© M. Mashabela et al., Published by EDP Sciences 2022

1 Introduction

The growth in the aerospace industry has led to a demand for lighter, stronger materials that can also function at elevated operating temperatures (up to 1000 °C). This has led to a search by design engineers and material scientists to explore alternative materials to the traditionally used materials such as steel and aluminium which have weaknesses such as high density in the case of steel and low strength in the case of aluminium [1]. As each material comes with specific strengths and weaknesses, there is a need to explore combining materials to form composites in order to enhance the mechanical properties of materials for aerospace applications [2].

Titanium alloy Ti6Al4V is an α+β alloy, that is made up of 6 wt.% aluminium which stabilises the α phase and increases the beta transus temperature (T_β ), and 4 wt.% vanadium which stabilises the β phase and decreases the beta transus temperature (T_β ) [1]. It is also made up of other alloying elements including iron (Fe), carbon (C), nitrogen (N), hydrogen (H), and oxygen (O) in various percentages [2]. The Ti6Al4V alloy is used in various industries including biomedical, aerospace, automotive, and consumer goods [3]. Its high specific strength (232–265 MPa m³/kg) and high corrosion resistance have made it a favourable alloy for aerospace applications [4].

The mechanical properties of strength, stiffness and hardness for Ti6Al4V can be enhanced to expand its applications in the aerospace sector. Composites offer structural advantages and better functional properties targeting desired mechanical properties for specific applications than the individual materials [5]. A composite may be considered to be made up of three phases which include the matrix, reinforcement, and interface. The matrix is the primary phase and is a continuous phase that holds the reinforcement phase in place, shares and transfers the applied load. The reinforcement is a second phase that is embedded in the matrix in a continuous or discontinuous form and is generally stronger and stiffer than the matrix [6]. The interface is a region in which the matrix phase and the reinforcement phase interact [7]. The mechanical properties (tensile strength and stiffness, flexural strength and stiffness, impact toughness, compressive strength, and hardness) of reinforced composites depend on factors such as type of matrix, type of reinforcement, interfacial bonding, as well as, methods of processing and post processing [8,9].

L3 Reinforcement materials come in the form of fibres and particles. Fibres are either continuous aligned or short and aligned or randomly oriented. Particles come in various sizes (macro >1 mm, micro 1μm − 1mm and nano <0.1 μm). A change in mechanical properties due to the size of particles is observed at the micro and nano scale level. Both micro and nano particles greatly affect the mechanical properties of composites [10,11]. Nano particles as reinforcements have shown significant improvement in the mechanical properties of metal matrix composites (MMCs) [12]. The selection of nano particles that can be used as reinforcements in the Ti6Al4V matrix is based on the required application of the resulting composite. Nano composites that can be used as reinforcements in the Ti6Al4V matrix should have melting points that are sufficiently high to prevent them melting during processing of the composite. The coefficient of thermal expansion (CTE) of the nano particle is required to be close to that of the alloy matrix to avoid CTE effects of thermal stress, which are important in aerospace and other applications [13,14]. The density of the reinforcement material should be equal to or lower than that of the matrix, in order to retain or improve on the specific properties of the matrix [9]. Nano particles can be classified into different groups including fullerenes, metal nano particles, ceramic nano particles, and polymeric nano particles. The large surface area to volume ratio of nano particles imparts on them advantageous, distinctive physical and chemical properties [11].

Nano particles that can be used as reinforcement for the Ti6Al4V matrix are reviewed in this paper and their mechanical and physical properties contrasted against each other in order to select the best suited material for applications in the aerospace industry. This overview shows the existence of considerable research on the application of carbon nanotubes as reinforcements in Ti6Al4V matrix, and the related experimental data is presented in the paper. For the other materials discussed here, research studies are mainly focused on the use of micro sized particles, while little research exists for their nano scale particles. The ensuing material starts with a background on of composites, their uses, and different types of reinforcements, and eventually narrows down to different types of nano particulate reinforcements, their microstructural features and mechanical properties. A brief is presented on the elastic theory of composites, followed by briefs on manufacturing and synthesis of nano composites and challenges of additive manufacturing of nano composites, a discussion in which two nano particles are proposed for separate incorporation into a Ti6Al4V matrix, and finally conclusions.

2 Classification of composites

Composite materials are a combination of two or more materials which have different properties and are separated by a defined interface. The combination of different materials forms a new material that has more structural advantage and better functional properties than the individual materials. Composite materials may be considered to be made up of three phases which include a matrix phase, reinforcement phase, and interface [15,16] as illustrated in Figure 1, which shows a cross-section through a fibre reinforced composite material [17].

Composites may be categorised according to their type of reinforcement and type of matrix as shown in Figure 2. There are four main types of composites categorised according to the type of matrix used, including polymer matrix composites (PMCs), metal matrix composites (MMCs), ceramic matrix composites (CMCs), and carbon-carbon matrix composites (CMCs). Polymer matrix composites are polymer matrices with reinforcements lodged within the matrices [18]. Polymer matrix composites are easily processed, are lightweight, and possess different desirable mechanical properties. There are two main kinds of polymers used in PMCs, namely thermosets and thermoplastics [19]. Metal matrices normally consist of magnesium, aluminium, and titanium. In MMCs the objective of introducing reinforcements is the enhancement of stiffness, creep resistance, strength, hardness, wear resistance, and elevated temperature performance [20]. Ceramic matrix composites are made up of ceramic matrices and reinforcements which are particulates, continuous (long) fibers or discontinuous (short) fibers [21]. Reinforcing ceramics with continuous fibres provides better improvement in toughness than reinforcement with particulates or whiskers [22]. Ceramics on their own are prone to brittleness and the use of reinforcements in ceramics reduces this tendency by enhancing the toughness of the materials [21]. Carbon-graphite matrix composites consist of carbon or graphite as a matrix with fibres of graphite or carbon embedded in the matrix, respectively. Carbon fibre reinforcement of graphite makes tougher, stronger, and more resistant thermal shock materials than conventional graphite. Commercially available carbon fibres contain carbon in excess of 99.9% and function to reinforce graphite matrices. These composites have good fatigue and low creep resistance, are lightweight and have low density, high strength, high stiffness, and high fracture toughness. They also possess high strength at high temperatures (greater than 2000°). However, carbon composites are prone to oxidation at elevated temperatures of T > 500 °C in air, and T > 700 °C in steam, which is a disadvantage. They have high manufacturing costs because of the use of carbon fibres and require further processing steps following manufacturing [23].

Fig. 1 Cross section through a composite material showing the reinforcement, interface, and matrix [17].

Fig. 2 Classification of composites [17].

2.1 Types of matrices

Matrices hold the reinforcing materials and are the primary base for composites. There are four main types of matrices which include polymers, ceramics, and metals and also carbon-carbon matrices [24]. Polymers are materials consisting of a large number of atoms held together by covalent bonds and exist in long chains. Typical temperatures of application for polymers range between 0–150 °C and up to 250 °C for high performance polymers [25]. They are used mainly for their lightweight nature and ease of fabrication. Polymer matrices consist of thermoplastics (such as epoxies, and phenolic) and thermosets (such as polyethylene, polypropylene, and nylon) [26]. Metal matrices normally employ lightweight alloys of magnesium, aluminium, and titanium as matrices. Metal matrices have working temperatures ranging between 200 °C and 800 °C. Metal matrices have good values of ductility (10–25%), strength (240–460 MPa) and stiffness (45–128 GPa) and are mainly used in car engines and turbines. They have complex methods of processing compared to polymers and are less commonly used [20,27]. Ceramics are mainly used in high temperature applications. Ceramics are a combination of metals, or intermediate metals (such as silicon), with a non-metal (such as oxygen, nitrogen, or carbon). Ceramic matrices consist mainly of alumina (Al₂O₃), silica (SiO₂), and other inorganic non-metallic substances. Ceramics are resistant to oxidation and corrosion, and are hard, strong and able to resist high temperatures [28].

