Characterization of Aluminium Alloy LM6 with B₄C and Graphite Reinforced Hybrid Metal Matrix Composites ^†

Abstract

Hybrid metal matrix composites (MMCs) are increasingly important in aviation, marine, automotive, and industrial manufacturing due to their ability to enhance the mechanical and chemical properties of composites. The study aimed to understand the fabrication, mechanical properties and microstructural properties of LM6/B4C/Gr composites. An aluminium alloy (LM6) is the base metal, having properties of less weight, medium strength, and excellent castability. The addition of B4C and Gr enhanced the tensile strength, hardness, and wear resistance of the composites, while maintaining good ductility. Boron carbide is a lightweight and extremely hard material with excellent wear resistance and high thermal stability. It has a specific modulus that is almost two times higher than that of aluminium, meaning it can provide superior stiffness and strength while maintaining a low weight such as drive shafts, housings, and structural supports. The addition of graphite improves the lubrication properties of the composites. Composites were successfully fabricated through a stir casting process, with the uniform dispersion of boron carbide and graphite particles in the aluminium LM6 matrix. The hybrid metal matrix composites are fabricated by five different combinations of B4C (1, 2, 3, 4, 5 wt%) with constant wt% of graphite (1 wt%).The fabricated samples of hybrid composites used to find the mechanical properties and microstructure analysis. The test results reveal that the tensile strength and hardness of the composites increased with an increase in the weight percentage of reinforcements, and the percentage of elongation decreases with increasing the reinforcement particles. The boron carbide (B₄C) and graphite (Gr)particles in a matrix material are analyzed by a scanning electron microscope (SEM). Energy dispersive X-ray analysis (EDX) is used to evaluate the microstructure and chemical composition of the composites, providing valuable insights for their design and optimization.

1. Introduction

Monolithic metals and alloys are excellent in many applications, but they might fall short in meeting specific requirements, such as lightweight structures with high strength, corrosion resistance, or tailored electrical properties. This is where composite materials come into play. Composite materials offer a way to combine the best properties of different materials, often overcoming the limitations of individual constituents. By strategically arranging these materials, engineers can create materials that exhibit enhanced performance and efficiency that would be difficult to achieve with single-phase materials like metals. Metal matrix composites (MMCs) are indeed among the most promising advanced materials with a wide range of applications due to their unique combination of properties.

1.1. Metal-Matrix Composites

The design of composite materials, including metal matrix composites (MMCs), involves careful consideration of the reinforcing materials and their interaction with the matrix material. By selecting the appropriate reinforcing materials, volume fractions, shapes, and sizes, engineers can tailor the mechanical properties of the composite to meet specific performance requirements. Composites of a metal matrix include a wide variety of metal systems (for example, Al, Ti, Mg, Fe, Cu, Ni alloys) using several types of reinforcements in the form of whiskers (SiC), monofilament (SIC, W, B), continuous fibers (SiC, alumina, graphite), and particulates (TiC, SiC, Al₂O₃, B₄C, etc.). While metal matrix composites (MMCs) offer significant performance advantages, there are indeed barriers that need to be addressed before they can be successfully integrated into products.

1.2. Aluminium (LM6 Alloy) Based Metal Matrix Composites

Aluminum-based metal matrix composites (MMCs) are being developed and considered as potential substitutes for a wide range of conventional monolithic materials in various applications. The unique properties of aluminum-based MMCs make them attractive candidates for enhancing performance and providing new solutions in industries where traditional materials may fall short.

The selection of an appropriate matrix material is crucial in the development of aluminum-based metal matrix composites (MMCs), and commercially available aluminum-silicon alloy LM6 is a notable choice due to its exceptional castability. LM6 is an aluminum alloy that contains silicon as one of its main alloying elements. LM6 indeed stands out as a versatile and valuable material, especially in applications that require a combination of corrosion resistance, castability, mechanical performance, and machinability. The use of certain aluminum alloys like Al 6061, Al 2024, Al 7075, LM25, and LM26 in research work has been studied extensively by the researchers, while noting that characterization data for LM6 alloys are relatively scarce. Common reinforcement particles that are frequently employed in aluminum matrix composites (AMCs) to enhance their properties and performance. These reinforcement particles, including titanium carbide (TiC), titanium diboride (TiB2), silicon carbide (SiC), alumina (Al₂O₃), and boron carbide (B₄C), play a crucial role in tailoring the mechanical, thermal, and other functional characteristics of the composite material. B₄C is indeed a remarkable material that offers a range of properties that make it a valuable choice for strengthening Al-based composites. Its exceptional hardness, low density, chemical stability, strength, tribological properties, neutron absorption capability, thermal stability, and chemical inertness contribute to its widespread use in various industries. It’s clear that aluminum metal matrix composites (AMCs) reinforced with boron carbide (B₄C) particulates have captured significant attention due to their potential for the substantial enhancement of mechanical and thermal properties [1]. The addition of graphite to aluminum alloys highlights the potential for tailored material properties by combining different elements. This results in an enhanced performance, making the composite materials suitable for various applications, including those where wear resistance, low friction, and thermal conductivity are critical [2].

