- Email: [email protected]
Thermoplastic blow molding of metals
Thermoplastic blow molding of metals While plastics have revolutionized industrial design due to their versatile processability, their relatively low strength has hampered their use in structural components. On the other hand, while metals are the basis for strong structural components, the geometries into which they can be processed are rather limited. The “ideal” material would offer a desirable combination of superior structural properties and the ability to be precision (net) shaped into complex geometries. Here we show that bulk metallic glasses (BMGs), which have superior mechanical properties, can be blow molded like plastics. The key to the enhanced processability of BMG formers is their amenability to thermoplastic forming. This allows complex BMG structures, some of which cannot be produced using any other metal process, to be net shaped precisely. Jan Schroersa,*, Thomas M. Hodgesa, Golden Kumara, Hari Ramanb, Anthony J. Barnesb, Quoc Phamc, and Theodore A. Waniukc a Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511, USA b SuperformUSA, Riverside, CA 92517, USA c Liquidmetal Technologies, Rancho Santa Margarita, CA 92688, USA * E-mail: [email protected] Metals are the most widely used structural material, spanning
the process (Fig. 1). Such a desired processing window of forming
length scales from ~100 nm to ~100 m in applications where a
pressure lies between 10-5 and 1 MPa and can be readily accessed
combination of strength and ductility is required. Compared to
with (thermo)plastics. Plastic processing is typically carried out at
plastics, however, metals exhibit limited processability (Fig. 1).
viscosities of 103–106 Pa·s and strain rates of 10-2–101 sec-1. In the
The origin of the superb processability of (thermo)plastics is the
simplest case of a Newtonian flow, viscosity, η, translates into the flow stress (or forming pressure), σ, according to σ = η⋅3ε⋅, where
gradual softening from a solid-like material (glass) below the glass
14
transition temperature, Tg, to a liquid-like material (supercooled
ε⋅ is the strain rate. Most conventional metals cannot be processed
liquid) when heated above Tg. From a processing point of view,
within the ranges described above. They are either processed
an ideal material would flow under a forming pressure, which
in their crystalline state, where even at elevated temperatures
is low, yet sufficiently large that turbulent flow is avoided and
they possess high strength, or in a highly fluid liquid state
gravity and wetting effects can be neglected on the time scale of
above their melting temperature. This results in turbulent flow,
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
ISSN:1369 7021 © Elsevier Ltd 2011
Thermoplastic blow molding of metals
REVIEW
of handling low viscosity liquids. However, these improvements are marginal when compared to plastics and can only be realized in a limited family of alloys.
Bulk metallic glass Recent decades have witnessed the development of a new class of metallic alloys that solidify into an amorphous structure even at moderate (<100 K/s) cooling rates4. The amorphous structure of these bulk metallic glass (BMG) forming alloys results in strength and elasticity values that typically surpass those of conventional structural metals4. In order to harness their full range of attractive properties, BMGs should be used in geometries where at least one dimension is below approximately9 1 mm. This length scale depends on the critical crack length and varies among BMGs and is due to size effect on the BMGs’ mechanical properties5-8. For example, despite the Fig. 1 Properties vs. processability compared via the temperature-dependent strength for conventional steel, SPF alloys, plastics, and BMGs. Conventional metals are represented by a 1045 steel, SPF alloys by aluminum based 2004 SPF, BMGs by Zr44Ti11Cu10Ni10Be25 and Pt57.5Cu14.7Ni5.3P22.5, and PET (polyethylene terephthalate) was chosen as an example system for plastics. The temperature dependent strength (flow stress) is calculated for the fluids from σ = η⋅3ε⋅ for a strain rate of 10-1 sec-1. The ideal processing region is defined by the lowest processing pressure where effects such as turbulent flow, wetting, and gravitational influences can be neglected on the time scale of the experiment. This region can be accessed by plastics and also by some of the recently developed highly processable BMGs12-15, but not by conventional metals or even by SPF alloys. Compared to plastics, such BMGs exhibit a room temperature strength which is two orders of magnitude higher. Thus, BMGs can be considered high strength metals that can be processed like plastics.
fact that BMGs lack a strain hardening mechanism, which results in shear instability and has been considered the Achilles’ heel of BMGs4, significant bending ductility has been observed when one dimension is reduced to approximately5 1 mm,
Thermoplastic based processing of bulk metallic glass The main challenge associated with net shape processing of BMGs is to avoid crystallization. The stability against crystallization exhibited by BMG formers allows for two principally different processing methods10. One is direct casting, where the liquid BMG former must fill a mold and simultaneously be cooled sufficiently fast to avoid crystallization.
wetting effects, and possible segregation, as well as undesirable
Only a careful balance of processing parameters can satisfy these
microstructures on subsequent solidification. Thus, compared to
contradictory requirements and, as a consequence, this only allows
plastic processing, most metal processing methods yield inferior
the casting of some limited geometries10. Geometries with thin
results in terms of versatility of shapes, precision, and economics.
sections are particularly difficult to cast, a fact which has hampered the
These limitations have spurred research focused on processing
widespread use of BMGs. Alternatively and unique among metals, BMG formers can be thermoplastically formed (TPF)10. During TPF, the pre-
metals in a softer state1-3.