2.2 Metal matrix composites

Metal matrix composites (MMCs) are classified according to the type of matrix that is used. The most commonly used MMCs include aluminium-based composites, magnesium-based composites, titanium-based composites, copper-based composites, and super alloy-based composites. Metal matrix composites are used in various industries including aviation, automotive, and other applications such as electrical components. Aluminium matrix composites are the most widely used MMCs with applications in the automotive and aerospace industry. Magnesium-based composites are not widely used due to their low thermal conductivity and limitations in the fabrication process (ball milling, extrusion, and casting) compared to aluminium MMCs. Titanium matrix composites have proven useful in the aviation industry for the manufacture of propulsion components. Titanium alloys matrix composites have good strength at higher temperatures than aluminium MMCs, which is a good advantage in manufacturing aircraft and missile structures [29,30].

2.3 Types of reinforcements

There are three main types of reinforcements according to the aspect ratio of reinforcement including particulates, short fibres or whiskers, and continuous fibres. Figure 3 shows these types of reinforcements [31].

Fig. 3 Various types of reinforcement [31].

2.3.1 Natural and synthetic fiber reinforcement

Fibre reinforcement exists in the form of natural and synthetic fibres. Natural fibres occur in the form of plant and animal fibres. The source plants can be categorised as primary and secondary plants. Primary plants are plants that are grown for the purpose of producing fibres, while secondary plants are those plants where the fibres arise as by-products of other primary uses [32]. Applications for natural fibres include manufacture of thermoplastic and thermoset composites [33]. Natural fibres such as jute, sisal, silk, and coir are economical, abundant, and lightweight due to their low densities, have high toughness and are biodegradable. Jute can be used for applications where a high strength to weight ratio and weight reduction is required [34]. Natural fibres have low tensile strength, have a tendency to absorb moisture leading to odour and discoloration due to rotting, and begin to deteriorate at temperatures above 200 °C, which leads to a breakdown of their mechanical properties [35]. Synthetic fibres consist of glass, carbon and polymers such as aramid and Kevlar, and are man-made fibres. Glass fibres dominate amongst the class of synthetic fibres and their applications include large, low-cost structures such as wind turbine blades, ships, and civil engineering structures. Other applications include the manufacturing of circuit boards, windows, light bulbs, tableware, concrete, and cement [36,37]. Carbon fibres which are the most expensive amongst synthetic fibres are used for high-performance structures such as aerospace components, racing cars and general automotive applications. Aramid fibres are used in applications where good energy absorbing properties are required [38]. Some properties of carbon, aramid and other synthetic fibres are shown in Tables 1 and 2.

2.3.2 Natural and synthetic particulate reinforcements

Particle reinforcement fillers are mainly made up of ceramics or metal powders with small particle dimensions [40]. The use of particulate reinforcements in composites achieves less strengthening effects compared to fibre reinforcements in their longitudinal directions. Both particulate and fibre reinforcements give rise to increased stiffness, hardness, strength, and toughness. The advantages of using particulate reinforcement include low cost, as well as, ease of production and forming compared to fibre reinforcements. Particulate reinforcements find applications where high levels of wear resistance are required, and give rise to reduced friction, increased wear resistance, increased abrasion resistance, improved machinability, increased surface hardness, and reduced shrinkage [41]. The properties of particulate (filler) reinforcements are influenced by the size, shape, and surface chemistry of the particles. Particle reinforcements include carbon black which is important in the rubber industry, natural minerals, and synthetic minerals [42]. Mineral fillers usually contain silicon, aluminium, and other metallic elements in their chemical structure [43]. Common mineral fillers include silica, wollastonite, talc, mica, glass beads, and kaolin clay, some of which are shown in Table 3.

3 Particulate reinforced composites

A particulate composite is distinguished by particles dispersed within a matrix. The particles can be of various sizes, shapes, and orientation. Particulates can be further categorized into the two subclasses of flakes and filled/skeletal particles (Fig. 4). A flake composite is made of flakes with a larger contact area to enhances the thermal conductivity of composites [44,45]. A filled/skeletal composite is made of continuous skeletal filled by a second material such as a honey comb core filled with insulating material [44]. Particulate reinforced composites are made of natural and synthetic particles. Natural particles include minerals such as calcium carbonate, koaline, aluminum trihydrate, clay, feldspar, nepheline syenite, natural silica and mica, talc and agricultural waste. Synthetic fillers are made of processed mineral products which include carbon black, fumed silica, and aluminum hydroxide. Particles sizes range between 0.1 µm to 2 mm [45,46].

Magnesium has a high corrosion rate and as such is reinforced with nano particulates to enhance the necessary mechanical properties (including Young's modulus and yield strength) and to reduce its susceptibility to corrosion for biomedical applications. Niobium (Nb) and tantalum (Ta) particles are used to reinforce magnesium through a process of powder metallurgy [47]. When a copper matrix is reinforced with tungsten carbide (WC), hardness values of 360–370 HV are obtained, which are higher than the normal values for copper of 100 HV [48]. When natural rubber and styrene-butadiene are blended and reinforced with soy protein particles they show improved mechanical properties and faster rates of curing [49]. Other particulate composites include concrete which is composed of cement (matrix) and sand, as well as, gravel (particulates). Large particle composites are used in three types of matrices including metals, polymers, and ceramics. Cermet is an example of a ceramic-metal composite. A common type of cermet is cemented carbide which is made of hard particles of refractory carbide ceramic such as tungsten carbide (WC) or titanium carbide (TiC) rooted in a matrix of a metal such as cobalt (Co) or nickel (Ni) [50].

Fig. 4 Classification of a particulates [44].

3.1 Nano particulates

The reduction in the size of reinforcing particles to the nano scale level has a significant effect on the interaction between particles and dislocations in the matrix [51]. Reinforcements having smaller particle sizes improve the flexural strength because of increased particle surface area which causes a high surface energy at the interface between matrix and filler material [52]. Sachit et al. [53] reported significant improvement in mechanical properties at particle sizes below 20 nm, with three times more improvement observed at 11 nm in their experiments. Guo et al. [54] reported improved strength and increased interface shear stress at the nanoscale size. The studies carried out by Guo also observed a clean interface between the matrix and the reinforcement with no compounds forming at the interface. Unuoha et al. [55] conducted experiments on the effect of filler loading and particle size on the mechanical properties of the periwinkle shell-filled recycled polypropylene composite. The tensile strength of this composite was observed to increase with increasing volume fraction of filler up to 15% and to decrease at filler volume fractions of 20% and above. The decrease in strength at the higher volume fraction of the reinforcing filler was due to agglomeration of the filler material that prevents proper dispersion of particles within the matrix. The strength of the composite was also observed to increase with a decrease in particle size because smaller particles provide larger surface areas, which enhances dispersion and interfacial bonding between matrix and filler materials. Unuoha et al. [55], further observed improved yield strength and Young's modulus of the composite up to a volume fraction of the reinforcing filler of 15%, a decrease of the two properties above this value and an increase with decreasing particle size. The flexural strength and hardness of the composite were seen to increase up to volume fractions of the reinforcing filler of 20% and 25%.

