Microstructural modification of As-cast NiAl bronze by friction stir processing

The application of friction stir processing (FSP) to a cast NiAl bronze (NAB) material is presented as a means for selective modification of the near-surface layers by converting as-cast microstructures to a wrought condition in the absence of macroscopic shape change. This may enable selective surface hardening of cast components. The complex physical metallurgy of the NAB is reviewed, and microstructure changes associated with FSP for a selected set of processing parameters are examined by optical microscopy (OM) and transmission electron microscopy (TEM) methods. Direct temperature measurement in the stir zone is infeasible and, so, these microstructure changes are used to estimate peak temperatures in the stir zone. The persistence of a Fe3Al phase (kappa(ii)) indicates that peak temperatures are below the solvus for this phase, while the presence of transformation products of the beta phase, including fine Widmanstatten alpha, bainite, and martensite, indicates that peak temperatures exceed the eutectoid temperature for the reaction beta --> alpha + kappa(iii) throughout the stir zone.