Synthesis and Study on Effect of Parameters on Dry Sliding Wear Characteristics of AL-SI Alloys


Francis Uchenna OZIOKO


Mechanical Engineering Department, Federal University of Technology, Minna, Nigeria  


* Corresponding author: Phone: +2348028854727




The effect of parameters on dry sliding wear characteristics of Al-Si alloys was studied. Aluminium-silicon alloys containing 7%, 12% and 14% weight of silicon were synthesized using casting method. Dry sliding wear characteristics of sample were studied against a hardened carbon steel (Fe-2.3%Cr-0.9%C) using a pin-on-disc. Observations were recorded keeping two parameters (sliding distance, sliding speed and load) constant against wear at room temperature. Microstructural characterization was done using optical microscope (OM) and scanning electron microscope (SEM). Hardness and wear characteristics of different samples have shown near uniform behaviour. The wear rate decreased when the percentage of silicon increases. Wear was observed to increase at higher applied load, higher sliding speed and higher sliding distance. The wear characteristics of Al-14%Si was observed superior to those of Al-7%Si and Al-12%Si due to the degree of refinement of their eutectic silicon.


Al-Si alloys; Wear; Parameters; Microstructure; Vickers hardness.





The commercial usage of aluminium-silicon alloys promises much larger business opportunities than the aluminium itself due to the sheer size of tribological applications. The tribological properties of Al-Si alloys are affected by shape and distribution of silicon particles, and addition of alloying elements such as copper, magnesium, nickel, and zinc often combined with a suitable heat treatment [1-3]. The excellent tribological properties Al-Si alloys have led to their extensive uses in engineering application, particularly in plain bearings, internal combustion engine pistons, and cylinder liners [4,5].

The factors that affect wear have been grouped under the following headings [6, 7]. The load greatly affects wear rate, which is dependent on the direction of load application, either up or down [8].The influence of the Si content of the aluminium alloys on their wear resistance has been well documented and eutectic alloys are reported to have better wear resistance than those of hypoeutectic and hypereutectic composition [9]. Silicon is present as a uniformly distributed fine particle in the structure. However, when the primary silicon appears as coarse polyhedral particles, the strength properties decrease with increasing silicon content, but the hardness goes on increasing because of the increase in the number of silicon particles [10]. The aluminium alloys are reported to have an increase in wear rate as sliding speed increase. The phenomenon of increase in wear rate at longer sliding distances and increase in the applied load was also reported [3,10-12].

This research paper investigating of wear parameters effect on the dry sliding wear characteristics of aluminium alloys.



Material and Method


The materials used are commercially pure aluminium (99.7%) and commercially pure silicon (99.5%). The equipment used includes a coke fired graphite crucible furnace, a thermocouple, hexachloroethane, Al-Ti master alloy, optical emission spectrometer, optical microscope (OM), scanning electron microscope (SEM), electronic weighing machine, acetone, pin-on-disk machine and Vickers hardness machine.


Preparation of the Alloys

Al-Si alloys with varying silicon composition was prepared by melting commercially pure aluminium (99.7%) and commercially pure silicon (99.5%) in a coke fired graphite crucible furnace. The melt temperature was maintained at 10oC above 710oC pouring temperature. This was to attain homogeneous composition in the melt. Dissolved hydrogen reduction and microstructure modification was achieved by plunging 2% solid hexachloroethane and 0.1% Al-Ti master alloy into the melt. The weight of Si charged per 500gm of Al, for the preparation of Al-7% Si, Al-12%Si, Al-14%Si alloys are 38.5, 62.2 and 73.4gm respectively. Each melt was stirred for 20s after degassing and modification held for 5 min and then poured into a 200oC pre-heated metallic mould. The measuring of temperature was achieved using thermocouple that measure temperature up to 10000C. The cast samples were of 100 mm length and 12 mm diameter. The same size and shape was machined from the as cast Al-Si alloys to present uniformity in measurements.


Determination of Hardness Values

In determining the hardness of the sample, Vickers hardness testing machine was used. The applied load during the testing was 5 kgf with a dwell time of 20 seconds. Four indentations were made at random locations for all the samples. Two indentations were performed on the top face and two on the bottom and the average values of the lengths of diagonals of the impressions were used to calculate Vickers hardness number.


Determination of Wear Values

Wear test was carried out on pin-on-disc machine under dry sliding conditions at room temperature (25°C). The sample was mounted vertically on a still vice such that its face pressed against the rotating disc. The disc used was carbon steel (Fe-2.3%Cr-0.9%C) hardened to 65 HRC, 50mm track diameter and 8mm thick. Specimens of 10 mm diameter and 30mm length were prepared from the cast 12 mm diameter round bars. On completion of each sample testing the specimen was removed, cleaned with acetone, dried and weighed to determine the mass loss due to wear. The difference in the mass measured before and after the test gives the wear of the specimen. The mass loss of the pin (specimen) was measured in an electronic weighing machine with a least count of 0.001 g. The ratio of mass losses to sliding distance was defined as wear rate. The wear test was carried out by varying one of the three parameters (load, sliding speed, and sliding distance) and keeping other two constant. The constant values for load, sliding speed, sliding distance used were 20N, 20rpm and 1727mm respectively.



Results and Discussion


The chemical compositions of the Aluminium-Silicon castings are shown in Table 1.


Table 1. Compositions of the Aluminium-Silicon Alloys (wt %)

































*elements with wt% less than 0.001 are not shown


The weight percentage of silicon in Al-7% Si and Al-12% Si were found to be 7.001% and 12.002% very close to 7% and 12% respectively. This suggests that the cast structure made was very sound and that the pouring temperature was sufficient. It can be seen that there was no loss of silicon and aluminium evaporation. The weight percentage of silicon in Al-14%Si was found to be 13.98% which was still close to 14%. This may be due to loss of some silicon.

