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Tensile Deformation and Fracture Behaviour of an Aerospace Aluminium Alloy AA2219 in Different Ageing Conditions
Available online at www.sciencedirect.com
ScienceDirect Procedia Materials Science 6 (2014) 322 – 330
3rd International Conference on Materials Processing and Characterisation (ICMPC 2014)
Tensile Deformation and Fracture Behaviour Of An Aerospace Aluminium Alloy AA2219 in Different Ageing Conditions Ch.V.A.Narasayya1*, P. Rambabu1, M.K. Mohan2, Rahul Mitra3 and N. Eswara Prasad1 1 Regional Centre for Military Airworthiness (Materials), CEMILAC, DRDO, Hyderabad, India Department of Metallurgical and Materials Engineering, National Institute of Technology, Warangal, India 3 Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharaghpur, India 2
Abstract Aluminium alloys due to their high strength to weight ratio coupled with good corrosion resistance find many applications in aeronautical field. 2XXX series of aluminium alloys with their high specific strength are selected for structural applications of aerospace vehicles. AA 2219 alloy is widely used for fabrication of components for aircraft, missile and space vehicles. Fracture toughness of this alloy is the most important design criteria for some select aero applications. Therefore, the study of fracture toughness in different ageing conditions is taken up. The effect of ageing on the crack opening mode (Tensile or mode I) fracture toughness (KIC) was evaluated for Al alloy AA2219 in Limited under Ageing (LUA), Under Ageing (UA) and Peak Ageing (PA) conditions. Comparison was done in the peak aged condition (T6) and as received T652 tempers. Sheet tensile samples were also prepared and tested in all the ageing conditions. Optical microscopy has been carried out and fractographic studies with SEM have been conducted to know the deformation and fracture, which observations form the basis for the observed variations in fracture toughness. © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selectionand andpeer peer-review under responsibility ofGokaraju the Gokaraju Rangaraju Institute of Engineering and Technology Selection review under responsibility of the Rangaraju Institute of Engineering and Technology (GRIET) (GRIET).
Key words: Fracture Toughness (KIC); Limited under ageing (LUA); under ageing (UA); Peak ageing (PA); Tensile and plane strain fracture modes.
1.
Introduction
Among the heat treatable 2XXX series (according to Aluminium Association of USA) aluminium alloys, AA 2219 has a great potential for wide range of applications owing to their high specific strength, good fracture toughness and excellent stress corrosion resistance.
2211-8128 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer review under responsibility of the Gokaraju Rangaraju Institute of Engineering and Technology (GRIET) doi:10.1016/j.mspro.2014.07.041
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* Corresponding author.
E-mail address: [email protected] This alloy is often used in aerospace industry due to its excellent weldability. Excellent weldability for this alloy is due to the excess amount of copper than it’s solubility at eutectic temperature, which provides sufficient eutectic for the healing during welding and avoids the hot cracking during welding (Gupta et al. 2006). It is a precipitation hardenable Al-Cu alloy containing Cu as a principal hardening element with small addition of Mn, Zr, V and Ti (Sharma et al. 2009, Srivatsan et al. 2008, Chen et al. 2009). The mechanical properties and fracture toughness of this alloy are good over a wide temperature range for +250 0 C to -2500C (Venkat narayana et al. 2004). Because of these qualities this alloy has been globally in use for some of the structural applications and also as an important cryogenic material for the welded structures for fuel tanks to store liquid oxygen and liquid nitrogen. This alloy in the age hardened condition is in use for some of the structural components with marginally higher thickness. The Tensile and fracture properties are mostly depend on the ageing condition of the material. The study of the fracture properties under different ageing conditions will be useful as design data of structures. 2. Material and Experimental details An aluminium alloy AA2219 ring rolled forging with OD 790X ID 410 X 80mm in T652 temper was used for the present study. Chemical composition of the alloy is given in Table 1. Table 1. Chemical composition (Wt %) Cu Mn Ti Zr V Fe Si Mg Zn Al 6.65 0.33 0.04 0.16 0.08 0.15 0.09 0.01 0.09 Balance The forging was carried out in the temperature range of 380-4300C and solutionising was done at 5350C. After stress relieving by compression, artificial ageing was done at 177 0C for 18 hours. The ring forging has been sectioned using Electric Discharge Machining (EDM) wire cut machine into blocks of size 65X70X30mm in two different orientations viz circumferential –radial (C-R) and in thickness – circumferential (L-C). These cut blocks were solutionised at 535±50C and quenched in hot water (50-650C) followed by artificial ageing at 1910C to 2 hr for Limited Under ageing (LUA), 9 hr for Under ageing (UA) and 26 hr for Peak ageing (PA) which indicate T6 temper. After ageing the samples were subjected to air cooling. Sheet tensile samples with 4mm thickness are extracted as per ASTM E 804 from these blocks in the orientation as shown in Fig 3 and tested on INSTRON 5500R servo hydraulic testing machine having 10kN load cell with cross head velocity of 1mm/min. Strain hardening exponent (n) has been calculated by using Ludwik-Power law of stress strain relation. Vickers hardness measurement has been carried out with 10 kg load for a dwell time of 30s on LECO Vickers hardness tester. For different ageing conditions and in two different tempers tensile test was carried out. Optical microscopy has been carried out on samples in PA condition for T652 temper and also in all ageing conditions of T6 temper. Grain size has been measured by liner intercept method. Etchant used is Tuckers reagent with chemical composition HCl-15ml, HNO3-5ml, HF-5ml, H2O-8ml (Chan et.al 2009). Fractography of the tensile samples and CT specimens were performed using FEI- QUANTA 200-3D dual beam analyzer operated at 30 kV fitted with Energy dispersive spectrometer (EDS). 3. Results and Discussions 3.1
