formed and propagated at both of the two interfaces. Figure 11, C3 shows the deformed EGZ. The nugget can be considered as experiencing work hardening. It can be seen that the microhardness of EGZ increased with an increasing deformation degree. This causes the load to rise again. Figure 11D is the stage when only one interface failed. Figure 11, D1 shows the fracture occurred in the interior of the LCGZ. It is interesting to note that on the other interface, the deformed grain induced the crack propagated into the interior of weld nugget — Fig. 11, D2. As a consequence, a failed weld joint in the final stage of the loaddisplacement curve as shown in Fig. 11A is formed. The load-displacement curve of the Type IV weld joint that failed in the IF/PO mode is similar to that in the DIF mode. Figure 12 shows the loaddisplacement curve and microstructures of the Type IV weld joint that failed in the BMF mode. The curve has a “platform,” which indicates that the crack is propagating in the base metal and, therefore, the load is relatively stable. The weld nugget had very small deformation during the tensile process compared with those that failed in the DIF and IF/PO modes. This indicates that the weld nugget was large enough to resist being squeezed by the middle sheet and therefore the crack formed around the edge of the weld nugget and then propagated to the base metal. Figure 13 shows the effect of welding time on the peak load and energy absorption of joint design IV. The energy absorption in the Type IV joint is defined by the area under the loaddisplacement curve up to the second peak. Although a welding time of 250 ms should be used to guarantee the BMF mode, the peak load and energy absorption were almost unchanged after a welding time of 200 ms. This is because the hardening of the weld nugget compensated the relatively small size of the weld nugget when the weld joint failed in the DIF and IF/PO modes. From the macroscopic photos of the failed joint and the microstructure of the failed weld joint, it can be seen that the weld nugget either deformed (DIF and IF/PO modes) or became invisible (BMF mode) after the joint failed. As a result, the accuracy of the button size cannot be measured from the failed A C Fig. 10 — Photos of failure modes of the Type IV joint in the 1.0/1.0/1.0 mm stack: A — Double interfacial failure; B — one interfacial/one pullout failure; C — base metal fracture failure. B2 B3 Fig. 11 — Typical loaddisplacement curve and microstructures of Type IV weld joints in the 1.0/1.0/1.0 mm stack which failed by the interfacial mechanism (18 kA, 200 ms). weld joint, and the effect of button size on the peak load and energy of joint design IV cannot be obtained. Failure Mode Transition in Three Unequal Thickness Stacks The overall failure rules of the 1.5/1.0/2.0 mm stack were similar to that of 1.0/1.0/1.0 mm stack. Figure 14 shows the macrostructures of weld joints in 1.5/1.0/2.0 mm stack. For all four types of joints, the IF failure location moved from LCGZ to EGZ. In fact, no obvious LCGZ formed in the 1.5/1.0/2.0 mm stack. This is because WELDING RESEARCH 484-s WELDING JOURNAL / DECEMBER 2016, VOL. 95 AA B B1 C1 C2 C3 D1 D2 D3
Welding Journal | December 2016
To see the actual publication please follow the link above