Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December 2012 143
Tribological Properties of Aluminium 2024 Alloy–Beryl Particulate MMC’s
H.B. Bhaskar and Abdul sharief
Abstract--- Metal Matrix Composites (MMCs) are emerging as the most versatile materials for advanced structural, automotive, aviation, aerospace, marine, defense applications and other related sectors because of their excellent combination of properties. In the present investigation, Al2024-Beryl composites were fabricated by liquid metallurgy route by varying Weight Percentage (wt. %) of reinforcement from 0 wt. % to 10 wt. % in steps of 2 wt. %. The dry sliding wear tests were conducted to examine the wear behavior of the Al2024 alloy and its composites. The sliding wear tests were conducted for various loads, speeds and sliding distances. The result reveals that wear rates of the composite is lower than that of the matrix alloy and friction coefficient was minimum when compared to monolithic alloy. The incorporation of beryl particles as reinforcement material in Al2024 alloy improves the tribological characteristics.
Keywords--- MMCs, Al2024, Wear Rate, Beryl, Pin-on-Disc, Coefficient of Friction
I. INTRODUCTION LUMINIUM Metal Matrix Composites (AMMCs) are the new candidate materials used in varieties of
engineering applications. Aluminum and its alloys have been used as matrix material owing to its wide applications in industrial sector. To increase the mechanical and tribological properties, hard reinforcement phase such as particulate, fiber or whiskers are uniformly distributed. These materials have emerged as the important class of advanced materials giving engineers the opportunity to tailor the material properties according to their needs. Essentially these materials differ from the conventional engineering materials from the viewpoint of homogeneity.[1] AMMCs are a class of composite materials which are having desirable properties like low density, high specific stiffness, high specific strength, controlled co efficient of thermal expansion, increased fa1tigue resistance and superior dimensional stability at elevated temperatures etc. [2]. AMMCs have emerged as the advanced engineering materials giving engineers the opportunity to tailor the material properties according to their
H.B. Bhaskar, Research Scholar, Department of Mechanical Engineering, Sri Siddhartha Institute of Technology, Maralur Post, Tumkur-572105, Karnataka, India. E-mail: [email protected]
Abdul sharief, Professor and Head, Department of Mechanical Engineering, P.A. College of Engineering, Mangalore -574153, Karnataka, India. E-mail:[email protected] DOI: 10.9756/BIJIEMS.1845
needs. The mechanical and tribological properties of the matrix material was improved by reinforcing the various reinforcements ranging from very soft materials like Graphite, Talc etc., to high hardened ceramic particulates like SiCp, Al2O3, etc., [3]. In recent years, particulate reinforced aluminum alloy composites fabricated have shown significant improvement in tribological properties, including sliding wear, abrasive wear, friction and seizure resistance [4]. An excellent review on the dry sliding wear of discontinuously reinforced aluminum composites by Sannino et al., [5] reported about the principal tribological parameters that control the friction and wear performance of discontinuously reinforced aluminum composites. The mechanical and physical factors have been identified as sliding velocity and normal load whereas the material factors are volume fraction, type of reinforcement and size of reinforcement. The volume fractions of reinforcement have the strongest effect on the wear resistance and have been studied [6-9]. Many researchers have carried out the work to increase the wear resistance of MMCs by different types of reinforcements. The main outcome of these is that the reinforcement improves the resistance to sliding wear [10-11]. However, the research works on the dry sliding wear of aluminium alloys containing beryl particles were limited. Therefore in the present investigation an attempt is made to study the sliding wear behavior of Al2024-Beryl particulate composites for different weight percentages of beryl particles.
II. EXPERIMENTAL DETAILS
2.1 Material The matrix alloy selected for the development of
composite material is Al-Cu-Mg alloy and designated by the aluminium association as Al2024-T6. The chemical composition of the matrix material is given in the Table 1. Beryl, which is naturally occurring and chemically having beryllium–alumina–silicate [Be3Al2(SiO3)6] was used as the reinforcement material. The chemical composition of beryl particles used for development of the composite are mentioned in the Table 2.
Table 1: Chemical Composition of Al2024 Alloy (wt.%)
Table 2: Composition of Reinforcement Material (wt.%)
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2.2 Preparation of the Composite The Al2024 alloy and Al2024-Beryl reinforced composites
were fabricated by liquid metallurgy method. This method is the most economical to fabricate composites materials. The matrix material was first superheated to above its melting temperature by using 5 KW electrical resistance furnace and preheated beryl particulates were added into molten metal. The molten metal was stirred for duration of 8 min using a mechanical stirrer and speed of the stirrer was maintained at 350 rpm. The mixed molten metal at 710°C was poured into the pre-heated cast iron molds. Al2024-Beryl composites were casted by adding proportionate of weight percentage of reinforcement from 0 wt.% to 10 wt.% in steps of 2 wt.%. This liquid metallurgy method has been used for developing the MMCs by many researchers. [10-16]. The castings were tested to know the common casting defects using ultrasonic flaw detector.
