic effects to disperse nano sized particles in molten metal has been studied. Based on this technique, nano-sized SiC and B 4 C particles reinforced with AL7075 aluminium nanocomposites were fabricated. XRD analysis reveals that uniform structure of nano composites was formed in the Al alloy. The microstructure of the nanocomposites was investigated by high resolution scanning electron microscope (SEM). Experimental results show a nearly uniform distribution and good dispersion of the SiC and B 4 C nanoparticles within the aluminium metal matrix, although some small clusters were found in the matrix. The mechanical properties like tensile strength and micro hardness of nanocomposites have been improved significantly compared to that of pure alloy. The interaction between SiC and B 4 C nanoparticles with aluminium matrix was investigated. Keywords: Al7075, Ultrasonic cavitation, SiC, B 4 C, nanocomposites Introduction Aluminum (Al) based metal matrix composites (MMCs) have been extensively studied as an attractive choice of automotive, aerospace and military applications due to their light-weight, high strength, stiffness and resistance to high temperature 1. Generally, the micro-ceramic particles are used to improve the yield and ultimate strength of the metal 2. However the ductility of the MMCs deteriorates with high ceramic particle concentration. Nanoparticle reinforcements can significantly enhance the mechanical strength of the matrix by more effectively promoting the particle hardening mechanisms than micron size particles 3. A fine and uniform dispersion of nanoparticles provides a good balance between the strength and inter-particle spacing effects to maximize the yield strength and creep resistance. It is expected that MMCs reinforced with ceramic nanoparticles (less than 100 nm), termed as metal matrix nanocomposites (MMNCs), can overcome those disadvantages associated with the conventional MMCs. The properties of MMNCs would get enhanced considerably even with lower volume fraction of nanoparticles and MMNCs could especially provide a significantly improved performance at elevated temperatures 4. Solidification processing such as stir *Author for correspondence gopalakannans@yahoo.com casting employs mechanical stirring to produce aluminium metal matrix composites which are reinforced by micro ceramic particles. A combination of good distribution and dispersion of micro particles can be achieved by optimizing the process parameters of mechanical stirring 5. However to produce MMNCs, it is extremely challenging for the conventional mechanical stirring method to distribute and disperse nanoparticles uniformly with the matrix metals because of higher specific surface area exist in nanoparticles. In order to achieve a uniform dispersion and distribution of nanoparticles in aluminium metal matrix composites, Lan et al. and Yang et al. developed a new technique that combined solidification processes with ultrasonic cavitation based dispersion of nanoparticles in metal melts 6. Traditional nanomanufacturing methods for nanocomposites, such as high energy ball milling, rapid solidification, electroplating, sputtering etc., cannot be applied for mass production and net shape fabrication of complex structural components. The ultrasonic cavitation process is the most economical of all available processes for production of MMNCs as well. Furthermore, it is able to sustain high productivity rates and very large size components can be fabricated. The fabrication cost of nanocomposites using ultrasonic cavitation method is about one-third to one-half with that of other competitive methods; and for high volume production it is projected that the costs will fall to one-

2 282 J SCI IND RES VOL 74 MAY 2015 tenth 7. There are technical challenges associated in the manufacturing of homogenous high density composites. To achieve optimum MMNC properties, the distribution of the reinforcement particles in the matrix alloy must be uniform with good bonding strength. The mechanical properties of MMNCs are controlled to a large extent by the structure and properties of the reinforcement metal interface. A stronger interface permits transfer and distribution of load from the matrix to the reinforcement, resulting in an increased elastic modulus and strength 6,7. Particles distribution in the matrix material during the melt stage of cavitation process depend strongly on the wave frequency, sonication time, heating temperature, viscosity of slurry, particle wetting, effectiveness of mixing and minimizing gas entrapment. The present investigation aims to determine the mechanical properties and to analyze the microstructure of MMNCs as Al 7075 matrix with 0.5 wt% SiC and 0.5 wt% B 4 C reinforcements. Further evaluation of aforementioned properties for its corresponding monolithic alloy has also been done to make comparison between them. It has been