Friction Stir Processing of AA6061-T4 - Cold Rolled vs. as Cast

Mihaela Iordachescu, Jesus Ruiz, Danut Iordachescu, Pedro Vilaça, Jaime Planas

Friction Stir Processing (FSP) is a processing technique that may reduce stresses and strains, improving the mechanical properties of light alloys due to its pure solid-state transformation characteristics. It is used for inducing directed, localized and controlled material properties in any arbitrary location and pattern of components. The paper approaches the behaviour of AA6061-T4 cold rolled vs. as cast, both processed by Friction Stir. Despite significant advances, the fundamentals of FSP thermo-mechanical process are still not completely understood.  The peculiar behaviour of each material after FSP was assessed by using microstructural analysis, and HV micro-hardness and roughness tests, respectively. The correlation between the features of the residual stresses field and the heat and material flow in the processed zone was investigated. The microstructure of the processed layer is complex and highly dependent on the position within the processed zone. This is due to the large local variations in the plastic flow, and to the thermal history resulted from the material interaction with the tool. The X-ray diffraction (XRD) technique was used to measure the residual stresses. The correlations between the calculated residual stresses in the stirred zone and the process parameters, such as tool rotation and advancing speeds are presented, as too. Experimental results show that the residual stress on the longitudinal direction is greater than the one on the transversal direction, at the top surface of the processed zone. Higher stresses values are recorded in the region below the crown to the mid-thickness of the stirred zone.