Effect Of Pricipitates And Its Distribution On Mechanical Properties Of High-Strength Aluminum Alloy

The effect of pricipitates and its distribution on mechanicalproperties,electrical conductivity, SCC resistance of high-strength aluminumalloy have been investigated by tension test, Vickers hardness and electricalconductivity measurement, DSC analysis, XRD, OM,SEM and TEMobservation. The major conclusions are drawn as follows:1. Effect of thermomechanical treatment on the age-hardeningcharacteristics of 2024 aluminum alloys has been investigated.The resultsreveal the ageing characteristics as follows: The aging response is acceleratedafter large cold deformation, and the peak strength is attained after aging for 40min; double aging peaks can be found in the age-hardening curves,and the firstpeak appears when aging for 40 mins corresponding peak tensile strength (σ_b)and elongation is up to 580 MPa and 9.2% respectively. The second peakappears when aging for 120 min, but the peak tensile strength (520MPa) islower than the first one; in early stage of aging (＜40min), elongation slightlyincreases from 8% with the prolonging of aging time of the alloy. After agingfor 60 mins, however the elongation remarkably decreases to≤5% and shows aplateau with the prolonging of aging time in the curve of elongation-ageing time.It is indicated that the high density of dislocation introduced by largedeformation accelerate the precipitation of G. P zones and the aging response ofthe alloy. The first aging peak is due to the formation of GP zones and thedeformation strengthening caused by the high density of dislocation. And thesecond peak presented in the aging curve attributes to the nucleation and growthof S'phase. The offset between dislocation density decrease and precipitation ofS'-phase finally results in the phenomenon of double aging peaks when aged at190℃. Moreover, it is suggested that the formation of PFZ and coarsenequilibrium phase accompanying with the precipitation of S' phase decrease theelongation.2. The effects of progressive solid solution on microstrueture andmechanical property of 7A04 aluminum alloy have been investigated. Results indicate that after solutionized at condition of 470℃/5min+485℃/9min andsubsequent aging process of 140℃/6h+150℃/1h,σ_b,σ_0.2 andδ₅ of 7A04 alloyreach 545MPa, 499MPa and 11.1% respectivly. Metallography observationindicates that the grain size after short-time progressive(470℃/5min+485℃/3,9min) solid solution is smaller than that aftersingle-stage solution (485℃/8,14min). SEM observation indicates that theprogressivly solutionized sample in tension test behaves as typical dimpledfracture. X-ray diffraction analysis suggests that after progressive solution at thecondition of 470℃/5min+485℃/9min MgZn₂ as dominated strengthening phaseis totally dissolved except a small amount of Al₂Cu particle. EDS analysispresents that the main undissolved phases are inclusions full of Fe, Cu and Coatoms. It is indicated that the supersaturation degree by progressivlysolutionized at 470℃/5min+485℃/9min solution reaches that by single-stepsolutionized at 500℃/20min, and obtains the highest mechanical property aftersubsequent aging, which is equal to that attained in traditional solution process.3. Effect of thermomechanical treatment on the age-hardeningcharacteristics of 7A04 aluminum alloys has been investigated.The results arerevealed as follows: the grains in 7A04 alloy after thermomechanical treatmentare refined to 11μm in size, but keep as great as 30μm in diameter whenprocessed by conventional treatment. After thermomechanical treatment theelongation to failure is improved and reached upto 25.7%. When solutionizedand concurrently recrystallized at 470℃for 5mins plus 480℃for 12mins the7A04 aluminum alloy presents the optimum combination of tensilestrength(585MPa) and elongation(17.6%), because the overaged second phaseparticle is sufficiently dissolved into matrix and at the same time the excessivegrowth of recrystallized grains is avoided.4. The effect of cold deformation after solution treatment on theprecipitation characteristic and deformation strengthening of the 2024 and 7A04aluminum alloys has been investigated. The tension tests indicate that the ageingresponse of the 2024 aluminum alloy is accelerated when treated by colddeformation after solution treatment, and the tensile strength increase about 140MPa retaining 8% elongation. However, compared with 2024 alloy, althoughthe aging response of the cold deformed 7A04 aluminum alloy is accelerated,the tensile strength has not obviously increased and the elongation is evendecreased drastically. The results of TEM observation