Optical Microscopy
Microstructures have been observed in all ageing conditions for T6 temper and PA condition for T652 temper in different orientations. No much variation was observed in different ageing conditions and also in two different tempers. Grain size was measured and in the Circumferential direction it was found to be 125Pm, in the radial direction it is found to be 89 Pm and in the thickness direction it is 77Pm. Optical microstructure in three principal directions in peak aged condition in 3D form are shown in Fig 1.
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Fig 1: Optical micrographs in three principal directions in peak aged condition for T6 temper.
3.2
Hardness
The Vickers hardness (HV10) has been measured and represented in Fig 2.The hardness profile indicating increase in hardness due to change in precipitate morphology from LUA condition to PA condition through UA condition. There is slight variation in T6 temper to that of as received T652 condition. The T652 temper has slight higher hardness indicating the uniform distribution of precipitates due to stress induced uniformly during stress relieving operation between solutionising and artificial ageing. There is no much hardness variation has been observed in the two orientations of C-R & L-C for all the ageing conditions in T6 temper.
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Hardness (HV)
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130
120
110
100 0
5
10
15
20
25
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Fig 2. Hardness (HV) in different ageing conditions for T6 temper.
3.3
Tensile Properties
Tensile properties were evaluated in the circumferential and thickness direction for T652 temper and also for different ageing conditions in T6 temper. Samples were prepared in the orientation resembles the actual loading condition in fracture toughness testing as shown in fig. 3. The results are represented in Fig 4-7. The reported results
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are the average of two tests.
Fig. 3 : CT specimen with the orientation of sheet tensile specimen extraction.
Strength properties (Vys and Vult) increased with ageing from LUA to UA and to subsequent PA condition. Vys shows a more pronounced increase with ageing time than Vult. The increase in Vys is associated with decrease in ductility (% El). Except the precipitation density and distribution all the remaining microstructural features are same in different ageing conditions. And so the change in tensile properties can be attributed for the change in precipitation morphology and distribution. In C-R Orientation the Vys has indicated more pronounced improvement that Vult. There is around 32% increase in Vys from LUA to PA condition and for Vult it is observed 9%. The reverse trend has been observed for % El. Around 29% elongation has been decreased from LUA to PA ageing condition
500
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C-R PS C-R UTS L-C PS L-C UTS C-R --'-- % El L-C ---- % El
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Fig .4 : Mechanical properties in three different ageing conditions. (A) Tensile (B) Strain hardening exponent
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.
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Fig .5 : Tensile properties in different temper condition in two orientations.(A) circumferential (B) Thickness .
For L-C orientation the variation in tensile properties is less compared to the C-R direction. The Vys has shown an increase of 17% and for Vult it is 10%. % El has shown drastic change from LUA condition to PA condition with a decrease of around 61 % of elongation. T6 temper tensile properties observed marginally higher than T652 in both the orientation. The measured strain hardening exponent has shown the conventional decrease in value as the ageing time increases for LUA to PA the results were shown in Fig. 4B 3.4
Fracture Toughness
Though most of the studies of this alloy have been conducted in the J IC mode of testing, higher thickness structural components are also in use and so in the present study all the fracture toughness tests were conducted in the KIC mode based on LEFM by using the method described in ASTM E399 04 with a sample thickness of 25mm. The samples were fatigue precracked. The results are represented in Fig 6 A & B. The results are average of two tests. There is no much variation in fracture toughness in the case of C-R orientation for T652 and also T6 temper. For C-R orientation, fracture properties in both the tempers are almost same. But in the under ageing condition (LUA) and UA the fracture toughness is invalid in thickness, This can be attributed to low strength and high value of strain hardening coefficient in these ageing conditions due to inadequate precipitate size and distribution and also the elongated grains in this orientation.
