Superplastic such as thermoforming, blow forming and

Superplastic
sheet forming

J. Deschodt, J.
Vanheule and W. Vanoverberghe

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

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

<1 to 14 10?4 to 10?1 250 to 1500 100 to 550 Titanium base alloy 6 10?4 1600 950 Copper base alloys 4 to 8 10?4 to 10?2 200 to 5500 460 to 850 3        SPF processes The process makes use of a single-part and -operation pressing instead of multi-operation conventional method or even multi-part constructions. Many thermoplastics processes can be used for superplastic forming of metals. In thermoforming, a pressure causes the sheet to form the shape of a heated die. Blow forming, vacuum forming, deep drawing and combinations with diffusion bonding are other possibilities. 5 3.1       Advantages and disadvantages Products with large and complex shapes or curves can be produced without joints and rivets. Processes requiring a large amount of deformation can be performed in one operation. Multistage manufacturing processes can be avoided. The obtained precision is very good and fine details can be reproduced with great accuracy. There is a high reduction of residual stresses due to the temperatures used. By the latter, an absence of spring back, or elastic recovery, is also present because the yield strength is low in that case. Neck-free elongations of many hundred percent are achieved in contrast to conventional forming. There is a vast improvement in formability. By making the products larger and eliminating assembly operations often the weight of products can be reduced. There are less holes to start fatigue cracks. Forming pressures are also drastically reduced. Tooling and fabrication costs are lower and there is a shorter production step lead time. Close tolerances can be guaranteed which reduce machining costs. Wastage is minimized so that there is maximum use of the material. This is an important saving in energy intensive materials such as titanium or aluminium alloys. The products have a fine and uniform grain size, which leads to better strength, ductility, and fatigue resistance, but also uniform mechanical properties throughout the body of the finished product. 1 8 The major disadvantage with a controlled superplastic forming process is the required time due to the slow forming rate to maintain superplastic behaviour. Cycle times start from two minutes and goes up to two hours. In contrast to just a few seconds to perform conventional forming. Which is why applications are mostly limited to low volume products, such as those common in the aerospace industry. The process is also quite costly because of temperature and time needed. 4 9 3.2       Process parameters The process parameters having a major impact on the global achievable elongation are the strain rate sensitivity of the material and the processing temperature. The superplastic forming process consists of multiple steps of variables. The three most important process variables are temperature, pressure cycle and die size or geometry. These three main parameters have and influence on the process time as well. 10 3.2.1      Temperature Temperature is the most important parameter for superplastic behaviour. Depending on the material, a specific temperature value activates and balances the grain boundary sliding, diffusion and dislocation creep relevance. A high working temperature increases the grain boundary sliding part in the total elongation (relative sliding of the grains) and decreases the strain hardening phenomenon, so that the mechanical characteristics of the product are better. To improve thinning distribution, not evenly distributed temperature on a sheet can be used. A higher forming temperature value weighs on the product and installation costs on the other hand. The lifetime of dies and presses decrease with higher stresses caused by higher working temperatures. Table 1 shows the processing temperature range for aluminium, titanium and copper.  11 Strain hardening may be completely unwanted as an event that unintentionally happens during the manufacturing process. Making a product stronger this way is not always wanted, especially if the material is being heavily deformed, because the ductility will be lower. Also, a great deal of force is required as part of the process. The directional properties of the metal can be affected as well. 12 3.2.2      Pressure cycle Comparing SPF alloys with plastics shows a much higher temperature, but the same order of magnitude in pressure. This forming pressure must vary continuously in time for a constant strain rate during deformation. The pressure must increase because of extending contact surface, the material flow and strain rate lower. When pressure or strain rate increases too much, then the grain boundary sliding part lowers in respect to the dislocation creep contribution (because there is not enough time for sliding, so stretching of the grains occurs), causing a higher material strain hardening and worse mechanical properties of the product. The pressure cycle depends on the forming depth. The strain rate sensitivity