Modeling and numerical simulation of welding dynamic displacement field (2)

2 Establishment of displacement field analysis model

2.1 Establishment of geometric models

The geometric model of the displacement field is consistent with the temperature field. After the temperature field calculation of the step is completed, the unit is converted from the temperature field unit to the structure field unit by unit transformation, and the division of the unit is consistent with the temperature field, as shown in Fig. 1. Shown.

Figure 1 Division of finite element calculation unit

2.2 Processing of the molten pool

When the metal in the molten pool zone melts under the action of arc heat, the molten pool zone will enter a state of zero mechanical properties, that is, all stress and strain will disappear; when the molten pool is converted from a liquid to a solid, it enters the initial state of the unstrained history. In addition, the liquid bath metal exerts little force on the surrounding solids and has almost no effect on the stress-strain distribution around the bath. Therefore, in order to correctly simulate the stress-strain distribution in the high temperature zone, the occurrence and disappearance of the molten pool must be considered. Otherwise, the displacement field simulation will be invalid due to the pseudo deformation of the molten pool zone. In this regard, the "unit life and death" method is adopted. The principle is as follows: the temperature field value result of each substep is selected: the unit exceeding the melting point will cause it to die, and the unit below the melting point is "activated".

2.3 Nonlinear processing

There is a large nonlinearity in the welding process. It is manifested in the following aspects:

1 Geometric nonlinearity: Welding is a large strain problem. Large strain means that the strain generated is large enough to cause a change in the shape of the unit to cause a change in stiffness.

2 Material nonlinearity: refers to the nonlinear relationship between stress and strain, for example, plasticity is a nonlinear stress-strain relationship; while viscoplasticity, creep is a relationship between strain and other factors (time, temperature). In order to fully consider the properties of plastic materials in the analysis, the uniformity of yield criterion, flow criterion and hardening law must be considered.

For the above problems, the following methods are used:

1 Using the Full Newton-Raphson method, the stiffness matrix is ​​corrected once for each equilibrium iteration.

2 Simulate material nonlinearity using the bilinear isotropic enhancement model provided by Ansys. This type applies to isotropic materials, and the Von Mises yield criterion is applied together with the Prandtl-Reuss flow equation (but not the Bauschinger effect).

2.4 Analysis Process

This paper uses ANSYS software for finite element calculation. ANSYS provides two coupling methods for the analysis of different physics: direct coupling and indirect coupling. Strictly speaking, temperature field analysis and displacement field analysis are directly coupled, but since the test proves that this coupling effect is very small, it is ignored. The indirect coupling method established at the substep level is used in the calculation. That is, the time is divided into enough inter-cells (sub-steps), and the transient thermal analysis is performed first in each interval. After the solution is completed, the result of the maximum heat flow gradient is stored in the unit table; then the unit conversion is performed to the same geometry. The model and unit division are used for structural analysis, and the unit table result data is imported as the boundary condition of structural analysis, and the structural analysis of static free deformation is performed. This process was followed by a 5 second simulation of the welding heating process followed by a simulation of the cooling process for about 60 seconds. The command flow of the heating phase is shown in Figure 2.

Figure 2 Calculation analysis flow chart

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