This article summarizes a paper entitled, “An Evaluation of Friction Stir Spot Welding as a Method for Joining Ultra-thin Steel Sheet,” by Y. Hovansk, et al.H-10
The study analyzes Friction Stir Spot Welding (FSSW) as a process for assembly of two sheet stack ups. The steel sheet used for this study is CR4-GI, a hot dip galvanized ultra-low carbon interstitial free steel with a tensile strength of 280 MPa. Thicknesses of both 0.45 and 1.2 mm were used to create dissimilar thickness, two-sheet stack-ups. Preparation for joining via FSSW did not alter the zinc coating. FSSW joints were evaluated in lap shear tensile, T-peel, and cross tension.
FSSW welds were welded with an EKasin injection molded, silicon nitride tool shown in Figure 1. All welds were performed at 1600 rpm.
The zinc coating that originally covered the sheet surface was extruded beneath the FSSW tool to the outer edges of the weld as seen in Figure 2. Figure 3 shows a representative weld on a T-peel specimen.
A minimum of 25 specimens were produced for each geometry tested, however, these specimens were performed at various times throughout weld development and data is shown below. Figure 4 shows the load-extension curves for a set of nine friction stir spot welds. Figure 5 shows a representative fracture of lap-shear tensile specimen.
Figure 6 shows the load-extension curves for a set of eight friction stir spot welds tests in T-peel. A representative fracture of T-peel specimen is shown in Figure 7.
Figure 8 shows the load extension curves for a set of 13 friction stir spot welds tested in cross tension. Representative geometry and fracture of cross tension specimen are shown in Figure 9.
A table showing the overall results for the FSSW joints produced herein are shown in Table 1 below:
While each specific test orientation demonstrated the ability for the weld nugget to pull out of the ultra-thin top sheet and remain with the lower 1.2-mm-thick sheet, the overall ratios between fracture loads suggest there is an area for improvement with respect to T-peel.