Superplasticity: Recent Approaches and Trends
389
in the strain rate [12]. Higher the value of m, more the material will resist the neck
formation during stretching. If the value of m > 0.3, it can be inferred as the positive
sign of superplasticity phenomenon [13].
Efforts have come to the notice in producing superplastic stainless steels, but more
attention has been garnered by the advent of superplasticity in non-ferrous metals like
Al, Ti, Mg and Ni so as to accomplish the manufacturing of lightweight structures
in transportation and aerospace field as monolithic parts [14]. The applications of
superplastic components include:
• Producing prosthetic implants [15]
• Housings of missiles made from SP2004 alloy [16]
• Aircraft service panel from TiAl6V4 alloy [17]
• Winglets, engine nacelles, air stairs and engine inlet lip skins for aircraft made
from superplastic aluminum by Luxfer Superform, leading manufacturer of USA
[18]
• Metro and light rail applications [19]
Generally, there are two main categories of techniques for developing ultra-fine
grain structure in the non-ferrous metals based on the processing routes, which is
the first step toward achieving superplasticity. One is thermo-mechanical processing
(TMP), mainly rolling and extrusion and the second category is dynamic recrystallization via severe plastic deformation (SPD). Some other unconventional techniques
include consolidation of amorphous or nano-crystalline powder, mechanical alloying
and physical vapor deposition (PVD) [6].
The high strength Al 7475 alloy was subjected to thermo-mechanical processing
including hot rolling, solution heat treatment and rapid recrystallization in molten
salt bath followed by artificial aging and water quenching. These steps helped in
reducing the average grain size of the alloy to 10 μm. [20] Similar TMP technique
was applied on commercial Al 7075 alloy to obtain grain refinement in the range
of 8–14 μm [21]. TMP steps are rigorous and time-consuming and not suitable for
modern manufacturing plants having continuous production schedules. This paper
mainly discusses the recently trending SPD routes.
3 Severe Plastic Deformation Approaches
3.1 High Pressure Torsion
High pressure torsion (HPT) targets for the grain size reduction by both straining
and compression. This strategy was employed by Bridgman [22] for the very first
time, wherein certain material was subjected to mean hydrostatic pressure up to
50,000 kg/cm
2 combined with shearing stresses. As shown in Fig. 1, the material to
be deformed is held in between two anvils out of which one is stationary and the
other one can rotate. Pressure from the top direction is applied with simultaneous
389
in the strain rate [12]. Higher the value of m, more the material will resist the neck
formation during stretching. If the value of m > 0.3, it can be inferred as the positive
sign of superplasticity phenomenon [13].
Efforts have come to the notice in producing superplastic stainless steels, but more
attention has been garnered by the advent of superplasticity in non-ferrous metals like
Al, Ti, Mg and Ni so as to accomplish the manufacturing of lightweight structures
in transportation and aerospace field as monolithic parts [14]. The applications of
superplastic components include:
• Producing prosthetic implants [15]
• Housings of missiles made from SP2004 alloy [16]
• Aircraft service panel from TiAl6V4 alloy [17]
• Winglets, engine nacelles, air stairs and engine inlet lip skins for aircraft made
from superplastic aluminum by Luxfer Superform, leading manufacturer of USA
[18]
• Metro and light rail applications [19]
Generally, there are two main categories of techniques for developing ultra-fine
grain structure in the non-ferrous metals based on the processing routes, which is
the first step toward achieving superplasticity. One is thermo-mechanical processing
(TMP), mainly rolling and extrusion and the second category is dynamic recrystallization via severe plastic deformation (SPD). Some other unconventional techniques
include consolidation of amorphous or nano-crystalline powder, mechanical alloying
and physical vapor deposition (PVD) [6].
The high strength Al 7475 alloy was subjected to thermo-mechanical processing
including hot rolling, solution heat treatment and rapid recrystallization in molten
salt bath followed by artificial aging and water quenching. These steps helped in
reducing the average grain size of the alloy to 10 μm. [20] Similar TMP technique
was applied on commercial Al 7075 alloy to obtain grain refinement in the range
of 8–14 μm [21]. TMP steps are rigorous and time-consuming and not suitable for
modern manufacturing plants having continuous production schedules. This paper
mainly discusses the recently trending SPD routes.
3 Severe Plastic Deformation Approaches
3.1 High Pressure Torsion
High pressure torsion (HPT) targets for the grain size reduction by both straining
and compression. This strategy was employed by Bridgman [22] for the very first
time, wherein certain material was subjected to mean hydrostatic pressure up to
50,000 kg/cm
2 combined with shearing stresses. As shown in Fig. 1, the material to
be deformed is held in between two anvils out of which one is stationary and the
other one can rotate. Pressure from the top direction is applied with simultaneous