Abstract
Aluminum alloys with silicon, magnesium, and copper were extensively used alloying elements in various applications because of their excellent properties. In recent decades, aluminum matrix composites (AMCs) are an advanced engineering material widely utilized in diverse engineering applications, including aircraft, automobile, marine, and shipbuilding, owing to their low density, lightweight, good stiffness, superior strength, and good tribological properties. Aluminum is abundant and its use is as vast as the ocean. It is also the most used matrix material in the composite arena. Therefore, incorporating a ceramic particle into a relatively soft aluminum matrix improves hardness, strength, stiffness, creep, fatigue, and wear properties instead of the conventional materials. This article is an assay to review and spotlight some recent works on the mechanical behaviors of aluminum-based titanium diboride reinforced metal matrix composite. This review article concentrates on the mechanical properties and the fabrication processes of Al-TiB2 composites to provide a valuable reference to nurture future research precisely.
1. Introduction
In the past few decades, aluminum matrix composite has acted an essential role in material science, especially in aircraft, marine, automobile, transportation, and defense sectors [1]. Several investigations have reported that the inclusion of ceramic filler contents to the matrix improves the mechanical, physical, and tribological properties [2]. Aluminum matrices that are incorporated with hard ceramic filler contents expose the augmented mechanical properties as compared to the plain alloy materials [3]. Figure 1 exhibits the list of wrought aluminum alloy. Due to their high strength-to-weight ratio, high thermal conductivity, good corrosion resistance, and improved mechanical properties, aluminum metal matrix composites (AMCs) are increasingly used as structural materials. Composite materials are becoming more popular due to their unique properties and high strength-to-weight ratio. Ceramic particles provide exceptional strength and wear resistance to AMCs [4–6]. Figure 2 reveals the classification of aluminum composites fabrication process. A large range of filler particles such as SiC, Si3N4, ZrN, TiN, TiB2, Al2O3, BN, WC, and SiO2 has used the reinforcements for the manufacture of composites. Amid the other filler materials, titanium diboride (TiB2) is a promising candidate filler material for aluminum-based composites. It exhibits an enticing combination of mechanical and physical properties, superior strength, perfect hardness, high melting point, excellent corrosion resistance, and excellent wear protection [7–10]. Figure 3 illustrates the advantages and disadvantages of different techniques for composites.
Titanium diboride (TiB2) particle does not react with molten aluminum and cannot form reaction products at the intergap between matrix and reinforcement [11]. Compared with the other filler contents, TiB2 is a desirable strengthening agent for aluminum-based metal matrix composites [12]. Titanium diboride based aluminum matrix composites were recently employed in the manufacturing of automobile piston, vehicle drive shaft, cylinder liners, cutting tools, crank shaft, brake drum, and bicycle frames and were also employed in aerospace, marine, and automotive industries because of their good stiffness, superior strength, high temperature stability, and lightweight [13]. AMCs have been fabricated using a variety of methods like compocasting, melt stirring casting, powder metallurgy, in situ casting, squeeze casting, and spray forming and mechanical alloying methods. Figures 4(a)–4(c) reveal the schematic diagram of stir casting, powder metallurgy, and hot extrusion process.
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Liquid state processing method contains incorporation of ceramic particles externally or formed inside the molten metal. The former is known as ex situ (stir casting technique) while the latter is called as in situ (direct melt reaction technique or exothermic salt-metal reaction technique) fabrication. Liquid state casting techniques have shown some incomparable benefits like constant dissemination of filler particles in the matrix and strong interfacial attachment between the matrix and the filler particle [14]. Figure 5 reveals the classification of fabrication method of AMCs.
Figure 6 exhibits the influence of various parameters on hardness. A number of researchers have produced titanium diboride reinforced AMCs using various techniques. This scientific review article provides an aerial view of research efforts that are focused on mechanical properties and synthesizing techniques of aluminum-based titanium diboride composites. Table 1 reveals the physical and mechanical properties of several ceramics reinforcements. The SEM image of the TiB2 particles is depicted in Figure 7.
2. Fabrication Techniques of Al-TiB2 Composites
Among the several manufacturing techniques, the two techniques that are being used quite often are in situ casting and stir casting. Figure 8 reveals the process parameters influence the production of composites through the melt stirring route.
2.1. Stir Casting
The processing route is the most significant consideration in the fabrication of AMCs. In 1968, S. Ray dispersed alumina (Al2O3) ceramic filler materials into the Al melt, and in this process the incorporated filler material is blended with a molten state alloy by means of mechanical stirring [15]. For the processing of discontinuous rein-forced AMCs, several researchers prefer to employ stir casting route. The foremost objective of melt stirring is that it is trouble-free, flexible, unproblematic, reasonable, and appropriate for bulk production [16, 17]. Mohanavel et al. [18] utilized melt stirring to manufacture AA6351/SiC AMCs. SEM images of the resultant AA6351/SiC AMCs comprising 4%, 8%, and 12% SiC, respectively, are revealed in Figures 9(a)–9(c). The SEM images demonstrate a nearly homogeneous dispersion of the SiC in the AA6351 alloy. Moreover, the stir casting is affordable and provides an efficient stirring movement in the melts due to the sound particle-matrix association in the filler material. The cast-on route for the development of inhomogeneous filler in integrated MMCs is the most widely used liquid state casting process. Figure 10 reveals the experimental structure for the fabrication of composites through melt stirring route.
