Effects Of Short Coating Carbon Fiber On High Temperature Tensile Properties Of 7075 Aluminum Alloy For Drill Pipe

Aluminum alloy drill pipe has the characteristics of low density,high specific strength,and good acid corrosion resistance.It is widely used in deep wells,ultra-deep wells,extended reach wells,offshore drilling wells,and deep oil and gas wells.These environments require extremely high-temperature strength of aluminum alloy drill pipe.In the standard aluminum alloy materials for drilling pipe,there is a contradiction between high strength and high-temperature resistance,the highest operating temperature of high strength aluminum alloy(Al-Zn-Mg)is only 120°C,while the specific strength of the heat-resistant aluminum alloy(Al-Cu-Mg)is merely comparable to that of steel and titanium alloy.Thus,the high-temperature strength of traditional aluminum alloy materials should be further developed.To meet the requirements of geological engineering,carbon fiber reinforced aluminum alloy composites were selected and fabricated by high strength aluminum alloy(AA7075)as the matrix and higher strength-to-weight ratio carbon fiber as reinforcement.The strengthening mechanism of the composite materials was summarized,and several coating methods of carbon fiber were adopted and optimized.The results provide ideas for composite material interface design,guidance for research,and suggestions for field application of aluminum alloy drill pipe.In this paper,oriented uncoated carbon fiber reinforced 7075 aluminum alloy composites(SCF/AA7075)were firstly prepared by powder metallurgy method.The addition of carbon fiber can lower the density of the composites,and improve the specific strength and hardness of composites.The enrichment of aluminum carbide(Al₄C₃)and magnesium(Mg)provided a strong interface and enabled the composites to obtain desirable mechanical properties.At 150°C,the tensile strength of the composites increased from 441 MPa to 510 MPa(by 15%),and the yield strength increases from 414 MPa to 450 MPa(by 9%).After 10 minutes at 150°C,the maximum strength of the composite can maintain 93%of that at room temperature,which was 16 percentage points higher than that of the matrix.However,the brittle interfacial product Al₄C₃ also led to the shear failure of carbon fibers and poor plasticity of composites.The strengthening model proposed in this paper can better describe the mechanical properties of composites with different carbon fiber contents at different temperatures.In this model,load transfer was the main strengthening mechanism,and residual thermal stress played an important role.At room temperature,negative factor of residual thermal stress can be used to estimate the deterioration of material properties,while it can be transformed into a positive enhancement effect at high temperature.To reduce the influence of damage to fiber by interfacial products,a high temperature solid phase synthesis method was proposed to prepare carbon fiber with boron carbide coating.Boron coated carbon fibers with thickness of 100 to 400nanometers were prepared by using boron powder and boric acid(at 1050°C for 1?4hours).The thermogravimetric results show that the weight of carbon fiber does not decrease when exposed to 1000°C.Although the mechanical properties of the carbon fiber were decreased by the high temperature environment of the coating,but the strength of the composite was slightly reduced at room temperature,the mechanical properties of the composite were 13%higher than the strength of the matrix after being held at 150°C for 10 minutes,and the maintenance rate of mechanical properties was90%.After holding at 200°C for 10 minutes,the mechanical properties of the composite were 9%higher than that of the matrix,and the mechanical property maintenance rate was 72%.To reduce the thermal impact during the preparation,titanium carbide coating was prepared on carbon fiber surface by molten salt method,and the coating temperature and coating time were optimized.With the addition of titanium carbide coating in SCF/AA7075,the tensile strength at room temperature decreases from 577 MPa to 526 MPa due to the increase of coating thickness.At 150°C,the high temperature tensile strength first increased to 499 MPa and then decreased to465 MPa.Due to the strong interfacial bonding force of titanium carbide coating,the fracture mode of the composite is mainly by pulled-out failure when coating for 15minutes.The fracture mode of the composite is mainly shear failure when the coating is done for 60 minutes.To avoid the damage of titanium carbide coating thickness on carbon fiber,the idea of in-situ generation of titanium carbide interface was proposed.The titanium dioxide coating was synthesized on the surface of carbon fiber by chemical vapor deposition method,and the discontinuous interface was formed by the reaction between it and the element Mg segregated at the interface during the preparation of the composite material.The interfacial components are titanium dioxide(Ti O₂),titanium carbide(Ti C)and magnesium oxide(Mg O).With the discontinuous interface design,the tensile strength and yield strength of the composites increased to 590 MPa and 535MPa at room temperature,respectively.After introducing the coating,the tensile retention of the composite is about 90%at 150°C.The fracture mode changed from shear failure to pulled-out failure by adding Ti O₂ coating.Further extending coating time can degrade the tensile strength of the composite,but has no effect on the yield property.