- Email: [email protected]
Failure analysis of titanium alloy (Ti6Al4V) fastener used in aerospace application
Engineering Failure Analysis 17 (2010) 1457–1465
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of titanium alloy (Ti6Al4V) fastener used in aerospace application Abhay K. Jha *, Satish Kumar Singh, M. Swathi Kiranmayee, K. Sreekumar, P.P. Sinha Materials Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Indian Space Research Organization, Trivandrum 695 022, India
a r t i c l e
i n f o
Article history: Received 2 March 2010 Received in revised form 7 May 2010 Accepted 26 May 2010 Available online 1 June 2010 Keywords: Failure of titanium fastener Beta transus temperature Bulging of transformed alpha in Ti alloy
a b s t r a c t Titanium alloy fasteners are being used in space programme. These fasteners are coated with MoS2, which serves the purpose of solid lubricant. During the trial assembly of flight spin motor to the bracket mounted on subsystem, one of the two fasteners failed such that the head of the bolt had sheared off the shank. Metallographic analysis carried out on the failed fasteners revealed variations in the microstructures all along the shank axis. Microstructure consisted of equiaxed primary alpha in transformed beta matrix within lower portion of the shank, while it was elongated primary alpha with little bulging all along prior beta grain boundaries as well as acicular alpha at some other location towards the head side, features, typical of, as if worked above beta transus temperatures. This paper highlights the details of investigations carried out on the failed fasteners. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Titanium alloys find extensive application in aerospace industry due to their high strength to weight ratio, high elevated temperature properties up to 600 °C and excellent corrosion resistance. Pure titanium exists in two allotropic forms: Hexagonal Close Packed (HCP) alpha and Body Centered Cubic (BCC) beta phases. Beta phase is called high temperature phase and the allotropic transformation from beta-to-alpha phase occurs at 882 °C during cooling. Through suitable alloying additions to the titanium, the high temperature beta phase can be retained at ambient temperature. The temperature above which only beta phase is present is called beta transus temperature. The characteristics of titanium along with other metal are included in Table 1 [1]. As seen from the table thermal conductivity and specific heat is substantially lower that iron and nickel. These factors together lead to comparatively more heat generation due to adiabatic heating during mechanical working of the alloy compared to iron and nickel. Ti6Al4V alloy contains aluminum as alpha and vanadium as beta stabilizer and contains both alpha and beta phases at ambient temperature. Microstructures play an important role in determining the strength, ductility, crack resistance and fracture toughness of the alloy and these microstructures are closely related with processing history like super and sub beta transus processing or working and the subsequent heat treatment [2]. Microstructure can vary from equiaxed to elongated primary alpha (ap) in transformed beta matrix from heavy to light mechanical working below beta transus temperature respectively. Volume fraction of primary alpha is related with temperature of processing below beta transus temperature. Mechanical working above beta transus temperature results in completely transformed microstructure. The microstructure of the alloy processed above beta transus consists of colonies of a plate, the size of which depends on degree of working and thermal history [3]. Generally alloy is used in annealed condition. The alloy is in heat treatment (solution treatment and age-
* Corresponding author. Tel.: +91 471 2563628; fax: +91 471 2705048. E-mail address: [email protected] (A.K. Jha). 1350-6307/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2010.05.007
1458
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Table 1 Physical and thermal properties of titanium and other metals [1]. Sl. no.
