Abstract
In this work, we synthesize Al/TiB2 metal matrix composites (MMC) based on the effect of cooling rate in the melt while pouring into the permanent mold condition. The objective of this paper is to achieve the desired distribution pattern and increased TiB2 particles’ size in the Al/TiB2 MMC ingot. Two halide salts, viz., potassium hexafluorotitanate (K2TiF6) and potassium tetrafluoroborate (KBF4), are procured and measured. The two salts were mixed with the aluminium melt in the crucible, and it is stirred manually with help of a graphite rod. Because of the exothermic reaction, the melt reacts very quickly and that is what dropped the salts slowly. The salt particles were synthesized because of the exothermic reaction, and it will allow the particles to grow. The size and distribution of particles differ at different place in the MMC. An FEA tool ProCAST was used to analyze the cooling rate of the melt, and SEM is used to study the microstructure of the ingot at different places. The microstructures helped to identify the size of reinforcement in the MMC. The TiB2 particles are distributed more at this location at 810°C, and the TiB2 particles formed various clusters in this zone as 70%–80%. Also, the tribological characteristics are analyzed with the help of the results.
1. Introduction
The cast aluminium components are used in automotive industries due to its more strength-weight ratio, outstanding castability, and corrosion resistance [1, 2]. The ex situ method is involved for the fabrication of particulate matter reinforced metal matrix composites (PRMMCs) by conventional ex situ method due to its isotropic properties, ease of fabrication, and the lower cost. The reinforcement is added directly to fabricate the ex situ composites [3, 4]. In in situ method, a chemical reaction of reinforcements inside the composites takes place to synthesize the composites. To identify the behavior of in situ particles, a small work was carried out in the aluminium matrix composites [5].
The in situ metal matrix composites have a good attraction characteristic because of their good bonding strength and well distribution of fine reinforcement [6, 7]. It was found that the study was concentrated on fabrication and the mechanical properties of the reinforcement such as SiC, Al2O3, TiC, and B4C. Various researchers have performed high-performance applications focusing on TiB2 as the reinforcement as of its high elastic modulus and high thermal conductivity [8, 9]. Also, it does not react with the molten aluminium. The casting defects such as oxide films, porosity, and other inclusions will strongly disturb the mechanical behavior of the cast aluminium alloys [10, 11]. However, because of the stiffness, hardness, and improved tensile strength, the aluminium-based MMCs are preferred compared to the base matrix alloy [12–14]. The fabrication of aluminium-based MMCs is done by addition of SiC, Al2O3, TiC, CBN, and TiB2. Out of these ceramic-based reinforced particles, TiB2 is used mostly because they possess hardness, maximum tensile strength, and compressive strength [15, 16].
Because of the very clean and size of particles in the interface of the in situ method, the fabrication of Al/TiB2 MMCs is preferred. Also, the increased tensile strength and fatigue strength is due to the very fine particles [17].
In the in situ fabrication method, two salt powders such as K2TiF6 and KBF4 were used as reinforcement. They were mixed in measured proportion and then poured to the aluminium melt slowly. The mixture was stirred manually to endorse the reaction between salts with the help of a manual graphite rod. The floron gas was inserted in the melt to avoid formation of gases which will create the casting defects such as blow holes [18, 19]. Later, the melt was kept in hold to synthesize TiB2 particles that grow in size with holding time [20, 21].
The Al/TiB2 melt was poured into the mould, so the falling elevation will be the possible turbulence that occurs during filling. The fragmented TiB2 particles are created due to the turbulence, and also a variation in distribution occurred at different locations. It occurs because the cooling rate and turbulence were attributed and also due to the influence of melt fluidity [22, 23]. The above parameters were analyzed, and their effects are understood clearly from the SEM micrographs captured from the cast ingot at six different locations.
2. Experimental Work
An in situ method was used to fabricate Al/TiB2 MMCs through salt metal reaction. Three different melt temperatures were maintained, such as 750°C, 780°C, and 810°C. Also, three dissimilar holding times were maintained after mixing of entire salt such as ten minutes, twenty minutes, and thirty minutes before pouring into the permanent molds. Through the same procedure, totally nine ingots were fabricated with different combinations, pouring temperature, and holding time [24]. The size of the reinforced particle is not the same as in the melt because of the parameters’ fluidness and disorder at different places of the ingot and the local cooling rate of the casting [25].
