Study On Microstructure And Properties Of Nano-Carbon Aluminum Matrix Composites By Friction Stir Processing

The solid phase processing technology represented by powder metallurgy and friction stir processing is a common preparation method for aluminum matrix composites.But the defects such as porosity,inclusions and reinforcement agglomeration are easily found in the aluminum matrix composites prepared by powder metallurgy,and the process is more complicated and costly;meanwhile,the traditional friction stir processing is limited to open small holes and narrow slots,which makes this process not easy to achieve standardization and difficult to apply to manufacture products in batch.Inspired by the idea of powder metallurgy process and based on the friction stir processing and variable speed ball milling technology,a new process for the preparation of metal matrix composites is developed in this paper.The main features are: firstly,the slots created are sufficient to accommodate the stirring pins;secondly,a multi-pass variable speed friction stir processing solution has been developed.Fifteen aluminium matrix composites containing pure aluminium powder,silicon carbide of various particle sizes and contents and graphene nanosheets of various contents were successfully prepared.It was found that the homogeneity of the reinforcements mixed with the aluminium powder determined the grain refinement and the uniformity of the reinforcements distribution in the prepared aluminium matrix composites.The microstructure of the aluminium matrix composites containing 100wt% aluminium powder,20wt% 8 μm silicon carbide,10wt% 800 nm and 50 nm silicon carbide and 1wt% and 1.5wt%graphene nanosheets was found to be isometric with small grain size and uniform reinforcement distribution.Silicon carbide mainly enhances the tensile strength of aluminum matrix composites,while graphene nanosheets mainly enhance the elongation,both of which can enhance microhardness.20wt% 8 micron silicon carbide + 1wt% graphene nanosheet aluminum matrix composite exhibits the highest tensile strength and microhardness of 171.3MPa and 168.8±1.1 HV,respectively,which is 1.9 times the tensile strength(90 MPa)and5.6 times the microhardness(30±0.2 HV)of the 1060 aluminum alloy.1wt% Graphene nanosheet aluminum matrix composite exhibits highest elongation of 55.6%,which is 1.3times the elongation of 1060 aluminum alloy(42%).graphene nanosheets and silicon carbide have opposite effects on the average friction coefficient and wear mechanism of aluminum matrix composites,but the same effect on the wear volume of aluminum matrix composites.The 2wt% graphene nanosheet aluminum matrix composite has the smallest average friction coefficient of 0.409,which is seven tenths of the average friction coefficient of the 1060 aluminum alloy(0.567),its wear volume is the smallest of all graphene nanosheet aluminium matrix composites and its wear mechanism is abrasive wear.