Influence Of Bi And Nd Addition On The Microstructure And Mechanical Properties Of Casting Hypereutectic Al-Mg₂Si Alloy

Hypereutectic Al-Mg₂Si alloys with different amounts Bi and Nd are prepared by usingtraditional gravity casting. The microstructures of the samples are investigated by opticalmicroscopy （OM）, scanning electron microscopy （SEM） and X-ray diffraction （XRD）, themechanical properties are measured by universal material testing machine, the fracturesurfaces of the alloys are analysized by SEM and dry sliding wear behavior of Al-15%Mg₂Sialloy is studied by friction and wear testing machine as well in this paper. The results aresummarized as follows:The addition of an appropriate amount of Bi or Nd has significant modification effect onboth primary and eutectic Mg₂Si in the hypereutectic Al-Mg₂Si alloy. The morphology ofprimary Mg₂Si is changed from irregular or dendritic to polyhedral shape. Moreover, themorphology of the eutectic Mg₂Si phase altered from flake-like to a thin laminar, very shortfibrous or dot-like; and their average sizes are significantly decreased.With the increase of Bi content, the ultimate tensile strength （UTS） and percentageelongation （EL） of the hypereutectic Al-Mg₂Si alloy increase firstly and then decrease slightly.Compared with the unmodified alloy, the UTS of1.0wt.%Bi modified alloy was260MPawith an EL of7.0%. The UTS and EL of modified alloy increase51.2%and100%,respectively.At the same time, ductile characteristic appears after modification of Bi. The primary andeutectic Mg₂Si which have both higher yield strength and crack resistance result in theimprovement of the alloy’s tension strength.The UTS of the alloy increases from185MPa to245MPa, with an EL from0.57%to1.71%after adding1.0wt.%Nd, and the increasing percentages are32.4%and200%,respectively.The wear rates and friction factors of the alloy with4.0%Bi alter the wear mechamismof the alloy from the combination of abrasive, adhesive and delamination wear without Biinto single mild abrasion wear with4.0%Bi. The improvement of wear properties of thecomposites with Bi is originated from a combination of Bi modification on Mg₂Si andself-lubrication function of Bi particles in the friction process.