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Metallurgical analysis of cracks on an aluminum alloy ring used in a satellite separation system
Engineering Failure Analysis 9 (2002) 709–719 www.elsevier.com/locate/engfailanal
Metallurgical analysis of cracks on an aluminum alloy ring used in a satellite separation system Abhay K. Jha*, S.V.S.N. Murty, E. Jacob Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, PO ISRO, Trivandrum 695 022, India Received 12 November 2001; accepted 12 December 2001
Abstract High strength Al–Cu alloys of the 2XXX series suit the critical requirements of structural applications in satellite launch vehicles. Copper is the principal alloying element in this alloy, often with magnesium as a secondary, addition. These alloys do not have as good corrosion resistance as most other aluminum alloys, however, they find widespread use for parts and structures requiring high specific strength. Al alloy ring of AA 2014 grade is being used for a satellite separation system in the Indian space programme. This ring consists of two lugs at diametrically opposite locations and rotates through the required angle under thrust generated by pyro thrusters. Recently, after one of the functional qualification tests of the system, both the lugs of the ring were found to be cracked. Detailed analysis revealed that a notch created by the stopper against pyro actuation played a role in crack initiation. Insufficient working during the mandrel forging operation and the presence of iron-rich particles facilitated crack propagation as revealed by fragmentation of particles and dendrite lobes on the fracture surface. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ductile fracture; Stress concentrations; Precipitates; Dendrites
1. Introduction High strength aluminum–copper alloys of the 2XXX series suit the critical requirements of structural applications in satellite launch vehicles, due to their high specific strength. Aluminum alloy AA 2014 is being used for various structural applications in launch vehicles. One such application is in the microsatellite separation system of the vehicle upper stage. The system consists of an interface ring to the satellite; an interface ring to the vehicle; and a retainer ring to lock both the interface rings in position. This system works on a ball and lock mechanism and consists of two lugs at diametrically opposite locations. Two pyro thrusters are used to apply thrust on these lugs and rotate the ring through the required angle, releasing the interface ring to the satellite, which is jettisoned by using spring thrusters. There are stoppers, which stop the lugs after the required angle of rotation. A schematic of the system along a vertical section is shown in Fig. 1. * Corresponding author. Tel.: 91-471-563-237; fax: +91-471-415-348. E-mail address: [email protected] (A.K. Jha). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00004-3
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Recently, during one of the functional qualification tests of the system, both lugs of the retainer ring were found to be cracked. Both the cracked lugs were subjected to extensive metallurgical investigation and this paper highlights the details of analysis carried out.
2. Material Al alloy AA 2014 of cast ingot size 215 mm dia.700 mm height is used for production of the ring through mandrel forging route. The cast input material is upset to 200 mm height. Punching is done using a 120 mm dia. punch and stock is a mandrel forged to a size of 450230180 mm. The mandrel-forged ring in proof machined size (420 OD260 ID160 mm height) is parted into seven rings followed by individual machining to the required ring size. The achieved mechanical properties of the forged piece are UTS: 455–500 MPa, YS: 390–460 MPa.
3. Observations The retainer ring consisted of two lugs (henceforth referred to as lug ‘A’ and lug ‘B’) at diametrically opposite locations (Fig. 2). Lug ‘A’ was fully broken while lug ‘B’ was cracked up to 90% of the lug
Fig. 1. Schematic of the system along a vertical section.
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Fig. 2. Retainer ring with lugs A and B.
Fig. 3. Lug ‘B’ of the retainer ring showing the dent by the stopper and the roughness of the curvature.
Table 1 Chemical composition of the ring Element
Cu
Mn
Mg
Si
Fe
Cr
Zn
Al
Wt.%
4.50
0.60
0.50
0.20
0.20
<0.05
<0.05
Rest
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Fig. 4. Lug ‘B’ of the retainer ring showing the crack along the height of the ring.
Fig. 5. Crack on the face of lug ‘B’ towards the aft end ring showing the crack morphology, 10.
