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Grain Size Control in Fusion Welding and Wire-Arc Additive Manufacturing of Ferritic Stainless Steels

Alikhani, Ali Akbar | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54493 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid; Kokabi, Amir Hossein
  7. Abstract:
  8. Ferritic stainless steels, despite having a good combination of mechanical properties and corrosion properties and low price, are less used than austenitic stainless steels due to their low weldability. The main challenges of welding ferritic stainless steels are coarse grain formation in the molten area as well as grain growth in the heat affected zone. In this research, in the first phase, an attempt is made to change the shape and size of the grains in the melted zone by changing the GTAW fusion welding parameters, including welding current and welding speed. In the second phase, an attempt is made to prevent the formation of columnar grains by adding micron particles to the molten pool and to change the shape of the grains in a Equiaxed grains. In the third part, friction stir processing are performed before welding to prevent the growth of grains in the heat affected zone. In the final phase, the wall printing is done using the wire-arc additive manufacturing process and the effect of the process parameters is checked and the structure is modified using the addition of secondary material. The results of this project can be used both in fusion welding and in the additive manufacturing of ferritic stainless steel parts and improve the properties of the welded or printed part.In general, by simultaneously increasing the welding current and speed so that the penetration depth remains constant and the full penetration weld is obtained, to some extent, Equiaxed grains can be formed in the welding center and grain growth can be prevented in the heat-affected zone. Also, the addition of micron particles of titanium carbide and tungsten carbide to the weld pool to some extent prevents the formation of columnar grains and helps to reduce the grains size. However, the addition of these carbide particles enhances the formation of austenite, followed by the formation of the martensite phase, and reduces the impact properties in this area. The use of austenitic filler metal also prevents the formation of large columnar grains in the weld zone and increases the weld strength compared to the base metal. However, the addition of micron particles or the use of austenitic filler metal, although to some extent solve the problem of coarseness in the weld zone, but do not affect the growth of grains in the heat-affected zone. The use of friction-stir processing before welding causes the grains to become fine, and even after experiencing the heat of welding and the growth of these grains in the heat-affected zone, the grains are still smaller than the grains of the base metal. Wire and arc additive manufacturing were made from samples of ferritic stainless steel without any porosity. However, grain growth and martensite formation, which are the main problems of ferritic stainless steels, were also observed in the printing of parts in this process and caused a decrease in the properties of these parts. On the other hand, by adding pure titanium wire during the wire-arc additive manufacuring process, in addition to reducing the grain size in the walls due to the formation of titanium/nitride carbide particles, the formation of martensite phase in the grain boundary and loss of impact properties was limited
  9. Keywords:
  10. Columnar Grains ; Friction Stir Processing ; Equiaxial Grains ; Wire and Arc Additive Manufacturing ; Gas Tungsten Arc Welding (GTAW) ; Ferritic Stainless Steels ; Micron Particles

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