145
Smart Machining Processes
region make it possible to obtain a variety of micro-/nano-surface structures or
three-dimensional (3D) parts of BMGs with high fidelity. For instance, the width
of grooves formed by SPF is 18.45 ± 0.05 μm with a very small average error of ca.
0.27% related to the silicon mold, deviation of formed from designed angles is less
than 0.1°, and the depth of BMG channels in different areas is highly repeatable (Li
et al., 2020).
Among the most promising technologies that are based on superplasticity, superplastic forming/diffusion bonding (SPF/DB) can be named (Li et al., 2015). SPF/DB
uses material superplasticity and diffusion to manufacture hollow or honeycomb
structures of high complexity in a single step. Major advantages of this technology
include high design freedom, short lead time, little spring-back, reduced number of
parts, and increased structure integrity. It is also cost-saving, since 10–50% weight
savings and 25–40% cost reduction can be achieved by using an SPF/DB structure.
Such structures made of Al, Ti, and superalloys are mainly used as aerospace components such as ducts, aircraft wing access panels, rudders, nozzles, engine casings,
and blades. A single-layer SPF structure has been used as a rocket fuel tank with wall
thickness as low as 1.8 mm and variation within 0.15 mm. SPF/DB has significantly
reduced welding work in this sort of fuel tanks and increased structural integrity. In
optimization of SPF/DB structure, numerical methods are used to simulate stress,
strain, and thickness distribution, while processing parameters are analyzed to predict and avoid risks during processing. Future developments include simulation of
SPF/DB forming process, prediction of life-cycle-long properties of produced components, and optimization of structural parameters. These will have a large impact
on application of SPF/DB structures in the aerospace industry (Li et al., 2015).
3.5 SEVERE PLASTIC DEFORMATION IN
NANOSTRUCTURAL MATERIALS PROCESSING
Nanocrystalline (NC) materials with a grain size in the range of 1–100 nm have
emerged as a new class of materials with unusual structures (Mohamed and Li, 2001).
Because of such characteristics, these materials exhibit unique microstructures in
which the volume of grain boundary is significant and affects various physical, electrical, chemical, magnetic, and deformation properties in the ultrafine grain size range.
One of the most important features of NC materials is an interesting possibility to
address the low strain rates of the superplastic region, usually 10 −5 to 10 −2 s −1 , too slow
to be economically suitable for a variety of industrial applications. Mohamed and Li
(2001) emphasize that as the grain size decreases from micrometer to nanometer, the
superplastic region can be transposed to high strain rates or observed at lower temperatures, exhibiting a high strain rate and/or low-temperature superplasticity. Wang, Jiang
et al. (2020) report properties of bulk nanocrystalline Mg fabricated by cryomilling
and spark plasma sintering with an average grain size of 74 nm. They note superplastic
strain of ~120% during compression at room temperature with a strain rate of up to
10 −2 s −1 , as well as an increased strain rate sensitivity.
The recent development of third-generation materials is characterized by their
ultrafine-grained or NC structure produced via severe plastic deformation (SPD),
Smart Machining Processes
region make it possible to obtain a variety of micro-/nano-surface structures or
three-dimensional (3D) parts of BMGs with high fidelity. For instance, the width
of grooves formed by SPF is 18.45 ± 0.05 μm with a very small average error of ca.
0.27% related to the silicon mold, deviation of formed from designed angles is less
than 0.1°, and the depth of BMG channels in different areas is highly repeatable (Li
et al., 2020).
Among the most promising technologies that are based on superplasticity, superplastic forming/diffusion bonding (SPF/DB) can be named (Li et al., 2015). SPF/DB
uses material superplasticity and diffusion to manufacture hollow or honeycomb
structures of high complexity in a single step. Major advantages of this technology
include high design freedom, short lead time, little spring-back, reduced number of
parts, and increased structure integrity. It is also cost-saving, since 10–50% weight
savings and 25–40% cost reduction can be achieved by using an SPF/DB structure.
Such structures made of Al, Ti, and superalloys are mainly used as aerospace components such as ducts, aircraft wing access panels, rudders, nozzles, engine casings,
and blades. A single-layer SPF structure has been used as a rocket fuel tank with wall
thickness as low as 1.8 mm and variation within 0.15 mm. SPF/DB has significantly
reduced welding work in this sort of fuel tanks and increased structural integrity. In
optimization of SPF/DB structure, numerical methods are used to simulate stress,
strain, and thickness distribution, while processing parameters are analyzed to predict and avoid risks during processing. Future developments include simulation of
SPF/DB forming process, prediction of life-cycle-long properties of produced components, and optimization of structural parameters. These will have a large impact
on application of SPF/DB structures in the aerospace industry (Li et al., 2015).
3.5 SEVERE PLASTIC DEFORMATION IN
NANOSTRUCTURAL MATERIALS PROCESSING
Nanocrystalline (NC) materials with a grain size in the range of 1–100 nm have
emerged as a new class of materials with unusual structures (Mohamed and Li, 2001).
Because of such characteristics, these materials exhibit unique microstructures in
which the volume of grain boundary is significant and affects various physical, electrical, chemical, magnetic, and deformation properties in the ultrafine grain size range.
One of the most important features of NC materials is an interesting possibility to
address the low strain rates of the superplastic region, usually 10 −5 to 10 −2 s −1 , too slow
to be economically suitable for a variety of industrial applications. Mohamed and Li
(2001) emphasize that as the grain size decreases from micrometer to nanometer, the
superplastic region can be transposed to high strain rates or observed at lower temperatures, exhibiting a high strain rate and/or low-temperature superplasticity. Wang, Jiang
et al. (2020) report properties of bulk nanocrystalline Mg fabricated by cryomilling
and spark plasma sintering with an average grain size of 74 nm. They note superplastic
strain of ~120% during compression at room temperature with a strain rate of up to
10 −2 s −1 , as well as an increased strain rate sensitivity.
The recent development of third-generation materials is characterized by their
ultrafine-grained or NC structure produced via severe plastic deformation (SPD),