Finite element analysis of localised rolling to reduce residual stress and distortion

摘要

Fusion welding processes cause residual stress due to the uneven heatdistribution produced by the moving welding torch. These residual stresses arecharacterised by a large tensile component in the welding direction. Due to theself-equilibrated nature of the residual stress, compressive ones are present inthe far field next to the weld seam, which can cause different kind of distortionsuch as bending or buckling. Welding residual stress can be responsible ofpremature failure of the components, such as stress crack corrosion, buckling,and reduction of fatigue life. Localised rolling is a stress engineering techniquethat can be used to reduce the residual stress and distortion caused by welding.It induces plastic strain in the rolling direction, counteracting the plastic strainproduced during welding.In this thesis three techniques were investigated, pre-weld rolling, post-weldrolling, and in situ rolling. These techniques have been seldom studied in thepast, particularly pre-weld rolling; consequently the mechanisms are poorlyunderstood. Finite element models allow stress and strain development duringboth welding and rolling processes to be better understood, providing animproved understanding of the mechanisms involved and aiding processdevelopment.A literature survey was done to find the state of the art of the computationalwelding mechanics simulations, stress management, and the residual stressmeasurement techniques, as well as the knowledge gaps such as, the thermallosses through the backing-bar in the thermal simulation, the frictionalinteraction in the rolling process, and the material properties of the steel used inthe models. In the literature not many models that investigate the managementof welding residual stress were found.After this, the general considerations and assumptions for the welding thermalmechanical models presented in this thesis were discussed. The effect ofdifferent backing-bar conditions, as well as different material properties whereinvestigated. Both influenced the residual stress profile to varying degrees. Inparticular, temperature dependent heat loss to the backing-bar was necessaryto capture the improved heat loss near the weld. The distortion predicted by themodel was investigated to determine whether it was due to bending or bucklingphenomena. Lastly, the temperature distribution and residual stress predictionswere validated against thermocouple and neutron diffraction measurementsconducted by Coules et al. [1–3].Pre-weld rolling was the first of the three rolling methods considered, in whichrolling is applied to the plates before performing GMA butt-welds. The principlebehind this technique consisted in inducing tensile residual stress in the weldregion before welding; therefore, it is similar to mechanically tensioning theweld, which can significantly reduce the residual stress and distortion. However,there was no significant change in the tensile residual stresses. On the otherhand, it was possible to achieve a small reduction in the distortion, when theplates were rolled on the opposite surface to the weld; rolling in this wayinduced distortion in the opposite direction to the distortion induced by welding,reducing the magnitude of the latter. These results were compared withexperiments conducted by Coules et al. [1,4]. A subsequent investigationcombined pre-weld rolling with post-weld heating. With this additional processthe residual stress and distortion were significantly reduced, and flatter residualstress profile was achieved.The post-weld rolling and in situ rolling techniques were discussed afterwards.In the post-weld rolling models, rolling was applied after the weldment wascooled to room temperature. In in situ rolling the roller was applied on top of theweld bead at some distance behind the torch, while it was still hot. The principlebehind these techniques consisted in applying positive plastic strain to the weldbead region by a roller, counteracting the negative plastic strains produced inthe welding process. Two roller profiles were investigated, namely, grooved,and double flat rollers. The post-weld rolling on top of the weld bead models,which used the grooved roller, showed good agreement against experimentalresults, producing a large reduction of the residual stress and distortion. Somediscrepancies were present when the weld toes were rolled with the dual flatroller. The former roller was more efficient for reducing residual stress anddistortion. The influence of different friction coefficients (between the roller andweldment, and between the backing-bar and the weldment), were investigated.It showed significant dependency on the residual stress distribution when highrolling loads were used. The frictional interaction constrained the contact areainducing more compressive stress in the core of the weld bead; therefore itproduced more tensile residual stress in the surface of the weldment.Additionally, the influence of rolling parameters on the through-thicknessresidual stress variation was investigated. Low loads only influence the residualstress near the surface, while high loads affected the material through the entirethickness.When the dual flat roller was used to roll next to the weld bead, significantcompressive residual stress was induce in the weld bead; however, the residualstress reduction was very sensitive to the contact of the roller to the weld toes;therefore, when rolling a weld bead that varies in shape along the weld, theresidual stress reduction is not uniform and varies along the length. On theother hand, the in situ rolling did not produced significant residual stress ordistortion reduction in all the cases analysed. The rolling occurred when thematerial was still hot and the residual stress was subsequently formed as thematerial cooled to room temperature. Numerical modelling was a very usefultool for understanding the development of stress and plastic strain during thewelding and rolling processes.