| Al-Zn-Mg-Cu series alloys have been extensively used as the main structural material in the aviation industry.The development of Al-Zn-Mg-Cu alloys exhibits a trend to ultra-strength,high toughness and corrosion resistance alloys,high strength,high toughness and high hardenability alloys,and large-sized high-performance materials with low residual stress.In order to break the bottlenecks of preparation technology of large size high-performance aluminum alloy used in aerospace and meet the technical requirements of the fourth and fifth generation aluminum alloy,the effects of composition optimization via thermodynamic calculation and multistage aging on microstructure and performances of Al-Zn-Mg-Cu alloy were studied to solve the problem between strength and corrosion resistance.The influence of alloying elements on the equilibrium concentration and content of precipitates and matrix of 7050 alloy was analyzed using software package JMat Pro for material property simulation of Al-base alloys.Within the range of aging temperature,the amount of equilibrium ? phase increased with the increase of Zn contents and decreased with the increase of Cu contents.The weight-percent of Al,Cu and Mg for ? phase decreased with the increase of Zn and Cu contents,whereas the weight-percent of Zn for ? phase increased with the increase of Zn and Cu contents.Instead,the weight-percent of Al,Cu and Mg for ? phase increased with the increase of Mg contents,whereas the weight-percent of Zn for ? phase decreased with the increase of Mg contents.The solidification paths of different component alloys revealed that as the Zn and Mg contents increased,the amount of Mg Zn2 phase increased while the amount of Al2 Cu Mg phase decreased.High Cu content will induce high fraction of Al2 Cu Mg phase and had an adverse influence on Mg Zn2 phase.The 7050 alloys with contents of Zn,Mg and Cu in the ranges of 6.4%~6.7%,2.4%~2.6% and 1.9%~2.2% will obtain more than 4.5% ? phase and less than 0.5% S phase.Compared with the thermodynamic calculations of 5.85Zn-2.28Mg-2.13 Cu alloy and 6.42Zn-2.2Mg-2.17 Cu alloy,the increase of Zn content induced that the amount of Al2 Cu Mg phase decreased from 0.529% to 0.371%.Further,the amount of equilibrium Mg Zn2 phase increased from 8.37% to 9.15%,which resulted the increase of yield strength and tensile strength under T6 aging from 508 MPa up to 527 MPa and 581 MPa up to 604 MPa,respectively.The effects of pre-aging on microstructure and properties of the RRA treated 7050 has been investigated.The results revealed that the electronic conductivity and hardness of RRA-treated samples are sensitive to the pre-aging tempers.For under-aged specimens,the retrogression hardness appeared a stage of hardness increasing followed by a further decrease in hardness results.The stage of rapid dissolution of the fine precipitates resulting in a decrease of retrogression hardness vanishes.However,during retrogression the peak aged specimens' hardness exhibit a fall to a minimum following by a increasing again to a secondary peak,while a further decrease in hardness results from continued treatment.With prolonging retrogression times,the electronic conductivity increasing monotonously.The RRA-treated samples with 100?/24 h and 120?/2h pre-aging can be tailored towards a good combination of strength and the electrical conductivity higher than that of 120?/24 h pre-aging.The tensile and yield strength increased 3.8%,3.0% and 4.1%,3.2%,respectively.The multistage aging process involving 120 ? /24 h pre-aging,40 ? /h heating process from 120 to 200? and 120? /24 h re-ageing treatment was capable of enduing high strength equivalent to the T6 temper,together with the RRA corrosion resistance levels.The tensile and yield strength were 586.9MPa and563.7MPa,respectively.The strengthening mechanism was associated with ?' phase and ? phase,meanwhile the coarsening grain boundary precipitates was the main reason for the increase of corrosion resistance.The influence of repetitious non-isothermal aging(RNIA)temper on microstructure and strength of Al-Zn-Mg-Cu alloy was investigated.The results suggested that the variation of hardness and strength was associated with heating and cooling rate.While the heating and cooling rate was 40? /h,with the increase of repetitious times the growth and coarsening of intragranular precipitates took place and the hardness and strength deceased.After 2 cycles the strength still remained at the level of the peak strength.While the heating and cooling rate was as quick as 80?/h,with the increase of repetitious times the intragranular precipitates exhibited non-obvious coarsening accompanied by the occurrence of secondary precipitation and the strength remained at the level of the peak strength after 3 cycles.With the increase of repetitious times,the electrical conductivity exhibited a monotonous increase and the maximum depth of intergranular corrosion decreased.The coarsening of grain boundary precipitates and the increase of the solute concentration in the precipitates were the main reason for the improvement of corrosion resistance.The 2NIA40 and 3NIA80 tempers can acquire satisfactory corrosion resistance with the strength equivalent to T6 prak aging temper. |
PDF Full Text Request