Microstructure And Properties Of Aluminum Matrix Composites Reinforced By Copper-Coated Al₁₈B₄O₃₃ Whiskers

Electroless plating copper coating was deposited onto surfaces of aluminum borate whiskers for reinforcement in squeeze-cast whisker reinforced 6061Al composites. Microstructures of the copper coating and composite were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Mechanical properties of the composites were studied through tensile tests, thermal expansion tests and hardness tests.Because of its excellent properties, such as high strength, high module, low expansion coefficient etc, Al18B4O33w is added to metal-matrix composite as reinforcement. In this work, the process of electroless copper plating on Al18B4O33w was optimized with orthogonal experiments, and Al18B4O33w covered with electroless copper coating was obtained. The relationship between the influence factor of the bath and deposition rate was studied, meanwhile, the surface morphology of deposited coating was observed by SEM. The results show that the pH value and the content of stabilizing agent have a strong influence on the deposition rate, the whiskers are fine dispersed, and the surface of deposited coating is uniform.In order to study the effectiveness of the coating, the composites of the copper coated and uncoated whiskers were fabricated by using squeeze-casting method, and the solution treatment of composites was carried out at 490℃. The experimental result indicates, the Cu coating changes the interfacial state of composites and solution treatment has great effects on the interface state of coated ABOw/Cu/Al composites. The investigation shows that interfacial reaction exists between Cu coating and aluminum during solution treatment, although copper coating can not totally prevent the diffusion of Mg element from matrix onto whisker, but can evidently restrain the interfacial reaction between ABOw and 6061Al matrix, and decrease depletion of Mg element, back aging treatment is beneficial.Because the distribution of copper particles on whisker surfaces are not uniform, especially in the 1Cu-30ABOw/Al composite in which the copper content is the least , the MgAl₂O₄ reaction exists in coating of composite, but the size and the amount of MgAl₂O₄, the reaction product, is less. Effect of coating on restraining MgAl₂O₄ interfacial reaction is better with increase of copper content. In 1Cu-20ABOw/Al composite, which contains relatively more copper, and 1Cu-15ABOw/Al composite, in which the copper content is the most, no MgAl₂O₄ was observed in either as-cast state or solution treatment state. It shows that the composite of higher copper content can easily prevent the diffusion of Mg element from matrix into whisker.Under the as-cast condition, the interphases on whisker surface are not the same. In interfacial product containing only Cu and Al, the more the copper content in composites i, the more the copper content in metallic compounds is.Solution treatment has great influence on the tensile strength of coated ABOw/Cu/Al composites. The effects on them are different in the composites with different copper contents. In this work, we have fabricated four kinds of coated ABOw/Cu/Al composites, of which the volume ratio of copper to whisker are 1:15, 1:20, 1:25 and 1:30. The tensile strength of uncoated composite first increases then rapidly decreases with the solution treatment time increasing. The copper-coated whiskers reinforced6061Al composites with volume ratio of 1:15, 1:20, 1:25 and 1:30 between copper and whisker were denoted as 1Cu-15ABOw/Al, 1Cu-20ABOw/Al, 1Cu-25ABOw/Al and 1Cu-30ABOw/Al, respectively. The tensile strength of 1Cu-15ABOw/Al composite increases in large degree then a little decrease with the solution treatment time increasing. The tensile strength of 1Cu-20ABOw/Al composite firstly increases then decreases, which indicates a peak value with the solution treatment time increasing. However, the tensile strengths of both 1Cu-25ABOw/Al and 1Cu-30ABOw/Al composites increase then tend to be steady with the solution time increasing. But at as-cast condition not only the tensile strength of 1Cu-30ABOw/Al is the highest of all coated composites, but also after solution treatment, the tensile strength increases and the change of tensile strength remains small with solution time increasing. The property is steady. It shows that a little amount of copper has effect on enhancing tensile strength.