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Engineering Failure Analysis 15 (2008) 787–795

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Stress corrosion cracking in alumnium alloy AFNOR7020-T6 water tank adaptor for liquid propulsion system

Abhay K. Jha *, G. Naga Shiresha, K. Sreekumar, M.C. Mittal, K.N. Ninan

Materials Characterization Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Indian Space Research

Organisation, Trivandrum 695 022, India

Received 24 May 2007; accepted 24 May 2007Available online 9 June 2007

Abstract

A torroidal shaped tank in the liquid propulsion system provides water for initial run of the turbo pump and forcooling various components. This is made of high strength aluminium alloy Al–4.5Zn–1.5Mg designated as AFNOR7020. Pressurisation adaptors are initially shrink fitted to the openings provided in the water tank and later TIG weldedcircumferentially to configure the tank assembly. During one of the functional checks, a crack was noticed on the weldjoint of the adaptor. Detailed metallurgical analysis indicated that the failure was due to stress corrosion cracking(SCC).

This paper brings out the metallurgical analysis carried out which confirms the SCC.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Stress corrosion cracking; AFNOR 7020 aluminium alloy

1. Background

High strength aluminum alloy AFNOR 7020 ( Al–4.5Zn–1.5Mg) is extensively used for the fabrication oftorroidal tank, which provides water for initial run of turbo pump and for cooling various components in theliquid propellant system for space applications. Pressurisation adaptors used in the system are made ofAFNOR 7020 forgings. These adaptors are TIG welded circumferentially to the openings provided in thewater tank to configure the tank assembly. A typical water tank consists of four adaptors.

In the present case, one out of four adaptors used in water tank was fabricated from the forging in T6 con-dition, whereas forgings in T6511 condition were used for fabrication of remaining three adaptors. During oneof the functional checks of such a water tank, a crack was noticed on the weld joint of the adaptor, made offorging in T6 condition, while the other three adaptors were found intact. A systematic metallurgical investi-gation was carried out to understand the mechanism of failure of this adaptor.

1350-6307/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2007.05.009

* Corresponding author. Tel.: +91 471 2563237; fax: +91 471 2705048.E-mail address: ak_jha@vssc.gov.in (A.K. Jha).

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2. Analyses

2.1. Chemical analysis

A cut piece from the cracked adaptor was analyzed for chemical composition and results confirmed thematerial to be AFNOR 7020 (Table 1).

2.2. Visual observations

The visual observation indicated the presence of linear crack very close to weld fusion line. Fig. 1 shows thecracked adaptor indicating the location of crack.

The crack length was 35 mm on the reinforcement side, while it was 25 mm on the root side. This indicatedpossibility of crack initiation at the reinforcement side of the weld. The crack was discontinuous in nature andhad blunt tip.

2.3. Optical metallography

Specimens were sliced off from the lateral and transverse (across thickness) surfaces of the cracked region.The specimens were prepared using conventional metallographic techniques and etched with Keller’s reagent( 2 ml HF, 3 ml HCl, 5 ml HNO3, 180 ml water) to reveal the microstructure.

Discontinuous cracks were found to propagate and branch along the grain boundaries (Fig. 2). The crackmorphology was indicative of anodic dissolution of second phase particle, some of which were found deb-onded from the matrix.

The transverse plane of the specimen had two cracks, running across the thickness. These cracks were closeto each other (150 lm) and were almost parallel. The first crack initiated at the fusion junction on weld rein-forcement (Fig. 3) while the second one propagated through solute enriched phases within weld pool beforeproceeding along the grain boundaries of the parent material (Fig. 4).

Table 1Chemical composition of the cracked adaptor

Composition wt%

Zn Mg Fe Mn Cu Si Cr Zr + Ti Al

Specification 4.0–5.0 1.0–1.4 0.4 max 0.05–0.50 0.20 max 0.35 max 0.10–0.35 0.08–0.35 BalObserved 4.3 0.51 0.10 0.10 – 0.10 0.17 0.10 Bal

Fig. 1. The adaptor indicating the location of crack.

Fig. 2. Crack morphology along the lateral surface.

Fig. 3. Optical microphotographs showing crack initiation from the pit root at weld pool – adaptor parent junction and its propagationalong the grain boundary.

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De-bonding of few second phase particles through anodic dissolution during crack propagation was alsoobserved (Fig. 5). The typical microstructures of weld pool and fusion line are shown in Fig. 6. The parentmaterial confirmed elongated grains with reasonably high grain aspect ratio with second phase particles pres-ent along the grain boundaries (Fig. 7).

2.4. Scanning electron microscopy

The specimens were taken out from the cracked adaptor. The flange surface of the adaptor consisting thecracks and the fracture surface of the adaptors were viewed under OLYMPUS scanning electron microscopefor detailed metallurgical features.

2.4.1. Observations on the adaptor flange surface

Scanning electron microscopy (SEM) revealed that the crack was wide at the middle and narrower at theends with blunt tip. The de-bonding of the second phase particles at certain locations ahead of the crack tipswas due to anodic dissolution (Fig. 8a). Formation of cavities and their interlinking leading to crack forma-tion (Fig. 9) at certain locations on the lateral surface of the adaptor also gave evidence for anodic dissolutionof second phase particles.

Fig. 4. Optical microphotographs showing crack initiation from the solute enriched and its propagation along the grain boundary.

Fig. 5. Optical microphotographs showing second phase particles de-bonded from the matrix.

