Rotary friction welding of molybdenum components

Joining of TZM components by inertia rotary friction (IFW) welding is an established industrial process for welding cross-sections up to 1500 mm2. Up-scaling to medium-size components up to 5000 mm2 in a direct drive friction welding process requires a better understanding of the influencing factors of the weld procedure, e.g. machine parameters, weld preforms, and the clamping system. Based on the existing IFW process for TZM tubes, welding parameters were transferred to tubular components of pure molybdenum (OD: 150 mm, ID: 130 mm, 4400 mm2). Successful welds were produced showing a fine-grained, defect-free microstructure. However, molybdenum proved to be more challenging than TZM. Particularly high upset rates and motor overload occurred during the friction phase. Therefore, a study was carried out to reveal the underlying mechanisms with small-size samples under laboratory conditions. It was shown that extensive plastic deformation of the entire weld zone occurred due to higher thermal diffusivity and lower strength of molybdenum compared to TZM. This high upset rate reduces the process window for a reproducible welding procedure significantly. In conclusion, a concentrated energy input during the transition from friction to forge phase is required to countervail the high thermal diffusivity of molybdenum. Based on these observations, the feasibility of friction welding of medium-size molybdenum tubes will be discussed.