Alloy composite casting
Research into stir casting of a metal composite has shown increased resistance to dry sliding wear. Materials scientists Zohair Sarajan, Mehrdad Soltani and SeidAbbas Hoseininejad from Iran outline the process employed.
Aluminium matrix composites have a wide range of applications where high specific strength, high modulus and good wear resistance are crucial. Aluminium alloys have been gaining importance as structural materials, but for many applications improved wear resistance is needed. The main goal is to determine dry sliding wear behaviour in the context of the formation of a transfer layer on the sliding surface of cast aluminium-5 wt.% copper composites, which are synthesised by the addition of titanium oxide to the melt before solidification.
Composites have enhanced engineering materials beyond natural combinations by introducing man-made combinations, including across different categories of metals, ceramics and polymers. In many metallic components, wear resistance is the primary consideration. Normally, aluminium alloys have excellent mechanical properties, coupled with good corrosion resistance. But they often demonstrate poor resistance to wear and seizure.
In the mix
The liquid temperature of the aluminium alloy was calculated based on chemical composition. Pieces of aluminium-copper (composition given in Table 1) were heated in a graphite crucible kept in a vertical electric resistance muffle furnace to attain a molten state. When the temperature reached 800°C, the melt was mechanically stirred to create a vortex. Titanium oxide particles (Table 2) were added to the vortex for transfer into the melt, and melt-particle reaction was allowed to continue for 20 minutes.
The next step was the addition of titanium oxide particles as the reinforcement. The liquid metal was stirred under a hot argon atmosphere at around 100°C, at a maximum flow of 2,360cm3/min and the stir casting composite ingot was quickly cooled by immersion in water immediately after casting.
Dry sliding wear tests of different stir casting composites and un-reinforced alloys were then performed in ambient conditions, with relative humidity in the range of 40-60% and temperature between 17-25°C, using a pin-on-disc machine.
The images (Figure 1) are SEM micro-structures of the composite and are typical examples of the material synthesised by dispersion of titanium oxide in an aluminium copper alloy. By increasing the amount of oxide particles, more bright particles appear as fine spots in the microstructure. The bright, elongated and faceted precipitates of copper aluminide are shown in the matrix, which is surrounded by titanium oxide particles.
Figure 2 shows the results of pin-on-disc tests carried out normally (solid lines), allowing the wear debris to get between the sliding surfaces, contributing to three-body-wear and formation of a transfer layer. The dotted lines indicate the results of the tests when the wear debris has been removed continuously using a camel brush. For a normal load, there is a linear increase of cumulative volume loss with increasing sliding distance. This follows Archard’s Law, as commonly observed for metal matrix composites, although in dry sliding wear of the mechanism is not purely adhesion.
Figure 3 shows the variation of wear rate with load, and tests carried out with and without the removal of wear debris. The wear rates for the composites tested without removal of debris demonstrate a linear increase with increasing load, but their magnitude is relatively close to composites with different particle content. The slope of the variation of wear rate with load is less when the wear debris is not removed.
The wear coefficients have been estimated by multiplying the slope of variation of wear rate with load. The wear rate is estimated by dividing the normal load applied during dry sliding by the hardness of the material (Figure 3).
Wear debris of composite and cast aluminium has been compared (Figure 5). Both micrographs show smaller oxide particles, apart from agglomerates, which could be flaked off the transfer layer. The agglomerates are relatively smaller for the composites (Figure 5a), compared to those observed for cast aluminium (Figure 5b).
An increase in hardness is expected to reduce the real area of contact, which may be attributed to decreasing this area during dry sliding due to increased hardness of the composites with higher particle content. This micrograph also shows the accumulated volume loss is considerably higher when wearing debris is removed during dry sliding wear.
Right combination for wear reduction
The results of this study show the following –
- The hardness of the composites, developed by the addition of titanium oxide, increases by raising the particle content, resulting from the heightened reaction between the added oxide and the molten alloy during processing.
- During dry sliding against the counter face of the hardened steel disc, under conditions of load and sliding speed, the wear is primarily oxidative as shown from wear debris, although there have been some metallic fragments.
- The accumulated loss of volume in composites shows a linear increase with sliding distance during both types of tests, conducted with and without removal of wear debris. This establishes the beneficial role of the formation of transfer layer.
- Increasing the addition of oxides, resulting in higher particle content in composites, decreases the accumulated volume loss and wear rates, and has been attributed to the increased hardness with increasing oxide addition.
Zohair Sarajan, Department of Materials Science and Engineering, Islamic Azad University- Yazd Branch, Safaeeyeh, Yazd, IR of IRAN, PO Box 89195/155. Tel: +98 351 8219223. Email: firstname.lastname@example.org