The effects of Bi and Pb on the soft phase in Al82Sn15Si2.2Cu0.8 sliding bearing alloy

Abstract

Lead (Pb) is often used in AlSn based sliding bearing alloys to improve their tribological properties. However, due to the polluting and toxic nature, it is desired to replace Pb with other soft metals. Although the tribological properties of AlSn based alloy have been extensively studied, how the soft phase in alloy was influenced by alloying elements was rarely characterized. In this work, Bi is chosen for its thermodynamic similarity to Pb to investigate the effects of alloying elements on the constituents, 3D morphology, and hardness of the soft phase in AlSn based alloys. It was found that due to the immiscible characteristic with aluminum, Bi and Pb additions both tend to break the branching structure of the soft Sn-rich phase in Al82Sn15Si2.2Cu0.8 alloy and produce spherical Bi<Sn> and Pb<Sn> mixed phases respectively. It was also shown that Bi significantly increased the hardness of the Sn-rich phase by solution strengthening, but Pb has only a slight effect. The mixed phase Bi<Sn> produced by Bi addition is not softer than the Sn-rich phase in Al82Sn15Si2.2Cu0.8 alloy, but the Pb<Sn> phase produced by Pb addition is much softer. The results indicate that to replace the toxic Pb in AlSn based bearing alloys, efforts should be focused on the further reduction of the soft phase’ hardness in alloy.