Nanostructured Co–Cr–Fe alloy surface layer fabricated by combination of laser clad and friction stir processing

Friction stir processing (FSP) was carried out on a laser clad Co–Cr–Fe alloy layer in order to transform the coarse laser clad microstructure into nanostructure and improve the mechanical properties. In the microstructure of the laser clad Co–Cr–Fe alloy, coarse brittle carbides precipitated along the grain boundary and formed networks, along which micro-cracks tended to initiate. After FSP, nanostructured surface layers with 20–100 nm grain sizes and 30–65 μm thicknesses were obtained. The coarse network carbides were significantly crushed to nanosize particles uniformly distributed in the nanostructured Co matrix. The plastic layer showed a gradient grain size that ranged from tens of nanometers at the top surface to tens of micrometers at the interior. A large number of the deformation nanotwins were found in the plastic zone which revealed the deformation mechanism. Moreover, FSP enabled the closure of micro-cracks in the laser-clad microstructure. The formation of nanostructures increased the hardness up to 819 Hv, compared with the 490 Hv of the laser parent alloy. The friction coefficients of the laser-clad Co alloy decreased from about 0.35 to 0.1–0.2 after FSP, with enhanced wear performance.