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Failure analysis and optimization of thermo-mechanical process parameters oftitanium alloy fasteners

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Failure analysis of Ti–6Al–4V fasteners was carried out and the failure was attributed to fatigue damage which occurred because socket head hole was pierced into the shank. Metallurgical analysis has confirmed heterogeneity in microstructure, due to uncontrolled process parameters which were followed during fastener manufacturing. An experimental study was conducted to optimize process parameters such that strain rates were optimized as 102 to 103 /s and heating was done in a controlled furnace at 900 8C. Metallurgical analysis with optimum process parameters revealed that there is no evidence of heterogeneity in microstructure.

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Articoli tecnico scientifici o articoli contenenti case history
Articolo Case Studies in Construction Materials, 2013

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Case study Failure analysis and optimization of thermo-mechanical process parameters of titanium alloy (Ti''6Al''4V) fasteners for aerospace applications Vartha Venkateswarlu *, Debashish Tripathy, K. Rajagopal, K. Thomas Tharian, P.V. Venkitakrishnan Liquid Propulsion Systems Center, ISRO, Trivandrum 695547, India 1. Introduction Ti''6Al''4V is the most important and widely used titanium alloy because of their high strength to weight ratio, good corrosion resistance, excellent fracture toughness and attractive mechanical properties which make it an ideal choice for many aerospace applications. It contains aluminum as alpha ( a) stabilizer and vanadium as high temperature beta ( b) phase stabilizer at room temperature. Alloy exhibits different microstructure depending on the chemical composition, processing history and thermal treatment procedures. Forging of titanium alloy is critical as, the more difficult to deform a phase is usually present at low temperatures, while more easily deformable b phase is present at high temperature. The temperature at which alloy transforms completely from a to b is termed as b transus ( bt) and is a critical temperature in titanium alloy forging process. The bt of a''b alloys typically ranges from 870 to 1010 8 C [1]. Flow stress in Ti''6Al''4V is more sensitive to forging temperature as strain-softening takes place when it forged below bt and is much lesser if it is forged above bt as illustrated in Fig. 1. Also, Strain rates are more sensitive to 'ow stress ('ow stress at 900 8 C, 50% strain, strain rate of 10/s is 209 MPa and 0.001/s, the 'ow stress is 50 MPa) as shown in Fig. 2. Tensile properties and fatigue behavior of the Ti''6Al''4V are highly in'uenced by its microstructure and process parameters followed during manufacturing cycle. Titanium microstructure depends on the thermal cycle wherein the Case Studies in Engineering Failure Analysis 1 (2013) 49''60 A R T I C L E I N F O Article history: Received 15 December 2012 Accepted 14 April 2013 Available online 3 May 2013 Keywords:
Ti''6Al''4V
Solution treated and aged Strain rate Fasteners
Fatigue failure * Corresponding author at: SRCPES/SRQA, LPSC-Valiamala, Trivandrum 695547, Kerala, India. Tel.: +91 0471 2567693/7259; fax: +91 0471 2567259. E-mail addresses: vartha_v@yahoo.com, varthavenkateswarlu@lpscv.dos.gov.in (V. Venkateswarlu). Contents lists available at SciVerse ScienceDirect Case Studies in Engineering Failure Analysis j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c s e f a 2213-2902 ' 2013 Elsevier Ltd. http://dx.doi.org/10.1016/j.csefa.2013.04.003 Open access under CC BY-NC-ND license. materials are subjected to both during heat treatments and during manufacturing process. The thermal cycle governs the distribution of a and b phases in the titanium microstructure. Ti''6Al''4V worked above the beta transition temperature (995 8 C), shows only b phase which is characterized by a cubic centered crystal structure. Under b transus temperature, a and b phase are present together in the microstructure [2]. Another important factor is the heat treatment: (i) Annealing treatment consists of heating the materials at a temperature of 700''850 8 C for 1''4 h followed by slow cooling. (ii) Solution treatment at 950 8 C followed by quenching in water, aging at 500 8 C for 3''6 h followed by air cooling [3]. The objective of present study is metallurgical analysis of the failed bolt, fractography study to find out the root cause of the failure, and optimization of processing parameters for fastener manufacturing. 