| 7B04 aluminum allo y has important applicat ion value in the aerospace industry,military equipment and other fields owing to its high specific strength,toughness,excellent welding propert ies.However,it is sensit ive to stress corrosion cracking（SCC）.Therefore,it is vital to improve the SCC resistance of 7xxx series aluminum allo ys and thus optimize their comprehensive performance in order to expand their applicat ions.Moreover,the retrogressio n and re-aging（RRA）heat treat ment has attracted more attent ion as a method that can improve the comprehensive properties of allo ys in recent years.However,almost no study has been reported on the effect o f pre-aging in RRA on SCC of 7B04 allo y.In this paper,the effects of external factors,i.e.strain rate,eviromental temperature and corrosion medium,on the SCC resistance of 7B04 aluminum allo y sheet（O temper）are investigated by slow strain rate tensile test.The sheet is then subjected to solut ion and aging treat ment.The characterist ics of the precipitates are adjusted by changing the process parameters of pre-aging and retrogressio n in RRA.The relat ionships between SCC&electrochemical behaviors,mechanical propert ies and microstructure of the allo ys in RRA state are studied and the specific act io n mechanism is clarified.The main research results of the thesis are as follows:（1）The effect of external factors on the SCC resistance of 7B04 aluminum allo y was clarified.The SCC suscept ibilit y of 7B04 aluminum allo y sheet（O temper）in3.5%Na Cl at 25°C increases firstly and then decreases wit h the increasing strain rate in the strain rate range from 3×10-7s-1 to 3×10-6s-1.The highest SCC suscept ibilit y o f the allo y is achieved wit h the strain rate of 6.67×10-7s-1.The SCC suscept ibilit y o f the allo y at 25°C with the strain rate of 1×10-6s-1 increases with the higher content of H2O2（0-0.5%）in 3.5%Na Cl.The SCC suscept ibilit y of the allo y also increases wit h the higher eviromental temperature（25°C-40°C）.However,the suscept ibilit y index of the allo y in 3.5%Na Cl wit h the strain rate of 1×10-6s-1 is slight ly higher at 40°C than25°C.The appropriate SSRT parameters are obtained,with the strain rate,corrosio n medium and eviromental temperature are 1×10-6 s-1,3.5%Na Cl and 25°C,respectively.（2）The effect of pre-aging temperature（60°C-120°C）on the corrosio n behavior and mechanical properties of 7B04 aluminum allo y was studied.The SCC resistance of the RRA-treated 7B04 aluminum allo y gradually decreases as the pre-aging temperature increases.The highest SCC suscept ibilit y of the allo y is achieved wit h the pre-aging temperature of 120℃and the ISSRT value is 0.048.And the thick phase within the allo y also increases,result ing in a decrease in the electrochemical corrosio n performance of the allo y.The strength of RRA-treated 7B04 allo y shows a mo notonic increasing trend as the pre-aging temperature increases and is lower than the peak aging state（594 MPa）.The allo y treated with the pre-aging temperature of120°C shows the best combinat ion of mechanical propert ies,with the tensile strength,the yield strength and the elongat ion of 584MPa 512MPa and 13.9%,respect ively.Alt hough the MPt is coarsened with the increasing pre-aging temperature,the volume fraction increases significant ly,result ing in the enhanced strength.（3）The effect of pre-aging t ime（12h-24h）on the corrosion behavior and mechanical propert ies of 7B04 aluminum allo y was studied.The SCC suscept ibilit y o f the allo y decreases first ly and then increases with the prolonged pre-aging t ime.The lowest SCC suscept ibilit y of the allo y is achieved with the RRA treat ment o f120℃/20h+200℃/15min+120℃/24h and the ISSRT value is 0.020.Compared with the pre-aging treat ment for 24h,although the size of the GBPs is slight ly smaller that at20h,the PFZ is narrower（55.3nm）and the Cu content of the GBPs is also higher（6.4at.%）,the SCC suscept ibilit y is higher at 24h.The electrochemical corrosio n performance of the allo y also decreases with the prolonged pre-aging t ime.The strength of RRA-treated 7B04 allo y increases and then decreases with the increasing pre-aging t ime.The highest strength is obtained at 20h,with the tensile strength,the yield strength and the elo ngat ion of 618MPa,554MPa and 12.0%,respectively.As compared with the T6 temper,the tensile strength increases by 4.2%.（4）The effect of retrogression t ime（5min-25min）on the mechanical properties of 7B04 aluminum allo y was studied.Wit h the increase of the retrogressio n t ime,the strength of the allo y increases first ly and then decreases.Wit h the retrogressio n t ime of 15min,the allo y shows the best combinat ion of mechanical propert ies.The opt imized RRA treat ment is 120℃/20h+200℃/15min+120℃/24h,the RRA-treated allo y exhibits the tensile strength of 618MPa,the yield strength of 554MPa,the elongat ion of 12.0%and the ISSRT value is 0.020. |
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