Effect Of Cryogenic Treatment On Microstructure And Mechanical Properties Of 2.5D-C_f/Al Composites

The three-dimensional woven carbon fiber reinforced aluminum matrix composite material has the advantages of light weight,excellent temperature resistance,high specific stiffness and specific strength,corrosion resistance,impact resistance and damage resistance.At the same time,the three-dimensional woven structure avoids the anisotropy of the traditional unidirectional composite material and the problem that the two-dimensional composite material is easy to crack and delamination,the interlaminar shear strength is low,and the impact resistance is poor.It is one of the most promising structural materials in the field of advanced engineering technology such as aerospace and national defense.As a branch of 3D braided composites,2.5D fabric composites,like 3D composites,are increasingly being studied and applied.In recent years,a lot of research has been carried out on reinforcing materials,preparation processes and post-treatment of composite materials in order to obtain composite materials with better performance.This paper mainly studies the effects of subsequent cryogenic treatment on the microstructure and mechanical properties of 2.5D-C_f/Al composites.In this paper,M40-6K carbon fiber is used as the reinforcement,and ZL301aluminum alloy is selected as the matrix material.Prepared by the vacuum pressure infiltration of Vol.49%2.5D-C_f/Al composites.The cryogenic treatment technology of 2.5D-C_f/Al composite was studied.In addition,the density,microstructure,element distribution and phase composition,residual stress and tensile properties of the composite after cryogenic treatment were tested and characterized.Study the effects of cryogenic cooling rate and cryogenic treatment time on the microstructure and mechanical properties of 2.5D-C_f/Al composites.And mechanism of cryogenic treatment is also discussed.The following research results:The tensile stress of the matrix side of the 2.5D-C_f/Al composites under the as-cast condition is 77.6MPa,after cryogenic treatment,the residual stress on the matrix side of the composites decreases and eventually transforms into residual compressive stress.With the slowing down of the cryogenic cooling rate,the residual stress tends to decrease first and then rise.When the cooling rate is 10?/min,the residual stress is most completely released,which is-41Mpa.With the prolongation of the cryogenic treatment time,the residual stress continues to decrease and eventually the residual stress state changes,that is,the matrix is transformed from the tensile stress to the compressive stress,and at the same time,the reinforcing phase is transformed from the compressive stress to the tensile stress.When the cryogenic treatment time is further extended to 48h,the residual stress value fluctuates less,indicating that the effect of further prolonged cryogenic treatment on the release of residual stress is no longer significant.Cryogenic treatment can significantly increase the density and tensile strength of2.5D-C_f/Al composites,and as the cryogenic cooling rate slows down,the tensile strength of composites first increases and then decreases,but the density difference is small between different cryogenic cooling rates of composites.The density of the cryogenically treated composite at a temperature drop rate of 10?/min reached a maximum of 96.33%.At this time,the tensile strength also reached a peak of 475MPa,which was 16.42%higher than that of the as-cast state.In this state,more fibers were pulled out in the composite fracture,it is a typical ductile fracture.After cryogenic treatment,the elastic modulus of the composites is reduced to some extent compared with the as-cast condition,but the difference in elastic modulus between different cooling rates is small.With the continuous deepening of cryogenic treatment time,the tensile strength of 2.5D-C_f/Al composites showed a law of increasing first and then decreasing,and the tensile strength reached the peak after 32h cryogenic treatment.The elastic modulus of 2.5D-C_f/Al composites was almost unchanged after16h of short-term cryogenic treatment.After 32h of cryogenic treatment,the elastic modulus decreased to some extent,and after 48h of deep cryogenic treatment,the elastic modulus of the composites is increased and reaches a maximum of 91.4 GPa,which is 6.8%higher than that in the as-cast state.Cryogenic treatment can improve fiber segregation and impregnation micro-defects in 2.5D-C_f/Al composites,increase the density of composites,and restore the spheroidal deformation of fibers.And with the prolonged cryogenic treatment time,composites has fewer defects,the density is higher,and the fiber is more rounded.After 32h of cryogenic treatment,there was no impregnation defect in the composites,and the density reached a maximum of 95.73%.There was no significant improvement in the microstructure and density of the composites after continuing to extend the cryogenic treatment time to 48h.Based on the above analysis of the microstructure,density,residual stress,phase and mechanical properties of 2.5D-C_f/Al composites,the optimum process parameters for cryogenic treatment of 2.5D-C_f/Al composites are as follows:the cryogenic cooling rate is 10?/min,the holding time is 32h,and the cryogenic temperature is-190?.