Superplastic forming using induction heating

Apparatus and method for superplastic forming. The workpiece is held between a pair of dies that are electrically and thermally nonconductive, and is heated inductively by a coil embedded within the dies.

FIELD OF THE INVENTION 
The present invention relates to superplastic forming of workpieces. 
BACKGROUND OF THE INVENTION 
Under certain conditions, some materials can be plastically deformed 
without rupture well beyond their normal limits, a property called 
superplasticity. This property is exhibited by certain metals and alloys, 
within limited ranges of temperature and strain rate. For example, 
titanium and its alloys are superplastic in the temperature range 
1450.degree.-1850.degree. F. 
Superplastic forming (SPF) is a fabrication technique that relies on 
superplasticity. A typical SPF process involves placing a sheet of metal 
in a die, heating the sheet to an elevated temperature at which it 
exhibits superplasticity, and then using a gas to apply pressure to one 
side of the sheet. The pressure stretches the sheet and causes it to 
assume the shape of the die surface. The pressure is selected to strain 
the material at a strain rate that is within its superplasticity range at 
the elevated temperature. 
One advantage of SPF is that very complex shapes can be readily formed. In 
addition, the SPF process is generally applicable to single and 
multi-sheet fabrication, and can be combined with joining processes such 
as diffusion bonding to produce complex sandwich structures at a 
relatively low cost. The simplicity of the SPF process leads to lighter 
and less expensive parts with fewer fasteners, and higher potential 
geometric complexity. Common applications of SPF include the manufacturing 
of parts for aircraft, missiles and space vehicles. 
In a typical prior art SPF process for titanium, a titanium sheet is placed 
between steel dies, one of which has a contoured surface corresponding to 
the shape to be imparted to the titanium sheet. The dies are then placed 
on platens or plates which are heated through the use of electrical 
resistance type heating elements embedded within the platens. The platens 
heat the dies through conduction heating to about 1650.degree. F. To avoid 
oxidation of the titanium at the elevated temperature, the sheet is 
immersed in an inert atmosphere such as argon gas. The dies conduct heat 
into the titanium until its temperature reaches the superplastic range. At 
that time, the pressure of the argon gas on the side of the sheet away 
from the contoured surface is elevated sufficiently to deform the titanium 
sheet against the contoured surface, whereupon the sheet acquires the 
shape of the surface. 
The high temperature at which the SPF operation must be carried out causes 
it to be a slow and cumbersome process. In particular, because of their 
large thermal mass, the dies are typically maintained at forming 
temperature throughout a production run. Failure to maintain the dies at 
superplastic forming temperatures during part loading and unloading would 
result in unacceptable process times for each part. Thus, blank sheets 
must be inserted, and formed parts removed, while the SPF dies are at 
forming temperature. Because the parts are loaded and unloaded from the 
dies while still at forming temperature, the parts must be very carefully 
handled in order to minimize bending of the part. Even with careful 
handling, some parts may be distorted during unloading and require 
subsequent processing steps to achieve proper part tolerances. 
Furthermore, the elevated temperature of the forming dies and parts 
requires operators to wear protective clothing and use special equipment 
to insert the metal sheets between the dies, and particularly to remove 
the formed parts. 
SUMMARY OF THE INVENTION 
The present invention provides an improved apparatus and method for 
superplastic forming. 
A preferred apparatus according to the invention comprises a die having a 
nonmetallic forming surface that has a shape corresponding to the desired 
shape for the workpiece. Means are provided for positioning the workpiece 
such that it overlies the forming surface, with a first side of the 
workpiece facing the forming surface. Inductive heating means are provided 
for subjecting the workpiece to a time varying magnetic field, such that 
the workpiece is heated to a temperature at which it is superplastic. 
Finally, means are provided for producing a pressure differential between 
the first and second sides of the workpiece, while the workpiece is at 
said temperature, such that the workpiece deforms and assumes the shape of 
the forming surface. The dies are preferably formed from a dielectric, 
thermally insulating material such as a castable ceramic. Thermal energy 
is therefore applied to the workpiece but not the dies, leading to much 
shorter heating and cooling cycles as compared to the prior art technique 
of using resistance-type heating with metal dies. The inductive heating 
means of the invention preferably comprises a plurality of electrical 
conductors embedded within the dies, and means for interconnecting such 
conductors into a coil that surrounds the workpiece.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 schematically illustrates a preferred embodiment of a superplastic 
forming apparatus according to the present invention. The apparatus 
includes a pair of dies 12 and 14 between which a workpiece 16 may be 
positioned. Both dies are preferably formed from a dielectric, thermally 
insulating material, such as ceramic. A plurality of metallic tubing 
sections 18 are embedded within the upper and lower dies, and 
interconnected to one another to form a single coil, as further described 
below. The apparatus also includes coil driver 20 connected to two of the 
tubing sections 18, and pressure source 22. The apparatus may also include 
means (not shown) for pumping a cooling fluid through tubing sections 18. 
