Keywords

1 Introduction

Aluminium metal matrix composites are extensively used in various applications due to its improved properties with low density. These materials can be used in aircraft, automotive, construction, packing, electronics and military industries. The type of reinforcements like metals, ceramic and oxide particles can be used in aluminium alloy, depending upon their applications. These materials are the encouraging materials to give better properties with low density and also cheaper than the other materials like Mg and Ti. In this present work, Al2024 is selected on the basis of usage of material in aerospace and automobile applications, which will lead to high strength-to-weight ratio and also having good mechanical properties. B4C is used as one of the reinforcements, which is having high hardness value as compared to the other materials. The graphite is used as another best reinforcing material, having less density, and can be used as solid lubricant. The major usage of graphite can be seen in the composites to reduce the wear in between the contacting elements. Different methods like stir casting, liquid infiltration, squeeze casting, diffusion bonding and powder metallurgy technique can be used for the fabrication of composites; among all these methods, powder metallurgy (PM) technique [1,2,3,4,5,6, 10, 13, 14] is selected for the fabrication of the composites. The PM technique can be used for mass production and uniform distribution of particles with less wastage of materials. The hardness of the composite increases with the increase in B4C content, and then, it is very difficult for machining process. To overcome this effect, small amount of Gr is used for the fabrication of composites. The monocomposites with single reinforcement restrict some of their properties; hence, adding more number of reinforcements called as hybrid composites [5,6,7,8,9,10,11,12,13,14,15,16,17,18] improves their properties. The corrosion behaviour of various metal matrix composites [14,15,16,17,18,19,20,21] is explained by many researchers. Hanji et al. [15] studied the corrosion behaviour of heat-treated hybrid metal matrix composites reinforced with B4C and graphite particles, and the results revealed that the corrosion resistance of hybrid composite was the least since there is a formation of aluminium carbides which leads to galvanic and inter-granular corrosion. Muthazhagan et al. [16] fabricated successfully hybrid metal matrix composites by powder metallurgy route and concluded that the corrosion rate increased with the increase in B4C and Gr in aluminium due to the localized attack on the grain boundaries. Ravindranath et al. [17] explained the degradation of hybrid metal matrix composites reinforced with boron carbide and graphite due to pit corrosion. The corrosion resistance of B4C reinforced composite is higher due to the presence of carbide, the least corrosion resistance for hybrid composites and the corrosion resistance is maximum for the base aluminium alloy. Bihari et al. [18] examined the corrosion behaviour of Al7075/Al2O3/graphite hybrid composite in 3.5% sodium chloride solution. The corrosion behaviour of Al7075 and Al7075/Al2O3/Gr MMCs was tested by weight loss, potentiodynamic polarization and electrochemical impedance method. An increase in the percentage of Al2O3 particles reduces the potential, corrosion current density and corrosion rate. Singh et al. [19] investigated the influence of SiC particle addition on the corrosion behaviour of Al2014-Cu alloy in 3.5% NaCl solution; the addition of SiC content to the base alloy decreases their corrosion resistance. The increase of SiC is likely to increase the cathodic sites in composite, and it leads to start the galvanic effect which results in the decrease in corrosion resistance.

2 Materials and Methods

2.1 Matrix and Reinforcement Materials

The matrix material used in this work is Al2024 alloy of grain size 50 µm, and it is received from Parshwamani Metals, Mumbai. The elemental compositions of this composite are given in Table 18.1. The major contribution in this composite is copper, and it is used in aerospace and automobile applications. These alloys have good strength-to-weight ratio and corrosive properties. Two types of reinforcements are used to fabricate the required mono- and hybrid metal matrix composites. In the initial stage, B4C of grain size 100 µm is added to the aluminium alloy to obtain aluminium metal matrix composite. In the lateral stage, fixed amount of Gr (3%) of particle size 100 µm by varying B4C (0, 3, 6, 9, 12, 15%) is added to the aluminium alloy to fabricate hybrid metal matrix composites. Graphite is used widely in industrial application because of its self-lubricating and dry lubricating properties. The particle size of used materials is given in Table 18.2, and SEM pictures of received powders are shown in Fig. 18.1. The required quantity of composite powders (Al-3%B4C, Al-6%B4C, Al-9%B4C, Al-12%B4C and Al-15% B4C) and (Al-3%Gr-3%B4C, Al-3%Gr-6%B4C, Al-3%Gr-9%B4C, Al-3%Gr-12%B4C and Al-3%Gr-15%B4C) is weighed in a simple balance of an accuracy 0.0001 g and is taken in a tungsten carbide bowl of planetary ball mill for proper mixing. The SEM images of received individual powders are shown in Fig. 18.1.