C-R Orientation L-C Orientation
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Ch.V.A. Narasayya et al. / Procedia Materials Science 6 (2014) 322 – 330 Fig 6: Fracture properties (A) In different ageing conditions (B) Two different orientations. 30
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Circumferential 1/2 KIC (MPa m )
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0.2% PS (MPa)
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0.2% PS (MPa)
Fig 7: Fracture toughness vs 0.2% PS in different orientations. (A) Circumferential (B) Thickness.
For L-C orientation valid KIC has been observed in all the ageing conditions. As the ageing time increases the fracture toughness decreased from LUA to PA indicating the normal trend of decrease fracture toughness as strength increases [Hahn et al. 1975]. 3.5
Fractography
Fig : 8. Fractographs of sheet tensile samples in (A) LUA (B) UA (C) PA conditions
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Ch.V.A. Narasayya et al. / Procedia Materials Science 6 (2014) 322 – 330 Fig : 9. Fractographs of CT samples in different conditions of T6 temper (A) LUA (B) UA (C) PA.
3.6
Microscopic fracture behavior
A careful and comprehensive examination of the tensile fractured surfaces does provide useful information on the specific role of intrinsic microstructural effects on strength and ductility properties. A careful observation of the fracture surfaces revealed minute differences in overall fracture morphology at the macroscopic level and intrinsic fracture features. The fractographs of tensile samples has shown in Fig 8 A, B &C. At the macroscopic level, tensile fracture of the samples was essentially normal to the stress axis. The overall morphology of the fracture was rough revealing regions of locally ductile and brittle failure of the constituent particles. Higher magnification observations of the tensile fracture surfaces revealed microscopic cracking along the recrystallized grain boundaries. An array of very fine microscopic cracks was clearly evident along the recrystallised grain boundaries reminiscent of locally brittle failure mechanism. Trans granular fracture regions observed are due to the presence of the small second phase particles, which causes the formation of pockets of shallow dimples and microscopic voids of varying size. Coarse constituent particles were observed to have fracture/de-cohesion from the matrix initiating the micro voids. Secondary cracks were also observed. The predominant mechanism of failure has been observed as micro void nucleation, growth, coalescence.
Fig 10: Fractograph at higher Magnification indicating the mechanism of failure. In both the orientations the same features have been observed. Typical features of ductile fracture, generated by nucleation of dimples mainly from the coarse intermetallic particles fractured in a brittle manner as shown in Fig 10. Dimples observed on fracture surfaces nucleated from the CuAl 2 second phase particles. Typical KIC tested samples were also subjected to fractography analysis at low and high magnifications which are shown in Fig 9 A, B & C. The analysis was made next to the fatigue precracked area and also at 3mm away from the boundary of the fatigue precracked area. The macroscopic fracture path across the specimen thickness revealed the flatness of the fracture surface and the absence of shear lips for all the PA samples in L-C orientation. But in case of C-R orientation under aged (LUA and UA) samples, which are invalid in thickness have been observed with small shear lips. The formation of these lips are due to prevailing of plain strain conditions due to low strength and high “n” values in the under aged conditions. Two sizes of dimples were observed at the higher magnification. The first population of dimples are in the size range of 10-30Pm, which were associated with fractures / decohesion of coarse second phase particles of
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329
undissolved CuAl2 particles. Which are due to the excess amount of Cu than it’s solubility at solutionising temperature. The large primary voids were caused by the fracture of constituent particles. This is because the larger particles experience higher stress during loading and causes to fracture of the particles rather than decohesion (Hahn et al. 1975), whereas primary voids of small sizes were the results of particle matrix decohesion. Other size of dimples have found to be are finer in size (0.6-3 Pm) and formed due to the formation of ligament breaking between primary voids causing shear voiding between the primary voids. The breaking of the particles has been observed more in CT specimens than tensile specimens. Similar features were observed in all ageing conditions. Fine particles void sheeting had been observed predominantly in the PA condition than in the LUA and UA condition 3.7
