NASA/TP-1998-208421 Evaluation of the Transient Liquid Phase (TLP) Bonding Process for Ti3A1-Based Honeycomb Core Sandwich Structure R. Keith Bird and Eric K. Hoffman Langley Research Center, Hampton, Virginia National Aeronautics and Space Administration Langley Research Center Hampton, Virginia 23681-2199 June 1998 https://ntrs.nasa.gov/search.jsp?R=19980201176 2020-03-17T14:05:05+00:00Z
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Evaluation of the Transient Liquid Phase (TLP) Bonding ... · RohrBond, known generically as transient liquid phase (TLP) bonding, is described in detail in ref. 7-12. This process
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NASA/TP-1998-208421
Evaluation of the Transient Liquid Phase (TLP)
Bonding Process for Ti3A1-Based HoneycombCore Sandwich Structure
R. Keith Bird and Eric K. Hoffman
Langley Research Center, Hampton, Virginia
National Aeronautics and
Space Administration
Langley Research CenterHampton, Virginia 23681-2199
The authors would like to express their appreciation to Stephanie Chiras for her contribution to this researchproject. Ms. Chiras was a summer student from Harvard University, and she was responsible for thetesting and data analysis for a large number of the edgewise compression tests. Ms. Chiras conducted anextensive amount of testing over a short period of time to help complete this research project.
Available from the following:
NASA Center for AeroSpace Information (CASI)
7121 Standard Drive
Hanover, MD 21076-1320
(301) 621-0390
National Technical Information Service (NTIS)
5285 Port Royal Road
Springfield, VA 22161-2171
(703) 487-4650
Summary
The suitability of using transient liquid phase
(TLP) bonding to fabricate honeycomb coresandwich panels with Ti-14A1-21Nb (wt%) tita-
nium aluminide (Ti3A1) face sheets for high-
temperature hypersonic vehicle applications wasevaluated. The TLP bonding process involves
using a foil filler metal between the mating
titanium honeycomb sandwich components
that produces a eutectic liquid phase to assist in
the diffusion bonding. Three titanium alloyhoneycomb cores (Ti-3A1-2.5V, Ti-6A1-4V, and
Ti-15V-3Cr-3Sn-3A1) and one Ti3A1 alloyhoneycomb core (Ti-14A1-21Nb) were
investigated. Structural properties of the
honeycomb core sandwich panels were
determined using edgewise compression (EWC)
tests and the strength of the joint between the face
sheet and honeycomb was evaluated usingflatwise tension (FWT) tests. In addition, tensiletests were conducted on the Ti-14A1-21Nb face
sheet material to determine the effects of
processing on the face sheet properties. FWT,EWC, and tensile tests were conducted at tem-
peratures ranging from room temperature to1500°F.
EWC tests indicated that the honeycomb cores
and diffusion bonded joints were able to stabilize
the face sheets up to and beyond the face sheet
compressive yield strength for all temperatures
investigated. The Ti- 15V-3Cr-3Sn-3A1 specimens
produced the highest FWT strengths over the
temperature range from room temperature to1000°F. At temperatures above 1000°F, the
Ti-14A1-21Nb specimens produced the highest
FWT strengths.
Tensile tests of the Ti-14A1-21Nb face sheet
material indicated that the processing conditions
used to fabricate the three titanium alloy honey-
comb core sandwich panels caused a decrease inface sheet strength, an increase in modulus, and a
minor decrease in ductility compared to unproc-essed ot/13 solution treated Ti-14A1-21Nb sheet
material. The processing conditions used to
fabricate the sandwich panel with Ti-14A1-21Nb
honeycomb core resulted in a significant loss of
face sheet ductility and tensile strength at room
temperature.
Microstructural examination showed that the
side of the face sheets to which the filler metals
had been applied was transformed from equiaxed
5 2 grains to coarse plates of 5 2 with intergranular[3. Fractographic examination of the tensile
specimens showed that this transformed regionwas dominated by brittle fracture.
The results from this study indicate that TLP
bonding has good potential for fabricating light-weight honeycomb core sandwich structure withtitanium aluminide face sheets and titanium alu-
minide or conventional titanium alloy honeycombcore suitable for high-temperature hypersonic
vehicle applications.
