3. Results and discussion
The macrostructures of the weld nuggets for the AA5754 to Aural2 spot welds with and without the addition of adhesive are
shown in Fig. 2. Both the weld diameter and penetration are less on the AA5754 side compared to the Aural2 side of the nugget.
This is common and caused by the much higher electrical resistivity of the Aural2 alloy compared to that of the AA5754 alloy
[13–14]. The addition of adhesive increased penetration into both substrates and enlarged the weld nugget (Fig. 3b). Note that
the nugget diameter on the AA5754 side is less than that on the Aural2 side, which results in the notch root being located at
the perimeter of the AA5754 side of the nugget (red arrows).
Microhardness measurements in Fig. 3 show that the weld nuggets have higher hardness than the other regions of the weld,
i.e. heat affected zone (HAZ) and base metal, for both conditions. However, the microhardness is lower in the center of the weld
nugget with more variation when welding with the addition of adhesive suggesting that porosity exists within the nugget that
may reduce nugget strength. This is further confirmed by the 3D XCT scanning results (Fig. 4) where a significantly higher volume
fraction of defects exists in the weld nugget with the addition of adhesive.
Despite the differences in nugget size and penetration, the maximum tensile load from the tensile shear testing is slightly
higher in the spot weld without the addition of adhesive (Table 3). Further shear test results confirm that the nugget strength
is indeed lower in the dissimilar alloy spot welds with the addition of adhesive (Fig. 5). This was most likely caused by the fracture
mode since the flow curves up to the point of fracture are very similar. Fracture involves nugget pullout which initiates at the
notch root and propagates into the nugget volume on the Aural2 side of the weld. Additional porosity in the weld nugget with
adhesive then results in premature fracture and reduced strength.
The fatigue testing results are shown in Fig. 6. For a given load range, fatigue life is longer with the addition of adhesive than
those without.
XCT scan results (Fig. 7) show some evidence of damage growth and coalescence within the nuggets (see Steps 2–4 in Fig. 7a
and b). However, the main fatigue crack initiates at the edge of the nugget (see the red circles in Step 4 in Fig. 7a and b) and
penetrates through the Aural2 die casting sheet in the thickness direction. This is consistent with the observations of all the fatigue
fractured specimens.