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International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of......

International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.

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  • 1. Aezeden Mohamed / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1116-11201116 | P a g eThe Effect Of Cooling Rate On Cyclic Stress Strain Response OfAl-2024Aezeden MohamedFaculty of Engineering and Applied Science, Memorial University, St. John’s, NL, A1B 3X5AbstractCyclic stress strain curves (CSSC)response of Al-2024 Annealed condition wasconducted under symmetric tension-compressionat room temperature and constant frequency,using a servo-hydraulic testing machine, to studythe effect of cooling rate on cyclic stress straincurves (CSSC) response. The fatigue response ofAl-2024 alloy was evaluated macroscopically interms of cyclic stress strain response andmicroscopically in terms of appearance ofprecipitations free zones.It was found that the cyclic stress strainresponse of air-cooled specimens group exhibiteda higher saturation stress with plastic strain andsmaller PFZ regions whereas the cyclic stressstrain response of furnace cooled specimensgroup exhibited a lower saturation stress andlarger PFZ region.Both types of the cooling rates wereshowed precipitates free zones (PFZ’s) wereobserved adjacent to the grain boundaries.However, (PFZ’s) were more pronounced infurnace cooled specimens group than that in air-cooled specimens group.Keywords: cyclic stress strain curve, plastic strainrate, precipitate free zone, air-cooled, furnacecooled.1. INTRODUCTIONThe important of fatigue study stemsdirectly from the need to investigate cyclic stressstrain response of metals and their alloys whensubjected to cyclic loading under controlled cyclicstrain. The special interest is to develop mechanismsfor explaining how microstructures can lead to theobserved deformation and fracture characteristics,and to acquire quantitative information to predictthe life of engineering components subjected to acyclic loading at which case the fatigue stress iscompletely dominated by cyclic plastic deformationthat causes irreversible changes in the materialmicrostructure.In the last two decades, many papers havebeen published, for example, Lee and Sanders [1, 2]conducted strain-controlled fatigue tests onpolycrystalline Al-2024 alloy and demonstrated thatthe alloy can either strain harden or soften,depending on heat treatment. They concluded thatthe nature of precipitates, not the level of monotonicyield strength, determined the character of cyclicresponse between Al-2024 and overaged Al-2024.They concluded that overaged Al-2024 cyclicallyhardened to much less extent as compared to Al-2024 aged at room temperature to develop G-Pzones and exhibit no plateau in both heat treatmentsat any plastic strain amplitudes.Although the CSSC for polycrystallinematerials is similar to that of single crystalmaterials, there is a controversy on whether theintermediate region exhibits plateau or quasi plateauor no plateau in polycrystalline materials. RecentlyLiu [3] showed that polycrystalline copper exhibiteda quasi-plateau region in CSSC instead of a plateau,which was observed by Kuokkala and Hirofumi [4,5] who studied cyclic polycrystalline pure copper.Test conditions such as stress or strain control,wave shape and frequency may affect the fatiguecharacteristics. The plateau-like behavior was morepronounced when the test was performed under lowstrain rate, while the cyclic stress strain response ofpolycrystals tested at constant high frequency showsless plateau behavior or even no plateau behavior[3]. Previous studies by Winter [6] have suggestedthat the frequency range from 0.05 to 5 Hz does notaffect cyclic deformation behavior.The objective of this paper is to investigateeffect of two different cooling rates on the cyclicstress strain response of Al-2024 alloy and addressthe evolution of microstructure during cyclicdeformation.2. MATERIAL AND EXPERIMENTALPROCEDUREThe material used for the current study is Al-2024 alloy rod stock was machined into cylindricalfatigue specimens with gauge length of 20 mmaccording to ASTM E466 specifications [7] asshown in Fig. 1. In order to study cooling rate effecton cyclic stress strain response of Al-2024 alloy.Two groups of specimens were carried out on fatiguespecimens. Both groups, the specimens wereannealed at 400C for 3 hours followed by air-cooledfor the first group and followed by furnace cooledfor the second group.