Figure 1(a) shows the twenty-four various locations in cast ingots from top to bottom, and based on variation of local conditions of ingot, six locations were selected from twenty-four various locations. The distribution of TiB2 particles and size were compared for all nine ingots through the SEM micrographs; Figures 1(b)–1(d) show the SEM micrographs.
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2.1. Finite Element Analysis
Figure 2(a) shows the simulation model, and Figure 2(b) describes the mesh diagrams of ingot. The temperature vs. time curves were generated at twenty-four different locations of each ingot by using the FEA tool. Out of the twenty-four different locations, selected six locations were shortlisted for simulation. From the simulation, a drastic variation occurred in the melt due to the turbulence of melt, cooling rate, and fluidness.
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Figures 3(a)–3(f) show the temperature vs. time curves at selected six different locations.
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3. Results and Discussions
At location 23, the temperature-time curve indicates maximum cooling rate and turbulence during filling out of twenty-four different locations. Also, it was found that the fluidness reaches its maximum range at the maximum pouring temperature.
Also, here we discuss the effect of cooling rate at location 23. This location is marked in the bottom surface of the ingot, and the cooling rate and the fluidity are maximum at this location at 810°C pouring temperature. Additionally, the falling height of the melt during the filling process is considered to be maximum. Because of the abovementioned reasons, at location 23, the turbulence will be maximum. Because of these reasons, the circulation will be maximum at 810°C, and it will cause the TiB2 particles to fragment. Hence, the TiB2 particles get entrapped quickly because of maximum cooling rate, and the casting at this location is freezed [26].
Moreover, more TiB2 particles are distributed at this location at 810°C and the TiB2 particles formed various clusters in this zone as 70%–80%.
Here, we discuss the effect of cooling rate at location 21. This location is marked in the middle surface of the ingot; the cooling rate and the fluidity are modest at this location at 810°C pouring temperature. Additionally, the falling height of the melt during the filling process is considered to be average. Because of the abovementioned reasons, at location 21, the turbulence will be average. Because of these reasons, the circulation will be maximum at 810°C, and it will cause the TiB2 particles to fragment. Hence, the TiB2 particles get entrapped quickly because of maximum cooling rate, and the casting at this location is freezed [27, 28].
Moreover, more TiB2 particles are distributed at this location at 810°C and the TiB2 particles formed various clusters in this zone as 60%–70%.
In this paragraph, we discuss the effect of cooling rate at location 19. This location is marked in the top surface of the ingot; the cooling rate and the fluidity are very low at this location at 810°C pouring temperature. Additionally, the falling height of the melt during the filling process is considered to be very less. Because of the abovementioned reasons, at location 19, the turbulence will be average. Because of these reasons, the circulation will be maximum at 810°C, and it will cause the TiB2 particles to fragment. Hence, the TiB2 particles get entrapped quickly because of very minimum cooling rate, and the casting at this location is freezed. Most of the TiB2 particles were trapped at locations 23 and 21 already. Hence, very small particles settled down in this particular location.
Analyzing at location 1, the falling height and the cooling rate were minimum. Due to the effect, most of the particles were trapped at the bottom of the ingot at location 1, which was found to have fewer TiB2 particles through SEM micrographs. Also, it covers 40%–50% area in the SEM micrograph.
The friction between the aluminium matrix and TiB2 particles was analyzed. The tribological characteristics showed better performance than conventional material.
Also, the distribution of TiB2 particles at different locations for less holding times and pouring temperatures was found to be low, which is tabulated in Tables 1 and 2.
4. Conclusions
The TiB2 particles were found to be more at location 23 because of the very high cooling rate.
The TiB2 particles were found more at locations 23 and 21 due to the very high turbulence. When the circulation of the fluid is more, the fluidity is also more.
Also, the tribological characteristics showed significant improvement on the basis of hardness. However, the reinforced particles get trapped quickly when the cooling rate was very high at these locations.
Data Availability
No data were used to support this study.
Conflicts of Interest
The authors declare that they have no conflicts of interest.