PSI - Issue 51

Liting Shi et al. / Procedia Structural Integrity 51 (2023) 102–108 L. Shi et al. / Structural Integrity Procedia 00 (2022) 000–000

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The two-sheet and three-sheet resistance spot welds were produced as tensile shear (Fig. 1(a)) and coach peel (Fig. 1 (b)) configurations using MRD electrodes and multiple solidification weld schedules. The welding schedules are outlined in Table 1. Resistance spot welds of 1.2 mm AA6022 to itself were also welded and tested as a baseline. After welding, all RSW specimens were baked in an oven at 175 ℃ for 35 min to simulate ELPO bake operations that all vehicles undergo during production to cure adhesives and sealers.

Fig. 1 Configurations of three-sheet Al-steel-steel resistance spot welding specimens of (a) tensile shear, and (b) coach peel.

2.2 Weld characterization Standard metallographic specimens were prepared to characterize weld features including weld nugget diameter, intermetallic layer thickness and weld nugget in steel sheets according to the process, similar to those reported in previous research (Shi et al. (2020)). Focused Ion Beam (FIB) technology was used to prepare samples for scanning electron microscopy (SEM), which was used to characterize the morphology of intermetallic compounds. 2.3 Tensile and fatigue testing Displacement controlled tensile tests of tensile shear and coach peel specimens were conducted at a test speed of 3 mm/min. The average maximum tensile loads of seven tests were calculated and then used as reference load levels for fatigue testing. Load controlled fatigue testing was performed using an MTS Landmark hydraulic test frame with the maximum load of 20%~60% tensile load and load ratio R of 0.1. The fatigue test frequency was 20 Hz and 5 Hz for tensile shear and coach peel specimens, respectively. The run-out life was set to be 1 million cycles. After both tensile and fatigue testing, the fracture specimens were collected for fracture mode observations. Finally, fatigue life of all stack-ups was assessed using the structural stress method based on the Rupp’s approach (Rupp et al. (1995)). The equations for structural stress calculation in tensile shear and coal peel specimens were presented in our previous research (Shi et al. (2019)).

3. Results and discussion 3.1 Weld characterization

There are no significant defects being found in three-sheet RSWs (Fig. 2 (c)). Average weld nugget diameters were measured and shown in Fig. 2 as well. The thickness of the intermetallic compound layer at the periphery of the three sheet RSWs was less than 2 μm indicating sufficient shear strength could be reached based on our previous research (Kang et al. (2020)). The IMC morphology in three-sheet RSW is similar to that in two-sheet RSW (Shi et al. (2022)), i.e. a needle-like FeAl 3 adjacent to the AA6022 sheet and a tongue-like Fe 2 Al 5 adjacent to the steel sheet.