Ph.D. Tezi Görüntüleme

      In this study, friction stir processing (FSP) was applied to a shipbuilding low carbon Grade A steel and its effect on the structural and mechanical properties was investigated in detail. Friction stir welding (FSW) was also applied to that steel, and microstructure and mechanical properties of joined zone were investigated in comparison with the metal active gas (MAG) welding which is one of the welding methods frequently used in shipbuilding. It was observed that the grain size in the processed zone (PZ) decreased from 25 μm to about 3-4 μm after FSP applied under optimum parameters. In spite of small decrease in ductility and formability values, hardness, tensile and impact strength values as well as corrosion resistance increased, and ductile-to-brittle transition temperature decreased as a result of grain refinement with FSP. Finite element based ultimate strength analysis models of both single plate and ship mid-section grid system were established, and it was observed that ultimate strength values significantly increased after FSP. After FSW and MAG welding, grain size decreased in the welding zone (WZ) as in the case of FSP. Also, in spite of a negligible decrease in ductility and formability values in the WZ after FSW and MAG welding, the hardness, tensile strength values increased with significantly improvement in the formability and corrosion resistance. On the other hand, it was determined that FSWed joints showed higher hardness, tensile strength, formability and corrosion resistance values than those of MAG joints.

      Key Words: Shipbuilding steels, Friction stir processing (FSP), Friction stir welding (FSW), Metal