The mechanical properties of particulate composite materials including tensile strength, hardness, and Young's modulus improve with a decrease in the size of particles, whilst wear resistance increases with an increase in the size of particles at the micro and macro scale but not generally so at nano scale [56]. The rate of wear decreases as particle size increases because larger particle sizes cover greater wear surfaces and are, therefore, able to control the rate of wear. Smaller particles are only able to cover smaller surface areas and wear along with the matrix during increased load [57,58]. The wear resistance of a material decreases when volume content of nano particles is increased in the matrix material [59,60]. Nano particles offer higher rigidity and higher yield strength to matrices than macro and micro particles. They also yield higher fracture toughness in the resulting composites than macro and micro particles [61]. Common nano particles include carbon nanotubes, which can be single walled nano tubes (SWNTs) and multi walled nano tubes (MWNTs), silicon carbide (SiC), titanium carbide (TiC), titanium diboride (TiB₂) and metal nano particles made of either gold or silver nano particles [62]. In the next section, the formulation, processing, microstructures and mechanical properties of each of these nano particles are discussed to some detail.

3.1.1 Carbon nanotubes

Carbon nanotubes are a class of carbon-based nanoparticles. Carbon nanotubes are classified as single walled carbon nano tubes (SWCNTs), shown in Figure 5A, and multi walled carbon nano tubes (MWCNTs), shown in Figure 5B, according to their structure [63].

Carbon nanotubes are made of graphite sheets with their diameters in the nanoscale. In single walled carbon nanotubes (SWCNT), a single layer of graphite (called graphene) is made up of carbon atoms with a diameter range between 0.4 and 2 nm depending on synthesising temperature and forms a consistent honeycomb lattice in which each atom connects to three other neighbouring atoms through strong chemical bonds. Single walled carbon nanotubes are mostly used in the electrical field for applications in sensors and wires because of their twisting properties [64,65]. Multi walled carbon nanotubes have diameters between 2 and 25 nm. They are differentiated from single walled nano tubes based on the multi walled Russian-doll structure in which layer upon layer is inserted inside the previous structure. They also have a more rigid structure compared to SWNT. Their structure is made of carbon nano fibres having small outer diameters and a hollow centre. Multi walled carbon nanotubes offer increased mechanical properties compared to single walled carbon nanotubes due to the multiple layers of graphene, which makes them useful in composite applications [64]. Carbon nanotubes are non-biodegradable, biocompatible, ultra-lightweight, have high specific strengths and stiffness, all which make them advantageous for manufacturing of ultra-lightweight composite structures. They can be joined with different biological molecules including drugs, proteins and nucleic acid to offer bio functionality [66]. They have low values of density (1.2–2.1 g/m³) and coefficient of thermal expansion (0.613 W/m K). Carbon nanotubes have high tensile strength (11–63 GPa), high Young's modulus (∼1 TPa) and high hardness (1210 HV) [67,68]. These properties are of particular interest in the aerospace sector due to the demand for strong and lightweight components and structures.

To highlight some outcomes of reinforcement of Ti6Al4V matrix with MWCNTs, the work of Topcu et al. [69] is now detailed, in which investigations on the creep behavior and hardness of Ti6Al4V reinforced with multiwalled carbon nanotubes under static loads was carried out. The MWCNTs used in this study had particle sizes between 10 and 30 nm and were dispersed into the Ti6Al4V matrix at volume fractions between 0.5% and 5%. In this work, homogeneity of powder particles was seen to depend on the volume fractions of MWCNTs in the Ti6Al4V matrix. This was achieved at 1% volume fraction of MWNCTs and was not achieved at larger volume fractions (4% and 5%), the latter due to the large aspect ratios of the nanotubes and also their large surface areas [70]. The 1% and 2% volume fraction MWCNT composites showed higher values of creep that those of 4% and 5% volume fraction. Hardness was observed to increase with increasing volume fraction of the MWCNTs, with values of hardness between 400 HV and 500 HV at volume fractions of 4% and 5%, respectively, which compares favorably to that of the matrix of 341 HV. The authors attributed this trend to the rising agglomeration with increasing volume fraction of the MWCNTs and the attendant formation of hard and brittle titanium carbide [71]. Scanning electron micrographs of the Ti6Al4V/MWCNTs presented by the authors (Figs. 5 and 6) show no porosity in the microstructure of the composite, which is a result of the good sintering process, and to re-agglomeration of MWCNTs that was observed at 4% and 5% volume fractions of MWCNTs. The authors observed that re-agglomeration of MWNTs gave rise to interfacial reaction between Ti6Al4V and MWCNTs leading to creation of clusters of brittle titanium carbide (TiC), which are identified by the red arrows in Figure 6 [69].

The SEM micrographs taken at 1% volume fraction showed a firm microstructure with α and β phases homogenously dispersed within the Ti6Al4V matrix. Figure 7 shows the microstructure of the Ti6Al4V-MWCNTs composite. The grey-colored image depicts the Ti6Al4V matrix having α phase and the β phase (in the white strips) at the grain boundaries. The microstructure at 1% volume fraction had both α and β phases without interfacial reaction compounds.

Fig. 5 Classification of carbon nanotubes (a) single walled carbon nanotubes; (b) multiwalled carbon nanotubes [63].

Fig. 6 SEM image of a Ti6Al4V-MWCNTs composite at 4% and 5% volume fraction [69].

Fig. 7 SEM image of Ti6Al4V-MWCNTs composite at 1% [69].

3.1.2 Titanium diboride (TiB₂)

Titanium diboride (TiB₂) is a non-oxide ceramic material that exhibits good properties of tensile strength (338.8–373.6MPa), hardness (3059HV), ductility, corrosion resistance, Young's modulus (410–430 GPa), and electrical conductivity. Titanium diboride (TiB₂) has a thermal coefficient of expansion (7.15 × 10⁻⁶/K) that is not very different from that of titanium (8.6 × 10⁻⁶/K) [61]. The mismatch of the coefficient of thermal expansion of materials has become an important factor in aerospace applications [13]. A large difference in the coefficient of thermal expansion of materials leads to the generation of thermal stresses due to changes of temperature changes at their joints and possible failure [72]. Mismatch of the coefficient of thermal expansion is, therefore, relevant in the application of hybrid frames, and repair of composite structures and components [13]. The ceramic is chemically (no unwanted reaction products arise at the interface during processing or high temperature applications) and mechanically compatible with the titanium matrix [74]. It has a high melting point (3127 °C), much higher than that of Ti6l4V of 1667 °C, and a low density (4.451 g/m³) [61], not much different from that of Ti6Al4V of 4.25 g/m³. The properties of titanium diboride make it a possible candidate for forming composites with other materials such as Ti6Al4V, for aerospace applications to produce light structures that can function at elevated temperatures.

