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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frame at a constant frequency of 40 Hz. Doublers were employed during the fatigue testing to maintain the specimen alignment and eliminate any additional bending during the testing. Metallurgical analysis was performed on tested and untested specimens by cross-sectioning along the centreline of the weld nugget parallel to the loading direction. The untested specimens were cold mounted in epoxy and finely polished using 0.5 μ m colloidal silica. For metallographic analysis, the AA6022-T4 was etched with 20% NaOH solution and the IF steel was etched with 2% Nital solution. Macrographs of the specimens were imaged under a Zeiss digital optical microscope and fractography of the tested specimens were observed under the FEI Nova Nano 650 scanning electron microscope (SEM). Chemical composition analysis of the intermetallic compounds (IMCs) were performed on an FEI Tecnai Osiris transmission electron microscope (TEM) equipped with X-FEG gun.

Fig. 1. Schematic representation showing the geometrical features of the test specimen and mechanical test shims

Table 1. Welding schedules used in the present study. Stack-up

Weld force

Weld current, rms

Weld time

1.2-mm 6022-T4 to 2.0-mm 6022-T4 1.2-mm 6022-T4 to 2.0-mm HDG LCS

3114 N 3114 N

24.1 kA 12.2 kA

141 ms 1665 ms

3)

Results and discussion

3.1. Metallurgical examination

Fig. 2 is a series of photomicrographs of the untested AA6022-T4 to AA6022-T4 RSW specimen cross-section along the center line of the weld parallel to loading direction and subjected to metallurgical preparation.

Fig. 2. A representative cross-section of an untested AA6022-T4 to AA6022-T4 resistance spot welds (a) overall view, (b) magnified view of the weld nugget indicating regions 1, 2, 3, of further study, (c) magnified view of region 1, of the heat affected zone, (d) magnified view of region 2, of the notch slit terminating in the weld nugget (dark arrow indicates the termination point) and, (e) magnified view of region 3, of the grain structure in the weld nugget.