Influence of Boron Fiber Powder and Graphite Reinforcements on Physical and Mechanical Properties of Aluminum 2024 Alloy Fabricated by Stir Casting

In this study, boron fiber powder and graphite is reinforced to Al 2024 alloy to develop hybrid metal matrix composite by stir casting process. Hybrid MMCs developed with different weight fraction for 4%, 6%, 8% and 10% of boron fiber and 2% of graphite. Stirring parameters are optimized to obtain solid casting. Reinforcements are poured into molten aluminium at 15 g/min and stirrer is rotated for 5 minutes at 250 rpm with two stages stirring. 1% of magnesium was added to improve the wettability of Al 2024. Cast samples are machined as per the standards to investigate the microstructure, physical and mechanical properties. Optical and SEM analysis was carried out on machined sample to study the uniform distribution of particles. XRD and EDAX analysis is carried out to confirm the dispersion of particles into the matrix. Uniform distribution of the particles is found in optical and SEM images for these stirring parameters. The peak representation of boron and graphite particles is also observed in XRD and EDAX analysis. Theoretical and experimental density of the cast sample is determined by rule of mixture and Archimedes principle. Result shows the density of the composite decreases by increasing percentage of reinforcements. Micro Vickers hardness was tested on the cast composites and the result showed Al 2024 alloy hardness was increased by 31.25% by reinforcing boron and graphite. Similarly, tensile and compression strength increased by increasing the percentage of reinforcement. Tensile and compression strength of Al 2024 alloy increased by 45.23% and 29.18% respectively. The ductility of the composites decreased by increasing the percentage of reinforcements. How to cite this paper: Kumaraswamy, H.S., Bharat, V. and Rao, T.K. (2019) Influence of Boron Fiber Powder and Graphite Reinforcements on Physical and Mechanical Properties of Aluminum 2024 Alloy Fabricated by Stir Casting. Journal of Minerals and Materials Characterization and Engineering, 7, 103-116. https://doi.org/10.4236/jmmce.2019.73008 Received: March 24, 2019 Accepted: April 16, 2019 Published: April 19, 2019 Copyright © 2019 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access H. S. Kumaraswamy et al. DOI: 10.4236/jmmce.2019.73008 104 J. Minerals and Materials Characterization and Engineering


Introduction
Structural applications require high specific strength and therefore matrix material having light weight like magnesium, titanium or aluminum is mainly used. Also, the same matrix metal is used for very high temperature application. In the recent past, research is mainly focused on aluminum matrix composites (AMCs) due to its light weight and high specific strength which is an essential property for any structures.
Generally, there are three types of composites, namely metal matrix composites, polymer matrix composites and ceramic matrix composites. Metal matrix composites (MMCs) are composite materials consisting of metals as matrix and metal or non-metal as reinforcement. Reinforcements are used to improve mechanical and tribological properties. MMCs with more than one reinforcement are called as hybrid MMCs. Aluminum is reinforced with ceramic particles and fibers to improve mechanical and tribological properties like high strength, good malleability, good machinability, good corrosion resistance, improved wear resistance, improved hardness and high thermal conductivity. MMCs are fabricated by solid state and liquid state processes [1]. The selection process for fabricating MMCs plays a very important role. Stir casting is the most economical method for manufacturing MMCs compared with other manufacturing techniques. Generally, stir casting technique (liquid state) is employed as it is economical and can provide enhanced properties by attaining uniform distribution of reinforcements in the matrix. Stir casting technique has many challenges in developing the metal matrix composite. Uniform distribution of particles is the biggest challenge in this technique. Stirring process parameters play an important role to achieve the uniform distribution of particles. In this study, initially the following stirring parameters are selected initially to optimize. At 250 rpm of stirring speed, 10 minutes of stirring time at 800/°C, it gives the better distribution particles into the matrix [2]. Selection of stirring parameters like stirring speed and stirring time will have effect on the uniform distribution of particles and also affect the mechanical properties. At 600 rpm for 10 minutes, duration will give the uniform distribution of particles and good mechanical properties for different volume fraction of silicon carbide with aluminum [3]. The parameters considered for developing a MMC are 600 & 700 rpm of stirring speed and 5, 10, 15 min stirring time. The effects of these parameters on the microstructure and mechanical properties of the produced composites were analysed using re- show that the composite specimen with 15% alumina, 5% graphite and 3% boron carbide showed highest hardness of 70 HV; however, with increasing the content of reinforcements, the variations in micro hardness also increased. This variation is due to the non-uniform dispersion of reinforcements, which allowed agglomeration of particles and causes spots of variable hardness. Though, hardness of higher reinforced composites is much higher, but higher variations in different locations were obtained. The tensile strength of composites increased with increase in the reinforcement content up to 10% aB 4 C composite with 186.12 MPa and was 54.82% higher strength than the base aluminum alloy (120.79 MPa). The presence of higher amount of reinforcements in the matrix increases the discontinuities, which leads to lower strength. The hard phases present in the matrix prevents the higher order deformations and percentage elongation of material gets reduced. Tribological studies revealed that hardest composite revealed least wear and the presence of appropriate amount of graphite (5%) leads to uniform tribo layer. At higher loads and higher sliding velocity, the formed layer gets smeared; resulting in increased coefficient of friction and wear rate of composites [5].
In the present study Aluminium hybrid composite is developed stir casting by selecting Al 2024 as a matrix and reinforced with boron fiber powder and graphite developed by stir casting process by optimizing the stirring parameter.

