Dr Ajay Kumar

This paper reviews the progress of solidification processing of cast metal matrix composites over 50 years since their original discovery, which was first published in 1969. Current use of metal matrix composite components in automotives, railways, space, computer hardware, and recreational equipment are presented. Some cast metal matrix composites which are discussed include aluminum reinforced with graphite, silicon carbide, alumina, and fly ash. Several critical issues in solidification processing of metal matrix composites , including nucleation, growth, the effects of reinforcements of fluidity, and the thermo-physical properties of the melts on micro-segregation, and particle pushing or engulfment during solidification of the are discussed. Current and future directions and challenges in solidification processing of cast metal matrix composites are presented, including the synthesis of self-lubricating, self-healing, and self-cleaning composites using solidification processing. INTRODUCTION There is an increasing demand for lightweight , high-performance materials for next-generation engineering applications, 1 and metal matrix composites are the preferred choice of lighter-weight engineered materials. In 2004, 3.5 Mkg of metal matrix composites were used, and this number is increasing with an annual growth rate of more than 6%. 2,3 According to the Global Metal Matrix Composites Market Report, there has been a linear growth of metal matrix composites production from 5 Mkg to 7 Mkg after 2012, and revenue has increased from 228.8 million USD to 400 million USD (Fig. 1a). The papers published on metal matrix composites have increased exponentially since 1988, as shown in Fig. 1b. Metal matrix composites refer to the family of materials in which a hard ceramic phase is incorporated in a ductile metallic matrix in order to achieve tailorable material properties which are not obtainable in conventional monolithic materials. Metal matrix composites are broadly classified into three categories based on the aspect ratio of the reinforcing phase, i.e., continuous fibers, whiskers, and particulates. The common ceramic reinforcements are silicon carbide, graphite, aluminum oxide, titanium carbide (TiC) and titanium boride (TiB 2), while the matrix metals include aluminum, copper , titanium, or magnesium. These composites can be synthesized by vapor phase, liquid phase, or solid phase processes. 4,5 This paper focuses mainly on liquid phase solidification processes where the matrix, in the form of a liquid, is mixed with the reinforcements and allowed to solidify to form a composite. Issues relating to the influence of reinforcements on the thermophysical properties of the melts on nucleation growth, micro-segregation and particle pushing or engulf-ment of particles and structure formation during solidification of metal matrix composites are discussed.

Thin Sn-based solder microjoints, typically less than 25 lm thick, attached to Cu bond pads on both sides in three-dimensional electronic packages are often converted completely into intermetallic compounds (IMC) during service or accelerated testing. The rate of IMC growth and the proportions of the IMCs relative to the unreacted Sn in the joint have strong implications on solder joint reliability, and hence on the performance of the entire package. Although IMC growth at the interface between two semi-infinite slabs of different components (e.g., Cu and Sn) has been previously modeled, to date, no analytical treatment of intermetallic growth at the interfaces of a thin joint (Sn) sandwiched between two massive substrates (Cu), with concurrent growth of two different intermetallics (Cu 6 Sn 5 and Cu 3 Sn) at each interface, has been proposed. In this work, a multicomponent diffusion-based model is developed for IMC growth in a thin joint between two semi-infinite substrates, with each interface possessing a multiphase structure. Because the Sn layer is thin, after prolonged aging, Sn may be completely depleted by reaction with Cu from the substrates, with the entire joint becoming IMC. Considering the joint as a finite source of Sn and the substrates as semi-infinite sources of Cu, general solutions of Fick's second law of diffusion were applied to obtain the concentration of Cu within each phase. Values of interdiffusion coefficients available in literature were used to solve for the interfacial position parameters in the multiphase system, and thence to determine the instantaneous thicknesses of Cu 6 Sn 5 (g), Cu 3 Sn (e), and Sn. The predictions of the model were validated against experimental data on the growth of IMCs in thin solder joints at 180°C and 210°C. Finally, the model was used to simulate the effects of aging temperature, joint thickness, and initial IMC thicknesses on the growth kinetics of IMCs during isothermal aging.