Property
Nickel
Iron
Titanium
1 2 3 4 5 6
Melting point (°C) Density (g/cm3) Heat capacity (J/g/°C) Thermal conductivity (W/cm/°C) Thermal diffusivity (Cm2/s) Heat of fusion per unit volume (J/Cm3)
1453 8.90 3.95 0.907 0.230 5742
1535 7.87 3.54 0.802 0.227 5427
1662 4.54 2.37 0.219 0.092 3946
ing) condition has tensile properties 18% higher than annealed condition. The solution treatment above beta transus temperature results in martensitic structure (acicular alpha) while solution treatment below beta transus temperature results in primary alpha and retained beta in martensitic structure. Gas bottles and propellant tank of Ti6Al4V alloy for ambient application for launch vehicle and satellites are used for storing gaseous Helium and liquid propellants. It also finds application as high strength fasteners. The bolts are made from rods. Head and threads of the bolt are realized through hot forging and through thread rolling operations respectively. The fillet radius of bolt is realized through cold-working-machining operation [4]. During the trial assembly of flight spin motor to the bracket, one of the two fasteners failed such that the head of the bolt had sheared off from the shank portion of bolt. The failure was noticed after various functional qualification tests were over. The head of the sheared off bolt was not available and hence the analysis could be carried out with the failed shank portion only. 2. Observations The bolt used was full-length thread and the failure occurred at the head–shank junction. The thread root, next to the failure edge had cracks (Fig. 1). The thread roots had pits and their interlinking resulted in crack formation (Fig. 2). The presence of such cracks were found in almost 3–4 threads next to fracture end. The crack path (Fig. 3) appeared to be along grain.
CRACK
Fig. 1. The failed bolt, presence of crack at thread root, next to the fractured edge.
Fig. 2. Presence of pits/cavities, their interlinking and formation of crack at thread roots of the failed bolt.
1459
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Isolated pits/cavities at thread root
Cracking along grain boundaries Fig. 3. Pits/cavities at thread root of failed bolt.
Crack at thread root
Crack at thread root
Fig. 4. The crack across the bulk of shank material, absence of crack branching.
Fig. 5. Optical photomicrograph showing presence of predominantly martensitic alpha with no globular primary alpha phases at fracture edge.
1460
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Fig. 6. Optical photomicrographs showing presence of predominantly martensitic alpha with few globular primary alpha phases near fracture edge.
Fig. 7. Martensitic alpha phase.
The shank portion of the failed fastener was sliced off along the shank axis and polished using conventional metallorgraphic techniques. The polished un-etched specimen was observed under optical microscope for crack depth. There were no branching of crack and the depth of such crack/defect was estimated to be maximum 100 lm (Fig. 4). The polished specimen duly etched with Krolls’ reagent revealed predominantly martensitic alpha (alpha prime) with few globular primary alpha phase (Fig. 5) near the fracture edge. This structure is typical of material worked very close to beta transus temperature of 990 °C and followed by very fast cooling. The acicular martensitic alpha could be seen very clearly at higher magnification (Fig. 6). Microstructure observed from the fracture edge all along the shank revealed variation in microstructures. Microstructure slightly away from the fracture edge consisted of martensitic alpha (Fig. 7) and primary alpha and alpha along prior beta grain boundaries, whereas equiaxed primary alpha in transformed beta matrix was seen at locations away from the fracture edge (Fig. 8, location 3). Bulging of few primary alpha phase present at prior beta grain boundaries was also noticed, an indicative of working at higher temperature than the solution treatment temperature 950 °C (Fig. 9). Also, extent of bulging was more as it moved away from fracture edge of the bolt. A variation in volume fraction of primary alpha was seen from near or to away the fracture edge. Fracture surface studies of failed bolt under scanning electron microscope indicated failure initiated at thread root and propagated inward (Fig. 10). Fracture surface had shear lip, which was typical of ductile failure. Features seen on fracture surface were in consistent with the morphology of martensitic alpha (Fig. 11). Few fasteners from the same batch of supply were tested for mechanical properties. The fasteners exhibited tensile strength in the range of 115–131 kg/mm2. One of the tensile tested fasteners was observed for surface crack at thread root, if any, microstructure and fracture surface study. Thread roots were devoid of any pits and their interlinking. Variation in microstructure was also seen in similar manner to that of failed one. Martensitic alpha at and near head–shank junction was seen, while at locations away from the junction, it was equiaxed alpha and transformed beta structure (Fig. 12). Fracture
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
1461
1 2
3
Location 1
Location 2
Location 3
Fig. 8. Optical photomicrographs showing elongated primary alpha along prior beta grain boundaries as well as acicular alpha at locations 1 and 2, and equiaxed primary alphas in transformed beta matrix at location 3.
