Advanced Materials Research Vol. 1042 (2014) pp 58-64 Submitted: 14.08.2014 © (2014) Trans Tech Publications, Switzerland Accepted: 14.08.2014 doi:10.4028/www.scientific.net/AMR.1042.58

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Ultrasonic Cavitation Based Processing of Metal Matrix Nanocomposites: An Overview

Santanu Sardar1, a, Santanu Kumar Karmakar1,b and Debdulal Das2,c 1Department of Mechanical Engineering, Indian Institute of Engineering Science and Technology,

Shibpur, Howrah – 711103, West Bengal, India 2Department of Metallurgy and Materials Engineering, Indian Institute of Engineering Science and

Technology, Shibpur, Howrah – 711103, West Bengal, India asan_becme@yahoo.co.in, bskk@mech.becs.ac.in, cdebdulal_das@metal.becs.ac.in

Keywords: Nanocomposite, metal matrix composite, ultrasonic cavitation, nanoparticle, dispersion, casting, microstructure, mechanical property.

Abstract. Metal matrix nanocomposites (MMNCs) have emerged as an important class of materials for structural applications specifically in the automobile and aerospace sectors; however, development of cost effective mass production technique of MMNCs with requisite operational and geometrical flexibilities is still a great challenge. Focused research in the last decade has highlighted that ultrasonic cavitation based processing is the most promising method for manufacturing of MMNCs with nearly uniform distribution of nanoparticles, having added advantage of being a liquid-phase route. This article presents an overview on the basic principles and recent advances in the ultrasonic cavitation based processing of MMNCs with a particular emphasis on identifying relationships amongst processing variables, microstructural parameters and mechanical properties. Critical issues of MMNCs fabrication are discussed.

Introduction Metal matrix composites (MMCs) are advanced hybrid materials in which tailored properties such as excellent strength-to-weight ratio, which is not easily attainable in base materials, are realized by utilizing the better ductility and toughness of the metal/alloy matrix in presence of higher modulus and hardness of ceramic reinforcement [1,2]. In order to achieve substantial improvement of strength, conventional MMCs are generally reinforced with 15-60 vol.% of micron-sized Al2O3/SiC particles [3]. Such high concentration of reinforcement, however, markedly reduces the ductility of MMCs and hence, limits their widespread application [4,5]. Similar level of strength can be achieved with much lower concentration of reinforcement (say, 1-5 vol.%) if the size of the ceramic particles is reduced to the nanometer scale (typically less than 100 nm) as the nanoparticles more effectively promote particle hardening mechanisms than the micron-sized particles [5-8]. For example, tensile strength of an Al-1 vol.% Si3N4 (10 nm) composite has been found to be comparable to that of an Al-15 vol.% SiC (3.5 µm) composite [1]. Since, metal matrix nanocomposites (MMNCs) require less amount of reinforcement; these are expected to exhibit higher ductility and formability as compared to the micron-sized MMCs [9]. For Mg-SiC microcomposite (25 µm, 10 vol.%) and nanocomposite (50 nm, 1.1 vol.%), Wong et al. [10] have shown that the tensile strength of the nanocomposite i.e. 203 MPa is not only higher than that of the microcomposite i.e. 165 MPa, but also the total elongation of the former i.e. 7.6% is significantly higher than of the latter i.e.1.5%. Therefore, significant efforts have been directed in recent years to develop MMNCs considering their enormous potential for application in aerospace, automobile and military industries.

Development of structural components of MMNCs remains great technological challenges. The common processing technologies are neither reliable nor cost effective to enable mass production of complex MMNC components with reproducible microstructures and properties [2,11]. Manufacturing of nanocomposites in comparable with microcomposites can also be divided into

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Fig. 1. Schematic presentation of basic set-up used in ultrasonic cavitation based method.

