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JMEPEG https://doi.org/10.1007/s11665-018-3277-y

Wear Behavior of AZ31/Al2O3 Magnesium Matrix Surface Nanocomposite Fabricated via Friction Stir Processing Mahdi Azizieh, Arsham Norouzi Larki, Mehdi Tahmasebi, Mehdi Bavi, Ehsan Alizadeh, and Hyoung Seop Kim (Submitted December 11, 2016; in revised form February 8, 2018) The aim of this study was to produce magnesium-based surface nanocomposites via friction stir processing and to investigate the effect of tool rotational speed on the microstructure, hardness and wear behavior. The surface of the nanocomposites was characterized using optical and scanning electron microscopes, as well as through microhardness and wear tests. The results indicated that with the increase in rotational speed, the grain size of the surface nanocomposites increased, but its hardness decreased despite the improved distribution of Al2O3 nanoparticles. It was also found that the wear resistance has a direct relation to the distribution of the Al2O3 nanoparticles. Furthermore, the addition of nano-Al2O3 changed the wear mechanism from the adhesive mode in the as-received AZ31 to the abrasive mode in the nanocomposite specimens. The rotational speed of 1400 rpm was an optimum parameter to achieve a suitable composite layer with the highest wear resistance. Keywords

friction stir processing, hardness, magnesium, metallic matrix composite, wear

1. Introduction Magnesium alloys are among the lightest structural materials which have a unique combination of high strength-to-weight ratio, good machining qualities, ease of recyclability and high dimensional stability (Ref 1). However, these alloys also have poor mechanical properties such as hardness and wear resistance that limit their usage (Ref 2, 3). Surface modification is an effective approach to overcome these limitations, and coating processes are commonly used (Ref 4). However, these processes do not produce a local surface improvement. Recently, there has been much attention on friction stir processing (FSP) to achieve the desired surface modification. FSP is an innovative method, based on the principles of friction stir welding (FSW), in which frictional heat from a rotating tool is used to soften and deform the material, thereby providing local improvement on the surface properties. Hence, FSP could be used for surface composite fabrication (Ref 5). In this case, the rotating tool mixes introduced particles of other materials to the substrate to produce surface composites. There have been a number of studies on fabrication of magnesium base composite by FSP, to induce grain refinement, improve hardness and increase wear resistance (Ref 6-11). Morisada et al. (Ref 12) showed that in the AZ31 matrix, the distribution of multi-wall carbon nanotubes (MWCNTs) changed with the travel speed of the FSP tool. Lee et al. (Ref 8) showed that SiO2 nanoparticles were uniformly dispersed after four FSP passes. They produced a composite with hardness nearly double that of the base material Mahdi Azizieh, Arsham Norouzi Larki, Mehdi Tahmasebi, Mehdi Bavi