PSI - Issue 5

Jidong Kang et al. / Procedia Structural Integrity 5 (2017) 1425–1432 Jidong Kang/ Structural Integrity Procedia 00 (2017) 000 – 000

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Fig. 4. Transmission electron microscopic images of (a) interface showing the top AA6022-T4 sheet, intermetallic layer and bottom IF steel sheet, (b) the ‘needle’ like shape of the FeAl 3 formed closer to top AA6022- T4 sheet and, (c) the ‘tongue’ like shape of the Fe 2 Al 5 formed closer to the bottom IF steel sheet.

3.2. Quasi-static tensile test

The quasi-static lap-shear load-displacement curves of the AA6022-T4 to AA6022-T4 and AA6022-T4 to IF steel RSWs are plotted and presented in Fig. 5(a). The RSWs in AA6022-T4 to IF steel produced an average lap-shear strength of 3900 N, while the RSWs in AA6022-T4 to AA6022-T4 exhibited an average lap-shear strength of 3100 N. In both the joints, the weld nugget fracture mode was interfacial. On an average, the nugget diameter in AA6022 T4 to AA6022-T4 RSWs measured 6.1 mm while in AA6022-T4 to IF steel RSWs measured 8.3 mm. This suggests that the much greater strength of the AA6022-T4 to IF steel RSWs compared to the AA6022-T4 to AA6022-T4 ones is most likely due to the larger nugget created in the dissimilar metal joints.

Fig. 5. Lap-shear load displacement curves for the AA6022-T4 to AA6022-T4 and AA6022-T4 to IF steel RSWs welded with MRD electrodes, which generated an average weld nugget diameter of 6.1mm and 8.3mm, respectively and (b) Fatigue test results of the AA6022-T4 to AA6022 T4 and AA6022-T4 to IF steel resistance spot welds.

3.3. Fatigue test results

The load controlled fatigue test results of the AA6022-T4 to AA6022-T4 and AA6022-T4 to IF steel RSWs are presented in Fig.5(b). All the tests presented here were conducted with a load ratio R=0.1 at 40 Hz. In general, scatter for the fatigue data was fairly limited. The AA6022-T4 to IF steel RSWs exhibited much greater fatigue strength compared to AA6022-T4 to AA6022-T4 RSWs. For example, at 100,000 cycles to failure, the dissimilar metal joints ’ fatigue strength was approximately twice that of the aluminum spot weld fatigue strength. Irrespective of the stack configuration, the majority of the test specimens fractured due to kinked crack growth in the top aluminum sheet. Although top sheet fracture was the dominant fracture mode, additionally in test specimens that were tested at a maximum fatigue load above 70% of the ultimate lap-shear strength, interfacial shear fracture of the weld nugget was