.~,REPORT No: NMLIMST/IAF/t.13/80/2007 Restricted Circulation
October, 2007-
METALLURGICAL ANALYSIS OF THE DAMAGED
WHEEL HUBS OF A VRO HS-748 H-1517
Sponsored by
Air Force Station BorjharKamrup District
Guwahati -781017Assa m
l I~
"h~~
Materials Science & Technology Division
National Metallurgical Laboratory
(Council of Scientific and Industrial Research)
Jamshedpur - 831007
\
, NATIONAL METALLURGICAL LABORATORY(Council of Scientific & Industrial Research)
JAMSHEDPUR..,PROJECT COMPLETION REPORT
Project Title: METALLURGICAL ANALYSIS OF Project No. :THE DAMAGED WHEEL HUBS OF AVRO HS-748 H- Report No. NML/MST/IAFIl. I3/80/2007, Oct' 20071517 Date of Project Initiation:27.08.2007
Date of Completion: 05.10.2007Project Team Members: Class: Publ ic document (Free/Priced)Dr. M. Ghosh (PL) Restricted circulation (X)Dr. S. R. Singh (Co PL) Only to clientMr. S. K. Das SecretCustomer / Client's name and address: Classification:
Wg. Cdr. V. Mathur In-house
Technical Member of Court of Enquiry Grant-in-aid
Air Force Station, Borjhar Sponsored (X)Kamrup District, Guwahati - 78\ 0 17 ConsultancyAssam Collaborati ve
Date of Report: 05.10.2007Area: Material Characterization. Specify Type:Sub Area: Failure analysis Ongoing Area: (X)
New Area Initiated:
Key Words: Avro aircraft, wheel hub, aluminium alloy, precipitation, dispersoid, fusion, flow lines.Abstract: There was a Cat-D accident to Avro aircraft HS-748 H-1517 at Dimapur in July'07. In thataccident as soon as STBD side touches the ground, both the tyres bursted along with one of the tyres onports ide. It causes rubbing and scraping of the wheel hubs leading to permanent damage to thecomponent. It has been explored after the incident that, the brake unit and the bearings of the wheel huhswere perfectly all right.Chemical composition of base metal of wheel hub of the aircraft is close to British specification BS 2L77 of AI base alloys; however, Cu concentration is marginally higher for both the samples in comparisonto the specification. Microstructure of the base alloy exhibits grain size of ] 5-20lm with fine dispersionof AI-Cu intermetallic phases. 2nd phase occurs pre-dominantly at the grain boundary. Apart from theselarge dispersoid of Cu-O and AI-Si-Cu-O are also found. The deformed part of the wheel hub consists ofthe substrate, bend and flow region. The substrate contains both precipitate free and dencely precipitatedregion. Dissolution of precipitate is caused by localised heat generation owing to friction. Co-existenceof both grain boundary and without grain boundary region is also observed. With grain boundary regioncontains dispersoid of Cu-O and AI-Si rich phase. The bend and flow regions consist of precipitate freeregion, dencely precipitated region, finer distribution of AI-Cu phase and large 2nd phase of AI-Si andCu-O. Minor structural difference or loss of directionality at the three regions could be attributed to thelocalised incipient fusion of the alloy due to excessive fricti6nal heat. The root cause of the accident iswheel jamming during landing. Jamming is evident from the flow lines on the deformed part of the hub.Jamming results in excessive stress over the wheel hub. Hence the wheel hub deformed immediatelyafter the tire burst. The jamming is also evident from the dissolution of 2nd phases and microstructure ofthe deformed region. Actual chemical composition, processing root, mechanical properties, heattreatment schedule of that particular wheel hub were not available; hence it is difficult to remarkconclusively the exact reason of structural heterogeneity found in the investigation.Details of IPRs (PI tick): NA If no IPR taken, reasons: NAPatent
CopyrightTrade Marks
Report Issuance Authority: Dr.S.R.Singh Signature: .R.
NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLURGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-1517.
