Scripta mater. 44 (2001) 409 – 414
www.elsevier.com/locate/scriptamat
TEMPERATURE DEPENDENT CREEP EXPANSION OF
Ti-6Al-4V LOW DENSITY CORE SANDWICH STRUCTURES
Douglas T. Queheillalta, Kevin A. Gableb and Haydn N.G. Wadleya
a
Department of Materials Science and Engineering, University of Virginia,
Charlottesville, VA 22904, USA
b
Materials Science and Engineering Department, University of Florida, Gainesville, FL 32601, USA
(Received June 27, 2000)
(Accepted in revised form September 6, 2000)
Keywords: Creep; Hot isostatic pressing; Powder processing; Titanium
Introduction
The application of lightweight, structurally efficient metal based honeycomb structures has been limited
by their high manufacturing costs. Interest has therefore developed in an entrapped gas expansion
process for the low cost manufacture of porous metal sandwich structures. These porous cored sandwich
structures are of potential interest for applications such as aircraft door, wing and stiffener skins.
The approach is based upon work by Kearns et al. [1,2] and Martin et al. [3,4]. They investigated
a powder metallurgy technique for the production of Ti-6Al-4V porous cored sandwich structures. Their
process began with the filling of a thick walled Ti-6Al-4V canister with Ti-6Al-4V powder and back
filling with between 1 and 7 atm. of argon gas. The canister was then sealed and consolidated by hot
isostatic pressing (HIPing) to create a 95–98% dense material with gas filled pores. The consolidated
samples were then hot rolled to create a plate consisting of a core with a finely dispersed pore
distribution. The structure contains pores with high internal gas pressures and thin, fully dense face
sheets (inherited from the original canister). The small pores are then re-expanded using a high
temperature isothermal annealing step. This process results in a low density cored sandwich structure
with the core containing up to 35% porosity. The mechanical performance of these panels are governed
by the cores relative density, the elastic properties of the core, the face sheet thickness and microstructure and are therefore dependent on the degree of expansion.
Recently, in-situ sensors [5], microstructural modeling [6] and superplastic foaming under temperature cycling [7] have been used to gain insight into the expansion response of Ti-6Al-4V low density
core sandwich structures. These studies investigated the role of variables such as the initial pore
pressure, the relative density of the core and the pore morphology upon the expansion process. During
entrapped gas expansion processing the driving force governing the re-expansion step is the internal
pore gas pressure which is controlled by the initial gas pressure, the degree of consolidation, the
increase in applied temperature and resultant volume expansion. It is resisted by the matrix which
deforms by creep. The creep constitutive response is a function of stress, temperature, accumulated
strain and the microstructure of the matrix (particularly its grain size). Therefore the maximum
attainable porosity is limited by the matrix constitutive response and the loss of gas pressure, which
occurs either as a result of the expansion itself (an increase in void volume lowers the gas pressure) or
because of gas lost through the external surfaces of the expanding body [8]. The gas loss through
external surfaces arises as a result of either failure to isolate voids properly so that interconnected paths
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for gas flow to the surface exist from the start of the re-expansion step, or because of void coalescence
resulting in the evolution of long-range gas flow paths through the porous core. It is assumed that no
gas is lost through the fully dense face sheets. Here, we experimentally explore the effects of heating
rate and isothermal annealing temperature on the expansion response of similar Ti-6Al-4V low density
core sandwich structures and their resultant matrix microstructure.
Sample Preparation
Low density core Ti-6Al-4V sandwich panels were prepared by the Boeing Company (St. Louis, MO).
