The critical nugget diameter for the Type I joint can be obtained as follows This predicted value is very close to the experimental result of 9.1 mm. For the Type II joint of the 1.5/1.0/2.0 mm stack, all the joints failed in IF mode, assuming that the PO failure location of Type II joint is the PMZ and the rotation angle is the same as Type I joint. Note that the IF failure location was the EGZ rather than the LCGZ because the nugget will shift to the thicker sheet. The critical nugget diameter for Type II should be However, the maximum button size obtained from experiments was about 10 mm. Therefore, the prediction for the Type II joint is also reasonable. For the Type III joint of the 1.5/1.0/2.0 mm stack, the IF failure location was the LCGZ, while the PO failure location was the PMZ. The average indentation was about 90% of the original sheet thickness, i.e., tID was 1.35 mm. The average rotation angle was about 3 deg when the joint failed in the IF mode, while it was 10 deg when the joint failed in the PO mode. Thus, the critical nugget diameter for the Type III joint can be obtained as follows: The predicted value is very close to the experimental result of 8.2 mm. For the Type IV joint of the 1.5/1.0/2.0 mm stack, it can be seen that in the DIF failure (Fig. 14), both of the two interfaces failed through the EGZ. Accordingly, all the HLCGZ in Equation 17 should be replaced by HEGZ. Using W = 25 mm, t = 1 mm, f = 0.6, HPMZ = 75 Hv, HBM = 95 Hv, and HEGZ = 60 Hv, the critical nugget diameter of the Type IV joint is The predicted value is close to the predicted result of the 1.0/1.0/1.0 mm stack. This is reasonable because the BMF failure is dependent on the property of the middle sheet. Since the middle sheets in the two thickness combinations were the same, the experimental and predicted results should be similar. Effect of Joint Design on the Failure Mode Transition The effect of joint design on the failure mode transition is shown in Fig. 19. The data point for the Type II joint in the 1.5/1.0/2.0 mm stack comes from the predicted results. For the 1.0/1.0/1.0 mm stack, the tendency to fail in the IF mode is increased in the order Type III, Types I and II, and Type IV. This is consistent with Pouranvari and Marashi’s work (Ref. 2). The failure of the weld joint is the competition between shear stress at the sheet/sheet interface (i.e., IF failure) and the tensile stress at the nugget circumference (i.e., PO failure) (Ref. 20). The higher the shear stress at the sheet/sheet interface, the higher the tendency to fail in the IF mode. The Type III joint has the maximum rotation angle and the minimum shear stress at the sheet/sheet interface. Therefore, it has the minimum critical diameter DC to fail in the PO mode. In contrast, the sheet/sheet interfaces in the Type IV joint experienced pure shear. The weld joint has virtually no rotation and therefore, it has the largest critical diameter DC to fail in the PO mode (BMF mode). For the 1.5/1.0/2.0 mm stack, the tendency to fail in the IF mode is increased in the order of Type III, Type IV, Type I, and Type II. Without considering the Type IV joint, i.e., the pure shear condition, the failure rules for the two thickness combinations are similar. The Type III joint experienced the maximum rotation while the Type II joint has the minimum rotation angle. However, although the Type IV joint experienced pure shear, the strength of the middle sheet was lower than the shear strength of two sheet/sheet interfaces. Therefore, for the three unequal thickness stacks, the thickness of the middle sheet should control the critical weld nugget size of pure shear joint. Conclusions and Future Work In this paper, the failure mode transition of three-sheet aluminum alloy resistance spot welds (RSWs) during tensile-shear tests were investigated through experiments and an analytical model. Four types of joints were investigated. The following conclusions can be drawn: 1) The microstructure in the threesheet 6061 aluminum alloy RSWs consists of a partially melted zone (PMZ), columnar grain zone (CGZ), and equiaxed grain zone (EGZ), where the columnar grain zone is divided into the columnar grain with large secondary dendrite arm spacing (LCGZ) and the columnar grain with small secondary dendrite arm spacing (SCGZ). The hardness test indicates that the LCGZ has the lowest hardness. 2) Three failure modes in Types I, II, and III joints, named the interfacial (IF) failure, partial thickness-partial pullout (PT-PP) failure, and pullout (PO) failure, were observed. There is no critical welding parameter or nugget diameter to separate the PT-PP and PO failures. The formation of the LCGZ in the weld nugget contributes to the PT-PP failure. There is a competition between the two interfaces in the Type III joint, and failure will occur on the weaker one. 3) Three failure modes in the Type IV joint, named the double interfacial (DIF) failure, one interfacial/one pullout (IF/PO) failure, and the base metal fracture (BMF) failure were identified. In the case of the DIF and IF/PO failures, the nugget was squeezed and experienced work hardening. In the DIF failure, one interface failed through the LCGZ first, and then the other interface failed through the interior of the weld nugget. In the case of IF/PO failure, the weld nugget experienced less deformation due to its larger nugget size. In the case of BMF failure, the weld nugget had a very small deformation and the crack formed around the edge of the weld nugget and then propagated to the base metal. 4) The LCGZ is the weak area in (DC )Type III = 3tID Pf HPMZ HEGZ cos��IF cos��PO = 3��1.35 0.6 75 60 cos 3 deg cos 10 deg ��8.4 mm (DC )Type IV ��6.0 mm (DC )Type II = 3tID Pf HPMZ HEGZ cos��IF cos��PO = 3��1.8 0.6 75 60 1 cos 2 deg ��11.6 mm (DC )Type I = 3tID Pf HSCGZ HEGZ cos��IF cos��PO = 3��1.3 0.6 85 60 1 cos 2 deg ��9.2 mm WELDING RESEARCH DECEMBER 2016 / WELDING JOURNAL 489-s
Welding Journal | December 2016
To see the actual publication please follow the link above