REPORTNo. NML/MST/IAF/I .13/80/2007, OCTOBER 2007
BACKGROUND
There was a Cat-D accident to Avro aircraft HS-748 H-1517 at Dimapur in July'07. In
that accident as soon as STBD side touched the ground, both the tyres bursted along
with one of the tyres on ports ide. It causes rubbing and scraping of the wheel hubs
leading to permanent damage to the component. It has been told that, such kind of
accident i.e, in which three wheels was burst at a time, is very rare all over the world.
Moreover, it has been explored after the incident that, the brake unit and the bearings
of the wheel hubs were perfectly all right. The cause of failure could not be
ascertained at the operating base. Hence the two wheel hubs of STBD side have been
sent to NML for metallurgical failure analysis vide letter No. 19W/S.IO/9/HS-748/H-
1517/16 Aug 07/FS, dated 22ndAug, 2007 through Sgt. R.N. Tripathy of 59th Sqn of
IAF. The damaged components were received at NML on 27.08.2007 for the
investigation.
SCOPE OF WORK
Based on the information provided and discussions held at NML, it is decided to carry
out following assignments for detailed investigation of the wheel hubs:
I. Visual observation, photographic recording and determination of sampling
locations.
2. Microstructural analysis of the wheel hub by optical and scanning electron
microscopy.
3. Determination of chemical composition of the materials by spark erosion
technique.
4. SEM characterization of the deformed region of the failed component.
5. Hardness measurement.
6. Find out the root cause of failure.
COMPONENT'S MATERIAL AND SERVICE EXPOSURE
The Avro HS-748 H-1517 aircraft has two sets of wheel hubs, one set is on portside
and the other set on the STBD side. At the time of landing, the STBD side wheels
touched the ground ahead of few fractions of seconds than portside. The maximum
landing weight of the aircraft is 4.3x I041bs and the hubs are capable enough to
withstand the static as well as impact load during touch down. The life completed
NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLURGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-1517.
REPORT No. NMLlMST/IAF/I.13/80/2007,OCTOBER 2007
since new is 124:45hrs with 101LOGS and the TBO of the same components is
1500hrs with 1500LOGS. Life completed since O/H is also 124:45hrs with 101LOGS
for the wheel hub. The under carriage of the main wheel assembly has the serial
number 5057EX and 5216EY for the damaged two wheel hubs. It has been also
reported that, the hub is made off Mg-base alloys. As per their inspection report tyre
burst during landing causes rubbing of the wheel hubs leading to severe damage to the
components.
EXPERIMENT AL RESULTS & DISCUSSIONS
From the as received damaged wheel hubs, sampling was done from one of the
components considering the symmetry for the location of failure. The sampling
location for fractographic and microstructural analysis with their nomenclature are
shown in Figs.! (a-f). All the deformed/rubbed surfaces along with the periphery of
the hubs were examined visually in reflected light to find out the appearance of the
affected regions with the extent of damage on macro scale. Bulk chemical
composition of the wheel hub was determined by Spark Erosion Technique (PMI
Master Plus). The samples were prepared by conventional metallographic technique
and etched with Keller's reagent. The samples were observed in optical microscope
(Correct SOME TRS) and scanning electron microscope (JEOL JSM 840A) to reveal
the microstructural features of the service exposed alloy. The hardness of the samples
was measured in Vickers scale (Leica) using 200gf load with ISsecs dwelling time.
The deformed region of the hubs was examined in scanning electron microscope
(SEM) to reveal the characteristics of deformation. The details of the investigation are
presented in the following subsections.
1. Visual Examination
Figs.1(a-f) exhibit the damaged wheel hubs of the aircraft. Shining deep scratch marks
along the periphery of the hubs clearly indicates heavy rubbing action (Fig.] a and b).
The span of the rubbed surface exists over a curvature length of -285mm for both the
hubs. The rubbing also causes change of plane over the periphery and the length of
each segment varies with in the range of 40-80mm (Fig.l e and f). Thus the rubbed
area consists of multiple number of planes having an angularity of 10-15° with respect
to the tangent at that location of the hub. The change of plane in rubbed zone indicates
that, wheels have been undergone rotational motion of few degrees while it dragged
2
NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLURGICAL ANALYSIS OF THE DAMAGED WHEEL HUBS OF AVRO HS-748 H-ISI7.