observed that tensile strength and hardness of MMNCs have shown significant improvement. Ultrasonic cavitation method Ultrasonic waves are the waves of frequency above khz and generated by mechanical vibrations of frequencies higher than 18 khz. When these waves propagate into liquid media, alternating compression and expansion cycles are produced. During the expansion cycle, high intensity ultrasonic waves make small bubbles grow in the liquid. When they attain a volume at which they can no longer absorb enough energy, they implode violently. This phenomenon is known as cavitation 7. The high intensity ultrasonic waves generate nonlinear effects in liquids, namely transient cavitation and acoustic streaming, which are mostly responsible for refining microstructures, degassing of liquid metals for reduced porosity and dispersive effects for homogenizing. Transient cavitation involves the formation, growth, pulsating and collapsing of micro-bubbles in liquids under cyclic high intensity ultrasonic waves 8. Experimental details Work material and ceramic reinforcements Aluminum 7075 (Al-Zn-Mg-Cu alloy) is used as the base matrix alloy. Its chemical composition (%) is Si = 0.2, Fe = 0.22, Cu = 2.0 max, Mn = 0.1, Mg = , Zn = , Ti = 0.1 max, Cr = 0.2, and the balance is aluminium. It is a very high strength material used for highly stressed structural parts. The applications of Al 7075 are Aircraft fittings, gears and shafts, fuse parts, meter shafts and gears, missile parts, regulating valve parts, worm gears, keys, aircraft, aerospace and defense applications; bike frames, all terrain vehicle (ATV) sprockets 9. Since boron carbide is (B 4 C) is a highperformance monolithic ceramic particle, it is added as reinforcement. It combines low specific weight with high Young s modulus and an extremely high hardness, as well as the capability of maintaining its mechanical and wear properties even at high temperatures. Common applications of B 4 C are armour and wear protection, neutron absorbers in the nuclear industry, bearings, nozzles and turbines. B4C offers more life of components while comparing to Al 2 O 3. Silicon carbide (SiC) has excellent hightemperature strength, a very high oxidation ability and good chemical resistance. Its thermal conductivity is four times that of steel and it has low thermal expansion coefficient, hence it is preferred for high temperature heat exchangers. The above said materials have wide applications in various fields and hence it is chosen for the present study 10. Heat treatment of reinforcement particles and permanent mold Most ceramic particles are visibly rejected by melt in the absence of heat treatment. Heat treatment conditions are important in order to improve the wettability and bonding between the substances. Heat treatment removes absorbed surface contaminations and raises the surface energy of the solid thereby improving their wettability with the metal. Heat treatment of SiC and B 4 C particles may form a surfaces oxide which also improves their wettability with molten metal 11. Hence, the silicon carbide particles were heated in an oven at C for one hour to improve the wettability. The mild steel permanent mold was preheated to C in a muffle furnace in order to ensure the flowability of molten metal and formation of dendrite structure. Ultrasonic cavitation fabrication of nanocomposites The SiC and B 4 C nanoparticles reinforced with Al 7075 were fabricated by ultrasonic cavitation based solidification processing. The microstructures and mechanical properties of nanocomposites were studied to understand the effect of nanoparticles in as cast Al 7075 alloys. The Ultrasonic Cavitation setup was used for the fabrication of nano-sized SiC and

3 GOPALAKANNAN & SENTHILVELAN: SYNTHESIS AND CHARACTERISATION OF Al B 4 C reinforced aluminium metal matrix nanocomposites. It mainly consists of a resistance heating furnace for melting aluminium, protection gas system and ultrasonic processing system. A stainless steel crucible of 110 mm inside diameter and 150 mm height was used for melting and ultrasonic processing. The ultrasonic probe made of niobium alloy was used to generate an 18 khz and a maximum of 4 kw power output for melt processing. The melt temperature for ultrasonic processing was controlled at about C above the alloy melting temperature (610 0 C). Niobium is a high temperature element and does not react with aluminium at the melt temperature. Nano sized SiC and B 4 C particles were fed into the aluminium melt through a steel tube. The average size of the SiC and B 4 C particles used was about 50 nm. For each casting, about one kg of Al 7075 was first melted in the crucible to a temperature of C. The aluminium alloy melt pool was protected by argon gas. SiC and B 4 C nanoparticles of 0.5 wt% (5g) preheated to C for one hour in a muffle furnace to improve the wettability 11. When nanoparticles