Preparation,Microstructure And Properties Of Continuous/Short Carbon Fiber Reinforced Aluminum Matrix Composites

Carbon fiber reinforced aluminum matrix(Cf/Al)composites have great application potential in equipment manufacturing and aerospace fields because of its high specific strength,specific stiffness,good dimensional stability and other excellent properties.Among many composite preparation processes,although little research has been carried on the preparation of Cf/Al composite by liquid solidification method,it is still the most convenient and efficient way to achieve industrial-grade Cf/Al composite.However,this process needs to solve the problems of poor wettability between base metal and carbon fiber as well as adverse reactions at high temperatures.With the development of plating technology,carbon fiber surface modification technology can effectively improve the wettability of carbon fiber and the adverse reactions between carbon fiber and matrix,which makes it possible to prepare Cf/Al composites by liquid solidification method.Therefore,the main objective of this paper is to prepare the Cf/Al composites with uniform dispersion of carbon fiber in the matrix by a simple liquid solidification method.The preparation process of continuous Cf/Al composites and short Cf/Al composites is studied,which will lay a foundation for the application of the Cf/Al composites in the future.According to the different characteristics of continuous fiber and short fiber,two new technologies for preparing composite materials were proposed,namely continuous Cf/Al composites prepared by ultrasonic vibration method and the short Cf/Al composites prepared by mechanical combined with electromagnetic stirring method.In the preparation process of the continuous Cf/Al composites,it is difficult to disperse the fiber bundles without external field,and the defects of carbon fiber agglomeration are very obvious,which can be seen almost macroscopically.In order to solve this problem,the ultrasonic vibration was used to promote the dispersion of carbon fiber in the matrix.Ultrasonic vibration can generate high energy in the melt and accelerate the flow of the melt,the fibers bundle can be completely infiltrated when the melt has enough pressure.Since the addition of different alloy elements has different effects on melt viscosity and fluidity,inorder to eliminate interference and simply study the effect of process parameters on the dispersion of carbon fibers in the matrix,so industrial pure aluminum was used as the matrix of the continuous Cf/Al composites in this paper.The results show that the excessive pressure causes serious damage to carbon fiber with weak shear force.When the distance between carbon fiber and the ultrasonic vibration head is less than 15cm,the carbon fiber will cause obvious damage.Through analysis of attenuation characteristics of energy produced by ultrasonic vibration in the melt,when the distance between carbon fiber and ultrasonic vibration head is adjusted to 15 mm,carbon fibers can be completely infiltrated by aluminum melt,and obvious fiber damage can be avoided.Therefore,when the melt has a certain turbulence intensity,it has a positive effect on the uniform dispersion of carbon fiber in the matrix.During the preparation of continuous Cf/Al composites,the process parameters such as the amplitude and the carbon fiber binding force have an important influence on the dispersion of carbon fibers in the matrix.The results show that when the amplitude is 35 ?m,the infiltration effect of aluminum melt on carbon fiber bundles will be seriously affected by small infiltration pressure,which will lead to un-infiltrated defects in the matrix;When amplitude reaches 41 ?m,excessive amplitude will cause the velocity of melt to exceed the turbulent threshold,resulting in the generation of carbon fiber clusters in the matrix.When the binding force is less than 2 N,carbon fibers will shift and entangle in the melt,which will seriously affect infiltration effect;when the binding force is greater than 6 N,the dispersion of carbon fiber will be hindered,and there are also un-infiltrated defects in the matrix.No matter the un-infiltrated defects or the existence of fiber clusters,it is found through analysis of the fracture process that the performance of the composite material will be reduced.The Cf/Al composites with uniform distribution of carbon fibers in the matrix and no obvious casting defects can be prepared under the condition that the ultrasonic amplitude range is 37-39 ?m,the binding force is 4 N and the sequential solidification method is adopted.In this case,the maximum tensile strength of the composites can reach 203 MPa,which is 271%higher than that of the matrix.Compared with continuous Cf/Al composites,the short Cf/Al composites have a wider application prospect,but its preparation is more difficult,and the uniform