The Studies Of Al Surface Composite Layer By Surface Mechanical Alloying

Surface mechanical alloying, one of the surface severe plastic deformation methods, has attracted more attentions due to its improvement of the performance and the service cycle of the alloy. Aluminum and aluminum alloy, with a large quantities of light weight, heat conduction and electrical conductivity, good formability and good weldability, corrosion resistance and high strength, is one of the most useful non-ferrous metal material. However, the application of aluminum alloy has limit to a large extent for its shortcomings of the low hardness, large linear expansion coefficient, poor wear resistance, low electrode potential. Therefore, the specific surface treatment technology, given the new characteristics of the aluminum surface, was very important to further the application of aluminum alloy.The nano-structure Al/Fe, Al/Zr, Al/FeZr gradient surface composite layer were prepared by using the method of surface mechanical alloying. The microstructure, morphology, composition and hardness of the surface composite layers were analyzed and determined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy spectrum analysis (EDX), microhardness (HV) testing and other analysis techniques. The effect of the process parameters, the size, microstructure and composition of the particles on the properties of the as-fabricated layers were also been discussed. The phase evolution of the coatings was also investigated by the subsequent annealing.The results indicate that the microstructure and properties of the surface composite layers, for its gradient non equilibrium crystal defect introducing by the flying balls, change with the depth of the layers. The microstructure and microhardness evolution strongly depend on the process parameters, the particle size and its initial microstructure of the mixture powder. The thickness of the composite surface layers increased with the increasing milling time and the size of milling balls. The smaller the particles’ size and the better adhesion the particles, the more uniform structure and morphology the composite layer is. The early sandwich lamellar structure of the composite layer eventually evolved into the uniform alloying composite layer. The microhardness of as-fabricated Al surface composite layer can be effectively improved by surface mechanical treatment. Its strengthening mechanism is the combination of the fine grain strengthening, solid solution strengthening, work-hardening, precipitation strengthening and particles reinforcement.