shaped BMG is reheated into its supercooled liquid region and formed
Superplastic formable alloys and semi-solid processing
into its final shape. The supercooled liquid region is the temperature
Superplastically formable (SPF) metallic alloys were developed to
before it eventually crystallizes. Despite the early recognition and
withstand plastic deformations of several hundred percent before
utilization of TPF11, its potential as a net shape process to enable a
failure2,3,
wider array of forming methods was not explored10 until the recent
far beyond the plasticity range of 25 % and less normally
associated with metals. However, the flow stresses of SPF alloys,
region where the BMG former first relaxes into a supercooled liquid
development of BMGs with high formability12-15.
though significantly lower than those observed in conventional metals, are still significantly higher than those of plastics and do not fall into
Blow molding of bulk metallic glass
the ideal processing region (Fig. 1) even under strain rates as low as
Even though fast cooling and forming are decoupled during TPF of
10-3 sec-1.
BMGs, thin sections with a high aspect ratio remain challenging to
In a separate development, alloys were developed which can be processed in a semi-solid state, at temperatures where they are partially liquid and partially
solid1.
As a result, the processing
create when using techniques where the BMG is in physical contact with the mold. This is due to stick conditions between the BMG and the mold and the resulting parabolic flow patterns16. In order to
temperature can be reduced, yielding benefits such as lower energy
eliminate such stick conditions, physical contact between the BMG and
consumption, reduced oxidation, and elimination of the necessity
the mold must be avoided, at least while significant tangential strain is
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
15
REVIEW
Thermoplastic blow molding of metals
(a)
(b)
(c)
(d)
Fig. 2 (a) Zr44Ti11Cu10Ni10Be25 BMG disc 35 mm in diameter and 0.8 mm in thickness which was deformed using blow molding by approximately 400 % at 460 °C for 50 sec under a pressure difference17 of 105 Pa. (b) When deforming beyond ~500 % overall strain, the deforming BMG ruptures due to non-uniform thinning of the BMG caused by non-uniform initial stress conditions. (c) Finite element modeling, assuming Newtonian behavior, predicts the same latitude for the rupture as was experimentally observed. (d) Overall strain can be increased by using a non-uniform initial thickness distribution. The ratio of the disk thickness at the center, dC, to the thickness at the edge, dE, varies from 1 to 4.2. By increasing dC/dE up to 3, the overall strain was increased from 470 % for dC/dE =1 to 686 % for dC/dE =3. For dC/dE >3, the geometry becomes unstable, as evidenced by the fact that rupture occurs at a different latitude than predicted by FEM (Fig. 2c).
generated. We will show that this can be achieved by TPF-based blow
Newtonian behavior. The condition m = 1 represents the highest
molding.
possible thinning resistance for a metal. In comparison to BMGs, SPF
The required minimum pressure for TPF-based blow molding is defined by the flow stress of the BMG. From a processing point of
typical m values ranging from 0.4 to 0.718. As a result, the overall strain
view, ideal conditions are those under which strain exceeds 100 % at
that can be achieved in the SPF process is limited.
strain rates of ≤10-1 sec-1 using a forming pressure of 1 atm. We have
16
metals are significantly more prone to thinning, reflected in their
To achieve larger overall strains the thickness distribution in the
concluded from theoretical considerations17 that recently developed
initial pre-shape must be modified such that locations which will
BMG formers with high formability12-15 fulfill these requirements.
undergo large strains have an increased thickness. This is demonstrated
Fig. 2a shows a BMG disc which was deformed by approximately
in Fig. 2d, where BMG discs with varying thickness at the edge, dE,
400 % using blow molding as experimental verification. However,
and center, dC, were used. The overall strain increases from 470 % for
deforming beyond 500 % caused rupture, terminating the forming
dC/dE = 1 to 686 % for dC/dE = 3. This strategy is limited because for
operation and thereby limiting experimentally achievable overall
dC/dE > 3 the geometry becomes unstable, as evidenced by the fact
strains (Fig. 2b). This is due to geometrical thinning: a non-uniform
that it ruptures at a different latitude than predicted by FEM (Fig. 2c).
