An analysis of through-thickness residual stresses in aluminium FSW butt joints

In the paper, the results of a wide experimental campaign on friction stir welding (FSW) of aluminum alloys are reported. The attention was focused on the through-thickness residual stresses that occur on aluminum joints, after the welding process. In detail, using the hole-drilling method the residual stresses distribution in the zone close to the tool shoulder border of the joint advancing side, has been investigated; four different aluminum alloys and three different process conditions have been considered. The experimental analysis has shown that unlike traditional welding processes, the residual stresses are negative in the surface of the examined zone, and increase with depth until values of about 100–150 MPa that occur at a depth of about 0.5–1.0 mm. As expected, the maximum value of the residual stresses induced by the FSW process influences the mechanical behavior of the joint significantly, as it has been observed for the AA6082-T6 aluminum alloy by considering its static and fatigue resistance. Such results corroborate that the hole-drilling method, widely employed in the industrial field due to its simplicity and low cost, can be used for an accurate estimation of the maximum residual stresses that occur in an aluminum butt joint obtained by friction stir welding.