Microstructures And Properties Of High-Strength,Heat And Corrosion-Resistant Al-7.0Zn-2.4Mg-1.7Cu Alloy Extruded Pipe With Variable Cross-Section For Oil Drilling

Al-Zn-Mg-Cu alloy is considered as a potential ideal material for oil drilling due to its high specific strength,good fatigue and acid（H₂S and CO₂）corrosion resistance.With the gradual development of oil drilling to deep well,the harsh service environment of deep well drilling requires higher on the strength,thermal stability and corrosion resistance of material for oil drilling.In present work,based on the analysis of the microstructure of Al-7.0Zn-2.4Mg-1.7Cu alloy by electron backscattering diffraction（EBSD）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,X-ray diffractometer（XRD）and finite element numerical simulation,the formation and evolution laws of grain structure,second phase during hot deformation,heat treatment and friction stir welding（FSW）,and their effects on mechanical properties,corrosion resistance and welding properties of alloy are systematically studied.The parameters for hot extrusion,pre-stretching,aging treatment and welding of Al-Zn-Mg-Cu alloy extruded pipe with variable cross-section with high strength,high thermal stability,excellent corrosion resistance and welding performance have been determined.The main conclusions of this thesis can be drawn as follows:（1）The hot deformation behavior of Al-Zn-Mg-Cu alloy during hot compression is studied.A high precision flow stress constitutive model considering strain compensation and the hot processing map of studied alloy are developed,and the optimal hot processing parameters of studied alloy are determined.The results show that both dynamic recovery（DRV）and continuous dynamic recrystallization（CDRX）occur during hot deformation,but DRV is the main dynamic softening mechanism.The optimal hot processing parameters at a strain of 0.6 are at hot deformation temperature range of 390～440℃and strain rate range of 0.01～0.0316 s^-1.（2）Based on the flow stress constitutive model of Al-Zn-Mg-Cu alloy and finite element numerical simulation,the optimum hot extrusion parameters of the pipe with variable cross-section are determined,and the influence of deformation heat on microstructure and hardness value during hot extrusion is revealed.The relationship between hardness value and strain at different positions of the hot extruded pipe is established.The results show that hardness of the pipe with variable cross-section decreases from 114.6 HV to 94.6 HV according to the exponential law with strain from 3.7 to 8.6 after hot extrusion with the optimal parameters（billet temperature of 410℃,tooling temperature of 390℃,and ram speed of 1.5mm/s）.This is attributed to the existence of different deformation heat leads to the increase of DRX fraction,the decrease of average grain size and dislocation density,and the increase in grain boundary strengthening is far from compensating for the decrease in dislocation strengthening.（3）The effect of strain gradient on the corrosion resistance of T6-aged hot extruded Al-Zn-Mg-Cu alloy pipe with variable cross-section was studied.The quantitative relationship between the maximum intergranular corrosion and the strain at various positions of hot extruded pipe is established.The results show that the maximum intergranular corrosion depth of T6-aged pipe with variable cross-section increases exponentially from 55.6μm to 98.5μm with the strain from 3.7 to 8.6.This is because the increase in strain leads to an increase in distortion storage energy and number density of Al₇Cu₂Fe phase,which promotes particle stimulated nucleation recrystallization and proportion of high-angle grain boundaries（HAGBs）.More HAGBs and Al₇Cu₂Fe phases can accelerate the galvanic corrosion rate between grain boundary,Al₇Cu₂Fe phase and aluminum matrix.（4）The mechanical properties and thermal stability of Al-Zn-Mg-Cu alloy extruded pipe with different pre-stretching deformation after aging at different conditions are studied in comparison.The mechanism of dislocation introduced by pre-stretching on precipitation behavior is revealed.The results show that with the increase of pre-stretching deformation,dislocation density increases,diffusion rate of solute atoms increases,aging precipitation kinetics increases,which is conducive to the coarsening of precipitates and the reduction of alloy strength.However,larger average size of precipitate consumes more solute atoms and vacancies,which reduces the coarsening rate of precipitate during thermal exposure and improves the thermal stability at 120℃.The level of alloy subjected to pre-stretching should be controlled at about 2%.（5）The effect of different regression re-aging（RRA）treatment conditions on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloy extruded pipe is studied.The results show that the precipitate in matrix of alloy coarsen and the strength of the alloy decreases with the increasing regression temperature or regression time.Meanwhile,the precipitate at the grain boundary coarsens,which reduces the concentration of solute atoms near the grain boundary,promotes the broadening of precipitation free zone（PFZ）,and improves the elongation of alloy.The optimum parameters of RRA are determined as:100℃/24 h+170℃/4h+water quench+100℃/24 h.Aged with this RRA condition,the tensile strength,yield strength and elongation at room temperature of the alloy pipe is 684.3 MPa,660.5 MPa and 10.6%,respectively.（6）The effect of post weld aging heat treatment on the microstructure and mechanical properties of FSW joint of Al-Zn-Mg-Cu alloy is studied.It is found that the tensile strength at room temperature and welding coefficient of T6-aged joint increase to 491.4 MPa and 91.8%,respectively.This is due to the rapid cooling of heat-affected zone,which makes it present a nearly saturated solid solution,and retains a high concentration of vacancies,prompting the dense of precipitation of GP zone andη’phase during the subsequent post weld aging heat treatment,increasing the strength at room temperature of the joint.