Symbols and Abbreviations
o_
52
ep
E
EDS
EWC
FWT
SEM
Ti-14-21
Ti-15-3
Ti-3-2.5
Ti-6-4
hexagonal close-packed titanium
ordered Ti3AI intermetallic
body-centered cubic titanium
plastic tensile strain to failure
tensile modulus of elasticity
energy dispersive x-ray spectroscopy
edgewise compression
flatwise tension
scanning electron microscopy
alloy Ti-14A1-21Nb (wt%)
alloy Ti-15V-3Cr-3Sn-3A1 (wt%)
alloy Ti-3A1-2.5V (wt%)
alloy Ti-6A1-4V (wt%)
:_"J_' .... , ¸¸¸¸:,4,:i?:::_-JL_'i_:
Z
TLP transient liquid phase
UTS ultimate tensile strength
WDS wavelength dispersive x-ray
spectroscopy
YS 0.2%-offset tensile yield strength
Introduction
The successful deployment of high-speed air-
craft having a sustained hypersonic cruise capa-
bility is highly dependent upon the development
of lightweight, high-temperature structures
(ref. 1-3). The structural concepts for this type of
vehicle call for high-stiffness, thin-gage product
forms that can be fabricated into efficient, load-
bearing components. Titanium-based materials
are considered prime candidates for large-scalestructural use in the airframe because of their low
density and good properties at temperatures up to
800°F. In sheet form they could provide the basis
for efficient, lightweight honeycomb core sand-wich structure. To achieve the performance goals
for a hypersonic vehicle airframe, however, it is
critical that the temperature capability of the tita-
nium honeycomb core sandwich structure be
extended to even higher operating temperatures
so that the need for structural cooling can beminimized. Over the last few years, a new class of
material based on the _2 titanium aluminide
intermetallic (Ti3A1) composition has emerged.
Ti3Al-based alloys have improved high-temperature properties and lower densities
compared to those for conventional titanium
alloys (ref. 1,4). Structures fabricated from Ti3A1
have the potential for high strength- and stiffness-to-weight ratios at the temperatures of interest for
hypersonic flight. The primary drawback with
utilizing Ti3A1 is the inherently low room
temperature ductility and toughness (ref. 1,5,6),
which can have serious implications for theintegrity of such a structure. Extensive research
(ref. 3,5,6), however, has resulted in an increase
in the room temperature ductility of the Ti3A1 to
acceptable levels through niobium
alloying additions. One such alloy of particularinterest is Ti-14A1-21Nb (Ti-14-21).
The development of processing and joining
techniques for the incorporation of Ti3A1 intohoneycomb core sandwich structure is critical to
the construction of hypersonic aircraft compo-
nents (ref. 1). The joining techniques must pro-
duce joints with adequate strength at service tem-
peratures without significantly degrading the
properties of the component materials. A state-of-
the-art commercial joining process that was
developed for fabricating titanium alloy honey-comb core sandwich structure is the RohrBond 1
process. RohrBond is a diffusion-assisted bonding
process that utilizes a eutectic liquid phase to
assist in the diffusion bonding of the mating
titanium honeycomb sandwich components.
RohrBond, known generically as transient liquid
phase (TLP) bonding, is described in detail in
ref. 7-12. This process has several advantages
over conventional diffusion bonding and brazing
and is a candidate for fabricating Ti3Al-basedhoneycomb core sandwich structure.
This study was conducted to evaluate the suit-
ability of the TLP bonding process for the fabri-
cation of honeycomb core sandwich panels with
Ti-14-21 components. Sandwich panels were
fabricated by Rohr, Inc., using Ti-14-21 face
sheets and various titanium alloy honeycomb core
materials. Specimens cut from honeycomb core
sandwich panels were tested in edgewise com-
pression (EWC) to measure the structural behav-
ior and in flatwise tension (FWT) to measure the
strength of the diffusion bonded joint between the
face sheet and honeycomb core. In addition, ten-sile tests were conducted on the Ti-14-21 face
sheet material to determine the effect of
processing on the face sheet properties. FWT,EWC, and tensile tests were conducted at tem-
peratures ranging from room temperature to
1500°F. Fractography and metallurgical analy-
sis were used to correlate properties andmicrostructure.
1RohrBond is a registered trademark of Rohr, Inc.
