Distortion Engineering for Assembly

In the production of welded sheet metal assemblies (car bodies and components, exhaust systems, power plant components, household appliances and much more) shape deviations occur. Both thermal distortion from the welding process, the occurrence of bulging problems, and distortion when clamping imperfect components contribute to this. DynaWeld can calculate this form deviation at the design stage and suggest compensation measures.

DynaWeld focusses on the thermal joining and manufacturing simulation. The manufacturing simulation helps to optimally design modern high-tech production methods. On the one hand, companies can save on manufacturing costs and on the other hand, they can realize ambitious production stages at all.

In the past, welding structure analysis for predicting deformation and residual stresses has been mostly applied to small components. Investigations and validations were done on small test examples. Meanwhile, a state of this simulation methodology has been established that achieves results with excellent agreement with reality. However, small assemblies are less of an interest in industrial applications because welding tests can be performed faster and at lower cost. The situation is different for large structures and multi-level assemblies. Test weldings are becoming increasingly costly, especially when large special tools need to be changed without having sufficient assurance of effectiveness. Here the simulation helps to massively save costs in the development of the production line and the manufacturing design.

Especially in car body manufacturing processes it is always a great challenge to insure the match between target geometrical design (CAD) and real shape in desired tolerances. In very early states of the development of new car models the design of the production processes has to be started simultaneously. Production facilities may be determined before the entire manufacturing process is ensured with respect to given target tolerances. For example production steps (stations), cycle times of manufacturing, robot types and their amount and even areal conditions are fixed. So if problems appear in the later phase of development, the possibilities of intervention become smaller. That leads to the necessitiy of strong simulation tools for:
  • finding the reasons for certain distortion evolution
  • virtual testing of variations
Best practice would be the application of simulation in earlier states for:
  • approvement of the prearranged production
  • intervention in early states of development, if tolerances are not reached or visible buckling problems appear
DynaWeld has developed a simulation tool with special methods that do justice to the assembly simulation. The most important features are:
  • support of high performance FE codes
  • applied technology for weld filler modelling
  • clamping tools with start and release of the clamps
  • consideration of imperfect individual components
  • multi stage assembly
In standard simulations, the welding filler material is firmly connected to the components even for seams that have not yet been welded. Though one can lower by material-sided interventions the stiffness of these not yet existing connections, but the approach still falsifies decisively the result. DynaWeld has developed a special simulation method in which this error does not occur, but the still unwelded components can move freely. Thus, DynaWeld is able to detect gap formation during welding (Fig. 1). Clamping or stapling concepts can be laid out in such a way that no critical gaps appear. Conversely, this method can also be used to properly design the processes, because the locations for tack welds or clamping tools can be determined.
Fig. 2: Starting and releasing the clamp during welding of a car roof

The clamping of the components results in a further significant influence on the assembly distortion. Closing of the clamps causes an additional deformation and stress component as soon as the individual components or subassemblies already have geometry deviations from the CAD zero position from previous manufacturing steps. Such clamping distortions are "frozen" by the joining. In addition, pre-deformations can be deliberately included by clamps for distortion compensation. DynaWeld takes into account the clamping process in the simulation, including specified clamping distances or clamping forces. Figure 2 shows this by the example of welding a car roof with the body.

The assembly of a body or welded construction is usually carried out in several stations. In the process, subassemblies are first manufactured, which are joined together in further steps to form the main assembly (Fig. 3). This multi-step process is successfully simulated with DynaWeld. The state variables deformation, stresses, contact and resulting temperatures from the first calculation stage are taken over into the following simulation as input variables.
Fig. 3: Simulation sequence assembly

With the previous discussed methods the assembly deformation of large sheet metal structures can be appropriately calculated. The figure on the left side shows the buckling distortion on the car roof in the not yet optimized state and the same distortion in color representation. The assembly calculation can and should be done in the early design phase, since it can be used to design the concept for distortion compensation.

Distortion management with DynaWeld enables production planning with, in contrast to today´s practice, almost no later tool corrections of the assembly line.

Our software solutions: