Investigation of process parameters and tool design on friction stir channeling (FSC) of AA5083 monolithic plates and microstructural characterization

The present study investigates the feasibility of achieving subsurface internal closed channels in monolithic AA5083-H111 plates using the friction stir channeling (FSC) method with the shoulder–workpiece clearance approach. Different parameters were studied for the optimization of the process. As a result of varying shoulder-to-pin diameter ratios, two friction stir welding/processing tools were utilized, and the effects of varying shoulder clearances against various traverse speeds and rotation speeds were examined. The results revealed that the maximum channel cross-sectional area was obtained by the application of the tool with the smaller shoulder-to-pin ratio and with a weld pitch of 4.93 (1000 rpm rotational speed / 203 mm/min transverse speed). As for the examination of the shoulder–workpiece clearances, the results showed that by increasing the clearance, the cross-sectional area slightly increases up to a maximum value, after which large-scale cracks and voids are introduced in the zone between the channel and the workpiece. For the microstructural characterization, specimens were examined microscopically using optical microscopy (OM) and scanning electron microscopy (SEM). The examination revealed the presence of porosity and micro-crevices located in the area between the channel and the surface. The microstructure was found to be closely related to microhardness distribution analysis. The microstructural defects introduced a slight reduction in hardness values in the area between the channel and the surface with respect to the hardness values of the rest of the examined stir zone and the TMAZ/HAZ zones where the hardness values were found to be close to the values of the base material.