Distortion control in welding by mechanical tensioning

Abstract

This paper investigates the potential of mechanical tensioning (MT) to reduce the magnitude of residual stresses in welds and to eliminate buckling distortion. Both friction stir (FSW) and arc welds have been produced from the aluminium alloy AA2024, with different levels of tensile stress applied along the weld line either during or after welding. The resulting welds have been characterised in terms of out of plane distortion, residual stresses and microstructure. Buckling distortion was eliminated by stretching plates to between 35 and 70% of the yield stress of the material during welding. For each set of welding parameters investigated, an optimum tensioning stress has been identified, which eliminates the tensile residual stress peak across the weld zone, along with distortion. This optimum tensioning stress increases in line with the heat input of the welding process. When MT stresses are increased beyond this optimum value, then distortion arises once more and a band of compressive stress is formed across the weld zone.

Mechanical Tensioning of High-Strength Aluminum Alloy Friction Stir Welds

“…The fact that the bending distortion reduces with increased GMT is in accordance with previous observations, [10,16] but concave sample bending and an increase in angular bending with increased GMT has not been described before. At this point, it is not clear why the angular distortion increases, because GMT has little or no effect on the transverse residual stresses (Section C).…”

“…Alternatively, local mechanical tensioning can be applied directly, using rollers, [19,20] or indirectly, by thermal tensioning through localized heating and cooling techniques. [16,21,22] A detailed description of the precise mechanisms acting during welding with GMT is given by Richards et al [18] In essence, the in-situ GMT load applied along the welding direction works by reducing the level of local longitudinal misfit that accumulates between the soft metal in the weld vicinity and the cooler parent material (PM) further from the weld. This is because the level of compressive yielding upon heating in front of the tool is reduced, while tensile yielding just after the tool has passed by is increased.…”

“…[14][15][16][17][18] This loading can be performed during or after welding and can be categorized as global, in which the entire component is mechanically tensioned, or local, in which the loads are restricted to a small region around the location of the welding tool.…”

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].…”

Assessment of the Contour Method for 2-D Cross Sectional Residual Stress Measurements of Friction Stir Welded Parts of AA2024-T3—Numerical and Experimental Comparison

“…However, a more thorough comparison of the different residual stress measurements as well as the applied methods as the one presented here has not been given in literature before. Furthermore, Price et al [3] X-ray diffraction 100 120 Altenkirch et al [4] Neutron and synchroton X-ray diffraction 125 190 Reynolds et al [5] Neutron diffraction 80 90 Steuwer et al [6] synchroton X-ray diffraction 100 175 Dubourg et al [7] Neutron diffraction 150 175 Deplus et al [8] Cut-compliance 50 110 Carlone et al [1] Contour method 50 100…”