A A A A
a Grain boundary alpha phase
Martensitic alpha
b Fig. 9. Optical photomicrographs showing (a) martensitic alpha (location A) and (b) primary alpha along prior beta grain boundaries (as shown by arrows).
1462
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
surface of tensile tested fasteners revealed fine equiaxed dimples (Fig. 13) in consistent with that of alpha–beta microstructures.
3. Discussions Primary alpha in transformed beta matrix away from head–shank interface of bolt has indicated that rod used for fabrication of the bolt is processed in two-phase region of the alloy. Head of bolt is recommended to be hot forged in two-phase regions (below 995 °C) of the alloy for optimum combination of strength and ductility. Observation of absence of primary alpha near the shank–head junction and the head area indicates increase of temperature during head formation of the bolt semis above beta transus temperature. Whereas retention of primary alpha in shank portion of bolt has indicated only localized heating of the bolt semis and not complete semis during their head formation. The variation of volume fraction of primary alpha from head-to-shank area of the bolt is due to variation in temperature from head-to-shank area. Bulging or growth of transformed alpha grain in the microstructure (Fig. 9) is due to growth of un-dissolved acicular alpha during its next heating to a temperature lower than working temperature as volume fraction of beta phase increases with temperature. Solutioning of the alloy for solution treatment operation is carried out at 955 °C, two-phase regions of the alloy, and then cools at faster rate to form martensitic alpha and the typical microstructure is shown in Fig. 15. It can be seen from Fig. 14 [5] that when the solution treatment temperature reaches in two-phase field to 843 °C, Ms temperature reaches to ambient temperature and faster cooling from and below this temperature will not lead to martensitic transformation. Martensitic structure is observed in head area of both the bolts (Figs. 5, 6 and 12) while it could not be confirmed in shank area. This is indicative of the fact that solution treatment operation was not properly carried out on bolts. Fig. 12a and b shows flow lines, which is due to flow of materials during the hot forging operation of head formation of the bolt. Microstructure observed on failed and other bolts are similar and these variations in microstructure are mainly due to process deviations during the processing of the bolts.
Fracture initiated
a
b Fig. 10. SEM fractographs showing (a) fracture surface with fibrous feature and (b) fracture initiated at thread root.
Fig. 11. SEM fractographs showing (a and b) fracture surface morphology.
1463
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Johnson et al. [6] studied the influence of microstructure on deformation behavior of Ti6Al4V. They found that sample with widmanstatten microstructure failed at a smaller strain than the counterpart with equiaxed microstructure, and this difference increased with increasing strain rate. With equiaxed microstructure, short cracks or voids formed are usually blunted at grain boundaries whereas cracks can grow within the laths of a-phase more easily. Failure of the bolt is at shank–head interface. The fillet-radius at the interface is provided through cold-working and machining operations. It is likelihood of introduction of notch at the interface and it is probably presence of the notch would have caused failure of the bolt during its operation.
Head
Flow Lines Flow Lines
Hea .
Shank
a
b
c
d
e
f
Shank
Fig. 12. Optical photomicrographs showing microstructures at various locations of tensile tested fastener. (a and b) At head–shank junction showing flow lines, (c and d) acicular martensitic alpha and primary alpha along prior beta grain boundaries, (e) resolved martensitic alpha within coarsened transformed beta matrix and (f) equiaxed alpha–beta structure.
1464
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Fig. 13. SEM fractographs of tensile tested bolt.
Fig. 14. Schematic pseudo-binary phase diagram for Ti6Al–V alloy [5].
