Volume 5, Issue 5 (May 2008)
Mechanical Evaluation of Mixed As-Cast and Friction Stir Processed Zones in Nickel Aluminum Bronze
The performance of nickel aluminum bronze (NAB) propellers can be limited by the resistance of the alloy to fatigue. Friction stir processing (FSP) is a potential method for improving the fatigue life and fracture toughness of this material through grain structure refinement. As friction stir processing is a surface treatment, as-cast, thermo-mechanically affected zone (TMAZ), and FSP zone microstructures can all occur in components with thick cross sections and when FSP is performed on only selected areas of the component surface. The boundary between modified and unmodified microstructures produced by traditional processing techniques (i.e., heat affected zones produced by welding) are often the source of in-service failures as they can contain defects, residual stresses, deleterious microstructures or any combination thereof. In this paper, the mechanical behavior of FSP nickel aluminum bronze specimens containing as-cast, TMAZ, and FSP microstructures are evaluated using monotonic tensile tests and rotating bending fatigue tests. Analysis of the fatigue specimen fracture surfaces indicate that fatigue cracks initiated and propagated through the as-cast microstructure before penetrating the TMAZ and the FSP microstructures. The tensile specimens failed in the as-cast structure away from the FSP zone and the TMAZ. These results indicate that the as-cast material is weaker than both the FSP and the TMAZ, implying that localized friction stir processing is not detrimental to the mechanical behavior of a NAB component, even in the boundary region.