decrease in thickness upon deformation18. For pre-shapes other than
In order to increase the range of complexity that can be net shaped,
perfect spheres, the stress distribution is non-uniform. This leads to
the pre-shape has to more closely resemble the final shape. Thereby, a
non-uniform strains, and consequently the thickness decreases in
more homogenous stress distribution during deformation is established
a non-uniform manner throughout the pre-shape. The magnitude
and, in addition, the overall strain required is reduced. In the processing
of the thinning is controlled by the strain rate sensitivity exponent, d—, σ which reflects the material’s thinning resistance, i.e., its m= — dε⋅ strain rate dependent resistance to deformation. By extrapolating data
of (thermo)plastics, this strategy has led to a vast array of complex
on the effect of the strain rate on viscosity19 we can conclude that
disc, a strain of ~4500 % is required. Using the pre-shape shown in
under processing conditions suited for blow molding of BMG formers, m = 1, indicating Newtonian behavior (σ = η⋅3ε⋅ ). This assumption is
Fig. 3a, however, the maximum required strain is reduced to less than
supported by finite element modeling (FEM) (Fig. 2c), which predicts
of 0.1 MPa of the pre-shaped bottle with Zr44Ti11Cu10Ni10Be25
rupture at the same latitude as experimentally observed (Fig. 2b) at
material properties. The final shape is achieved after only 54 sec. When
processing conditions under which Zr44Ti11Cu10Ni10Be25 BMG exhibits
the same pre-bottle was experimentally expanded using even higher
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
shapes, one prominent example of which is the soda bottle. In order to blow mold the rectangular-shaped bottle shown in Fig. 3a from a
1000 %. Fig. 3a depicts the FEM expansion under a pressure difference
Thermoplastic blow molding of metals
REVIEW
(a) (c)
(e)
(d) (f)
(b)
(g)
Fig. 3 (a) FEM of the expansion of a Zr44Ti11Cu10Ni10Be25 pre-shape. Assuming the same processing conditions used in experiments (p = 105 Pa, T = 460 °C) and Newtonian behavior (σ = η⋅3ε⋅ ) as the constitutive equation with a viscosity of η = 3.1×106 Pa·s, complete expansion into the final shape is achieved after 54 seconds. (b) Even when the experimental pressure is increased to 3×105 Pa and the processing time is increased up to 300 seconds, filling is still incomplete. (c) Effect of processing environment on the expansion process. When processing in vacuum (outer surface), heat losses are significantly reduced, resulting in large strains and the FEM-predicted cross-sectional profile. When processing in air, convection causes the deforming BMG to cool, which increases its flow stress, thereby slowing down the expansion kinetics and limiting the achievable strain. Unlike the cross-sectional profile predicted by FEM in vacuum (Fig. 2c), here the thinnest sections are close to the edges where cooling is the lowest due to the proximity to the heater. (e-f) Schematics of the temperature distribution during blow molding of SPF metal, polymer, and BMG. The SPF alloy (e) is in thermal equilibrium with the environment due to the long processing time of approximately 30 minutes and the high thermal conductivity. The opposite is true for polymers (f) where the low thermal conductivity results in adiabatic conditions on the time scale of the process (<10 s). BMGs on the other hand are in-between these conditions. They develop a temperature gradient with a magnitude determined by the difference between the temperature of the heater and the environment (g).
pressures (up to 0.3 MPa) and longer times (up to 300 sec), complete
variations within the experimental setup affect a BMG’s viscosity
filling of the mold could not be achieved (Fig. 3c), despite a suggested
and therefore its deformation resistance. The temperature
theoretical maximum strain exceeding 2500 %.
dependence of the viscosity is quantified through the steepness index, d log (η /G) ⎥ ms = — —— ——– dT / T ⎥ g
⎥T=T
g
(G: Shear Modulus, Tg: glass transition temperature)20.
Blow molding comparison of BMGs, plastics, and SPF alloys
Within this categorization scheme, plastics are considered fragile,
To understand the limited formability observed experimentally during
BMG formers are considered moderately strong liquids21. The reduced
blow molding, the BMGs are compared with plastics and SPF metals
temperature sensitivity of BMGs’ viscosity suggests that BMGs should
in Table 1. The processing temperatures and pressures for some
hold a processing advantage over plastics for blow molding. The time
BMGs are comparable to those of plastics. Even though the maximum
scale over which temperature gradients in the processing environment
strain that can be achieved with BMGs is limited, its value can exceed
affect the blow molded material is controlled by the material’s
10 000 % and thus it does not impose a practical limitation. In
thermal conductivity. The thermal conductivity of BMG is about two
addition, resistance to thinning for BMGs is comparable to plastics, and
orders of magnitude higher than for plastics used in blow molding
significantly larger than for SPF metals19. This comparison suggests
(see, for example10). Therefore, a temperature gradient within the
that BMGs are as suited for blow molding as plastics are.
experimental setup translates significantly faster into a temperature
However, in the experimental realization of blow molding, temperature variations are always present. Such temperature
exhibiting a viscosity which changes rapidly with temperature, while
change within the deforming BMG than in the deforming plastic. Since the temperature gradients are always negative due to convective heat
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
17
REVIEW
Thermoplastic blow molding of metals
Table 1 Processing parameters of BMGs, plastics, and SPF alloys. Examples for BMGs are Zr44Ti11Cu10Ni10Be25, Pt57.5Cu14.7Ni5.3P22.5, and Au49Ag5.5Pd2.3Cu26.9Si16.3, for plastic is polypropylene, for SPF alloys are Ti (6AL-4V) and Al (2004 SPF) alloys. Strain rates are 4x10-4 sec-1 for Ti SPF alloys and 5x10-3 sec-1 for Al SPF alloys.
though the thermal conductivity for SPF alloys is even higher than the
Parameter
(Fig. 3f).
