VALIDATION OF WELD GEOMETRY AND RESIDUAL STRESSES IN WELD

Validation of Weld Geometry

Validation of nugget development can be seen in Fig. 6.1. The comparison of the nugget diameter between prediction and experience shows a qualitative agreement at the end of each pulse. The validation of the final nugget diameter size and penetration height can be seen in Fig 6.1d. From the experience, it is observed that nugget development is saturated since the end of the second pulse. Little nugget growth can be seen since the end of third and fourth pulses. However there is a discrepancy in the validation of nugget penetration, particularly at the first pulse as shown in Fig. 6.1a. Predicted nugget height is relatively small in comparison to that of experiment, but the validation of nugget diameter shows a qualitative agreement with the experience. Validation of the sheet deformation and electrode indentation on the sheet is in a good agreement with the experimental results. 

Residual Stresses and Deformations

Development of internal circumferential stress in the nugget during welding process is shown in Fig. 6.2. The formation of internal stress is because of local plastic deformation caused by the non-uniform temperature distribution during the welding process. Fig. 6.2 illustrates the circumferential stress and temperature distribution in the weld. It can be seen from this figure that a high and uniform compressive stress develops continuously around the nugget, as indicated by ‘Θ’ sign, near the isothermal contour of 1530°C. However, this compressive stress decreases as temperature increases. The development of the tensile stress can be also observed at the electrode/sheet and sheet/sheet interfaces. During the holding cycle, in which current is shut off, but load is maintained, the temperature drops rapidly and the weld metal in the nugget begins to ‘shrink’, which results in the development of the tensile stresses. At the end of the holding cycle, the squeeze load is removed, and the tensile stresses develop continuously in the weld. Fig. 6.3 shows different components of the residual stresses after 10 cycles of electrode holding. It can be seen that tensile stress predominates in the weld, and concentrates along the faying surface. Additionally, tensile residual stress can be observed just beneath the peripheral region of the electrode/sheet interface. Distribution of radial stress along the mid-thickness of the sheet at different stage illustrated in Fig. 6.4 indicates that tensile residual stress in the nugget increases since the end of welding. During welding stage, the nugget is subject to the compressive stress induced by the thermal dilatation in the weld and tensile stress appears increasingly as welding cycle increases. After the current shut off, it can be seen that the profile of tensile stress is uniform in the nugget and drops drastically in the region near the HAZ. However, tensile stress across the HAZ increases again and becomes relatively uniform in the base metal. Similar radial stress distribution with a high gradient of stress across the HAZ found experimentally using X-ray diffraction technique is also reported in-[ANA87]. Distribution of von-Mises or equivalent stress at 10 cycles after the end of holding is presented in Fig. 6.5. Maximum residual stress about 150-182 MPa occurs near the electrode/sheet interface and in the sheet. Using welding parameters at the upper limit of weldability domain, maximum stress values found is approximately the yield strength of steel, (e.g. RP0,2-room temp = 174MPa determined from the experiment). Note that von-Mises stress is always positive. Fig. 6.6 illustrates the plastic strain cumulated since the beginning of process until the end of welding, the end of electrode holding, and 10 cycles after the end of holding cycle. The simulation results indicate that cumulated plastic strain increases markedly during the welding, and there is a slight increase after the end of holding cycle, (i.e. the maintaining force is removed). Development of plastic strain can be observed in the regions locating from the notch to the periphery of the electrode/sheet interface. Maximum strain can be visible at the notch region, where the microcrack initiates during the fatigue tests. The highest value of plastic strain is around 0,175-0,2, which indicates that the resistance spot welding is a small deformation process. Other feature concerning the improvement of welds quality in terms of fatigue strength and weld toughness is to reduce the residual stress level in the weld, e.g. using the post-heat treatment technique. However according to the experience reported in the previous study-[ANA87], although this post-heat treatment helps reducing the residual stress level in the weld and thus the reduction of the stress gradient across the HAZ, but no significant amelioration in spot weld fatigue strength of low carbon steel is obtained for the industrial benefit.  

Conclusions

Analysis of residual stresses produced in RSW process is carried out using the numerical modeling technique. This is to understand the generation of stresses and the occurrence of residual stresses in the assembly. Following conclusions can be made from this study : ™ Using the novel electrical contact resistance model allows reducing the dispersion in the weld geometry validation at different stage of welding. Validation of final weld geometry in terms of nugget diameter/HAZ size and electrode indentation on sheet surface shows a good qualitative agreement with the experience. However, the dispersion between experiment results and that of prediction is observed for the nugget penetration, especially at the earlier stage of welding. The permissible value for the current passage at the faying surface in electro-thermal contact model, (§‘ε’), still plays a role on the development of weld geometry; the smaller the value of epsilon, the larger the nugget size, particularly at the beginning of welding. ™ Typical distribution of relative high tensile residual stresses in radial and circumferential directions is observed in the nugget, (§Fig. 6.3a and 6.3c). Significant stress gradient is observed across the HAZ and stress distribution is relatively uniform in the base metal. This finding is in a good agreement with the results of the previous study using X-ray diffraction technique for the determination of the residual stresses in weld in the case of low carbon steel joining. However, tensile residual stresses in some of the region, (§e.g. along the faying surface and the notch), could be favorable for the crack initiation, and further investigations should be conducted to evaluate the implications on weld quality and mechanical performance. Two critical zones of the stress gradient are found in the weld. The first zone is around the notch region and along faying surface, and the second one is beneath the electrode/sheet interface. Note that both regions locate in the HAZ. In this case of welding condition, it is found that the magnitude of equivalent residual stress in spot weld is approximately the yield strength of the sheet material. The development of the compressive stresses induced by thermal gradient or the thermal dilatation of the assembly under the squeeze force of the electrode can be visible in the spot weld during the welding operation. ™ Development of plastic zone is particularly found beneath the periphery of electrode/sheet contact and around the notch region. Final cumulated plastic strains ranging from 0,175 to 0,2 concentrate around the notch region.