Tribological properties of Aluminum Alloys: Part One

Various types of aluminum alloys are continually being developed to improve their wear resistance. Among these alloys, aluminum-silicon (Al-Si) alloys have been found to be beneficial in many industrial applications and considered to be appropriate substitutes for cast iron components.

The addition of silicon in aluminum alloys improves their wear, casting, machining and corrosion characteristics. Both hypoeutectic and hypereutectic Al-Si alloys are widely used in a variety of applications, including automotive and related equipment, air-conditioning equipment and home electrical appliances.

A large number of studies in recent years have been devoted to investigate the friction and wear behavior of Al-Si alloys. However, most of these studies were conducted under dry sliding conditions. The few studies that have been conducted under lubricated sliding conditions were mainly concerned with the effect of silicon content and silicon morphology on the friction and wear resistance of these alloys. Also, the friction and wear tests for these studies were conducted mostly in an air environment.

Al-Si alloys are widely used for critical components in refrigerant compressors, especially connecting rods in reciprocating compressors and swash plates in automotive air-conditioning compressors. The successful operation of compressors used in air-conditioning and refrigeration systems is mainly governed by the tribological behavior at the critical contacts within the compressors.

It is known that the environmental conditions around these contacts have a significant impact on their tribological performance. Thus, the friction and wear characteristics of Al-Si alloys in a refrigerant environment might be quite different from those of Al-Si alloys in an air environment.

At the original equipment level, the use of aluminum in main and rod bearings is growing for a variety of reasons. One such reasons is that aluminum bearings are less expensive to manufacturer and it removes the presence of lead, which is an environmental concern for manufacturers.

Friction and wear occur in machinery components which run together. The researchers investigate friction and wear behavior of materials because of the undesirable effect observed in the performance and life of machinery components. Aluminum alloys and other lightweight materials have growing applications in the automotive industry, with respect to reducing the fuel utilization and shielding the environment, where they can successfully reinstate steel and cast iron parts.

These alloys are extensively used in buildings and constructions, containers and packaging, marine, aviation, aerospace and electrical industries because of their lightweight, corrosion resistance in most environments or combination of theses properties. Aluminum alloys have higher conductivities (electrical and thermal) than most other metals, and they are usually cheaper than the alloys that are superior conductors (copper, silver, gold, and so on). Aluminum based alloy provides good combination of strength, corrosion resistance, together with fluidity and freedom from hot shortness.

Aluminum based bearing alloys commonly contain tin (6-40%) as a soft component. Some of aluminum based bearing alloys contain silicon. It has very high hardness and its inclusions distributed over the aluminum matrix serve as an abrasive particle polishing the mating surface. The aluminum matrix of engine bearing alloys may be strengthened by addition of copper, nickel, chromium, manganese, magnesium, zinc etc.

Henizleitner et al. studied the characterization of mechanical properties of sliding bearing materials based on aluminum and reported that the static strength as well as the fatigue strength shows a strong dependence on temperature. Prasad Rao et al. studied the improvement in tensile strength and load nearing capacity during dry wear of Al-7Si alloy combined grain refinement and modification.

The results of the investigations carried out, show a relationship between the nature of tribological wear of the Al + intermetallic phase type composites and the type of friction partner. In the investigations, two types of material were taken for the cooperation with the composite Al + intermetallic phases.

One of the materials were composite based on the AlSi12CuNiMg alloy, reinforced with ceramic particles (SiC, Al₂O₃), and the other was spheroidal cast iron. Comparing the results obtained during the investigation of wear and friction coefficient, it can be affirmed that both the friction coefficient value and the wear depend on the type of the material used. In the case of composites: AlSi12CuNiMg + ceramic particles, regardless of the type of composite, the tribological properties of the system are similar.

In spite of differences in the level of resistance to wear of the compared systems, their common characteristic feature must be stressed. Mainly, in both cases, plastic deformation of the pin occurred as a result of wear, which deformation contributed to an increase of the pin diameter in friction peripheries. This phenomenon was not present during the cooperation with spheroidal cast iron. For both systems, the increase in the diameter had a similar value of ca. 0.7 mm.

The tribological systems in which composites of the Al + intermetallic phases type are used, are characterized by a stable course of the friction coefficient value as a function of friction distance, irrespective of the type of material cooperating with them.

In the case of cooperation of composites Al + intermetallic phases with AlSi12CuNiMg aluminum alloy based composites, one should take into account the changes of friction conditions resulting from plastic deformation of the friction surface of the composite Al + intermetallic phases.

Most of the work has been devoted to the effects of silicon, copper, zinc, and lead. In addition, various aluminum composites have been evaluated. The major results from these studies are summarized in part two of this article (to be published October 2012).