distribution of carbon fibers in the composites is more difficult to control.The detailed analysis of melt flow field in the crucible during the preparation of short Cf/Al composites by numerical simulation technology shows that there is only a stable circulating flow field in the circular crucible under mechanical stirring,which is not conducive to carbon fiber dispersion.The study of continuous Cf/Al composites prepared by ultrasonic vibration shows that the uniform dispersion of carbon fibers can be achieved with a certain turbulence intensity in the melt.Therefore,a new idea for preparing short Cf/Al composites with square crucible under mechanical stirring was first proposed,that is,the square crucible was used to increase the turbulence intensity of the flow field and realize the uniform dispersion of carbon fiber in the metal melt.Meanwhile,in order to satisfy the use environment of the composites and enhance the fluidity of the matrix,Al-Si alloy was used as the matrix of the short Cf/Al composites.Through simulation and experimental research,it is found that the short fibers in the melt will continuously impact and rub the end wall,which results in the initial dispersion of carbon fibers in the square crucible.Then,the locally agglomerated fibers will be dispersed under convolution and shear of the secondary flow in the melt.The experimental results show that carbon fibers in the composite matrix prepared with a square crucible at a rotating velocity of 1000 rpm have a multi-orientation distribution state,which can effectively avoid the existence of un-infiltrated defects and uneven fiber distribution in the composites prepared by a round crucible.The tensile strength of the composite prepared by this process can reach 172 MPa,which is 48.3%higher than that of the composite prepared by a round crucible.Research on the microstructure of short Cf/Al composites prepared by asquare crucible combined with mechanical stirring found that the dispersibility of carbon fiber in the matrix of short Cf/Al composites prepared by mechanical stirring in square crucible is obviously improved than that in a round crucible,and the amount of agglomerated carbon fibers is obviously reduced.However,a certain agglomeration of carbon fiber was still found in the four right angles of the crucible,which is due to the existence of stirring dead zone in the four right-angle edge regions of the square crucible.Through simulation and research on the electromagnetic induction process,it is found that the electromagnetic force can produce radial and axial pressure gradients inside the melt,and the unbalanced pressure gradients are conducive to the flow of the melt in the axial direction,which is beneficial to the improvement of carbon fiber agglomeration in the right angle region of the square crucible.Based on this,the new process of preparing the short Cf/Al composites by mechanical combined with electromagnetic stirring was put forward.Firstly,the numerical simulation of the flow field distribution under the action of mechanical stirring,electromagnetic stirring and mechanical combined with electromagnetic stirring was carried out.The results show that there is a complex 3D flow in the crucible under the effect of electromagnetic force.Under the mechanical combined with electromagnetic stirring,the axial flow velocity of melt in the "dead zone" of can reach to 0.98 m/s,which has a positive effect on improving the agglomeration of carbon fibers in the right angle region.The numerical simulation results show that the ideal parameters of mechanical combined with electromagnetic stirring process are that the current in the coil is 900 A and stirring speed is 1000rpm.The experimental results under the condition of technological scheme show that the mechanical combined with electromagnetic stirring can make the carbon fibers in the melt achieving a multi-oriented and uniform distribution in all regions,which can significantly improve the phenomenon of local fiber aggregation under pure mechanical agitation,so that the overall performance in the composite can be improved,and the tensile strength of the composites can reach to at least 177 MPa.At the same time,the addition of carbon fibers would divide the melt into several regions to hinder the solute transport in the melt,which would significantly reduce the size of ?-Al in the Al-Si alloy matrix.The addition of carbon fibers would also promote the nucleation of Si elements in the melt on the surface of the carbon fibers,and caused eutectic silicon to change from coarse needle flakes to fine flakes or short rods.In addition,under liquid conditions,because of the low mixing enthalpy between Si element and Ni coating and the carbon fiber,Si element would continuously move to carbon fibers and exist around the fiber,which would have a positive effect on inhibiting the formation of Al₄C₃ brittle phase.