Investigation on Mechanical and Wear Behaviors of LM6 Aluminium Alloy-Based Hybrid Metal Matrix Composites Using Stir Casting Process

In this investigation


Introduction
Aluminium-based metal matrix composite (AMMC) is widely used in automobiles, sports, aerospace, marine, and other engineering fields.It is because of high strength-toweight ratio, wear resistance, good corrosion resistance, and better machinability.Aluminium alloy-based ceramic composites fabricated through liquid metallurgy technique offered superior mechanical and tribological properties [1].Wear is one of the vital parameters for considering aluminium hybrid metal matrix materials.e main drawback of materials is that they exhibit poor tribological properties.Tribological performance of aluminium reinforced with different ceramic particles using a pin on disc tribometer exhibited better wear characteristics [2].Aluminium alloy reinforced with B 4 C composites was prepared by means of liquid metallurgy techniques with various particulate weight fractions.When the density of composites decreased, the tensile power, hardness, compressive power, and fracture toughness increased with the addition of ceramic particles.Aluminium (LM6) matrix reinforced with aluminium oxide and fly ash was fabricated which increased the wear resistance [3][4][5].e LM6 aluminium alloy is a eutectic alloy containing 85% of aluminium and 12-15% of silicon.It has the ability to resist corrosion and hot cracking.Aluminiumbased metal matrix composites reduce the cost of the material because of their lesser weight, durability, and recyclability.Easy availability of reinforcements and higher production volumes can minimize the cost of metal matrix composites.Addition of ceramic particles like silicon, Al 2 O 3 , graphite, and lead oxide glass provides better castability and machinability to the composites [6].Metal matrix composites can be produced by liquid metallurgy techniques to produce complex net shapes.In the stir casting process, discontinuous reinforcements in the matrix result in poor wettability, porosity, and interfacial reactions.e LM6 aluminium-based composites offered better wear resistance and high strength by the addition of adding short steel fiber in various weight fractions [7].Different weight fractions of B 4 C reinforcement (3,6,9, and 12 wt.%) with the addition of 3 wt.% graphite (Gr) were added to the matrix material to develop aluminium hybrid composites.e newly developed mixtures were created by the stir casting method.Aluminium hybrid composites were tested under pin on disc for different loads and sliding distances.Addition of graphite materials provided self-lubricating effect, while the ceramic particles increased the strength of the matrix material.Worn surface shows fine grooves with delamination at minimum loads and sliding speed.Al-B 4 C-Gr composites with 9% B 4 C and 3% Gr enhanced better wear resistance compared with other reinforcements [8].Moreover, the LM6 aluminium is reinforced with the addition of fly ash using the stir casting technique.
e mechanical behavior of composites was investigated, and the ultimate tensile strength was increased from 35% to 45%.However, the density and wear loss of composites were decreased.e accumulation of fly ash into LM6 matrix material increased the mechanical features of composites [9].e metal matrix (AA6061) reinforced with silicon carbide and B 4 C was fabricated by the powder metallurgy process.e hardness, strength, and coefficient of friction increased, while the wear rate was reduced with the addition of 10% volume of ceramic particles.B 4 C particles of various weight % (2, 4, 6, 8, 10, and 12 wt.%) and average particles size of 25 μm were reinforced to the base metal by the stir casting method.It was observed that the mechanical features of samples improved by the addition of reinforcements [10].
e LM6 alloy with varying weight fractions of B 4 C increased the hardness and decreased the density [11], and the LM6 alloy with copper and SiC composition increased the density and hardness [12].e addition of Al 2 O 3 reinforcement particles into the LM6 alloy increased the hardness and compressive strength [13].Similarly, the addition of Al nickel particles in LM6 alloy offered higher tensile and yield strength compared with pure alloy.
Gowtham et al. [15] reported the fabrications of LM6 alloy with B4C which were prepared by the stir casting method.e mechanical behaviors were investigated using tests like the tensile test, hardness test, and compression test.
e new composite materials of spur gear can be designed and analyzed by FESEM.Finally, the gear model represented an increase in module, and pressure angle decreased the contact stress among the mating gears.Pio et al. [16] investigated the effect of Al-Ti-B and compared it with LM6 alloy by the sand casting method.e performance of the sand casting process was analyzed by considering different moduli of materials.
e authors reported the correction between cooling rates with grain refinement.e authors found that LM6 aluminium alloy produced by lower modulus offered better mechanical properties.Kaur Sandhu [17] investigated the LM6 reinforced with SiC and Al 2 O 3 .
e mechanical properties were improved because of the scattering of ceramic particles.
Sivaprakash and Sathish [18] investigated the LM6 alloy with the addition of Al 2 O 3 and Si 3 N 4 composites fabricated by a squeeze casting route.
e main advantages of this method were maximum utilization of materials, low porosity, and good surface finish.Faisal and Prabagaran [19] investigated that LM6 alloy composites with the addition of B 4 C particles, and the mechanical properties were examined.
e graphite particles provided self-lubricating properties, which reduced the consumption of fuel and lubricating oil with additional advantage of energy expenditure in industries and automobile modules.e new composite material of the piston was designed, and an analysis was carried out by FEM.Further, it was compared with the base alloy, and the result revealed that hybrid composites produced better static structural properties.Mandala et al. [20,21] examined Al-2 Mg alloy matrix with uncoated copper and nickelcoated steel fiber.Copper-coated steel fiber indicated good bonding strength and interface between the matrix and fillers.
ese composites offered higher strength than the parent metal.e fracture mechanisms of composites like dimple formation, fiber breakage, and pull out were identified.From the above literature survey, it was observed that the mechanical properties of base alloy increased with the addition of filler materials and ceramic particles.
In this work, the hybrid composite is prepared by reinforcing copper-coated short steel fiber and B 4 C particles into LM6 alloy using the stir casting method.B 4 C particle has been selected because of its higher hardness with excellent resistance to wear and corrosion.e earlier research works carried out by the authors report the mechanical and wear behaviors of single steel fiber-reinforced LM13/LM6 aluminium alloy-based composites [7,[22][23][24][25][26].So far, the authors studied single steel fiber-reinforced composites.e present work presents the mechanical and wear behaviors of particle (B 4 C) and fiber (copper-coated steel fiber)-reinforced hybrid metal matrix composites.e combination of particle and fiber-reinforced composites will provide better properties rather than single particle/fiber-reinforced composites.e mechanical properties such as hardness and 2 Advances in Materials Science and Engineering tensile properties, microstructures, and tribological properties of the fabricated samples are investigated as per ASTM standards and reported.