REpORTNo. NML/MST/IAF/1.I3/80/2007, OCTOBER 2007
on the runway. Both the end portion of the rubbed surface has been folded (Fig.1c and
d). The shining edge of the folded region is wavy / serrated in nature having cracks of
2-4mm length. The cracks are perpendicular to the edge of the folded region. Fig. 1a
and c shows the sampling location for fractography and metallography.
"'"
Figs. 1(a- f): Photographs of wheel hubs (5057 Ex and 5216 Ey) showing (a) and (b) asreceived damaged components having scratch marks over the periphery, (c) and (d)
folded edges at end of the failed component, (e) and (f) change of plane over theperiphery of the rubbed surface.
3
NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLtJRGICAL ANALYSIS OF TI-IE DAMAGED WI-IEEL I-IUBS OF AVRO I-IS-748 1-1-1517.
REPORTNo. NML/MST/IAF/113/80/2007, OCTOBER 2007
2. Chemical Composition
The concentration of major alloying elements present in the alloy is determined by
spark erosion technique. The data is furnished in Table-I. The chemical composition
resemblances with British specification of BS 2L 77 and Indian specification of IS
7902, Grade 24345.
Table 1: Chemical composition of the wheel hub (wt%).
Though it has been told that, the affected wheel hubs are made off Mg alloys, yet
chemical composition exhibits that, the alloy belongs to 2XXX series of AI-alloys.
For both the samples concentration of alloying elements varies insignificantly.
However, Cu concentration is marginally higher for both the samples in comparison
to the BS specification. Si content of sample A I is little higher with respect to
specification. Compositional deviation cannot be assessed owing non-availability of
actual alloy specification as per the OEM.
3. Microstructural Analysis
The microstructure of the base alloy as well as deformed region of the wheel hub was
examined in optical and scanning electron microscopy. Optical microstructure
(Figs.2a and b) of the base alloy exhibits fine grain structure (l5-20~m) with the
dispersion of the second phase both at grain body and grain boundary. Some shaded
and gray chunky islands were also found within the matrix. At higher resolution in
SEM. grain boundary thickening (Figs.2c and d) is quiet evident owing to second
phase precipitation. Within the grain body the second phases are finer in nature. These
4
Elements As per British Sample Al Sample A2specification of BS2L 77
AI Bal Bal BalSi 0.5-0.9 0.95 0.87Fe 0.5 max 0.42 0.42Cu 3.9-5.0 5.04 5.06Mn 0.4-1.2 0.73 0.68Mg 0.2-0.8 0.45 0.49Zn 0.2 max 0.078 0.091Cr 0.1 max 0.021 0.024Ni 0.2 max 0.008 0.004Ti Not specified 0.049 0.058Ti + Zr 0.2 max Zr not found Zr not foundPb 0.05 max 0.01 0.018Sn 0.05 max 0.043 0.040
NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLURGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-1517.
REPORTNo. NML/MST/IAF/I .13/8012007, OCTOBER 2007
2ndphases are the intermetallics of Cu-AI combination. The large islands within the
matrix are of two types; one is bright (Fig.2c, marked as 'I') and the other is gray
(Fig.2d, marked as 'II') in appearance. In qualitative EDS analysis, it has been found
that bright islands is the combination of Cu and 0, whereas gray regions are the
complex combination of Al-Si-Cu-O.
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Figs.2 (a-f): Microstructure of base material [a] and [b] optical microscopy, [c]and [d] scanning electron microscopy, [e] EDS from region marked 'I' of
Fig.2c and [fJ EDS from area 'II' marked on Fig.2d.
5
NATIONAL METALLURGICAL LABORATORY,JAMSHEDPURMETALLURGICAL ANALYSIS OF THE DAMAGED WHEEL HUBS OF AVRO HS-748 H-1517.