The possibility of pits resulted from corrosion is very remote, as titanium alloys posses good corrosion resistance in many environments. The assembly had experienced various functional qualification tests, which included a virational test. There is
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
1465
Fig. 15. Optical micrograph, showing primary alpha in martensitic alpha matrix, typical of solution treated at 955 °C.
every possibility of fatigue crack initiation at grain boundaries triple points resembling like corrosion pits and the formation of crack along the grain boundaries to a certain depth of approximately 100 lm, as measured, under vibrational load. Jha et al. [7] has correlated microstructure consisted of a-phase along the prior b grain boundaries with a lower value of elongation in unidirectional loading condition. They established that a-phase along the prior b grain boundaries had assisted the material to yield at smaller strain and cracks to grow with laths of a-phase. 4. Conclusions
1. 2. 3. 4.
Analysis of bolt revealed process deviation during the processing of the bolt. The process deviation resulted in variation in microstructure from head-to-shank area of the bolt. The microstructure confirmed material working (head formation) at temperature above beta transus temperature. Such microstructure deteriorates the properties of material and hence the failure, especially in presence of sharp fillet corner at head–shank junction.
Acknowledgement The authors are indebted to Sri P.S. Veeraraghavan, Director, VSSC for permission to publish this work. References [1] Siegle SR et al. Consideration in processing titanium. Mater Sci Eng 1999;A263:237. [2] Chesnutt JC, et al. Relationship between mechanical properties, microstructure, and fracture topography in alpha + beta titanium alloys, fractographymicroscopic cracking process. ASTM STP 600, pp. 99. [3] Williams JC. Titanium alloys: production, behavior and application, high performance materials in aerospace. In: Harney M, editor. New York: Flower, Chapman and Hall; 1995. p. 85. [4] Aerospace –Bolts with MJ threads, in titanium alloys, strength class 1100 MPa-procurement specification, ISO 9152. [5] Smith Williams F. structure and properties of engineering alloys. McGraw-Hill Book Company.; 1981. p. 411. [6] Wagoner Johnson AJ, Bull CW, Kumar KS, Briant CL. Metal Mater Trans A 2003;34A(February):295. [7] Jha Abhay K, Diwakar V, Pant Bhanu, Sreekumar S. Eng Fail Anal 2006;13:P843–856.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of titanium alloy (Ti6Al4V) fastener used in aerospace application Abhay K. Jha *, Satish Kumar Singh, M. Swathi Kiranmayee, K. Sreekumar, P.P. Sinha Materials Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Indian Space Research Organization, Trivandrum 695 022, India
a r t i c l e
i n f o
Article history: Received 2 March 2010 Received in revised form 7 May 2010 Accepted 26 May 2010 Available online 1 June 2010 Keywords: Failure of titanium fastener Beta transus temperature Bulging of transformed alpha in Ti alloy
a b s t r a c t Titanium alloy fasteners are being used in space programme. These fasteners are coated with MoS2, which serves the purpose of solid lubricant. During the trial assembly of flight spin motor to the bracket mounted on subsystem, one of the two fasteners failed such that the head of the bolt had sheared off the shank. Metallographic analysis carried out on the failed fasteners revealed variations in the microstructures all along the shank axis. Microstructure consisted of equiaxed primary alpha in transformed beta matrix within lower portion of the shank, while it was elongated primary alpha with little bulging all along prior beta grain boundaries as well as acicular alpha at some other location towards the head side, features, typical of, as if worked above beta transus temperatures. This paper highlights the details of investigations carried out on the failed fasteners. