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CASTABILITY
THE
OF TI-5553 ALLOY
The microstructure and properties of
cast titanium alloy Ti-5553 were
evaluated in a joint program by
Howmet and Boeing.
Stewart Veeck* and David Lee
Howmet Corporation
Whitehall, Michigan
Rodney Boyer* and Robert Briggs
The Boeing Company
Seattle, Washington
T
itanium alloy 5553 (Ti-5Al-5V-5Mo-3Cr)
is a near-beta alloy that exhibits excellent hardenability and strength characteristics, which make it attractive as a
structural titanium-casting alloy. To evaluate its
castability, Boeing and Howmet conducted a joint
program. The input stock was supplied in the
form of large pieces of a forged component that
had been cut up at Boeing as part of an internal
evaluation.
Prior to the initial casting trials, a sample of the
forged alloy was taken for chemical analysis. The
results of the analysis are shown in Table 1. The
data show that the oxygen content of the alloy
is low relative to many titanium alloys, which
most likely enhances the ductility and toughness
of the alloy. In addition, examination of the chemistry of this material indicates that, relative to the
nominal composition, the alloy composition is
shifted toward the alpha-rich portion of the presumed range by virtue of the beta-forming elements being lower than the nominal composition.
This article summarizes the experimental
casting studies carried out by both companies,
including castability, microstructure/property
correlation, evaluation of a bulkhead casting,
property assessments, and manufacturability.
Castability studies
The experimental casting studies were divided
into several areas. Initial efforts focused on establishing the castability of the alloy relative to
Ti-6Al-4V, which is the baseline standard for the
industry. In these studies, the fill capabilities of
the two alloys were compared directly by casting
a fluidity mold (Fig. 1). Cast Ti-5553 material was
then examined to determine the microstructure/
tensile property behavior of the alloy using both
the standard heat treatment developed for the
wrought material, and alternative heat treatments
designed to enhance the mechanical properties.
Based on the preliminary castability and mechanical property data, which both looked very prom*Member of ASM International
ising, several bulkhead components were cast,
hot isostatically pressed (HIP’ed), and heattreated for more detailed physical and mechanical property evaluations. These components also
were tested to examine the response to processing
methods such as weld repair and chemical
milling.
As is typical for titanium alloys, the Ti-5553
alloy was not as castable as Ti-64; however, the
relative castability was comparable to both Ti6242 and Ti-15-3-3-3 alloys, which are routinely
selected for structural titanium castings.
Microstructure/property correlations
The cast material from the fluidity molds was
HIP’ed, and selected pieces were chemically
milled to remove alpha case and then vacuum
heat-treated by a heat treatment Boeing developed for high durability wrought product. This
heat treatment included an above-beta solution
cycle, followed by cool down at a controlled rate
to the desired age temperature. The beta transformation temperature for the alloy was reported
as 860°C (1580°F).
The initial tensile results associated with the baseline heat treatment indicated good strength and
very high ductility values. Therefore, engineers thought that
somewhat higher strengths, along
with acceptable ductility, were
possible by modifying either the
cooling-rate parameters from the
solution temperature, or the aging
temperature to control the nucleation density and/or size of the
precipitated alpha phase.
Consequently, a study was initiated to examine a range of
cooling rates and age temperatures to determine the effect on
the tensile characteristics. Sections
of down-feed were cut out for the
Fig. 1 — This is the standard fluinitial evaluations, and Vickers idity mold for castability trials.
Table 1 — Ti-5553(Ti-5Al-5V-5Mo-3Cr)
compositional analysis
Element
Carbon
Iron
Molybdenum
Aluminum
Titanium
Vanadium
Silicon
Chromium
Oxygen
Nitrogen
Hydrogen
ADVANCED MATERIALS & PROCESSES/OCTOBER 2004
Result, wt%
Nominal
0.009
0.31
4.71
5.12
Bal
4.72
0.05
2.77
0.1282
0.0055
0.0038
—
0.50
5.00
5.00
—
5.00
—
3.00
—
—
—
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1200
60
50
1100
900
Strength, MPa
1000
40
UTS
800
700
30
20
YS
Elongation, %
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El.
600
100
200
300
400
500
Temperature, °C
hardness tests (300-gram load) were conducted
on the sections after each set of heat treat parameters to establish the impact on hardness (which
is related to tensile strength).
After generating curves plotting the Vickers
hardness as a function of age temperature and
cooling rate, several of the parameters were selected to heat-treat and test tensile test bars to determine the correlation between hardness and
tensile properties. These data served as the basis
for establishing a heat treatment that would provide the best combination of strength and ductility in the alloy.
The data from these studies suggest that increasing the cooling rate from the solution temperature and lowering the age temperature relative to the baseline heat treatment, would provide
higher strengths while maintaining adequate ductility. Based on a target hardness of 380 Vickers,
these conditions should provide ultimate tensile
strength of about 1100 MPa (160 ksi), yield
strength of 1035 MPa (150 ksi), and elongations
in the range of 6 to 8%.
Bulkhead component
To further demonstrate the castability of the
alloy, a second set of casting trials was conducted
on a generic bulkhead component. This component was a moderately sized part having a finished weight of approximately 11.3 kg (25 lb),
which represented a typical structural titanium
casting.
After casting, the shell was removed from the
two cast parts, and they were then HIP’ed at
900°C/100 MPa/2 h (1650°F/14.5 ksi/2 h). The
gates were removed and the parts were chemically milled. After weld repair and blending, the
castings were cleaned and vacuum heat-treated
together at the Howmet Research Center (HRC)
using the optimized heat treat cycle established
above.
