【正文】 . The whole welded tubes were then tested by traction on a MTS testing machine. The final dimensions of the welded tubes are measured on a CMM machine at many points on the tubes. The distortion of the welded tubes is determined by paring the final positions with the initial positions of the tubes. Numerical analysis In Sysweld, a welding analysis is performed based on a weakcoupling formulation between the heat transfer and mechanical problems. Only the thermal history will affect on the mechanical properties, but not in reverse direction. Therefore, a thermal metallurgical mechanical analysis is divided into two steps. The first step is a thermal metallurgical analysis, in which the heat transferred from the welding source makes phase changes during the welding process. The results of temperature and phase changes from the first step are then used as input for the second analysis. It is a pure thermoelastoplastic simulation [6]. Heat source model identification Before running a welding simulation, it is necessary to determine the parameters of the heat source model. This is called heat source fitting. Actually, it is a thermal simulation using this heat source model in the steady state, which isbined with an 大連交通大學(xué) 2021 屆本科生畢業(yè)設(shè)計(jì)（論文）外文翻譯 4 optimization tool to obtain the parameters of the heat source. Figure 3 presents the form of a 3D conical heat source of which the energy distribution is described in Eq (1) as follows: F=Q0exp(r178。v the welding speed and t the time. In this study, a metallographic crosssection has been used to identify the heat source parameters as shown in Figure 4. The use of a 3D conical heat source fits very well the weld crosssection. The mesh size in the crosssection is around mm for this case. The finer is the mesh, the more accurate is the shape of the melting pool, but the longer is the simulation. Figure 3: 3D conical heat source (Sysweld). 大連交通大學(xué) 2021 屆本科生畢業(yè)設(shè)計(jì)（論文）外文翻譯 5 a) b) Figure 4: (a) Metallographic crosssection, (b) Melting pool crosssection. Analysis model The mesh of the tubes was created in Hypermesh . Sysweld 2021 has been used as solver and pre/post processor. A full 3D thermal metallurgical mechanical analysis with brick and prism elements. Two welding sequences have been done such as W1/W2/W3/W4 and W1/W3/W2/W4. The tubes are clamped using four vblocks during the welding, two for each tube. In the simulations, the positions where the tubes are in contact against the surfaces of the vblocks are considered as fixed conditions (. Ux = Uy = Uz = 0). In the release phase, the tubes are free from the vblocks. Results The distortion of the welded tube is measured when it is released from the 大連交通大學(xué) 2021 屆本科生畢業(yè)設(shè)計(jì)（論文）外文翻譯 6 constraints. The distortion is determined by measuring the displacement of the small tube on the top and lateral surfaces along the centre line of the tube. These measures are re