Materials BMG
Plastics
Processing temperature [°C]
160 - 260
SPF alloys 18
160 (Au-based)
900 (Ti6Al4)
280 (Pt-based)
465 (Al 2004)
350 (Pd-based)
a timescale of ~30 minutes allows the alloy to equilibrate with the processing environment (Fig. 3e). For the BMG (Fig. 3g) the timescale (~1 minute) is too short to equilibrate, yet too long, considering its thermal conductivity, to achieve adiabatic conditions like for plastics Fig. 4 illustrates the application of strategies to reduce temperature gradients; complex geometries can be net shaped (within 1 min) through BMG blow molding when temperature uniformity is established (Figs. 4a & b). The low forming pressure required, together with the BMG’s ability to replicate extremely small features22, and its
430 (Zr-based
drastically reduced solidification shrinkage compared to conventional
Processing pressure [Pa]
1 - 10 x 105 Pa 1 - 4 x 105 Pa
1 - 4 x 105 Pa
Maximum strain [%]
∞
~10 000
<400
Typical strain rate [s-1]
10-1 - 1
10-1
10-3
m
~1
alloys (<0.5 % vs. 5 %10), result in the highest dimensional precision of blow molded BMG parts.
Integration of shaping, joining, and finishing into one processing step Blow molding of BMGs also results in very smooth surfaces while
d log (η/G) ⎥ ms = —————⎥ dTg / T ⎥ T=Tg κ [W/mK]
thermal conductivity for BMGs, the fact that forming is carried out on
1 20
137
0.4 - 0.7 21
52 (Pt-based)
Not applicable
21
10
strain conditions takes place during the free expansion stage. During this expansion stage the action of surface tension alone smoothes perturbations to approximately23 10 μm. Once the BMG touches
70 (Zr-based) 0.3
providing the ability to pattern surfaces. Expansion under plane
170
the mold, no more lateral strain takes place due to stick conditions between the BMG and the mold. However, even when the BMG is in contact with the mold, the normal stress component still results in
transfer to the environment, the temperature of the BMG specimen
normal strains as long as the length scales involved are small compared
decreases during blow molding (Fig. 3b). Consequently, in order to
to the thickness of the deforming BMG24. The presence of a normal
blow mold BMG formers into shapes with a complexity comparable to
component results in an outstanding surface finish (see, e.g., Fig. 2a),
plastics, temperature gradients within the setup must be reduced to
and it can also be utilized to pattern the surface, as demonstrated in
a significant larger extent than is required for plastic processing. This
Fig. 4c, allowing functionalization of the surfaces to be integrated into
is not only required to achieve large overall strains but also replicate
the blow mold process.
small features, where the stress decreases according to σ = ΔpS/4t
Besides surface patterning, joining can also be integrated as a
(Δp: forming pressure, S: span width, t: BMG’s thickness), leading
processing step during blow molding. In conventional metal processing,
to a reduced strain rate in those regions. One approach to reducing
an additional processing step is required to join parts, which creates
the temperature gradients involves thermally decoupling the setup
stresses and affects mechanical properties. Applying conventional
from the environment by processing in a vacuum (Fig. 3c), which
joining techniques to BMGs is particularly problematic due to the
results in larger possible strains than when processed in air (Fig. 3d).
metastable nature of their amorphous structure25,26. Alternatively
Alternatively, a liquid medium can be used, which reduces temperature
joints can be created by blow molding around fastener sites, resulting
variations due to its high thermal mass and convective flow. Even
in a mechanical bond (Fig. 4c). With blow molding, high strength bulk metallic glasses can be formed in a manner similar to plastics when the specifics of these
Instrument citation Instron, Instron #5569 with a 50 000 kN and 5000 kN load cell Shimadzu Lab-x, XRD-6000 diffractometer
alloys are considered. This allows one to net shape complex geometries in an economical and precise manner, including shapes, which can not be produced with any other metal processing method. Furthermore, blow molding of BMGs enables combination of three traditional processing steps (shaping, joining, finishing) into one processing
Perkin Elmer Diamond DSC
step. The superior properties of BMGs relative to plastics and typical structural metals, combined with the ease, economy, and precision
18
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
Thermoplastic blow molding of metals
(a)
(c)
REVIEW
(b)
(d)
Fig. 4 By reducing the heat losses during deformation, shapes that cannot be produced with any other metal processing method can be precisely net shaped within < 1 minute through TPF based blow molding (a and b). Expansion of Zr44Ti11Cu10Ni10Be25 pre-shapes as shown in Fig. 3a result in hollow, low symmetry shapes that are seamless. (b) These include thin-walled shapes incorporating undercuts. Surface patterning and functionalization can be integrated into the blow molding processing step. (c) Joints can be created by blow molding around fastener sites, resulting in a mechanical bond. (d) Surface patterning or finishing can also be integrated into one processing step with the blow molding.
of blow molding, have the potential to impact society in a manner
Acknowledgments
similar to the development of synthetic plastics and their associated
The authors thanks NSF CMMI (MPM#0826445) for financial support and
processing methods.
Robert Fers and Robert Martinez for their preparation of test samples.
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14. Schroers, J., et al., Appl Phys Lett (2005) 87, 61912.
2. Pearson, C. E., J I Met (1934) 54, 111.
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3. Backofen, W. A., et al., JOM (1964) 16, 763.