Materials and Methodology
In the present study aluminium 2024 is used as a matrix, Boron fiber powder and graphite is used as a Reinforcements. The chemical composition of Al 2024 is shown in Table 1 mechanical properties of boron fiber powder is as shown in Table 2.

Experimental Detail
The liquid metallurgy route also called as stir casting technique was used for the development of hybrid metal matrix composite. Aluminum 2024, boron fiber powder and graphite is used as a matrix and reinforcements. The volume fraction of 4%, 6%, 8% and 10% aluminum composites are developed separately and Then the temperature range in the stir casting setup is set upto 850˚C, the temperature will raise slowly, and once temperature reaches 700˚C, Al 2024 ingots start melting, once the aluminum become completely molten state, C 2 Cl 6 degasification tablets are dispersed in the molten aluminum to remove the slag, wettability is one of the biggest challenge for aluminum 2024 alloy, addition of 1% of Magnesium improve the wettability of the aluminum molten [6] [7]. On the other side cast iron die is preheated to 500˚C for easy flow of molten metal and to maintain the temperature of molten metal constant. Once the temperature reaches 850˚C, pour the preheated reinforcements at 15 g/min, simultaneously

Density
Rule of mixture was used to calculate the theoretical density of the composites for the different weight fraction from 4% to 10% and 2% of graphite similarly experimental density is calculated using Archimedes' principle for the different composites

Tensile and Compression Strength
The tensile and compression strength of Al 2024 hybrid MMCS is tested using a computerized universal testing machine. The specimens are prepared as per the ASTM E-8 standards.
The specimen was machined as per ASTM E8 with 50 mm gauge length, 12.5 mm diameter, 25 mm length for holding the specimen on both side and fillet of 10 mm radius was developed to analyze the tensile strength. Similarly, 20 mm diameter and 50mm length specimens are machined to study the compression strength. Figure 3 shows the specimens used for testing the tensile strength and compression strength [12].

Result and Discussion
The following test has carried out on the developed hybrid MMC to analyze the different characterization. The following testing is carried out on the developed MMC.

Optical Micrograph
Optical microstructure study is carried out on Al 2024 hybrid composites and ticles. This is due to the optimized stirring speed and two stage stirring of molten during pouring of reinforcements. The images also show that the particles are strongly bonded with the matrix.

XRD Analysis
For 8% of boron fiber powder and 2% of graphite, the reaction products Carbide

Micro Vickers Hardness
The

Tensile Strength
The tensile test is conducted on hybrid MMCs using computerized universal testing machine as per ASTM -E8 standard specimens, the result reveals that the tensile strength of the composites increases with increasing the percentage of reinforcements. Cast Al 2024 is having a tensile strength of 120 MPa without reinforcing boron and graphite. Figure 8 shows tensile test specimens after failure.

Compression Strength
The specimens are prepared as per ASTM -E8 standard to study the compression strength of hybrid MMCs using computerized universal testing machine, Figure 10(a) and Figure 10

Fracture Analysis of Tensile Samples
Achieving uniform distribution in the stir casting is the biggest challenge. The strength of the composites is depending on the uniform distribution of the particles in the matrix, stirring parameters is one of the key points in stir casting to achieve the uniform distribution of the particles. Since the density of the ceramic material used is less than aluminium chances are more either floating of reinforcement or settling of reinforcement particles in the bottom of crucible is a reason for uneven distribution of particles. As a result of optimized stirring parameters, reinforced particles are distributed uniformly in the matrix and supporting the base metal at the time failure [13]. Figure 11 shows the reinforcement are bonded together and supporting base matrix to improve the strength while fracturing. Proper bonding is observed between the matrix and reinforcement. There is a pull out of particles while breaking has found. This is due to the level of affinity between Al 2024 alloy and boron fiber powder and graphite. Also grain size and shape of reinforcement particle determine bonding ability. If the reinforcement does not mix and bond with metal matrix, the strength of the composites will decrease. It is observed during tensile test due to the proper bonding between the matrix and reinforcement The reason for increasing the strength of Al 2024 by increasing percentage of reinforcements was more particles bonding and supporting the matrix Figure   11(b) number of particles are more since the percentage of reinforcement is more and this cause supporting the matrix will increases hence the strength of the Al 2024 alloy increases by increasing the percentage of reinforcements EDAX analysis shows that there is a presence of boron and graphite particle near the fracture surface.

Conclusions
The following conclusions are drawn after the completion of testing: 1) The density of the Al 2024 alloy s is decreased by increasing the percentage of reinforcement.
2) The porosity of the Al 2024 alloy is increases by increasing the percentage of reinforcement.
3) The optical and SEM analysis shows there is uniform distribution of reinforce particle in the hybrid MMCs.