A 10 ⫻ 10 ⫻ 2 cm3 Ti-6Al-4V HIP canister (1.5 mm wall thickness) was filled with Ti-6Al-4V powder
(⬍500 ␮m particle diameter) produced by the plasma rotating electrode method. The HIP canister was
hot vacuum outgassed and pressurized with three atmospheres (⬃0.3 MPa) of argon gas. The gas
charged powder compact was HIPed at 1040°C for 6 hours at a pressure of 103 MPa. After HIPing, the
sample was heated in air to 900°C, held for 30 minutes, and rolled (5% reduction per pass), with a 3
minute reheat at 900°C between rolls, to a final thickness of about 3.3 mm. Twenty uni-directional
rolled passes were used for the rolling step. During deformation strong diffusion bonds formed between
the HIP canister and the powder core, resulting in an integrally bonded sandwich structure. After rolling
the a relative density of the core was ⬃0.98 with fully dense face sheets that were ⬃300 ␮m thick.
Samples 10 ⫻ 10 ⫻ 3.3 mm3 in size were machined from the plate and encapsulated in evacuated quartz
ampoules (10⫺6 torr). The samples were annealed by heating at varying heating rates of 5°, 10°, 20° and
30°C per minute to an isothermal annealing temperature in the 800°–1150°C range. The isothermal
annealing cycle was adjusted such that the total heating and soak time was 600 minutes. The samples
were then cooled at a constant rate of 10°C per minute. Each samples density was measured prior to
and after annealing.
Results
Microstructural Characteristics
The as-rolled microstructure exhibited a porosity distribution consisting of pore sizes ranging from a
few microns in diameter up to ⬃200 ␮m; including morphologies that varied from spherical to prolate
shaped pores. The uni-directional rolling process aided in producing prolate shaped pores which were
aligned in the rolling direction. The porous core exhibited a heavily deformed lamellar microstructure
of elongated, interconnected ␣-phase grains with intergranular transformed ␤-phase. Figure 1 shows a
composite micrograph of the core microstructure in the as-rolled condition exhibiting an anisotropic
pore morphology.
Figure 2 shows scanning electron micrographs of the porous core for the Ti-6Al-4V low density core
sandwich structures. These are representative micrographs showing the evolutionary trends of the
expanding porous core for a series of samples heated at a rate of 10°C per minute. The inset shown for
each annealing temperature are representative micrographs of the porous cores base microstructure.
Since the uni-directional rolling process resulted in an anisotropic porous core, i.e. the pores tended
to be elongated in the rolling direction, the micrographs shown in Fig. 2 were taken parallel to the
rolling direction. The pore morphology perpendicular to the rolling direction tended to be more circular
in nature (a direct result of uni-directional rolling) and not shown here. It can be seen from Fig. 2, that
the porosity increased as the isothermal annealing temperature was increased from 850° to 1150°C.
Because the cores expansion was constrained by the fully dense face sheets, expansion occurred only
in a direction normal to the rolling plane. It should also be noted that as the annealing temperature
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LOW DENSITY CORE SANDWICH STRUCTURE
411
Figure 1. Composite micrograph in the as-rolled condition showing an anisotropic porosity distribution and base microstructure.
increased, the degree of void coalescence observed increased lending to the potential long-range escape
route for the entrapped argon gas.
The inset micrographs of Fig. 2 show the corresponding base microstructures. It was observed that
samples annealed below 1000°C, Figs. 2(a)–(d), exhibited a two-phase ␣ ⫹ ␤ microstructure which had
recrystallized to a fine equiaxed structure of primary ␣-phase and intergranular retained (metastable)
␤-phase grains. The sample annealed at 1000°C, Fig. 2(e), exhibited a mixed mode microstructure of
primary ␣-phase and intergranular retained (metastable) ␤-phase grains in addition to a small amount
of ␣ ⫹ ␤ Widmanstätten structure. The samples annealed above 1000°C, Figure 2(f)–(h), exhibited
rapid ␤-phase grain coarsening with an ␣ ⫹ ␤ Widmanstätten structure.