Materials and Methods
2.1.Material Selection.LM6 aluminium alloy is mainly used in automotive applications because of its higher silica content [27,28].Hence, it was chosen as matrix.Boron carbide particles have been chosen because of their higher hardness and excellent resistance to wear and corrosion.
To have an advantage of particle and fiber-reinforced composites, steel fibers have been chosen because of their higher tensile strength and hardness with better resistance to wear.Chemical structure of LM6 aluminium alloy is presented in Table 1.Steel fibers with chemical structures are given in Table 2. 5 wt% of B4C with particle size less than 100 microns is reinforced in LM6 aluminium alloy.3 mm steel fiber is used as secondary reinforcement, and its wt% is varied as 0, 2.5, 5, and 10.Steel fibers are copper coated using the electrode method [7].Steel fibers with an average fiber diameter of 190 μm and 500-3500 μm length are reinforced in LM6 aluminium alloy.

Experimental Procedure
3.1.Stir Casting Method.LM6 Al alloy hybrid metal matrix mixtures were prepared by liquid metallurgy route-stir casting process, as shown in Figure 1.800 g of LM6 Al alloy was dissolved in a graphite crucible and warmed up to 750 °C.
en, the B 4 C and short steel fiber were stirred at 950 °C for 2-3 hours with 850 rpm, respectively.Ceramic particles and short fibers are split due to shear force exerted by the impeller while stirring process.e melt is continuously stirred to achieve uniform dispersion of ceramic particles and fibers in the matrix.Consequently, the molten melt is discharged into a preheated cast iron die at 250 °C for required 300 mm length with 30 mm diameter (Figure 2).According to ASTM standards, the samples were made and the microstructure, hardness, tensile strength, and wear were investigated.

Tensile Test.
Tensile samples were prepared as per ASTM E8 standard, as shown in Figure 3. e tensile tests were conducted by computerized UTM Model TECSOL and TMC Engineering model, India, with a 10 kN load cell for each sample.e tensile strength was investigated using a crosshead speed of 2.5 mm/min.Figure 4 shows the prepared tensile samples and broken tensile samples tested to explore the tensile strength of mixtures.

Brinell Hardness Test.
e hardness test was performed in Brinell hardness verifier to check the hardness of LM6 Al alloy hybrid metal matrix composites.In this test, a hardened steel ball indenter is forced into the surface of a metal to be tested.e test was performed by using 5 mm ball indenter with a load of 500 kgf and dwell time of 10-15 S.

Wear Test.
Wear specimens were prepared for the required dimensions of 10 mm diameter with 35 mm length as per ASTMG99 standard.
e contact surface of the pin against the rotating disc (EN31 stainless steel) wear tester is shown in Figure 5.To study the wear behavior of samples, a load of 10 N-40 N with a sliding distance of 500-2000 m was considered.Weight losses were measured during the experiment through a single pan electronic machine with a precision of 0.0001 g. e sample was cleaned using acetone solution to calculate the wear loss for each sample.e worn surfaces were investigated using FESEM.

