PSI - Issue 19

M. Duchet et al. / Procedia Structural Integrity 19 (2019) 585–594 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Moreover, if the fatigue resistance is expressed as a load versus the thickness, the obtained trend is a reduction of the strength while the thickness decreases. All these results comply with the state of the art of fatigue behaviour of GMAW [3][4]. Without any improvement of the fatigue resistance of welded joints, the promotion of the use of AHSS could appear quite complex in certain conditions where the design could not recover enough fatigue strength. Local reinforcement of the welding is then necessary. One of the key issues is to obtain improvement at the most affordable cost, to be accepted by carmakers In automotive industry, carmakers are prone to reduce the weight of components but remain cost sensitive. They proceed only if the new proposed solutions can be applied at an acceptable cost. In this paper we have arbitrary limited the cost of the weight saving to a maximal value at 3 euros per saved kilogram (3 € / kg saved) . In other words, a mass reduction of one kilogram should not exceed a price of 3 euros. In the specific case of material changes in favor of AHSS for example, the acceptable cost must include both costs, for the material change and the weld reinforcement solution as well. In this article are introduced the different local reinforcement solutions to improve the fatigue behaviour of welds. Few of them are first applied to lap welded joints and experimentally tested in fatigue. In a second part, specific technological specimens, surnamed demonstrators, more representative to a real chassis component than a simple lap joint specimen, are introduced. In a third part, reinforcement solutions are applied to these demonstrators manufactured under industrial conditions and tested in fatigue too. Finally, a cost analysis is provided for each solution of reinforcement in term of cost by saved kilogram as a function of length of treated weld. 2. Reinforcement solutions Different studies have been made in the past decade for a better understanding of the fatigue behavior of lap joints (Fig. 2). In particular, it has been pointed out that changing the welding wire (with higher mechanical properties than the current G3Si wire) has no clear impact on the fatigue behaviour. Furthermore, a relaxation of residual stresses by a heat treatment at 480°C 1 hour could induce lower fatigue properties, as well as increasing the gap between the two sheets highly decreases the fatigue strength (increase of the stress concentration factor). Improving the fatigue resistance of such welds goes through realistic solutions compatible with the industrial requirements such as TIG remelting, shot-peening or transversal oscillating welding. These solutions are first exposed and few of them are then applied to the lap joint specimens for evaluation. 2.1. Description TIG remelting Using TIG (Tungsten Inert Gas) process in second pass on lap joint provokes a remelting of the weld, changing thus the size of the microstructure, potentially decreasing the level of residual stresses and principally smoothing the geometry of the weld. Affecting the local geometry of the weld, this process reduces the local stress concentrations obtained during the initial GMAW process. Transversal oscillating welding Transversal oscillating welding is applied by a mechanical oscillation of the welding torch or by applying a magnetic field that modifies the position of the arc from the right to the left periodically. It permits to enlarge the initial weld width which changes the local geometry and then reduces the local stress concentration (Fig. 4). The fact that the welding torch oscillates induces a reduction of the welding speed. • •