first grows with the strain rate up to a maximum value and then decreases again (  figure 4). There also is an influence of temperature here and total elongation is a function of m. Pressure determines the installation forming and working time. 3.2.3      Die size or geometry The final thickness distribution depends on the die size and geometry. Friction and thereby lubrication plays a role. E.g. a small radius hinders material flow and reduces superplastic behaviour. Generally, high friction forces between the sheet and die lead to the phenomenon of cavitation. This can be the result of a complex shape and introduces microscopic holes in the material. Seriousness of cavitation grows with the strain rate (pressure). When the process has an uncontrolled temperature and pressure cycle, the thickness along the surface of a die as not uniform at all. For the design of sheet formed parts, design guidelines are available from manufacturers. The most important parameters are depth of draw, draft angles and corner radii. 3 3.2.4      Time Time is a crucial parameter for profitable production. It is dependent on the type of material, thickness and die shape. A method of trial and error is still used to determine the minimal time needed. Finite element analyses are available to determine this parameter without real-life tests. They also determine other variables like stress and deformation. But there is still a lack of fundamental knowledge about the SPF process. 13 3.3       Forming techniques 3.3.1      Single sheet thermoforming Many different methods can be used to realize a component by means of single sheet thermoforming. Figure 5 illustrates an example of such a process. This specific process is called pressure forming or blow forming. The sheet is fixed between an upper and lower die. Inert gas under pressure is used to stretch the sheet into the die chamber. The process progresses further until the deformed sheet contacts the lower die. Of course, this results in considerable thinning of the sheet. To ensure superplasticity the die and sheet are maintained at the same temperature within a heating press. 1 Figure 5 Single sheet blow forming of SPF materials showing the cross-section of die and sheet: (a) initial flat sheet inserted in between upper and lower dies; (b) progression of forming under gas pressure; (c) final shaped part in contact with lower die; and (d) removal of the part 1 During the forming process, the pressure is dynamically changed. With finite element analysis, the pressurization profile can be calculated as well as the accompanying thickness distribution. The pressure control algorithm keeps the maximum strain rate in the deformation zone of the sheet. 14 The pressure control algorithm can be optimized for a better control of the right strain-rate induced in the material. Because of this optimisation the thickness distribution is more constant. 15 Besides blow forming, single sheet thermoforming has many other variations. These variations introduce a more uniform thickness distribution. In contrast to optimizing the pressurization profile, these variations use mechanical deformation for a better thickness distribution. The initial mechanical deformation makes the regions that normally remain thick, already thinner. 3, pp. 231-237 3.3.2      Multi-sheet forming with diffusion bonding Multi-sheet forming is a technique were many sheets are formed to a single component. Diffusion bonding (DB) is the process that connects the different sheets. This technique can form complex shapes that would normally be made by several parts, for example wings with internal reinforcements. The reduced number of parts are beneficial for weight reduction and cost saving. In some cases, blow forming SPF can be beneficially combined with diffusion bonding, which offers a benefit to fabricating high stiffness multi-sheet structures such as honeycomb components. Two sheets or more are placed in a heated press. Blow forming SPF makes sure that the sheet stretches over the die and the press assures diffusion bonding, see Figure 6. The temperature requirements for SPF and DB are essentially the same which leads to the natural combination of both processes. In addition, many of the superplastic alloys can use diffusion bonding within the small range of temperatures used for SPF. 1 Figure 6 Two-sheet SPF blow forming and DB 3.3.3      Recent rapid forming processes Before SPF can be used in the automotive industry the cycle times are required to be lower than a few minutes. Rapid forming processes have been in development for over 15 years to make mass production possible. Two companies have described their processes in open literature for closure panel production. 1 • Quick Plastic Forming developed by General Motors • High Cycle Blow Forming developed by Honda 3.4       Applications Aerospace and automotive industries make the most use of superplastic forming. Others are rail transport, architecture, medical and communication fields. 