1.3. Literature Review

M.H. Faisal et al. [3] performed different tests on LM6/B₄C (3 and 5% of B₄C) and LM6/B₄C 7%/Gr 2% composites. Mechanical properties like yield stress, tensile strength, wear, hardness, COF, and frictional force were analyzed using tensile, hardness, and pin on disc tests. Tests inferred that, by improving the weight % of B₄C flakes in the LM6 Al matrix, the hardness of the produced composites gets raised. SM. Sutharsan et al. carried out the studies on the development of aluminum alloys with boron and graphite reinforcement additions in varying percentages of volume and carried out hardness, tensile, impact, and wear tests by mechanical testing. In this research, the following points are concluded: composite displays the highest hardness value and lower wear rate of 32 HRB and 0.295 mm³ Nm⁻¹ with 7.5% B₄C + 2.5% Gr. C.Muthazhagan et al. [4] conducted tests on reinforcement particles, such as boron carbide and graphite, which were uniformly distributed in the Al matrix. The hardness of the composite is increased with the increasing of boron carbide particles in the aluminum matrix. The hardness of the composite is decreased with the increasing of graphite particles in aluminum matrix. R. Sathish et al. [5] performed a characterization of the aluminum alloy LM6 with B₄C and a fly ash reinforced composite. The optimum results were observed in an aluminum alloy (LM6) with 12% of fly ash and 3% of boron carbide (B₄C) reinforced matrix from impact strength. The fly ash and boron carbide (B₄C) particles in a matrix material were analyzed by a scanning electron microscope (SEM). Energy dispersive X-ray analysis (EDX) was used to observe the presence of reinforced particles in the hybrid metal matrix composite, forming an optimum result of a specimen. Ramachandra Raju K et al. [6] investigated the properties of the LM26 hybrid composite material with ceramic reinforcements. The experimental results show that the composite with the 2% SiC, 4% B₄C and 94% LM26 combination has a higher hardness and tensile strength. B.N. Sharath et al. [7] performed machinability studies on boron carbide and graphite reinforced aluminum hybrid composites. Incorporating B₄C and Gr reinforcements helps the material to shear easily and the formation of the discontinuous chip during the drilling of composites results in lower thrust forces. Rajkumar et al. [8] investigated the mechanical behavior of B₄C and graphite particles reinforced AL2117 alloy hybrid metal composites. The hardness of the aluminum matrix increased with the addition of 3% B₄C-5% graphite particulates and 6% B₄C-5% graphite particulates by 3.1% and 10.41%, respectively. The ultimate tensile strength of the Al 2117 matrix increased by 4.06% and 9.6% with the addition of 3% B₄C-5% graphite particulates and 6% B₄C-5% graphite particulates. B Jayendra et al. [9] studied the mechanical characterization of stir cast Al-7075/B₄C/graphite reinforced hybrid metal matrix composites. The hardness of the composite is an increasing trend with the increase of graphite percentage and is found at a maximum of 159 for 2% B₄C, 3% graphite and 95% Al7075 MMC. The hardness of the MMCs were found to be much higher than the base material alloys. K Sekar et al. studied the [10] mechanical characterization of stir cast Al-7075/B₄C/graphite Reinforced hybrid metal matrix composites. The hardness of hybrid composites is increased from 92.05 HV to 104 HV by the addition of 1 wt% of B₄C and 1.5 wt% of graphite and the hardness increased up to 12.98% compared to the base alloys.

2. Materials and Methods/Methodology

2.1. Materials Used

Matrix material—LM6; Reinforcement 1—boron carbide microparticles & Reinforcement 2—graphite microparticles.

Table 1. Chemical constitution of the aluminum LM6 alloy [11].

Alloy	Chemical Composition (Mass Percentage)
LM6	Cu	Si	Mg	Zn	Fe	Mn	Ni	Sn	Pb	Ti	Al
	0.1 max	10 to 13	0.1 max	0.1 max	0.1 max	0.5 max	0.1 max	0.05 max	0.1 max	0.2 max	Remainder

shows the chemical constitution of the aluminum LM6 alloy,

Table 2. Mechanical properties of the aluminum LM6 alloy [12].