Ultrasonic generator

Nanoparticles

Ultrasonic power supply

Furnace controller

Furnace

Molten metal

Thermocouple

Dispersed nanoparticles

Acoustic streamlines

Cavitation Crucible

Inert gas

Power source

Nano-particles cluster

Ultrasonic probe

two general categories: solid-state and liquid-state. Amongst the solid-state techniques, powder metallurgy and mechanical milling are the most popular ones and capable to generate uniform nano-scale structure [12]. However, these techniques are very costly and require extensive post processing. In addition, these methods are also limited to simpler shapes with smaller sizes which are often inadequate for structural application [1,2]. Amongst the liquid phase techniques, stir casting method is the most economical one. It has been widely used for near-net-shape processing of MMCs [6]. Being liquid-phase, this route offers better matrix-particle bonding, easier control of matrix structure, simplicity in production, vivid selection of materials, flexibility and applicability to large quantity production of structural components of complex geometry [2,13]. Nearly uniform dispersion of micron-sized particles is easily achieved by mechanical stirring arrangement in the stir casting method [2]. However, the similar stirring arrangement fails to disperse nanoparticles even in small quantity in the liquid metal because of their large surface-to-volume ratio and poor wettability in melts [6,7,14]. Moreover, the force generated by mechanical stirring is insufficient to break nanoparticle agglomerates generated due to strong Van der Waals attractive force [1,9,11,15]. In this regard, it has been observed in the last decade that ultrasonic cavitation based (UTCB) is the most effective technique to overcome the common problems relating to dispersion of nanoparticles in the liquid metal. In recent times, UTCB processing has emerged as the most promising method for cost effective mass production of MMNCs mainly due to the pioneering works by Li and co-workers [2,7,8,11,16]. They have successfully synthesized several high-performance nano Al2O3 [14,17,18] / SiC [2,4,16,19,20] particle reinforced Al [7,11,17,18] and Mg [2,4,6,19-21] matrix composites by UTCB method. This article presents the state-of-the-art of UTCB technique for manufacturing of MMNCs.

Ultrasonic Cavitation based Manufacturing of MMNCs A General Setup Two types of UTCB systems have been used till date for manufacturing of MMNCs, these are contact [2,6,20] and noncontact-type [15]. In case of contact-type, the ultrasonic probe is dipped into the liquid melt that directly aids to disperse the nanoparticles in the melt and the developed slurry is subsequently solidified by conventional route. Whereas for noncontact-type, solidification of liquid melt with added nanoparticles is performed in an ultrasonic chamber [15]. The contact-type system is the most preferred one considering its simplicity in operation and effectiveness to disperse nanoparticles in melt. Typical contact-type UTCB system is schematically illustrated in Fig. 1. In general, it consists of an electric resistance heating furnace for melting light metals/alloys in graphite [6,11,17,22] or steel [4,5] crucible, nanoparticle feeding system, ultrasonic wave generator with probe and inert gas, like argon [2,11,14,18] or CO2+ SF6 [4-6,21] purging arrangements. The ultrasonic probe made of titanium [6,7,13,16] or niobium [4,11,17] alloy is activated by a transducer based on either of the permendur magnetostrictive alloy [4,11,20] or the piezoelectric [13]. The transducer converts high power electrical energy to mechanical motion and generates power varying from 450W [21] to 4kW [11] with frequency ranging from 17.5 kHz [4,11,20] to 20 kHz [5-7,14,23]. In general, loose nanoparticles are added in the melt, however, Choi et al. [17,18] have introduced a double-capsule feeding method where the nanoparticles are first wrapped with a thin Al foil and then encapsulated

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Fig. 2. Influence of ultrasonic (a) power and (b) stirring time on mechanical properties of MMNCs; data taken from the reports of Li et al. [11] and Nie et al. [24], respectively.

in an Al tube. The Al tube is then slowly fed into the melt. Nie et al. [24] have preferred feeding of preheated nanoparticles into the liquid melt to enhance their wettability with the molten metal. In order to enhance the dispersion of nanoparticles and to minimize their segregation during solidification, a few researchers [21,24] have attempted addition of nanoparticles in the semi-solid state over fully liquid state.