REPORTNo. NMLlMST/IAF/1.l3/80/2007, OCTOBER 2007
From the deformed region, two samples, i.e, A I and BI have been chosen and their
overall appearance in transverse section is shown at low'magnification in Fig.3a and
b. In each sample three regions have been identified, i.e, substrate, bend and flow
region. Substrate indicate the region attached to the main part of the wheel hub, bend
means along which material deformation took place during accident and flow region
refers to the area of thin section owing to plastic flow of the alloy. These three
sections have been examined separately and salient features are given below.
Figs.3 (a & b): Transverse SEM micrograph of deformed part of the wheel hub[a] sample A I and [b] sample B I
The substrate microstructure of sample A I and BI exhibits more or less same
morphology (FigsAa and b). The matrix is decorated with bright and shaded islands.
Directionality is evident owing to mechanical working of the alloy during fabrication.
For sample BI, the microstructure exhibits a marked difference at the opposite side of
the bend with respect to the area near the bend. In case of the former, increment in the
number quantity of white islands with irregular shape occurred in comparison to the
streaking like appearance of the white islands of the latter.
6
NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLURGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-1517.
REPORTNO. NML/MST/IAF/I. I3/80/2007, OCTOBER 2007
Figs. 4(a-c): Optical micrographsof the substrate of deformed wheel
hub showing [a] sample A I withbright elongated islands in theshaded matrix [b] sample B Iexhibiting same bright elongatedislands in the shaded matrixadjacent to the bend and [c]sample B1 irregular shaped brightislands in combination with shaded
region opposite to the bend region..
The general view of the bend region is exhibited in Figs.Sa and b. Heavy deformation
is evident from the curvature of the material flow. For sample A I (Fig. Sa) the volume
fraction of bright regions is more than the sample B I (Fig.5b).
Figs. S(a & b): Optical microstructure of the bend region of the wheel hub [a]sample A I showing material flowing owing to heavy deformation and [b]
sample B 1 exhibiting less pronounced material flow.
Figs. 6a and b shows the flow region of the two samples. In case of the sample B 1, the
bright regions are few in number quantity with respect to the sample A I. From the
alignment of bright region in sample AI (Fig.6a), the direction of the material flow is
found to be diametrically parallel to the wheel hub. In case of sample B] (Fig.6b), no
such clear alignment has been found, rather the bright islands are distributed in a
scattered fashion.
SEM micrographs of the sample A I are illustrated in Figs.7a-h. The substrate
microstructure is heterogeneous in nature. Two distinct regions are evident in the
substrate; one is densely precipitated zone and the other is precipitate free zone where
7
NATIONAL METALLlRGICAL LABORATORY,JAMSHEDPURMETALLURGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-ISI7,
REPORTNo. NM L/MST/IAF/\. I3/80/2007, OCTOBER 2007
the second phases have dissolved completely (Figs,7a and b), The size of the
precipitate free region is -I O-151lm,
Figs, 6(a & b): Optical microstructure of the wheel hub on the flow region [a]sample Al showing alignment of bright region and [b] sample 81 exhibiting
scattered distribution of bright region.
Apart from the above, the substrate contains bright large 2nd phases having size -1-
IOllm and small grey phase having size -1-5Ilm within the matrix (Fig.7c), The
qualitative EDS analysis indicates that the former is basically copper oxide (Fig,8a)
and the latter is AI-Si rich phase in association with Cu and 0 (Fig,8b).
i/::-'. f.'~;~;':):};;i~/.~;.;;0> .::.~;/:}\)~/:'t}~:.:. tj)l~ "','..;,:, ;j.~. .'".f .1'" '.. , . ' ,',": :"'" ,jP".' "'., ..;.'f:':"':' ;,.}~.. 1't; ..;";)i:'.":",~:,/,;~:~",,;,"":X';;"'>' {' ,\:,.t.>(;{~''''.':/ ".,':, ,:;',:,""
,;(;~)..\::,:J ,::; ;.~::~.;/:,: "\J~.;'.:~;;'0:;::'\':~, .'Ir"'.. ! ,;" '.j (""'~.J'( , ,\'",; ,,'\", , '...'> ,
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8
NATIONAL METALLURGICAL LABORATORY,JAMSHEDPURMETALLURGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-1517.