16. Chiu, H. M., et al., Scripta Mater (2009) 61, 28.
4. Greer, A. L., Mater Today (2009) 12, 14.
17. Schroers, J., et al., Scripta Mater (2007) 57, 341.
5. Conner, R. D., et al., J Appl Phys (2003) 94, 904.
18. Barnes, A. J., J Mater Eng Perform (2007) 10, 11665.
6. Guo, H., et al., Nat Mater (2007) 6, 735.
19. Lu, J., et al., Acta Mater (2003) 51, 3429.
7. Volkert, C. A., et al., J Appl Phys (2008) 103, 083539.
20. Bohmer, R., et al., J Chem Phys (1993) 99, 4201.
8. Jang, D. C., and Greer, J. R., Nat Mater (2010) 9, 215.
21. Busch, R., et al., MRS Bull (2007) 32, 620.
9. Ashby, M. F., and Greer, A. L., Scripta Mater (2006) 54, 321.
22. Kumar, G., et al., Nature (2009) 457, 868.
10. Schroers, J., Adv Mater (2010) 22, 1566.
23. Kumar, G., and Schroers, J., Appl Phys Lett (2008) 92, 031901.
11. Patterson, J. P., and Jones, D. R. H., Mater Res Bull (1978) 13, 583.
24. Rowland, H. D., et al., J Micromech Microeng (2005) 15, 2414.
12. Schroers, J., and Johnson, W. L., Appl Phys Lett (2004) 84, 3666.
25. Kawamura, Y., and Ohno, Y., Scripta Mater (2001) 45, 279.
13. Zhang, B., et al., Phys Rev Lett (2005) 94, 205502.
26. Swiston, A. J., et al., Scripta Mater (2003) 48, 1575.
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
19
the process (Fig. 1). Such a desired processing window of forming
length scales from ~100 nm to ~100 m in applications where a
pressure lies between 10-5 and 1 MPa and can be readily accessed
combination of strength and ductility is required. Compared to
with (thermo)plastics. Plastic processing is typically carried out at
plastics, however, metals exhibit limited processability (Fig. 1).
viscosities of 103–106 Pa·s and strain rates of 10-2–101 sec-1. In the
The origin of the superb processability of (thermo)plastics is the
simplest case of a Newtonian flow, viscosity, η, translates into the flow stress (or forming pressure), σ, according to σ = η⋅3ε⋅, where
gradual softening from a solid-like material (glass) below the glass
14
transition temperature, Tg, to a liquid-like material (supercooled
ε⋅ is the strain rate. Most conventional metals cannot be processed
liquid) when heated above Tg. From a processing point of view,
within the ranges described above. They are either processed
an ideal material would flow under a forming pressure, which
in their crystalline state, where even at elevated temperatures
is low, yet sufficiently large that turbulent flow is avoided and
they possess high strength, or in a highly fluid liquid state
gravity and wetting effects can be neglected on the time scale of
above their melting temperature. This results in turbulent flow,
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
ISSN:1369 7021 © Elsevier Ltd 2011
Thermoplastic blow molding of metals
REVIEW
of handling low viscosity liquids. However, these improvements are marginal when compared to plastics and can only be realized in a limited family of alloys.
Bulk metallic glass Recent decades have witnessed the development of a new class of metallic alloys that solidify into an amorphous structure even at moderate (<100 K/s) cooling rates4. The amorphous structure of these bulk metallic glass (BMG) forming alloys results in strength and elasticity values that typically surpass those of conventional structural metals4. In order to harness their full range of attractive properties, BMGs should be used in geometries where at least one dimension is below approximately9 1 mm. This length scale depends on the critical crack length and varies among BMGs and is due to size effect on the BMGs’ mechanical properties5-8. For example, despite the Fig. 1 Properties vs. processability compared via the temperature-dependent strength for conventional steel, SPF alloys, plastics, and BMGs. Conventional metals are represented by a 1045 steel, SPF alloys by aluminum based 2004 SPF, BMGs by Zr44Ti11Cu10Ni10Be25 and Pt57.5Cu14.7Ni5.3P22.5, and PET (polyethylene terephthalate) was chosen as an example system for plastics. The temperature dependent strength (flow stress) is calculated for the fluids from σ = η⋅3ε⋅ for a strain rate of 10-1 sec-1. The ideal processing region is defined by the lowest processing pressure where effects such as turbulent flow, wetting, and gravitational influences can be neglected on the time scale of the experiment. This region can be accessed by plastics and also by some of the recently developed highly processable BMGs12-15, but not by conventional metals or even by SPF alloys. Compared to plastics, such BMGs exhibit a room temperature strength which is two orders of magnitude higher. Thus, BMGs can be considered high strength metals that can be processed like plastics.
fact that BMGs lack a strain hardening mechanism, which results in shear instability and has been considered the Achilles’ heel of BMGs4, significant bending ductility has been observed when one dimension is reduced to approximately5 1 mm,
Thermoplastic based processing of bulk metallic glass The main challenge associated with net shape processing of BMGs is to avoid crystallization. The stability against crystallization exhibited by BMG formers allows for two principally different processing methods10. One is direct casting, where the liquid BMG former must fill a mold and simultaneously be cooled sufficiently fast to avoid crystallization.
wetting effects, and possible segregation, as well as undesirable
Only a careful balance of processing parameters can satisfy these
microstructures on subsequent solidification. Thus, compared to
contradictory requirements and, as a consequence, this only allows
plastic processing, most metal processing methods yield inferior
the casting of some limited geometries10. Geometries with thin
results in terms of versatility of shapes, precision, and economics.