Ductile iron samples with similar compositions and varying microstructures were uniformly abraded, and the effects of phase fractions (ferrite, pearlite, and graphite) on the apparent contact angle (with water) and corrosion characteristics of ductile iron were investigated. We also investigated the effect of droplet volume on the apparent contact angle of ductile iron. Irrespective of the droplet size, the ductile iron system followed the Wenzel model of wetting, and the contact angle increased with increasing droplet volume. The Wenzel and Cassie−Baxter contact angles were calculated, and the calculated results agreed well with the experimental results. It was experimentally proven that pearlite is more susceptible to corrosion than ferrite and graphite, and a higher portion of pearlite in the microstructure can be detrimental to the corrosion resistance of the material. Understanding the relationship between the microstructure, contact angle, and corrosion can be used to develop materials with higher contact angle and corrosion-resistant microstructures. Using metal pipes that have high contact angles is desirable because artificial coatings on metal pipes can degrade over time leading to high cost of replacement and contamination to water systems. ■ INTRODUCTION Ductile iron is widely used for piping 1 and other components by the water industry due to its low cost, durability, and mechanical properties. However, these pipes often corrode 2 and suffer from accumulation of unwanted buildup of deposits. With high volume of wastewater passing through sewerage systems combined with the increasing number of pipes, this buildup becomes costly and difficult to remove. Self-cleaning materials have been used to address these issues as they minimize water contact angle and therefore can potentially minimize crucial environmental degradation processes such as corrosion, scaling, biofouling, and accumulation of dirt on the components. 3−9 Self-cleaning materials typically employ the lotus effect, which is the surface roughness-induced super-hydrophobicity, common on the leaves of water-repellent plants. The wettability of surfaces is often characterized by water contact angle measurement and it depends upon the relation between interfacial energies of phases present at the triple solid−liquid−gas line, as well as on surface roughness, heterogeneity, and physical and chemical processes occurring at the solid−liquid interface. Therefore, microstructural parameters of metallic alloys such as phases, alloying elements, grain size, grain orientation, texture, state of strain, and state of oxidation can affect the contact angle by altering the interfacial energies of the solid surface 10−12 with air and water. To date, little work has been done on studying the effect of these microstructural parameters of an alloy on its wetting and corrosion characteristics. The experimental data in the literature show that an increase in roughness increases the contact angle, 13−16 while an increasing contact angle decreases corrosion. 6 Generally, internal coatings (e.g., polyurethane, cement-mortar, and glass) are used, which can protect metal pipes from fouling and corrosion. However, these coatings may be short-lived, expensive, or may even contaminate the water system by leaching. Therefore, it is imperative to develop and study alloy systems that have high water contact angles by virtue of their microstructure, eliminating the need for any coating or secondary surface treatment to reduce fouling and corrosion. The theoretical value of the equilibrium contact angle of a liquid droplet on a flat solid surface is determined by Young's equation, as given by eq 1. It shows a geometrical relation between the cosine of the intrinsic contact angle (i.e., the contact angle that a liquid would make with a rigid, flat, chemically homogeneous, insoluble, and nonreactive solid surface 17) as a function of the interfacial energies of the solid− vapor (γ sv), solid−liquid (γ sl), and liquid−vapor (γ lv) interfaces. θ γ γ γ = − cos y sv sl lv (1) Two classical models, Wenzel and Cassie−Baxter, describe the wetting conditions of the surface in contact with the liquid forming either a homogeneous (solid−liquid) or inhomoge

Although significant effort has been made on using graphene as a reinforcement in metal matrix nanocomposites due to its extraordinary high strength and high modulus, metal-matrix graphene nanocomposites still exhibit large scatters in experimentally measured physical and mechanical properties. In this paper, recent progress in research on the synthesis of metal-matrix graphene nanocomposites using powder metallurgy technique involving milling, compaction, and extrusion or rolling with special emphasis on the agglomeration of graphene, interfacial bonding, and reaction between metal-matrix and graphene has been critically reviewed. Strengthening mechanisms such as grain refinement, oxide dispersions, strengthening, impeding of dislocations by reinforcement, load transfer between the matrix and graphene, and CTE mismatch in the metal-matrix graphene nanocomposites has been discussed. Existing theoretical models on the effects of graphene on mechanical properties including tribological behavior will also be discussed and compared with experimental observations. Potential future research directions in the area of graphene-reinforced MMNC will be outlined.