Matrix Material	Properties
	Density (g/cm3)	Thermal Conductivity (W/mK)	Tensile Stress (N/mm2)	% of Elongation in 50 mm	Hardness (BHN)	Young’s Modulus (×103 N/mm2)	Coff. of Thermal Expansion (μm/m-C)
LM6	2.65	0.34	160–190	5	50–55	71	21 × 10−4

shows the mechanical properties of the aluminum LM6 alloy,

Table 3. Properties of the boron carbide reinforcement [13].

Reinforcement	Properties
	Density (g/cm3)	Elastic Modulus (GPa)	Vickers Hardness (Kg/mm2)	Compressive Strength (MPa)	Thermal Conductivity (W/Mk)	Coefficient of Thermal Expansion (10−6/K)	Specific Thermal Conductivity (W·m2/Kg·K)
B4C	2.52	450	3770	3000	29	5.0	11.5

shows the chemical constitution of boron carbide reinforcement, and

Table 4. Properties of the graphite reinforcement.

Reinforcement	Properties
	Atomic Weight (g/mol)	Appearance	Density (g/cm3)	Melting Point (°C)	Boiling Point (°C)	Elastic Modulus (GPa)	Thermal Conductivity (W/mk)
Graphite	12.01	Black Powder	1.8	3652–3697	4200	12	90

shows the chemical constitution of graphite reinforcement.

2.2. Material Composition

In Figure 1, a graphical representation illustrates the formulation, combination, and preparation of the selected materials for the development of a new metal matrix composite. When a large volume of boron carbide is used in the composite, it becomes more difficult to achieve an even distribution of the reinforcing particles within the aluminum matrix. The denser aluminum tends to settle at the bottom of the casting, while the lighter boron carbide particles float towards the top. To overcome this challenge, various techniques can be employed to enhance the dispersion of boron carbide particles within the aluminum matrix. Some common approaches are mechanical mixing, particle surface treatment, optimization of process parameters and pre-alloying, etc. It is reported that incorporating a small volume fraction of reinforcing particles in aluminum alloys can provide advantages for machining, including reduced tool wear, improved chip formation, reduced cutting forces, and better surface finish. Considering this, the maximum percentage of reinforcement employed is limited to 5% in this investigation for the fabrication of composites. Based on the literature study, adding a smaller volume fraction of graphite, typically up to 2%, can provide a self-lubricating effect and improved wear performance to aluminum matrix composites [14]. Potassium hexafluoro titanate (K2TiF6) is indeed chosen as an additive to enhance the wettability of boron carbide (B₄C) in an aluminum LM6 matrix. The addition of K2TiF6 has two key effects on the composite material. Titanium improves grain refinement and superior bonding with the aluminum matrix alloy is the effect of the titanate (Ti) component. Gas trapping in the melt can be shunned through potassium and fluoride components [15].

2.3. Procurement of Matrix and Reinforcement Materials

The aluminum LM6 used in the study was procured from Rajeswari Metal Casting and Alloys located in Koorgalli, Mysore. The material was obtained in the form of billets, with each billet weighing approximately 7 kg. Matrix and reinforced materials are as shown in Figure 2. In this current study, boron carbide (B₄C) is selected as one of the reinforcement material. The boron carbide used in this research work was procured from the Nano Research Lab, located in Jamshedpur. The material was obtained in powder form with a high purity level of 99%. The boron carbide powder obtained from the Nano Research Lab has an atomic particle size (APS) that varies from 1 to 10 μm. In addition to boron carbide, graphite is also used as a reinforcement material in the current study. The graphite used in this research work was procured from the Nano Research Lab, located in Jamshedpur.

2.4. Fabrication—Stir Casting Method

Stir casting is a widely used method for producing metal matrix composites by achieving a uniform dispersion of reinforcement particles in a molten matrix alloy through mechanical stirring [16].

2.5. Fabrication Procedure

The composites were prepared in the electrical resistance furnace shown in Figure 3 with the following specifications: Rating: 6 KW: Power: 640 V; 50 Hz AC: Max Temp: 900 °C.

Illustration of complete stir casting process is elaborated in Figure 4.