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Titanium alloys find extensive application in aerospace industry due to their high strength to weight ratio, high elevated temperature properties up to 600 °C and excellent corrosion resistance. Pure titanium exists in two allotropic forms: Hexagonal Close Packed (HCP) alpha and Body Centered Cubic (BCC) beta phases. Beta phase is called high temperature phase and the allotropic transformation from beta-to-alpha phase occurs at 882 °C during cooling. Through suitable alloying additions to the titanium, the high temperature beta phase can be retained at ambient temperature. The temperature above which only beta phase is present is called beta transus temperature. The characteristics of titanium along with other metal are included in Table 1 [1]. As seen from the table thermal conductivity and specific heat is substantially lower that iron and nickel. These factors together lead to comparatively more heat generation due to adiabatic heating during mechanical working of the alloy compared to iron and nickel. Ti6Al4V alloy contains aluminum as alpha and vanadium as beta stabilizer and contains both alpha and beta phases at ambient temperature. Microstructures play an important role in determining the strength, ductility, crack resistance and fracture toughness of the alloy and these microstructures are closely related with processing history like super and sub beta transus processing or working and the subsequent heat treatment [2]. Microstructure can vary from equiaxed to elongated primary alpha (ap) in transformed beta matrix from heavy to light mechanical working below beta transus temperature respectively. Volume fraction of primary alpha is related with temperature of processing below beta transus temperature. Mechanical working above beta transus temperature results in completely transformed microstructure. The microstructure of the alloy processed above beta transus consists of colonies of a plate, the size of which depends on degree of working and thermal history [3]. Generally alloy is used in annealed condition. The alloy is in heat treatment (solution treatment and age-
* Corresponding author. Tel.: +91 471 2563628; fax: +91 471 2705048. E-mail address: [email protected] (A.K. Jha). 1350-6307/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2010.05.007
1458
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Table 1 Physical and thermal properties of titanium and other metals [1]. Sl. no.
Property
Nickel
Iron
Titanium
1 2 3 4 5 6
Melting point (°C) Density (g/cm3) Heat capacity (J/g/°C) Thermal conductivity (W/cm/°C) Thermal diffusivity (Cm2/s) Heat of fusion per unit volume (J/Cm3)
1453 8.90 3.95 0.907 0.230 5742
1535 7.87 3.54 0.802 0.227 5427
1662 4.54 2.37 0.219 0.092 3946
ing) condition has tensile properties 18% higher than annealed condition. The solution treatment above beta transus temperature results in martensitic structure (acicular alpha) while solution treatment below beta transus temperature results in primary alpha and retained beta in martensitic structure. Gas bottles and propellant tank of Ti6Al4V alloy for ambient application for launch vehicle and satellites are used for storing gaseous Helium and liquid propellants. It also finds application as high strength fasteners. The bolts are made from rods. Head and threads of the bolt are realized through hot forging and through thread rolling operations respectively. The fillet radius of bolt is realized through cold-working-machining operation [4]. During the trial assembly of flight spin motor to the bracket, one of the two fasteners failed such that the head of the bolt had sheared off from the shank portion of bolt. The failure was noticed after various functional qualification tests were over. The head of the sheared off bolt was not available and hence the analysis could be carried out with the failed shank portion only. 2. Observations The bolt used was full-length thread and the failure occurred at the head–shank junction. The thread root, next to the failure edge had cracks (Fig. 1). The thread roots had pits and their interlinking resulted in crack formation (Fig. 2). The presence of such cracks were found in almost 3–4 threads next to fracture end. The crack path (Fig. 3) appeared to be along grain.
CRACK
Fig. 1. The failed bolt, presence of crack at thread root, next to the fractured edge.
Fig. 2. Presence of pits/cavities, their interlinking and formation of crack at thread roots of the failed bolt.
1459
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Isolated pits/cavities at thread root
Cracking along grain boundaries Fig. 3. Pits/cavities at thread root of failed bolt.
Crack at thread root
Crack at thread root
Fig. 4. The crack across the bulk of shank material, absence of crack branching.
Fig. 5. Optical photomicrograph showing presence of predominantly martensitic alpha with no globular primary alpha phases at fracture edge.
1460
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Fig. 6. Optical photomicrographs showing presence of predominantly martensitic alpha with few globular primary alpha phases near fracture edge.
Fig. 7. Martensitic alpha phase.