This is the Golden Anniversary paper of the 1969 AFS paper “Dispersion of Graphite Particles in Aluminum Castings through Injection of the Melt.” This paper reviews the progress in cast metal matrix composites (MMCs) over 50 years. Property motivation and current use of MMC components in automotive, railways, space, computer hardware, and recreational equipment are presented. Information on the MMC industry including the total volume of major producers of cast MMCs is listed. Some cast MMCs discussed include aluminum–graphite, aluminum–silicon carbide, aluminum–alumina, and aluminum–fly ash. Current and future directions in cast MMCs, including the manufacture of foundry-produced nanocomposites, functionally gradient materials, syntactic foams, self-healing, and self-lubricating composites, are presented. Recent progress in the manufacture of lightweight self-lubricating cylinder liners for compressors, piston, and rotary engines in Al–graphite and Al–graphite–SiC composites are discussed. Future foundry-produced prospects of MMCs are presented.

Friction stir processing (FSP) was applied to graphene nanoplatelets (GNPs) physically compacted on the surface of squeeze cast A356 alloy to incorporate GNPs within the matrix and to improve its mechanical properties. Squeeze casting resulted in finer size silicon and intermetallic compounds in cast microstructure, and subsequently FSP further refined the microstructure of squeeze cast A356 alloy, and GNP reinforced A356 alloy. The finer Si particles, intermetallics and graphene dispersed in the matrix increased the yield and ultimate tensile strength of FSP squeeze cast A356 alloy compared to the results reported in prior literature for FSP A356 alloy. Eutectic Si needles have been converted to fine spherical particles during FSP and were uniformly distributed within the nugget zone. The crystallite size of GNPs which were physically adhered to the surface of squeeze cast alloy prior to FSP decreased after FSP as a result of deformation. Thus, a combination of squeeze casting, and friction stir processing and incorporation of GNPs reinforcement in the A356 matrix is a promising route to further improve its mechanical properties.

Welding of shape memory alloys without deterioration of shape memory effect could vastly extend their applications. To retain shape memory behavior, a solid-state welding technique called friction stir welding was employed in this study. Austenitic NiTi alloy sheets of thickness 1.2 mm were joined at tool rotational speeds of 800, 1000, and 1200 rpm. Due to dynamic recrystallization, the grain refinement has occurred in the weld region. The tensile testing has shown superelastic plateau for the welds at 800 and 1000 rpm. The phase transformation behavior of different weld regions was studied in detail using differential scanning calorimeter. A marginal drift in transformation temperatures was observed in the weld. To understand the drift in phase transformation temperatures, finite element analysis was carried out with focus on temperature distribution during welding. Finally, time-dependent shape recovery of a FSW welded joint was studied and it was found that the original position was completely recovered after 27 s at a temperature of 65 C.

This is the Golden Anniversary paper of the 1969 AFS paper "Dispersion of Graphite Particles in Aluminum Castings through Injection of the Melt." This paper reviews the progress in Cast Metal Matrix Composites (MMCs) over the past 50 years. Property motivation and current use of MMC components in automotive, railways, space, computer hardware, and recreational equipment are presented. The information on MMC industry including the total volume of MMC industry major producers of cast MMCs is listed. Some cast MMCs discussed include aluminum-graphite, aluminum-silicon carbide, aluminum-alumina, and aluminum-fly ash. Current and future directions in Cast MMCs, including the manufacture of foundry, produced nano-composites, functionally gradient materials, syntactic foams, self-healing, and self-lubricating composites are presented. Recent progress in the manufacture of lightweight self-lubricating cylinder liners for compressors, piston and rotary engines in Al-Graphite and Al-Graphite-SiC composites are discussed. Future foundry produced prospects of metal matrix composites are presented.

