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Investigation on the Effect of Friction Stir Processing on the Fusion Weldability of Magnesium Alloys

Jiryaei Sharahi, Hassan | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52688 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid; Movahedi, Mojtaba
  7. Abstract:
  8. Magnesium alloys, as the lightest structural alloys having a density of about one-fourth that of steels, offer significant potential for improving the energy efficiency of various transportation systems. Welding is a critical enabling manufacturing technology; therefore, the widespread application of a material is highly influenced by their weldability. Liquation and Liquation cracking are crucial weldability issues for fusion welding of cast AZ91 Mg alloy. The constitutional liquation of the coarse eutectic Mg17Al12 intermetallic phase is responsible for the occurrence of liquation cracking during fusion welding of this alloy. Here, friction stir processing (FSP) treatment, a successful pathway to microstructure refinement, prior to fusion welding, is utilized, aiming at tailoring initial microstructure of the base metal (BM) for enhancing its resistance to liquation cracking. Therefore, this study on the cast AZ91 magnesium alloy and its weldability during fusion welding were done in four sections. First, the liquation phenomena in the as-cast alloy was investigated. Second, the effect of FSP on the microstructural modifications of the as-cast BM was investigated. Third, the effect of using FSP as a pre-treatment to the fusion welding, on the liquation and liquation cracking of the fusion weld was examined. To investigate the effect of this technique on mechanical properties of the fusion weld the edges of the sheets are FSPed in through thickness direction using three different heat inputs. It is demonstrated that FSP pretreatment to fusion welding enables producing crack-free welds with lower susceptibility to liquation. The formation of ultra-fine sub-micron Mg17Al12 particles in FSP-treated BM translates the liquation mechanism from sub-solvus constitutional liquation to super-solvus melting of alloy, reducing the size of susceptible partial liquation zone to cracking. Moreover, the higher grain boundary density induced by dynamic recrystallization phenomena during FSP treatment can enhance the resistance to liquation cracking of the alloy in terms of decreased grain boundary film thickness, faster solidification rate of the liquid film before developing sufficient welding stresses to cause cracking, and the lower level of strain accumulation at each grain boundary interface. It is shown that this pre-welding strategy enables a significant enhancement in the weld mechanical properties compared to the conventionally produced welds
  9. Keywords:
  10. Liquation Cracking ; Magnesium Alloy ; Friction Stir Welding ; Weldability ; Fusion Welding ; Microstructure Modification

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