2.3 Testing of Composites The metallographic and hardness tests were carried out on
Al2024 alloy and Al2024 / Beryl reinforced composites as per ASTM standards. A pin-on-disc apparatus was used to investigate the dry sliding wear characteristics of the composite. The wear specimens were machined to the pin size of diameter 8 mm and height 30 mm and then polished metallographically as per American Society for Testing and Materials (ASTM) G99-95 standards. During the test, the pin was pressed against the rotating counterpart carbon case hardening steel disc with hardness 60 Rockwell C Hardness by applying the load and speed. After running through a fixed sliding distance, the wear and frictional force are tabulated. The specimen is cleaned with acetone and dried. The each experiment was repeated thrice and means response values were considered. Also the wear of the Al2024 alloy and its composites can be determined by measuring the weight loss in specimens before and after the test for different load and speed. The specimens were tested as per the procedure reported in papers Uyyuru et al., [19] and C.S.Ramesh et al., [20].
III. RESULTS AND DISCUSSION
3.1 Microstructure Analysis The samples for the microscopic examination were
prepared by standard metallographic procedures etched with killer’s agent and examined under optical microscope. Figure 1 (a & b) shows the optical micrograph of as-cast Al2024 alloy and Al2024/4wt.% of beryl particulate composite respectively. Micrograph indicates the nearly uniform distribution of the reinforcement particles in the composite.
Figure 1a: Micrograph of as-cast Al2024 alloy
Figure 1b: Micrograph of as-cast Al2024/4 wt.% of beryl Composites
3.2 Hardness The variation of hardness of Al2024 alloy and its
composite materials are shown in the figure 2. This graph explains the effect of particulate reinforcement on the micro hardness. The hardness of the Al2024/10wt.% of beryl composite increased by around 26% as compared to Al2024 matrix alloy. It is observed that with increased content of beryl in the matrix alloy, there was a significant improvement in the hardness of the composites. This trend is similar to the result of other researchers [21-25].
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Bonfring International Journal of Industrial Engineering and Management Science, Vol. 2, No. 4, December 2012 146
Figure 6: Variation of Wear Rate at Different Sliding Speed at Constant Load of 40N
The Figure 6 shows the variation of wear rate for different sliding speeds at constant load of 40N for the unreinforced alloy and its composites. The wear rate of both unreinforced alloy and the composites decreases as sliding speed increases from 1.152 m/s to 3.456 m/s for a load of 40N and further, the wear rate of the composite decreases as the amount of reinforcement content increases. The noise and vibration were observed during the process and transfer of the pin material to the disc was also observed. The figure 7 shows the variation of coefficient of friction with sliding speed at constant load. The coefficient of friction of both unreinforced alloy and the composites decreases as sliding speed increases from 1.152 m/s to 3.456 m/s for a load of 40N. From the figure 8 it is observed that the coefficient of friction of the unreinforced material is more than that of the composites for all different loads and coefficient of friction increases as the load increase for matrix alloy and its composites. Further, as the percentage of reinforcement increases, the coefficient of friction of the composite decreases. [17]
Figure 7: Variation of Coefficient of Friction with Sliding Speed at Constant Load of 40N
Figure 8: Variation of Coefficient of Friction at Different Load at Constant Speed 400rpm
Figure 9: SEM of Worn Surface of as-cast Al2024 Alloy
Figure 10: SEM of Worn Surface of as-cast Al2024-6 wt.% Beryl Composite
Figure 9 and 10 shows the Scanning Electron Micrograph (SEM) of worn surfaces of as-cast Al2024 alloy and Al2024-6 wt.% Beryl composite respectively. The mechanism of material removal during wear process of the Al2024 alloy was by plastic deformation and gouging. The wear resistance of the composite material is improved due to the presence of hard
reinforcement particles in the matrix material. The wear process in the composite material is by plastic deformation, gouging and reinforcement particles will crush to very minute particles and form a very thin sub surface layer designated as Mechanically Mixed Layer (MML) which provides protection to the matrix material. The MML forms a layer between the work hardened pin and the counter face which withstands high stresses and is very effective in reducing the sliding wear. It is evident that the influence of beryl particles will reduce the cutting and ploughing in the composite material. There are some slight traces of ploughing to be seen in the matrix area and the hardness of the particles was the reasons for wear resistance [18]. It is evident that the composite material indicates superior wear resistance than that of Al2024 alloy. This is because of improved hardness and strength of composites results in excellent wear resistance of the composite material.
IV. CONCLUSION The Al2024-Beryl particulate reinforced composites were
successfully developed using liquid metallurgy method. The Microstructural study clearly reveals nearly uniform distribution of beryl particulates in the Al2024 matrix alloy. The micro hardness of composites increases as reinforcement content increases in the matrix alloy. The incorporation of beryl particles into the matrix alloy improves the sliding wear resistance of the composites as compared to the unreinforced matrix alloy. The wear rate and coefficient of friction of the composites as well as the unreinforced material increases as the sliding distance and applied load increases. Further, as the percentage of reinforcement increases, the wear rate and coefficient of friction of the composite decreases. The wear rate and coefficient of friction of both unreinforced alloy and the composites decreases as the sliding speed increases up to 3 m/s for a load of 40N, after that wear rate and coefficient of friction slightly increases as the sliding speed increases.
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