Mechanism of failure:
For the both tensile and CT specimens the predominant mode of failure has been observed as micro void nucleation, growth and coalescence type. The size and distribution of the large second phase particles controls the initial void nucleation stage. The nature of matrix precipitate/ second phase particles structure plays an important role in the fracture behavior of the alloy. The fine scale of the precipitate governs the yield strength, strain hardening rate and thus energy absorption during fracture (Sharma et al. 2009). In the LUA condition most of the structure consists of GP zones only and as the ageing time increases to UA condition the formation of Ts from GP zones takes place, which is a major hardening phase. GP zones are fully coherent metastable two dimensional clusters normally in the size of a100-1500A of Cu atoms that are disk like in shape from the {100}Al planes (Hatch et al 1983). The under ageing treatment results in a structure consisting of Ts , which is also coherent with the Al matrix, although only few atoms layer in thickness is considered to be three dimensional. These Ts has the same plate like shape and orientation as the GP zones but it is slightly larger in diameter (200-500A0) [Porter et al 2009]. As the size of the precipitates increases with increasing ageing time the volumetric strains will be increased due to increasing size of the precipitates, which could not able to compensate the coherency strains. As the size of the precipitates increases, to accommodate huge volumetric strains energy, coherency will be disturbed and be disappeared gradually. In the over aged condition complete incoherent precipitates causes to reduction in mechanical properties. Both GP zones and Ts precipitates are coherent with the matrix lattice and are easily shearable by sliding dislocations resulting in planer slip. The planer slip is also promoted by the presence of solute atoms in the Almatrix. The solute atoms decreases the stacking faulty energy of the Al lattice and makes the cross slip more difficult. The strain will be localized in narrow slip bands. [Sharma et al 2009]. The strain hardening exponent (n) influence the deformation through its ability to accommodate strain in LCA and UA condition, the higher values of “n” results in large ability to accommodate strains. This causes in higher fracture toughness. As the ageing time precedes the Ts precipitates converted to Tc in the PA condition which are semi coherent in nature with BCT structure, when these precipitates are sheared, areas of strain concentration will be developed. At the crack tip, where the shearable precipitates are encompassed by the plastic zone, the ability of the materials to accommodate strains will be limited. This localisation of strains will be strongest for the PA condition and these precipitates are highly difficult to shear and cause to attain maximum strength. 4. Conclusions In AA2219 alloy ring rolled forgings, the plane strain fracture toughness (K IC) is 27.7 MPa m½ in L-C orientation and this KIC value is the highest in the LUA condition and 21.4 MPa m½ in the peak aged condition, which indicate the conventional inverse relation with decreasing fracture toughness with increasing yield strength. For C-R orientation also the same trend has been observed. For C-R direction in the peak aged condition only valid
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KIC has been observed where as in the under aged condition KIC has been found to be invalid as the specimen thickness employed does not comply with plane strain condition. Higher fracture toughness in C-R orientation can be attributed to elongated grain in C-R direction.For C-R & L-C orientations fracture toughness is almost similar in T652 and T6 Orientations for all conditions of ageing and thus orientations. The predominant mode of failure has been observed as the nucleation of micro voids, followed by its growth and subsequent coalescence. Both tensile and fracture toughness samples indicated the same nature of failure. 5. Acknowledgements The authors would like to thank Dr K.Tamilmani, DG (Aero) & CE,CEMILAC for permission to publish the results and constant support during this research work. The team would also like to thank Dr Vikas Kumar (Group Head-Mechanical behavior Group- DMRL) for his useful suggestions while carrying out the experiments and also to Dr P.Goshal (Group Head- Electron Microscopy group-DMRL) for his constant support and help during this research work. The authors thank DRDO for the financial support to conduct these studies.