Table 18.1 Elemental composition of Al 2024
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Table 18.2 Particle size and density of various elements
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Fig. 18.1
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SEM images of received, a Al 2024, b B4C, c graphite

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2.2 Ball Milling

Powder metallurgy technique is adopted for the fabrication of all the composites. In the primary stage, various weight-based measured composite powders (Al-3%B4C, Al-6%B4C, Al-9%B4C, Al-12%B4C and Al-15% B4C) are well mixed in the planetary ball mill and taken out from the bowl of the mill. In the lateral stage, aluminium metal matrix hybrid composites (Al-3%Gr-3%B4C, Al-3%Gr-6%B4C, Al-3%Gr-9%B4C, Al-3%Gr-12%B4C and Al-3%Gr-15%B4C) are well mixed in the ball mill itself. For the fabrication of all these composites, mechanical milling and alloying (RETSCH 100 Ball Mill Germany make) as shown in Fig. 18.2 is used for proper mixing of composite powders. Tungsten carbide balls of 10 mm diameter are added to these powders during mixing with ball-to-powder ratio of 10:1 [13, 14], and this milling process is carried out for about 30 min selected from the literature. FESEM analysis is conducted on mixed powders by using FESEM machine (Fig. 18.3) to know the elemental composition in each blend as shown in Fig. 18.4.

Fig. 18.2.
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RETSCH 100 ball milling machine

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Fig. 18.3
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FESEM for microstructure analysis

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Fig. 18.4
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FESEM micrographs of produced powder mixtures after ball milling

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Fig. 18.5
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Hardened steel die with plunger

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2.3 Compaction of Mixed Powders

Compression process is carried out on each blend by using hydraulic compression testing machine as shown in Fig. 18.6. For each composition, five numbers of samples are considered for different analyses. The mixed compositions are consolidated in a hardened steel die along with a punch as shown in Fig. 18.5. These powder mixtures are compressed up to a pressure of 400 Mpa [13, 14]; maintaining same pressure for about 5 s, then the pellets are ejected from the die. After compaction, the green pellets having 15 mm diameter and 25 mm length are obtained from the die. For each run, the die is filled with approximately same quantity of well-mixed powders. Silicon spray was sprayed into the die before the powder is put into the dye, so as to provide proper lubrication between the die walls and the powder. Then, the punch was placed over the filled powder and gradual pressure was applied over the powder.

Fig. 18.6
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Compression testing machine

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2.4 Sintering

The sintering process is used to convert the green pellets into the required specimens. The grain growth and grain bonding of the green pellets are taken place after the sintering process. The specimens are placed in a muffle furnace as shown in Fig. 18.7; the temperature is increased up to 600 °C which is the bonding temperature [13, 14] and is maintained at this temperature for about 3 h by following 10 °C per min as the increase rate of the temperature. The sintering process is carried out under an inert atmosphere (Organ gas) to avoid oxidization during the process. The cooling is carried out in the furnace itself by following 5 °C per minute to reach room temperature. The final samples after the sintering process are shown in Fig. 18.8.

Fig. 18.7
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Muffle furnace for sintering

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Fig. 18.8
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Specimens after sintering

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3 Microscopic Examination

Microscope is used for microscopic examination. Aluminium, boron carbide and graphite are detected in the analysis. The fabricated specimens of sintered preforms are examined in metallurgical microscopic, before going to conduct the examination, the fabricated samples are prepared by using standard hand polishing with 240, 600, 800 and 1000-grit silicon carbide papers. The etch–polish–etch procedure by using Keller’s etchant is used to attain mirror-like surface. The images obtained from microscope show the uniform distribution of particles throughout the surface of the specimen without any agglomerations.

4 Corrosion Test

4.1 Electrochemical Analysis

4.1.1 Preparation of Test Solution

The test solution of 3.5 wt% NaCl is prepared by dissolving 35 g of NaCl powder in 1 L distilled water to obtain 0.6 M solution, in which chlorine is the active pitting agent during corrosion test [16, 22]. All the experiments are conducted in aerated 3.5% NaCl solution with PH adjusted to 10 by adding potassium hydroxide.

