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RESEARCH PRODUCT

Manufacturing and characterization of Al matrix composites with nano reinforcements via cold spraying

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This study aims at developing new routes for manufacturing Al matrix composites (AMCs) strengthened with nano reinforcements by solid-state cold spraying (CS) process. Three different AMCs including CNTs/Al, TiB2/AlSi10Mg and TiB2/7075Al were successfully fabricated by CS of the composite powders prepared using different approaches. The microstructure evolution of as-sprayed composite samples was investigated by means of different characterization methods like X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM) and electron backscatter diffraction (SEM/EBSD). The adhesion strength, tribological behaviour, corrosion properties as well as the mechanical performance including microhardness and tensile properties of cold sprayed composites were evaluated. The particle plastic deformation behaviour and the formation mechanisms of composite deposits were investigated with the assist of single particle compression tests and FEA simulation. Moreover, coatings were post-treated by heat treatments and friction stir processing (FSP) and the influence of process conditions on the microstructure evolution and mechanical properties were investigated.Experimental results revealed that uniform distribution of CNTs into Al matrix was achieved by using shift speed ball milled (SSBM) composite powders. However, the tensile tests on as-sprayed SSBM CNTs/Al composite presents a brittle feature, with inter-splat interface and intergranular fracture. A slight improvement in both ultimate tensile strength (UTS) and elongation was obtained after annealing treatment as a result of enhanced interfacial bonding and defects healing effect. Fully dense and thick TiB2/AlSi10Mg and TiB2/7075Al composite components were obtained by CS of gas-atomized composite powders reinforced with in-situ TiB2 particles. The presence of poor inter-splat bonding and TiB2 clusters on the deformed splats surface results in a brittle fracture of the as-sprayed TiB2/AlSi10Mg composites. Significant improvement in ductility but reduction in tensile strength was obtained for the annealed TiB2/AlSi10Mg samples mainly due to the improved interfacial bonding between the deformed splats and grain growth after annealing treatment, respectively. A simultaneous enhancement in both strength and ductility has been achieved by FSP treatment. The strengthening mechanisms are related to the homogenous distribution of reinforcements, matrix grain refinement, and robust interfacial bonding. In the case of TiB2/7075Al composite, nanosized TiB2 particles are uniformly distributed in Al matrix. A higher particle impact velocity and temperature lead to a denser structure and greater particle deformation, enhancing metallic bonding between deformed splats and grain refinement. Consequently, higher microhardness and tensile strength were acquired. However, electrochemical tests revealed that a greater plastic deformation in the helium-processed samples results in inferior corrosion resistance compared to the air-processed samples due to the greater generation of defaults like strain stress and dislocations. The microhardness can be largely improved by heat treatment due to the formation of abundant precipitates in Al matrix. However, few improvements in tensile properties were found by applying post-heat treatments, and a brittle feature was still observed on the fracture surface of the heat-treated TiB2/7075Al tensile samples.