References ASTM E-8 (2004)“Annual Book of ASTM standards, American Society for Testing and Materials; Philadelphia, PA, PP62-85. ASTM E-399, (2004) Annual Book of ASTM standards, American Society of Testing and Materials “, Philadelphia, PA, Vol.3.01, PP- 461-494. Chen YC, Fang JC, Liu HJ (2009) “Precipitate evolution in friction stir welding of 2219-T6 aluminium alloys”, Mat charctzn vol.60, PP476-481. Gupta RK , Narayana Murthy SVS, (2006) “ Analysis of crack in aluminium alloy AA2219 weldment” ; Eng. Failure Analysis ; Vol.13; PP:1370-1375. Hahn GT and Rosenfield AR, (1975), “Metallurgical factors affecting fracture toughness of Al alloy,” Met Trans A; vol.6A, PP:653-668. Johan E Hatch (1983), “Aluminium –properties and physical metallurgy”; ASM; Metal park, ohio, I st Ed.., p153-183. Narayana GV, Sharma VMJ, Diwakar V and Kumar KS; (2004) “Fracture behavior of aluminium alloy 2219-T87 welded plates”, Sci & Tech weld and Join, Vol.2, PP-121-130. Porter DA, Esterling KE and Sherif MY, (2009); “Phase transformations in Metals and Alloys” 3rd Edition, CRC Press; Boca Raton, pp:276304. Sharma VMJ, Sree Kumar K, Nageswra Rao B, pathak SD (2009), “Effect of microstructure and strength on fracture behavior of AA2219 alloy”, Mat Sci and Eng A502; PP45-53. Srivatsan T.S, satish vasudevan, Lisa park, Lederich R.J (2008), “The quasi-stastic deformation and final fracture behavior of Aluminium alloy 2219” Mat Sci & Eng A, Vol.497,PP270-277
ScienceDirect Procedia Materials Science 6 (2014) 322 – 330
3rd International Conference on Materials Processing and Characterisation (ICMPC 2014)
Tensile Deformation and Fracture Behaviour Of An Aerospace Aluminium Alloy AA2219 in Different Ageing Conditions Ch.V.A.Narasayya1*, P. Rambabu1, M.K. Mohan2, Rahul Mitra3 and N. Eswara Prasad1 1 Regional Centre for Military Airworthiness (Materials), CEMILAC, DRDO, Hyderabad, India Department of Metallurgical and Materials Engineering, National Institute of Technology, Warangal, India 3 Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharaghpur, India 2
Abstract Aluminium alloys due to their high strength to weight ratio coupled with good corrosion resistance find many applications in aeronautical field. 2XXX series of aluminium alloys with their high specific strength are selected for structural applications of aerospace vehicles. AA 2219 alloy is widely used for fabrication of components for aircraft, missile and space vehicles. Fracture toughness of this alloy is the most important design criteria for some select aero applications. Therefore, the study of fracture toughness in different ageing conditions is taken up. The effect of ageing on the crack opening mode (Tensile or mode I) fracture toughness (KIC) was evaluated for Al alloy AA2219 in Limited under Ageing (LUA), Under Ageing (UA) and Peak Ageing (PA) conditions. Comparison was done in the peak aged condition (T6) and as received T652 tempers. Sheet tensile samples were also prepared and tested in all the ageing conditions. Optical microscopy has been carried out and fractographic studies with SEM have been conducted to know the deformation and fracture, which observations form the basis for the observed variations in fracture toughness. © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selectionand andpeer peer-review under responsibility ofGokaraju the Gokaraju Rangaraju Institute of Engineering and Technology Selection review under responsibility of the Rangaraju Institute of Engineering and Technology (GRIET) (GRIET).
Key words: Fracture Toughness (KIC); Limited under ageing (LUA); under ageing (UA); Peak ageing (PA); Tensile and plane strain fracture modes.
1.
Introduction
Among the heat treatable 2XXX series (according to Aluminium Association of USA) aluminium alloys, AA 2219 has a great potential for wide range of applications owing to their high specific strength, good fracture toughness and excellent stress corrosion resistance.
2211-8128 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer review under responsibility of the Gokaraju Rangaraju Institute of Engineering and Technology (GRIET) doi:10.1016/j.mspro.2014.07.041
Ch.V.A. Narasayya et al. / Procedia Materials Science 6 (2014) 322 – 330
323
* Corresponding author.
