Microstructural modification of as-cast NiAl bronze by friction stir processing
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6/30/04
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Microstructural Modification of As-Cast NiAl Bronze by Friction Stir Processing KEIICHIRO OH-ISHI and TERRY R. McNELLEY 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 (ii) indicates that peak temperatures are below the solvus for this phase, while the presence of transformation products of the phase, including fine Widmanstätten , bainite, and martensite, indicates that peak temperatures exceed the eutectoid temperature for the reaction b S a kiii throughout the stir zone.
I. INTRODUCTION
FRICTION stir processing (FSP) is an emerging metalworking technology that can provide localized modification and control of microstructures in near-surface layers of processed metallic components.[1,2,3] This technology represents an adaptation of the principles of friction stir welding, a solid-state joining process originally developed at The Welding Institute.[4] In FSP, a cylindrical, wear-resistant tool consisting of a smaller-diameter pin with a concentric, larger-diameter shoulder is rotated and forced into the surface of a single piece of material. The rotating tool penetrates until the shoulder comes into contact with the workpiece surface. Then, the tool is traversed along a path on the surface, as illustrated in Figure 1, in a single- or multipass process. The tool depicted in Figure 1 has spiral threads, and tool designs involving stepped or fluted pins have also been employed.[1] The tool shoulder acts to constrain upward metal flow caused by insertion of the pin, and metal flow in the workpiece may include vertical (parallel to the tool axis) as well as horizontal (in the plane of tool rotation) components (Figure 1). Upon initial insertion of the rotating tool, friction between the tool and the surface of the material results in heating and a “stirring” action that, in turn, produces adiabatic heating and local softening of the material. A processed zone, or stir zone, is produced by severe plastic deformation with movement of material about the pin. The combination of very large deformations and adiabatic heating results in microstructure refinement and homogenization, leading to improved strength and ductility of material in the stir zone.[1–3,5–10] The FSP may also result in closure of porosity and inclusion redistribution in castings and, thereby, contribute to improved m
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