Titanium diboride is not a naturally occurring compound and is formed through processes such as sintering, hot pressing, hot isostatic pressing, microwave sintering, and dynamic compaction. Pressureless sintering is a commonly used process to densify titanium diboride [73,75]. The high melting point of TiB₂ makes it difficult to produce through the sintering process as it requires high processing temperatures of over 2000 °C. However, sinter agents such as nickel (Ni), iron (Fe), and cobalt (Co) are able to lower the sintering temperature but lead to the deterioration of the properties TiB₂ [61]. The high temperature sintering process also promotes grain growth and sintering agents such as chromium (Cr), chromium boride (CrB2), carbon (C), nickel (Ni), nickel boride (NiB), and iron (Fe) are used to limit grain growth and increase densification. The anisotropy of its crystal structure leads to harmful internal stresses and the beginning of microcracking during cooling [72].

3.1.3 Titanium carbide (TiC)

Titanium carbide is made of large titanium atoms and small carbon atoms. Titanium carbide has a face centered cubic (FCC) crystal structure. Titanium carbide is formed through interstitial solid solution, with carbon atoms occupying the interstices within the Ti crystal structure [76]. Titanium carbide (TiC) is a hard, ceramic material that functions as a good reinforcement for Ti6Al4V by increasing the yield strength, hardness, and ultimate compressive strength of the alloy. Titanium carbide (TiC) exhibits superior hardness of 3200 HV, density of 4.94 g/m³, tensile strength of 258 MPa, Young's modulus lying between 448 and 451 GPa, coefficient of thermal expansion of 7.70 × 10⁻⁶W/m K and a melting point of 3160 °C [77].

A significant amount of research on fabrication of TiC/Ti6Al4V composites has led to remarkable improvement in hardness, yield strength, ductility and higher ultimate compressive strength. The methods used consist of powder metallurgy and ingot metallurgy which lead to interfacial reaction and poor wettability between matrix and reinforcement, that decrease the ductility of the TiC/Ti6Al4V composites. Hao [78] investigated the spark plasma sintering technique for fabrication of TiC/Ti6Al4V composites and further determined their mechanical properties of hardness, compressive yield strength and compressive strength. The samples were fabricated at 1500 °C and 50 MPa with holding times of 10 min. and 20 min [78].

Figures 8a and 8b shows SEM micrographs of a TiC/Ti6Al4V composite fabricated at 1500 °C and holding times of 10 min and 20 min, respectively. Comparing Figures 8a to 8b, it is evident that the quantity of TiC precipitates was more for the 20 min. holding time. Figure 7c shows an enlarged image of the TiC precipitates and exhibits no porosity and cracks which indicates good adhesion between TiC and Ti6Al4V matrix [78].

Figure 9 shows that the average Brinell hardness of 355HB for the TiC/Ti6Al4V composite, is significantly more than that of Ti6Al4V matrix of 270HB.

The ultimate compressive strength and yield strength of TiC/Ti6Al4V under quasi-static compression were determined as 1490 MPa and 1150 MPa, respectively, compared to corresponding values for Ti6Al4V of 1300 MPa and 980 MPa, respectively. Under dynamic compression, the Tic/Ti6Al4V composite showed a yield strength of 1520 MPa compared to a value of 1330 MPa for the Ti6Al4V matrix, and an ultimate compressive strength of 1530MPa for TiC/Ti6Al4V compared to 1410 MPa for the Ti6Al4V matrix. Clearly, the addition of titanium carbide into the Ti6Al4V matrix led to an improvement of the static and dynamic compressive yield strength and compressive ultimate strength, as well as, hardness.

Fig. 8 SEM micrographs of fabricated TiC/Ti6Al4V specimens [78].

Fig. 9 Brinell hardness of TiC/Ti6Al4V and Ti6Al4V matrix [78].

3.1.4 Silicon carbide (SiC)

Silicon carbide is a hard, ceramic material that has a lower impact resistance compared to aluminium oxide (AlO₂). Silicon carbide (SiC) exhibits high hardness (2400–2800 HV), high corrosion resistance, and is mechanically stable at high temperatures. Silicon carbide has a density of 4.36 g/m³, Young's modulus of 90–137 GPa, coefficient of thermal expansion of 7.9 × 10⁻⁶ W/m K, and a melting point in the range 3103 °C and can be used in high temperature applications to over 1500 °C [79]. Silicon Carbide crystals appear either with hexagonal or trigonal symmetry [80,81]. The properties of silicon carbide make it a possible candidate for the reinforcement of Ti6Al4V to form composites for aerospace applications.

Reinforcing Ti6Al4V with SiC promises an increase in the specific modulus, however producing the composite via the method of powder metallurgy produces composites that are porous and that have brittle reaction zones. Poletti [82] investigated various methods of producing Ti6Al4V reinforced with SiC with less porosity and decreased interfacial reaction. The methods include compressive induction heating, spark plasma sintering, conventional hot pressing, compressive induction heating, and equal channel angular pressing (ECAP) [83]. The conventional method of hot pressing produced samples with high porosity and a large reaction zone as shown in Figure 10a. The samples were reinforced with 20 vol.% SiC particles which were sintered at 1000 °C for 30 min. When the holding time was reduced to 5min., the reaction zone decreased as shown in Figure 10b. However, the porosity increased due to incomplete sintering [82,84].

The EAP method as represented by Figure 11 produced samples with minimum micro-cracks and no reaction zone and no porosity observed. Samples were placed into the ECAP die and preheated to consolidated temperatures of 200 °C and 300 °C for 15 min .

The hot extrusion method produced samples with the lowest porosity and without a reaction zone as shown in Figures 12a and 12b. The samples were produced at temperatures between 850 °C and 950 °C.

The compressive induction heating method produced samples without a reaction zone however, some porosity was observed and was attributed to the low temperatures and short holding time prevailing in this method that are not sufficient to sinter the Ti powders. Of the different methods discussed, the hot extrusion method yielded the best samples without cracks, pores and reaction zone. Clearly, the method used to produce the SiC/Ti6Al4V composites has a significant effect on the way that the SiC particles behave within the Ti6Al4V matrix and the resulting microstructure [84].

Fig. 10 20 vol.% SiC particle reinforcements in Ti6Al4V at 1000° and (a) 30 min and (b) 5 min holding times [82].

Fig. 11 ECAP method shows no porosity and reaction zone at 300 °C [82].

Fig. 12 Composite produced by hot extrusion at (a) 850 °C and (b) 950 °C without cracks, pores and no reaction zone [81].

3.1.5 Titanium nitride (TiN)

Titanium nitride is a hard ceramic which is chemically and thermally stable. It has a composition of 77.5% Titanium and 22.6% Nitrogen [85]. It has a wide range of applications due to its good mechanical properties of high hardness, as well as wear, heat and corrosion resistance. Titanium nitride can be used as a coating agent to improve the hardness of components, for high temperature furnaces, and for producing artificial limbs and biological materials. Titanium nitride has a melting point of 2950 °C, density of 5.24 g/m³, and thermal conductivity of 19 W/m K. Titanium nitride exhibits high Young's modulus of 600 GPa and high hardness (2300HV) [86].

Experimental studies were carried out by Falodun et al. [87] on the synthesis of Ti6Al4V-TiN-nano particles using the spark plasma sintering technique. In the studies Ti6Al4V was used as the matrix and was reinforced with titanium nitride nano particles with a particle size of 20 nm and volume fraction of 4%. Hardness tests were carried out and surface fracture of the composite investigated using SEM to study the morphology of the composite.