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2.2. In Situ Casting
The in situ technique has been an attractive processing route for producing AMCs. In situ synthesizing technique was started in the early 1990s. In this process in situ reaction between the halide salts and molten metal takes place to form reinforcement particles. Exothermic method is more effective than the stir casting process [19, 20]. In situ formed reinforcement exhibits homogeneous dissemination of fine sized filler materials. It is effortlessly accomplished without the need for incorporation of wetting agent. The in situ reactions, as provided in the following equations, resulted in the generation of TiB2 particles. Consider the following:
The formation pattern of TiB2 can be classified as follows. (a) The incorporation of K2TiF6 (potassium hexafluorotitanate) and KBF4 (potassium tetrafluoroborate) to molten aluminum generates intermetallic compounds, specifically Al3Ti and AlB2, which serve as for Ti and B atoms. (b) Boron atoms travel in the direction of particles from Al3Ti. (c) The reaction occurred between atoms Ti and B in a gap from the surface of Al3Ti to form TiB2. (d) Boron atoms begin diffusing into TiB2 particles because of the smaller scale. (e) Dissolution of Al3Ti particle owing to normal cracking and fragmentation of Al3Ti particles, which contribute to enriched TiB2 generation rate. (f) Generation of TiB2 particles, after the entire reaction. Moreover, the KBF4 inorganic salt was incorporated slightly in excess of the stoichiometric ratio to avoid the formation of titanium trialuminide (Al3Ti). Figure 11 reveals the manufacturing of in situ composite employing inorganic salts and aluminum matrix alloy.
In preparing AMCs, “in situ” techniques offer significant advantages over the conventional processing routes and in situ casting route is more economical. In situ type of processing is now in commercial use for TiB2 particle reinforced aluminum matrix composites [21–23]. Moreover, the TiB2 reinforcement particles in the composite inhibit dislocations, resulting in higher tensile strength. This interface allows for an efficient load transfer between the matrix alloy and the reinforcement. As the TiB2 particle content increases, this composite has higher mechanical properties than pure aluminum alloy [22]. Figure 12 displays the experimental arrangement of in situ casting. In [23], the dry sliding wear parameters on LM4/TiB2 composites were analyzed using the Taguchi method. Particulates reinforced aluminum matrix composite has better tribological properties compared to unreinforced aluminum matrix composite. Mohanavel et al. [21] employed in situ casting to manufacture AA6351/TiB2 AMCs. Microhardness and strength increase with the enhancement of the weight percentages of TiB2 particles. SEM micrographs of the resultant AA6351/TiB2 AMCs comprising 0%, 4%, and 8% TiB2, respectively, are revealed in Figures 13(a)–13(e). The SEM micrographs demonstrate a homogeneous dispersion of the TiB2 in the AA6351 alloy.
3. Comparison of In Situ versus Stir Casting
Table 2 explicates results obtained by various researchers where TiB2 is the reinforcement and the processes involved mostly are in situ and stir casting and the percentages at which superior mechanical properties emerge are spotted in Table 3. Both in situ and stir casting method can fabricate Al-TiB2 composites with strong mechanical and wear properties. Most of the explorations have been done on titanium diboride filler particle for aluminum based in situ AMCs. Titanium diboride reinforced aluminum matrix composites are fabricated using halide salt reaction technique and it has proven to be cost effective [24]. Al-TiB2 in situ composites are usually synthesized by the incorporation of inorganic salts like K2TiF6 and KBF4 which react with molten aluminum and form TiB2 in the Al melt [25–30]. The Al-TiB2 in situ AMCs reveal superior mechanical and tribological properties compared with the base matrix alloy. In situ technique offers high mechanical bonding, homogeneous dissemination of small size of fillers in the matrix, thermodynamic stability, and clear interfacing between the solid and liquid in contrast to stir casting [31, 32]. TiB2 involved composites have utilized stir casting, squeeze casting, centrifugal casting, and compocasting rarely. It is also more economical when compared to in situ casting. Mechanical and physical properties like density, porosity, flexural strength, and compression strength are analyzed by a few researchers only, where TiB2 plays a vital role as reinforcement.
4. Conclusions
This review article examines the impact of titanium diboride (TiB2) on aluminum matrix composites (AMCs). As seen in the previous section, various authors have discussed the mechanical properties of fabricated Al/TiB2 composites, such as hardness, tensile strength, compressive strength, and yield strength. It shall be opined that the stir casting technique is the rare one in making Al-TiB2 composite, whereas the in situ technique is the often-used technique due to its formation of more uniform TiB2 particles in the matrix. The TiB2 particles involved composites have rarely utilized squeeze casting, centrifugal casting, stir casting, and compocasting. Al alloys containing TiB2 reinforcements are to reveal enriched mechanical properties compared to monolithic alloys. Reported works also indicate the linearly augmented mechanical properties with the incorporation of TiB2. Several research works recommended that tensile strength and hardness of the AMCs were enriched with the inclusion of a rise in mass fraction of TiB2 particle contents.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this article.