| With the development of economy and technology,the requirements on the strengths,elasticity modulus,dimensional stability and wear resistance etc.of aluminum matrix composites have been much higher.Therefore,the development of high performance lightweight aluminum matrix composites is very important.Due to the low density,excellent mechanical,electrical and thermal properties,graphene reinforced Al matrix composites has a vast application prospect.But the difficult dispersion and poor wettability of graphene in Al matrix make it dfficult to reach its ideal reinforcement effect.In this thesis,graphene/Al composites were prepared by electroless nickel-plating,powder metallurgy and extrusion process.Graphene was prepared using exfoliation method and then modified by chemical nickel-plating,which can improve the dispersion and wettability.The dispersion of graphene in Al matrix was investigated.Microstructure and properties of graphene/Al composites were investigated.The following results are obtained.(1)Ni nanoparticles were successfully adhered to the graphene surface(nickel coated graphene).Graphene was prepared by modified Hummers method,high-temperature expansion,and ultrasonic exfoliation process;then the surface of the graphene was modified by electroless nickel-plating.Graphite oxide was expanded with different temperatures increased from 400℃ to 800℃.Specific surface area increased from 462 m2/g(400℃)to 644 m2/g(800℃).After ultrasonic treatment,the number of layers of graphene was small and its surface was clean.Electroless nickel-plating of graphene process includes sensitization pretreatment,activating pretreatment,and chemical plating treatment.Optimal chemical nickel plating process parameters are obtained whose water bath temperature is 60℃ and pH is 12.5.After chemical nickel-plating process,graphene surface dispersed Ni nanoparticles with the size of～50 nm.(2)The graphene is relatively uniformly dispersed in the aluminum matrix.After low energy ball milling,Ni nanoparticles still adhered to the surface of graphene.With the increase of the milling time,the dispersion of graphene in the aluminum matrix is more uniform,and the graphene is also peeled off by the shearing force of the mixture.When the content of graphene is 1.0 wt.%,The mixed powder with good dispersion of graphene in aluminum matrix can be obtained after ball milling at 72h.Under the condition of 72h low energy ball milling,the graphene was uniformly dispersed in the aluminum matrix and no agglomeration appeared with the graphene content was below 1.5 wt.%.Compared with the traditional milling method of graphene/aluminum composites which graphene occurred agglomerated at less than 1.0 wt.%,electroless nickel-plating on graphene surface method can effectively disperse graphene in aluminum matrix composites.However,when the content of graphene is 2.0 wt.%,a little agglomeration of graphene has occurred.(3)The influence of powder metallurgy parameters on microstructure,density and hardness of graphene/aluminum composites was obtained.With the prolongation of the mixing time,the aspect ratio of the composite grains increases,the hardness of the composite gradually increases,and the density of the graphene increases first and then decreases.With the increase of sintering temperature,the hardness and the density of the composites are improved.When the sintering temperature is 580℃,the composites have been densified.As a result of the large number of dislocations introduced during the milling process,the grains of the composites were recrystallized at temperatures above 600℃.When the sintering temperature reaches 630℃,Ni nanoparticles on the graphene surface react with the aluminum matrix to form Al3Ni phase,thus improving the interfacial bonding strength between graphene and aluminum matrix.(4)The graphene/aluminum composites were prepared by powder metallurgy and extrusion.The effects of graphene content on the microstructure and mechanical properties of the aluminum matrix composites were obtained.The strengthening mechanism of graphene on aluminum matrix composites was investigated.With the increase of graphene content,the density of the composite decreases,and the hardness and tensile strength of the composites showed a trend of first increasing and then decreasing.When the content of graphene is 1.5 wt.%,the hardness,tensile strength and elastic modulus reach the maximum value,which is 20.2%,30.5%and 37.5%higher than that of aluminum matrix.Graphene dispersed in the aluminum matrix,and did not show significant agglomeration.The fracture modes of the composites are ductile fracture.The graphene showed pulled out and overlapped phenomenon,and there were no nanoparticles on the graphene surface,which indicated that the Ni coating formed a good interface with the substrate.The strengthening mechanism of graphene on aluminum matrix is load transfer,dislocation strengthening and grain refinement.(5)Effects of the graphene content on the thermal expansion coefficient of the graphene/aluminum composites were investigated.When the temperature increased from 50℃to 500℃,the average thermal expansion coefficient of graphene/aluminum composites increased.The coefficient of thermal expansion of pure aluminum is 23.38×10-6/K at 25℃,the coefficient of thermal expansion of pure aluminum is 28.81×10-6/K at 300℃.And the thermal expansion coefficient of graphene/aluminum composites decreases with the increase of graphene content.When the content of graphene increases to 2.0 wt.%,the thermal expansion coefficient is 23.29×10-6/K,reaching 80.8%of pure aluminum.The main reason is that the coefficient of thermal expansion of aluminum is higher than that of graphene in the temperature rising process.In the meantime,the difference in thermal expansion coefficient makes graphene produce tensile stress to Al matrix,which has an inhibitory effect on the expansion of aluminum matrix,so the thermal expansion coefficient of composites is reduced. |
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