2
TLP Bonding
In the TLP bonding process, an intermediate
foil filler metal, usually based on copper, is
placed between the titanium components to be
joined. When heated above the titanium/filler
metal eutectic temperature for a sufficient periodof time, the titanium and filler metal interdiffuse
to form a eutectic composition. As a result, a
eutectic liquid forms and fills the gaps at the joint
interface. This liquid phase enhances diffusivityand ensures intimate contact at the joint interface
without the high bonding pressures and close
surface tolerances required by conventional dif-
fusion bonding. The components are held at this
temperature during which time the liquid solute
elements diffuse into the titanium components.
With continuing solute diffusion, the melt com-
position becomes titanium-enriched resulting in
an increase in the melting temperature and even-tual isothermal solidification. Additional time at
temperature permits solid-state diffusion to dilute
the solute concentration even further, resulting inimproved mechanical properties of the joint. After
this bonding period, the components are allowedto slow cool in the vacuum furnace.
Experimental Procedures
Materials
Four honeycomb core sandwich panels were
fabricated by Rohr, Inc., Chula Vista, California.
Each panel was 21 inches by 30 inches and con-sisted of two 0.020-inch-thick Ti-14-21 face
sheets TLP bonded to a 0.6-inch-thick honey-
comb core. A different honeycomb core alloy was
utilized for each sandwich panel: t_ + _ titanium
alloys Ti-3A1-2.5V (Ti-3-2.5) and Ti-6A1-4V
(Ti-6-4); 1_ titanium alloy Ti-15V-3Cr-3Sn-3A1
(Ti-15-3); and Ti3A1 alloy Ti-14-21. The o_+ [3and [3 cores were formed into 0.25-inch corm-
gated square cells from 0.002-inch-thick foil. Due
to the limited ductility of the Ti-14-21 alloy, the
alloy could not be rolled to as thin a gage norfabricated into as small a cell size as that for
tx + 13 and 13 titanium alloys. Therefore, theTi-14-21 core was formed into 0.375-inch
corrugated square cells from 0.004-inch-thick
foil. Figure 1 shows a picture of the Ti-14-21
honeycomb core sandwich panel.
Rohr used two proprietary TLP bonding
processes to fabricate the sandwich panels. The
first process, designated LID, was used tofabricate the Ti-3-2.5, Ti-6-4, and Ti-15-3
honeycomb core sandwich panels. The LID filler
metal used to join the face sheets to the
honeycomb core was a Cu/Ni alloy. The second
TLP process, designated Pro-4, was used to
fabricate the Ti-14-21 honeycomb core sandwich
panel. The Pro-4 process used a different filler
metal composition (Cu/Sn alloy), and required a
higher bonding temperature and a longer bonding
time than the LID process. Table 1 summarizes
the physical description of each panel.
Metallurgical Analysis
Metallurgical specimens were sectioned from
the honeycomb core sandwich panels using adiamond-wheel saw and were mounted in an
epoxy medium. Following polishing, the speci-
men surface was etched using Kroll's reagent.
The specimens were examined using optical
microscopy and scanning electron microscopy
(SEM) to characterize the microstructure. Energyand wavelength dispersive x-ray spectroscopy(EDS and WDS) were used to determine the face
for 10 minutes prior to being loaded to failure at a
rate of 250 lbs/min. The FWT strength was
calculated by dividing the maximum load by theface sheet area.
4
Edgewise compression tests
The edgewise compression test is designed to
determine the compressive properties of the
specimens when loaded in a direction parallel to
the plane of the face sheets. EWC strength is a
measure of the ability of the core and the joints to
stabilize the face sheets in compression.The EWC tests were conducted in accordance
with ASTM specification C364 (ref. 15). The
specimens were tested in a specially designed
compression cage EWC test fixture using a
screw-driven test machine. Figure 3 shows an
EWC specimen mounted in the test apparatus.
The specimens were tested at temperatures
ranging from room temperature to 1500°F
depending on the projected maximum service
temperature for each honeycomb core material.
Elevated temperature tests were conducted in a
resistance-heated, three-zone, split tube furnace
mounted to the test stand. Six thermocouples
were distributed along the length of the front and
back face sheets to monitor specimen
temperature. Each specimen was held at the
desired test temperature (+10°F) for 10 minutes
prior to being loaded to failure at a rate of 2000lbs/min. Strain for both the front and back face
sheets was measured with foil strain gages at
room temperature and water-cooled
extensometers at elevated temperature. EWC
strength was calculated by dividing the failure
load by the cross-sectional area of both facesheets.
