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- Stress reduction (2) (entfernen)
The residual stresses in narrow electron or laser beam welds with high stress gradients are decreased without any contact surfaces or additional equipment by applying the welding beam after welding in a defocused mode for heating the material regions in a certain distance from the weld on both sides. In case of electron beam application, the beam is positioned and focused by the electromagnetic coil with high frequency. In case of laser beam application a laser scanner optics enables fast positioning by an optomechanic beam deflection, while defocusing of the laser beam is obtained by increasing the distance between scanner optics and workpiece. Dependent on the component geometry and on the beam power different process parameters are used. The adjustable process parameters are the radius and the power of the defocused beam and the transversal and longitudinal distances between the welding and the defocused beam. The mechanism and the influence of the process parameters are investigated by FEM-simulation and a number of experiments on a ferritic steel S355J2+N with 5 mm thickness. FEM-simulation is used to reduce the matrix of process parameters for the experiments. The best experimental result shows a stress reduction of about 70%.
Longitudinal residual stresses in beam welds ranging at the value of the local yield strength can diminish the lifespan of components. To extend the service life of welds special methods of welding residual stress reduction were developed earlier which are however not effective for beam welds in complex component geometries. Application of beam welding sources for postwelding heat treatment of components has become a flexible tool for reducing longitudinal stresses in beam welds. Such heat treatment in a specific transversal distance to the weld by a defocused beam results in huge stress reductions depending on the used process parameters. Experimental results for ferritic and austenitic steels reveal weld stress reductions to up to compressive stresses. For different materials and diverse material thicknesses special process parameter regions have to be used in this procedure. At a transmission component this procedure shows a stress reduction by >300 MPa.