Hot deformation behaviour of 10wt% SiCp reinforced 2014 Al alloy cast composite is studied by hot rolling process at varying rolling speed 3-7m/min at a temperature of 400 o C. During rolling operation, the material is deformed with strain rates of 2-3 s-1. It is observed that the hardness of the Al-SiCp composite material decreases with increasing rolling passes. This happens mainly due to softening of the material. Fracturing of SiCp particles are observed after 65-73% deformations and the dynamic recrystallization and superplasticity are seen under high deformation. The SiCp particles in the Al MMCs flow along with the grains and Al phase is always around the SiCp particles. This is possible only when the material gets softer at higher temperature i.e. at 400 o C. A detailed microstructural examination indicates that different deformation mechanisms such as dynamic recrystallization, void formation, flow localization, and superplasticity are present.

Nickel Aluminum Bronze (NAB) alloy specimens subjected to Friction Stir Processing (FSP) with and without particulate addition were subjected to tribological testing in a reciprocating sliding tester which can perform sliding test on specimens immersed in sea-water. Stainless steel ball was slid against flat coupons of NAB as cast, and Friction-Stir-Processed NABs, under sea-water to study the wear performance considering their prospective use in marine bearing applications. The wear expressed in terms of the wear scar area measured using an image processing method indicates that the performance of the Friction-Stir-Processed NABs in sea-water environment is inferior to that of the un-processed NAB, even though the processed NABs have relatively superior hardness. While wear tests in dry condition indicate that the wear resistance for the friction stir processed samples is high, under saline environment the trend was found to be reversed, as their wear resistance was found to be low. The reduction in wear resistance in sea-water is explained in terms of the (electrochemical) corrosive wear mode being active in the saline medium. The surface damage as revealed from the SEM characterization of wear scars were found to correlate with the experimental deductions.

Glass microballoon filled ZA8 alloy matrix syntactic foams are studied for the effect of heat treatment on the microstructure, compressive properties and energy absorption capacity. Normalizing and quenching resulted in reduction or dissolution of eutectic (α + η) phase in the matrix alloy. Blocky Al 3 Ni precipitates were observed in the matrix due to the reaction between matrix and the nickel coating of the particles. The average density and porosity of the syntactic foam were around 3 g/cm 3 and 51.5%, respectively. The heat-treated composites had higher yield strength, compressive strength, plateau stress, densification strain and energy absorption capacity than the as-cast composite. The normalized and quenched composites showed the highest compressive strength, plateau stress and energy absorption capacity. In fact, their highest values were 216.8 MPa and 211.9 MPa, 226.9 MPa and 223.4 MPa, and 125.3 MJ/m 3 and 117.7 MJ/m 3 , respectively. The improvement in the com-pressive properties is attributed to composition homogenization of alloying elements and relief of the residual stresses. The superior properties of syntactic foams compared to those of the conventional metal foams suggest their potential applications in marine vessels and submarine structures.

In 3D electronic packages, stacked dies are connected vertically using through-silicon vias and solder micro-bumps, which are typically between 1 μm and 50 μm thick. Solder micro-joints undergo significant shear deformation due to various loading conditions, which can occur during usage of microelectronic devices, such as thermal cycling, mechanical bending, and drop impact. A limited amount of work has been done in shear deformation and failure mechanism of these joints. To explore this, 25-μm-thick joints of SAC305 solder between two Cu substrates were tested, containing three different Cu6Sn5-to-Cu3Sn ratios, in shear at strain rates from 1 s−1 to 100 s−1. The joint shear strength is correlated with observed failure mechanisms such as Sn, Cu6Sn5, Cu3Sn and Cu6Sn5/Cu3Sn interface failure. The growth kinetics of intermetallic compounds (IMCs) in thin Sn-3Ag-0.5Cu joints attached to Cu substrates have been analyzed, and empirical kinetic laws for the growth of Cu6Sn5 and Cu3Sn in thin joints are reported. By combining the shear deformation results, we infer that increased IMC content due to heat treatment deteriorates the mechanical properties of the joint due to the presence of disconnected incipient micro-cracks. Deformation and damage are controlled by the intermetallics, and not the strain-rate sensitive solder for the aged samples. Both Cu6Sn5 and the Cu6Sn5/Cu3Sn interface fracture are the dominant mechanisms with increasing aging under the shear deformation.

