Superplastic such as thermoforming, blow forming and

Superplastic
sheet forming

J. Deschodt, J.
Vanheule and W. Vanoverberghe

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Ghent
University, Belgium

Abstract

This
paper discusses superplastic sheet forming. First there is a concise
introduction on the phenomenon of superplasticity. The introduction on
superplasticity discusses the basic characteristics, the basic mechanism behind
the superplasticity and the influence of the strain rate on the superplastic
flow. Some examples of material used during superplastic sheet forming will be given.
The most important phenomenon of the total deformation is the grain boundary
sliding, which is accommodated by the dislocation creep and grain boundary
diffusion. A summary of advantages and disadvantages is present, as well as an
explanation for the three most important process variables. Those being
temperature, pressure cycle and die size or geometry. Following, different
forming techniques are discussed. Every super plastic sheet forming (SPF)
technique begins with the heating of the material to half its melting point. The
material becomes so soft that processes normally used on plastics can be
applied to metals, such as thermoforming, blow forming and vacuum forming. 1, p. 16 2

Keywords Superplastic sheet forming • SPF • Superplasticity • Process
parameters • Forming techniques

1       
INTRODUCTION

Superplastic sheet forming
(SPF) is a process that is mostly used to construct precise and complex parts,
out of specific types of materials who possess superplastic behaviour. The term superplasticity became
common after 1945, but the first spectacular experiment was already conducted
in 1934 by Pearson. 3, pp. 2-3
An elongation of 1950% was demonstrated for tin-bismuth and 1500% for lead-tin
eutectic alloys.

Conventional deformation
techniques typically reach an elongation of 10 to 30%, while the superplastic
sheet forming technique can reach an elongation of 2000 up to 3000% at an
increased temperature. Where very low strain rates are applied, and it takes
several minutes to hours to make one product. 4

In the first section, the superplasticity
phenomena will be discussed. The text gives an answer to the needed
requirements of superplasticity and which materials can be used. Superplastic
forming processes follow in the second section, described with advantages and
disadvantages, process parameters, some techniques and a couple of applications.

2       
Superplasticity IN GENERAL

The term
superplasticity is applicable when polycrystalline solids can undergo an
extremely large elongation at high temperature. Mostly a temperature higher
than 0.5 times the melting temperature is used. A uniaxial tension elongation
of ~200% is an indication that superplasticity occurs. Some materials can reach
a total elongation above 1000%. 1
The deformation is performed at low strain rates in the range 10-4 up
to 10-1s-1. The grain size of the materials subjected to
the deformation are below 15 µm. 1

The
previous paragraphs describe the structural superplasticity; a second type of
superplasticity is environmental superplasticity. This last type of deformation
is based on a phase transformation of the material. The deformation process is
divided in several steps of small deformation followed by a heat treatment. In
the following only structural superplasticity is discussed 3, pp. 4-5.

 

2.1      
Mechanism

The microstructural requirements for the material to reach
superplasticity are well determined, but the exact mechanism is less
understood. It is a combination of three phenomena, the first phenomenon is
grain boundary sliding, which is the motion of grains or groups of grains relative
to each other. A second phenomenon is dislocation creep, this is the motion of
dislocations in the lattice or in the metallic structure of the grains. This
phenomenon results in grain elongation. A third and last phenomenon is grain
boundary diffusion. This phenomenon is the migration of atoms from high stressed
zones to low stressed zones.

Several studies have shown that most of the strain takes place by the
motion of grains or groups of grains (grain boundary sliding) relative to each
other. 5
This vision is supported by experimental studies on the motion of marker lines
placed on a superplastic deformed specimen. In experiments performed by Langdon,
it has been shown that rotation of the grains occurs, but there isn’t a
build-up rotation of the grains. Some grains rotated to the left and others
rotated to the right. The impact of the rotation on the strain is very small.
Other researchers studied the impact of the motion of intergranular
dislocations on the strain, but the impact on the total deformation is very
small. When the displacement of the grains happens in a completely rigid
microstructure, a void will occur in the microstructure. To fill this void the
material needs to cavitate, this should be avoided during superplastic flow. 1 The total
deformation process is the realignment of grains, where the grain boundary
sliding phenomenon determines the total deformation and the other phenomena accommodates
the grain boundary sliding. 6

The
microstructural difference between conventional plastic flow and superplastic flow
is that the grains elongate in the tensile direction with conventional plastic
flow, but with superplastic flow the shape of the grains doesn’t change a lot.

2.2      
Strain
rate sensitivity

The flow stress is a function of rate of deformation and the deformation
itself, it can be expressed as:

With  the flow
stress, k a constant, strain,  the
strain rate, n the strain hardening
coefficient and m a factor for the
strain rate sensitivity of the flow stress. A higher value of m corresponds to a material with a high
resistance against neck propagation; for superplastic flow, a value of m larger than 0.3 is required and for
increased temperatures the factor n equals zero. 1

The factor m depends
on the strain rate, in      figure 2 the flow stress is
plotted in function of the strain rate. The factor m is the slope of the curve.
In stage I the impact of diffusion creep increases. In stage III the slope
decreases and during the deformation process, grain elongation occurs due to
dislocation creep. Both stages results in an inefficient superplastic
deformation. When the strain rates are in the range of stage II, the curve
reaches a maximum slope, which results in a maximum of the strain rate
sensitivity index m. In    figure 3 the strain rate
sensitivity index m is plotted in function of the total elongation. On this figure,
the highest total elongation results in the highest strain rate sensitivity
index m. In accordance with      figure 2, the total elongation
will reach a maximum in stage II. 3 1

2.3      
Materials

The materials that can be super plastically deformed, must have a very
small, stabilized grain size. For this reason, the material is modified to
decrease the grain size. This can be obtained in two ways. The first way is for
pseudo single phase materials where a small contribution of
precipitates gives a smaller grain size. This grain refinement can be executed
by recrystallization of the material before the SPF process, or some other
materials develop a smaller grain size at higher temperatures at the start of
the SPF process. The second way is for materials with about the same
contribution of two phases; in such materials, an allotropic transition is used
to reach a smaller grain size. The temperature at which the SPF process is
carried out is about 0.5 times the absolute melting temperature. A few examples
of superplastic materials are given in table 1. 1

Table 1: materials for
superplastic flow 7

Type material

Grain Size range in
?m

Strain rate range in
S-1

Max. Elongation
range in %

Temperature range in
°C

Aluminium based alloys