Expansion Characteristics
Figure 3 shows the expansion response (i.e. porosity evolution) of the Ti-6Al-4V low density core
sandwich structures as a function of heating rate and isothermal annealing temperature. It can be seen
from Fig. 3 that the volume fraction of porosity increased with increasing isothermal annealing
temperature. This occurs as a result of two competitive mechanisms; the driving force for expansion
(the internal pore pressure) increases with temperature and the materials resistance to flow which is
reduced via grain growth. It is interesting to note that a distinct difference in the slope of the
expansion–temperature relation occurs just below the ␤ transus (⬃980°C).
The creep constitutive response of the Ti-6Al-4V matrix has been described by a power law creep
(PLC) and diffusion accommodated grain boundary sliding (DAGS) potential [6]. In addition, fine
grained Ti-6Al-4V exhibits superplastic behavior between 750° and 950°C and grain boundary sliding
is assumed to be the dominant mechanism for the superplastic behavior [9 –11]. It has been shown that
partitioning of the substitutional alloying elements between the ␣- and ␤-phase (Al to ␣, V to ␤) retards
grain growth and increases the superplastic temperature range up to 900 –950°C [9]. At temperatures
above 950°C the ␣-phase volume fraction decreases, rapid coarsening of the ␤-phase occurs and
superplastic behavior is lost. It is believed that rate of expansion is retarded in the ␤-phase field due to
rapid grain growth [12] and gas lost through external surfaces of the expanding body because of void
coalescence and the evolution of long-range gas flow paths.
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Figure 2. Scanning electron micrographs of the porosity evolution for Ti-6Al-4V low density core sandwich structures (heating
rate ⫽ 10°C/min.). The inset micrographs for each annealing temperature shown are representative of the base microstructure.
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Figure 3. Expansion characteristics of the porous core as a function of heating rate and isothermal annealing temperature.
Figure 4 shows the porosity evolution of the Ti-6Al-4V low density core sandwich structures as a
function of heating rate, isothermal annealing temperature and soak time. It can also be seen from Fig.
4 that expansion in the ␣ ⫹ ␤ phase field was nearly independent of heating rate, whilst expansion in
the ␤ phase field increased with increasing heating rate. This is consistent with earlier observations that
creep expansion below 950°C occurs rapidly and then saturates before completion of the isothermal
anneal [5]. The observed heating rate dependence above the ␤-transus has been attributed to longer soak
times at the isothermal annealing temperature.
Concluding Remarks
The influence of heating rate and isothermal annealing temperature on the expansion response of
Ti-6Al-4V low density core sandwich structures and their resultant core microstructures were examined. It was observed that the maximum attainable porosity was limited by the loss of gas pressure,
which occurs either as a result of the expansion itself (an increase in void volume lowers the gas
pressure), or because of gas lost through the external surfaces of the expanding body. This latter effect
was attributed to a failure to isolate voids properly so that interconnected paths for gas flow to the
Figure 4. Expansion characteristics of the porous core as a function of heating rate and isothermal annealing time.
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surface exist from the start of the expansion step, or because of void coalescence and the evolution of
long-range gas flow paths.
It can also be concluded that expansion incurred during annealing in the ␣ ⫹ ␤ phase field occurred
rapidly and independent of heating rate and isothermal soak time. However, annealing in the ␤-phase
field has been retarded due to a decrease in the DAGS deformation mechanisms due to rapid coarsening
of the ␤-phase and gas lost through the external surfaces of the expanding body because of void
coalescence and the evolution of long-range gas flow paths. Although, annealing above the ␤-transus
leads to greater expansions, the higher processing temperature also leads to an undesirable Widmanstätten microstructure.
Acknowledgements
This work has been performed as part of the research of the Multidisciplinary University Research
Initiative (MURI) program on Ultralight Metal Structures. We are grateful for the many helpful
discussions with our colleagues in these organizations. The consortium’s work has been supported by
DARPA/DSO under contract N00014-96-I-1028 monitored by Dr. Steve Wax (DARPA) and Dr. Steve
Fishman (ONR).
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