Ultra-mild wear in eutectic Al–Si alloys

Abstract

Al–Si alloys that are intended for extended durability applications must operate under ultra-mild wear conditions. Understanding the mechanisms responsible for ultra-mild wear in cast Al–Si alloys is essential to any effort to accelerate the process of using these alloys in components operating under these conditions. Two eutectic grade Al–Si alloys with similar silicon percentages (11 wt% Si and 12 wt% Si) were tested. The alloys differed in matrix hardness, but had comparable silicon particle sizes and morphologies. Tests were conducted using a ball (AISI 52100 steel)-on-disc tribometer under light load (0.5 N) in lubricated conditions corresponding to ultra-mild wear. The sample surfaces were chemically etched to expose hard particles. No material removal was detected by mass loss measurements in either of the alloys after sliding to 2.2 × 10 3 m (10 5 cycles). Sliding occurred on the top of silicon particles at first. Evidence of both the sinking-in of silicon particles, and the pile-up of plastically deformed material around the sunken-in particles was observed. A contact stress analysis based on the Greenwood and Tripp model suggests that the applied normal pressure on the hard phases was approximately equal with the matrix hardness of the harder alloy, but exceeded the matrix strength of the softer alloy. This is consistent with the observation that the Al–11% Si with the higher matrix hardness suffered less surface damage – less significant amounts of particle sinking-in and aluminum piling up – when compared with the Al–12% Si with softer matrix.