Influence of Bi addition on microstructure and dry sliding wear behaviors of cast Al-Mg2Si metal matrix composite

The influence of addition of Bi (0.2%-8.0%) on the microstructure and dry sliding wear behaviors of cast Al-15%Mg2Si in situ metal matrix composites was investigated. The microstructure features of the composites were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), and their wear characteristics were evaluated at different loads. The results show that the proper addition of Bi has significant modification effect on the morphologies of both primary and eutectic Mg2Si in the Al-15%Mg2Si composites. With the increase of Bi content from 0 to 4%, the morphology of primary Mg2Si changes from irregular or dendritic to polyhedral shape and its average particle size significantly decreases from 70 to 6 mu m. Moreover, the morphology of eutectic Mg2Si phase also alters from flake-like to very short fibrous or dot-like shape. When the Bi content exceeds 4.0%, the primary Mg2Si becomes coarse again; however, the eutectic Mg2Si still exhibits modified morphology. The wear rates and coefficient of friction of the composites with Bi addition are lower than those without Bi. Furthermore, the wear mechanism of the composite changes from the combination of abrasive, adhesive and delamination wear without Bi into mild abrasion and adhesive wear with 1% Bi; the addition of 4% Bi effectively restrains the cracks in primary Mg2Si and on the worn surface of the composite and avoids adhesive and delamination wear, and thus only a single mild abrasion occurs. The improvement of wear performance of the composites with Bi is originated from the combination of Bi modification on Mg2Si and self-lubrication function of Bi particles in the friction process.