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Fusion Welding of Dissolvable Magnesium Alloy

Salehi, Mohammad | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56981 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba
  7. Abstract:
  8. Magnesium alloys as the lightest structural alloys, have a strong potential for improving the energy efficiency of different transportation systems such as automobiles, aerospace and electronics due to their high strength to weight ratio. Welding plays a crucial role as a foundational manufacturing technology. Hence, the extensive utilization of materials is greatly impacted by their ability to be welded.These alloys are highly susceptible to hot cracking such as liquation cracking during the fusion welding processes.Various mechanisms play a role in the occurrence of hot cracking, but the constitutional liquation of the coarse eutectic mg17al12 intermetallic phase is accountable for the development of liquation cracking in the fusion welding process of the AZ91-0.5Ni alloy. In recent years, many Mg alloys with high corrosion rate and high strength called dissolvable magnesium alloys have been developed for fracturing temporary plugging tools in the oil exploitation and they use for repairing or well stimulation in some cases and they will dissolve with controled corrosion rate after the operation to eliminate the need for drilling. This study aims to investigate the impact of incorporating MgO nanoparticles into a composite and utilizing friction stir processing (FSP) prior to welding on the weldability of a dissolvable magnesium alloy. The welding process being utilized is gas tungsten arc welding (GTAW) and it is performed using an autogenous technique. The weldability of the alloy was assessed using transverse tensile testing and microstructural analysis. It was observed that the presence of nanoparticles in the composite resulted in grain refinement in the fusion zone and partially melted zone, leading to improved weldability and suppression of liquation cracking. Additionally, the incorporation of nanoparticles in the composite has led to the elimination of the dendritic structure. This has facilitated the transformation and dissolution of coarse eutectic mg17al12 into sub-micron particles. Furthermore, there has been a reduction in the constitutional liquation of this phase, and a shift in the partial melting mechanism from sub-solidus to super-solidus, reducing the size of susceptible partial melting zone to cracking. Hence, the mechanical characteristics of the weld are enhanced, with an increase in ultimate tensile strength from 130 to 190 MPa when compared to the weld produced on homogenized base metal. Additionally, there is a shift in the fracture location from the partially melted zone (PMZ) to the fusion zone (FZ) and in the fracture mechanism from brittle to ductile fracture. This alteration contributes to an increase in the reliability of the alloy for use in sensitive applications. The increased resistance to liquation cracking can be attributed to the reduction in the thickness of the grain boundary liquated film and the accelerated solidification of this film before developing enough welding stresses to induce cracking, as well as the decreased level of stress accumulation at each grain boundary interface
  9. Keywords:
  10. Weldability ; Liquation Cracking ; Friction Stir Processing ; Gas Tungsten Arc Welding (GTAW) ; Magnesium Oxide ; Dissolvable Magnesium Alloy

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