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thickness. The crack had propagated along the plane perpendicular to the radial direction of the ring i.e. the axial plane of the ring or short transverse plane of the upset block. Both the lugs were taken out from the ring and observed under stereomicroscope, optical microscope and scanning electron microscope. A cut piece was analysed for its chemical composition and the results tabulated in Table 1. The silicon content is found to be lower than that for AA 2014 Al alloy. Stereo observations revealed that the radius of curvature on either side of the lugs was not smooth. The side towards the stopper end indicated machining/tool marks resulting in irregular curvature. In both the lugs, the vertical face facing the stopper revealed a deep dent. These dents were the result of plastic yielding of material due to the force exerted by the stopper against pyro actuation. Fig. 3 shows the crack and roughness of curvature as well as the dent caused by the stopper. The crack along the height of the ring for
Fig. 6. Crack on the face of lug ‘B’ towards the fore end ring showing the crack initiation from the severely mechanically damaged region, 10.
Fig. 7. Stereo photograph of the crack on the face of lug ‘B’ towards the fore end ring, 40.
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Fig. 8. Stereo photograph of the crack on the face of lug ‘B’ towards the fore end ring, 40.
Fig. 9. Stereo photograph of the crack of lug ‘A’ at the fracture end showing the damaged region and the crack morphology, 20.
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the lug ‘B’ is shown in Fig. 4. The two horizontal faces of the lug mating with two different interface rings are shown in Figs. 5 and 6. Evidence of crack initiation from the severely yielded/damaged region was clearly seen (Fig. 6). The crack morphology on both faces of the lug are shown in Figs. 7 and 8. Deviation in crack path as well as debonding from the matrix at the deviation point was seen (Fig. 7). The crack path followed in lug ‘A’ is shown in Fig. 9. The morphology of the crack path and initiation of crack for both the lugs i.e. lug ‘A’ and lug ‘B’ were found to be identical. Cut pieces from both the lugs were metallographically polished using conventional polishing techniques and etched with Keller’s reagent to reveal the microstructure. The microstructure revealed the typical structure of a forging that received inadequate working (Fig. 10) [1]. The ingot cast structure has not been fully broken-up, as revealed by the presence of a dendrite pattern (Fig. 11). Nonuniform distributions of
Fig. 10. Optical micrograph showing the microstructure typical of forging that received inadequate working, 60.
Fig. 11. Optical micrograph showing dendritic pattern as cast structure not fully broken-up, 120.
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precipitate CuAl2 (white outlined) and (Fe, Mn)3 SiAl12 (dark) preferentially along the grain boundaries were seen (Fig. 12). Grain size measurements were carried out on both the 1ugs. The grain size near the crack in lug ‘B’ varies up to 515315 mm while in lug A, grains were still coarser (600 mm). Typical photographs for the grain sizes of lugs A and B are shown in Figs. 13 and 14 respectively. Scanning electron microscopic observations made on the fracture surface of both the lugs revealed a predominantly ductile mode of failure (Fig. 15). However, the presence of particles on fracture surfaces and their fragmentation were seen at a few locations (Fig. 16a and b). These fragmented particles were confirmed by energy dispersive spectroscopy as iron-rich particles (Fig. 16c). Dendrite lobes within a shrinkage cavity were also seen at few locations (Fig. 17).
Fig. 12. Optical micrograph showing the distribution of precipitates, 60.
Fig. 13. Optical micrograph showing the coarse grain—lug ‘A’, 120.
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4. Discussion The observations indicate that the material has undergone plastic yielding under the resistance/force exerted by the stopper against the pyro actuator. This resulted in a dent, which caused a notch effect and crack initiation at the severely deformed region. This is seen in Figs. 6 and 7. The crack has initiated from the severely deformed region and propagated in the short transverse plane for which material deformability is poor as usually indicated by a low value of elongation. In this present case the microstructure revealed evidence of inadequate working while forging, which provides very poor ductility especially in the ST plane. The presence of coarser grains as well as dendrite pattern/arms indicated that even the cast structure was not fully broken-up. This is further in agreement with the observation of dendrite lobes within
Fig. 14. Optical micrograph showing the coarse grain—lug ‘B’, 60.
Fig. 15. SEM photograph showing the dimpled mode of failure.
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Fig. 16. SEM photographs showing (a) the presence of Fe rich particles and their fragmentation, (b) the location in ‘a’ at higher magnification and (c) the EDS spectrum of the particle.
Fig. 17. SEM photograph showing the presence of dendritic lobes within the shrinkage cavity.
