A B Fig. 2 — An example of the random pattern used on the shear specimens for digital image correlation: A — Before testing; B — after testing. The shear deformation is evident in the shear zone (highlighted in boxes in red color). Table 2 — Nominal Main Mechanical Properties of Aural2T7, AA5754O, and AA6022T4 Material Yield Strength, MPa Ultimate Tensile Strength, MPa Elongation, % Aural2T7 150–165 180–220 14–20 AA5754O 90–130 Minimum 250 23 AA6022T4 100–150 Minimum 200 24 Fig. 1 for the weld nuggets was determined based on weld nugget macrographs to ensure the entire shear zone was within the corresponding weld nugget. Wire electric discharge machining (EDM) was used to machine the specimen contour while the notch of the shear zone was machined using sink EDM with a specially designed electrode to ensure the notch edges at the bottom were at 90 deg to the shear zone on both sides of the sample. More details on shear test specimen preparation and shear test procedure for aluminum resistance spot welds are described in Ref. 12. A commercially available optical digital image correlation (DIC) strain mapping system, Aramis, was used to follow the shear strain development during the shear tests. In the present study, the DIC system was used to directly measure shear angles as they are the integral part of the full-field strain measurements during the shear tests (Refs. 11, 15). A random ink pattern was placed on the sample surface using an airbrush prior to testing for shear specimens. The DIC measurements were made from a set of specimen surface images collected automatically at 1-s intervals during the shear tests. After the tests, the captured images were processed to obtain full-field strain maps using the DIC system software (Aramis). A set of ink pattern images before and after testing are shown in Fig. 2. The instantaneous shear stress () was calculated using = F/A0 where F is the instantaneous force and A0 the initial shear area. The instantaneous average shear angle over the shear zone was calculated and converted to shear strain using = tan (), where is the shear strain and the shear angle. The von Mises yield criterion was used to convert the shear stress-shear strain curves into effective stresseffective strain curves, i.e., s = √3 , e = /√3, where s is the effective stress and ethe effective strain. The TEM samples were prepared using ion milling (Gatan PIPS) or FIB (FEI Helios Nanolab DualBeam). All TEM studies were carried out using FEI’s Tecnai Osiris TEM equipped with an X-FEG gun at 200 keV. Conventional bright-field/dark-field imaging, SAD, and CBED techniques were used for phase identification and crystallography. STEM mode using bright field (BF) and high angle annular dark field (HAADF) detectors were used in combination with EDS point detection and elemental mapping. The HAADF images contain chemical information as at higher angles, scattering is related to atomic Z-number. Results and Discussion Macrostructures along with microhardness line scans of all the spot welds are shown in Figs. 3–5. From Figs. 3–5, it is seen that the penetration is much greater into the Aural-2-T7 side of the stack-up in both the Aural-2-T7 to AA5754-O welds and Aural-2-T7 to AA6022-T4 welds. Figure 5 exhibits the unusual microstructure formed with very little penetration into the AA6022-T4 side for the Aural-2-T7 to AA6022-T4 spot welds. The bonded area between the two sheets is difficult to discern in the microstructure since wetting between the two sheets occurs beyond the area where shallow penetration can be seen. The weld nugget size is easily revealed during peel testing and was found to meet the target. It is also interesting to note in Fig. 4 that the diameter of the weld nugget is noticeably smaller at the AA5754-O side (e.g., 6.5 mm) than the Aural-2-T7 side (e.g., 8.0 mm). It can be concluded from the microhardness data presented in Figs. 3–5 that all the weld nuggets have significantly higher microhardness than the base alloy sheet. In addition, for the baked welds shown in Figs. 4 and 5, the nugget hardness remains unchanged after the e-coat, i.e., the thermal cycle does not impact the hardness of the weld nuggets. No apparent WELDING RESEARCH 250-s WELDING JOURNAL / JULY 2016, VOL. 95
Welding Journal | July 2016
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