Basic Principle The UTCB processing involves mainly two important phenomena, i.e., transient cavitation and acoustic streaming [6,7,19]. The acoustic cavitation is initiated by the high-intensity ultrasonic waves (above 25W/cm2) that generate strong non-linear effects in liquid [1,2,6]. The effect of such cavitation includes the formation, growth, pulsating, and collapsing of micro air bubbles that tend to be trapped inside nanoparticle clusters during the negative and positive pressure cycles [17,20]. The strong cavitation can create transient (in the order of nanoseconds) micro 'hot spots' that generate temperature of about 5000 ºC, pressure of above 1000 atms, and heating and cooling rates above 1010 K/s [4,5,11]. The severe implosive impact in conjunction with local transient high temperature can effectively fracture nanoparticle clusters, clean the particle surface and enhance the wettability between melts and particulates [6,7,16,24]. Acoustic streaming flows throughout the melt and helps to circulate nanoparticles all over the melt (Fig. 1). The acoustic cavitation starts to develop in the melt when the acoustic pressure exceeds the cavitation threshold [23]. This threshold value or the acoustic energy, required to initiate the cavitation, depends upon the purity of the melt with respect to non metallic solid inclusions and dissolved gases. Eskin et al. [23] have shown that the more polluted the melt, the lower is the cavitation threshold value. Compared to conventional castings, UTCB method proves to be more reliable for producing nanocomposites even with system having extreme difference in thermal coefficients between metal matrix and ceramic particulates [11,25].

Influence of Processing Parameters Although extensive research has been conducted to develop MMNCs by UTCB method, only a few studies have been reported to understand the influence of different process variables on the microstructure -specifically the dispersion of nanoparticles as well as the resultant mechanical properties of nanocomposite in particular [11,24]. Li et al. [11] have shown that the ultrasonic power of 3 kW yields maximum improvement of strength for A356-SiC nanocomposite, further increase of power decreases both strength as well as ductility (Fig. 2a). For AZ91-SiC nanocomposites, Nie et al. [24] have demonstrated that ultrasonic stirring time of 5 min shows best combination of mechanical properties whereas further increase of stirring time is detrimental for mechanical properties possibly due to increasing formation of micro-cavities (Fig. 2b).

Microstructure of MMNCs High intensity ultrasonic treatment creates cavitation

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Fig. 3. Variation of grain size of UTCBprocessed AZ31B nanocomposites in as cast and after extrusion [21,26]. Note that mixed particle sized composite consists of 1 vol.% of 60 nm and 14 vol.% 10 µm particles.

that is sufficient to break nanoparticle clusters in addition to the enhancement of wettability of ceramic particles in the liquid metal, whereas the generated acoustic streaming helps to disperse nanoparticles uniformly in the melt within very short duration [6,19]. Figure 3 summarizes the reported influence of size and amount of particles on grain size of SiC reinforced Mg-based composites prepared by UTCB method [21,26]. It is evident that the grain size of composites reduces with increasing volume of reinforcement and the same is more pronounced with decreasing size of the particles (Fig. 3). This is because of reinforcing particles act as homogeneous nucleating sites during solidification composite slurry. It is worth to mention here that ultrasonic melt treatment of alloys prior to solidification, refines melt by eliminating solid non-metallic inclusions and by degassing to lessen the cavities during casting [7,23]. However, there would be a chance of formation of micro-cavities in the cast samples after solidification, if the acoustic streaming becomes too wild on the melt surface, shielded by inert gas [7].

Shen et al. [21] have shown that mixture of micron and nano-sized particles is more effective to reduce grain size when compared to that of individual particles either of micron or nano-size (Fig. 3). Particles are pushed to the grain boundary by the solidification fronts during casting leading to the formation of undesirable agglomerates and clusters that adversely affects mechanical properties of composite materials [11]. Formation of typical necklace-typed particle distribution has been observed more in nanocomposites in comparison with sub-micron sized composites [5,26]. Such casting defects can be eliminated to a large extent by suitable post-processing like hot extrusion [24,26]. Decrease in grain size after extrusion as compared to as-cast condition illustrates the fact that during hot extrusion, dynamic recrystallization takes place due to an accumulation of dislocations at the grain boundaries [21,24,26].

Mechanical Properties of MMNCs Figure 4 illustrates the variations of tensile properties with respect to amount of nanoparticles in Al-alloy [17,18], commercial pure Mg [16,20] and Mg alloys [4,5,19] matrix composites, all prepared by UTCB method. Yield strength of MMNCs increases almost linearly with nanoparticle content within its investigated range (up to 4 wt.%). Tensile strength is also found to be increased significantly for all nanocomposites as compared to the base alloy (Fig. 4). Cicc et al. [22] have reported that incorporation of 0.5 wt.% β-SiC nanoparticles (30 nm) in semi-solid state of Zn (AC43A) alloy by UTCB process increases strength; however, the reported degree of improvement in strength is marginal when compared to that of Al or Mg- matrix nanocomposites. One of the unexpected facts of MMNCs produced by UTB based method is that the ductility of the composites either increases considerably or remains almost same level when compared to that of the unreinforced base metals/alloys. This is barring the results reported by Erman et al. [16] where addition of 1 wt.% SiC nanoparticle (55 nm) is found to diminish the ductility of commercial pure Mg considerably in as cast condition. It has been reported that uniform distribution of nanoparticles by UTCB technique in the Mg-matrix activates additional slip systems in otherwise limited number of slip systems of the Mg-alloys, resulting in improved ductility in the MMNCs.