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shrinkage cavities at a few locations (Fig. 17). This may impair further the formability of the material. The presence of such particles and their fragmentation in some of the AA 2014 alloy components have been reported earlier by Jha et al. [2], where the presence of such particles has been attributed to the segregation of Cu, as well as impurities like Fe, Mn and Si present in the melt during the solidification stage. It has been reported that an Fe addition in Al alloys has a deleterious effect on the fracture toughness due to the formation of iron-rich compound [3,4]. Such particles are incoherent with the matrix and have multiple cracks due to their brittle nature. The presence of Fe-rich particles has assisted the crack propagation as revealed by their fragmentation on the fracture surface. A sufficient amount of material yielding as caused by the stopper is indicative of the ductile nature of the material, which is supported by the evidence of a dimpled mode of failure. Stress raisers at the curvature like dent/tool marks at crack initiation sites played a significant role and the crack has initiated from the mechanically damaged region caused by the stopper against pyro actuation.
5. Conclusion Cracking of lugs is due to a notch effect created by the hitting of the lugs against the stopper. Insufficient working during forging and the presence of iron-rich particles has facilitated crack propagation.
Acknowledgements The authors are thankful to Dr. T.S. Lakshmanan, Head, Material Characterisation Division, Shri M.C. Mittal, Group Director, Materials and Metallurgy Group and Shri K.S. Sastri, Deputy Director (PCM), VSSC for their valuable technical guidance during the work. The authors wish to thank Shri Madhavan Nair, Director, VSSC to permit us to publish this paper.
References [1] Metals hand book, vol. 7, 8th ed. Metals Park (OH): American Society for Metals, 1972. p. 245. [2] Jha AK, et al. Failure of AA 2014 aluminum alloy bracket, vol. 10. Germany: Practical Metallography, XXIX, 1992. p. 534. [3] Murali S, et al. In: Proc. of the Second International Conference on Aluminum, INCAL-91, vol. 2. Bangalore: The Aluminum Association of India, 1991. p. 644. [4] Zinkham RE, Dedrick JH. In: Liebowitz H, editor. Fracture—an advanced treatise, vol. VI. New York: Academic Press, 1969. p. 369.
Metallurgical analysis of cracks on an aluminum alloy ring used in a satellite separation system Abhay K. Jha*, S.V.S.N. Murty, E. Jacob Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, PO ISRO, Trivandrum 695 022, India Received 12 November 2001; accepted 12 December 2001
Abstract High strength Al–Cu alloys of the 2XXX series suit the critical requirements of structural applications in satellite launch vehicles. Copper is the principal alloying element in this alloy, often with magnesium as a secondary, addition. These alloys do not have as good corrosion resistance as most other aluminum alloys, however, they find widespread use for parts and structures requiring high specific strength. Al alloy ring of AA 2014 grade is being used for a satellite separation system in the Indian space programme. This ring consists of two lugs at diametrically opposite locations and rotates through the required angle under thrust generated by pyro thrusters. Recently, after one of the functional qualification tests of the system, both the lugs of the ring were found to be cracked. Detailed analysis revealed that a notch created by the stopper against pyro actuation played a role in crack initiation. Insufficient working during the mandrel forging operation and the presence of iron-rich particles facilitated crack propagation as revealed by fragmentation of particles and dendrite lobes on the fracture surface. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ductile fracture; Stress concentrations; Precipitates; Dendrites
1. Introduction High strength aluminum–copper alloys of the 2XXX series suit the critical requirements of structural applications in satellite launch vehicles, due to their high specific strength. Aluminum alloy AA 2014 is being used for various structural applications in launch vehicles. One such application is in the microsatellite separation system of the vehicle upper stage. The system consists of an interface ring to the satellite; an interface ring to the vehicle; and a retainer ring to lock both the interface rings in position. This system works on a ball and lock mechanism and consists of two lugs at diametrically opposite locations. Two pyro thrusters are used to apply thrust on these lugs and rotate the ring through the required angle, releasing the interface ring to the satellite, which is jettisoned by using spring thrusters. There are stoppers, which stop the lugs after the required angle of rotation. A schematic of the system along a vertical section is shown in Fig. 1. * Corresponding author. Tel.: 91-471-563-237; fax: +91-471-415-348. E-mail address: [email protected] (A.K. Jha). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00004-3
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A.K. Jha et al. / Engineering Failure Analysis 9 (2002) 709–719
Recently, during one of the functional qualification tests of the system, both lugs of the retainer ring were found to be cracked. Both the cracked lugs were subjected to extensive metallurgical investigation and this paper highlights the details of analysis carried out.
