Microstructure and wear resistance of a multi-layered aluminium structure developed by friction stir additive manufacturing

Metallic structures fabricated through additive manufacturing (AM) techniques are promising as they exhibit superior mechanical properties with improved tribological performance. However, their suitability for developing aluminium/magnesium-based lightweight structures remains limited due to high-temperature processing conditions and solidification defects. Therefore, in this study, an AA7075 build structure was developed by employing a novel solid-state friction stir additive manufacturing route at a fixed tool rotation speed and varied traverse speeds. The cross-section was analysed using an optical macroscope, while microstructural details were investigated using the electron backscattered diffraction method (EBSD) and transmission electron microscopy (TEM). Hardness was measured using a Vickers hardness tester, while a pin-on-disc apparatus was employed for tribological characterisation. The build structure produced with 20% overlap resulted in a wider nugget zone with a well-refined microstructure, dense dislocations and finer precipitates. Grain size reduced to 1.54 μm, and the hardness values reached a maximum of 151.9 HV at the top of the build, which is 123% greater than that of the base material. Tribological behaviour varied with respect to operating conditions. Minimum friction coefficient and wear rate were observed at 5N load, with abrasion being the dominant wear mechanism. At 15 N load, friction level and wear rate increased drastically, and the surface indicated significant material removal in the form of delaminations and abrasion.