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Remanufacturing and Advanced Machining
boundary sliding, grain rotation, grain rearrangement, dislocation activity, diffusional creep, and dynamic recrystallization, on the assumption that an actual superplastic deformation should involve a combined, not a single, mechanism. Depending
on the sigmoidal relationship between stress and strain rate, the superplastic flow
behavior is divided into three mechanism zones (Langdon, 1982):
1. Diffusion creep (region I)
2. Grain boundary sliding (region II)
3. Dislocation creep (region III)
At present, a consensus has been reached on the dominant mechanism of grain
boundary sliding in region II at intermediate strain rates, where it accounts for 50%
to 80% of the total deformation during the superplastic deformation process. This
ratio varies with material, composition, grain size, and temperature (Ren et al., 2019).
It has been found that at high temperatures, a superplastic alloy is characterized by
a low flow stress below 10 MPa and a high resistance to nonuniform thinning, which
allows for near-net-shape forming of sheet material using techniques similar to those
for forming of thermoplastics (Humphreys et al., 2017). Superplastic forming (SPF)
is an attractive option for forming complex shapes from the sheet at low stresses with
substantially reduced tooling costs compared to conventional cold-pressing operations. However, the slow strain rates often necessary for superplastic forming and the
higher material costs have so far restricted the commercial exploitation of SPF to a
relatively small number of specialized applications (Humphreys et al., 2017). Pupan
and Kononenko (2008) highlight spectacular results of investigations on deformation
increase, such as elongation close to 8,000% in case of aluminum bronze, more than
800% for zirconia ceramics, and 1,400% for metal matrix composites. The authors
also draw attention to increasing deformation rate and provide examples of superplasticity at rates typical for traditional metal forming processes including explosive
forming. This sort of high-strain-rate superplasticity has been observed in aluminumbased and magnesium-based alloys (10 –2 to 10 –1 s –1 ). For ceramic materials consisting
of tetragonal zirconium oxide, magnesium aluminate spinel, and α-alumina phases,
superplasticity is achieved at strain rates of up to 1 s –1 (Kim et al., 2001).
Process modeling has been widely used in the SPF industry in order to perform
proper optimization of complex forming processes, possible only with a precise
characterization of superplasticity. At present, there are two recognized forms of
constitutive models that describe superplastic behavior, namely mechanism-based
and phenomenological constitutive equations. The mechanism models mentioned in
most references are based on the grain boundary sliding phenomenon with one or
more coordination mechanisms (Ren et al., 2019).
Pupan and Kononenko (2008) point out that superplastic forming is effective in
the following conditions:
• When non-ductile alloys are processed, impossible to be subject to plastic
deformation in normal conditions
• When a formed part consists of thin elements such as reinforcing ribs or
thin sheets
Remanufacturing and Advanced Machining
boundary sliding, grain rotation, grain rearrangement, dislocation activity, diffusional creep, and dynamic recrystallization, on the assumption that an actual superplastic deformation should involve a combined, not a single, mechanism. Depending
on the sigmoidal relationship between stress and strain rate, the superplastic flow
behavior is divided into three mechanism zones (Langdon, 1982):
1. Diffusion creep (region I)
2. Grain boundary sliding (region II)
3. Dislocation creep (region III)
At present, a consensus has been reached on the dominant mechanism of grain
boundary sliding in region II at intermediate strain rates, where it accounts for 50%
to 80% of the total deformation during the superplastic deformation process. This
ratio varies with material, composition, grain size, and temperature (Ren et al., 2019).
It has been found that at high temperatures, a superplastic alloy is characterized by
a low flow stress below 10 MPa and a high resistance to nonuniform thinning, which
allows for near-net-shape forming of sheet material using techniques similar to those
for forming of thermoplastics (Humphreys et al., 2017). Superplastic forming (SPF)
is an attractive option for forming complex shapes from the sheet at low stresses with
substantially reduced tooling costs compared to conventional cold-pressing operations. However, the slow strain rates often necessary for superplastic forming and the
higher material costs have so far restricted the commercial exploitation of SPF to a
relatively small number of specialized applications (Humphreys et al., 2017). Pupan
and Kononenko (2008) highlight spectacular results of investigations on deformation
increase, such as elongation close to 8,000% in case of aluminum bronze, more than
800% for zirconia ceramics, and 1,400% for metal matrix composites. The authors
also draw attention to increasing deformation rate and provide examples of superplasticity at rates typical for traditional metal forming processes including explosive
forming. This sort of high-strain-rate superplasticity has been observed in aluminumbased and magnesium-based alloys (10 –2 to 10 –1 s –1 ). For ceramic materials consisting
of tetragonal zirconium oxide, magnesium aluminate spinel, and α-alumina phases,
superplasticity is achieved at strain rates of up to 1 s –1 (Kim et al., 2001).
Process modeling has been widely used in the SPF industry in order to perform
proper optimization of complex forming processes, possible only with a precise
characterization of superplasticity. At present, there are two recognized forms of
constitutive models that describe superplastic behavior, namely mechanism-based
and phenomenological constitutive equations. The mechanism models mentioned in
most references are based on the grain boundary sliding phenomenon with one or
more coordination mechanisms (Ren et al., 2019).
Pupan and Kononenko (2008) point out that superplastic forming is effective in
the following conditions:
• When non-ductile alloys are processed, impossible to be subject to plastic
deformation in normal conditions
• When a formed part consists of thin elements such as reinforcing ribs or
thin sheets