Feasibility of solid-state additive manufacturing in fabrication of functionally graded AA7075/Al2O3 nanocomposite

Today, while functional-gradient materials with controlled gradual changes in microstructure and/or chemical composition have unique mechanical, thermal and optical properties in specified directions, the functional limitations in the traditional methods of processing these materials, on the one hand, and also the challenges ahead for the purpose of additive manufacturing of these materials through melting-based technologies, on the other hand, has attracted the attention of researchers in order to develop solid-state additive manufacturing technologies. In this study, a functional-gradient nanocomposite has been made using the friction surfacing technology (FS) as a solid-state additive manufacturing process. In the present research, firstly by using the FS technology as an emerging solid-state method for metal additive manufacturing (MAM), successful layer depositions of AA7075-T6 aluminum alloy consisting of 5 layers were carried out on a substrate of the same material. Then, to make a functional gradient nanocomposite of this alloy using the same processing parameters, another deposition was done by adding Al2O3-γ spherical powder particles with an average size of 20 µm as a reinforcing agent. During thermo-mechanical processing, many particles of alumina powder were decomposed and reached a size below 100 nm. The mechanism of transferring the powder to the layers was by drilling the holes of the consumables with specific patterns and then embedding the powder particles into these holes. The gradual increase in the amount of applied powder particles in each layer realizes the construction of functional-gradient material. The severe plastic deformation and also the heat generated by the friction caused the occurrence of dynamic recrystallization (DRX) and the formation of fine and coaxial grains in the range of 5-8 µm for both layer depositions. In the case of FGM sample, fine alumina particles with activation of the particle-stimulated nucleation (PSN) mechanism, caused the formation of smaller grains. The maximum amount of hardness in the upper layers of both the simple alloy and the functional-gradient one was obtained and equal to 112% and 141%, respectively, while the minimum hardness related to the heat-affected zone in the substrate was observed at 90 HV. Electron backscattered diffraction (EBSD) results also indicated a decrease in the number of low-angle grain boundaries (LAGBs) in the middle and lower regions compared to the upper layers. In such a way, the number of LAGBs for the middle and upper layers of the simple alloy was observed to be equal to 33.1% and 52.1%, respectively, and for the composite alloy, 29.4% and 53.5%, respectively. The maximum value of the ultimate strength and yield stress was also obtained for the simple alloy was equal to 482 MPa and 310 MPa, and for the composite alloy, it was equal to 449 MPa and 341 MPa. The hardness and strength of both layer deposition have increased significantly compared to the AA7075-TO aluminum alloy.