Mechanical and Tribological Properties of Al2024/Al2O3 Functionally Graded Spur Gears Synthesized by Centrifugal Casting Process

The mechanical and tribological properties of spur gears are important because they can affect the efficiency and wear life of these gears. Thus, this study's primary goal is to use functionally graded material (FGM) to enhance the properties of Al2024 aluminium alloy spur gears. The mechanical and tribological properties of aluminium alloy Al2024 with graded Al2O3 reinforcement particles spur gears synthesized by the vertical centrifugal casting (VCC) process were investigated experimentally. Different weight percentages were used for the reinforcement particles, including 0, 2, 5, and 10 wt.% of Al2O3. Microstructural and mechanical properties were determined by SEM, EDS, and hardness tests. The Al2024/Al2O3 FGM spur gears were subjected also to dry sliding wear conditions. It was seen that hardness, strength, and wear resistance values increased by adding Al2O3 reinforcement particles to the Al2024 base material. In addition, the wear rate of FGM gears decreased by increasing the Al2O3 reinforcement particles in Al2024 alloy.


Introduction
P ouring liquid metal into a mould cavity and waiting for it to solidify results in the finished casting during the metal casting process. Numerous factors, including the part's minimum section thickness, the existence of corners, and the crossnon-uniformity sections of the cast, affect how the molten metal flows into the mould cavity (El-Mahlawy et al., 2020). Conventional moulding, precision moulding (investment casting, permanent die casting mould), special moulding (centrifugal casting, vacuum casting), chemically bonded self-setting sand moulding (sodium silicate moulding), and innovative moulding (squeeze casting, and electro slag) are the five categories into which metal casting processes can be divided. Casting methods are particularly effective because they generate complicated integrated pieces with great strength and rigidity, the cavity size and die shape are important considerations in design (Stefanescu et al., 1988). There are many factors that influen`ce the selection of casting processes such as shape, number, size or weight, and complexity of produced casting, quality of finish machining needed, casting design considerations: draft, wall thickness, mechanical property requirements: strength and ductility, hardness, fatigue strength, toughness and specification limits (Kaufman and Rooy, 2004).
In the centrifugal casting process, a permanent mould is swiftly rotated about its axis to produce cylindrical components with various mechanical qualities. Under the influence of centrifugal force, the molten metal is repelled to the interior wall of the mould, where it cools and solidifies. Moreover, design specifications, product cost, and the type of gear material required all have an impact on the manufacturing process for gears. Casting is mostly used to create blanks for gear that will have cut teeth (Davis, 2005).
Functionally graded material (FGM) refers to a class of engineered materials that demonstrates gradual changes in composition and microstructure. Due to the smooth transition in thermal stresses across the thickness and the low-stress concentration at the interface between different materials, this causes the required variance in the functional performance (Arsha et al., 2015). Centrifugal casting is a useful technique for creating FGM, but there are a number of process variables that can affect the characteristics of the castings. These include the temperature at which the metal is poured, the temperature of the die wall, the speed at which the die rotates, and centrifugal force (Narendranath and Kumar, 2013). Chirita et al. (2008) demonstrated that when using centrifugal casting instead of gravity casting, considerably improved mechanical and fatigue properties are obtained throughout the casting. Rajan et al. (2008) investigated that the functionally graded composite disc made from Ale20SiC composite produced by vertical centrifugal casting (VCC) has shown improved structural and mechanical properties. The particles are gradually segregated towards the casting's outer periphery, which results in high strength, hardness, and wear resistance, as opposed to the inner periphery, which has a region of depleted particles. Sarkar et al. (2009) indicated that particle size, rotational speed, relative density differential between particles and melt, beginning pouring temperature, and initial mould temperature all have a significant impact on how reinforcing particles are distributed as a result of the natural liquid flow and solidification pattern. Komaraiah et al. (Shailesh et al., 2014) utilized the Taguchi method of experiment design, to examine the impact of process variables on the mechanical properties of aluminium alloy (Al4600) during centrifugal casting. Their findings showed that while an increase in die speed increases mechanical characteristics and density, an increase in pouring temperature decreases mechanical qualities. Radhika and Raghu (2016) used Al/SiC, Al/Al 2 O 3 , and Al/TiB 2 composites with a constant 12 wt.% of reinforcement, they were tested for their microstructural features, hardness, tensile strength, and abrasive wear qualities on hollow cylindrical components made by centrifugal casting. The results revealed that all composites have higher hardness and tensile strength in their outer peripheries. Mohapatra et al. (2020) indicated that several processing variables affected the properties of centrifugal casting. The rotational speed, the mould's temperature, the pouring temperature, the material used to make the mould, and the cooling system for the mould. They indicated that the increase in rotational speed improves grain refinement by increasing the solidification rate, while a low rotational speed causes instability and vibrations in the melt. The viscosity of the melt is reduced at high temperatures, and the cooling rate of the casting is also determined by the pouring temperature. Verma et al. (2021) used mould preheating temperatures ranging from 250 to 350 C, centrifugal speed is maintained between 600 and 1300 rpm, and pouring temperatures are in the range of 740e760 C. These conditions produce the highest levels of hardness and tensile strength while also having reinforcement particles of 10e15 wt.% and an average particle size of 18e50 mm. Elkotb et al. (2021) fabricated functionally graded materials for internal combustion engine piston models. They used a mixture of two pure aluminium alloys, A336 and A242, and applied the centrifugal casting technique to produce the FGM pistons. They found that the silicon percentage varies noticeably from the top of the piston (12%) to its bottom (1.5%). At the same time, the percentage of copper has an opposite gradient (0.78%e3.15%). The micro-hardness test of the FGM piston yielded positive findings, with a high hardness value on the top and a gradually decreasing hardness value along the length of the piston at the bottom. The manufactured FGM pistons demonstrated a high resistance to wear at their surface during the wear test, and this resistance steadily reduced at the bottom of the FGM pistons.
From the above literature, it can be noted that there is no focus on the application of FGM in the field of gear manufacturing. Therefore, the current research discusses the impact of using Al2024/Al 2 O 3 FGM in the manufacturing of spur gears to improve their mechanical and tribological properties.