show that the S' phaseinside the dislocation cells and at the boundaries of the dislocation cells of the2024 aluminum alloy has an uniform distribution. But in 7A04 aluminum alloyhas formed club-shapedη'phase at the boundaries of dislocation cells even ondislocation lines, while still small spheric GP zone in the region with lessdislocation. In addition the precipitates in samples of 7A04 alloy with coldrolling present more obvious tendency of growth and coarsening than thatwithout cold rolling. It is indicated that the coherent strain energy of the discformed GPâ… zones and cylinder formed GPâ…¡zones with matrix in the 2024aluminum alloy is small, causing a relative small tendency of non-uniformnucleation of the GPâ… zones and GPâ…¡zones on dislocation line and theboundaries of dislocation cell. Therefore, after large cold deformation, theprecipitates in the alloy are homogeneously distributed.And the alloy canacquire high strength due to combination of deformation strengthening andprecipitation hardening. Whereas, the coherent strain energy of the sphericformed GPâ… zones with matrix in the 7A04 aluminum alloy is so high that thetendency of non-uniform nucleation of the GPâ… zones on the boundaries ofdislocation cells and dislocation lines becomes great, for which a non-uniformgrowth and distribution of precipitates are created. As a result, there is noobviouse improvement of the tensile strength of the 7A04 alloy whenundergoing large cold deformation after solid solution treatment.5. The effect of heating method during retrogression on RRA (retrogressionand re-ageing) of extrusion bar with large cross section of 7A04 alloy isinvestigated. Experimental results show that a tensile strength upto 620-640MPafrom central part to surface layer is exhibited for extrusion bar ofΦ60mm indiameter size retrogressed for 4min at 200℃by electric current heating,550MPatensile strength for that retrogressed for 10min by electric current heating and575MPa tensile strength and unhomogeneous tensile elongation for that retrogressed by panel heating. The measurement results of resistance toSCC(stress corrosion cracking) indicate that both sample retrogressed for 4minand 10min by electric current heating stand for 30 day without fractured under aload stress of 375MPa in 3%NaCl+0.5%H₂O₂ corrosion solution,whereas thesample retrogressed by panel heating only stands for 19 day under samecondition.It is suggested that the extrusion bar in diameter size uptoΦ60mm of7A04 alloy retrogressed for 4min by electric current heating not only has a hightensile strength close to T6 temper condition, but also shows an excellentresistance to SCC due to having more dispersed precipitates of grain interior andmore precipitatedη'phase, and coarser precipitates with lager interspace at grainboundary compared to 10min retrogression by electric current heating and panelheating.6. The effect of retrogression time on the microstructrue and mechanicalproperties in RRA treatment of Al-9.99%Zn-2.5%Mg-1.72%Cu-0.13%Zr alloyhas been investigated. The results of mechanical property test show that whenthe sample is pre-aged at 100℃for 24h and retrogressed at 200℃for 7min andre-aged at 100℃for 24h, the tensile properties of the alloy can reach up toσ_b=795MPa;σ_0.2=767MPa;δ₅=9.1%, and the fracture time in the test ofresistance to stress corrosion is longer than 720h under the condition of 210MPaload stress and 3.0%NaCl+0.5%H₂O₂ standard corrosion environment. Theresults of TEM observation show that when the sample undergoes RRAtreatment of 100℃/24h+200℃/7min+100℃/24h the precipitate particle ofinterior of grain is the most dispersed,which is constituted by a large amount ofGP zone particle and a small amount ofη' particle, and the microstructure atgrain boundary is composed ofηequilibrium phase with large size in diameterand interspacing, and dispersed particle precipitated in re-ageing. It is suggestedthatâ‘ the dispersed particles of interior of grain and grain boundary ensuresuper-high strength at room temperature when being RRA treated by100℃/24h+200℃/7min+100℃/24h;â‘¡the resistance to stress corrosiondepands on the effect preventing H atom from migrating to grain boundary bygreatly reducing the number of dislocation near grain boundary in RRA treatment, and the un-reversible trap function for large size and interspacingparticles at grain boundary to catch H atom and release H molecules.7. The evolution of second phases and its effect on tensile mechanicalproperty in artificial ageing and RRA process of super-high strength aluminumalloy is investigated.The result of tensile test shows that the samples ofAl-Zn-Mg-Cu-Ag alloy aged at lower