REPORTNo, NML/MST/IAF/l,13/80/2007, OCTOBER 2007
.' j" ,d.." "',.'," :', '", ''', ., -', '
, ,(~, '.\-, '.""'1 "~'.' ~,;
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Figs. 7(a-h): SEM micrograph of the sample A I (a) BSE image of the substrateexhibiting dissolved second phase in the matrix (b) Same region as indicated inFig.7a in SE mode (c) BSE image ofCu distribution on the substrate (d) BSEimage of 2ndphase distribution in the bend region (e) BSE image of large Cu
2ndphase with the fine distribution of8' precipitate at bend region (f) SE imageof precipitate free region at the bend (g) SE image at bend exhibiting secondphase distribution and (h) BSE image showing 2ndphases in the flow part,
As stated before, the bend region is also consists of densely precipitated zone
(Fig.7h),precipitate free region (Fig.7h),Cu-O rich phase (Figs.7d and e), AI-Si rich..." ~". "''''..r'
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"""""""'= ..~... ". ..,,-.. " ,-
NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLURGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-lSI7.
REPORTNo. NMLlMST/IAF/J.l3/80/2007. OCTOBER 2007
.,1.",.",,-", 8 ',...[c]"". ("..
..."..,,,0".
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X (.~I
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[d]I .
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Figs. 8 (a-d): Qualitative EDS analysis (a) Cu-rich phase containing oxygen ofFig.7c and e, (b) Fe-Si rich phase in association with Cu and 0 of Fig.7d, (c)EDS of a complex compound consisting ofCu-AI-Si-Mg-O ofFig.7fand (d)Aluminium oxide as shown in Fig.7h.
2ndphase. One marked difference has been observed for the bend region with respect
to substrate that the number density of copper oxide particles are quiet large having
more or less same size distribution as stated before for the substrate. At an enlarged
magnification for a single copper oxide dispersoid, it has found that around the
particle near about IJ..lmarea is free from any 2ndphase (Fig.7e), however outside of
the region dence distribution of the same occurred. The bend region also contains
complex combination of Cu-Al-Si-Mg-O as shown in Figs.7f and 8c. However they
are few in number and occurred as clusters of small size distribution.
The flow region of sample Al (Fig.7h) contains both precipitate free and with
precipitate zone along with the distribution of bright copper oxide phase (-1-7J..lm),
AI-Si rich phase (-2-8J..lm)and grey aluminium oxide phase (1-3J..lm).
NATIONAL METALLURGICAL LABORATORY,JAMSHEDPURMETALLlIRGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AYRO HS-748 H-1517.
REPORTNo. NML/MST/IAF/113/80/2007, OCTOBER 2007
,."".I...".~", 8 ',...[c],,",. ,-.1.,
....
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Figs. 8 (a-d): Qualitative EDS analysis (a) Cu-rich phase containing oxygen ofFig.7c and e, (b) Fe-Si rich phase in association with Cu and 0 of Fig.7d, (c)EDS of a complex compound consisting ofCu-AI-Si-Mg-O ofFig.7fand (d)Aluminium oxide as shown in Fig.7h.
2nd phase. One marked difference has been observed for the bend region with respect
to substrate that the number density of copper oxide particles are quiet large having
more or less same size distribution as stated before for the substrate. At an enlarged
magnification for a single copper oxide dispersoid, it has found that around the
particle near about] I-lmarea is free from any 2ndphase (Fig. 7e), however outside of
the region dence distribution of the same occurred. The bend region also contains
complex combination of Cu-AI-Si-Mg-O as shown in Figs.7f and 8c. However they
are few in number and occurred as clusters of small size distribution.