Materials and methods
Non-commercial aluminium alloy (Al2024) was employed in this research as the metal matrix of the developed FGM composite. Al2024 alloy has numerous applications and is frequently used in airplanes, particularly in the wing and fuselage structures under tension, due to its high strength and fatigue resistance. The chemical composition of the nominated aluminium alloy is shown in Table 1.
The Al 2 O 3 reinforcement particles with different weight fractions such as 0, 2, 5, and 10 wt.% were used; having a mean grain size of 60 mm. Centrifugal casting process utilized to manufacture the FGM spur gears. Therefore, the matrix material (Al2024 alloy) was melted in the furnace at 670 C. After the complete melting of the matrix alloy, the reinforcement particles (Al 2 O 3 ) were added. Following that, the reinforcement particles were thoroughly combined utilizing the stir method.
As depicted in Fig. 1, the molten metal was poured into a cylindrical spur gear mould that had been heated to 250 C and was rotating at a constant speed of 1300 rpm. With different weight percentages of Al 2 O 3 reinforcement particles, the FGM spur gear production method has been repeated. The final FGMs spur gear cast products have an outer diameter of 112 mm, a thickness of 20 mm and an inner core diameter of 25 mm. Fig. 2, shows one of the fabricated FGM spur gears.

Results and discussions
As is well knowledge, the pressure created by direct gear contact exposes the gear tooth to the greatest mechanical stresses and wear values. Four gears have been manufactured in this work using the VCC method, one of which is made of pure Al 2024 alloy (high copper Alealloy), and the other gears from the FGM Al 2024 alloy with the addition of 2, 5, and 10 wt.% Al 2 O 3 particles as reinforcement.
EDS along the surface of the four gears, as well as SEM, hardness and pin-on-disc wear tests, were carried out to examine the effect of the functional gradient of the Al 2 O 3 particles on improving the gear qualities to withstand the pressures that it was exposed to.

