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.