Study On The Improvement Of Microstructure And Properties Of Lightweight High Strength Aluminum Alloy Drill Pipe

The insufficient mechanical properties of conventional steel drill pipes pose a challenge to the deep and ultra-deep oil and gas wells industry because of the high density of steel.Traditional drilling pipes,such as G105 and S135 steel drill pipes,suffer from their large weight and non-corrosion resistance and no longer fulfill the need of safe and high efficient drilling,and cause significant amounts of drilling tool failure accidents occasionally,thus aluminum alloy drill pipe outstands due to its low weight and high strength and is going to be an alternative.In this dissertation,in order to fulfill the urgent need of deep and extra-deep wells in oil and gas exploitation and scientific drilling,reinforcements of particles on 7000 series Al matrix composites were investigated based on several outstanding properties of ceramics and intermetallic compounds,such as high melting temperature,corrosion resistance and wear resistance.The objective of this study was to modify the performance of regular Al alloy drill pipe materials and extend its application.Through investigating the strength and the creep behavior of Al alloy matrix under high temperature,as well as the effect of hot deformation on mechanical properties and microstructure,theoretical basis was established to determine the reasonable deformation processing temperature of Al alloy matrix and corresponding composites.Grain refinement was achieved via adding the modifier Sr,and it improved the mechanical properties of Al alloy matrix after extrusion.Reinforcements of particles on mechanical properties of Al alloy matrix were further investigated through in-situ forming Ti B₂ ceramic reinforcements with low density,high specific modulus and wear resistance.The results showed that through controlling the morphology of Ti B₂ in melt,a fined microstructure of Al alloy matrix and a further improvement of mechanical properties at both room and high temperature were achieved.On the other hand,the results also proved the effect of Ti B₂ on improving the wear resistance of 7075 Al.Through complex modification of intermetallic compound Mg₂ Si in aluminum alloys and controlling the morphology and size of Mg₂ Si during consolidation,the stress concentration between the second phase and matrix was decreased and a further improvement in mechanical properties of composites was achieved under room temperature and high temperature.Lastly,laser melting was adapted to modify the surface of Al alloy drill pipes.This technology was demonstrated to effectively alleviate the wearing between aluminum alloy drill pipes and stratum and extend the service life of drill pipes.This study mainly concentrated on the mechanism of grain refinement improving mechanical properties of aluminum alloy,as well as reinforcements of aluminum matrix composites?AMCs?on mechanical properties and wear resistance.It provided the theoretical basis for fabricating aluminum alloy drill pipes with low weight and high strength.The conclusions of this study are as follows:1)Mechanical properties of 7075 Al were investigated via high temperature tensile tests and creep tests,and the mechanism of high temperature softening was analyzed based on microstructures.Different rolling temperatures were used to determine the temperature range of dynamic recrystallization and to provide the theoretical basis of choosing proper hot deformation temperatures for 7075 Al and corresponding composites.The results showed that the 7075 Al slab fabricated by rolling under 400 oC possessed the ultimate tensile strength?570MPa?and elongation?16.9%?simultaneously due to its fined microstructures and dynamic recrystallization.2)Mechanical properties and microstructures of 7075 Al were improved through adding modifier Sr.The contents of Sr were optimized.The results showed that after adding 0.1 wt.% Sr,the average grain size of casting 7075 Al was decreased from 157 mm to 105 mm,and the grain and residual second phases were also fined after extrusion.At this content,the tensile strength,elongation and microhardness were improved to 598 MPa,24.9% and 195 Hv respectively.The ductility was also improved through the analysis of fracture morphology.3)Particle reinforced 7075 Al matrix composites were fabricated through in-situ synthesizing Ti B₂ using molten salt method.The microstructures,mechanical properties and wear resistance were all improved.The results showed that with 1 wt.% Ti B₂ a finer microstructure was achieved.The tensile strength and friction coefficient were improved due to the uniform dispersion of Ti B₂ in matrix.At the same time,the friction coefficient and mass wear rate are reduced.The results also revealed the growth regulation of six prism Ti B₂ from outside to inside in the melt.4)Effect of complex modifying the intermetallic compound Mg₂ Si on mechanical properties and microstructures were investigated.Through adding 0.2 wt.% complex modifier Sr–Sb,the morphologies of primary Mg₂ Si transformed from equiaxed-dendrite to cube and eutectic Mg₂ Si changed from Chinese script to short rod.The average size of second phases also decreased from 50 mm to 20 mm,the tensile strength of the as-fabricated AMCs was improved by 26% and 17.8% respectively.