Effect of Rate of Heat Input on Thermomechanical Tensioning for Distortion Mitigation
Authors: Gadagi Amith , Mandal N. R. , Podder Debabrata 
Published: December 2017

Thin plates are extensively used for fabrication of stiffened panels in weight-sensitive ship construction. These panels being inherently low in critical buckling stress, when welded, suffer from weld-induced buckling distortions. The tendon force, caused by welding of the stiffeners, is mainly responsible for the occurrence of buckling due to welding. The reduction in the tendon force will decrease the tendency of the plate to buckle. thermomechanical tensioning (TMT) is an active, in-process distortion mitigation technique which reduces this tendon force and thereby reduces weld-induced buckling distortions. The weld-induced distortions are strongly influenced by the rate of heat input. Its effect on the TMT pull required to reduce the out-of-plane distortions was investigated in the present work through finite element (FE) simulations. To validate the thermomechanical model used in the FE analysis (FEA), experiments were conducted considering bead-on-plate welding. The results of numerical simulations and experiments were found to be in good agreement with each other. The validated FEA model was subsequently used to investigate the effect of rate of heat input on TMT in stiffened panels. It was found that the TMT pull required in case of a higher rate of input was more compared with the case of lower rate of heat input for the same percentage reduction in the out-of-plane distortions. The present work also showed that the tendon force gets reduced on application of TMT to a welding process, reducing the tendency of plate buckling due to welding. In the present work, an artificial neural network (ANN)-based modeling of the TMT process was also carried out to predict the weld-induced out-of-plane deformation for different rates of heat inputs and TMT pulls. The ANN-predicted welding distortions were found to be in fair agreement with the FEA-computed values

1. Introduction
Fusion welding is a key joining process widely used in manufacturing industries. It involves local intense heating of the parts to be joined followed by normal cooling (Wang et al. 2013a; Mandal et al. 2015). The nonuniform expansion and contraction of the weld metal and the surrounding parent metal induces distortions and residual stresses in the material. Because of this, the dimensional accuracy and the structural integrity of the welded structures are affected. It demands a lot of postweld rework, thus affecting the economy and productivity. To avoid this, active, inprocess distortion mitigation techniques are required to be implemented. Thin plates are extensively used in fabrication of ship structures to reduce the overall weight and thereby improving the efficiency and fuel economy (Huang et al. 2004). At the same time, these thin plates pose a serious problem of weld-induced buckling. Tajima et al. (2007) observed that the tendon forces due to welding generate compressive stresses in the far-field zone, away from the weld zone, resulting in buckling distortions. Wang et al. (2013b) also found that the tendon force is a dominant cause of weldinduced buckling through elastic FEA and eigenvalue analysis. Michaleris and Sun (1997) employed the concept of thermal tensioning to mitigate the weld-induced buckling distortions. In their work, the thermal tensioning was achieved by cooling the bottom of the weld and simultaneously heating the areas adjacent to the weld. It was observed that a high temperature gradient had detrimental effect increasing the distortion, whereas a low value of temperature gradient caused insufficient tensioning. Deo and Michaleris (2003) adopted a technique of transient thermal tensioning to reduce the weld-induced distortion. The tensile stresses induced because of the thermal tensioning reduced the weld-induced compressive residual stresses less than the value of critical buckling stress. Soul and Yanhua (2005) performed numerical study to investigate the effect of trailing heat sink on the welding residual stresses. It was found that the welding residual stresses were drastically reduced. Adak and Mandal (2010) investigated the effect of heat sinking method on the welding distortions in submerged arc butt-welded plates through experimental and numerical techniques. They achieved a noticeable reduction in the weld-induced distortions. Holder et al. (2011), by employing a technique of dynamically controlled low stress no distortion achieved a drastic reduction in the distortions of gas metal arc-welded plates. Yang and Dong (2011) numerically investigated the impact of in-process rolling along with the trailing heat sink method on the weld-induced distortions. It was concluded that a minimum of 5.5 KN rolling force was required to eliminate the welding distortions completely. Price et al. (2007) employed the mechanical tensioning technique to reduce the weld-induced out-of-plane distortions in friction stir and tungsten inert gas-welded plates. It was observed that the tensioning of the magnitude 25–40% of the yield completely eliminated the welding distortions. Richards et al. (2008) used the concept of global mechanical tensioning to achieve the reduction in the welding distortions by controlling the welding residual stresses. It was observed that the tensioning of the magnitude 40% of the yield reduced the magnitude of welding residual stresses to zero. A similar work was carried out on friction stir welding of high-strength aluminum alloys (Altenkirch et al. 2008). It was concluded that the excessive tensioning leads to the reversal of stress field in the welding zone. For thicker plates, the decrease of weld-induced residual stress was very less with the increase of distance from the welding zone. Mandal et al. (2014) introduced the Thermomechanical tensioning (TMT) method as an active, in-process distortion mitigation technique to reduce the occurrence of weld-induced buckling distortions. In their work, they carried out experiments to show the effect of TMT on weld-induced buckling distortions. Podder et al. (2017) carried out numerical studies to investigate the effect of TMT on the reduction of weldinduced residual stresses in 4.8-mm-thick steel stiffened panels. It was observed that the TMT process reduces the peak tensile stress in the weld zone, thereby reducing the compressive residual stress in the far-field zone. Gadagi et al. (2017) conducted numerical and experimental investigations on the effect of TMT on welding-induced out-of-plane distortions. The influence of restraining and tensioning lug removal sequence was also investigated in the study.