Mechanism of Grain Refinement of Pure Titanium by Friction Stir Welding

Abstract

Reliable connection of 5 mm pure titanium was achieved by friction stir welding, and the microstructure of the welded joint was studied by optical microscopy(OM), scanning electron microscopy(SEM) and transmission electron microscopy(TEM). The results show that the welded area can be divided into the weld nugget zone, thermo-mechanically affected zone, heat affected zone and base metal. According to the structure of each district, the dynamic recrystallization process of pure titanium during friction stir welding was analyzed. Because of high stacking fault energy, dislocations generated by friction stir of titanium cannot be completely decomposed. When there is a hindrance, it can continue to move only through slip and climb. By the effect of friction stir repeatedly, the dislocation tangles are accumulated and piled up, so the dislocation density continuously increases, which will produce new grain boundaries and form fine grains, and then the grain refinement is achieved.

Keywords

• pure titanium
• friction stir welding
• dynamic recrystallization