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

摘要

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 (Gexcess liq ) 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 & laquo;-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 (& laquo;-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.