were added in the Al alloy melts, the viscosity of the Al alloy significantly increased. Thus, after efficient ultrasonic processing, a higher casting temperature of C was used to ensure the flowability inside the mold. Al MMNCs with 0.5wt% of SiC and B 4 C were fabricated and for comparison 0wt% of Al alloy were also prepared. XRD analysis The X-ray diffraction (XRD) patterns of the samples were recorded on a Philips PANALYTICAL X' PERT PRO X-ray powder diffractometer using Cu K (λ = Å) radiation. Slow scans of the selected diffraction peaks were carried out in step mode (step size 0.05, measurement time 5 seconds, measurement temperature 25 C, standard: Si powder). The diffraction angle (2θ) was maintained between10 0 to Microstructure samples For microstructural study, samples of cast Al MMNCs ingots were cut to produce the samples for microscopic examination. Coolant was used during the cutting to avoid overheating of samples. Conventional grinding and polishing techniques were used for grinding and polishing the samples. Samples were mounted, mechanically polished down to 0.1µm with emery papers and etched by Keller s reagent (2 ml HF 48%), 3 ml HCL (conc.), 5 ml HNO 3 (conc.) and 190 ml of water 1. The microstructure of the samples was studied by scanning electron microscopy (SEM). Tensile test and micro hardness From the cast Al MMNC the standard tensile specimens were prepared by machining as per dimensions of ASTM E8 standard. To obtain mechanical properties, specimens with thickness of 6 mm and a gauge length of 32 mm were tested in a INSTRAN tensile testing machine. The hardness of the samples was measured using a UHL Vickers micro hardness measuring machine by applying a load of 0.5kg. This load was applied for 20 seconds. In order to eliminate the possibility of error a minimum of five hardness readings were taken for each sample 12. Results and Discussions Characterization of nano composites Generally, XRD can be used to characterize the nanoparticles present in Al nano composites, and it gives the average diameters of all the nanoparticles. The cast samples of size 13mm X 17mm X 3mm were characterized by XRD for determination of nano particle size. The particle size of the nano composite samples was calculated from the XRD using Debye- Scherrer formula given in Equation (1): D XRD = 0.89 cos (1) where λ - wavelength of X-ray used in Å, b FWHM (full width half maximum) in radians in the 2θ scale, θ - the Bragg angle, D XRD - particle size in nm. The particle size (D XRD ) was estimated by the Debye -Scherrer formula 13 using the full width at half maximum value of the respective indexed peaks and the average particle size (D avexr ) can be calculated.. Based on the 2θ value from the samples (Al 7075 with 0.5wt% SiC and Al wt% B 4 C nanocomposites) the average particle size (D avexr ) was found to be 50 nm for both SiC and B 4 C particles. In both the samples it can be seen that besides Al reflections, there appeared a broad peak at about 2θ = 45 0 and some smaller peaks indicating the SiC and B 4 C nano particles. The broad peaks whose positions are little lower than the hexagonal η phases which are formed from the metastable hexagonal η phase 14. The smaller peaks at 2θ = 36 0, 60 0, 72 0 and 2θ = 34 0, 38 0 and 74 0 indicates the presence of nano SiC and B 4 C particles respectively and concludes that uniform structure was formed in Al 7075 alloy. Microstructural Study During the experiment, when SiC particles were added into the molten alloys they tend to float on the

4 284 J SCI IND RES VOL 74 MAY 2015 surface of the melt. Even though SiC has a slightly large specific density than that of Al 7075 molten alloy, it floats because of high surface tension of the melt and poor wetting between the particles and the melt 11. The good wettability of B 4 C in aluminium has been found due to the formation of boron oxide film around the particles 12. By applying high intensity ultrasonic waves the acoustic streaming trapped the nanoparticles into the melt efficiently 6-8. The SEM micrographs were taken at high resolution in order to verify the dispersion of nanoparticles. Figure 1 and 2 shows the SEM micrograph of Al 7075 with 0.5wt% of SiC and Al 7075 with 0.5wt% of B 4 C. The nanoparticles are reasonably well distributed within Al matrix except some micro clusters are seen in the micrograph. Among these two nanocomposites, particle distribution in Al-B 4 C nanocomposites is found to be better. This is due to the fact that the wetting of ceramics particles in aluminium controls the quality of Al MMNCs castings. The wettability of B 4 C nanoparticles were found to be good while comparing to SiC nanoparticles 11. This is reflected in SEM micrograph. The micro clusters found in the Al-B 4 C nanocomposites were less and the nanoparticles dispersion was also reasonably good. However nano SiC is dispersed well in Al matrix still