Effect of Welding Parameters On Residual Stress Distribution in AA6056-T4 Friction Stir Welds

Friction Stir Welding (FSW) is the leading joining technique in the aeronautical industry for producing integral airframe structures, because of the advantages of lower cost and weight, and reduction in widespread fatigue damage compared to standard riveting.

FSW is a solid state process that involves the plunging of a rotating tool, comprising a profiled probe protruding from a shoulder, which is inserted and traversed along abutting sheets, to soften and stir the material around its pin, while its shoulder consolidates the joint.

Despite the undeniable advantages of this key technology, thermally induced tensile residual stresses can still negatively affects the integrity of a FSWelded structure. This study presents the relationship between the residual stress distribution and the main heat inducing FSW parameters, i.e. tool rotational and transverse speeds.

Longitudinal residual stresses of standard aeronautical aluminum alloy welds, produced with a Triflat FSW tool, using rotational and transverse speeds from 1250-2000rpm and 300-500mm/min, were measured using a strain-gage based destructive method.

Residual stresses exhibited an “M” distribution, with local minimum tensile residual stress occurring in the weld nugget and maxima near the shoulder radius in the thermo-mechanically affected zone (TMAZ).

Maximum tensile residual stresses were higher at the advancing side than on the retreating side of the weld due to a higher shearing rate and heat generation.

However, the retreating side TMAZ’s were found to be the weakest zones in the welds as indicated by micro-hardness and tensile fracture tests. On the retreating side, maximum and minimum tensile residual stresses (which were 39.24% and 16.92% of the base material yield strength respectively) showed a significant reduction with advance per revolution of the tool.

Hence, optimization of cyclic mechanical and stress corrosion properties of a potential FSWelded integral airframe structure in AA6056-T4, 1.6mm thick, would require an increasing advance per revolution.