Results and Discussion
Metallurgical Analysis
LID-processed panels
Figure 4 shows representative optical photo-
micrographs of LID-processed honeycomb core
sandwich panel joints fabricated with Ti-3-2.5,
Ti-15-3, and Ti-6-4 honeycomb cores. Large fil-lets formed at the face sheet/core nodes as a result
of capillary attraction of the molten filler metal.
No deleterious erosion of the foil gage honey-comb core due to core/filler metal interaction was
observed. The majority of the joints that were
examined in these panels were well-diffused and
had a fine multiphase structure. A small number
of the joints that were examined had regions
where the molten filler metal pooled at thecore/face sheet interface and formed undiffused
regions which may result in localized regions of
lower joint strength. A small undiffused region
within the joint can be seen in the photomicro-
graph of the Ti-6-4 honeycomb core panel shown
in figure 4.
Figure 5 shows the microstructure of the face
sheet remote from the joint in the Ti-3-2.5 honey-comb core sandwich panel. The microstructure ofthe Ti-14-21 face sheet bulk material consisted of
equiaxed 5 2 grains with 13at the grain boundaries.The surface of the face sheet to which the LID
filler metal had been applied was characterized by
a 50-_an-thick transformed region consisting of
coarse tx2 laths separated by intergranular 13. Thistransformed region resulted from diffusion of the
13-stabilizing Cu and Ni constituents of the fillermetal into the face sheet. Cu- and Ni-enrichment
of the 13phase contained within the transformed
region was verified by EDS x-ray mapping. (Seefig. 6.)
All three of the LID-processed panels exhib-
ited similar face sheet and joint microstructures.
Only the microstructures of the honeycomb core
remote from the joint differed. Figure 7 shows thehoneycomb core microstructures remote from the
joint for LID-processed Ti-3-2.5, Ti-6-4, and
Ti-15-3 honeycomb core sandwich panels. The
apparent thickness of the cell walls for the three
honeycomb core samples is a function of the
angle at which the metallurgical sections weretaken. All three of the honeycomb core cell wallswere 0.002 inch thick. The Ti-3-2.5 and Ti-6-4
honeycomb core microstructures consisted of
coarse tx plates separated by intergranular 13.The
microstructure of the Ti-15-3 honeycomb core
consisted of large 13grains extending through the
entire thickness of the foil gage cell wall, with
fine plates of tx decorating the grain boundaries.
Figure 8 shows an SEM photomicrograph of a
joint from a LID-processed Ti-6-4 honeycomb
core sandwich panel. Also shown is a WDS ele-
5
mental concentration profile of the LID filler
metal constituents Cu and Ni across the joint
interface. The original interface was assumed to
correspond to the edge of the face sheet. The
scans began 100 gm into the face sheet and
extended 200 gm into the honeycomb core with a
scan step distance of 10 _n.
The plot shows that the maximum Cu and Ni
concentrations were shifted from the original jointinterface approximately 50 gm into the hon-
eycomb core. The concentrations tended to varysharply within the joint. This behavior occurred
because of differences in composition ofindividual phases contained within the fine multi-
phase joint microstructure. The relative concen-
trations and depths of diffusion of Cu and Ni were
much greater in the Ti-6-4 honeycomb core than
in the Ti-14-21 face sheet. At the LID processing
temperature, the Ti-6-4 honeycomb core con-sisted of all [3 phase whereas the Ti-14-21 face
sheet contained ordered tx2 phase. The greater
concentrations of Cu and Ni in the honeycomb
core were attributed to Cu and Ni diffusing more
rapidly into 13-Ti than into ordered tx2 and capri-
lary flow of the molten LID filler metal along the
honeycomb core cell walls.