E-mail address: [email protected] This alloy is often used in aerospace industry due to its excellent weldability. Excellent weldability for this alloy is due to the excess amount of copper than it’s solubility at eutectic temperature, which provides sufficient eutectic for the healing during welding and avoids the hot cracking during welding (Gupta et al. 2006). It is a precipitation hardenable Al-Cu alloy containing Cu as a principal hardening element with small addition of Mn, Zr, V and Ti (Sharma et al. 2009, Srivatsan et al. 2008, Chen et al. 2009). The mechanical properties and fracture toughness of this alloy are good over a wide temperature range for +250 0 C to -2500C (Venkat narayana et al. 2004). Because of these qualities this alloy has been globally in use for some of the structural applications and also as an important cryogenic material for the welded structures for fuel tanks to store liquid oxygen and liquid nitrogen. This alloy in the age hardened condition is in use for some of the structural components with marginally higher thickness. The Tensile and fracture properties are mostly depend on the ageing condition of the material. The study of the fracture properties under different ageing conditions will be useful as design data of structures. 2. Material and Experimental details An aluminium alloy AA2219 ring rolled forging with OD 790X ID 410 X 80mm in T652 temper was used for the present study. Chemical composition of the alloy is given in Table 1. Table 1. Chemical composition (Wt %) Cu Mn Ti Zr V Fe Si Mg Zn Al 6.65 0.33 0.04 0.16 0.08 0.15 0.09 0.01 0.09 Balance The forging was carried out in the temperature range of 380-4300C and solutionising was done at 5350C. After stress relieving by compression, artificial ageing was done at 177 0C for 18 hours. The ring forging has been sectioned using Electric Discharge Machining (EDM) wire cut machine into blocks of size 65X70X30mm in two different orientations viz circumferential –radial (C-R) and in thickness – circumferential (L-C). These cut blocks were solutionised at 535±50C and quenched in hot water (50-650C) followed by artificial ageing at 1910C to 2 hr for Limited Under ageing (LUA), 9 hr for Under ageing (UA) and 26 hr for Peak ageing (PA) which indicate T6 temper. After ageing the samples were subjected to air cooling. Sheet tensile samples with 4mm thickness are extracted as per ASTM E 804 from these blocks in the orientation as shown in Fig 3 and tested on INSTRON 5500R servo hydraulic testing machine having 10kN load cell with cross head velocity of 1mm/min. Strain hardening exponent (n) has been calculated by using Ludwik-Power law of stress strain relation. Vickers hardness measurement has been carried out with 10 kg load for a dwell time of 30s on LECO Vickers hardness tester. For different ageing conditions and in two different tempers tensile test was carried out. Optical microscopy has been carried out on samples in PA condition for T652 temper and also in all ageing conditions of T6 temper. Grain size has been measured by liner intercept method. Etchant used is Tuckers reagent with chemical composition HCl-15ml, HNO3-5ml, HF-5ml, H2O-8ml (Chan et.al 2009). Fractography of the tensile samples and CT specimens were performed using FEI- QUANTA 200-3D dual beam analyzer operated at 30 kV fitted with Energy dispersive spectrometer (EDS). 3. Results and Discussions 3.1
Optical Microscopy
Microstructures have been observed in all ageing conditions for T6 temper and PA condition for T652 temper in different orientations. No much variation was observed in different ageing conditions and also in two different tempers. Grain size was measured and in the Circumferential direction it was found to be 125Pm, in the radial direction it is found to be 89 Pm and in the thickness direction it is 77Pm. Optical microstructure in three principal directions in peak aged condition in 3D form are shown in Fig 1.
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Fig 1: Optical micrographs in three principal directions in peak aged condition for T6 temper.
3.2
Hardness
The Vickers hardness (HV10) has been measured and represented in Fig 2.The hardness profile indicating increase in hardness due to change in precipitate morphology from LUA condition to PA condition through UA condition. There is slight variation in T6 temper to that of as received T652 condition. The T652 temper has slight higher hardness indicating the uniform distribution of precipitates due to stress induced uniformly during stress relieving operation between solutionising and artificial ageing. There is no much hardness variation has been observed in the two orientations of C-R & L-C for all the ageing conditions in T6 temper.
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Hardness (HV)
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Fig 2. Hardness (HV) in different ageing conditions for T6 temper.
3.3
Tensile Properties
Tensile properties were evaluated in the circumferential and thickness direction for T652 temper and also for different ageing conditions in T6 temper. Samples were prepared in the orientation resembles the actual loading condition in fracture toughness testing as shown in fig. 3. The results are represented in Fig 4-7. The reported results
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are the average of two tests.
Fig. 3 : CT specimen with the orientation of sheet tensile specimen extraction.
Strength properties (Vys and Vult) increased with ageing from LUA to UA and to subsequent PA condition. Vys shows a more pronounced increase with ageing time than Vult. The increase in Vys is associated with decrease in ductility (% El). Except the precipitation density and distribution all the remaining microstructural features are same in different ageing conditions. And so the change in tensile properties can be attributed for the change in precipitation morphology and distribution. In C-R Orientation the Vys has indicated more pronounced improvement that Vult. There is around 32% increase in Vys from LUA to PA condition and for Vult it is observed 9%. The reverse trend has been observed for % El. Around 29% elongation has been decreased from LUA to PA ageing condition
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Fig .4 : Mechanical properties in three different ageing conditions. (A) Tensile (B) Strain hardening exponent
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Ch.V.A. Narasayya et al. / Procedia Materials Science 6 (2014) 322 – 330
.
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Thickness Orientation UTS & PS (MPa)
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10
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% El 200
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Fig .5 : Tensile properties in different temper condition in two orientations.(A) circumferential (B) Thickness .