The shank portion of the failed fastener was sliced off along the shank axis and polished using conventional metallorgraphic techniques. The polished un-etched specimen was observed under optical microscope for crack depth. There were no branching of crack and the depth of such crack/defect was estimated to be maximum 100 lm (Fig. 4). The polished specimen duly etched with Krolls’ reagent revealed predominantly martensitic alpha (alpha prime) with few globular primary alpha phase (Fig. 5) near the fracture edge. This structure is typical of material worked very close to beta transus temperature of 990 °C and followed by very fast cooling. The acicular martensitic alpha could be seen very clearly at higher magnification (Fig. 6). Microstructure observed from the fracture edge all along the shank revealed variation in microstructures. Microstructure slightly away from the fracture edge consisted of martensitic alpha (Fig. 7) and primary alpha and alpha along prior beta grain boundaries, whereas equiaxed primary alpha in transformed beta matrix was seen at locations away from the fracture edge (Fig. 8, location 3). Bulging of few primary alpha phase present at prior beta grain boundaries was also noticed, an indicative of working at higher temperature than the solution treatment temperature 950 °C (Fig. 9). Also, extent of bulging was more as it moved away from fracture edge of the bolt. A variation in volume fraction of primary alpha was seen from near or to away the fracture edge. Fracture surface studies of failed bolt under scanning electron microscope indicated failure initiated at thread root and propagated inward (Fig. 10). Fracture surface had shear lip, which was typical of ductile failure. Features seen on fracture surface were in consistent with the morphology of martensitic alpha (Fig. 11). Few fasteners from the same batch of supply were tested for mechanical properties. The fasteners exhibited tensile strength in the range of 115–131 kg/mm2. One of the tensile tested fasteners was observed for surface crack at thread root, if any, microstructure and fracture surface study. Thread roots were devoid of any pits and their interlinking. Variation in microstructure was also seen in similar manner to that of failed one. Martensitic alpha at and near head–shank junction was seen, while at locations away from the junction, it was equiaxed alpha and transformed beta structure (Fig. 12). Fracture
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
1461
1 2
3
Location 1
Location 2
Location 3
Fig. 8. Optical photomicrographs showing elongated primary alpha along prior beta grain boundaries as well as acicular alpha at locations 1 and 2, and equiaxed primary alphas in transformed beta matrix at location 3.
A A A A
a Grain boundary alpha phase
Martensitic alpha
b Fig. 9. Optical photomicrographs showing (a) martensitic alpha (location A) and (b) primary alpha along prior beta grain boundaries (as shown by arrows).
1462
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
surface of tensile tested fasteners revealed fine equiaxed dimples (Fig. 13) in consistent with that of alpha–beta microstructures.
3. Discussions Primary alpha in transformed beta matrix away from head–shank interface of bolt has indicated that rod used for fabrication of the bolt is processed in two-phase region of the alloy. Head of bolt is recommended to be hot forged in two-phase regions (below 995 °C) of the alloy for optimum combination of strength and ductility. Observation of absence of primary alpha near the shank–head junction and the head area indicates increase of temperature during head formation of the bolt semis above beta transus temperature. Whereas retention of primary alpha in shank portion of bolt has indicated only localized heating of the bolt semis and not complete semis during their head formation. The variation of volume fraction of primary alpha from head-to-shank area of the bolt is due to variation in temperature from head-to-shank area. Bulging or growth of transformed alpha grain in the microstructure (Fig. 9) is due to growth of un-dissolved acicular alpha during its next heating to a temperature lower than working temperature as volume fraction of beta phase increases with temperature. Solutioning of the alloy for solution treatment operation is carried out at 955 °C, two-phase regions of the alloy, and then cools at faster rate to form martensitic alpha and the typical microstructure is shown in Fig. 15. It can be seen from Fig. 14 [5] that when the solution treatment temperature reaches in two-phase field to 843 °C, Ms temperature reaches to ambient temperature and faster cooling from and below this temperature will not lead to martensitic transformation. Martensitic structure is observed in head area of both the bolts (Figs. 5, 6 and 12) while it could not be confirmed in shank area. This is indicative of the fact that solution treatment operation was not properly carried out on bolts. Fig. 12a and b shows flow lines, which is due to flow of materials during the hot forging operation of head formation of the bolt. Microstructure observed on failed and other bolts are similar and these variations in microstructure are mainly due to process deviations during the processing of the bolts.