  • 2. Aezeden Mohamed / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1116-11201117 | P a g eFig. 1. Dimensions of fatigue test specimen in (mm)This type of heat treatment will producesoftness, allow diffusion processes to occur fast, torelieve stresses, and to produce a specificmicrostructure.Fatigue testing was performed on servo-hydraulic testing machine. Fatigue test were rununder a symmetrical unixial tension-compressionloading with total strain control. Triangularwaveform was employed for all the fatigue tests. Thetotal strain was measured using a dynamicextensometer, which was attached to the specimenwithin the gauge length. Fatigue test was carried outat constant frequency of 0.025 Hz for both groups ofspecimens were cyclically saturated and thensectioned longitudinally as well as cross sectionalong the gauge length, polished and etched insodium hydroxide (NaOH) solution for optical andelectron microscopy observations.3. RESULTS AND DISCUSSIONS3.1 Cyclic Stress–Strain BehaviorBoth groups, the air cooled specimens andfurnace cooled specimens were subjected to fatiguetest with plastic strain range from 1.81E-05 to3.68E-02 up to saturation with constant frequency0.025 Hz. The cyclic stress strain curves CSSC ofAl-2024 alloy is shown in Fig. 2.From the cyclic stress-strain curve, it wasfound that air-cooled specimens exhibited highersaturation stress at low plastic strain amplitudesranging from 1.81E-05 to 3.75E-03 than furnace-cooled specimens. The saturation stress of air-cooling specimens is approximately 125 MPawhereas; saturation stress of furnace coolingspecimens is approximately 50 MPa at the sameplastic strain amplitude of 1.81E-05. This differencein saturation stress between two cooling rateaccounts for the hardening behavior of air cooled ascompared to furnace cooled. This hardeningbehavior is due to the nature of the precipitate freezones formed adjacent to grain boundaries, whichare generally weak region in nature resulting in highinitial deformation stress.Deforming a specimen that has beensoftening by annealing heat treatment requires lessenergy as compared to a specimen that has beenstrengthened by cold worked. Heating allowsrecovery and recrystallization but is usually limitedto avoid excessive grain growth and oxidation. Thecurrent study, comparing different between slow tovery slow cooling rates which may lead to graingrowth.0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010050100150200250300350400450SaturationStress(MPa)Plastic Strain AmplitudeCSS Curves of Al-2024Furnace CooledAir CooledFig. 2. CSS Curves of Al-2024 of air and furnacecooled3.2 Microstructural AnalysisMicrostructural features that control theproperties of Al-2024 alloys processes, generallycoarsening, discontinuous coarsening, and formationof PFZ’s.Al-2024 alloys suffer a great deal from theformation of precipitate-free zones, partly becausethey are frequently strengthened using precipitationhardening, but also because they can contain highconcentrations of vacancies.Optical microscope, scanning and transmissionelectron microscopy (SEM) and (TEM) analysis offatigued specimens was conducted for observationsof PFZs and precipitates. Fig. 3 and Fig. 4 areoptical micrographs of air cooled and furnace cooledspecimens showed indication of precipitate freezones (PFZ’s) respectively.The presence of precipitate free zones(PFZs) adjacent to grain boundaries of both coolingrate are confirmed by the scanning electronmicroscopy (SEM) as shown in Fig. 5 for specimensair and furnace cooled. As grain boundaries maylower the strain energy of precipitates, nucleation ofthe stable non-coherent phases occurs mostly at thegrain boundaries.
  • 3. Aezeden Mohamed / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1116-11201118 | P a g eFig. 3 Optical micrograph of air cooled fatiguedspecimen showing PFZ (a) cross section and (b)longitudinal section.Fig. 4 Optical micrograph of furnace cooled fatiguedspecimen showing more pronounced PFZ (a) crosssection and (b) longitudinal section
  • 4. Aezeden Mohamed / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1116-11201119 | P a g eFig. 5 SEM micrograph (a) air cooled fatiguedspecimen showing PFZ, (b) furnace cooled fatiguedspecimen showing more pronounced PFZSuch nucleation is enhanced by a very slowcooling rate as in the case of furnace cooledspecimens, which leads to solute-depleted precipitatefree zones (PFZs) adjacent to the grain boundary.Since PFZ have lower strength than the strength ofmain grains, lead to a favourite site for crackinitiation, the presence of PFZ leads to high stressconcentrations at the grain boundaries, it becomemore obvious at triple point junctions.Microstructure can have a greater influence onfatigue properties of all Al-Cu alloys than the levelof tensile properties.It is often the case that precipitation doesnot occur uniformly throughout the microstructureduring the heat treatment of a supersaturated phase.The current study showed that the precipitatesrearranged themselves and become more uniformmore or less parallel direction as they become closermoving towards the grain boundaries. If further, anincrease in the number of cycles may initiate slipbands