sections are particularly difficult to cast, a fact which has hampered the
These limitations have spurred research focused on processing
widespread use of BMGs. Alternatively and unique among metals, BMG formers can be thermoplastically formed (TPF)10. During TPF, the pre-
metals in a softer state1-3.
shaped BMG is reheated into its supercooled liquid region and formed
Superplastic formable alloys and semi-solid processing
into its final shape. The supercooled liquid region is the temperature
Superplastically formable (SPF) metallic alloys were developed to
before it eventually crystallizes. Despite the early recognition and
withstand plastic deformations of several hundred percent before
utilization of TPF11, its potential as a net shape process to enable a
failure2,3,
wider array of forming methods was not explored10 until the recent
far beyond the plasticity range of 25 % and less normally
associated with metals. However, the flow stresses of SPF alloys,
region where the BMG former first relaxes into a supercooled liquid
development of BMGs with high formability12-15.
though significantly lower than those observed in conventional metals, are still significantly higher than those of plastics and do not fall into
Blow molding of bulk metallic glass
the ideal processing region (Fig. 1) even under strain rates as low as
Even though fast cooling and forming are decoupled during TPF of
10-3 sec-1.
BMGs, thin sections with a high aspect ratio remain challenging to
In a separate development, alloys were developed which can be processed in a semi-solid state, at temperatures where they are partially liquid and partially
solid1.
As a result, the processing
create when using techniques where the BMG is in physical contact with the mold. This is due to stick conditions between the BMG and the mold and the resulting parabolic flow patterns16. In order to
temperature can be reduced, yielding benefits such as lower energy
eliminate such stick conditions, physical contact between the BMG and
consumption, reduced oxidation, and elimination of the necessity
the mold must be avoided, at least while significant tangential strain is
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
15
REVIEW
Thermoplastic blow molding of metals
(a)
(b)
(c)
(d)
Fig. 2 (a) Zr44Ti11Cu10Ni10Be25 BMG disc 35 mm in diameter and 0.8 mm in thickness which was deformed using blow molding by approximately 400 % at 460 °C for 50 sec under a pressure difference17 of 105 Pa. (b) When deforming beyond ~500 % overall strain, the deforming BMG ruptures due to non-uniform thinning of the BMG caused by non-uniform initial stress conditions. (c) Finite element modeling, assuming Newtonian behavior, predicts the same latitude for the rupture as was experimentally observed. (d) Overall strain can be increased by using a non-uniform initial thickness distribution. The ratio of the disk thickness at the center, dC, to the thickness at the edge, dE, varies from 1 to 4.2. By increasing dC/dE up to 3, the overall strain was increased from 470 % for dC/dE =1 to 686 % for dC/dE =3. For dC/dE >3, the geometry becomes unstable, as evidenced by the fact that rupture occurs at a different latitude than predicted by FEM (Fig. 2c).
generated. We will show that this can be achieved by TPF-based blow
Newtonian behavior. The condition m = 1 represents the highest
molding.
possible thinning resistance for a metal. In comparison to BMGs, SPF
The required minimum pressure for TPF-based blow molding is defined by the flow stress of the BMG. From a processing point of
typical m values ranging from 0.4 to 0.718. As a result, the overall strain
view, ideal conditions are those under which strain exceeds 100 % at
that can be achieved in the SPF process is limited.
strain rates of ≤10-1 sec-1 using a forming pressure of 1 atm. We have
16
metals are significantly more prone to thinning, reflected in their
To achieve larger overall strains the thickness distribution in the
concluded from theoretical considerations17 that recently developed
initial pre-shape must be modified such that locations which will
BMG formers with high formability12-15 fulfill these requirements.
undergo large strains have an increased thickness. This is demonstrated
Fig. 2a shows a BMG disc which was deformed by approximately
in Fig. 2d, where BMG discs with varying thickness at the edge, dE,
400 % using blow molding as experimental verification. However,
and center, dC, were used. The overall strain increases from 470 % for
deforming beyond 500 % caused rupture, terminating the forming
dC/dE = 1 to 686 % for dC/dE = 3. This strategy is limited because for
operation and thereby limiting experimentally achievable overall
dC/dE > 3 the geometry becomes unstable, as evidenced by the fact
strains (Fig. 2b). This is due to geometrical thinning: a non-uniform
that it ruptures at a different latitude than predicted by FEM (Fig. 2c).