The flow region of sample A] (Fig.7h) contains both precipitate free and with
precipitate zone along with the distribution of bright copper oxide phase (-l-htm),
AI-Si rich phase (-2-8I-lm) and grey aluminium oxide phase (1-3I-lm).
10
NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLURGICAL ANALYSIS OF THE DAMAGED WHEEL HUBS OF AVRO HS-748 H-tSt7.
REPORT No. NML/MST/IAF/l.13/80/2007,OCTOBER2007
Figs. 9 (a-g): SEM micrograph of the sample B1 (a) Fine grain and without grainboundary region co-existence in the substrate (b) junk Cu rich and other 2nd
phases in the no grain boundary region of the substrate (c) BSE image ofFig.9b(d) on bend region exhibiting grain boundary (e) co-existence of precipitate freeand densely precipitated region on the flow part (f) Distribution ofCu bearing
phase.
The SEM micrographs of different parts of sample B 1 is shown in Figs.9a-g. The
presence of without grain boundary and with grain boundary region is clearly revealed
(Fig.9a). The size of the former is -5-7j..lm containing fine distribution of 2nd phase
mainly at the grain boundary. However like sample A I (Fig.7b) precipitate free region
in the substrate has not been found. Without grain boundary region contains bulky
copper oxide phase (-5-20j..lm), tiny AI-Si rich phase (-I-2j..lm) and clusters of AI-
oxide phase (Figs.9b and c). Definite alignment of these two regions indicates
direction of material flow during forming operation.
The SEM microstructure of bend region of sample B1 resemblances with the bend
region of sample A I exhibiting Cu-O, Al-Si rich second phase distribution in the
II
NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLURGICAL ANAL YSIS OF THE DAMAGED WHEEL HUBS OF AVRO HS-748 H-1517.
REPORTNo. NMLlMST/IAF/\.l3/80/2007, OCTOBER2007
heterogeneous matrix of precipitate free and dencely precipitated region (Fig.9d). The
flow region of sample B I does not reveal any grain boundary region; rather the zone
contains denced distribution of 2nd phase and precipitate free region (Fig.ge). The
flow region also contains Cu-O and Cu-Al-Si-Mg-O phase as shown in Fig.9f.
4. SEM Characterization of Deformed Surface
The micrographs of A I and BI are shown in Figs.IO(a-d). Scratch marks over the
periphery once again endorses rubbing action (Fig.1Oaand c). At the folded region the
direction of scratches changes and makes an angel of 60-70° with the original scratch
marks (Fig.l Ob). Intense impact causes bending of the component at the edges and the
residual stress causes cracking (Fig.l Ocand d). Material flow near the edge of the hub
is evident (Fig.1Ob). The flow lines indicates that at the moment of the accident, the
wheel was not in rotation, i.e. may be due to some mechanical reason wheel was
jammed.
Figs. 10(a-d): SEM micrographs of the deformed wheel hubs (a) scratch marks
12
NATIONAL METALU'RGICAL LABORATORY, .JAMSHEDP1:RMETALLURGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-ISI7.
REPORT No, NML/MST/IAF/l13/80/2007. OCTOBER2007
over the surface of sample A I (b) multiple flow lines at the edge of fold ofsample A I, (c) scratch mark and cracking over the surface of B I and (d)
scratches and flow lines over the 'surface of B I.
5. Hardness Evaluation
Hardness is measured on the transverse section of the as-received service exposed
wheel hub and the data are collated in Table-2. The data exhibits wide variation
between the substrate and the bend region, whereas the flow region and the bend
exhibits more or less same value. As per specification. for AI alloy of BS 2L 77 at T6
condition, the specified hardness is minimum 134 BHN. With respect to that, base
material hardness is as per speci fication. The hardness of the substrate of sample A I
and Blare more or less at par with the base alloy, However lower values (as compared
with the specified limit) are observed for bend as well as flow region. In both the
regions. from the microstructure it is evident that dissolution of precipitate takes place
which indicates loss of strength and hence the lowering of the hardness. More over
localised grain boundary free regions on the deformed region indicates
recrystallization of the alloy owing to frictional heat. In the recrystallized region new
strain free grains are formed, which are also responsible for the reduction in hardness.