Chemical composition analysis of FGM gears
Spectroscopy was used as the foundation for this analysis. EDS Analysis was carried out using an SEM Oxford X-Max 20 at Microscopic Examination Unit at the faculty of Agriculture e Mansoura University. For the Spectro-test, each specimen was produced 10*10*10 mm 3 from the gear tooth. Table 2 shows the chemical composition for the four specimens with the different weight% of Al 2 O 3 reinforcement particles.   Three readings were taken along the surface of each Al2024 and FGM specimen from the tooth's outside and radially down to its interior part. Table 2 lists the chemical composition of the four specimens with various AL 2 O 3 weight percentages. The most significant material compositions are aluminium and aluminium oxide. Fig. 3 illustrates the gradient distribution of oxygen and aluminium as an indication of functionally gradation of Al 2 O 3 reinforcement particles within the Al alloy. It is worth to be noted that, the O 2 content decreased from outer to inner which indicates the aluminium oxide gradient distribution. At contrarily, the aluminium wt.% moves from gear center toward the gear teeth.

Hardness testing
In order to analyse the impact of gradients in the alloying elements with the addition of Al 2 O 3 particles weight percentage of FGM gear, hardness is measured along the surface of the gear for each of the four gear types. As illustrated in Fig. 4, readings were taken for six positions on the FGM cast gears from the outer to the inner toward the gear centre. As a precursor to the hardness test, small specimens were cut from each gear. To complete this task, a Micro-Hardness Tester HV-1000 instrument was used. The gadget was configured with a 9.8 N load and a 10-s timer. Fig. 5, illustrates the Micro Vickers hardness test findings for FGMs specimens. The micro-hardness test revealed the highest values at the outer edge of the cast gear tooth because it contains more reinforcing particles than the inner side does due to centrifugal force, this is a sign of the FGM gear's increased capacity to endure existing thermal and mechanical pressures. It is obvious that the hardness of the outer region of the specimen is higher than the inner region for all specimens and hardness increases due to increasing reinforcement particle's wt.% of Al 2 O 3 .

Wear behaviour testing
Wear testing is typically done to assess a material's wear performance. In this study, it is attempted to determine how the gradient of aluminium oxide  particles in the chosen Al-Alloys from the top of the gear tooth to its base affects the enhancement of wear resistance of the casted FGM gear. This experiment followed the pin-on-disk apparatus weight loss theory. As depicted in Fig. 6, An MMW1A vertical wear testing machine was used.
Following the machine's instructions, miniature pin specimens for the test with dimensions of 4.8 mm in diameter and 12.7 mm in length were created from the produced gears. The appropriate final weight before and after the examination was measured using a precision balance with 0.1 mg of accuracy (see Fig. 6b). In accordance with ASTM G99, the test was carried out for 15 min using a disc with a diameter of 46 mm, a loading of 30 N, and a rotational speed of 200 rpm. In order to calculate the weight-loss average for each measurement, the Al2024 alloy and FGM pins with various weight percentages of Al 2 O 3 reinforcement particles were tested at three locations along the radial surface as shown in Fig. 7. Additionally, three pins were cut axially parallel to the gear axis; the first at the tooth near the outer surface of the gear, the third near the gear central hole, while the second pin at a middle position between the first and third pins (see Fig. 7). Fig. 8 shows the weight-loss percentage values of the pins and disc for the pure Al2024 alloy and FGM   gear specimens wear tests, while the error bars in the figures represent the standard deviation in the results. In the wear test conducted on a pure Al2024 gear specimen, it was noticed that the average weight-loss percentage for pure Al2024 pins was 1.833%, while the average weight loss percentage for the disc was 0.0005%. However, the tests showed the lowest values of weight loss percentages for the FGM gear with 10 wt.% Al 2 O 3 reinforcement particles as shown in Fig. 8a.
Furthermore, it is obvious to note that, within each FGM gear there is a significant gradation in wear test results from the outer to the inner of the manufactured FGM gears. This is indicated clearly as the weight loss percentage increases gradually from the outer to the inner of the FGM gear due to the progressive distribution of Al 2 O 3 reinforcement particles caused by centrifugal force. Thus, the Al 2 O 3 reinforcement weight percent and its distribution inside the alloy appears to have an impact on the variation in weight loss values and accordingly on the wear of the manufactured FGM gears.
Moreover, it is clear from Fig. 8b, which shows the weight-loss in the disc, that the weight loss rate in the disc decreases as the amount of Al 2 O 3 wt.% increases within the FGM gear specimens. Also, at contrarily to the pins results, it is obvious to note that, the disc weight-loss percentage results decrease gradually from the outer to the inner within each FGM gear. The radically different nature of wear changes may be the cause of the disc wear rate variations. Since the environmental, material, and dynamic characteristics may all be modified relatively insignificantly without significantly altering the wear mode, and the wear rate value.
Additionally, Fig. 9 displays the weight-loss percentages for the pins cut axially parallel to the gear axis. As shown in the figure, there is a ramp increase in weight-loss percentages from the gear's outer surface to its inner toward the center. Also, it is obvious that the weight-loss rate and accordingly the wear for the axially cut pins decreased as Al 2 O 3 wt.% increased. Noting that, the missed result of the 3rd pin of the gear containing 5 wt.% of Al 2 O 3 was due to some problems in extracting the pin from the gear. In general, from the above results and discussions it can be concluded that, the addition of Al 2 O 3 reinforcement particles to the aluminium alloy contributes to improving its wear resistance. As a result, the life span of the FGM gear will be increased.
Additionally, for any system, wear and friction occur simultaneously. Therefore, for each specimen, the friction coefficient was noted during the wear test. Fig. 10 provides an illustration of the reported friction coefficient values using the radially and axially cut pins. The friction coefficient values can be seen to progressively grow from the outer gear tooth to its inner toward the gear center. Also, it can be noted that, the coefficient of friction values decreased by increasing the Al 2 O 3 reinforcement  It is crucial to compare the results of the FGM gear specimen's friction coefficient and wear rate. The FGM spur gear specimens with greater Al 2 O 3 wt.%; i.e., at gear teeth; have the greatest wear resistance and the smallest friction coefficient, as can be shown in Figs. 8e10, respectively. The FGM spur gear specimens with reduced Al 2 O 3 wt.%; i.e., near the gear central hole; on the other hand, have the greatest friction coefficient and the greatest weight loss. As a result, the tooth head's lifespan was extended and increasing the gear efficiency.