The authors presented the SEM micrograph of the Ti6Al4V powder in Figure 13a highlighting the spherical shape of the powder. The accompanying SEM by the authors in Figure 13b shows particles of Ti6Al4V reinforced with TiN nano particles (the very much smaller speckles on the surfaces of the Ti6Al4V particles). The authors reported that there was adequate dispersion of the nano powder particles within the Ti6Al4V matrix which they attributed to the tubular mixing process used. The hardness of the TiN/Ti6Al4V composite was determined as 531 HV, irrespective of the sintering temperature and length of time for sintering, and is significantly higher than that of the Ti6Al4V used of 389 H.

Fig. 13 SEM morphology Ti6Al4V-TiN powders [87].

3.1.6 Titanium boride (TiB)

Titanium boride (TiB) is a hard ceramic and is considered a good reinforcing compound due to its high elastic modulus, a density similar to that of Ti6Al4V matrix, high stability, and good interfacial bonding with the Ti6Al4V matrix. When TiB is used as a reinforcement in the Ti6Al4V matrix no intermediate phase forms as is the case with TiB₂ and in addition, less boron is required in the formation of TiB compared to TiB₂. However, to produce TiB reaction sintering of Ti and TiB₂ powders must be carried out in the chemical reaction Ti + TiB₂ → 2TiB [88]. The sintering temperature and time determine the transformation of TiB₂ into TiB and as these parameters vary, it is challenging to obtain TiB. This transformation of TiB₂ particles produces TiB whiskers. In most cases TiB does occur as a result of interfacial reaction between titanium diboride and Ti6Al4V. Titanium boride is considered a suitable reinforcement because it is thermodynamically and mechanically stable in the Ti6Al4V matrix, and it increases strength and stiffness without increasing the density or generating significant thermal stresses in Ti6Al4V matrix due the close matching of the CTE of both materials [89].

In the work of Huang et al. [90], Ti6Al4V and TiB₂ powders were mixed in a mechanical process using a planetary blender. The blended powders were hot pressed in a vacuum, where TiB₂ and Ti6Al4V were synthesised in a reaction resulting in titanium boride (TiB) with excess Ti. In general, the TiB phase is more thermodynamically stable compared to the TiB₂ phase with excess Ti. In this work, TiB₂ whiskers with 5% volume fraction were observed to be dispersed in the Ti6Al4V matrix. Using SEM, it was observed that in the presence of TiB, Ti6Al4V formed a widmanstatten microstructure (Fig. 14) when cooled slowly (furnace cooling) above the beta transus temperature (T_β ). The authors noted that, having TiB as a reinforcement led to the formation of a branch like symbiosis structure, with alpha platelets and intergranular beta phases. It was inferred that TiB stopped the growth of α + β lathes which directly promoted growth of the widmanstatten structure [91].

Figure 15 from the work of Huang et al. [90] shows the 5 vol.% TiB/Ti6Al4V composite at various magnifications and highlights the morphologies of TiB and other grain structures in the composite. Figures 15a and 15b shows the TiB whiskers distributed around the Ti6Al4V particles, also forming a grain boundary structure having a grain size of around 200 µm similar to that of the as received Ti6Al4V particles. Figure 15c shows TiB whiskers with neighbouring Ti particles, which induces strong bonds between Ti particles. Figure 15d shows that during the in situ reaction process TiB branches (the branched structures). Huang et al. [90] reported that the reaction of Ti and TiB₂ results in consumption of Ti. The authors further reported that this led to an increase in the percentage of aluminium (Al) and vanadium (V) in the matrix. However, they were not able to measure these increases accurately. The Al content increased from 6.00% to 6.12% and V content from 4.00% to 4.08 which leads to a slightly stronger matrix of the composite compared to the Ti6Al4V alloy on its own. The symbiosis structure shown in Figure 15c was observed to significantly contribute to the strengthening of the TiB/Ti6Al4V composite.

Table 4 shows the mechanical properties of the Ti6Al4V alloy and the TiB/Ti6Al4V composite, in order to further investigate the contribution of the TiB whiskers to the mechanical properties of the Ti6Al4V matrix. From the table a conclusion can be drawn that the presence of TiB whiskers within the Ti6Al4V matrix significantly improves the mechanical properties of yield strength and tensile strength of the resulting composite [92]. Hardness tests measured between 330 and 520 HV which higher compared to Ti6Al4V which has hardness of 341 HV.

Fig. 14 Widmanstatten microstructure for Ti6Al4V [90].

Fig. 15 Micrograph of TiB/Ti6Al4V composite [90].

3.2 The interfacial bond

The interaction between the nano particle and the matrix takes place at the interface, which is referred to as the interfacial bond. [94]. The interfacial bond is a weak link for most composite materials and controlling the interfacial bond strength is a complex problem [95]. Improved interfacial bonding can be obtained by using satisfactory agents for organic and inorganic composites, as well [96].

The interfacial bond between the fibre and matrix plays an important role when it comes to determining the mechanical properties of a composite. Good bonding between the fibre and matrix produces high strength and stiffness in the resulting composite [97]. Bonding mechanisms can be associated to various underlying processes including adsorption and wetting, electrostatic attraction, reaction bonding and exchange reaction bonding as shown in Figure 16. Interface bonds are formed by (a) molecular entanglement, (b) electrostatic entanglement, (c) interdiffusion of elements, (d) chemical reaction between group A and group B atoms of the surfaces of two different materials, (e) chemical reaction forming of a new compound and (f) mechanical interlocking. Other low energy bonding mechanisms include Van der Waals forces, and hydrogen bonding [94].

Wetting is a form of bond which involves slightly short-range interactions of electrons on the atomic scale. This takes place when the atoms of the constituents approach each other within a few atomic diameters or are in direct contact. Wetting of a solid by a liquid matrix is enhanced when surface tensions of both the reinforcement material and liquid material are nearly the same [98]. Interdiffusion occurs when atoms or molecules of two different adjacent materials diffuse across their interface, thus forming a bond. Diffusion theory describes the bonding effect between polymers and reinforcements, which is influenced by several factors including contact time, thermal treatment, and compatibility of polymers and reinforcements [99]. The bond strength in a polymer matrix depends on the amount of molecular entanglement, number of molecules involved and strength of bond between molecules. Electrostatic bonding arises from opposite charges existing at the interface between adjacent constituents. Chemical bonding occurs between atoms on the surfaces of reinforcement and matrices. Chemical bonds may be covalent, ionic, or metallic. Mechanical bonding involves interlocking on the surfaces of the fibres and matrices [94].

The strength of the interface determines the ability of the composite to transfer stresses from the matrix to reinforcement. In the case of bonding based on wetting, the strength of the interface is maximum when the surface tension of the reinforcing material which is a solid and the surface tension of the matrix being a liquid are nearly the same. In the case of interdiffusion, the interfacial bond is strengthened by an increase in the number of interdiffused atoms or molecules and the strength of the bonds between the atoms or molecules. The strength in mechanical bonding is promoted by surface oxidation treatments and is particularly important in polymer matrices and carbon fibres. However, the strength of such an interfacial bond is weak against transverse shear but strong in longitudinal shear depending on the degree of roughness [100].

Treatment is required to improve the strength of interfacial bonds, which in turn improves the properties of the composite [101]. Natural fibres can be modified physically to alter their structural and surface properties, which affects mechanical bonding with polymer matrices, using methods such as thermo treatment, stretching and calendaring. Electric discharge with corona, a method that oxidizes the surface, changes the surface energy of cellulose fibres in natural fibres and is an effective way to enhances the interfacial bond [101]. Bonding promoters and coupling agents can be used to improve the interfacial bond in polymers, metals, and inorganic filler materials. These bonding promoters can be applied as a coating or included in the reinforcements. Silanes are typical bonding promoters. Others include zircoaluminates, chartwell adhesion promoters, functional organic polymers, titanates, and zirconates [102,103].

