Accelerating Formation Of Nanocrystallite Al-coatings By Ball Peening Process

A novel technique has been developed to produce Al-rich intermetallic compound coatings on metals. This technique applied mechanical vibration to a retort, which was loaded with Al powder, alumina filler, ammonium chloride activator and FeCrAl alloy balls. The operation temperature was below 600oC for 0.25～4h treatment. The Al-rich intermetallic compound coatings have been successfully produced on the carbon steel, Cr5Mo, stainless steel and Ni-base superalloy. The coatings appear to be homogeneous, with a high density and free of porosity, and have excellent adherence to the substrate. The coatings mainly consist of Al rich phases and greatly enhance the high-temperature resistance of metals. Microstructure studies suggest that the formation of the intermetallic phases at the relative low temperature has a complex mechanism. The growth process of the Al-rich intermetallic coatings consists of four steps: 1. Ball impact causes Al particles adhering onto the substrate. An Al-adhered layer is formed on the surface of the metals. 2. Severe plastic deformation induces refinement of top layer and local high temperature, which promote the atomic diffusion and result in the formation of the initial alloy layer. 3. Subsequent ball impact refines the Al-rich phases in the initial alloy layer, and causes a new Al adhered layer formed on the surface of metals. 4. Diffusion of Al and alloy elements takes place between the newly formed Al layer and the initial alloy layer. The inward diffusion of Al is hindered by the coarse-grains of the substrate, while the outward diffusion of alloy elements is enhanced along the large amount of grain boundaries and defects in the alloy layer. The intermetallic coatings grow outward by the repeated ball impact. With this mechanism, Al-rich intermetallic coatings are formed at a much lower temperature in a much shorter time, compared with the conventional aluminizing process. This concept provides a new approach that can be used in other diffusion coating processes. We apply this technique to produce nanocrystallite ODS coatings. Pure Al powder was mixed with 1wt% Y2O3 powder by ball milling. The ultrafine Y2O3 powder was well dispersed in the Al particles. The modified Al particles were welded onto the surface of metals by ball impact, causing the refinement of coarse grains and acceleration of atomic diffusion. Nanocrystallite ODS alloy layer with Y2O3 grew outward at a much low temperature and in a short treatment duration. We also apply this technique to clad inner surface of tubes with Al foils at room temperature. Al foils were welded onto the inner surface of steel tubes by ball attrition in planetary rotation, followed by annealing at 660°C for 20 min. Studies on the interface microstructure and bonding indicate that the bonding strength is a function of the rotation speed, diameter of alloy balls and treatment time. A large amount of plastic deformation increases the bonding degree but may break Al foils and separate them from the inner surface. Subsequent annealing allows diffusion of Al into the substrate, transfers the Al foil into multi-layers of intermetallic compound and alloy phases. This technique provides an easy and fast way to improve the properties of inner surface of steel tubes. The nanocrystallite Al-coatings and ODS coatings greatly improve the high temperature corrosion properties. In high temperature environment, Al2O3 scale is formed on the surface of the coatings with spinel of FeAl2O4,NiAl2O4, and Cr2O3,TiO2. Ball peening causes stress and cracks in the coatings, and results in internal oxide and intrusive oxide in high temperature oxidation. Dispersed Y2O3 in the coatings reduces the growth rate of oxide scale, and enhances the resistance to spallation.