WELDING RESEARCH lowest microhardness appears in the LCGZ, which has a coarser structure and less alloy content. This low hardness zone has a detrimental effect on the mechanical properties of the joint, which will be discussed in the following sections. Failure Mode Transition in Three Equal Thickness Stacks Failure of Joint Types I and II Three types of failure modes, interfacial (IF) failure, partial thicknesspartial pullout (PT-PP) failure (Ref. 16), and pullout (PO) failure, are observed in joint Types I and II, as shown in Fig. 4. Only the load-displacement curves of the Type II joint are given here due to the similarity of the mechanical behavior of the Type I and II joints, as shown in Fig. 5. The load smoothly dropped to zero after it reached its maximum value in the IF failure, while a residual force remained after the force began to drop in the PT-PP and PO failures (Ref. 17). More details about the failure process are shown in Fig. 6. Figures 6A and 6B show the macro/microstructures of the fracture surface cross section of welds that failed in the IF mode. Figure 6A locates the section where the force achieved its maximum value, and a crack formed, explaining the subsequent load reduction. The crack initially formed between the PMZ and LCGZ and then propagated through the interior of the LCGZ, and finally failed as an interfacial characterization — Fig. 6B. The suboptimized welding A B parameters (16 kA, inadequate heat input) contributed to the formation of the LCGZ, which has a low hardness and strength to resist the crack propagation. C B Note that the weld size in Figs. 6A,B is inconsistent. This variation may be caused by local differences in contact resistances of workpiece/workpiece and electrode/workpiece, which will alter the heat input and affect the nugget formation (Ref. 18). Figures 6C and 6D show fracture initiation location of the welds that failed in the PT-PP and PO mode, respectively. 482-s WELDING JOURNAL / DECEMBER 2016, VOL. 95 In the PT-PP mode, the failure location was the PMZ while the failure of the PO mode was due to necking of the base metal. This suggested that the PT-PP mode is a suboptimized failure mode. Figure 7 shows the effect of button size on the peak load and energy absorption of joint designs I and II. Simple linear regression was applied to both the data obtained from joints I and II, and a best fit line with a coefficient of determination (R2) of 0.878, was obtained. The relatively high value of R2 suggested that a linear relationship exists between the peak load and button size. This phenomenon is also observed by Han et al. (Ref. 18) and Sun et al. (Ref. 19). However, a twoorder polynomial relation exists between the energy absorption and button size, indicating that a larger weld nugget could not only improve the peak load, but also the microstructure (less LCGZ due to suitable heat input) and relieve the stress concentration around the weld nugget and, in turn, improve the ductility of the weld joint. Failure of Joint Type III Similar to joint Types I and II, IF, PT-PP, and PO failures were observed in the Type III joint. Only the PO failure mode will be discussed here because the analysis of the PT-PP failure mode is similar to that for joint Types I and II. Figure 8 shows the macro/mi- Fig. 5 — Typical loaddisplacement curves of the Type II joint in the 1.0/1.0/1.0 mm stack. Fig. 6 — Macro/microstructures of Type II weld joints in the 1.0/1.0/1.0 mm stack that failed in A, B — IF mode (16 kA, 200 ms); C — PTPP mode (20 kA, 200 ms); D — PO mode (20 kA, 200 ms).
Welding Journal | December 2016
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