Pro-4-processed panel
Figure 9 shows an optical photomicrograph of
a joint from the Pro-4-processed Ti-14-21 honey-
comb core sandwich panel. The large fillets at the
face sheet/core nodes and the well-diffused,
multi-phase joints were similar in appearance to
the joints in the LID-processed panels. In addi-
tion, no deleterious erosion of the honeycomb
core was observed. Figure 10 shows the Ti-14-21
face sheet microstructure remote from the joint.The microstructure of the Ti-14-21 face sheet
bulk material was characterized by equiaxed t_2grains with 13at the grain boundaries. This micro-
structure was similar in appearance to that of the
LID-processed Ti-14-21 face sheet bulk material
(fig. 5), except that the [3 phase in the grainboundaries had coarsened and become more con-
tinuous. The surface of the face sheet to which the
Pro-4 filler metal had been applied exhibited a
transformed region which consisted of finer t_2
laths with higher aspect ratios than those in the
LID-processed Ti-14-21 face sheets. In addition,
the depth of this transformed region extended to a
greater depth (90 gm) due to the higher tempera-ture and longer time associated with the Pro-4
processing cycle.
Figure 11 shows the Pro-4-processed Ti-14-21
honeycomb core microstructure remote from the
joint. The outer edges of the core cell walls
consisted of tx2 laths and intergranular 13while the
interior consisted of equiaxed _2 with 1_ at thegrain boundary triple points. This transformed
microstructure was caused by capillary flow of
the molten filler metal along the surface of the
honeycomb core cell wails and its subsequent dif-
fusion into the outer edges of the honeycombcore.
Figure 12 shows an SEM photomicrograph of
a Pro-4-processed Ti-14-21 honeycomb core
sandwich panel joint. Also shown is a WDS
chemical concentration profile of the Pro-4 filler
metal constituents Cu and Sn across the joint in-
terface. The original interface was assumed to
correspond to the edge of the face sheet. The
scans began 200 gm into the face sheet and
extended 300 gm into the honeycomb core with a
scan step distance of 10 gm.
The plots show that the maximum Cu and Sn
concentrations occurred in the honeycomb core
approximately 50 gm from the joint interface.
The concentrations tended to vary sharply within
the fine multiphase joint microstructure. Thedepth of Cu and Sn diffusion in the Ti-14-21 hon-
eycomb core was greater than in the Ti-14-21
face sheets. This was attributed to capillary flow
of the molten Pro-4 filler metal along the honey-comb core cell walls and the slower bulk diffu-
sion into the face sheets than surface diffusivity
into the honeycomb core. Within the joint, a
greater concentration of Cu was detected than Sn.This concentration difference was attributed to
differences in the starting composition of the two
filler metals and to differences in diffusivity ofthe two elements.
In comparing the concentration profiles for the
6
Pro-4- and LID-processedjoints, the Pro-4processresultedin greaterconcentrationsandgreaterdepthof diffusionof Cuin thefacesheetandhoneycombcore.Thedifferencein thedepthof diffusion was attributed to the highertemperatureandlongertimeassociatedwith thePro-4processingcyclewhile the concentrationdifferencewasattributedto the differencesinstartingCu compositionbetweenthe LID andPro-4filler metals.
Face Sheet Tensile Tests
The Ti-14-21 face sheet tensile data for the
LID- and Pro-4-processed honeycomb core sand-
wich panels are shown in figures 13-16. Acomplete tabulation of the tensile test data is
shown in Appendix A. Figure 13 shows the aver-
age room and elevated temperature UTS and YS
of the Ti-14-21 face sheet specimens from the
three LID-processed panels. Also shown for com-
parison are the average UTS and YS at room
temperature and 1200°F for unprocessed Ti- 14-21sheet in the cc/13solution treated condition. The
room temperature UTS and YS of unprocessed
Ti-14-21 sheet were 91 ksi and 82 ksi, respec-
tively. As compared to the unprocessed sheet, the
LID-processed face sheets exhibited lower UTSranging from 78 ksi to 81 ksi and lower YS
ranging from 64 ksi to 70ksi. At 1200°F, the
unprocessed sheet had UTS of 51 ksi and YS of
40 ksi. The LID-processed face sheets exhibited
lower YS ranging from 28 ksi to 33 ksi, but the
UTS (43-53 ksi) was approximately the same asthat for the unprocessed sheet. The face sheets
from the three panels had similar strength behav-
ior because they all were exposed to the same
processing conditions. The UTS of the LID-
processed sheets increased slightly from room
temperature to 800°F, then decreased sharply withincreasing temperature. The YS for the LID-
processed face sheets decreased continuouslywith increasing temperature.
