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Investigation on Friction Stir Lap Welding of Al-5083 Sheet to St-12 Sheet and to Al-1100 Clad St-12 Sheet

Movahedi, Mojtaba | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 43203 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Seyed Reihani, Morteza
  7. Abstract:
  8. In this study, friction stir lap welding of Al-5083 alloy sheets with 3 mm thickness to St-12 steel sheets with 1 mm thickness (joint without intermediate layer) and to Al-1100 clad St-12 sheets (joint with the Al-1100 intermediate layer) was investigated. The cladding process was done by rolling. For each type of joints, the bonding mechanism, optimum process parameters to reach the joints with the highest fracture loads and also the microstructure evolutions at the joint region were studied. Furthermore, for the joints without intermediate layer, the effects of the Al-5083 annealing treatment before welding and the post-weld heat treatment of the joints on the fracture loads were explored.
    In the case of the joints without intermediate layer, the results indicate that the bond obtained between the Al-5083 and the St-12 sheets by friction stir welding is due to the combination of five mechanisms including macroscopic, microscopic and nanoscopic mechanical locks, atom-to-atom bond formation between two sheets at the interface and diffusion and chemical bonding. At a constant rotation speed, the weld defects decrease with reducing the travel speed from 23 to 7 cm.min-1. Generally, decreasing the tool travel speed (in the range of 23 to 7 cm.min-1) and increasing the rotation speed (in the range of 750 to 1125 rpm) enhance the fracture loads of the joints, so in the samples welded at 7 and 11 cm min-1 travel speed, for all tool rotation speeds, fracture occurs in the St-12 base sheet far from the joint region. The joints fracture loads enhance with the increase in the duration of the post-weld annealing treatment from 30 to 180 min at the temperatures of 300 and 350oC. In comparison with the atomic diffusion through the joint interface, the formation and development of a thin intermetallic layer in the joint (with the thickness of less than 0.77 m) may have a higher impact on the improvement of the joint strength. However, the joint fracture load decreases with the increase in the annealing duration at 400oC, and from the fracture load point of view, an intermetallic layer thickness between 1.8 and 2.6 m is a critical thickness. The growth of this intermetallic layer composed of Al5Fe2 and Al3Fe is not just a diffusion-controlled growth. Moreover, in the St-12 ultra-fine grains region, there are evidences of the extended dynamic recovery, geometric dynamic recrystallization and grain boundary bulging mechanisms.
    In the case of the joints with the Al-1100 intermediate layer, the results show that the bond obtained between the Al-5083 base metal and the Al-1100 intermediate layer is due to the macroscopic mechanical locks as well as the atom-to-atom bond formation between two sheets through the interface. Additionally, contrary to the Al-1100 layer thickness (in the range of 500 to 150 m), the welding rotation and travel speeds variation does not considerably impact the fracture loads. The fracture loads of the samples improve as the thickness of the Al-1100 layer is decreased. In the samples with the Al-1100 thickness of 150 µm, the fracture loads reaches up to 94% of the fracture load of the St-12 base metal
  9. Keywords:
  10. Friction Stir Welding ; Heat Treatment ; Aluminum Alloy 5083 ; Roll Bonding ; St-12 Steel ; Shear Fracture Load

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