Cao et al. [4] for Mg-SiC and Nie et al. [5] for AZ91-SiC systems have systematically studied the influence of nanoparticle content of tensile properties of MMCs. Results of these investigators assist to infer that the best combination of tensile properties corresponds to composite with 2 wt.% of nanoparticles. When nanoparticle content is increased to beyond 2 wt.%, both tensile strength

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Fig. 4. Variations of (a) tensile strength and (b) elongation to failure with amount of nanoparticles for Al- and Mg-nanocomposites prepared by UTCB method.

and elongation are reduced markedly; these observations have been attributed to increased particle agglomeration, resulting in degradation of mechanical properties. In other words, experimental results relating to microstructure and mechanical properties indicate that the ultrasonic system, used till date for manufacturing of MMNCs, could disperse nanoparticle uniformly up to 2 wt.%. Therefore, dispersion of higher amount of solid nanoparticles in the liquid metals still remains a challenge even for UTCB processes. It is worth to mention here that 2 wt.%. nanoparticles is often considered sufficient to impart desirable strength in composite materials considering their higher effectiveness in promoting particle strengthening mechanisms, as briefly outlined below.

The enhanced strength and hardness in particulate reinforced MMNCs can be attributed primarily to the following four types of strengthening mechanisms: (i) Orowan strengthening from dislocation bowing by reinforcing particles, (ii) Hall–Petch strengthening from grain refinement, (iii) Taylor strengthening due to modulus difference between the matrix and the particle, and (iv) Dislocation forest strengthening resulting from the mismatch of thermal coefficient between the matrix and the particulate [25]. Extent of strengthening by Orowan mechanism depends on the amount and size of reinforcing particles that effectively determine the inter-particle spacing. More homogenous distribution of nanoparticles by UTCB processing is expected to create the highest strengthening effect by providing the strongest barriers for a moving dislocation line per volume percentage of particles [16,19]. Reinforcing particle acts as heterogeneous nucleating sites during solidification of MMCs, leading to the refinement of grain size. This effect increases with decreasing particle size for same volume of reinforcement. Therefore, MMNCs exhibit higher grain size strengthening effect as per Hall–Petch relationship [16,25]. The contribution from the Taylor strengthening mechanism that determines the load-bearing capacity of the composite is more pronounced for reinforcement with higher modulus value and for better interfacial bond strength between the particle and the matrix. The interfacial bond strength is expected to be higher for MMNCs processed by UTCB method, because it is known to develop cleaner particle surface with enhanced wettability [1,25]. Dislocation forest strengthening is considered to have smaller contribution than Hall–Petch and Orowan strengthening, specifically in case of cast MMNCs [16,25]. Improved strength of nanocomposites over microcomposites is commonly attributed to the enhanced contribution from one or more of the above-mentioned strengthening mechanisms often without systematic analyses to delineate their real contribution, which should be the focus of the future research endeavour.

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Concluding remarks The ultrasonic cavitation based manufacturing of MMNCs has been briefly reviewed. Ultrasonic method develops nanocomposites with more uniform dispersion of nanoparticles due to combined influence of transient cavitation and acoustic streaming. Being liquid-phase route, this technique is considered as one of the most promising methods for manufacturing of MMNCs for vivid structural applications. Recent research has established the utility of this method via laboratory scale manufacturing of several Al- and Mg-based nanocomposites with significantly improved strength and ductility. Scale-up of the process in preparing structural components, however, imposes new challenges. Furthermore, considerable basic research is necessary to understand the relationships of process variables, developed microstructures and mechanical properties of MMNCs so as to realize the full potential of this novel processing method. Acknowledgement The assistance received from the Centre of Excellence on Microstructurally Designed Advanced Materials Development, TEQIP-II to carry out a part of this work is gratefully acknowledged.

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