temperature(100℃) not only have highertensile strength up to 753MPa but also relatively higher tensile elongation above9% than normal temperature(120℃) in artificial ageing. The sample ofAl-Zn-Mg-Cu alloy has the highest tension strength upto 788MPa when aged at100℃for 48 hours in single step of artificial ageing. Further more in RRAprocess samples of Al-Zn-Mg-Cu alloy preaged at 100℃for 24 hoursretrogressed at 200℃for 7min and reaged at 100℃for 24 hours present the besttensile strength of 795MPa than others. The tensile strength of Al-Zn-Mg-Cualloy after RRA treated decreases with prolonging of retrogression time andreageing time starting from 7mins and 24 houres respectively. SEM observationshows that crack of the samples in tensile test is created at large particles in thefracture while there are more particles of undissolved phase in Al-Zn-Mg-Cualloy containing Ag. TEM observation shows that the dominant strengtheningparticle corresponding to the peak strength of Al-Zn-Mg-Cu alloy containing Agwhen aged at 120℃for 8 hours isη' phase while dominant strengtheningparticle is GP zone when aged at 100℃for 80 hours. However,η' phase as thedominant strengthening particles corresponds to the peak strength ofAl-Zn-Mg-Cu alloy without content of Ag when aged at 100℃for 48hours.TEM observation also shows that GP zone as strengthening particle is dominantin the samples of Al-Zn-Mg-Cu alloy reaged at 100℃for 24 hours, andstrengthening particles is coarsened when the sample is retrogressed at 200℃and reaged for a longer time. It is suggested that whether at the presence ofcoarse particles of undissolved phases or when GP zone andη'particle grow upin the retrogression,sample needs deformable GP zone instead of undeformableη' in subsequent artificial ageing and reageing as dominant strengtheningparticle, in order to present a larger freedom spacing for dislocation to slip and let the sample not to behave too brittle to display high resistance to imposedplastic deformation or high tensile strength.8. The results of tensile strength test indicate that after pre-aging at 100℃or 120℃for 24 houres and retrogressing at 200℃for various time and reagingtreatment, the strength of studied alloys are greater than that of pre-agingcondition, some of them even exceed the peak strength of single-step artificialaging(T6),reach up to 795MPa. And the alloys exhibit a desired combination oftensile strength and elongation after RRA(retrogression and re-aging) treatment.TEM and EDS analyses show that the PFZ(precipitates free zones) formedduring retrogressing in short period become narrowed and even disappearedafter reaging treatment. The corresponding microstructure of second phase ingrain interior has a uniform dispersoid distribution. All of the alloys in T6temper have obvious characteristic of PFZ, in spite of uniform dispersoid ofsecond phase in grain interior. It is suggested that the behavior exceedingpeak-aging strength in the RRA condition ascribes to narrowing and evendisappearing of PFZ, which reinforces the grain-broundary, while theprecipitates in the matrix still maintain a more dispersed distribution, resultingin a higher effect of precipitation strengthening.9. The relation of the microstructure, electrical conductivity with stresscorrosion resistance of three kind of Al-Zn-Mg-Cu alloy treated by retrogressionand reageing has been investigated. Experimental results indicated that thesamples of the alloys having greater electrical conductility more than or equal to35.46%IACS displayed excellent resistance to stress corrosion, correspondingmicrostructural distribution was large size and separateηparticles on grainboundary, and dispersoid coherent precipitated particles in grain interiorpresenting higher tensile strength, whereas when having lower than or equal to34.63%IACS the samples presented lower resistance to stress corrosion despitehigher tensile strength. The corresponding microstructure was dispersoid andcoherent GP zone andη' phase particles in grain interior, small size andcatenulateηphase particles on grain boundary. It is suggested that comparedwith T73 etc. overageing tempered condition that shows high electrical conductivity upto 38-42%IACS, dispersoid and coherent particals in graininterior of the alloys treated by retrogression and reageing result in lowerelectrical conductivity but high tensile strength, while excellent resistance tostress corrosion is still obtained due to separate precipitates on grain boundarywith large size in dimension. Consequently a moderate electrical conductivityranging from 35.46 to 36.82%IACS, high resistance to stress corrosion andtensile strength is characterized.