Tribological Properties of Aluminum Alloys: Part Two

Silicon has received the most attention among all alloying elements studied. This is due to the fact that Al-Si alloys are corrosion resistant, strong, have low thermal expansion coefficients, and have superior tribological characteristics compared to the other aluminum alloys. These alloys have been successfully used as substitutes for cast iron in applications such as pistons and cylinder linings for internal combustion engines, swash plates, connecting rods, and sockets in refrigerant compressors.

Most researchers agree that the wear rate of Al-Si alloys goes through a minimum at certain Si content. The higher wear rates at low Si contents are attributed to the lower hardness of these alloys, while the increased wear rates at high Si contents are due to reductions in their ductility and fracture toughness. Although there is some controversy in the literature about the location of this minimum, it seems that for weaker and more brittle binary Al-Si alloys, the minimal wear rate is achieved with alloys having Si content at or slightly higher than the eutectic composition (12.6%).

For the commercial alloys which have other alloying elements to improve their strength and toughness (e.g. Cu and Mg), the minimum is usually shifted to higher Si contents (e.g. 17%). Probably the most popular commercial alloy with 17% Si is the 390 alloy which is used in numerous applications where high wear resistance is required.

There is also a controversy about the optimal size of the silicon particles. The results differ depending on the conditions of the tests conducted. Researchers, who have conducted tests under dry conditions, tend to agree that the initial size of Si particles is not important. Under dry conditions, the rubbing surfaces are subjected to severe traction which often causes plastic flow of the subsurface layer. Under these conditions, the silicon particles fracture and attain some equilibrium size (1-5 µm) and shape (spherical), irrespective of their initial size and shape.

The strain hardening of the subsurface, together with some compaction of wear debris, form a work-hardened layer which is characterized by much higher hardness (240 HV compared to 100 HV of the bulk alloy). This layer protects the surface from further damage.

If the sliding occurs under lubricated conditions, a work-hardened layer usually is not formed and the microstructure remains unchanged up to the surface. Under these conditions, the size and shape of the silicon particles becomes important. Larger silicon particles are more effective in modifying the counter face by removing the preferred sites for aluminum transfer and in polishing away of any adhered aluminum.

On the other hand, larger Si particles cause higher temperature spikes which may lead to thermally driven seizure. Alloys with finer silicon particles also have a higher hardness, which is an important parameter for better wear resistance.

Copper increases the strength and wear resistance of aluminum alloys through a mechanism of precipitation hardening. The wear and seizure resistance of Al-Cu alloys were found to increase up to Cu contents of about 4%, after which it levels off.

Lead containing aluminum alloys are used for journal bearings. The friction and wear of alloys containing up to 50% lead have been studied. The optimum performance was achieved with a lead content of 25%. Higher Pb contents decrease the strengths of the alloy, and bring about higher wear. If the Pb content is low, the alloy is not able to form a smeared lead layer at the interface and is, therefore, more prone to seizure and higher wear. Zinc reduces both wear resistance and seizure resistance of aluminum alloys. The reason is the reduced strength of the alloys at elevated interfacial temperatures.

The addition of magnesium up to 1 %is also useful in reducing wear. It is added to provide strengthening through precipitation of Mg₂Si in the matrix. However, large amounts of magnesium degrade the mechanical properties of aluminum alloys.

Zinc reduces both wear resistance and seizure resistance of aluminum alloys. The reason is the reduced strength of the alloys at elevated interfacial temperatures. The addition of magnesium up to 1% is also useful in reducing wear. It is added to provide strengthening through precipitation of Mg₂Si in the matrix. However, large amounts of magnesium degrade the mechanical properties of aluminum alloys.

Iron is the most common alloying element for aluminum. It can be tolerated up to levels of 1.5-2.0%. The presence of iron modifies the silicon phase by introducing several phases.

Al-Fe-Si phases: If present alone, iron forms intermetallic compounds with aluminumat grainboundariesandimpairs mechanical properties. When elements such as manganese, chromium, cobalt and molybdenum are present, iron combines with them to form intermetallic compounds which are less harmful. Other elements usually added to aluminum include nickel, titanium and zirconium (grain refiners), sodium and strontium (eutectic silicon modifiers) and phosphorus (primary silicon refiner).

The results can be summarized as follows:
• The 2024-T351 and 6061-T61 alloys have approximately two orders of magnitude higher wear than the other materials tested. The friction coefficient of these alloys is also significantly higher and the contact resistance is lower than that of other alloys.
• In general, the amount of wear decreases as the amount of silicon content increases. This trend, however, is complicated by the presence of other alloying elements and the different heat treatment processes.
• The addition of bismuth and higher amount of copper reduces wear for alloys of otherwise similar composition and heat treatment.
• The lowest wear is obtained with the 390DC-T6 alloy. The HV4-T61 alloy also gives very good wear resistance.
• Conventional anodizing (356CM-AN) does not improve the wear resistance of the 356 aluminum alloy under concentrated contacts. The hard layer cracks under the high contact stress causing an increase in wear.
• Hard anodizing and SiC particle reinforcement provide very good wear resistance. However, they cause increased wear on the counterface due to the rough hard surfaces generated by the hard anodizing processes and the hard SiC particles on the surface of a SiC-AI composite.
• The capped PAG (PAG 2) seems to be a better lubricant for 356CM-T61 alloy than the uncapped lubricant (PAG 1). However, for the 390PM-T61 alloy, the lubricity of the PAG's is about the same.
• R407C and R410A used with alkylbenzene lubricantprovidesimilar(with 390PM-T61) or slightly better (with 356CM-T61) wear resistance when compared to R22. The lower wear obtained with the 356CM-T61 alloy is probably due to the higher viscosity of alkylbenzeneIR407C or R410A mixtures compared to the aikylbenzene/R22 mixture. However, the relative wear differences between these mixtures are small, therefore, more tests need to be conducted to examine their lubricity difference, if any exists.
• The R134a refrigerant does not always behave as an inert environment. Under the conditions of this study, a chemical reaction between the freshly exposed aluminum and R134a occurred. The reaction product was identified as AlF₃.
• R134a and other HFC's at sufficiently high partial pressures increase the brittleness of aluminum alloys. Corrosion cracking and/or hydrogen embrittlement are viewed as possible mechanisms. The formation of AlF₃ on freshly exposed aluminum surfaces may be another reason for the enhanced brittleness. However, the actual causes and mechanisms for the surface fatigue on the 356 alloys are not clear at this time.