Fracture initiated
a
b Fig. 10. SEM fractographs showing (a) fracture surface with fibrous feature and (b) fracture initiated at thread root.
Fig. 11. SEM fractographs showing (a and b) fracture surface morphology.
1463
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Johnson et al. [6] studied the influence of microstructure on deformation behavior of Ti6Al4V. They found that sample with widmanstatten microstructure failed at a smaller strain than the counterpart with equiaxed microstructure, and this difference increased with increasing strain rate. With equiaxed microstructure, short cracks or voids formed are usually blunted at grain boundaries whereas cracks can grow within the laths of a-phase more easily. Failure of the bolt is at shank–head interface. The fillet-radius at the interface is provided through cold-working and machining operations. It is likelihood of introduction of notch at the interface and it is probably presence of the notch would have caused failure of the bolt during its operation.
Head
Flow Lines Flow Lines
Hea .
Shank
a
b
c
d
e
f
Shank
Fig. 12. Optical photomicrographs showing microstructures at various locations of tensile tested fastener. (a and b) At head–shank junction showing flow lines, (c and d) acicular martensitic alpha and primary alpha along prior beta grain boundaries, (e) resolved martensitic alpha within coarsened transformed beta matrix and (f) equiaxed alpha–beta structure.
1464
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
Fig. 13. SEM fractographs of tensile tested bolt.
Fig. 14. Schematic pseudo-binary phase diagram for Ti6Al–V alloy [5].
The possibility of pits resulted from corrosion is very remote, as titanium alloys posses good corrosion resistance in many environments. The assembly had experienced various functional qualification tests, which included a virational test. There is
A.K. Jha et al. / Engineering Failure Analysis 17 (2010) 1457–1465
1465
Fig. 15. Optical micrograph, showing primary alpha in martensitic alpha matrix, typical of solution treated at 955 °C.
every possibility of fatigue crack initiation at grain boundaries triple points resembling like corrosion pits and the formation of crack along the grain boundaries to a certain depth of approximately 100 lm, as measured, under vibrational load. Jha et al. [7] has correlated microstructure consisted of a-phase along the prior b grain boundaries with a lower value of elongation in unidirectional loading condition. They established that a-phase along the prior b grain boundaries had assisted the material to yield at smaller strain and cracks to grow with laths of a-phase. 4. Conclusions
1. 2. 3. 4.
Analysis of bolt revealed process deviation during the processing of the bolt. The process deviation resulted in variation in microstructure from head-to-shank area of the bolt. The microstructure confirmed material working (head formation) at temperature above beta transus temperature. Such microstructure deteriorates the properties of material and hence the failure, especially in presence of sharp fillet corner at head–shank junction.
Acknowledgement The authors are indebted to Sri P.S. Veeraraghavan, Director, VSSC for permission to publish this work. References [1] Siegle SR et al. Consideration in processing titanium. Mater Sci Eng 1999;A263:237. [2] Chesnutt JC, et al. Relationship between mechanical properties, microstructure, and fracture topography in alpha + beta titanium alloys, fractographymicroscopic cracking process. ASTM STP 600, pp. 99. [3] Williams JC. Titanium alloys: production, behavior and application, high performance materials in aerospace. In: Harney M, editor. New York: Flower, Chapman and Hall; 1995. p. 85. [4] Aerospace –Bolts with MJ threads, in titanium alloys, strength class 1100 MPa-procurement specification, ISO 9152. [5] Smith Williams F. structure and properties of engineering alloys. McGraw-Hill Book Company.; 1981. p. 411. [6] Wagoner Johnson AJ, Bull CW, Kumar KS, Briant CL. Metal Mater Trans A 2003;34A(February):295. [7] Jha Abhay K, Diwakar V, Pant Bhanu, Sreekumar S. Eng Fail Anal 2006;13:P843–856.