Microstructures obtained from optical microscope at 100x magnifications are shown in Figure 1(a-c) for Al-7% Si, Al-12% Si and Al-14% Si respectively. It was observed that with increase in silicon percentage in the alloy, the microstructure was different for Al-7%Si and Al-12%Si alloys compared with Al-14%Si alloy. It showed light areas with rounded particles (aluminium) are crystallized, which are surrounded by networked structure of dark fine areas (eutectic silicon). The micrograph of Al-12%Si alloy showed the refinement of the dark networked structure of eutectic silicon particles. The silicon has long rod like structure. It may be seen in Al-14%Si that the degree of refinement of the eutectic silicon increased as the silicon content of the alloy increased beyond the eutectic composition. Here the primary silicon appears as coarse polyhedral particles. Although the presence of primary silicon was also observed in the Al-12%Si, but its size and volume fraction was less compared to Al-14% Si alloys.





Figure 1. Microstructure of as Cast alloys: (a) Al-7%Si (b) Al-12%Si and (c) Al-14%Si  


Figure 2 (a-f) shows the micrographs taken at low and high magnifications respectively for Al-7%, Al-12% and Al-14%Si alloys. The deep separated grooves of Al-7%Si alloy shown in figure 2 (a-b) was more compared to Al-12%Si in figure 2(c-d) and was lowest in Al-14%Si shown in figure 2(e-f). It revealed wear was highest in Al-7%Si and lowest in Al-14%Si. The presence of deep separated grooves may be that the hard dispersoid particles or fractured pieces are removed during abrasion by entrapped debris, hard asperities on the hardened steel counter face or work hardened deposits on the counter face. For Al-12%Si the deeper grooves was observed to be smooth in nature. This may be due to the fact that some wear debris of the material might have flown off. In Al-14%Si, shown in figure 2 (e-f) as Si content increases hardness of the material also increases. Deeper grooves may be due to the abrasion of SiC particles that have forced the silicon in platelet form, for which deeper grooves are produced. The worn surface of the alloys from the wear test observed under scanning electron microscope at magnification of x500 and x1500 are presented below.

The Vickers hardness numbers for Al-7%Si, Al-12%Si and Al-14%Si are found to be 52.05, 65.3 and 69.35 respectively. Figure 3 showed that Al-14%Si alloy has more strong effect on hardness than Al-7%Si, Al-12%Si. The sequence of silicon contents in the order of significant effects are Al-14%Si, Al-12%Si and Al-7%Si. The net result showed that hardness of the Al-Si alloy increased with increasing weight percentage of silicon and this may be due to the harder nature of silicon.








Figure 2. SEM micrograph of worn surface of alloys: Al-7%Si (a,b), Al-12%Si (c,d)

and Al-14%Si (e,f)


Figure 3. Variation of Hardness with Silicon Content


Figures 4-6 shows the wear test of Al-Si alloys at different operating parameters. The result of wear rate on all samples with varying loads with increasing silicon content is shown in Figure 4. It may be noted that wear rate decreases when the percentage of silicon increases and the wear rate increases with increasing load. The increases in wear rate throughout the tests confirm that increasing load increases wear rate. This finding is in agreement with that of [13,14].For Al-7%Si alloy, the wear rate was, whereas for Al-12%Si alloy, the wear rate decreased to and for Al-14%Si alloy, the wear rate was only for 15N loads. Comparable trend in wear rate for all other loads was observed.


Figure 4. Variation of wear of Al-Si alloys with applied load


Variation of wear rate with increase in sliding speed with increasing silicon content is shown in Figure 5. With increase in sliding speed, an increase in the wear rate was observed. This finding is in agreement with that of [13-14]. For Al-7%Si alloy, the wear rate was, whereas for Al-12%Si alloy, the wear rate was decreased to and for Al-14%Si alloy, the wear rate was only for 15rpm sliding speed. Comparable trends in wear rate for 20 rpm, 25rpm and 30 rpm sliding speeds was observed. Wear rate plotted against sliding distance at a constant load of 20N and speed 20rpm for different sliding distances like, 1099mm, 1727mm, 2355mm and 2983mm is shown in Figure 6.


Figure 5. Variation of wear of Al-Si alloys with sliding speed


Figure 6. Variation of wear of Al-Si alloys with sliding distance


The wear rate increased with increasing sliding distance for all condition. The wear rate was highest in Al-7% Si alloy and lowest in Al-14%Si alloy, which was due to the presence of hard silicon particles.

The increase in wear rate with increasing applied load can be attributed to the consequential effect of increased strain-hardening of the materials in contact, resulting in increase in the resistance to abrade or erode. The wear rate increase at higher sliding speed is the resulting effect of the interface temperature in increasing the wear rate due to plastic deformation of the material. The wear rate increase with increasing sliding distance is due to more amount of time in wearing for all conditions. These trends are also consistent with the results of the earlier research in the field.





The variation of silicon in Al-Si led to more degree of refinement of the eutectic silicon as the silicon content of the alloy increased beyond the eutectic composition. The amount of primary silicon increased with the increase in silicon amount in the cast. Hardness of the Al-Si alloy increased with the increase in amount of silicon present. The wear rate decreased when the percentage of silicon was increased. Wear was observed to increase at higher applied load and at higher sliding speed. Effect of load and sliding speed are more pronounced on the wear of the Al-Si alloys than sliding distance.





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