Fig. 16 Interface bonds.

3.3 Theoretical models for predicting the elastic properties of composites and their use in designing nanoparticulate composites

The models used to determine the stiffness of particulate composites are the same as those used for composites in general. In these models, the assumption is made that the constituent components of the composite and the composite are all isotropic. It is also assumed that both the particulate and matrix phases exhibit linear elasticity in deformation and therefore, obey Hooke's law. The models for stiffness and stress assume elastic behaviour of both phases. Although a number of models exist, only the lower and upper bounds represented by the Reuss, and Voight models, respectively, and the Halpin and Tsai semi-empirical equations are mentioned in this paper. It is noted that for particulate composites, the first model gives better estimates of the properties of composites than the second model, while the last model is more adaptable, with the limitation that it is semiempirical.

3.3.1 The Voigt rule for strength and stiffness

The Voigt rule also referred to as the Rule of Mixtures (RoM) or Iso-Strain rule is based on the axial stiffness of continuous fibre reinforcement [93].

In this rule, the axial stress in a continuous longitudinally aligned fibre reinforced composite stress is given by the equation:

$${\sigma }_c={\upsilon }_f{\sigma }_f+\left(1-{\upsilon }_f\right){\sigma }_m$$(1)where the subscript c, f and m refer to the composites, fibre and matrix, respectively and the symbols σ and υ stands for direct stress and volume fraction, respectively. In this model, the axial strain in the reinforcement (ϵ _f ), matrix (ϵ _m )and composite (ϵ _c ) are all equal. Considerations of Hooke's Law and taking into account this equality converts equation (1) to the following equation for stiffness:

$$E_c=E_m{V}_m+E_p{V}_p$$(2)where the symbol E stands for elastic modulus.

3.3.2 The Reuss rule for strength and stiffness

The Iso-Stress or Reuss model is also known as the Inverse Rule of Mixtures (IRoM) and gives a prediction of the stiffness in directions orthogonal to the axial direction of continuous longitudinally aligned fibre reinforced composites. In the rule, the total transverse strain (ϵ _c ) of the composite is taken to be equal to the volume weighted sum of the strains in the fibre and matrix [93] thus:

$${ϵ}_t=\upsilon {ϵ}_f+\left(1-\upsilon \right){ϵ}_m$$(3)where the symbol (ϵ) refers to strain.

Introducing Hooke's Law into equation (3), noting that from the Iso-Stress rule σ _c  = σ _f  = σ _m , and making the stiffness in the composite the subject leads to:

$$E_c=\frac{E_f{E}_m}{E_f{v}_m+E_m{v}_f}(\text{this\hspace{0.22em}defines\hspace{0.22em}the\hspace{0.22em}lower\hspace{0.22em}bound}).$$(4)

Figure 17 shows such an example of the Voight and Reuss models. In Figure 16, tungsten concentration percentages are shown to have different values of modulus of elasticity. The scatter points of this graph fall within the Voight model (upper bound) and IROM (lower bound).

Fig. 17 Modulus of elasticity vs. volume fraction percent of tungsten for tungsten particles dispersed within a copper matrix.

3.3.3 Halpin and Tsai semi-empirical equations

The Halpin–Tsai equations are commonly used for determining the elastic properties of composites. Of these elastic properties the most important one is the transverse stiffness E ₂ of a laminar with oriented continuous fibres. An approach based on the strength of the material underestimates the true value of E ₂, which leads to the lower bound or the Reuss model. The Halpin–Tsai model provides a simpler and more practical way of calculating E ₂ which is beneficial for design [105]. The Halpin–Tsai equation for transverse stiffness E ₂ is:

$$\frac{E_2}{E_m}=\frac{1+\xi \mathrm{\Delta }{V}_f}{1-\mathrm{\Delta }{V}_f},$$(5)

where

$$\mathrm{\Delta }=\frac{E_f/E_m-1}{E_f/E_m+\xi },$$(6)and E _f and E _m stand for the stiffness of the fibre and matrix, respectively, Δ is a stress partitioning factor, ξ is a shape parameter that is dependent on the geometry, distribution or packing arrangement, and loading direction of the reinforcing fibres [105,106]. For the conditions where ξ → 0 equation (1) reduces to the lower bound values (E ₂) and where ξ→ ∞ equation (1) reduces to the upper bound values (E ₁), V _f denotes the fibre volume fraction [105]. For composites reinforced with randomly oriented fibres, the following modified Halpin–Tsai equations are used [106]:

$$E_c=E_m\left[\frac{3}{8}\left(\frac{1+\xi {\mathrm{\Delta }}_L{V}_f}{1+{\mathrm{\Delta }}_L{V}_f}\right)+\frac{5}{8}\left(\frac{1+2{\mathrm{\Delta }}_T{V}_f}{1-{\mathrm{\Delta }}_T{V}_f}\right)\right],$$(7)

where

$${\mathrm{\Delta }}_L=\frac{E_f/E_m-1}{E_f/E_m+\frac{2l}{d}},$$(8)and

$${\mathrm{\Delta }}_T=\frac{E_f/E_m-1}{E_f/E_m+2},$$(9)and E _c represents the composite stiffness, l is the length of the fibre and d is the diameter of the fibre.

The Halpin–Tsai equations for predicting the elastic properties of composites are the preferred choice over other models due to their flexibility and adaptability also, for allowing sensible interpolations between the lower and upper bounds of composite properties. Though they are good for predictions at lower volume fractions of reinforcement, they lead to underestimates at larger volume fractions. They offer the advantage of simplicity but however, have the drawback of dependency of their predictions on parameters that need to be determined experimentally [106].

3.3.4 Influence of particle loading on mechanical properties of a composite

The content of fillers has an influence on the mechanical properties of composites. The presence of a particulate in the matrix leads to an increase of strength due to higher strength of fillers and transfer of stress through interfacial bonding between the fillers and matrix. The impact strength of the composite can also improve as more filler content is increased due to the ability of the filler particles to absorb more energy [107]. However, some mechanical properties might deteriorate as the content of filler is increased. Where the strength and stiffness of filler particle are less than those of the matrix, or in cases of inefficient interfacial bonding, the modulus of elasticity and tensile strength will drop with increasing filler content as was reported by Fan [108] for clay in a polypropylene-montmorillonite nanocomposite. As noted earlier, the Voigt and Reuss models can be used to predict the elastic behaviour of a composite for different contents of filler. The Hashin and Shtrikman theory is used to predict more accurately, values of the coefficient of thermal expansion compared to the rule of mixtures. Van der Poel's theory provides an accurate method for determining the shear modulus of a particulate composite. Originally this method was complicated and had errors. It provided a table of values for which the Poisson's ratio of materials for the matrix was limited to 0.5. It was, however, been re-examined, modified and errors corrected to allow materials with any value of Poisson's ratio to be used [109]. Figure 18 represents the relative modulus E _c / E _m of glass sphere particles in a rigid epoxy matrix plotted as a function of filler volume fraction.

