Influence of surface and heat treatment on the fretting wear of an aluminium alloy reinforced with SiC particles

Abstract

Because particulate reinforced metal matrix composites cannot be welded without unacceptable disruption of local particle distribution and matrix microstructure, they are normally joined using mechanical fasteners or adhesives, and consequently they are susceptible to fretting wear and fretting fatigue. The fretting wear behaviour of a 2124 AlCuMg alloy reinforced with a uniform distribution of 17 vol% of 3 μm silicon carbide particles has been compared with that of the equivalent monolithic alloy. With the matrix alloy in the naturally aged (T4) condition, it was shown previously that for test durations up to ∼1 × 10 6 fretting cycles (40 μm stroke) the composite possessed significant advantages over the monolithic alloy, with a much reduced level of debris loss and transfer to the steel counterface. For longer test durations a reversal in behaviour took place with the wear rate of the composite increasing above that of the monolithic alloy. In an attempt to improve the longer duration fretting performance of the composite, two approaches have been adopted: (1) artificial ageing (14 h at 170°C) of the matrix alloy to peak strength (T6 condition), and (2) application of an anodizing surface treatment to the composite. The artificial ageing treatment has little influence on the wear rate of the composite. In contrast, the effect of the anodising surface treatment is most marked. Anodizing of the composite (T4 condition) was carried out in 1.5M sulphuric acid at a current density of 5 mA cm −2 at 20°C, after a light alkali etch. A period of 45 min was sufficient to produce a 10 μm thick layer into which the reinforcing particles were assimilated. Testing was undertaken with the anodized layer not sealed. Shorter duration tests (≤ 1 × 10 6 fretting cycles) showed increased levels of material transfer from the anodized composite to the steel counterface. However, over longer duration tests (≤ 1 × 10 7 fretting cycles) anodizing of the composite achieved an appreciable reduction (factor of ≥ 3) in its wear rate, and in the wear rate of the steel counterface. The behaviour of the anodized layer is interpreted in terms of its early fragmentation introducing oxide debris into the fretting interface which restricts metal-to-metal contact long after the anodized layer has been consumed.