In recent years, many structures were made by welding. On the other hand, new materials are used to improve the performance of the structure. However, the welding of a new material is difficult and it is difficult to predict the best condition of welding. It is hoped to develop a numerical analysis method to predict welding condition of a new material. The welding arc can be calculated by some grid method. But it is difficult to calculate the surface shape of welding pool by grid method because the surface shape of welding pool and weld penetration is deformed violently by some force. Moving particle semi-implicit (MPS) method is the numerical method which can be applied because the shape of surface is calculated easily. This method is good at calculation of large deformation and matches numerical analysis of welding process. In the present study, a particle simulation model is developed. The convection in welding pool was analyzed using these models. The Marangoni force is worked toward low temperature side in calculation. Then, the calculated weld penetration became wide and shallow. On the other hand, in calculation, the Marangoni force is worked toward high temperature side. The calculated penetration becomes narrow and deep.

Abstract. In marine and offshore industries, aluminium welding processes are often used especially for floating LNG (Liquefied Natural Gas) production, storage and LNG fuelled vessels. Welding distortions of aluminium plates are known to be larger than that of steel, so it is desired to mitigate the distortion. Also, numerical models of welding distortion prior to a manufacturing of components are expected in order to improve products. Finite element models of welding residual stresses and distortions on aluminium plates by using the commercial finite element code ABAQUS were developed in this study. A mixed material hardening model was employed in order to simulate aluminium material behaviours. In addition, a mitigation technique of distortions by in-process additional heating of plate bottom side was investigated by using the developed numerical models. The finite element models were validated by experiments which distortions and temperature histories of MIG fillet welding were measured. Experiments of in-process additional heating of plate bottom side were also conducted and results were compared with the simulation. The simulations and the experiments showed that the mitigation technique proposed in this study was able to achieve a reduction of distortions for aluminium fillet welding. IntroductionA5083-O aluminium alloy is often used for marine and offshore industries especially for floating LNG (Liquefied Natural Gas) production, storage and LNG fuelled vessels since it has better strength to weight ratio and corrosion resistances compared with steel. Welding distortion of aluminium alloy is larger than that of steel and more difficult to remedy after the weld. Therefore, a prediction of distortions prior to manufacturing processes and mitigation techniques are desired.A development of finite element models for welding distortion was originated from 1970's [1] and many researchers contributed the development [2,3]. Throughout the studies of the prediction of weld distortion, prediction accuracies were dependent on weld heat source models used in finite element models. Effect of material constitutive model was studied by the round robin project [4]. This project investigated experimental and numerical techniques for characterisation of residual stresses on a single bead on plate problem. This research concluded that predicted residual stresses and distortion are sensitive to material constitutive models.Demands of mitigation techniques of welding distortion are increased. Several mitigation methods, which are mechanical constraint, pre-heating, in-process cooling and in-process heating [5,6,7,8], have been proposed, then it is important to choose an appropriate method for individual components. The mechanical constraint technique is the simplest method, but this technique requires