Traditional alloys are designed on the basis of one or more majority of
materials which is Fe-based, Ni-based, and Co-based super alloy for mechanical properties enhancement but results with poor ductility at room temperature. New alloy design of materials requires more alloying elements for improving the material properties consistently. High entropy alloy consists of at least five principal elements with the concentration of each material between 5 and 35 at.% on the basis
of maximum configurational entropy at equi-atomic composition with more stable than intermetallic at elevated temperatures. In this paper, FeCrNbVMn alloy phase formed by mechanical alloying. The mechanically alloyed powders were subsequently consolidated by cold pressing and sintered in tube furnace. Analysis of microstructure and mechanical properties were carried out with the help of XRD, SEM, TEM and nanoindetation tests. The bulk sample showed hardness of ∼19 GPa at nano scale.

Nano-porous ceramics have potential applications as diverse as biomedical implants, catalysis, and armors. This work shows that in-situ Nano-porous Polymer Derived Ceramics (PDC) can be produced in Metal Matrix Composites (MMCs) using solid state Friction Stir Processing (FSP). Direct insertion of cross-linked polymer into the metal by FSP in solid state is a significant step toward inserting different chemistry of polymer precursors to generate a variety of in-situ porous structures in Polymer Derived (PD)-MMC. The PDC route is an efficient and cost effective way to produce SiCN-based PD-MMC and tailored pore architecture suitable for high temperature applications. Microstructural observations indicate a uniform distribution of ~100 nm size pores in the ceramic phase after pyrolysis.

The authors recently reported a novel in-situ method of fabricating nano-polymer derived metal matrix composite (PD-MMC) by friction stir processing (FSP) and addressed the issues of tool wear and particle agglomeration. In the present work, the microstructural evolution and tensile properties of the processed composite are reported. The microstructure during FSP evolved by discontinuous dynamic recrystallization. In the composite, fine ceramic particles pin the grain boundaries, preventing grain growth resulting in a fine grain (2 μm) structure being
retained. FSPed Cu (processed with the same process parameters as that of the composite) exhibited a grain size of 100 μm compared to 400 μm in the base Cu. The composite microstructure was characterized by equiaxed grains with narrow grain size distribution and a high fraction (N80%) of high angle grain boundaries. The combined effect of grain refinement and ceramic particle incorporation lead to a twofold improvement in the proof stress of the composite (201 MPa compared to 98 MPa of base copper). The ultimate tensile strength improved by 33% and there was small drop in the ductility of the composite when compared to base Cu. Kocks-Mecking plot of the composite showed stage III of work hardening.

Friction stir processing (FSP) is a solid state technique used for material processing. Tool wear and the agglomeration of ceramic particles have been serious issues in FSP of metal matrix composites. In the present study, FSP has been employed to disperse the nanoscale particles of a polymer-derived silicon carbonitride (SiCN) ceramic phase into copper by an in-situ process. SiCN cross linked polymer particles were incorporated using multi-pass FSP into pure copper to form bulk particulate metal matrix composites. The polymer was then converted into ceramic through an in-situ pyrolysis process and dispersed by FSP. Multi-pass processing was carried out to remove porosity from the samples and also for the uniform dispersion of polymer derived ceramic particles. Microstructural observations were carried out using Field Emission Scanning Electron Microscopy (FE-SEM) and Transmission Electron Microscopy (TEM) of the composite. The results indicate a uniform distribution of ~100 nm size particles of the ceramic phase in the copper matrix after FSP. The nanocomposite exhibits a five fold increase in microhardness (260HV 100) which is attributed to the nano scale dispersion of ceramic particles. A mechanism has been proposed for the fracturing of PDC particles during multi-pass FSP.

The present disclosure pertains to the field of advanced composites. In particular, the present disclosure relates to preparation of in-situ polymer derived metal matrix composites of copper and copper alloys using friction stir processing. The method can be used for preparing polymer derived metal matrix composites of other metals like Fe, Al, Mg and their
alloys also. The disclosure also relates to polymer derived composites with exemplary mechanical properties.
(Application No- 4555/CHE/2014)