4.1.2 Corrosion Procedure

The corrosion phenomenon can be found by using several methods in which weight loss and spray test analysis are qualitative. In this experimental work, electrochemical analysis technique is used to get reproducible quantitative data. To study the corrosion behaviour of fabricated aluminium samples, software-based PAR electrochemical weld tester system is used to carry out potential dynamic polarization tests. The basic electrochemical system with electrochemical flat cell used in this work is shown in Fig. 18.9. A standard calomel electrode SCE and carbon electrode are used as reference and auxiliary electrodes, respectively. All the experiments are conducted in aerated 3.5% NaCl solutions with PH adjusted to 10 by adding potassium hydroxide. The potential scan is carried out at 0.166 mvs−1 with the initial potential of −0.25 V (OC) SCE to the final pitting potential. The exposed area for these specimens is 1 cm2. Polarization curve is plot between potential (mv) and current density (mA/cm2) which is having anodic and cathodic branches. Potential at which the anodic and cathodic branches are met is taken as corrosion potential (Ecorr); the typical polarization curves are shown in Figs. 18.11 and 18.12. Specimens exhibiting relatively more positive corrosion potential (or less negative potentials) are considered to have better corrosion resistance. The corrosion test is conducted by potential dynamic polarization tests as explained in the previous procedure; then, the same corrosion test is conducted on all the samples to know the corrosion behaviour. The pits and cracks are observed in every specimen after the corrosion test. According to the ASTM G 59, potentiodynamic anodic polarization technique is carried out on the fabricated composite samples to know the corrosion behaviour of tested samples. The values of Ecorr, Epit and corrosion current are calculated for all the test samples. The microstructure is visualized with optical microscope, and the micrograph is captured before the corrosion and FESEM analysis is conducted after the corrosion test.

Fig. 18.9
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Schematic representation of flat cell

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Fig. 18.10
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Images of specimens before and after corrosion

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Fig. 18.11
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Polarization plots for individual composite specimens

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Fig. 18.12
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Potentiodynamic curves of all the fabricated composites

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The optical microscopic and FESEM images shown in Fig. 18.10 describe the corrosion process taken over all the obtained samples before and after corrosion. The obtained images, number of pits and cracks formed over the surface of all the specimens after the corrosion test. Comparing these images, it has been observed that the pits and cracks formed over the surface of hybrid composites are more than the monocomposites and base aluminium alloy. The cracks and pits formed over the surface of the base aluminium alloy are less as compared with the other composites; this is because of the inter-granular corrosion which is the localized corrosion attack surrounding the B4C particles in monocomposites, whereas in hybrid composites, the corrosion is more due to the presence of graphite which leads to galvanic corrosion by forming a primary corrosion mechanism [17].

The obtained potentiodynamic polarization curve of potential versus current is known as Tafel plots (Figs. 18.11 and 18.12). Different regions like the active region, passive region, beginning of film removal and passive region can be known from the Tafel plot. These are also known as anodic and cathodic polarization curves. With the help of these graphs, the values of corrosion current (icorr), corrosion potential (Ecorr) and pitting potential (Epit) are given in Table 18.3.

Table 18.3 Icorr, Ecorr, Epit and corrosion values of the specimen in 3.5 wt% NaCl corrosion media
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From the above table, it is clear that the icorr values decreases for both mono- and hybrid composites but the values of monocomposites are little bit smaller than the hybrid composites having same composition. Similar trend is observed for Ecorr values, but the values are in an increased manner. For better corrosion resistance, icorr values should be maximum and Ecorr values should be minimum. The corrosion resistance decreases from the base metal to B4C-reinforced composite. The corrosion resistance further decreases upon addition of graphite.

5 Conclusions

The aluminium metal matrix mono- and hybrid composites are fabricated successfully by powder metallurgy technique. Microstructure and FESEM analysis shows the uniform distribution of the reinforcement in aluminium alloy. The electrochemical cell analysis by potentiodynamic anodic polarization technique is conducted on the fabricated composites. The corrosion is found from Tafel graphs. The results of the samples show that the corrosion resistance of the base aluminium alloy is least. The corrosion resistance of the monocomposite is higher due to the presence of carbide particles and the localized corrosion attack surrounding the B4C particles. The hybrid composite of B4C and Gr is least corrosive resistant among the three; the reason for this is the presence of graphite content, so the corrosion resistance decreased due to soft nature of graphite.