Figure 14 shows the average room and ele-
vated temperature UTS and YS values for the
Pro-4-processed Ti-14-21 face sheet specimens.Also shown for reference is the UTS and YS data
ranges for the LID-processed Ti-14-21 face sheet
specimens from figure 13. The Pro-4-processed
specimens were exposed to a higher temperature,
a longer time, and a different filler metal compo-sition than the LID-processed specimens. At room
temperature and 800°F, the UTS for the Pro-4-
processed Ti-14-21 specimens was significantly
less than that for the LID-processed specimens.
At temperatures of 1000°F and above, the UTS
values for the Pro-4- and LID-processedspecimens were equivalent. The YS of the Pro-4-
processed Ti-14-21 face sheet specimens corre-
lated well with that for the LID-processed
specimens at all temperatures tested.
The average room and elevated temperature
ductilities, as measured by ep, for the LID- andPro-4-processed Ti-14-21 face sheet specimens
and the unprocessed Ti-14-21 sheet in the c_/13
solution treated condition are shown in figure 15.
The room temperature ductility of unprocessed
Ti-14-21 sheet was 3.0%. After exposure to the
LID-processing cycle, the Ti-14-21 still retained
most of its room temperature ductility
(ep= 1.8-2.8%); however, Pro-4 processing
embrittled the Ti-14-21 (ep=0.2%). The facesheet ductility of the LID-processed specimens
increased rapidly with temperature up to 800°F.
At temperatures of 800°F and higher, the ep wasso high (>12%) that the tests were stopped prior
to specimen failure. Although the ductility of the
Pro-4-processed specimens gradually increased
with temperature, the ep was still relatively low(3.9%) at 1000°F. Only at 1300°F and 1500°F did
any of these specimens attain 12% plastic strainwithout failure.
Figure 16 shows the average room and ele-vated temperature E values for the LID- and Pro-
4-processed Ti-14-21 face sheet specimens and
the unprocessed Ti-14-21 sheet in the c_/15
solution treated condition. The room temperatureE of unprocessed Ti-14-21 was 10.4 Msi. After
exposure to the LID-processing cycle, the roomtemperature E of the Ti-14-21 increased to an
average value ranging from 11.6 Msi to 13.4 Msi.
Similarly, the room temperature E for the Pro-4-processed specimens increased to 12.7 Msi. All
four data curves show that E decreased gradually
with temperature to 800°F, then decreased rapidlyat the higher temperatures. At 1200°F, the E
for the LID- and Pro-4-processedsheetwasapproximately the same as that for theunprocessedsheet.
Figure17showstheroomtemperaturefracturesurfaceof a LID-processedTi-14-21facesheettensilespecimen.TheLID-transformedregionofthemicrostructure(seefig. 5) producedadiffer-ent fracturemorphologythan did the micro-structureremotefrom thetransformedregion.IntheLID-transformedregion,thefracturesurfacewascharacterizedbyintergranularfracturealongthecoarse_2 lathswith somedistinctregionsofductilemicrovoidcoalescencecorrespondingtothe[3phase. The remainder of the fracture surface
remote from the LID-transformed region
exhibited intergranular fracture of the equiaxed
_2 grains. These fracture surface features were
typical for all the LID-processed Ti-14-21 face
sheet tensile specimens, regardless of the type of
honeycomb core to which they were originallybonded.
Figure 18 shows the room temperature fracture
surface of a Pro-4-processed Ti-14-21 face sheet
tensile specimen. The Pro-4-transformed region
of the microstructure (see fig. 10) produced a
different fracture morphology than did the micro-
structure remote from the transformed region. The
specimen exhibited brittle fracture along the
entire Pro-4-transformed region, which comprised
approximately 20% of the face-sheet cross-
section. No signs of ductile microvoid coales-
cence were evident. The high Cu concentrations
in the transformed region may have contributed to
the brittle fracture morphology observed in this
region. The addition of Cu has been previouslyassociated with the formation of brittle com-
pounds such as Ti2Cu (ref. 16). The fracture sur-
face remote from the transformed region exhib-
ited intergranular fracture of the equiaxed _2grains that was similar in appearance to that
observed for the LID-processed face sheets.
LID-processing of the Ti-14-21 face sheet
material caused a decrease in room temperatureUTS and YS, an increase in E, and a minor
decrease in ep in comparison to unprocessed
Ti-14-21 sheet tensile properties. The Pro-4-
processed face sheets showed the same YS and E
trends as did the LID-processed face sheets.