2.6. Detailed Procedure for the Preparation of the Composite Are as Follows

In the experimental procedure, the electric resistance furnace was switched on and heated to a specific temperature of 750 °C. By preheating the graphite crucible to a relatively low temperature of 60–70 °C, any moisture or volatile substances can be evaporated and eliminated from the crucible’s interior. Both the base metal alloy (aluminum LM6) and the reinforcement materials (such as boron carbide and graphite) were weighed according to the desired composition for casting. Preheating the base metal alloy and reinforcement materials within the temperature range of 350 °C–450 °C allows for improved mixing, enhanced wetting, and reduced thermal shock, contributing to the successful fabrication of the composite material. The pre-weighed aluminum alloy LM6 was placed into the crucible and heated up to a temperature of 750 °C. This temperature is above the melting point of the aluminum alloy, which is approximately 660 °C [17]. A degasifier tablet containing hexa-chloro-ethane (HCE) was added to the molten metal, and it was held immersed for a few seconds until all the gases were removed. A stirring action is performed to disperse the reinforcement particulates uniformly throughout the molten metal matrix, resulting in a homogeneous distribution of the reinforcement particles within the melt. The mold boxes were heated using a blowtorch until they lost all moisture. This step is carried out to remove any moisture or residual moisture that might be present in the mold boxes. The pre-weighed and pre-heated reinforcement materials, such as boron carbide or graphite, were added to the molten metal in the furnace using a graphite spoon. Simultaneously, the molten metal was stirred at a stirring speed of 550 rpm to create a vortex for approximately 5 min. The wettability agent, potassium hexafluoro titanate (K2TiF6), was introduced into the molten metal gradually. The amount of additive used was 25% of the weight of the reinforcement quantity.

3. Results and Discussion

3.1. Static Mechanical and Microstructural Characterization

By conducting static mechanical and microstructural characterization, we get the comprehensive understanding of the composite material’s mechanical behavior, structural integrity, and microstructural features.

3.2. Tensile Test

Tension tests are a common method used to determine the mechanical properties of materials, including their strength and deformation characteristics. The tension tests were conducted on cylindrical specimens using a BISS make computerized 50 kN universal testing machine (UTM). Aluminum–boron carbide–graphite specimens are prepared as per the recommendations given in ASTM E8-04 [18]. The initial phase of the tensile test was conducted in load mode, where the universal testing machine (UTM) applies an increasing tensile force to the specimen. The load mode is used until the applied force reaches 0.5 kN. Figure 5 shows the geometry of tensile test specimen as per ASTM standards. Figure 6a shows the prepared specimens for the tensile test. Figure 6b shows the tensile test specimens after the test.

Tensile Stress Results

The mechanical properties of the aluminium–boron carbide–graphite test specimens for all compositions are determined and the values of peak load, ultimate tensile strength, percentage of elongation on gage length, modulus of elasticity, and 0.2% yield strength are presented in the following

Table 5. Tensile stress results of the specimens.

wt% of Aluminium LM6	wt% of Boron Carbide	wt% of Graphite	Composite Nomenclature	Peak Load (kN)	0.2% Yield Strength (MPa)	Ultimate Tensile Strength (MPa)	Young’s Modulus (GPa)	Elongation of Gage Length (%)
100% LM6	0	0	C1	9.23	97.23	135.20	67.82	7.04
98% LM6	1	1	C2	10.35	109.13	142.01	72.09	6.32
97% LM6	2		C3	10.97	114.87	148.20	76.54	6.06
96% LM6	3		C4	11.42	121.97	156.45	82.76	5.44
95% LM6	4		C5	12.01	135.23	168.23	86.55	5.09
94% LM6	5		C6	12.20	138.23	170.98	88.32	4.90

.

Figure 7 shows the effect of boron carbide and graphite reinforcements on tensile properties.

From Figure 8, it can be seen that the percentage of elongation of the composites decreases as the increase of B₄C content in the LM6 alloy matrix. Under tensile loading, B₄C particle may causes stress concentration around particles. Thus, particles dispersed at grain boundaries are the key for stress concentrations. So, the addition of B₄C particles reduced the elongation of the matrix. Thus, the elongation decreased with the increase of wt% of B₄C particulates.

3.3. Density Test

Density tests were conducted as per the Archimedes water displacement method (ASTM D792) [19] using a densometer according to which the test samples were weighed in the air, followed by weighing it immersed in distilled water at room temperature (assumed as 28 °C). The density of water at 28 °C is considered as 0.99626 g/cm³. Density was calculated using the relation shown in Equation (1).

ρc = (m_a/(m_a − m_w)) ∗ ρw

(1)

where ρw and ρc are the density of water and composite, respectively, at room temperature; m_w and m_a correspond to mass of composite specimen in water and air, respectively. A specimen of diameter 12 mm and thickness 3 mm was prepared for conducting the test for each composition as shown in Figure 9.

Theoretical density is calculated using the rule of mixture as shown in equation in (2).