2. Material Al alloy AA 2014 of cast ingot size 215 mm dia.700 mm height is used for production of the ring through mandrel forging route. The cast input material is upset to 200 mm height. Punching is done using a 120 mm dia. punch and stock is a mandrel forged to a size of 450230180 mm. The mandrel-forged ring in proof machined size (420 OD260 ID160 mm height) is parted into seven rings followed by individual machining to the required ring size. The achieved mechanical properties of the forged piece are UTS: 455–500 MPa, YS: 390–460 MPa.
3. Observations The retainer ring consisted of two lugs (henceforth referred to as lug ‘A’ and lug ‘B’) at diametrically opposite locations (Fig. 2). Lug ‘A’ was fully broken while lug ‘B’ was cracked up to 90% of the lug
Fig. 1. Schematic of the system along a vertical section.
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Fig. 2. Retainer ring with lugs A and B.
Fig. 3. Lug ‘B’ of the retainer ring showing the dent by the stopper and the roughness of the curvature.
Table 1 Chemical composition of the ring Element
Cu
Mn
Mg
Si
Fe
Cr
Zn
Al
Wt.%
4.50
0.60
0.50
0.20
0.20
<0.05
<0.05
Rest
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Fig. 4. Lug ‘B’ of the retainer ring showing the crack along the height of the ring.
Fig. 5. Crack on the face of lug ‘B’ towards the aft end ring showing the crack morphology, 10.
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thickness. The crack had propagated along the plane perpendicular to the radial direction of the ring i.e. the axial plane of the ring or short transverse plane of the upset block. Both the lugs were taken out from the ring and observed under stereomicroscope, optical microscope and scanning electron microscope. A cut piece was analysed for its chemical composition and the results tabulated in Table 1. The silicon content is found to be lower than that for AA 2014 Al alloy. Stereo observations revealed that the radius of curvature on either side of the lugs was not smooth. The side towards the stopper end indicated machining/tool marks resulting in irregular curvature. In both the lugs, the vertical face facing the stopper revealed a deep dent. These dents were the result of plastic yielding of material due to the force exerted by the stopper against pyro actuation. Fig. 3 shows the crack and roughness of curvature as well as the dent caused by the stopper. The crack along the height of the ring for
Fig. 6. Crack on the face of lug ‘B’ towards the fore end ring showing the crack initiation from the severely mechanically damaged region, 10.
Fig. 7. Stereo photograph of the crack on the face of lug ‘B’ towards the fore end ring, 40.
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Fig. 8. Stereo photograph of the crack on the face of lug ‘B’ towards the fore end ring, 40.
Fig. 9. Stereo photograph of the crack of lug ‘A’ at the fracture end showing the damaged region and the crack morphology, 20.
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the lug ‘B’ is shown in Fig. 4. The two horizontal faces of the lug mating with two different interface rings are shown in Figs. 5 and 6. Evidence of crack initiation from the severely yielded/damaged region was clearly seen (Fig. 6). The crack morphology on both faces of the lug are shown in Figs. 7 and 8. Deviation in crack path as well as debonding from the matrix at the deviation point was seen (Fig. 7). The crack path followed in lug ‘A’ is shown in Fig. 9. The morphology of the crack path and initiation of crack for both the lugs i.e. lug ‘A’ and lug ‘B’ were found to be identical. Cut pieces from both the lugs were metallographically polished using conventional polishing techniques and etched with Keller’s reagent to reveal the microstructure. The microstructure revealed the typical structure of a forging that received inadequate working (Fig. 10) [1]. The ingot cast structure has not been fully broken-up, as revealed by the presence of a dendrite pattern (Fig. 11). Nonuniform distributions of
Fig. 10. Optical micrograph showing the microstructure typical of forging that received inadequate working, 60.
Fig. 11. Optical micrograph showing dendritic pattern as cast structure not fully broken-up, 120.
