Microstructure and Mechanical Properties of the Al/AlN Composite Layer Formed In Situ on Aluminum by FSP in the Presence of Nitrogen Gas

For the first time, nitrogen gas was used in friction stir processing (FSP) to form an Al/AlN composite surface layer on aluminum plate. Investigating the microstructure of the layers by X-ray diffraction, scanning electron microscopy and optical microscopy showed the formation of aluminum nitride particles on the surface layer of commercially pure aluminum. These particles found to have non-uniform distribution in the layer. While the percentage of composite reinforcements is not more than 10%, the aluminum grain size reduced from 100 to 70 μm due to restoring processes resulting from severe plastic deformation and significant improvement in surface properties observed. Due to extreme thermo-mechanical process, the surface layers have a finer grain size and high levels of hardness. As a result, the grain size ranged from an average of 100 μm to 70 μm and a hardness of 22 to a maximum of 38 Hv, which is about 1.7 times the base metal hardness. The wear resistance of the sample also improved up to 1.1 times with respect to fine graining of the surface layer and the presence of hard nitride particles. The coefficient of friction reduced from 1.2 to 0.9, indicating an improvement in the tribological properties of the sample