Results and Discussion
4.1.Microstructure Analysis.Analysis of LM6 Al alloy-based hybrid composites was carried out by an optical microscope at various wt.% of reinforced materials.Various percentages of composite samples were polished by using an emery sheet.
ere are various types of emery sheets used to polish the materials like 120, 400, 800, 1200, 1500, and 2500 grit sheets.After completion of polishing process, the samples were etched for microstructure analysis.
Dispersion of reinforcements was examined in different samples.ose particles were clearly visible while examining through the metallurgical microscope.LM6 Al alloy strengthened by different weight % of steel fiber with the addition of 5 wt.% of B 4 C materials is shown in Figures 6(a) and 6(b).It shows uniform dispersion of reinforcements in AMMCs.Also, a good interface relation among steel fiber and parent material is achieved because of its coating of copper on fibers.Figures 6(c) and 6(d) show the microstructure of steel fiber in LM6 Al alloy, and micropores were obtained.is fiber was also coated with copper.Copper coating on steel fibers avoids the connection among liquid aluminium as well as steel fiber, hence feasible creation of intermetallic compounds.Copper responds to aluminium and creates a logical relation on the boundary.LM6 aluminium alloy-based hybrid metal matrix composites are produced by reinforcing ceramic particle and short steel fibers.

Hardness.
Hardness of samples increases with the addition of short steel fiber and constant B 4 C addition in matrix.is is due to a reformed grain size of material that increased the hardness of material as shown in Figure 7.
By observing the graph, it was found that without any addition of reinforcements, the hardness number is low.By varying copper-coated steel fiber (CCSF), the hardness of samples increased.e hardness of composites increased up to 146 BHN by reinforcing 10 wt% short steel fiber and 5 wt% boron carbide particles in the matrix.

Ultimate Tensile Strength.
e effect of ceramic particles and steel fibers on tensile strength of hybrid composites was determined from the tensile test.Change of ultimate tensile strength with varying steel fibers is shown in Figure 8.
is steel fiber is coated with copper.Tensile Advances in Materials Science and Engineering     4 Advances in Materials Science and Engineering strength of models increased along with increment of fiber content up to 5 wt.% CCSF.Addition of steel fibers reduces the ultimate tensile power of samples.Percentage of elongation is measured for checking the ductility, as shown in Figure 9.It was observed that the position of mixture material reduces with the addition of short steel fibers in the matrix material.

Analysis of Rupture Surface.
Rupture surface of LM6 alloy hybrid composites is shown as a crevice that produces ductile fracture.However, broken steel fibers are measured in the fracture layer of hybrid composites, as shown in          Advances in Materials Science and Engineering particles and fibers are observed.e wear method transforms from medium to serious through varying weights.At a load of 40 N, local delamination and extensive surface ploughing were examined.

Conclusion
Experimental investigations were conducted on LM6 Al alloy-based hybrid metal matrix materials that were prepared for various weight fractions of short steel fiber (CCSF) by the stir casting route.e above samples were investigated to check the tensile strength, hardness, and wear resistance, and the following results were obtained: (1) Microstructure of samples indicated good interfacial connection among matrix stage and support.(2) Brinell hardness increased with increasing short fiber content and constant amount of B 4 C into the base material.(3) Tensile strength of samples indicated that percentage of fiber reinforcement in LM6 Al alloy hybrid composites increased the tensile strength, and high strength was observed for 5 wt.% of short steel fiberreinforced composites.If fiber content increases beyond 5 wt.%, then the tensile strength of materials is reduced.(4) Wear weight loss of samples increased with the addition of weight and sliding distance.However, weight loss is reduced with the addition of weight percentage of reinforcements.(5) Fracture surface of samples showed broken steel fibers with depression on the layer.Ductile fracture was found for composites.(6) Worn surfaces of composites revealed fine grooves, continuous grooves, delamination, and steel fibers at different locations in samples.

Figure 4 :
Figure 4: Tensile samples before and after tensile test of LM6 Al alloy hybrid composites.

4. 3 .
Tribological Behavior of Hybrid Composites 4.3.1.Cumulative Wear Loss.Collective weight loss of LM6 Al alloy hybrid composites for a stable sliding length of 2000 m with various weight conditions 10 N, 20 N, 30 N, and 40 N is shown in Figure 11.Variation of collective weight failure of hybrid mixtures reduced with the increase of fiber wt.% addition with constant wt.% of B 4 C. Weight losses increase with a varying load of 10 N to 40 N.

Figure 6 :
Figure 6: (a, b) Optical micrographs of LM6 Al alloy hybrid composites with 5% of B 4 C. (c, d) Optical micrographs of LM6 Al alloy with 5% of CCSF and 5% of B 4 C.

Figure 13 (
Figure13(a)  shows unbroken depressions because of local division of many layers, and Figure13(b) shows comparatively fine grooves because of combination of steel fibers in LM6 Al alloy.e volume of the cut is found to be smaller when higher wt.% of fiber is observed.Figures13(c) and13(d)  show the damaged layer of 5 wt.% CCSF and 5 wt.% of B 4 C reinforcement tested at a load of 30 N.Figures13(e)-13(h) show the micrographs of 5 wt.% B 4 C and 5 wt.% CCSF for a moving length of 2000 m in various loads varying between 10 N and 40 N. e number of grooves increased with the increasing weight, and confined division was recorded at maximum weight.While increasing the sliding distance, the matrix loses weight, and unbroken

Figure 10 :Figure 13 Figure 12 :
Figure 10: (a) Rupture surface of LM6 Al alloy.(b) Rupture surface with dimples of LM6 Al alloy hybrid composites.(c) LM6 Al alloy hybrid composites with broken wires.(d) LM6 Al alloy hybrid composites with two broken wires.