decrease in thickness upon deformation18. For pre-shapes other than
In order to increase the range of complexity that can be net shaped,
perfect spheres, the stress distribution is non-uniform. This leads to
the pre-shape has to more closely resemble the final shape. Thereby, a
non-uniform strains, and consequently the thickness decreases in
more homogenous stress distribution during deformation is established
a non-uniform manner throughout the pre-shape. The magnitude
and, in addition, the overall strain required is reduced. In the processing
of the thinning is controlled by the strain rate sensitivity exponent, d—, σ which reflects the material’s thinning resistance, i.e., its m= — dε⋅ strain rate dependent resistance to deformation. By extrapolating data
of (thermo)plastics, this strategy has led to a vast array of complex
on the effect of the strain rate on viscosity19 we can conclude that
disc, a strain of ~4500 % is required. Using the pre-shape shown in
under processing conditions suited for blow molding of BMG formers, m = 1, indicating Newtonian behavior (σ = η⋅3ε⋅ ). This assumption is
Fig. 3a, however, the maximum required strain is reduced to less than
supported by finite element modeling (FEM) (Fig. 2c), which predicts
of 0.1 MPa of the pre-shaped bottle with Zr44Ti11Cu10Ni10Be25
rupture at the same latitude as experimentally observed (Fig. 2b) at
material properties. The final shape is achieved after only 54 sec. When
processing conditions under which Zr44Ti11Cu10Ni10Be25 BMG exhibits
the same pre-bottle was experimentally expanded using even higher
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
shapes, one prominent example of which is the soda bottle. In order to blow mold the rectangular-shaped bottle shown in Fig. 3a from a
1000 %. Fig. 3a depicts the FEM expansion under a pressure difference
Thermoplastic blow molding of metals
REVIEW
(a) (c)
(e)
(d) (f)
(b)
(g)
Fig. 3 (a) FEM of the expansion of a Zr44Ti11Cu10Ni10Be25 pre-shape. Assuming the same processing conditions used in experiments (p = 105 Pa, T = 460 °C) and Newtonian behavior (σ = η⋅3ε⋅ ) as the constitutive equation with a viscosity of η = 3.1×106 Pa·s, complete expansion into the final shape is achieved after 54 seconds. (b) Even when the experimental pressure is increased to 3×105 Pa and the processing time is increased up to 300 seconds, filling is still incomplete. (c) Effect of processing environment on the expansion process. When processing in vacuum (outer surface), heat losses are significantly reduced, resulting in large strains and the FEM-predicted cross-sectional profile. When processing in air, convection causes the deforming BMG to cool, which increases its flow stress, thereby slowing down the expansion kinetics and limiting the achievable strain. Unlike the cross-sectional profile predicted by FEM in vacuum (Fig. 2c), here the thinnest sections are close to the edges where cooling is the lowest due to the proximity to the heater. (e-f) Schematics of the temperature distribution during blow molding of SPF metal, polymer, and BMG. The SPF alloy (e) is in thermal equilibrium with the environment due to the long processing time of approximately 30 minutes and the high thermal conductivity. The opposite is true for polymers (f) where the low thermal conductivity results in adiabatic conditions on the time scale of the process (<10 s). BMGs on the other hand are in-between these conditions. They develop a temperature gradient with a magnitude determined by the difference between the temperature of the heater and the environment (g).
pressures (up to 0.3 MPa) and longer times (up to 300 sec), complete
variations within the experimental setup affect a BMG’s viscosity
filling of the mold could not be achieved (Fig. 3c), despite a suggested
and therefore its deformation resistance. The temperature
theoretical maximum strain exceeding 2500 %.
dependence of the viscosity is quantified through the steepness index, d log (η /G) ⎥ ms = — —— ——– dT / T ⎥ g
⎥T=T
g
(G: Shear Modulus, Tg: glass transition temperature)20.
Blow molding comparison of BMGs, plastics, and SPF alloys
Within this categorization scheme, plastics are considered fragile,
To understand the limited formability observed experimentally during
BMG formers are considered moderately strong liquids21. The reduced
blow molding, the BMGs are compared with plastics and SPF metals
temperature sensitivity of BMGs’ viscosity suggests that BMGs should
in Table 1. The processing temperatures and pressures for some
hold a processing advantage over plastics for blow molding. The time
BMGs are comparable to those of plastics. Even though the maximum
scale over which temperature gradients in the processing environment
strain that can be achieved with BMGs is limited, its value can exceed
affect the blow molded material is controlled by the material’s
10 000 % and thus it does not impose a practical limitation. In
thermal conductivity. The thermal conductivity of BMG is about two
addition, resistance to thinning for BMGs is comparable to plastics, and
orders of magnitude higher than for plastics used in blow molding
significantly larger than for SPF metals19. This comparison suggests
(see, for example10). Therefore, a temperature gradient within the
that BMGs are as suited for blow molding as plastics are.
experimental setup translates significantly faster into a temperature
However, in the experimental realization of blow molding, temperature variations are always present. Such temperature
exhibiting a viscosity which changes rapidly with temperature, while
change within the deforming BMG than in the deforming plastic. Since the temperature gradients are always negative due to convective heat
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
17
REVIEW
Thermoplastic blow molding of metals
Table 1 Processing parameters of BMGs, plastics, and SPF alloys. Examples for BMGs are Zr44Ti11Cu10Ni10Be25, Pt57.5Cu14.7Ni5.3P22.5, and Au49Ag5.5Pd2.3Cu26.9Si16.3, for plastic is polypropylene, for SPF alloys are Ti (6AL-4V) and Al (2004 SPF) alloys. Strain rates are 4x10-4 sec-1 for Ti SPF alloys and 5x10-3 sec-1 for Al SPF alloys.
though the thermal conductivity for SPF alloys is even higher than the
Parameter
(Fig. 3f).