However, Pro-4 processing caused a significant
reduction in room temperature face sheet ductilityand UTS in comparison to the LID-processed face
sheet tensile properties. This reduction in room
temperture properties is most likely due to the the
greater depth of Cu diffusion into the Pro-4-
processed face sheets than was observed in the
LID-processed face sheets. This embrittlement
could potentially be reduced by lowering the
processing temperature and time and minimizingthe amount of filler metal used to fabricate the
panel.
Flatwise Tension Tests
The average room and elevated temperature
FWT strengths for the LID- and Pro-4-processed
honeycomb core sandwich specimens are shown
in figure 19. A complete tabulation of the FWT
test data is shown in Appendix B. The plot shows
that the LID-processed Ti-15-3 honeycomb core
sandwich specimens had the highest FWT
strength at temperatures up to 1000°F. At tem-
peratures above 1000°F, the FWT strength of all
three types of LID-processed specimens
decreased rapidly. Although the Pro-4-processed
Ti-14-21 honeycomb core sandwich specimens
had the lowest FWT strength at room tempera-
ture, they also had the highest FWT strength at
temperatures above 1000°F. The FWT strength of
the Ti-14-21 specimens did not begin to decreaseuntil the temperature exceeded 1300°F.
The FWT strength behavior of the Ti-14-21 speci-
mens (low room temperature strength and high
elevated temperature strength) can be related to
the damage tolerance of the joint. The Pro-4
process most likely embrittled the Ti-14-21 hon-
eycomb in the vicinity of the diffusion bondedjoint (as was seen with the face sheet tensile
specimens). Poor damage tolerance characteristics
of the joint at room temperature resulted in failureat low FWT stress levels. At the elevated
temperatures, the joint became more ductile and
more damagetolerant, allowing higherFWTstresslevels to be attainedprior to specimenfracture.
Effect of temperature on average FWT strength of LID- and Pro-4-processed honeycomb core sandwich
Figure 20. Room and elevated temperature failure modes for Ti-14-21 honeycomb core FWT specimens.
28
EdgewiseCompression
Strength,ksi
10
8O
60
40
2O
- Honeycomb core sandwich panel
LID-processed Ti-3-2.5
[] LID-processed Ti-6-4
o LID_processed Ti-1.5-3_
I I I I I I I I I I I I I I I
0 500 1000 1500Temperature, °F
Figure 21. Effect of temperature on average EWC strength of LID- and Pro-4-processed honeycomb core sandwichspecimens.
Face Sheet Wrinkling Shear Crimping Shear Crimping Shear Crimping
Room Temperature 800OF 1000°F 1100°F
Figure 22. Room and elevated temperature failure modes for Ti-3-2.5 honeycomb core EWC specimens.
29
Face Sheet Wrinkling
Face Sheet Wrinkling Shear Crimping
Room Temperature
1000°F 1500°F
Figure 23. Room and elevated temperature failure modes for Ti-14-21 honeycomb core EWC specimens.
3O
.... " _'' , '" " _ i, '¸'_ i '
Stress,ksi
100
90
8O
70
60
5O
40
3O
20
10
I EWC Specimen.... Tensile Specimen
n
m
i i I0 0.5 1.0 1.5 2.0
RT
Strain, %
. .... 1200°F
Figure 24. Room temperature and 1200°F stress-strain curves for LID-processed Ti-3-2.5 honeycomb core EWCspecimens and Ti-14-21 face sheet tensile specimens.
31
Appendix A
Unprocessed, LID-Processed, and Pro-4-Processed Ti-14-21 Face SheetTensile Data
Table A-1. Tensile properties of unprocessed Ti-14-21 sheet material in the oc/13solution treated condition
Specimen No. Temp. (°F) UTS (ksi) YS (ksi) E (Msi) ep (%)
U- 1 Room Temp. 90.8 81.9 10.4 3.0
U-2 1200 51.0 40.0 7.0 >12 a
aSpecimen did not fail. Test stopped after this amount of plastic strain.
Table A-2. Tensile properties of Ti-14-21 face sheet material from LID-processed
Ti-3-2.5 honeycomb core sandwich panel
Specimen No. Temp.(°F) UTS (ksi) YS (ksi) E (Msi) ep (%)
3-41-1
3-41-2
3-41-3
3-41-4
Room
Temp.