1 SPF is widely used in aviation and aerospace for components such as: hollow blades, inlet lips, wing boxes and other parts. These components are manufactured through three-layer titanium alloy structure bonded together with DB. These multi layered components show a great advantage in weight reduction. The large design freedom, high load-bearing capability and fine structural integrity of SPF has great significance to the design and manufacturing of aircrafts, aircraft engines, missiles and other spacecraft parts. 16 Superplastic forming of aluminium alloys has become more popular in the automotive industry through the years. Companies are setting up their own facilities where part numbers approach 100 000 per annum. These facilities are using a rapid forming processes like GM and Honda to form their car panels. 1 Figure 7: Left: Ti–6Al–4V hollow, wide-chord fan blades on a Rolls-Royce Trent series gas turbine engine (copyright Rolls-Royce plc) 1, Right: One piece bodysides made by Superform 17 4        CONCLUSIONS The superplastic sheet forming is based on the phenomenon of superplasticity. This deformation process results in a grain realignment and almost none grain elongation. The exact mechanism behind the superplasticity is still less understood. In literature, it is noted that the mechanism behind a combination of grain boundary sliding, dislocation creep and grain boundary diffusion. Superplastic has a lot of advantages over conventional forming techniques. The metal can be deformed with less power therefor the dies may be cheaper. Better strength of the result is obtained with SPF and a conventional multi part component can be made in one part. All these advantages and more ensure that SPF is better for certain applications but it has still some big disadvantages because of which it is not generally used. The duration times of SPF are to long for mass production and together with the high temperature leads to costly manufacturing technique. General motor has made progress with quick plastic forming (QPF) on reducing the time. Other innovations in SPF are mainly about reducing heating costs and cutting heating-plates maintenance costs whilst increasing productivity through reduced cycle time 3. The three main process parameters have been discussed and their influence can be explained. A complex temperature and pressure cycle is needed to maintain the superplastic behaviour in ideal circumstances. Therefore, a lot of research is performed to make the computer models more accurate for different parameters 5        Acknowledgements The authors would like to acknowledge the support of prof. dr. ir. Wim De Waele for the review of this paper and thank each other for the good teamwork. 6        References 1 G. Giuliano, Superplastic forming of advanced metallic materials: methods and applications, Cambridge: Woodhead Publishing Limited, 2011. 2 "Wikipedia," Online. Available: https://en.wikipedia.org/wiki/Superplastic_forming. Accessed 14 12 2017. 3 K. Padmanabhan and G. Davies, Superplasticity Mechanical and Structural Aspects,Environmental Effects,Fundamentals and Applications, Berlin, Heidelberg: Springer, 1980. 4 E. Degarmo, J. Black and R. Kohser, Materials and Processes in Manufacturing (11th ed.), Wiley, 2012. 5 O. A. Kaibyshev, A. I. Pshenichniuk and V. V. Astanin, "Superplasticity resulting from cooperative grain boandary sliding," Acta materialia, vol. 14, pp. 4911-4916, 1998. 6 E. Alabort, P. Kontis, D. Barba, K. Dragnevski and R. Reed, "On the mechanisms of superplasticity in Ti-6Al-4V," Acta Materialia, vol. 2016, no. 105, pp. 449-463, 2016. 7 L. Ceschini and A. Afrikantov, "Superplastic Forming (SPF) of materials and SPF combined with diffusion bonding: technological and design aspects," Metallurgical science and technology, vol. 3, no. 10, pp. 41-55, 1992. 8 B. Ilschner, Materials Research and Engineering, 2 ed., Germany, 1980. 9 T. G. Nieh, J. Wadsworth and O. D. Sherby, Superplasticity in Metals and Ceramics, Cambridge University Press, 2005. 10 N. Capetti, L. Garofalo, A. Naddeo, M. Nastasia and A. Pellegrino, "A method for setting variables in Super Plastic Forming process," Journal of Achievements in Materials and Manufacturing Engineering, vol. 38, no. 2 (February), pp. 187-194, 2010. 11 L. Jun, T. Ming-Jen, A.-u.-l. Yingyot, E. W. J. Anders, F. Kai-Soon and C. Sylvie, "Superplastic-like forming of non-superplastic AA5083 combined with mechanical pre-forming," Verlag Londen Limited, p. 7, 2010. 12 NDT Resource Center, "Strengthening/Hardening Mechanisms," Online. Available: https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Structure/strengthening.htm. Accessed 3 12 2017. 13 S. F. Jarrar, M. Liewald, P. Schmid and A. Fortanier, "Superplastic Forming of Triangular Channels with Sharp Radii," ASM International, p. 8, 2014. 14 Y. Hwang and H. Lay, "Study on superplastic blow-forming in a rectangular closed-die," Elsevier, Taiwan. 15 L. Carrino, G. Giuliano and G. Napolitano, "A posteriori optimisation of the forming pressure in superplastic forming processes by the finite element method," 2002. 16 H. Xiaoning, D. Lihua, Z. Xingzhen, L. Zhen and J. Shao, "Optimal Design of Geometric Parameters for SPF/DB 3-layer Structures," 2017. 17 SUPERFORM, "http://www.superforming.com/bodysides," Online. Accessed 12 2017.