2. Experimental procedure The details of process of manufacture of fasteners are given below. 2.1. Input material The chemical composition of the input material is given in Table 1. Fig. 2. Effect of deformation rate on 'ow stress. Fig. 1. Effect of temperature on 'ow stress. Table 1 Chemical composition of Ti''6Al''4V (by %weight). Alloy C Al V Fe O N H Ti Ti''6Al''4V 0.01 6.28 4.03 0.030 0.169 0.008 0.006 Balance V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 50 2.2. Process of manufacture The input rod of size of F9.0 mm was processed by hot rolling at temperature 955 8 C followed by annealing at 700 8 C for 1 h thereafter by air cooling. The rods were further reduced to F5.6 mm by hot swaging. Finally, size of F4.5 mm was achieved by centerless grinding. 2.2.1. Head forging Head forging was done by 'ame heating the head portion of feed stock followed by swaging on the dies. The head forged feed stock was heat treated to solution treatment at 940  10 8 C with soaking period of 45 min followed by water quenching. Further, aging was done at 535  10 8 C for 5 h-air cooling. Socket head was punched by cold working. 2.2.2. Thread rolling Threads were processed on the heat treated head forged feed stock by thread rolling. 3. Observations 3.1. Clearance philosophy followed for fastener 3.1.1. Mechanical properties The input feed stock material was tested to confirm the mechanical properties in annealed condition. Further sample blanks were tested in solution treated and aged (STA) condition. The details of mechanical properties in annealed and STA condition are illustrated in Table 2. Mechanical properties in annealed and STA condition were found satisfactory. 3.1.2. Metallurgical analysis Metallurgical analysis was carried out using light microscopy. The specimens were prepared by grinding with 1000 grit SIC paper, fine polishing with 1 mm diamond paste, followed by etching with kroll (10 ml HF + 30 ml HNO3) reagent in annealed condition. It revealed fine alpha in transformed beta structure in annealed condition. 3.1.3. Acceptance test on the fasteners (a) Mechanical test on the fasteners was conducted and withstood for specified load of 9.6 kN further allowed to test up to the breaking load of 10.43 kN. (b) Torque test was conducted at 3.0 N m for 5 cycles and found satisfactory. (c) Metallurgical analysis was carried out on the finished fastener. It was revealed beta structure in head portion and alpha + beta structure in shank portion of the fastener in STA condition. (d) Further 100% dimensional inspection was carried out as per NFL22-220 with 6 g acceptance criteria. 3.2. Observations on failed bolt As a qualification of bolt for 'ight assembly, the bolt assembly has undergone proof pressure test at 2.4 MPa, Vibration test at frequency range of 20''100 Hz, 3 db/oct for 120 s in both lateral and longitudinal directions and 'ight acceptance test. During routine visual inspection before electrical check, it was observed in one of the three fasteners that the head of the bolt had got sheared off from the shank portion of bolt. Further, a failure analysis was carried out on the failed bolt. 3.2.1. Visual inspection The bolt used was having 25 mm long thread portion and 75 mm plain shank portion as shown in Fig. 3 and bolt assembly with valve housing shown in Fig. 4. Failure occurred at the head to shank junction as shown in Figs. 5 and 6. Further, crack portion of the head and shank portion was analyzed with stereo zoom microscope; it was observed that, a hole was pierced into the shank as shown in Fig. 7. Dye penetrant test was carried out on the fresh bolts as per ASTM E165 and it was observed that circumferential cracks were present at the bottom of the head and some cracks extending to shank side. Table 2 Mechanical properties in Annealed and STA condition. Heat treatment condition Yield strength (MPa) Ultimate tensile strength (MPa) %Elongation Annealed 950 1069 16 STA 1143 1240 15 V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 51 3.2.2. Metallurgical analysis Metallurgical analysis was carried out on the head portion, failure zone and shank portion of the bolt. In the head portion a crack was observed adjacent to the one corner of bolt. Microstructure revealed a fine lamellar structure with beta grains at head portion and coarse lamellar with beta grains at adjacent to crack region as shown in Figs. 8''11. Fig. 3. Good fasteners. Fig. 5. Fracture portion of the bolt. Fig. 4. Assembly of the bolt. Fig. 6. Head region. V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 52 In the shank portion of failure