Investigation On The Microstructure And Mechanical Properties Of Heat-resistant Short Carbon Fiber/Al Composites Applied In Drill Pipe

With the development of oil-gas drilling and scientific drilling technology,higher demand on drill pipe has been proposed.Traditional steel drill pipes,such as G105and S135 drill pipes,have no longer satisfied the requirement in deep and extra-deep drilling engineering,due to their limits such as large weight,high magnetism and poor corrosion resistance.In the recent few decades,aluminum alloy drill pipe has attracted many industry insiders,because of its incomparable advantages,such as low density,high specific strength and brilliant corrosion resistance.It is known that a series of aluminum drill pipe has been widely applied in extra-deep drilling and extended reach drilling in USA and Russia.Nevertheless,the mechanical properties of aluminum alloy degrade much due to the low softening temperature,causing the premature failure of drill pipe in deep formation with high temperature.It largely limits the application of aluminum alloy drill pipe.In this dissertation,in order to fulfill the urgent need of low-weight and heat-resistant drill pipe materials in deep and extra-deep oil-gas drilling and scientific drilling,we fabricated short carbon fiber reinforced aluminum matrix composites based on the available 2024 Al drill pipe materials.The strengthening mechanism of carbon fiber on the mechanical properties of SCFs/2024 Al composites was studied in order to provide theoretical foundation for developing a new series of carbon fiber reinforced aluminum matrix composites drill pipes with low weight and heat resistance.Firstly,short carbon fiber reinforced 2024 Al matrix composites?SCFs/2024 Al?with low fiber content were fabricated via powder metallurgy.The microstructure observation results showed that carbon fiber was dispersed randomly in the matrix with no obvious direction.The dramatic interfacial reaction with large amounts of interface product Al₄C₃ was found at the interface between carbon fiber and Al matrix.The phenomenon of segregation was also observed at the interface,with the formation of Al₂Cu and Mn-rich phase.The above interface products were all found coarsening at high temperature.The addition of carbon fiber efficiently improved the overall hardness of composites,and the enhancement in the matrix around fiber was more obvious.The results of nanoindentation showed that the most neighboring matrix to a single carbon fiber possessed the highest hardness and modulus,at 2.51 and 95.53GPa respectively.The results of tensile tests showed that,the yield strength and ultimate tensile strength of SCFs/2024 Al composites firstly improved and then decreased with the increasing carbon fiber content.4 vol.%SCFs/2024 Al had the highest yield strength at all testing temperature including room temperature,150oC and 250oC,which was 19.6%,45.6%and 36.7%higher than that of 2024 Al.Comparing to 2024 Al,the SCFs/2024 Al composites all showed brittle fracture behavior.At room temperature,carbon fiber was mostly broken in shear failure due to the strong interfacial bonding caused by thermal residual stress and interfacial reaction.At high temperature,with the recovering of residual stress and coarsening of interface products,the interfacial bonding between carbon fiber and Al matrix became weaken,causing the pull-out and debonding carbon fiber took up at the fracture surface.To overcome the unapparent improvement in ultimate tensile strength and dramatic decrease in plasticity of SCFs/2024 Al caused by the low-stress brittle failure of carbon fiber,hot extrusion was applied to achieve the directional dispersion of carbon fiber along extrusion direction.At the precondition of constant extrusion force,the extrusion speed decreased with the increasing fiber content,decreasing extrusion temperature and increasing extrusion ratio.The decreasing extrusion speed reflected the weakening in the ability of flowing deformation of Al matrix,causing the deviation angle between carbon fiber and extrusion direction became larger.The addition of carbon fiber resulted in the refinement of Al grains around carbon fiber.At high temperature,carbon fiber was also able to restrict the coarsening of Al grains.Large amounts of Al₄C₃ and segregations were still existent at the interface of as-extruded SCFs/2024 Al.At room temperature,the ultimate tensile strength of SCFs/2024 Al firstly increased and then decreased with the increasing fiber content.When the fiber content was 8 vol.%,and the extrusion temperature and ratio were 460oC and 14.6 respectively,the composite had the highest ultimate tensile strength at514 MPa.The strengthening mechanism of as-extruded SCFs/2024 Al composites included stress transfer,grain refinement,texture conversion and dislocation accumulation.Similarly,the as-extruded SCFs/2024 Al composites showed brittle fracture behavior,due to the brittle failure of carbon fiber caused by the strong interfacial bonding.At high temperature,the addition of carbon fiber could efficiently restricted the degradation in tensile strengths of composites,mostly benefited by the heat resistant carbon fiber and the restriction of Al grain coarsening by carbon fiber.In addition,the softening of Al matrix and recovering of interface residual stress relieved the brittle fracture of carbon fiber.Crack deflection occurring at the interface helped improve the plasticity and high temperature strength of SCFs/2024 Al composites.Lastly,in order to restrict the interfacial reaction between carbon fiber and Al matrix,Ti coating was fabricated on carbon fiber using Ti-I₂ system CVD processing.On the research basis of powder metallurgy and hot extrusion,the influence of Ti coating on the interface microstructure and tensile properties of SCFs/2024 Al was studied.Through the thermodynamic theoretical calculation on Ti-I₂ system,the molar ratio of Ti:I₂ was intended to be 1:1.6.At this molar ratio,TiI₃ and TiI₂ both had largest equilibrium concentration in reaction camber,which was beneficial for the Ti deposition.The results of XRD and Raman confirmed that the Ti coating on carbon fiber consisted of TiO₂ and amorphous carbon,because little O₂ was flowed into the reaction camber during cooling period.At the 900oC coating temperature,the coating thickness increasing with the increasing heat preservation time.The interface microstructure observation showed that the Ti coating on carbon fiber was able to alleviate the interfacial reaction between carbon fiber and Al matrix,the amount of Al₄C₃ at the interface decreased apparently.In Ti-coated-SCFs/2024 Al composites,the coating on carbon fiber was conversed into a complexed combination of TiC,TiO₂,MgO,Ti and amorphous carbon.Compared to the as-extruded uncoated-SCFs/2024Al composites,the coated-SCFs/2024 Al composites achieved simultaneous enhancement in ultimate tensile strength and plasticity,due to the transformation in interface structure.Through the crack deflection mechanism,the stress distribution was adjusted at the interface of composites,and the stress concentration was avoided.Carbon fiber was able to efficiently transfer the stress from matrix and improve the strength and plasticity.