Effects Of Cryogenic Treatment On Microstructure And Mechanical Properties Of 3D Orthogonal-C_f/Al Composites

As an important branch of 3D braided composites,3D orthogonal composites refers to the composites which inserts Z yarn in the thickness direction of the mutually perpendicular warp and weft surface,and the fiber presents mutually perpendicular orthogonal arrangement in the whole braided structure in ideal condition.It has the characteristics of simple structure,convenient production,high interlaminar shear strength and low notch sensitivity,and has bright development.The three-dimensional orthogonal C_f/Al composite was prepared by vacuum air pressure infiltration.It has the advantages of integration of preparation and forming and has good formability of complex structural parts.The composite prepared by this process has excellent comprehensive properties.However,at present,there are still some problems such as the difficulty in controlling the interface strength between the fiber and the matrix,and because of the difference of the coefficient of thermal expansion resulting in the large residual stress in the composite.They directly affects the performance and stability of the metal matrix composite.Cryogenic treatment can not only significantly improve the strength and wear resistance of the material,but also improve the uniformity and dimensional stability of the material,and has the characteristics of simple operation,low cost,clean and pollution-free.In this paper,the effects of cryogenic treatment on the microstructure and mechanical properties of 3D orthogonal C_f/Al composites was studied.3D orthogonal C_f/Al composites with 50%fiber volume fraction using M40J carbon fiber as reinforcement and ZL301 aluminum alloy as matrix which prepared by vacuum air pressure infiltration were processed by the cryogenic treatment.The density,microstructure,residual stress,microhardness,interface bonding strength and tensile strength of the composites after cryogenic treatment were measured and characterized.The effects of soaking time and circular index of cryogenic treatment on the microstructure and mechanical properties of 3D orthogonal C_f/Al composites were studied,and the mechanism of the effects was discussed.The results suggest:Some of fiber segregation phenomenon can be observed in the as cast condition.And the surface of fiber are uneven and presents as"pea-like"shape.With the improvement of the soaking time and the circular index of cryogenic treatment,the phenomenon of fiber segregation is relieved,the surface of fiber become round and smooth.And the"pea-like"shape gradually return to round.The distribution of residual stress on the matrix of the composites can be changed obviously by cryogenic treatment.The residual stress on the matrix of the composites is+185.4MPa in the as cast condition.With the increase of the soaking time and the circular index of cryogenic treatment,the state of residual stress of the composites gradually changs from tensile stress in the as cast condition to compressive stress.When the soaking time of the cryogenic treatment is 36h,the residual stress decreases to+115.7MPa and its decreasing rate has slow down;When the circular index of cryogenic treatment reaches 7,the residual stress of the matrix reaches-267.3MPa.The stress fringes in the as cast condition are large and irregular mainly distribute on the interface.After one single cryogenic treatment the stress fringes become tiny and show the trend of expanding from the interface to the matrix.When the circular index of cryogenic treatment reaches 7 the sub grain can be observed in the composites due to the tangle of dislocations.The XRD results show that the single cryogenic treatment does not change the position of the main diffraction peak of the composite;the main diffraction peaks of the composites changes obviously with the increase of the circular index of cryogenic treatment.The main diffraction peaks in the condition of as cast and other circular index of cryogenic treatment all appears at 38.5°position.While the diffraction peaks of seventh cryogenic treatment cycles is at 44.8°position.This indicates that the larger internal stress results in the grain turning in the aluminum matrix.Cryogenic treatment has little effects on the microhardness of matrix between the fiber bundles of composites.With the increases of the circular index of cryogenic treatment,the microhardness of matrix in the fiber bundle and interface of composites increases gradually.And reach the peak when the circular index of cryogenic treatment are 9 and 7,their value respectively are 305.9HV and 515.9HV.This may be due to the effect of dislocation strengthening to improve the microhardness in composites.The average transverse tensile strength(interfacial strength)of the consecutive C_f/Al composites increases with the increase of the circular index of cryogenic treatment.The average transverse tensile strength of the composite is 10.39MPa in the as cast condition.When the circular index of cryogenic treatment reaches 9,the average transverse tensile strength of the composite is 13.33MPa,28.3%higher than that in the as cast condition.The tensile strength of the composite can be improved by cryogenic treatment obviously.The average tensile strength of the as cast composite is 735.86MPa.The tensile strength of the composites gradually increase with the increase of the soaking time of the cryogenic treatment,and reaches the maximum value of 825MPa when the soaking time is 36h,which is 12.1%higher than the as cast condition;the tensile strength of the composites increase with the increase of the circular index of the cryogenic treatment,and reaches the maximum value of 885.89MPa when the circular index is 7,which is 20.39%higher than the as cast condition.The average elastic modulus of the as cast composites is 105.94GPa.It increases with the increase of the soaking time of the cryogenic treatment,and reaches the peak value of 130.24GPa when the soaking time of the cryogenic treatment is 36h.After multiple cycles of cryogenic treatment,the average elastic modulus of 3D orthogonal C_f/Al composite is stable at120GPa,which is 13.27%higher than that of as cast composites.For the three-dimensional orthogonal C_f/Al composite,the reduction of residual stress and the bridging of micro holes plays a leading role in the strengthening of mechanical properties of the single cryogenic treatment;for the multiple cryogenic treatment,the strengthening of mechanical properties may be mainly due to the transformation of the state of residual stress at the matrix,the moderate improvement of interface strength and the increase of dislocation density.