CALPHAD-guided alloy design of Bi-modified Al–Sn bearing alloy with multiphase composite structure and optimal self-lubricating property

Abstract

Through primary phase diagram analysis and thermodynamic calculation of Al–Sn, Al–Bi and Sn–Bi binary systems, Bi element was ensured to be a suitable alloying element possibly introducing granular Bi-rich precipitates and increasing the self-lubricating property of Al–Sn bearing alloy. Through calculation of excess free energy of liquid (Gexcessliq) in Al–Sn–Bi ternary system, it’s further proven that the segregation of Bi in Al can be suppressed by Sn element. Pseudo-binary phase diagrams of Al–Sn–xBi (wt%, x = 0, 1, 2, 3) were obtained through CALPHAD method, it’s found that Al–20Sn and Al–20Sn–1Bi alloys are composited of α-Al and Sn-rich secondary phases (SPs). However, Bi will be precipitated from the Sn-rich SPs in Al–20Sn–2Bi and Al–20Sn–3Bi alloys which are thus composited of three phases (α-Al, Sn-rich SPs and Bi-rich precipitates). The Al–20Sn–xBi (wt%, x = 0, 1, 2, 3) alloys were prepared with the guidance of CALPHAD. The microstructures match the calculation results well, fine and granular Bi-rich precipitates can be observed only in Al–20Sn–2Bi and Al–20Sn–3Bi alloys. Friction coefficients of alloys firstly decrease to 0.23 and then increase to 0.32 with Bi content increasing from 0 wt% to 3 wt%. The granular Bi-rich precipitates can improve the self-lubricating property of Al-Sn alloy. However, the increasing brittleness will lead to decrease of self-lubricating property when Bi content surpasses the critical value.