Sample ID
169 ::t2170 + I
112::t3128+ I
115::t2127+ 2
Base
Sample AISamole BI
From the above observation it may be inferred that, the root cause of the accident is
wheel jamming during landing. The flow lines at the edge of the folded region
indicate the phenomena. Jamming raises the frictional force over the wheel hub
assembly. The frictional stress perhaps exceeds the load bearing ability of the
component. Thus. during landing excessive load causes deformation of the wheel hub
after tire burst. The alloy of the wheel hub contains dispersoid of Cu-O and AI-Si-Cu-
0 within the matrix. This dispersoid are large in size hard in nature. Hence ductility of
the alloy becomes limited. The same features are also found at the deformed region of
the wheel hub. Hence, this microstructural heterogeneity is not caused by the
accident; rather it has been in-herited during processing. This structural heterogeneity
is responsible for the formation of cracks in the deformed region. The jamming of the
13
NATIONAL MET ALLlJRGICAL LABORATORY. JAMSHEDPlJRMETALLURGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-ISt7.
REPORTNo. NML/MST/IAF/ 1.13/80/2007. OCTOBER 2007
hub also increases friction against the ground manifold and temperature is raised. This
localised heating causes dissolution of precipitates in localised region as it may be
presumed that the temperature reaches near the solvus temperature of the 2ndphases
(-550°C). The frictional heat also promotes the formation of new strain free grains in
bend and flow region, which is manifested by the reduction in hardness at the bend
and flow region.
CONCLUSIONS
I. Chemical composition of base metal of wheel hub of Avro aircraft HS-748 H-
1517 is close to British specification of BS 2L 77; however, Cu concentration is
marginally higher for both the samples in comparison to the BS specification. Si
content of sample A I is Iittle higher with respect to specification. The exact alloy
specification is not available; hence it is not possible to compare the data obtained
in the present investigation.
2. Microstructure of the base alloy exhibits grain size of 15-20~m having fine
dispersion of AI-Cu intermetallic phases. 2ndphase occurs pre-dominantly at the
grain boundary and is responsible for grain boundary thickening. Apart from these
large dispersoid consisting of Cu-O and AI-Si-Cu-O are also found.
3. The deformed region of the wheel hub consists of three regions; the substrate, the
bend and the flow region. The substrate contains both precipitate free and dencely
precipitated region. Dissolution of precipitate is caused by localised heat
generation owing to friction. Co-existence of both grain boundary and without
grain boundary region is also observed. The grain size of the with grain boundary
region is :<:;:IO~Lm,i.e., lower than the base alloy. It also indicates localised heating
promotes recrystallization and new strain free grains are formed. With grain
boundary region contains large dispersoid of Cu-O and AI~Si rich phase.
4. The bend as well as flow regions consist of precipitate free region, dencely
precipitated region, finer distribution of AI-Cu phase and large 2ndphase of AI-Si
and Cu-O. Minor structural difference or loss of directionality at the three regions
could be attributed to the localised overheating leading to dissolution of
precipitate and recrystallization.
5. The root cause of the accident is wheel jamming during landing. Jamming is
evident from the flow lines on the deformed part of the hub. Jamming results in
14
<, NATIONAL METALLURGICAL LABORATORY, JAMSHEDPURMETALLlJRGICAL ANALYSIS OFTHE DAMAGED WHEEL HUBS OF AVRO HS-748 H-1517.
REPORTNo. NMLlMST/lAF/1.13/80/2007. OCTOBER 2007
,.
excessive stress over the wheel hub. Hence the wheel hub deformed immediately
after the tire burst. The jamm ing is also evident from the microstructure of the
deformed region as it raises frictional force and causes dissolution of 2ndphases
with in the matrix in a scattered fashion and recrystallization.
Actual chemical composition, processing root, mechanical properties, heat treatment
schedule of that particular wheel hub were not available; hence it is difficult to remark
conclusively the exact reason of structural heterogeneity found in the investigation.
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