One casting was submitted to Boeing for mechanical property evaluations, and one was retained at Howmet for a parallel evaluation. An
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Fig. 3 — This graph summarizes the tensile properties of
Ti-5553 as a function of temperature. The strength and ductility of the HIP’ed and heat-treated test component were
better than anticipated.
1100
1000
900
Max. stress, MPa
Fig. 2 — This Ti-5553 bulkhead component was cast, HIP’ed, and weldrepaired before testing.
800
700 600
500
R=0.1
T=70°F
400
104
Ti-5553
Ti-64
105
106
Cycles to failure
107
Fig. 4 — High-cycle fatigue behavior shows that Ti-5553
has significantly better run-out stress than Ti-6Al-4V. In
fact, its fatigue behavior is actually superior to many wrought
titanium alloys.
investment cast bulkhead component, successfully cast in Ti-5553, is shown in Fig. 2 after HIP
and post-HIP processing.
Property evaluations
The measured bulk density of the alloy was determined to be 4.67 gm/cm3. The average coefficient of thermal expansion (CTE) over the indicated temperature range was 10.8 x 10-6 mm/mm/
ºC. Figure 3 summarizes the tensile data as a function of temperature. Based on the original estimates from the hardness data, the strength and
ductility combination achieved in the HIP’ed and
heat treated test components at room temperature were actually better than anticipated. The
strength characteristics of the alloy at both room
temperature and elevated temperatures appear
to be excellent. Even at temperatures of 427°C
(800°F), an ultimate tensile strength of 896 MPa
(130 ksi) is maintained.
ADVANCED MATERIALS & PROCESSES/OCTOBER 2004
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Limited data showed the Table 2 — Static properties of Ti-5553
room temperature tensile mod(Ti-5Al-5V-5Mo-3Cr) versus Ti-6Al-4V
ulus of the material to be 118 x
3
Property
Cast and HIP’ed Ti-5553(1) Cast and HIP’ed Ti-64(2)
10 MPa (17.1 Msi). The remaining static test data, which Ultimate tensile strength, MPa (ksi)
1159 (168)
910 (1320
included compression, shear,
Yield strength, MPa (ksi)
1055 (153)
828 (120)
and bearing data, is shown in
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8.9
Table 2, along with comparative Elongation, %
1138 (165)
897 (130)
data for 19-mm cast, HIP’ed, Compressive strength, MPa (ksi)
and mill annealed Ti-6AL-4V. Max shear strength, MPa (ksi)
690 (100)
655 (95)
Clearly, the superior strength Bearing UTS (E/d=2.0), MPa (ksi)
2248 (326)
1862 (270)
characteristics of the alloy trans- Bearing YS (E/d=2.0), MPa (ksi)
1931 (280)
1648 (239)
late to significant improve- Room temperature properties. (1) Ti-5553 optimized heat treatment. (2) Standard mill anneal, 843°C/2h (19-mm section size).
ments in other static properties
ically Ti-6Al-4V alloy, which is the dominant
relative to Ti-6Al-4V.
The dynamic properties of Ti-5553 also looked casting alloy. The ability to use controlled heat
very attractive, particularly the high cycle fatigue treatment parameters to produce Widmanstätten
behavior of the alloy, which is shown in Fig. 4 type structures appears to be a key ingredient
along with comparative historical HCF data for in obtaining superior tensile strength properties
in the alloy.
cast.
Many more details about the studies are availThe observed run-out stress at 107 cycles of 758
MPa (110 ksi) is a significantly higher than that of able from the authors.
Ti-6Al-4V, which typically exhibits a run-out stress
of 414 to 448 MPa (60 to 65 ksi) under similar test For more information: Dr. Rodney Boyer , Boeing Co.,
conditions (R = 0.1). In fact, the fatigue behavior Seattle, WA 98124; e-mail: Rodney.r.boyer@boeing.com.
is actually superior to many wrought titanium al- David Lee, Howmet Corp., Whitehall, MI 49461; e-mail:
loys. This may be a consequence of the refined dlee@Howmet.com. Stewart Veeck, Howmet Corp.,
Widmanstätten microstructure, which has shown Whitehall, MI 49461; e-mail: sjveeck@msn.com. Robert
Briggs, Boeing Co., Seattle, WA 98124; e-mail: Robert.d.
superior high-cycle fatigue behavior in studies briggs@boeing.com.
conducted previously. Clearly, the high strength
and good ductility of the alloy contribute substantially to the excellent high-cycle fatigue behavior.
The fracture toughness of Ti-5553 was lower than
that normally observed in cast Ti-6Al-4V, but the
93 to 104 MPa√m values were consistent with the
higher yield strength of the material, and were
better than the wrought version of the alloy, when
heat treated under similar conditions. In addition,
although the fracture toughness values obtained
were invalid, the values would be allowed under
Boeing certification requirements.
The alloy exhibited excellent elevated temperature properties. However, the creep data indicated that the creep characteristics of Ti-5553 are
very similar to those of Ti-6Al-4V. This result is
not surprising, since it is a near-beta alloy. Therefore, it would not be expected to have good creep
resistance at elevated temperatures where the
lower-strength beta phase would dominate the
structure. Boeing data showed the times to 0.5%
creep at 538°C/379 MPa to be only 0.2 and 0.3
hours, whereas Ti-6242, which is known for its
creep resistance, typically has a creep life in the
50 to 80 hour range under similar test conditions.
Future possibilities
The results of the joint program between
Howmet and Boeing to evaluate Ti-5553 as a
casting alloy have been very encouraging. The
studies have shown Ti-5553 to be comparable to
Ti-6242 and Ti-15-3-3-3 in terms of castability. Furthermore, the mechanical property evaluations
have shown the alloy to exhibit substantially superior strength and fatigue characteristics relative to other cast titanium alloys and more specifADVANCED MATERIALS & PROCESSES/OCTOBER 2004
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