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Remanufacturing and Advanced Machining
represented by high-pressure torsion and equal channel angular pressing (ECAP),
or by bottom-up synthesis like electrodeposition in various alloys and even pure
metals (Masuda and Sato, 2020). These techniques enable low-temperature and/or
high-strain-rate superplasticity with grain growth restricted by a reduced deformation temperature and time.
In the high-pressure torsion (HPT) process, a specimen is held between a plunger
and support and is strained in torsion under an applied pressure p in the order of
1–10 GPa (Borodachenkova et al., 2017). As shown in Figure 3.5, the lower holder
(3) rotates causing deformation of the specimen (2) by the contact surface friction
forces under a quasi-hydrostatic pressure. Torsion under such pressure results in the
reduction of the grain size (Pupan and Kononenko, 2008).
In practice, there are two main types of HPT processing distinguished according
to the shape of the anvils, namely unconstrained and constrained. In unconstrained
HPT, samples are placed between two anvils and their material is free to flow outward when the pressure and torsion are applied. As a result, much thinner elements
can be obtained than in constrained HPT, where specimens are placed into a cavity of the lower anvil or both anvils that prevent a material from flowing outward.
However, the constrained HPT is a more common method (Borodachenkova et al.,
2017). The main disadvantage of this method is limited dimensions and forms of
processed specimens that must have diameters between 10 and 20 mm and thickness
0.2–0.5 mm (Pupan and Kononenko, 2008).
Among modifications of the HPT methods, Segal (2018) points out the following :
• High-pressure torsion of long samples, where long billets are processed
using an incremental or continuous material transfer through a torsion zone
• High-pressure tube twisting, in which a sample is compressed by two
punches between a die and mandrel and is then additionally twisted by a
rotating die
• High-pressure sliding, where sheet material is compressed between anvils
and sheared by the movement of the lower anvil in forward and backward
directions, instead of the rotationary movement shown in Figure 3.5.
The equal channel angular pressing (ECAP) is the most popular and widely studied
SPD technique schematically shown in Figure 3.6. As described by Kapoor (2017),
a workpiece with a square or round cross section is pressed into a die with two
intersecting channels of an equal cross-section area. The workpiece is pressed with
a plunger and all its material exits through the second channel, experiencing shear
strain in the intersection area of the two channels. Since the cross section is symmetric, either square or circular, the sample can be rotated before reentry into the
die. Passing the sample through an ECAP die produces a simple shear deformation,
each pass through a die with a 90° angle will result in an effective strain of 1, and n
such passes will result in an effective strain of n (Kapoor, 2017).
The fine nanocrystalline structure can be obtained after two or three passages
through the die, but usually ca. eight passages are required to obtain uniform grains
of dimensions 100 nm and smaller (Pupan and Kononenko, 2008). ECAP can be
Remanufacturing and Advanced Machining
represented by high-pressure torsion and equal channel angular pressing (ECAP),
or by bottom-up synthesis like electrodeposition in various alloys and even pure
metals (Masuda and Sato, 2020). These techniques enable low-temperature and/or
high-strain-rate superplasticity with grain growth restricted by a reduced deformation temperature and time.
In the high-pressure torsion (HPT) process, a specimen is held between a plunger
and support and is strained in torsion under an applied pressure p in the order of
1–10 GPa (Borodachenkova et al., 2017). As shown in Figure 3.5, the lower holder
(3) rotates causing deformation of the specimen (2) by the contact surface friction
forces under a quasi-hydrostatic pressure. Torsion under such pressure results in the
reduction of the grain size (Pupan and Kononenko, 2008).
In practice, there are two main types of HPT processing distinguished according
to the shape of the anvils, namely unconstrained and constrained. In unconstrained
HPT, samples are placed between two anvils and their material is free to flow outward when the pressure and torsion are applied. As a result, much thinner elements
can be obtained than in constrained HPT, where specimens are placed into a cavity of the lower anvil or both anvils that prevent a material from flowing outward.
However, the constrained HPT is a more common method (Borodachenkova et al.,
2017). The main disadvantage of this method is limited dimensions and forms of
processed specimens that must have diameters between 10 and 20 mm and thickness
0.2–0.5 mm (Pupan and Kononenko, 2008).
Among modifications of the HPT methods, Segal (2018) points out the following :
• High-pressure torsion of long samples, where long billets are processed
using an incremental or continuous material transfer through a torsion zone
• High-pressure tube twisting, in which a sample is compressed by two
punches between a die and mandrel and is then additionally twisted by a
rotating die
• High-pressure sliding, where sheet material is compressed between anvils
and sheared by the movement of the lower anvil in forward and backward
directions, instead of the rotationary movement shown in Figure 3.5.
The equal channel angular pressing (ECAP) is the most popular and widely studied
SPD technique schematically shown in Figure 3.6. As described by Kapoor (2017),
a workpiece with a square or round cross section is pressed into a die with two
intersecting channels of an equal cross-section area. The workpiece is pressed with
a plunger and all its material exits through the second channel, experiencing shear
strain in the intersection area of the two channels. Since the cross section is symmetric, either square or circular, the sample can be rotated before reentry into the
die. Passing the sample through an ECAP die produces a simple shear deformation,
each pass through a die with a 90° angle will result in an effective strain of 1, and n
such passes will result in an effective strain of n (Kapoor, 2017).
The fine nanocrystalline structure can be obtained after two or three passages
through the die, but usually ca. eight passages are required to obtain uniform grains
of dimensions 100 nm and smaller (Pupan and Kononenko, 2008). ECAP can be