The H1 and H2 curves in this figure represent the upper and lower bounds, respectively, predicted from the Hashin and Shtrikman's theory. The predicted values from Budiansky's and Van der Poel's theory (B and V curves) and least upper curve (H2) and highest lower bounds of (H1) of the Hashin–Shtrikman theory are shown as pairs of solid and dashed lines in Figure 18. The solid and dashed lines in this figure represent predicted values using these three theories based on experimental values of stiffness and Poisson's ratio for the fresh (1–2 weeks) and age hardened (200 days) samples, respectively. The solid and open circles represent experimental values for the fresh (1–2 weeks) and age hardened (200 days) samples, respectively. The values obtained using van der poel's theory are seen in this figure to be in good agreement with the experimental values up to 35% filler content and fit the experimental data better than both the Hashin-Shtrikman and Budiansky curves. However, above this value and up to a volume fraction of 50% filler content, the experimental values exceed van der poel's predictions [109]. It is evident from the curves and data shown in Figure 18 that Van der Poel's predictions offer the best fit to experimental data compared to the Hashin-Shtrikman and Budiansky theories.

Figure 19 shows an example of the effect that filler content on the mechanical properties of silicon dioxide (SiO₂) reinforced with carbon nanotubes. Figure 19a shows the bending strength and fracture toughness for the composite to increase with increasing CNT volume content up to a volume of fraction of CNTs of 5% and to decrease beyond this value. In Figure 19b, the values of hardness increase from a volume fraction of CNTs of 0.5% up to 4% but decrease as CNT volume % increases beyond 4%. In Figure 19c wear loss experiences a decrease from a volume fraction of CNTs of 0.5% till 4% and then increases as the volume fraction of CNTs increases beyond here. The friction coefficient remains constant between a volume fraction of CNTs of 0.5–3% and then drops slightly between volume fractions of CNTs of 3–4%, remains constant between volume fractions of CNTs of 4–10%, then finally drops beyond here. It is expected that at the lower volume fractions there is good interaction between particles and the matrix thus the formation of strong interfacial bonds and the observed attendant increase in strength and hardness. However, as reinforcement loading is increased the surface area of the reinforcing particles available for interaction with the matrix reduces, possibly due to agglomeration of particles and hence the observed decrease of strength and hardness. The reverse trend observed for wear loss is likely to have been a result of increasing available particle surfaces to resist wear with increasing particle volume fraction and agglomeration of particles.

Another example of the variation of mechanical properties due to reinforcement of aluminium reinforced with silicon carbide is shown in Figures 20a, 20b. As seen in Figure 20a, the trend of hardness of the composite with increasing volume content of SiC % is different from the trend that was observed in Figure 19a. Increasing the volume fraction of SiC leads to an increase in the values of hardness of the composite for all volume fractions Similarly, the curves presented in Figure 20b show that increasing the content of SiC leads to an increase in the values of both shear modulus and Young's modulus of the composite for all volume fractions. In this case SiC is in the micro scale as opposed to CNTs in the nano scale, which seemingly implies that the size scale of reinforcing particles has different reinforcing trends. This is likely to arise from less agglomeration at the micro sale than at the nano scale. Therefore, the challenge is to determine the correct volume filler % specific to reinforcement size. Modelling will, therefore, be beneficial in determining the optimum volume content % of filler material. The models previously discussed in Sections 3.3.2 and 3.3.3 are important in this respect.

Fig. 18 A plot of the relative young's modulus E c / E m versus the filler volume fraction [109].

Fig. 19 The effect of the content of carbon nanotube filler on selected properties of silicon dioxide (SO2). (a) CNT volume content % versus bending strength and fracture toughness. (b) CNT volume content % versus hardness (c) CNT volume content % versus wear loss and friction coefficient [54].

Fig. 20 SiC volume content % in Aluminium (a) SiC content % vs Vicker’s hardness (b) SiC content vs Young’s modulus and Shear modulus [54].

3.4 Manufacturing and synthesis of the Nano composite

Various technologies exist for the production of composite materials including hand manufacture, rotor moulding, injection moulding, compression moulding and additive manufacture [110]. Conventional techniques include injection moulding for polymer-based composites or casting for metal matrix composites. Polymer matrix composites can be produced by a spay up method which consists of short fibres mixed with resin are sprayed against a form and cured. For continuous fibres which are either unidirectional aligned, mat or fabric form some special techniques have been devised to produce them which include using the hand lay-up technique, pressure bag moulding, and matched die moulding. Using the hand lay-up technique, the tapes, mats, or fabrics are placed against a form which, saturated with a polymer resin, rolled to ensure good contact and freedom from porosity and finally cured. Bag moulding uses tapes and fabrics which can be placed in a die using a high pressure or die to force the individual plies together so that good bonding is achieved during curing. Skins of military aircraft made from large polymer matrix components can be produced using bag moulding. Matched die moulding uses fibres or mats which are placed into a two-die part; upon closing the die the composite shape is formed. Pultrusion is another processing technique that is used to form a simple product with a constant cross section including round, plate, rectangular, pipe, or sheet plates. The fibres or mats are drawn from spools and then passed through a polymer resin for impregnation and put together to produce a particular shape before entering a heated die for curing. Producing metal matrix composites with continuous fibres is more difficult to achieve than polymer matrix composites. Casting process that force the liquid around the fibres using capilarity, vacuum infiltration pressure casting, or continuous casting are used. These processes are illustrated in Figure 21 [107].

Additive manufacturing provides flexibility, reduces production costs and lead times, reduces component parts and assembly, and the ability to manufacture complex structures [111]. A number of additive manufacturing technologies exist, including selective laser melting (SLM), direct metal laser sintering (DMLS), selective laser sintering (SLS), electron beam melting (EBM), fused deposition modelling (FDM), laser metal deposition (LMD), laser engineered net shaping (LENS), and laminated object manufacturing (LOM). Each technology has advantages and disadvantages. For the case of DMLS and SLM, the technologies are able to accommodate selected metallic powders for manufacturing, whereas FDM is not amenable to additive manufacturing of powder materials [104,111].

The selective laser melting technologies are governed by various process parameters that determine the quality of the parts produced. These include laser power, laser scanning speed, hatch distance, scan pattern, beam diameter, scan angle, beam spatial distribution, point overlapping, particle size distribution, powder deposition, shape of powder particles, flowability of powder, thickness of deposited layer, and application of protective gas atmosphere [112]. Aboulkhair et al. [112] grouped 15 important process parameters in SLM into, scan related, powder-related and temperature related categories. Gajera et al. [113] identified laser power, scanning speed, layer thickness and hatch spacing as the most important process parameters in the DMLS manufacturing in terms of their effect on the mechanical and physical properties of built parts. Thomas et al. [114] identified fifty key process parameters in SLM, clustered in the four categories of laser scanning parameters, material properties of the powder, powder bed and recoater parameters and built environment parameters.

The process parameters required to process the composite material take on a different approach to that of when a single element powder is used. In the case of the composite material two materials having different thermophysical properties will behave differently under the laser beam as opposed to a single element powder. The right amount of laser energy intensity is required to sufficiently melt the Ti6Al4V (ELI) matrix however without melting the nanoparticles. Melting of the nanoparticles is not required; they need only be embedded in the matrix. To achieve this careful modelling is required in order to account for the differences in the thermophysical properties of the two materials [115].

