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Welding Journal | January 2014

dissimilar configuration spot welds tested in cross-tension. It shows that the CTS for such configurations are greater than predicted by the minimum rule, largely due to changes in the solid mechanics governing joint failure. Materials and Procedure Used for the Investigation The materials chosen for this study are a low-carbon equivalent 980-MPa dual-phase (DP980 LCE) cold-rolled steel with a hot-dip galvanized coating, and a 800-MPa transformationinduced plasticity (TRIP800) steel. Each steel grade was obtained in different thicknesses. Material details are given in Table 1. It can be seen that although there are slight differences in chemistry or mechanical properties from sample to sample, due to slightly different processing parameters, the metallurgical concept remains the same for each grade. Experiments were carried out on two- and threesheet stackups. Both the DP980 and TRIP800 were used for the two-sheet stackups and only the DP980 was used for the threesheet stackups. All of the joints were made using similar grade material of both similar and dissimilar material thickness to focus on geometrical effects. The material thickness combinations for all of the two-sheet joints are shown in Table 2. All three-sheet stackups were made using the 1-mm DP980 LCE. These configurations were designed to study what happens in such cases, knowing that three-sheet welding is very common in car body manufacturing. The four configurations tested, shown in Fig. 2, are as follows: A — a square DP980 coupon (patch) is inserted between the two classical cross-tension coupons for welding (1+patch+1 mm); B — two coupons oriented the same way welded with one coupon oriented in the transverse direction to form a cross-tension sample (1+1+1 mm); C — same configuration as A but the external coupon is removed by manual torsion before cross-tension testing (1+1+0 mm); D — same configuration as A, but the two coupons oriented the same way are first spot welded together strongly (with several spots) in the extremities, before the actual three-sheet spot weld is done (1++++1+1 mm). Spot welding was carried out based on using the procedure and parameters described in ISO 18278-2 (Ref. 13). For each welded configuration, a welding current range test was carried out using a pedestal spot single-phase 50-Hz welding machine, with the welding parameters chosen to correspond to the thinnest sheet in the assembly (Table 3). Each three-specimen cross-tension test was welded at the multiple current levels using 38- × 125-mm coupons as specified by ISO 18278-2. Cross-tension testing was then carried out with a tensile machine equipped with a special hydraulic clamping system, allowing the cross-tension specimens to be held without sliding. After testing, the maximum load was recorded, and the weld diameter was measured according ISO 14329 (Ref. 14), using a caliper gauge for button diameters and a magnifying glass for weld diameters in case of partial or full interfacial failures. Only welds without expulsion are considered in the following WELDING JOURNAL 39 analysis. Information Obtained from Testing Cross-tension strength is strongly dependent on weld diameter — Fig. 3. In spite of the scatter, the CTS for the dissimilar configuration is clearly above that of the similar 1-mm configuration, rather in the trend of the thicker similar 2-mm configuration. Fig. 1 — Example of dissimilar configuration with CTS matching the “minimum rule” (Ref. 7). Fig. 2 — Three-sheet configurations based on 1-mm DP980 LCE sample. Fig. 3 — Cross-tension strength for TRIP800 configurations. Fig. 4 — Cross-tension strength for DP980 1+1, 1+2, and 2+2 configurations.


Welding Journal | January 2014
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