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precipitate CuAl2 (white outlined) and (Fe, Mn)3 SiAl12 (dark) preferentially along the grain boundaries were seen (Fig. 12). Grain size measurements were carried out on both the 1ugs. The grain size near the crack in lug ‘B’ varies up to 515315 mm while in lug A, grains were still coarser (600 mm). Typical photographs for the grain sizes of lugs A and B are shown in Figs. 13 and 14 respectively. Scanning electron microscopic observations made on the fracture surface of both the lugs revealed a predominantly ductile mode of failure (Fig. 15). However, the presence of particles on fracture surfaces and their fragmentation were seen at a few locations (Fig. 16a and b). These fragmented particles were confirmed by energy dispersive spectroscopy as iron-rich particles (Fig. 16c). Dendrite lobes within a shrinkage cavity were also seen at few locations (Fig. 17).
Fig. 12. Optical micrograph showing the distribution of precipitates, 60.
Fig. 13. Optical micrograph showing the coarse grain—lug ‘A’, 120.
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4. Discussion The observations indicate that the material has undergone plastic yielding under the resistance/force exerted by the stopper against the pyro actuator. This resulted in a dent, which caused a notch effect and crack initiation at the severely deformed region. This is seen in Figs. 6 and 7. The crack has initiated from the severely deformed region and propagated in the short transverse plane for which material deformability is poor as usually indicated by a low value of elongation. In this present case the microstructure revealed evidence of inadequate working while forging, which provides very poor ductility especially in the ST plane. The presence of coarser grains as well as dendrite pattern/arms indicated that even the cast structure was not fully broken-up. This is further in agreement with the observation of dendrite lobes within
Fig. 14. Optical micrograph showing the coarse grain—lug ‘B’, 60.
Fig. 15. SEM photograph showing the dimpled mode of failure.
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Fig. 16. SEM photographs showing (a) the presence of Fe rich particles and their fragmentation, (b) the location in ‘a’ at higher magnification and (c) the EDS spectrum of the particle.
Fig. 17. SEM photograph showing the presence of dendritic lobes within the shrinkage cavity.
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shrinkage cavities at a few locations (Fig. 17). This may impair further the formability of the material. The presence of such particles and their fragmentation in some of the AA 2014 alloy components have been reported earlier by Jha et al. [2], where the presence of such particles has been attributed to the segregation of Cu, as well as impurities like Fe, Mn and Si present in the melt during the solidification stage. It has been reported that an Fe addition in Al alloys has a deleterious effect on the fracture toughness due to the formation of iron-rich compound [3,4]. Such particles are incoherent with the matrix and have multiple cracks due to their brittle nature. The presence of Fe-rich particles has assisted the crack propagation as revealed by their fragmentation on the fracture surface. A sufficient amount of material yielding as caused by the stopper is indicative of the ductile nature of the material, which is supported by the evidence of a dimpled mode of failure. Stress raisers at the curvature like dent/tool marks at crack initiation sites played a significant role and the crack has initiated from the mechanically damaged region caused by the stopper against pyro actuation.
5. Conclusion Cracking of lugs is due to a notch effect created by the hitting of the lugs against the stopper. Insufficient working during forging and the presence of iron-rich particles has facilitated crack propagation.
Acknowledgements The authors are thankful to Dr. T.S. Lakshmanan, Head, Material Characterisation Division, Shri M.C. Mittal, Group Director, Materials and Metallurgy Group and Shri K.S. Sastri, Deputy Director (PCM), VSSC for their valuable technical guidance during the work. The authors wish to thank Shri Madhavan Nair, Director, VSSC to permit us to publish this paper.
References [1] Metals hand book, vol. 7, 8th ed. Metals Park (OH): American Society for Metals, 1972. p. 245. [2] Jha AK, et al. Failure of AA 2014 aluminum alloy bracket, vol. 10. Germany: Practical Metallography, XXIX, 1992. p. 534. [3] Murali S, et al. In: Proc. of the Second International Conference on Aluminum, INCAL-91, vol. 2. Bangalore: The Aluminum Association of India, 1991. p. 644. [4] Zinkham RE, Dedrick JH. In: Liebowitz H, editor. Fracture—an advanced treatise, vol. VI. New York: Academic Press, 1969. p. 369.