it has more micro clusters shown in Fig. 1. Mechanical Properties The tensile testing results are shown in Fig. 3 (a). With only 0.5wt% of SiC and B 4 C nano sized particles, the ultimate strength of as cast Al alloy 7075 was improved by 62%, which is significantly better than that of what Al alloy with same percentage of micro-particle reinforcement can offer 4,12. Whereas in SiC nanoparticles, still there are clusters shown in Fig. 1, because of their poor wettability property which affects the improved results in mechanical properties. An Al SiC nano composite offers 31% improvements in their ultimate strength which lesser than that of Al B 4 C nanocomposites. Fig. 1 - Al7075 reinforced with 0.5wt% of B 4 C Fig. 3- (a) Tensile strength of nanocomposites Fig. 2 - Al7075 reinforced with 0.5wt% of SiC Fig. 3- (b) Hardness of nanocomposites

5 GOPALAKANNAN & SENTHILVELAN: SYNTHESIS AND CHARACTERISATION OF Al It should be noted that there is a massive difference in their hardness values shown in Fig. 3 (b). The Al B 4 C nanocomposite offers 55% improvement in micro hardness. This is mainly because the B 4 C is harder ceramic material than SiC. Naturally the hardness of B 4 C is slightly lesser than diamond which is the hardest of all materials. The Al/SiC nanocomposites hardness improved by 35% which is lower than Al/B 4 C nanocomposites because of clusters was shown in the SEM micrograph Fig. 1. It is expected that with optimized process parameters the dispersion and mechanical properties of MMNCs will further be improved. Conclusion In this present work, the tensile strength and hardness of Al 7075 alloy reinforced with 0.5wt% of SiC and B 4 C nanoparticles were examined and compared with 0wt% of Al alloy. From the SEM micrograph the nanoparticles are well dispersed in Al matrix and offers improved results than 0wt% Al alloy. Out of B 4 C and SiC nanocomposites, the dispersion of nano B 4 C particles is better than SiC nanoparticles because of their wettability property. This offers improved results for B 4 C while comparing to SiC nanoparticles. It can be concluded that the use of nanoparticles gives better results in ultimate tensile strength and hardness. Acknowledgement The authors are very much thankful to Dr N Satyanarayana, Professor, Department of Nanoscience and Nanotechnology, Pondicherry University, Puducherry, India for the help rendered in characterization of the nanocomposites. References 1 RajeshKumar B & Sudhir Kumar, Influence of SiC particles distribution and their weight percentage on 7075 Al alloy, J Mater Engg Perform, 20 (2011) Kalkanh A & Yilmaz S, Synthesis and characterization of aluminium alloy 7075 reinforced with silicon carbide particulates, Mater Desig, 29 (2008) Choi S M & Awaji H, Nanocomposites-a new material design concept, Sci Technol Adv Mater, 6 (2005), Yang Y, Lan J & Li X, Study of bulk aluminium matrix nano-composite fabricated by ultrasonic dispersion of nano-sized SiC particles in molten aluminium alloy, Mater Sci Engg A, 380 (2004) Gopalakannan S, Senthilvelan T & Ranganathan S, Statistical optimization of EDM process parameters on machining of aluminum hybrid metal matrix composite by applying Taguchi based grey analysis, J Sci Ind Res, 72(6) (2013) Lan J, Yang Y & Li X, Microstructure and microhardness of SiC nanoparticles reinforced magnesium composites fabricated by ultrasonic method, Mater Sci Engg A, 386 (2004) Coa G, Konishi H & Li X, Mechanical properties and microstructure of Mg/SiC nanocomposites fabricated by ultrasonic cavitation based nanomanufacturing, J Manuf Sci Engg, 130 (2008) Suneel D, Nageswara Rao D, Satyanarayana C H & Jain P K, Estimation of cavitation pressure to disperse carbon nonatubes in aluminium metal matrix nanocomposites, Asian Int J Sci Technol Prod Manuf Engg, 2(1) (2009) Dasgupta R & Meenai H, SiC particulate dispersed composites of an Al-Zn-Mg-Cu alloy property comparisons with parent alloy, Mater Character, 29 (2005) Senthilvelan T, Gopalakannan S, Vishnuvardhan V & Keerthivaran K, Fabrication and characterization of SiC, Al 2 O 3 and B 4 C reinforced Al-Zn-Mg-Cu alloy (AA 7075) metal matrix composites: A study, Adv Mater Res, (2013) Shorowordi K M, Laoui T, Haseeb A S M A, Celis J P & Froyen L, Microstructure and interface characteristics of B 4 C, SiC and Al 2 O 3 reinforced Al matrix composites, J Mater Process Technol, 142 (2003) Suneel D, Nageswara Rao D & Jain P K, Ultrasonic cavitation assisted fabrication and characterization of a356 metal matrix nanocomposite reiforced with Sic, B4C, CNTs, Asian Int J Sci Technol Prod Manuf Engg, 2(2) (2009) Velmurugan K, Venkatachalapathy V S K & Sendhilnathan S, Synthesis of nickel zinc iron nanoparticles by coprecipitation technique, Mater Res, 13(3) (2009) Zhao Y H, Liao Z X, Jin Z, Valiev R Z & Zhu Y T, Structure property evaluations of ECAPed 7075 Al alloy during annealing, Min Metal Mater Soc, 4 (2004)