For L-C orientation the variation in tensile properties is less compared to the C-R direction. The Vys has shown an increase of 17% and for Vult it is 10%. % El has shown drastic change from LUA condition to PA condition with a decrease of around 61 % of elongation. T6 temper tensile properties observed marginally higher than T652 in both the orientation. The measured strain hardening exponent has shown the conventional decrease in value as the ageing time increases for LUA to PA the results were shown in Fig. 4B 3.4
Fracture Toughness
Though most of the studies of this alloy have been conducted in the J IC mode of testing, higher thickness structural components are also in use and so in the present study all the fracture toughness tests were conducted in the KIC mode based on LEFM by using the method described in ASTM E399 04 with a sample thickness of 25mm. The samples were fatigue precracked. The results are represented in Fig 6 A & B. The results are average of two tests. There is no much variation in fracture toughness in the case of C-R orientation for T652 and also T6 temper. For C-R orientation, fracture properties in both the tempers are almost same. But in the under ageing condition (LUA) and UA the fracture toughness is invalid in thickness, This can be attributed to low strength and high value of strain hardening coefficient in these ageing conditions due to inadequate precipitate size and distribution and also the elongated grains in this orientation.
C-R Orientation L-C Orientation
30
35
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30
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T6 T652 Circumferential Thickness
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28
Fracture Toughness 1/2 KIC (MPa m )
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Fracture toughness KQC(MpPa m )
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5
20
0
18 0
5
10
15
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Ageing Time (hr)
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30
Orientation
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Ch.V.A. Narasayya et al. / Procedia Materials Science 6 (2014) 322 – 330 Fig 6: Fracture properties (A) In different ageing conditions (B) Two different orientations. 30
32
B
A
28
0.2% PS
0.2% PS
26
Thickness 1/2 KIC (MPa m )
Circumferential 1/2 KIC (MPa m )
30
28
24
22
20
26
18
240
260
280
300
320
340
260
0.2% PS (MPa)
280
300
320
340
0.2% PS (MPa)
Fig 7: Fracture toughness vs 0.2% PS in different orientations. (A) Circumferential (B) Thickness.
For L-C orientation valid KIC has been observed in all the ageing conditions. As the ageing time increases the fracture toughness decreased from LUA to PA indicating the normal trend of decrease fracture toughness as strength increases [Hahn et al. 1975]. 3.5
Fractography
Fig : 8. Fractographs of sheet tensile samples in (A) LUA (B) UA (C) PA conditions
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Ch.V.A. Narasayya et al. / Procedia Materials Science 6 (2014) 322 – 330 Fig : 9. Fractographs of CT samples in different conditions of T6 temper (A) LUA (B) UA (C) PA.
3.6
Microscopic fracture behavior
A careful and comprehensive examination of the tensile fractured surfaces does provide useful information on the specific role of intrinsic microstructural effects on strength and ductility properties. A careful observation of the fracture surfaces revealed minute differences in overall fracture morphology at the macroscopic level and intrinsic fracture features. The fractographs of tensile samples has shown in Fig 8 A, B &C. At the macroscopic level, tensile fracture of the samples was essentially normal to the stress axis. The overall morphology of the fracture was rough revealing regions of locally ductile and brittle failure of the constituent particles. Higher magnification observations of the tensile fracture surfaces revealed microscopic cracking along the recrystallized grain boundaries. An array of very fine microscopic cracks was clearly evident along the recrystallised grain boundaries reminiscent of locally brittle failure mechanism. Trans granular fracture regions observed are due to the presence of the small second phase particles, which causes the formation of pockets of shallow dimples and microscopic voids of varying size. Coarse constituent particles were observed to have fracture/de-cohesion from the matrix initiating the micro voids. Secondary cracks were also observed. The predominant mechanism of failure has been observed as micro void nucleation, growth, coalescence.
