Post-Weld Residual Stress Mitigation by Scanning of a Defocused Laser Beam

Abstract:

High welding residual stresses can cause service life reducing consequences. Even though many processes have been developed to reduce these stresses, they are only applicable for wider welds and simple component geometries or are cost-intensive, respectively. The presented method uses a defocused beam after welding for heating the material regions on both sides of the weld. In this way, the welding stresses are decreased without contacting the surfaces using the available equipment. Different process parameters could be used depending on the component geometry and the laser power. The mechanism and the influence of the process parameters were investigated by FEM-simulation and experiments on S355J2+N steel and showed a stress reduction of about 73 % .

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The mechanism of penetration increase in A-TIG welding

ABSTRACT:

The mechanism of the increasing of A-TIG welding penetration is studied by using the activating flux we developed for stainless steel. The effect of flux on the flow and temperature fields of weld pool is simulated by the PHOENICS software. It shows that without flux، the fluid flow will be outward along the surface of the weld pool and then down، resulting in a flatter weld pool shape. With the flux، the oxygen، which changes the temperature dependence of surface tension grades from a negative value to a positive value، can cause significant changes on the weld penetration. Fluid flow will be inward along the surface of the weld pool toward the center and then down. This fluid flow pattern efficiently transfers heat to the weld root and produces a relatively deep and narrow weld. This change is the main cause of penetration increase. Moreover، arc construction can cause the weld width to become narrower and the penetration to become deeper، but this is not the main cause of penetration increase. The effect of flux on fluid flow of the weld pool surface and arc profiles were observed in conventional TIG welding and in A-TIG welding by using high-speed video camera. The fluid flow behavior was visualized in real-time scale by micro focused X-ray transmission video observation system. The result indicated that stronger inward fluid flow pattern leading to weld beads with narrower width and deeper penetration could be apparently identified in the cese of A-TIG welding. The flux could change the direction of fluid flow in welding pool. It has a good agreement with the simulation results.

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