zone, it was observed that a crack was branched from the cracked region and microstructure revealed coarse lamellar structure near the crack region and fine lamellar structure slightly away from the failure zone as shown in Figs. 12 and 13. The shank portion of bolt was revealed grain boundary alpha phase at prior beta grain boundaries and martensitic alpha structure as shown in Figs. 14 and 15. The heterogeneity in microstructure is also illustrated in Jha et al. [4]. Fracture surface of the failed bolt under Scanning Electron Microscope indicated that there was shift in the socket head hole from the center and also machining marks of pierced hole of socket head into the shank as shown in Figs. 16 and 17. At Fig. 7. Hole pierced in the head. Fig. 8. Crack at head region. Fig. 9. Edge of the failure zone. V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 53 head location, the nucleation of cracks was started at notch zone and further propagated to the outer periphery as shown in Fig. 18. At shank portion, cracks were confined to circular hole as shown in Fig. 19. In addition, fracture surface had fatigue striations as shown in Figs. 20''22 occurred due to vibration loads on the bolts. Also observed, dimple type of zones at periphery of the bolt is shown in Fig. 23. Fig. 10. Coarse lamellar structure in beta grains. Fig. 11. Fine lamellar structure in beta grains. Fig. 12. Branching of crack from cracked zone. V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 54 Fig. 13. Coarse and fine lamellar structure in beta grains. Fig. 14. Grain boundary alpha. Fig. 15. Martensitic alpha structure. V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 55 Fig. 16. Shift in socket head hole from the center. Fig. 17. Pierced zone of hole in the shank. Fig. 18. Nucleation crack in head. V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 56 Fig. 19. Propagation crack in shank. Fig. 20. Striations due to fatigue loading at 750. Fig. 21. Striations due to fatigue loading at 900. V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 57 4. Results and discussions In general, failure in mechanical fasteners occurs due to overload of static loading (tension, shear, bending, or torsion) and dynamic forces result from impact loading and cyclic fatigue loading including vibration loads. In addition, other common cause of failure includes environmental effects, manufacturing discrepancies, improper use and incorrect installation. For present case, primary reason for failure was socket head hole. The shank portion got pierced thereby resulting in the reduction of effective load bearing area, which allowed more stress concentration zones at interface zone, and it was compared with thread profile of metric socket head bolt standard [5]. The cracks started at interface region as pierced zone acted as notch and this region experienced dynamic loads due to vibration test. Surface cracks present in interface region were due to uncontrolled process parameters followed during head forging and socket head hole punching. Microstructure analysis of the bolt revealed from the head to shank had heterogeneity in microstructure. Heterogeneity in microstructure has indicated the localized increase in temperature during head formation, as titanium alloys are very sensitive to temperature and strain rate. Observation of absence of primary alpha in head portion and coarse lamellar structure in the failed zone indicated that increase of temperature during head forging. Presence of fine and coarse lamellar structure in the interface zone was due to transformation decomposition of the high temperature beta phase. Shank portion microstructure shows martensitic alpha, grain boundary alpha with prior grain boundaries occurred due to improper processing after heating to beta field. When Ti''6Al''4V is processed improperly after heating into the beta field; alpha phase can form preferentially along the prior beta grains. Extensive hot work is required to break up such structures. Because cracks tend to propagate in or near interfaces, this type of structure can provide loci for crack initiation and propagation and thereby lead to premature failure [6]. Fig. 22. Striations due to fatigue loading at 1400. Fig. 23. Dimples type fracture zones observed 5000. V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 58 The forging temperature plays an important role in fatigue performance, if Ti''6Al''4V is forged just below the phase transition temperature, the microstructure obtained will have a fatigue limit higher than the fatigue limit of the microstructure obtained forging above bt [7]. Finally, it was accomplished that bolt has failed as a result of fatigue; which transpired due to manufacturing discrepancies of head forging and socket hole. Further, heterogeneity in microstructure aided for further propagation of cracks. Based on the failure analysis, the process parameters followed for manufacturing of the fasteners were studied. Possible microstructure during thermo-mechanical processing with different temperature and cooling medium are illustrated in Fig. 24 [2]. It was understood that, head forging of bolt had manufacturing discrepancies as it was done by localized heating using torch 'ame and there was no control of temperature. Strain rate followed are very high of the order of 10 1 to 10 2/s. Repeated experiments were conducted on head forging to finalize the process parameters. The details of process parameters for manufacturing fasteners are given in Table 3. Head forging was done by controlled heating below the beta transus temperature (<950 8 C) in a furnace. Forging was done at a strain rate of 10 2 to 10 3/s by removing the counter weight of 256 kg. Dye penetrant test results indicated the interface region is free of surface cracks. No significant discontinuities were found during radiography. Further, metallurgical analysis was carried out on the head portion, interface region and shank area. It was revealed that there is no evidence of heterogeneity in microstructure. Also further, socket hole is removed from the design, and plain hexagonal socket head bolt is recommended for future applications. 5. Conclusion Failure analysis of Ti''6Al''4V fasteners was carried out and the failure was attributed to fatigue damage which occurred because socket head hole was pierced into the shank. Metallurgical analysis has confirmed heterogeneity in microstructure, due to uncontrolled process parameters which were followed during fastener manufacturing. An experimental study was conducted to optimize process parameters such that strain rates were optimized as 10 2 to 10 3/s and heating was done in a controlled furnace at 900 8 C. Metallurgical analysis with optimum process parameters revealed that there is no evidence of heterogeneity in microstructure. Fig. 24. Microstructure of Ti''6Al''4V during thermo mechanical processing [2]. Table 3 Process parameters of manufacturing of fastener. Process of manufacture Process parameters Hot rolling Hot rolled to the size F9.0 mm at temperature of 955 8 C Annealing 700 8 C-1 h-air cooling Swaging Reduced to F4.5 mm by hot swaging Head forging Heated to 900 8 C, forged at strain rate of 10 2 to 10 3/s Solution treatment and aging Solution treatment at 895 8 C-1 h-water quenched, aging at 520 8 C-4 h-air cooling Thread rolling One single pass thread rolling V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 59 6. Scope of future work Further, fatigue analysis and vibration test are to be carried out on the realized bolts. Acknowledgements The authors are highly thankful to M/s Midhani, Hyderabad Resident Office, MCD/VSSC, QCIG & PREG of LPSC for providing facilities for manufacturing of fasteners, and characterization during failure and qualification. Authors are highly indebted to Shri. S Ramakrishnan, Director, LPSC for permission of publishing this work. References [1] Forming and forging ASM handbook, vol. 14. OH: ASM International; 1984. [2] Matthew Jr JD. Titanium, a technical guide. 2nd ed. OH: ASM International; 2000. [3] Heat treating, vol. 9. ASM metals handbook. ASM Publications, OH; 2000. [4] Jha AK, et al. Failure analysis of titanium alloy (Ti''6Al''4V) fastener used in aerospace application. Engineering Failure Analysis 2010;17(6):1457''65. [5] NFL22-220, metric threads tolerances. [6] Fatigue and fracture ASM handbook, vol. 19. OH: ASM International; 1984. [7] Manes A, et al. Failure analysis of a composite main rotor helicopter hub. Engineering Failure Analysis 2011;18:97''109. V. Venkateswarlu et al. / Case Studies in Engineering Failure Analysis 1 (2013) 49''60 60

Document Outline

Failure analysis and optimization of thermo-mechanical process parameters of titanium alloy (Ti-6Al-4V) fasteners for aerospace applications Introduction Experimental procedure Input material Process of manufacture Head forging Thread rolling Observations Clearance philosophy followed for fastener Mechanical properties Metallurgical analysis Acceptance test on the fasteners Observations on failed bolt Visual inspection Metallurgical analysis Results and discussions Conclusion Scope of future work Acknowledgements References


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