80.3
81.2
80.2
77.5
64.7
64.5
63.7
63.9
11.9
11.8
11.7
11.7
2.51
2.77
2.591.92
58.9
59.2
3-41-5 84.0 46.2 11.3 12.048OO
3-41-6 83.6 45.8 10.9 12.01
1000
1100
35.9
36.2
32.3
32.6
28.4
28.0
49.4
50.2
8.6
9.1
7.6
7.4
6.6
6.2
3-41-11 43.11200
3-41-12 42.3
>12.12 a
>11.85 a
>11.80 a
>11.80 a
>11.81 a>11.72 a
aSpecimen did not fail. Test stopped after this amount of plastic strain.
32
TableA-3.Tensilepropertiesof Ti-14-21 face sheet material from LID-processed
Ti-6-4 honeycomb core sandwich panel
Specimen No. Temp. (°F) UTS (ksi) YS (ksi) E (Msi) ep (%)
6-33-7
6-33-8
6-33-9
6-33-10
Room
Temp.
76.9
77.3
78.3
78.6
64.9
65.3
65.5
64.5
11.7
11.5
11.5
11.7
2.72
2.53
3.09
2.82
6-33-11 79.1 44.1 11.0 > 12.04 a8O0
6-33-12 88.1 43.2 10.2 >18.03 a
6-34-13 64.4 38.3 9.3 >12.10 a1000
6-34-14 64.0 38.2 9.1 >11.85 a
6-34-15 55.7 34.2 7.8 > 11.78 a1100
6-34-16 56.6 34.9 7.8 >11.74 a
6-34-17 48.0 30.7 6.9 >11.72 a1200
6-34-18 48.2 30.9 6.6 >11.66 a
aSpecimen did not fail. Test stopped after this amount of plastic strain.
Table A-4. Tensile properties of Ti-14-21 face sheet material from LID-processed
Ti-15-3 honeycomb core sandwich panel
Specimen No. Temp. (°F) UTS (ksi) YS (ksi) E (Msi) ep (%)
I Form ApprovedREPORT DOCUMENTATION PAGE OMB No. 07704-0188
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1. AGENCY USE ONLY (Leave blank)12. REPORT DATE 3. REPORT TYPE AND DATES COVERED
I June 1998 Technical Publication
4, TITLE AND SUBTITLE
Evaluation of the Transient Liquid Phase (TLP) Bonding Process forTi3A1-Based Honeycomb Core Sandwich Structure
6. AUTHOR(S)R. Keith Bird
Eric K. Hoffman
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
NASA Langley Research Center
Hampton, VA 23681-2199
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Washington, DC 20546-0001
5. FUNDING NUMBERS
522-12-11-01
8. PERFORMING ORGANIZATION
REPORTNUMBER
L-17717
10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
NASA/TP- 1998-208421
11. SUPPLEMENTARY NOTES
12a. DISTRIBUTION/AVAILABILITY STATEMENT
Unclassified-Unlimited
Subject Category 26 Distribution: StandardAvailability: NASA CASI (301) 621-0390
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13. ABSTRACT (Maximum 200 words)
The suitability of using transient liquid phase (TLP) bonding to fabricate honeycomb core sandwich panelswith Ti-14A1-21Nb (wt%) titanium aluminide (Ti3A1) face sheets for high-temperature hypersonic vehicle applica-tions was evaluated. Three titanium alloy honeycomb cores and one Ti3A1 alloy honeycomb core were investigated.Edgewise compression (EWC) and flatwise tension (FWT) tests on honeycomb core sandwich specimens and ten-sile tests of the face sheet material were conducted at temperatures ranging from room temperature to 1500°E
EWC tests indicated that the honeycomb cores and diffusion bonded joints were able to stabilize the face sheetsup to and beyond the face sheet compressive yield strength for all temperatures investigated. The specimens withthe Ti3A1 honeycomb core produced the highest FWT strengths at temperatures above 1000°E Tensile tests indi-
cated that TLP processing conditions resulted in decreases in ductility of the Ti-14A1-21Nb face sheets.Microstructural examination showed that the side of the face sheets to which the filler metals had been applied
was transformed from equiaxed _2 grains to coarse plates of c_2 with intergranular [3. Fractographic examination of
the tensile specimens showed that this transformed region was dominated by brittle fracture.