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Global mechanical tensioning for the management of residual stresses in welds
D.G. Richards, P.B. Prangnell, P.J. Withers, S.W. Williams,
Materials Science and Engineering: A. 2008;489(1-2):351-361.
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Abstract
Welding processes typically introduce substantial residual stresses, which can have a detrimental impact on service life of the joint. In this paper, the general principles behind the global mechanical tensioning technique for controlling weld residual stresses are examined. The method involves tensioning a plate along the weld line during welding. A finite element model has been developed to follow the evolution of the residual stresses throughout the welding process, focusing specifically on friction stir welding. In the model the welding process was represented simply as a heat source, with the mechanical action of the tool neglected. As a result, the findings have relevance to a wide range of welding processes. A kinetically dependent material softening model was used to capture the local yield stress behaviour during welding. For Al alloy friction stir welds the maximum longitudinal weld stresses have been reported to fall approximately linearly, reaching zero for tensioning levels of approximately 40???50% of the material???s yield stress. At still higher levels of tensioning, the state of stress in the weld region has been found to become increasingly compressive. This behaviour is explained in terms of the reduction in compressive plastic straining ahead, and an increase in tensile plastic straining behind, the heat source as the tensioning level is increased. In addition the model is used to examine the sensitivity of the longitudinal weld stresses to heat input and traverse speed as a function of tensioning level. Finally, it is shown that tensioning during welding is much more effective than post weld tensioning.