Microstructure testing
Scanning electron microscopy (SEM) is a powerful tool for the study of microstructural features in materials. SEM (JEOL JSM 6510 L, see Fig. 11); is used to inspect the surfaces of samples and give a high-quality and clear stereoscopic image. The magnification capacity reaches 300,000 times. One gear tooth was taken from each spur gear to examine the microstructure of the gears with different weight percentage of Al 2 O 3 . Five scanning micrographs are taken at five different positions on the gear tooth, i.e., at the gear tooth crest, 2.5, 5, 7.5, and 10 mm from the tooth crest. Sample were examined at the Microscopic Examination Unit at the faculty of Agriculture e Mansoura University.
The SEM results of the Al 2 O 3 reinforcement particles in Al2024 aluminium alloy shown in Fig. 12. The SEM micrographs provide valuable information about the microstructure and distribution of the Al 2 O 3 reinforcement particles in Al2024 aluminium alloy. Additionally, the SEM images also provide insight into the size and shape of the Al 2 O 3 particles, which can be used to further optimize the FGM spur gears properties. One of the key findings from the SEM results is the gradually dispersion of the Al 2 O 3 particles in the aluminium alloy matrix. This is a critical factor in determining the mechanical properties of the FGM spur gear, as it influences the distribution of stress and strain during gear meshing as well as the wear rate. It was observed that aluminium oxide particles increasely concentrated at the outer zone of the gear tooth and their concentration decreased gradually toward the gear central hole. This can be explained by the high rotational speed of vertical centrifugal casting process.

Conclusions
Vertical centrifugal casting (VCC) technique has been used to produce the functionally graded material (FGM) spur gears. Spur gears were produced from AL2024 aluminium alloy reinforced with different weight percentages of Al 2 O 3 particles using a centrifugal rotational speed of 1300 rpm. The mechanical characteristics of the FGM spur gears, as well as their microstructural performance were examined. Present study findings lead to the following conclusions: (1) Chemical analysis and SEM results showed that for all specimens there is a pronounced gradient in the distribution of Al 2 O 3 reinforcement particles within the FGM gears. The concentration of