WELDING JOURNAL 55 Fig. 2 — Testing device used to measure upsetting force. Results Joint appearance, property and fracture surface. Figure 4A, B shows the appearance of the joints produced by friction stud welding with and without upsetting, respectively. Like general friction stir welding, the steel stud was easily plunged into the Al plate and the flash was uniformly formed at the periphery of the studs by extruding part of the metal beneath the shoulder (depending on plunge depth). The remarkable flash formation suggested the following key issues could be favorably achieved: • Breaking the oxide film by direct friction at the interface between the two workpieces • Removing disrupted oxide flakes by extruding them out of the joint interface • Intimate contact at the joint interface by extrusion force of the steel stud toward the Al plate. Moreover, for the joint with upsetting, flash was greater than that for the joint without upsetting, indicating that manually applying upsetting was effective in enhancing plastic deformation in the weld region. When upsetting was not applied, the measured failure loads for the three joints were 1.540, 1.166, and 1.132 kN, respectively. In this case, the average failure load of the friction stud welded joints was only 1.279 kN (~16.3 MPa). When upsetting was applied, the pressure was measured and the results are shown in Fig. 5. It can be seen that the upsetting force reached ~1.95 kN (~24.8 MPa). In the case with upsetting, the measured failure loads for the three joints were 2.934, 3.990, and 2.827 kN, respectively, and the average failure load can be significantly increased to 3.25 kN (~41.4 MPa), as high as 2.5 times of that without upsetting. Figure 6 shows the fracture surface appearances of the friction stud welded joints after tensile testing. For the joint without upsetting, the outer region of the fracture surface of the Al side was smoother than the central region, and a small quantity of aluminum adhered to the central region of the steel stud end after tensile testing. The results showed that stronger bonding was achieved preferentially in the central region. It can be attributed to a higher heating temperature and lower cooling rate in the inner region than those that occurred in the outer region, which were beneficial to avoiding debonding to some extent during the stopping stage. For the joint with upsetting, although the outer region of the fracture surface was smooth, the Al base metal in the central region was pulled out and adhered to the end surface of the steel stud, leading to a hole with a diameter of about 6 mm on the Al side. The information revealed: 1) stronger bonding was achieved preferentially in the central region as with the joint without upsetting, and 2) bonding in the central region was so strong that even 2.2-mm-thick Al could be pulled out. Both the higher tensile strength and much more favorable fracture path demonstrated that the upsetting action can significantly improve bonding behavior, especially in the central region. Microstructure of friction stud welded joints. Figure 7A, B shows the BSE macrographs of the friction stud welded joints. As shown in Fig. 7A, when upsetting was not used, the intimate contact between Al plate and steel stud was achieved well, and part of the plasticized Al adhered to the steel stud end. However, a large crack 10 mm in length (same as the diameter of the steel stud) and 100 μm in maximum width was present within the Al base metal near the interface, and ran roughly parallel to the Al/steel interface. In addition, it should be pointed out that, due to the inevitable error in machining and assembling, a microgap was present at the side interface between the Al and steel periphery. Therefore, the joint properties will depend primarily on Fig. 5 — Measured axial pressure when upsetting was used. Fig. 3 — Schematic of tensile test. Fig. 4 — Appearance of the joints produced by friction stud welding: A — Without upsetting; B — with upsetting applied. 4 2 3 A B
Welding Journal | January 2013
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