Investigation On Thermomechanical Treatment Of Al-Zn-Mg-Cu Alloy

A1-Zn-Mg-Cu high-strength a luminum a llo ys whic h are a very cruc ia l k ind o flight and high-strength structura l materia ls, have a wide application in the fie ld ofaerospace and aspect of transportatio ns or vehic les. It is very va luable to researchhigh-strength, high-toughness and corrosion resista nce aluminum allo ys as the ma indeve lopme nt direction in the future. Not only the high-strength with goodcooperation to high-toughness allo ys can be obtained, but a lso the productionprocess of compone nts can be greatly s imp lified which bringing about cons iderab leecono mic bene fit by us ing thermo mecha nica l treatment. The thesis stud ies andoptimizes the fina l thermo mecha nical treatment (FTMT) whic h is the most benefic ia lto improve strength, meanwhile investigates the effect of high-te mperaturethermomec hanica l treatme nt (HTMT) on microstructures and properties of allo ys.The ma in difference between two processes is whether it has gone throughhigh-temperature deformation or not before preageing and warm deformation. Thepaper also discusses the influences o f allo ys with d ifferent origina l orga nizations onmicrostructures and properties of materia ls in fina l states. It investigates the e ffectlaws of ma in a lloy e le ments and the influence degree of precip itated phases andsub-structure on strength of a lloys.The results o f the stud y show that: The optimum process cond ition is30%deformatio n on temperature460℃in high-temperature deformation, whic h mak ingthe alloy ma inta in a certain a mount of dis location configuratio n. Then the optimumcond ition for the stage of warm deformation is30%deformation on te mperature120℃, after which it e xists the further increas ing o f the d is locatio n configuratio n inalloys meanwhile it can extreme ly avo id no nho moge ne ity o f precip itated phases.The gra ins ha ve a stretching morpho logy, present reverting characteristics and existsub-structure organization a fter HTMT. Due to dis locatio n promoting the nuc leationof precip itated phases, the time of pre-ageing(8h) and fina l-age ing(9h) in HTMT areboth shorter than those(12h,10h respective ly) in FTMT. The strength of a lloy islower owing to poor distributio n uniformity o f precip itated phases and the p lastic ityis higher owing to increasing a mount o f sub-structure a fter HTM T comparing withthose after FTMT. In the same HTMT a nd FTMT cond itions, allo y Al-8.2Zn-2.05Mg-2.2Cu-0.1Mn-0.25Zr plates ha ve optima l mec hanica l properties by us ing vac uuminductive me lting a nd copper mould water-cooled techno logy in casting te mperature730℃. It exists uniform dispersion strengthening phase η’ of three a lloys(Al-8.2Zn- 2.05Mg(10Zn-2.5Mg,12Zn-3Mg)-2.2Cu-0.1Mn-0.25Zr) in T6treatment(120℃ageing24h), where precip itation strengthe ning plays a leading role. With theincreasing of ma in a lloy ele me nt Zn, Mg conte nt, the amo unt of precip itated phasesincreases gradua lly mak ing precip itatio n strengthening effect enhancedsimultaneo us ly. The tens ile stre ngth o f t hese alloys are646.2MPa,697.4MPa,732.5MPa respectively, while the elongation are13.0%,10.6%,7.1%respective ly.The application o f thermo mecha nica l treatme nts are more favorable to improve thestrength o f allo ys tha n adding so le ly main a lloy e le me nt contents. The a lloys on o nehand acquire maximum precip itation strengthening effect, on the other ha ndma inta in s ubgra ins stre ngthening effect in FTMT cond itio n, where the organizationis the most advantageous strengthe ning orga nization to improve its tens ile strength.The application o f HTMT can further increase the sub-structure stre ngthening e ffectwhile a ffect the d istributio n uniformity o f precip itated phases, although theorganization is not the best strengthe ning organizatio n but can keep a good plastic ity.Subgrains with uniform d ispersion strengthening phases distrib uting ins ide are themost favorable to improve the strength of a lloys, howe ver the nonho moge ne itydistributio n of precip itated phases hinders the impro veme nt of its strength. Thetens ile strength and the e longation of a lloy Al-8.2Zn-2.05Mg-2.2Cu-0.1Mn-0.25Zrare752.3MPa and6.7%after FTMT. The comprehens ive performa nces of thosealloys are better after HTMT, where the tens ile strength and the elo ngatio n are736.5MPa and9.0%.