This study shows that tack welding procedures have a significant influence on the final deformations caused by final butt welding. Typically, the final mode of deformation (sagging versus hogging) is influenced, but not always completely determined, by the profiles resulting from the tack welding operation. Based on the geometry and boundary conditions assumed in this study the following general conclusions can be formulated: Tack welds are certainly necessary to prevent free movement of the assemblies during butt welding. However, it was observed that tacking the plate at fewer points as in the case of 40mm tacks, generates less restraint during welding and this may provide a means to accommodate transient thermal expansion, leading to less final distortion. This observation is somewhat similar to the case when the plates are clamped in the out-of-plane sense during welding and later unclamped during or after cooling. Previous research has shown that higher out-of-plane deformation is developed when plates are fully clamped during welding instead of being left as free-standing unclamped assemblies. The problematic issue is to determine an appropriate overall tacking percentage that compromises between offering enough structural stiffness to counter gravity, for example, while allowing relatively free expansion during the transient phase. In the models analysed, tack welding corresponding to 10% of the overall length of the assembly was optimum. Placing the tacks on the opposite side to the butt weld did not significantly influence the overall out-of-plane deformation. This observation may not apply in all configurations. It depends on whether longitudinal bending, angular deformation or buckling predominates. Angular distortion reduces when tacks are placed on the opposite side to the butt weld and this strategy will be of value when the predominant deformation is angular. However for certain geometries and boundary conditions (particularly in long assemblies) longitudinal bowing predominates, tending to buckling deformation. In these cases balanced tacking does not significantly influence the final out-of-plane deformation. In this case and to reduce handling during manufacture lead time, the tacks may be placed on the same side as the butt weld. The use of more frequent but smaller tacks gave rise to higher deformations. This study shows that three 40mm tacks as opposed to thirteen 10mm tacks (i.e. similar tacking length ratio) resulted in a nominal 13% reduction in out-of-plane deformation. Longer less frequent tacks provide similar structural rigidity but allow greater free expansion during butt welding. Initial clamping during tack welding has a significant effect on the final distortion. In particular the plates should be clamped in order to attain a consistent weld gap and prevent free movement of the assemblies.


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    Title :

    Procedural tacking fabrication influences on welding distortion


    Contributors:


    Publication date :

    2008


    Size :

    6 Seiten, 7 Bilder, 15 Quellen



    Type of media :

    Conference paper


    Type of material :

    Print


    Language :

    English




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