Materials BMG
Plastics
Processing temperature [°C]
160 - 260
SPF alloys 18
160 (Au-based)
900 (Ti6Al4)
280 (Pt-based)
465 (Al 2004)
350 (Pd-based)
a timescale of ~30 minutes allows the alloy to equilibrate with the processing environment (Fig. 3e). For the BMG (Fig. 3g) the timescale (~1 minute) is too short to equilibrate, yet too long, considering its thermal conductivity, to achieve adiabatic conditions like for plastics Fig. 4 illustrates the application of strategies to reduce temperature gradients; complex geometries can be net shaped (within 1 min) through BMG blow molding when temperature uniformity is established (Figs. 4a & b). The low forming pressure required, together with the BMG’s ability to replicate extremely small features22, and its
430 (Zr-based
drastically reduced solidification shrinkage compared to conventional
Processing pressure [Pa]
1 - 10 x 105 Pa 1 - 4 x 105 Pa
1 - 4 x 105 Pa
Maximum strain [%]
∞
~10 000
<400
Typical strain rate [s-1]
10-1 - 1
10-1
10-3
m
~1
alloys (<0.5 % vs. 5 %10), result in the highest dimensional precision of blow molded BMG parts.
Integration of shaping, joining, and finishing into one processing step Blow molding of BMGs also results in very smooth surfaces while
d log (η/G) ⎥ ms = —————⎥ dTg / T ⎥ T=Tg κ [W/mK]
thermal conductivity for BMGs, the fact that forming is carried out on
1 20
137
0.4 - 0.7 21
52 (Pt-based)
Not applicable
21
10
strain conditions takes place during the free expansion stage. During this expansion stage the action of surface tension alone smoothes perturbations to approximately23 10 μm. Once the BMG touches
70 (Zr-based) 0.3
providing the ability to pattern surfaces. Expansion under plane
170
the mold, no more lateral strain takes place due to stick conditions between the BMG and the mold. However, even when the BMG is in contact with the mold, the normal stress component still results in
transfer to the environment, the temperature of the BMG specimen
normal strains as long as the length scales involved are small compared
decreases during blow molding (Fig. 3b). Consequently, in order to
to the thickness of the deforming BMG24. The presence of a normal
blow mold BMG formers into shapes with a complexity comparable to
component results in an outstanding surface finish (see, e.g., Fig. 2a),
plastics, temperature gradients within the setup must be reduced to
and it can also be utilized to pattern the surface, as demonstrated in
a significant larger extent than is required for plastic processing. This
Fig. 4c, allowing functionalization of the surfaces to be integrated into
is not only required to achieve large overall strains but also replicate
the blow mold process.
small features, where the stress decreases according to σ = ΔpS/4t
Besides surface patterning, joining can also be integrated as a
(Δp: forming pressure, S: span width, t: BMG’s thickness), leading
processing step during blow molding. In conventional metal processing,
to a reduced strain rate in those regions. One approach to reducing
an additional processing step is required to join parts, which creates
the temperature gradients involves thermally decoupling the setup
stresses and affects mechanical properties. Applying conventional
from the environment by processing in a vacuum (Fig. 3c), which
joining techniques to BMGs is particularly problematic due to the
results in larger possible strains than when processed in air (Fig. 3d).
metastable nature of their amorphous structure25,26. Alternatively
Alternatively, a liquid medium can be used, which reduces temperature
joints can be created by blow molding around fastener sites, resulting
variations due to its high thermal mass and convective flow. Even
in a mechanical bond (Fig. 4c). With blow molding, high strength bulk metallic glasses can be formed in a manner similar to plastics when the specifics of these
Instrument citation Instron, Instron #5569 with a 50 000 kN and 5000 kN load cell Shimadzu Lab-x, XRD-6000 diffractometer
alloys are considered. This allows one to net shape complex geometries in an economical and precise manner, including shapes, which can not be produced with any other metal processing method. Furthermore, blow molding of BMGs enables combination of three traditional processing steps (shaping, joining, finishing) into one processing
Perkin Elmer Diamond DSC
step. The superior properties of BMGs relative to plastics and typical structural metals, combined with the ease, economy, and precision
18
JAN–FEB 2011 | VOLUME 14 | NUMBER 1-2
Thermoplastic blow molding of metals
(a)
(c)
REVIEW
(b)
(d)
Fig. 4 By reducing the heat losses during deformation, shapes that cannot be produced with any other metal processing method can be precisely net shaped within < 1 minute through TPF based blow molding (a and b). Expansion of Zr44Ti11Cu10Ni10Be25 pre-shapes as shown in Fig. 3a result in hollow, low symmetry shapes that are seamless. (b) These include thin-walled shapes incorporating undercuts. Surface patterning and functionalization can be integrated into the blow molding processing step. (c) Joints can be created by blow molding around fastener sites, resulting in a mechanical bond. (d) Surface patterning or finishing can also be integrated into one processing step with the blow molding.
of blow molding, have the potential to impact society in a manner
Acknowledgments
similar to the development of synthetic plastics and their associated
The authors thanks NSF CMMI (MPM#0826445) for financial support and
processing methods.
Robert Fers and Robert Martinez for their preparation of test samples.
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