| Casting metallurgy method was used in this study to prepare 7075 alloys containing 0.22 % Sc(mass percent),7075 alloys containing 0.18% Ti and 7075 alloys containing 0.18% Ti and 11.2% Zn.The influence of Sc,Ti,and the content of Zn on the properties,especially the stress corrosion cracking of 7075 alloy were studied.In the end,the effect of retrogression and reaging treatment on the properties of 7075 alloy containing 0.18% Ti and 11.2% Zn was also investigated and the optimal heat treatment parameter obtained.Optical microscope(OM)and Scanning electron microscope(SEM)were used to observe the microstructures of the alloys with different compositions and heat treatments.The results showed that:(1)With the addition of 0.22% Sc,the grain size of 7075 alloy is refined effectively,and the grain boundary is cleaned.Compared with the conventional 7075 alloy,the hardness of 0.22%Sc7075 under T6,RRA,T73 and ORRA conditions is improved by 22.9%,29.5%,24.8% and 21.5% respectively,the tensile strength of the alloy is improved by 5.77%,6.32%,3.17% and 2.4%,the elongation is increased by 17.39%,20.59%,12.94% and 9.6% respectively,and the stress corrosion cracking susceptibility index is reduced by 30.28%,26.09%,33.87% and 4.41% respectively.(2)With the addition of 0.18% Ti,the grain size of 7075 alloy is refined in some degree,the grain boundary is cleaned and the grain boundary segregation is reduced.Compared with the conventional 7075 alloy,the hardness of 0.18%Ti7075 is also increased by 11.2%,19.6%,15.2% and 12.9% respectively,the tensile strength of the alloy under T6,RRA,T73 and ORRA conditions is improved by 2.3%,3.2%,1.4% and 3.2% respectively,the elongation is increased by 10.1%,18.9%,5.9% and 7.2% respectively and the stress corrosion cracking susceptibility index is reduced by 39.5%,55.2%,51.2% and 38.1% respectively.(3)With the Zn content increasing to 11.2%,the age-strengthening effect of MgZn2 is increased and the grain boundary of 7075 alloy is cleaned and the segregation in the matrix is reduced.Compared with the 0.18%Ti 7075 alloy,the hardness of high Zn content Ti containing 7075 alloy under T6,RRA,T73 and ORRA conditions is increased by 29.2%,17.2%,21.3% and 16.3% respectively,the tensile strength is improved by 9.4%,13.4%,7.0% and 6.2% respectively,the elongation is reduced by 15.8%,29.5%,6.7% and 1.1% respectively,while the stress corrosion cracking susceptibility index is increased by 111.4%,47.8%,42.9% and 54.1% respectively.(4)For 7075 alloy containing 11.2% Zn and 0.18% Ti,with the retrogression time increasing at 180?,the tensile strength and hardness of high Zn content Ti containing 7075 alloy reduced,but the stress corrosion cracking resistance improved.With the increase of retrogression time at 210?,the hardness reduced and the tensile strength increased to a peak value within short time and then reduced,and the stress corrosion cracking resistance improved.(5)Comparing all the results in the study we can see that when the retrogression temperature is at 180? and the retrogression time is for 1800 s,the strength of 0.18%Ti 7075 alloy containing 11.2% Zn reached 647 MPa,which is 13.51% higher than 7075 alloy with RRA treatment,and reached an excellent stress corrosion cracking susceptibility index of 8.34%,which is 70.53% lower than 7075 alloy with RRA treatment,and had an overall cost-effective performance. |
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