it can be drawn that although upsetting tended to slightly enhance interdiffusion between the two base metals via further plastic deformation during the stopping stage, the formed reaction layer at the interface did not grow excessively due to the lower bonding temperature. Since upsetting could rebond the torsion crack and did not result in excessive growth of intermetallic compounds at the void-free initial interface, the joint with upsetting action fractured neither along the secondary friction interface nor along the initial interface during tensile test, and exhibited a higher failure load. Summary In friction stud welding of a steel stud to an Al plate, even when an upsetting action was not introduced, intimate contact at the initial interface could be achieved. However, a crack roughly parallel to the initial interface was present within the softer Al plate, but not along the initial interface. A small amount of intermetallic compound indicated the crack did not result from embrittlement of the Al base metal, but was related to the torsion of the steel stud. This torsion crack could be the result of a combination of factors, including the following: 1) shift of actual friction interface from initial interface into the softer Al plate to form a secondary friction interface within the Al plate when strong interfacial bonding was established at the initial interface, 2) very long stopping stage time and, in particular, 3) the lack of joining pressure. When upsetting (24.8 MPa) was introduced manually, the debonding crack at the secondary friction interface could be closed at/after the moment of dead stop of the motor and the joint became so strong that most of the Al in the central bonded region adhered to the steel stud end after the tensile test. ◆ References 1. Dawes, C. J., and Thomas, W. M. 1996. Friction stir process welds aluminum alloys. Welding Journal 75(3): 41–45. 2. Mishra, R. S., Mahoney, M. W., Mc- Fadden, S. X., Mara, N.A., and Mukherjee, A. K. 1999. High strain rate superplasticity in a friction stir processed 7075 Al alloy. Scripta Materialia 42(2): 163–168. 3. Information on www.twi.co.uk/news events/videos/friction-stud-welding/ 4. Zhang, G. F., Su, W., Zhang, J., Zhang, J. X. 2011. Visual observation of effect of tilting tool on forging action during FSW of aluminium sheet. Science and Technology of Welding and Joining 16(1): 87–91. 5. Rathod, M. J., and Kutsuna, M. 2004. Joining of aluminum alloy 5052 and low-carbon steel by laser roll welding. Welding Journal 75(1): 16-s to 26-s. Leading Through Innovation worldwide vailable WELDING JOURNAL 57 Unique and Advanced Hardfacing Products Available through our worldwide distribution network for fast delivery i Inn gh a A www.postle.com Fig. 9— The measured thermal cycles during friction stud welding with or without upsetting action. For info go to www.aws.org/ad-index
Welding Journal | January 2013
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