Effect Of Solution Treatment And Aging Treatment On Microstructure And Properties Of Al-Zn-Mg-Cu Alloy For Oil Drilling

Compared with traditional steel drill pipes,aluminum alloy drill pipes have the advantages of low density,high specific strength and strong corrosion resistance.Among them,the high-strength Al-Zn-Mg-Cu alloy have great potential for application.In this paper,based on the special conditions of aluminum alloy oil drill pipe(Such as high temperature,Cl-solution and H2S,CO2 atmosphere,etc.),the Al-Zn-Mg-Cu alloy with three different compositions of low Cu,high Cu,and high Cu plus Er were studied.The effect of solution treatment and aging treatment on the microstructure and properties of the alloys were systematically studied,and the preferred composition of the alloy and the solution and aging process were determined.After the three alloys are deformed by extrusion,the microstructures of the alloys are squashed and elongated in the direction of extrusion and are streamlined.The second phases are the fine dispersed AIZnMgCu quaternary phases and the coarse Fe-rich phases,and high Cu plus Er alloy also have coarse Er-containing phases.The number of AIZnMgCu quaternary phases in high Cu alloy and high Cu plus Er alloy are higher than that of low Cu alloy.The optimized solution treatment is 470?/(1?1.5)h.After solution treatment,the AlZnMgCu quaternary phases are fully dissolved,the residual phases are coarse Fe-rich phases and a small amount of Er-containing phases,and the overall performance are good.After aging treatment of the three alloys,the precipitating strengthening phases are mainly GP ? and ?' phases.When in the single-stage aging,the size of the precipitated phases in the grain are small,and the precipitated phases in the grain boundary are semi-continuously distributed.The strength are high,and the exfoliation corrosion are serious.The size of the precipitated phases in the grain increases after heat exposure,and the yield strength decreases by 8?14%.When in the two-stage aging,the size of the precipitated phases in the grain increases,and the precipitated phases in the grain boundary increases and are discontinuously distributed.The strength are greatly reduced,the exfoliation corrosion resistance are improved,and the yield strength retention after heat exposure are slightly increased.When in the three-stage aging,the size of the precipitated phases in the grain are small,and the precipitated phases in the grain boundary are discontinuously distributed.The strength are reduced less,the exfoliation corrosion resistance are improved,and the yield strength retention after heat exposure are similar to the single-stage aging.Under different aging conditions,the average corrosion rates of the alloy in H2S and CO2 environments are between 0.0016mm/a and 0.0026mm/a,which are much less than 0.12 mm/a.There only have a small amount of pitting around the coarse second phases.The H2S and CO2 corrosion resistance both are very good.After three-stage aging at 120?/24h+170?/1h+120?/24h,all three alloys show good overall performance.Among the three alloys,the low Cu alloy have good exfoliation corrosion resistance.And the exfoliation corrosion resistance of high Cu alloy and high Cu plus Er alloy have declined;however,the intensity level are higher,and the yield strength retention after heat exposure are greater.The performance of high Cu and Er alloy are similar to that of high Cu alloy,but the corrosion resistance to H2S and CO2 are slightly lower than that of high Cu alloy.Overall,high Cu alloy have better comprehensive performance.After 470?/1h solution treatment and 120?/24h+170?/1h+120?/24h aging treatment at high Cu alloy,the yield strength,tensile strength,elongation,and hardness are 650 MPa,686 MPa,12.0%,and 192 HV,respectively.The yield strength retention after heat exposure is 92%.The exfoliation corrosion level is EB.The average corrosion rate in H2S and CO2 environments is 0.0024mm/a.And it is insensitive to H2S and CO2 environments.