the domed electrodes and hard norm welding parameters lead to a more concentrated heat generation and less heat dissipation, which is beneficial to inhibiting the formation of LCGZ, as discussed in the section of “joint microstructure.” For the Type I joint, the PO failure location was located at the SCGZ. All the Type II joints failed in IF mode, although the button size reached to about 10 mm. This will be discussed in the following text. For the Type III joint, the PO failure location was the PMZ. For the Type IV joint, the BMF failure location was also located at the PMZ. Figure 15 shows the effect of button size on the peak load of joint designs I, II, and III. Similar to the 1.0/1.0/1.0 mm stack, good linear relationships exist between the peak load and button size. The critical button sizes for the Type I joint and Type WELDING RESEARCH III joint were 9.1 and 8.2 mm, respectively. However, all the Type II joints failed in IF mode during the tensileshear test. The failure mode of joint design IV in the 1.5/1.0/2.0 mm stack was similar to that in the thickness combination of 1.0 /1.0 /1.0 mm. The critical button size was about 6.2 mm, which is nearly the same as that in the 1.0/1.0/1.0 mm stack (6.25 mm). This indicates that for the joint design of pure shear, the critical weld nugget size or button size may be controlled by the thickness of the middle sheet. Analytical Model to Predict Failure of ThreeSheet Aluminum Spot Welds Pouranvari et al. proposed a simple analytical failure model for the RSW of steel (Refs. 21, 22). However, the weld rotation was not considered in their mode. In this paper, an analytical model considering the weld rotation was developed based on weld area stress analysis. VandenBossche analyzed the stress distribution when a spot weld failed in the IF and PO modes (Ref. 23). As shown in Fig. 16B, once the weld rotates, the load on the weld interface can be decomposed to two components: the force N normal to the faying surface and the force S parallel to it. They are related to F by S=F cos (1) N=F sin (2) In the tensile-shear test, the driving force for the IF mode is the shear stress at the sheet/sheet interface (Ref. 24). The shear load S generates a shear stress S distributed across the interface. If the average value of the shear stress is V/A, then the maximum value is (Ref. 23) IF = 3S τSMAX 2A = 6FcosθIF πd2 (3) where IF is the weld rotation angle when the joint experiences IF failure. The driving force for the PO mode is the tensile stress around the nugget (Ref. 24). As shown in Fig. 16C, the tensile stress due to S is DECEMBER 2016 / WELDING JOURNAL 485-s Fig. 12 — Typical loaddisplacement curve of Type IV weld joints in the 1.0/1.0/1.0 mm stack that failed in BMF mode (22 kA, 200 ms). Fig. 13 — Effect of welding time on the peak load and energy of joint design IV in the 1.0/1.0/1.0 mm stack (20 kA).
Welding Journal | December 2016
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