The metallurgical and mechanical properties of ODS alloy MA 956 friction welds

The metallurgical and mechanical properties of friction welded MA 956 oxide dispersion strengthened (ODS) iron-based superalloy material were investigated. The mechanical properties of friction welded joints were evaluated using a combination of room temperature and elevated temperature tensile testing and creep rupture testing. The microstructural features and particle characteristics were examined using optical and transmission electron microscopy. The distribution of residual stress in completed joints was analyzed using FEM analysis. The room temperature and elevated temperature tensile strengths of friction welded joints were similar to those of as-received MA 956 base material and, in all cases, failure occurred away from the weld interface. However, the creep rupture properties of friction welded joints were much poorer than those of as-received MA 956 base material. The friction welding operation created low-aspect-ratio, fully recrystallized grains at the joint centerline and substantially altered the oxide particle chemistry, dimensions and shape in the joint region. It is speculated that the coarse, irregularly shaped particles in regions immediately adjacent to the weld interface were produced as a result of a strain-induced agglomeration of small-diameter yttria dispersoids with larger-diameter alumina and Ti(C,N) particles. Thermal elastic-plastic FEM modeling was used to analyze the residual stress generated by the friction welding operation. Close to the weld interface, the residual stress component perpendicular to the axial direction was much higher than the circumferential and radial direction components. However, its magnitude was less than the yield stress of the as-received MA 956 base material. The calculated results suggest that no plastic strain was produced as a direct result of cooling following friction welding.