Preparation,Microstructure And Thermal-mechanical Properties Of Continuous Carbon Fiber Reinforced Aluminum Matrix Composites

Continuous carbon fiber（CF）reinforced aluminum（Al）matrix composites have the advantages of light weight,high modulus,low expansion and good thermal conductivity.Therefore,they have a wide application prospect in the fields of aerospace,transportation and electronic communication.In the preparation process of the composites,there are two situations:easy and difficult interfacial reaction.In the easy interfacial reaction,the structure of CF is easy to be damaged,which affects their performance.In the difficult interface reaction,the degree of interfacial bonding plays a key role in their performance.Therefore,only by effectively regulating their interface can their performance be brought into play.Based on this,this dissertation studied the Al matrix composites reinforced by polyacrylonitrile（PAN）based CF with low graphitization degree and mesophase pitch based CF（MPCF）with high graphitization degree.For PAN-CF,vacuum pressure infiltration（VPI）method was used to prepare Al₂O₃-coated PAN-CF/Al-Si composites,and the interface was regulated by regulating the coating process.The preparation process parameters,microstructure characterization and thermal-mechanical properties of Al₂O₃-coated PAN-CFs and the composites were studied.The effects of Al₂O₃ coating thickness on the microstructure and thermal-mechanical properties of composites were revealed,and the composites with high modulus and low coefficient of thermal expansion were formed;For MPCF,the vacuum hot pressing（VHP）method was used to prepare MPCF/Al composites,and the interface was regulated by optimizing the preparation process parameters.The effects of hot pressing process parameters and MPCF volume fraction on the microstructure,interface and thermal-mechanical properties of the composites were studied.The characteristics of interface structure under the optimized process parameters and the influence law of MPCF volume fraction on the thermal-mechanical properties of the composites were revealed.A composite material system with high modulus,excellent thermal properties and adjustable was formed.In order to further meet the industrial demand for high-performance materials with integrated structure and function,MPCF/2024Al composites were prepared by VHP.The effects of hot pressing process parameters,MPCF volume fraction and T6 heat treatment on the microstructure,interface and thermal-mechanical properties of the composites were studied.The characteristics of interface structure under optimized process parameters and the effect of MPCF volume fraction on the thermal-mechanical properties of the composites were revealed,and the interface structure evolution process of T6 heat treatment on promoting interfacial bonding was clarified.A composite system with excellent thermal-mechanical properties and adjustable was formed.The main conclusions are as follows:（1）The Al₂O₃ coating on the surface of PAN-CF was prepared by sol-gel method.The effects of heat treatment temperature,the immersion time per cycle and the immersion-drying cycle on the microstructure and mechanical properties of Al₂O₃-coated PAN-CFs were systematically studied.The results showed that the better coating and mechanical properties of Al₂O₃-coated PAN-CFs could be obtained at 500℃for 2immersion-drying cycles by the immersion time of 5 min per cycle.Towards the Al₂O₃-coated PAN-CFs fabricated at 500℃for 2 immersion-drying cycles by the immersion time varying from 5 to 40 min per cycle,the average coating thickness ranged from 100 to 240 nm.（2）Al₂O₃-coated PAN-CF/Al-Si composites with 40 vol.%PAN-CF were prepared by VPI method.The effect of coating thickness on the microstructure,thermal-mechanical properties of PAN-CF/Al-Si composite was studied.The results showed that Al₂O₃ coating improves the the uniform dispersion and interfacial wettability of of PAN-CFs in Al matrix.With the increase of coating thickness,the diffusion of carbon in aluminum was hindered and the interfacial bonding strength decreased.Thus,the TC and mechanical properties of the composites decreased,while the CTE increased.Only proper coating thickness（100 nm）can effectively improve the thermal-mechanical properties of the composite.（3）40 vol.%MPCF/Al composite was prepared by VHP method.The effects of sintering temperature,sintering pressure and sintering time on microstructure and TC of the composite was investigated.The results showed that the composite with high longitudinal TC could be successfully prepared at 650℃/45 MPa/60 min.The high longitudinal TC resulted from the following microstructural characteristics:（1）suitable interfacial bonding,（2）relatively intact and parallel MPCFs,（3）controllable interfacial reaction.（4）Under the optimized process conditions,the effect of MPCF volume fraction（20～50%）on the microstructure,interface and thermal-mechanical properties of MPCF/Al composites was studied,and their thermal-mechanical properties were compared with those of other kinds of Al matrix composites and Al alloys.The results showed that the interface between MPCF and Al was mainly composed of an amorphous interface layer of 2～5 nm.At the same time,there was a very small amount of lath-like carbide crystal at the interface.With the increase of MPCF volume fraction,the longitudinal TC of the composites increased first and then decreased slightly,but they all exceeded 90%of the predictions by the rule of mixture,and the highest was 288.3 W/（m·K）;The longitudinal average CTE decreased all the time,which was closer to the value of Schapery’s model,and the lowest was-0.22ppm/K;The tensile strength first increased and then decreased,up to 177.2 MPa,and the elastic modulus increased all the time,up to 324 GPa.Compared with other Al matrix composites and Al alloys,the TC values of the composites in this work were at a medium level,but their CTE values and specific modulus had more advantages.（5）To meet the high performance requirements of the integration of material structure and function,40 vol.%MPCF/2024Al composite was prepared by VHP method.The effects of sintering temperature and sintering pressure on the microstructure and tensile properties of MPCF/Al composites were studied.The results showed that the composite with high tensile strength could be successfully prepared prepared at 570℃/35 MPa/60min,which was 356.3 MPa.At this time,the microstructure was relatively uniform and the continuity of MPCFs was relatively intact.（6）Under the optimized process conditions,the effect of MPCF volume fraction on the microstructure,interface,thermal-mechanical properties of MPCF/2024Al composites was studied.The results show that the interfacial bonding was good,the interface was clean and there was a continuous amorphous layer without interfacial reaction.With the increase of the volume fraction of MPCF,the longitudinal TC of the composites increased all the time,and the highest was 258.3 W/（m·K）;The longitudinal average CTE first decreased and then increased slightly,and the lowest was 0.1 ppm/K;The tensile strength increased first and then decreased.When the MPCF volume fraction was 30%,the highest was 406.3MPa,and the elastic modulus increased all the time,up to 343.5 GPa.（7）To further improve the tensile strength,MPCF/2024Al composites were treated with T6 heat treatment.The effect of T6 heat treatment on the microstructure,interface,thermal-mechanical properties of the composites was studied.The results showed that after T6 heat treatment,the amorphous layer at the interface changed from continuous to discontinuous,there was no interfacial reaction,and more and smaller precipitates were precipitated in the matrix.Compared with the as-sintered composites,the TC of the heat-treated composites was slightly reduced,the CTE was further reduced,the elastic modulus changed little,and the tensile strength was further improved,up to 499.3 MPa.To sum up,aiming at the system with easy and difficult interfacial reaction in for continuous CF/Al matrix composites,this dissertation adopts the methods of fiber surface coating treatment,composite preparation process optimization and T6 heat treatment to regulate the interface respectively,studies the effective control technology of composite interface,and explores the response law of thermal-mechanical properties,It provides a new idea for the preparation of high-performance continuous CF/Al matrix composites with integrated structure and function.