Abstract
Al-base alloys in general possess a low resistance against sliding wear, compared e.g. with many steels. Cast Al-alloys containing high amounts of Si are more resilient than conventional wrought alloys and can be used in applications involving tribological loading, e.g. in automotive engines. The wear resistance of these alloys is determined by hard Si phases in the microstructure, which can support the contact pressure, reduce adhesion of the soft Al-rich solid solution and hinder ploughing by a harder counterbody.
Friction surfacing (FS) is a solid-state manufacturing technique, which allows applying thick coatings from metals through friction and plastic deformation. The deposited material undergoes severe plastic deformation at elevated temperatures, resulting in a recrystallized microstructure in the coating. FS allows depositing thick layers of Si-rich Al-base alloys on metal substrates, which afterwards possess a wrought microstructure.
In this study, cast Al-base alloys with three different contents of Si (6.6%, 10.4% and 14.6%) are deposited by FS on AA 2024 substrate. The wear behaviour of such processed materials, as well as each respective cast state, is investigated in a pin-on-disk configuration against steel 42CrMo4, using motor oil 5W30 as lubricant. Wear volumes and coefficient of friction are measured. Changes of the surface and subsurface material are studied by X-ray photoelectron spectroscopy and scanning electron microscopy combined with a focused ion beam.
At 14.6 % Si, large primary Si phases are present in both material states. Here, higher coefficients of friction were observed (≈ 0.1), than for the other two materials (≈ 0.04). At the same time, the wear volume is lowest for the high-Si alloy, and shows no difference between the cast and FS-processed state. Microscopy shows fracturing of Si phases as well as deformation and cracking in the adjacent Al-rich matrix. Spheroidization of the Si phases during FS in the two hypoeutectic alloys (6.6 and 10.4% Si) leads to a reduction in wear by more than 50 % compared to the cast state. This study shows that applying wear resistant coatings from Si-rich Al-base alloys by FS is feasible, with comparable wear behaviour for hypereutectic and improved wear resistance for hypoeutectic alloys.
