Microstructure development of molybdenum during rotary friction welding

Rotary friction welding is a solid state welding process where the required heat for welding is generated by friction caused by the relative movement between a stationary part and its rotary counterpart. As the melting temperature is not reached, this welding technique does not produce typical welding defects known from fusion welding. In the case of molybdenum, grain coarsening and re-distribution of impurities on grain boundaries are the main problems during fusion welding that can partly be avoided by rotary friction welding. Pure molybdenum and the precipitation strengthened alloy TZM (titanium-zirconium-molybdenum) were subjected to rotary friction welding and a systematic parameter optimization led to successful welds. However, Mo and TZM proved to be very sensitive to small process parameter changes. Microstructural analysis shows a transition from elongated grains with intense substructure in the base material to nearly equiaxed grains at the weld interface is observed. Typical macroscopic zones in the joint can be distinguished and are investigated by electron backscatter diffraction (EBSD). Inverse pole figures maps, grain reference average misorientation maps and resulting grain sizes as well as orientation distribution functions (ODF) are considered to investigate the deformation state of the different zones. Continuous dynamic recrystallization and the competing dynamic recovery were observed as key mechanisms; Intensive subgrain formation and the onset of recrystallization played the major role on the microstructure modification due to rotary friction welding. Grain refinement is observed in the weld interface for the TZM, while coarse grains are observed in the same zone for the pure Mo but comparable crystallographic texture is observed for both materials.