in regions where strain has become localizedand may occur at localized soft regions such asprecipitate-free zones (PFZs). It should be pointedout that the type of precipitates observed in thegrains are slightly bigger and arranged asWidmanstatten type were more pronounced infurnace cooled specimens than in air-cooledspecimens which mean that if further slow coolingwe may observe larger precipitations free zone at thegrains. Fig. 5 is an SEM micrograph of cross sectiontaken from gauge length show PFZ’s adjacent to thegrain boundaries.Fig. 6 Transmission electron microscopy (TEM)showing distinct elongated precipitates asWidmanstatten precipitates growing at the expenseof the smaller precipitates and were observed inarranged in two directions perpendicular to eachother in the middle of the grains as showed in Fig. 6(a) and as moving towards grain boundaries startedrearranged in to parallel directions as they becomecloser to the grain boundaries as showed in Fig.6(b). The mechanism behind the phenomena ishowever unclear.In addition, it was also found that cooling rateof the alloy from furnace cooling to air coolingshowed a rapid increase in the saturation stress atlow plastic strain amplitudes (100 MPa different)and increase rapidly at high plastic strainamplitudes (175 MPa different) The increase ofapproximately 25%. The softening matrix of bothcooling rates explains this increase in saturationstress during cyclic. As a result, the alloy exhibitedhardening behavior at all plastic strain amplitudes.The absence of slip bands maybe resulted from thetype of annealing heat treatment, low number ofcycles and from the precipitates type, which wereformed in two-direction morphology. Suchmorphology would suppress the easy glide of slipbands within the grain and avoid the formation ofslip bands. The rapid increase in the saturationstress at high plastic strain amplitudes accounts forthe activation of secondary slip systems resulting inthe formation of dislocation cell structures in highstaking fault energy of aluminum alloy leading tosecondary hardening in saturation stress in air-cooled specimens as compared to furnace cooledspecimens at high plastic strain amplitude is
  • 5. Aezeden Mohamed / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1116-11201120 | P a g eexplained by the fact that rate of cooling faster inair-cooling than in furnace cooling, leads to thedislocation density relatively increases, producingstrain hardening.It should be noted that the cyclic deformation of Al-2024 alloy obtained in this work is locallyinfluenced by the presence of precipitates, their typeand morphology and cannot be generalized for othermetals and their alloys.Fig 7. TEM micrograph showing elongatedprecipitates (a) precipitates in two directions (b)precipitates parallel to the grain boundary6. CONCLUSIONSCyclic stress strain response of Al-2024 has beenstudied in terms of cooling rate effect. The followingconclusions were obtained: Cyclic stress strain response of Al-2024 isdependent on cooling rate such that air-cooledspecimens exhibited higher saturation stress thanthat furnace cooled specimens. Cyclic stress-strain response of Al-2024 of air-cooled specimens exhibits higher saturation stressespecially at high plastic strain amplitudes. This ismaybe due to narrow zone of PFZ’s and hence theprecipitates in matrix are more and homogeneous.However, in furnace-cooled specimens exhibitedlower saturation stress, zones of PFZ’s wereobserved to be larger and the precipitates are bigger.This is due to slow cooling rate as compared to air-cooled and lead to drop saturation stress. Precipitate free zones (PFZs) were observedadjacent to the grain boundaries in both cooling rate,which allowed the precipitates at grain boundaries togrow at the expense of the precipitates adjacent tothe grain boundaries. Widmanstatten precipitates growing at the expenseof the smaller precipitates and were observed in twodirections perpendicular to each other within thegrain and parallel directions closer to grainboundaries.REFERENCES[1] L., Jia-Kuen and C. Laird. Cyclicdeformation in Al 4wt. % Cu alloy singlecrystals containing coherent θ″ precipitatesI: Cyclic stress-strain response, MaterialsScience and Engineering, vol. 54, no.1, pp.39–51, 1982.[2] T. H. Sanders, J. T. Staley, and D. A.Mauney, Strain Control Fatigue as a Toolto Interpret Fatigue Initiation of AluminumAlloy, Presented at 10thAnnualInternational Symposium on MaterialScience, Seattle, Wash, 1975.[3] C.D. Liu, D.X. You, M.N. Bassim Cyclicstrain hardening in polycrystalline copper,Acta Metallurgica et Materialia, vol. 42,no. 5, pp. 1631–1638, 1994.[4] V. T. Kuokkala, T. Lepisto and P.Kettunen. Random strain cycling of large-grained polycrystalline copper. ScriptaMetallurgica vol. 16, no. 10, p p. 1149-1152, 1982.[5] H. Inoue and T. Takasugi, Texture Controlfor Improving Deep Drawability in Rolledand Annealed Aluminum Alloy Sheets,Materials Transactions, vol. 48, no. 8, pp.2014-2022, 2007.[6] A. T. Winter, A model for the fatigue ofcopper at low plastic strain amplitudes,Philosophical Magazine, vol. 30, no. 4,1974.[7] American Society for Metals (ASM),Metals Handbook Desk Edition, 1985, p.7.