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2.4.2. Observations on the fracture surface

The cracked piece was taken out to expose the fracture surface for detailed investigation under scanningelectron microscope (SEM). The fracture morphology was predominantly fibrous and intergranular in nature.Features were in consistent with the grain orientation (elongated grains) of adaptor parent (Fig. 10). At cer-tain locations, the crack morphology was intergranular. This was in agreement with the optical metallo-graphic observation of the equiaxed grain zones in HAZ (Fig. 11). Secondary cracks were found topropagate in a direction normal to fracture surface (Fig. 12). They were resultant of anodic dissolution of

Fig. 6. Typical microstructures showing weld pool, fusion line and HAZ regions.

Fig. 7. Typical microstructure of adaptor flange parent material.

Fig. 8. SEM photographs depicting crack morphology.

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second phase particles present along grain boundaries, which was more predominant at certain locations(Fig. 13).

3. Discussions

AFNOR 7020 is a weldable alloy in 7XXX series (Al–Zn–Mg system) without Cu. The major alloyingelements are Zn and Mg with minor addition of Zr, Cr, Mn and Ti. The main hardening constituent is the

Fig. 10. Fracture morphology consistent with the grain orientation of parent material.

Fig. 9. SEM photographs showing dissolution of particles their interlinking.

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intermetallic precipitate MgZn2 which is a semi coherent phase. The precipitate MgZn2 is more anodic than Alalloy matrix (as the potential of MgZn2 is �1.05 V and pure aluminium has potential of �0.85 V) and henceeasily dissolved in humid atmosphere. A potential difference of 0.20 V could exists between pure aluminiumand the precipitate present. The precipitation occurs preferentially at grain boundaries causing formationof alloying element (Zn, Mg) depleted zones surrounding the precipitates which creates a galvanic cell betweenthe two i.e narrow depleted zones and Zn/Mg-rich region [1].

These precipitates, being anodic in nature get dissolved in a corrosive environment and the crack propa-gates under synergistic effect of stresses and corrosive environment. Such stresses are either present withinthe material as locked in stresses or applied one. In the present case, the component was fabricated out of

Fig. 11. Fracture morphology consistent with the grain orientation in HAZ region.

Fig. 12. Fractographs showing the presence of secondary crack.

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a forging in T6 temper condition. Solution treatment followed by water quenching induced considerable ther-mal stresses in the material. The alloy, in super saturated condition, was aged to get precipitates for improvedstrength. The residual thermal stresses due to solution treatment redistributed within the material during theprecipitation treatment. Usually the material is used in T651/652 condition where stretching/compression ofthe alloy after solution treatment and before aging is done to relieve the stresses. In the present case, the mate-rial was not given any such stress relieving treatment.

Further, the component was shrink fitted to the openings provided in the water tank, followed by circum-ferential manual TIG welding. For this, the component was dipped in liquid nitrogen and on contraction fittedwithin the opening. Then on reaching the ambient temperature, uniform radial expansion of the adaptor tookplace. Expanding against the restraint imposed by neighbouring sheet material at ambient temperature causedstresses to develop within the material.

Metallurgical investigations indicated that cracking occurred on the HAZ of adaptor (forging) side. Crackhad blunt tip and evidence for dissolution of second phase particles were seen. Crack initiated at weld fusionjunction where solute enriched phases provided more anodic sites than the matrix. This caused the crack tofollow the path, provided by solute enriched phases.

The grain aspect ratio (l/d) of the material was of the order of 10 and above, which was higher than thedesirable ratio of three and less. Further, their orientation was such that continuous grain boundaries acrossthickness hosted for second phase precipitate and ultimately caused fast anodic dissolution. Intergranularmode of fracture consistent with grain orientation and evidence of dissolution of MgZn2 present along thegrain boundaries inferred that failure has occurred due to stress corrosion cracking [2]. This was synergisticeffect of the stresses induced by thermal treatment/shrink fitting/welding and the corrosive environmentdue to the presence of chloride in saline atmosphere.

Huang et al. [3] explained three possible conditions to induce high tensile strain in the HAZ. The most pre-dominant one was the restraint imposed on the material from contracting freely during welding. Stresses,

Fig. 13. Fractographs showing dissolution of second phase particles (at location A).

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under humid and saline environment caused SCC to initiate and heavily oriented microstructure with secondphase particles along grain boundaries facilitated cracking as reported by Meletis et al. [4].

Similar failures of aerospace components in the past have been reported earlier [5,6].

4. Conclusions

To summarize, the pressurization adaptor cracked due to the phenomenon of stress corrosion cracking(SCC), which occurred due to the combined effect of the following:

(1) Stresses, caused by strain induced during thermal treatment/shrink fitting/welding.(2) Anodic dissolution of MgZn2 even under minor corrosive environment, which provided conducive path

for crack propagation.

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Acknowledgements

The authors express their sincere gratitude to Dr. B.N. Suresh, Director, VSSC, Trivandrum for his kindpermission to publish this work.

References

[1] Davis JR, editor. Corrosion of alumnium and alumnium alloy. Materials Park, OH: ASM International; 1999. p. 64.[2] Metals Hand Book, 8th ed., vol. 10. OH: ASM International; 1975. p. 212.[3] Huang, Kou. Liquation cracking in full penetration Al–Si weld. Weld Res 2004:111S.[4] Meletis EI. Stress corrosion cracking properties of 2090 Al–Li alloy. In: Goel VS, editor. Corrosion cracking, conference proceedings.

American Society of Metals; 1986. p. 315.[5] Jha Abhay K, Murty SVSN, Diwakar V, Sreekumar K. Eng Failure Anal, vol. 10. UK: Elseveir; 2003. p. 265–73.[6] Sreekumar K, John KM, Natrajan A, Lakshmanan TS. Pract Metallogr 1994;31:586–95.