Fig 10: Fractograph at higher Magnification indicating the mechanism of failure. In both the orientations the same features have been observed. Typical features of ductile fracture, generated by nucleation of dimples mainly from the coarse intermetallic particles fractured in a brittle manner as shown in Fig 10. Dimples observed on fracture surfaces nucleated from the CuAl 2 second phase particles. Typical KIC tested samples were also subjected to fractography analysis at low and high magnifications which are shown in Fig 9 A, B & C. The analysis was made next to the fatigue precracked area and also at 3mm away from the boundary of the fatigue precracked area. The macroscopic fracture path across the specimen thickness revealed the flatness of the fracture surface and the absence of shear lips for all the PA samples in L-C orientation. But in case of C-R orientation under aged (LUA and UA) samples, which are invalid in thickness have been observed with small shear lips. The formation of these lips are due to prevailing of plain strain conditions due to low strength and high “n” values in the under aged conditions. Two sizes of dimples were observed at the higher magnification. The first population of dimples are in the size range of 10-30Pm, which were associated with fractures / decohesion of coarse second phase particles of
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undissolved CuAl2 particles. Which are due to the excess amount of Cu than it’s solubility at solutionising temperature. The large primary voids were caused by the fracture of constituent particles. This is because the larger particles experience higher stress during loading and causes to fracture of the particles rather than decohesion (Hahn et al. 1975), whereas primary voids of small sizes were the results of particle matrix decohesion. Other size of dimples have found to be are finer in size (0.6-3 Pm) and formed due to the formation of ligament breaking between primary voids causing shear voiding between the primary voids. The breaking of the particles has been observed more in CT specimens than tensile specimens. Similar features were observed in all ageing conditions. Fine particles void sheeting had been observed predominantly in the PA condition than in the LUA and UA condition 3.7
Mechanism of failure:
For the both tensile and CT specimens the predominant mode of failure has been observed as micro void nucleation, growth and coalescence type. The size and distribution of the large second phase particles controls the initial void nucleation stage. The nature of matrix precipitate/ second phase particles structure plays an important role in the fracture behavior of the alloy. The fine scale of the precipitate governs the yield strength, strain hardening rate and thus energy absorption during fracture (Sharma et al. 2009). In the LUA condition most of the structure consists of GP zones only and as the ageing time increases to UA condition the formation of Ts from GP zones takes place, which is a major hardening phase. GP zones are fully coherent metastable two dimensional clusters normally in the size of a100-1500A of Cu atoms that are disk like in shape from the {100}Al planes (Hatch et al 1983). The under ageing treatment results in a structure consisting of Ts , which is also coherent with the Al matrix, although only few atoms layer in thickness is considered to be three dimensional. These Ts has the same plate like shape and orientation as the GP zones but it is slightly larger in diameter (200-500A0) [Porter et al 2009]. As the size of the precipitates increases with increasing ageing time the volumetric strains will be increased due to increasing size of the precipitates, which could not able to compensate the coherency strains. As the size of the precipitates increases, to accommodate huge volumetric strains energy, coherency will be disturbed and be disappeared gradually. In the over aged condition complete incoherent precipitates causes to reduction in mechanical properties. Both GP zones and Ts precipitates are coherent with the matrix lattice and are easily shearable by sliding dislocations resulting in planer slip. The planer slip is also promoted by the presence of solute atoms in the Almatrix. The solute atoms decreases the stacking faulty energy of the Al lattice and makes the cross slip more difficult. The strain will be localized in narrow slip bands. [Sharma et al 2009]. The strain hardening exponent (n) influence the deformation through its ability to accommodate strain in LCA and UA condition, the higher values of “n” results in large ability to accommodate strains. This causes in higher fracture toughness. As the ageing time precedes the Ts precipitates converted to Tc in the PA condition which are semi coherent in nature with BCT structure, when these precipitates are sheared, areas of strain concentration will be developed. At the crack tip, where the shearable precipitates are encompassed by the plastic zone, the ability of the materials to accommodate strains will be limited. This localisation of strains will be strongest for the PA condition and these precipitates are highly difficult to shear and cause to attain maximum strength. 4. Conclusions In AA2219 alloy ring rolled forgings, the plane strain fracture toughness (K IC) is 27.7 MPa m½ in L-C orientation and this KIC value is the highest in the LUA condition and 21.4 MPa m½ in the peak aged condition, which indicate the conventional inverse relation with decreasing fracture toughness with increasing yield strength. For C-R orientation also the same trend has been observed. For C-R direction in the peak aged condition only valid
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KIC has been observed where as in the under aged condition KIC has been found to be invalid as the specimen thickness employed does not comply with plane strain condition. Higher fracture toughness in C-R orientation can be attributed to elongated grain in C-R direction.For C-R & L-C orientations fracture toughness is almost similar in T652 and T6 Orientations for all conditions of ageing and thus orientations. The predominant mode of failure has been observed as the nucleation of micro voids, followed by its growth and subsequent coalescence. Both tensile and fracture toughness samples indicated the same nature of failure. 5. Acknowledgements The authors would like to thank Dr K.Tamilmani, DG (Aero) & CE,CEMILAC for permission to publish the results and constant support during this research work. The team would also like to thank Dr Vikas Kumar (Group Head-Mechanical behavior Group- DMRL) for his useful suggestions while carrying out the experiments and also to Dr P.Goshal (Group Head- Electron Microscopy group-DMRL) for his constant support and help during this research work. The authors thank DRDO for the financial support to conduct these studies.
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