Mechanical Tensioning of High-Strength Aluminum Alloy Friction Stir Welds

Abstract

The extent to which in-situ global mechanical tensioning (GMT) can be used to modify the residual stress state in friction stir (FS) welds is investigated in this article. Residual stress distributions have been determined by synchrotron X-ray and neutron diffraction for four sets of FS welds in high-strength AA7449-W51 and lithium containing AA2199-T8 aerospace aluminum alloys subjected to a systematic range of GMT levels. For the cases studied, the results indicate that the level of residual stresses present in the as-welded state is a function of the alloy. The rate of residual stress reduction brought about by GMT, however, is basically alloy independent; indeed, it is essentially linear with respect to the GMT load, so that the tensioning required to reduce the weld stresses to zero can be calculated directly from the stresses present in the untensioned case. For thin plates, proximity to the yield stress in the hot-softened zone means that a guideline rule is that 1 MPa of tensioning during welding reduces the tensile stress by approximately 1 MPa. The GMT was found to be less effective at greater depths in thick plates. Furthermore, a reduction in bending distortion and an increase in angular distortion was observed with increased GMT, while no effects on the weld microstructure and hardness were observed.

Contemporary approaches to reducing weld induced residual stress

“…Depending on the longitudinal tensioning load used, GMT can greatly reduce residual stresses at the weld line, or even introduce compressive stresses in this region [145,146,147,148], see Figure 15b. For welds in 2000-series aluminium alloy, the magnitude of tensioning stress that must be applied during welding to reduce the longitudinal residual stress to approximately zero, has been reported as 25-40% of the room temperature yield stress of the parent material [146,148].…”

Effect of cooling media on residual stresses induced by a solid-state welding: underwater FSW

“…It is also revealed by Hatamleh et al [9] that the maximum extent and location of the RS is dependent to the fixture support. Furthermore, Altenkirch et al [10,11] showed that global tensioning can decrease the RS and a reverse linear relationship holds between the RS and the forces applied on thin plates.…”

Ultrasonic Measurements of Residual Stresses Caused by Severe Thermomechanical Deformation during FSW

“…In the case of FSW process, the residual stresses have been experimentally obtained by measuring in longitudinal and transverse states through destructive methods [12] and nondestructive methods such as ultrasonic as well as X-ray diffraction [13][14][15][16][17][18][19][20][21][22].…”

“…They have concluded that such location is within the thermo-mechanically affected zone (TMAZ). Furthermore, Altenkirch et al [18,19] have demonstrated that global tensioning is required to reduce the residual stress and a reverse linear relationship holds between the residual stress and the applied force in the case of thin sheets.…”