ρc = ρm V_m + ρ_r V_r

(2)

where ρc, ρm and ρr are density of composite, matrix, and reinforcement, respectively; Vm and Vr are volume of matrix and reinforcement, respectively. The theoretical density calculated, and experimental density obtained are listed in

Table 6. Theoretical and experimental density test results.

Composite Nomenclature		Theoretical Density (kg/m3)	Experimental Density			Void (%)
			Mass in Air (g)	Mass in Water (g)	Density (Kg/m3)
Al LM6	C1	2680	1.9964	1.2558	2689	0.34
98% Al–1% B4C–1% Gr	C2	2674.2	2.1528	1.3532	2686	0.44
97% Al–2% B4C–1% Gr	C3	2672.6	2.1351	1.3398	2678	0.21
96%Al–3% B4C–1% Gr	C4	2671	1.9561	1.2235	2662	0.34
95%Al–4% B4C–1% Gr	C5	2669.4	1.8669	1.1668	2659	0.39
94%Al–5% B4C–1% Gr	C6	2667.8	2.0173	1.2555	2642	0.97

. Figure 10 shows the density test results of aluminium–boron carbide–graphite composites.

3.4. Hardness Test

Hardness tests were performed on a Micro-Vickers hardness tester according to the ASTM E384 standard [20]. Performing hardness tests using a Micro-Vickers hardness tester and following the ASTM E384 standard is a well-established method for measuring the hardness of materials. ASTM E384 is a standard test method for micro-indentation hardness testing, and it provides guidelines for conducting Vickers hardness tests at small loads, specifically in the micro-hardness range. Proper surface preparation is a fundamental aspect of hardness testing and is vital for obtaining accurate and meaningful data, leading to better material characterization and understanding of its mechanical properties. The maximum load applied during indentation was 0.3 Kgf and the load was maintained for a duration of 10 s. The average values were computed for each sample and are presented in

Table 7. Hardness test results of aluminium–boron carbide–graphite composites.

Composite Nomenclature		Average Hardness Value
Al LM6	C1	83.03
98% Al–1% B4C–1% Gr	C2	91.93
97% Al–2% B4C–1% Gr	C3	103.33
96%Al–3% B4C–1% Gr	C4	115.67
95%Al–4% B4C–1% Gr	C5	123.33
94%Al–5% B4C–1% Gr	C6	127.67

. From Figure 11, it can be observed that, when the quantity of boron carbide is increased while keeping the quantity of graphite reinforcements fixed, the hardness values increases.

The maximum hardness value has increased by nearly 54% with respect to the base alloy. The increase in hardness is often associated with the presence of reinforcement particles acting as barriers or obstacles to the movement of dislocations within the material and they play a crucial role in the plastic deformation of materials.

3.5. Microstructure Analysis Using Scanning Electron Microscopy (SEM)

The specimens are prepared for testing or analysis using abrasive paper and polishing techniques as shown in Figure 12. This preparation ensures that the specimens have smooth surfaces, are free from imperfections, and are suitable for various characterization methods. Figure 13 shows the SEM (Backscattered) images of (a) Al LM6 base alloy, (b) Al-1%B₄C &1%Gr, (c) Al-2%B₄C &1%Gr, (d) Al-3%B₄C &1%Gr, (e) Al-4%B₄C &1%Gr, (f) Al-5%B₄C &1%Gr Composites.

3.6. Energy Dispersive Spectometry (EDS) Analysis

EDS is particularly useful for identifying elements on the surface of a sample and for providing qualitative and semi-quantitative information about the composition. The EDS spectrum for the aluminum LM6 alloy are shown in Figure 14. It shows the existence of a high percentage of aluminum along with silicon, carbon, oxide and magnesium. The EDS spectrum of composite specimens containing 5% boron carbide and 1% graphite is shown in Figure 15. Elemental composition of the sample confirms the presence of several more elements: boron (B) and carbon (C), in addition to silicon (Si), aluminum (Al), magnesium (Mg), and oxygen (O).

4. Conclusions

The investigation into the effects of increases in different percentages pf reinforcement of boron carbide particulates along with fixed percentage of graphite on the matrix alloy LM 6, revealed the improvement in tensile strength, modulus of elasticity when compared to the matrix alloy. Density values were measured using a densometer and the void is within 1% indicating homogeneous dispersion of reinforcement in the aluminum matrix. The microstructure of the specimens were examined through scanning electron microscopy. Backscattered SEM showed uniform dispersion of boron carbide and graphite in the aluminum matrix with a decrease in inter-particle spacing. Small clustering and voids were observed, supporting the decrease in the strength of composites. Energy dispersion spectroscopy (EDS) confirmed the presence of boron carbide particles in the aluminum matrix.