Correlation Research Of Hardenability-composition And Strength-texture Based On The 2195 Al-Cu-Li Thick Plate

The development of the launch vehicle has put forward a high demand for lightweight of the rocket body,which has promoted the engineering application of the 2195 Al-Cu-Li alloy with lightweight and high-strength.Moreover,the lifting of the launch vehicle capacity requires the 2195 Al-Cu-Li alloy tank develop towards the larger size and thickness.The performance uniform of the 2195 Al-Cu-Li alloy thick section is restricted by the texture at different thickness positions and the hardenability.There are few study on the hardenability of 2195 Al-Cu-Li alloy.Although some research focus on the correlation between the strength and texture of 2195 Al-Cu-Li alloy thick section,its mechanism is still unclear.Therefore,this dissertation established the evaluation criterion of 2195 Al-Cu-Li alloy hardenability by using the strength varying.The quench sensitivity of 2195 Al-Cu-Li alloy was analyzed by using the TTP（time-temperature-property）and TTT（time-temperature-transformation）curves.The mechanism of hardenability and quenching sensitivity was analyzed through the evolution of microstructure.The influence of the Cu and Li content on hardenability as well as quench sensitivity of 2195 Al-Cu-Li alloy were based on this evaluation criterion.The extruded plate with higher volume fraction of deformation texture and cold-rolled sheet with lower volume fraction of deformation texture were prepared to simulate the difference of texture between the surface and the center of the Al-Cu-Li alloy thick plate.The synergistic mechanism of texture and precipitates on strength properties of Al-Cu-Li alloy was researched.The main research results are as follows.（1）A novel end quenching method by using cold-rolled thin sheets combined with strength varying after subsequent T8 aging was used to research the hardenability of 2195 Al-Cu-Li alloy with 4.25 wt%Cu and0.98 wt%Li content.With the increase of the distance away from the quenching end,the coarse Cu-enriched phase particles nucleated and grew both inside grain and at the grain boundaries during the end quenching treatment,which impeded the nucleation and growth of T₁ andθ′precipitates during the aging treatment.Moreover,the coarse Cu-enriched phase particles at grain boundaries caused the width of PFZ increased with the increase of the distance away from the quenching end.The above microstructure features hence leaded to the strength decrease.The quenching depth was described by the distance of T8 sample which yield strength was 5%lower than that of the quenching end.The quenching depth of the 2195 Al-Cu-Li alloy was estimated as 11.8 mm.（2）The TTP curves and TTT curves were measured and characterized through the interrupted quench technique,which were used to research the quench sensitivity of 2195 Al-Cu-Li alloy with 4.25 wt%Cu and 0.98 wt%Li content.The noise nose temperature of the TTP and TTT curves were both about 380°C,the time of the supersaturated solid solution start decomposes to form coarse phases was the shortest at this temperature.The critical time of the nose temperature in TTP curve was about 0.15 s and the critical temperature range above the nose temperature for the 2195 Al-Cu-Li alloys was about 256.6°C～474.2°C.（3）The influence of Cu and Li content on the hardenability of 2195Al-Cu-Li alloy were different.When the Li content was about 1.20 wt%,the quenching depth of the 2195 Al-Cu-Li alloy decreased from 18.4 mm to 13.5 mm with the Cu content increased from 3.72 wt%to 4.20 wt%.With the increase of the Cu content in 2195 Al-Cu-Li alloy,the number and size of the Cu-enriched coarse particles both inside grain and at the grain boundaries increased at the same location with a certain distance far away from the quenching end during end quenching process.The alloy with higher Cu content therefore had less nucleation driving force of T₁ andθ′precipitates and wider PFZ,which caused the larger strength reduction fraction.However,the Li content had less effect on the hardenability of 2195 Al-Cu-Li alloy,when the Cu content was similar.（4）The quenching sensitivity of 2195 alloy increased with the Cu content increased from 3.72 wt%to 4.20 wt%,when the Li content of 2195Al-Cu-Li alloy was about 1.20 wt%.The solid solubility of the supersaturated solid solution increased with the increase of the Cu content,the decompose of the supersaturated solid solution have the larger driving force.At the same holding temperature,the 2195 Al-Cu-Li alloy with higher Cu content had the shorter beginning time of supersaturated solid solution desolventize.With the increase of the Cu content,the critical times of the nose temperature decreased and the critical temperature range of TTP and TTT curves both increased.（5）For the as quenched alloy,the average Taylor factor（M value）caused by difference texture have a positive correlation with the yield strength.The M value of the 2195 Al-Cu-Li alloy and Al-Cu-Zr alloy extruded plate at as quenched state were higher than the cold-rolled sheet,the strength of the extruded plate was therefore higher than the cold-rolled sheet.By contrast,the M value of the as quenched Al-Li-Zr alloy extruded plate were lower than the as quenched cold-rolled sheet,the strength of the extruded plate was therefore lower than the cold-rolled sheet.（6）The synergistic strengthening effect of T₁ precipitate and texture was more significant thanθ″,θ′andδ′precipitate.The artificial aging state2195 Al-Cu-Li alloy extruded plate had higher density of T₁precipitate and M value than the cold-rolled sheet.Moreover,the calculation of strength effect of T₁precipitate needs to be combined with M value.The strength of the 2195 Al-Cu-Li alloy extruded plate was therefore more than 90.0 MPa higher than cold-rolled sheet at the same aging state.The texture has less improvement on theθ″,θ′andδ′precipitate.The density of aging precipitates between extruded plate and cold-rolled sheet of Al-Cu-Zr alloy or Al-Li-Zr alloy was similar.Therefore,the strength difference of their extruded plate and cold-rolled sheet at the same aging state was less than 30.0 MPa.