Some limitations exist in the synthesis of AM nanocomposites such as inhomogeneous dispersion, inconsistent feedstock material, and optimization of process parameters. Repeatability and printability need to be enhanced by optimizing the process parameters and starting material properties. The integrity of the interfacial bond, homogeneity of the composite and its constituents, and minimization of defects should be ensured to enhance performance of the final products [116]. The agglomeration of nano particles reduces the surface area for binding and creates stress concentrating defects which result in a deviation from the desired mechanical properties. Loading of nano particles in the composite affects the rheological properties of the nano composite, which then affects printability of the material [117]. The filler content directly affects the properties of the final product, including mechanical and physical properties. Therefore, it is important to determine the right filler volume fraction, particle shape, and size [118].

Fig. 21 Various traditional methods of producing the composite material [107].

4 Discussion

The material in Section 3.1.1 highlights the importance of volume fraction of reinforcing carbon nano particles within the Ti6Al4V matrix and their effect on the arising microstructure. Agglomeration of carbon nano tubes was observed using SEM at 4% and 5% volume fractions of the carbon nanotubes therefore, emphasising the need to determine the optimum volume fraction the particles that will result in enhanced mechanical properties of the composite. In Section 3.1.2 the properties of titanium diboride are identified as being important for the use of titanium diboride as a possible reinforcement material of Ti6Al4V. It is also noted that since it is not a naturally occurring compound various methods of processing are required to density this ceramic. large temperatures are required in the sintering process (which promotes grain growth) due to its high melting point which then requires sintering agents to lower the sintering temperature.

Titanium carbide is shown in Section 3.1.3 to be a possible reinforcement for Ti6Al4V matrix with SEM showing good bonding between the matrix and TiC, with no porosity. However, the high density of TiC is a disadvantage when targeting specific properties, which are influenced by low density. In Section 3.1.4 where silicon carbide (SiC) is discussed It has been shown that, different processes of fabricating silicon carbide/Ti6Al4V composites significantly influences the microstructure, which then affects their mechanical properties. It has been further noted that the processing temperatures and holding times affect the microstructure. This is a disadvantage as it implies that time must be spent testing different temperatures and holding times to arrive at suitable values that will produce a composite with no porosity and cracks in the microstructure.

Titanium nitride particles have been shown in SEM scans presented in Section 3.1.5 to be uniformly dispersed within the Ti6Al4V matrix regardless of the sintering temperature or holding time applied. However, this advantage is negated by the high density and high thermal coefficient of conductivity of TiN is. The presence of TiB whiskers in TiB/Ti6Al4V composites has been shown in Section 3.1.6 to lead to an increase in the mechanical properties of yield strength, hardness and tensile strength. However, it suffers the shortcomings that the Young's modulus is lower than that of TiB₂, in addition to the fact that to obtain TiB a synthesis reaction between Ti and TiB₂ has to take place which is challenging to achieve.

Table 5 gives details of the mechanical and physical properties of nano particles discussed in this paper so far to assist with comparison for use on one or more types of particles as reinforcing materials for Ti6Al4V.

Apart from selecting the two materials required for reinforcement, designing the composite is also an important factor and various modelling theories were discussed including the Reuss, Voight, Van der Poel's model which are important in determining the elastic properties of the composite. A section on the production of the composite highlighting traditional methods of producing the composite and the importance of additive manufacturing in achieving better results due to its advancement and the ability to accommodate the production of complex structures, reduce production costs due to the use of powder material, achieve near net-shape, and good surface finish that requires minimum post processing emphasized the need to use additive manufacturing as a processing method for producing the composite due to the complexity of producing metal matrix composites using traditional methods. The use of traditional methods is cumbersome, requires the use of more material which result in high production cost, increased post processing which also increases production costs.

Future work will include incorporating carbon nanotubes of different volume fractions into Ti6Al4V, using various dispersion agents and mixing techniques. Following onto this, optimisation of process parameters and printing of three-dimensional parts will be required to determine the physical and mechanical properties of the composite, as well develop a good understanding of its failure processes under different types of loads.

5 Conclusions

Ti6Al4V exhibits good mechanical properties of strength, hardness, and high Young's modulus that make it a good candidate matrix for composite materials for use in the aerospace industry.

However, to expand its use in the aerospace sector, nano scale size of particle reinforcements are used to improve the tensile strength, Young's modulus, and hardness of the matrix.

Of the six nano particles discussed in this overview, carbon nano tubes and titanium diboride generally show superior properties of strength, modulus of elasticity, low density, high hardness, and low coefficient of thermal expansion.

Carbon nanotubes have a density which is significantly lower than the Ti6Al4V matrix, whilst titanium diboride has a density much closer to that of the Ti6Al4V matrix compared to the other nano particles, except for SiC.

Because of availability, carbon nanotubes were selected for use in the work being carried out.

Agglomeration of MWCNTs in carbon nanotubes and the formation of brittle titanium carbides are disadvantages.

Titanium carbide has a higher value of Young's modulus than the two selected nano particles, while silicon carbide has a lower density than them. Clearly, then the selection of a particular nano particle for reinforcement must be specific to a particular need and will involve some give and take of the mechanical, physical and thermophysical properties.

References

All Tables

All Figures

	Fig. 1 Cross section through a composite material showing the reinforcement, interface, and matrix [17].
In the text

	Fig. 2 Classification of composites [17].
In the text

	Fig. 3 Various types of reinforcement [31].
In the text

	Fig. 4 Classification of a particulates [44].
In the text

	Fig. 5 Classification of carbon nanotubes (a) single walled carbon nanotubes; (b) multiwalled carbon nanotubes [63].
In the text

	Fig. 6 SEM image of a Ti6Al4V-MWCNTs composite at 4% and 5% volume fraction [69].
In the text

	Fig. 7 SEM image of Ti6Al4V-MWCNTs composite at 1% [69].
In the text

	Fig. 8 SEM micrographs of fabricated TiC/Ti6Al4V specimens [78].
In the text

	Fig. 9 Brinell hardness of TiC/Ti6Al4V and Ti6Al4V matrix [78].
In the text

	Fig. 10 20 vol.% SiC particle reinforcements in Ti6Al4V at 1000° and (a) 30 min and (b) 5 min holding times [82].
In the text

	Fig. 11 ECAP method shows no porosity and reaction zone at 300 °C [82].
In the text

	Fig. 12 Composite produced by hot extrusion at (a) 850 °C and (b) 950 °C without cracks, pores and no reaction zone [81].
In the text

	Fig. 13 SEM morphology Ti6Al4V-TiN powders [87].
In the text

	Fig. 14 Widmanstatten microstructure for Ti6Al4V [90].
In the text

	Fig. 15 Micrograph of TiB/Ti6Al4V composite [90].
In the text

	Fig. 16 Interface bonds.
In the text

	Fig. 17 Modulus of elasticity vs. volume fraction percent of tungsten for tungsten particles dispersed within a copper matrix.
In the text

	Fig. 18 A plot of the relative young's modulus E c / E m versus the filler volume fraction [109].
In the text

	Fig. 19 The effect of the content of carbon nanotube filler on selected properties of silicon dioxide (SO2). (a) CNT volume content % versus bending strength and fracture toughness. (b) CNT volume content % versus hardness (c) CNT volume content % versus wear loss and friction coefficient [54].
In the text

	Fig. 20 SiC volume content % in Aluminium (a) SiC content % vs Vicker’s hardness (b) SiC content vs Young’s modulus and Shear modulus [54].
In the text

	Fig. 21 Various traditional methods of producing the composite material [107].
In the text