Lower die 14 includes a tool box 17 and a tool insert 15. The tool insert 
15 has a forming surface 30 that has a shape corresponding to the desired 
shape for workpiece 16. The tool insert 15 and tool box 17 are preferably 
formed from a dielectric, thermally insulating material such as ceramic. 
Tool insert 15 and tool box 17 can be separate pieces as shown in FIG. 1 
or they can be a single integral part. The two piece design shown in FIG. 
1 reduces cost by allowing different tool inserts 15 to be interchangeably 
used within the tool box 17. 
In operation, coil driver 20 energizes the coil formed by sections 18, such 
that workpiece 16 is inductively heated. When the workpiece reaches a 
temperature at which it is superplastic, pressure is applied to the upper 
surface of the workpiece from pressure source 22 via conduit 32 that 
passes through upper die 12. Suitable pressures vary depending upon the 
workpiece, with pressures in the range 50-600 psi being typical. In 
response to the pressure, the workpiece deforms until it assumes the shape 
of forming surface 30, as illustrated in FIG. 2. Small pinholes (not 
shown) may be formed in the tool insert 15 and tool box 17 to allow the 
venting of gas trapped between the workpiece and the forming surface as 
deformation proceeds. Such pinholes are often coupled to a flow meter to 
monitor the progress of deformation. When the workpiece has the desired 
shape, the coil is de-energized, and the pressure source removed. The dies 
may then be separated to remove the formed workpiece. 
One of the key concepts of the present invention is the use of inductive 
rather than resistive heating to heat the workpiece. Inductive heating is 
accomplished by applying an alternating electrical current to the coil 
within which the workpiece is positioned, to thereby produce an 
alternating magnetic field in the vicinity of the coil. The alternating 
magnetic field heats the metallic workpiece via eddy current heating. 
However the dies are constructed from a dielectric material that is not 
heated by the time varying magnetic field. Furthermore, the material from 
which the dies are constructed is also thermally insulating, so that it 
traps and contains the heat transferred to the workpiece. Other desirable 
properties for the dies are a low coefficient of thermal expansion, good 
thermal shock resistance, and relatively high compressive strength. The 
preferred die materials are castable ceramics, and particularly fused 
silica castable ceramics. 
FIG. 3 illustrates a suitable support structure for the apparatus shown in 
FIGS. 1 and 2. The support structure includes base 40 from which threaded 
shafts 42 extend upwardly from gear boxes 44 which rotate the threaded 
shafts 42. Dies 12 and 14 are supported above and below by metal 
strongbacks 50 and 52, respectively. Strongback 52 loosely receives shafts 
42, and rests upon reinforcing bars 54. Upper strongback 50 is threadably 
supported on shafts 42, such that the position of upper strongback 50 can 
be varied vertically by rotation of the shafts, to thereby open or close 
the dies. Each strongback consists of a metal structure whose purpose is 
to provide a stiff, flat surface backing the ceramic die, to transfer the 
load between the die and the support structure. The strongback must be 
flat and stiff enough to prevent the ceramic die from bending and 
cracking. Preferably, the strongback should be capable of holding the 
ceramic die to a surface tolerance of .+-.0.003 inches per square foot of 
die surface. Because relatively little of the magnetic field is produced 
outside the coil, the strongback remains substantially at room temperature 
during the SPF process, despite its metallic construction. 
Dies 12 and 14 are supported laterally by box 60 that encloses the dies on 
all sides. Box 60 is preferably formed from a dielectric material, so that 
it will not be heated by any inductive field that extends outside the 
dies. A suitable material for box 60 is a phenolic resin. The phenolic box 
may further be connected with preloaded tie rods, in a manner similar to 
prestressed concrete, or an external load fixture may be used to push 
against the phenolic box. Utilizing either method, the phenolic box sides 
function as pressure plates that maintain compressive forces on the 
ceramic dies. When the dies are formed from a castable ceramic material, 
the phenolic box may also provide the sidewalls of the mold for casting 
the dies. 
FIGS. 4 and 5 illustrate a preferred method for interconnecting tubing 
sections 18 (FIG. 1) into a single coil. Referring initially to FIG. 4, 
coil 70 comprises straight sections 72 and curved sections 74. Each 
straight section 72 is cast into one of the dies, while each curved 
section 74 extends between the upper and lower die, as illustrated in FIG. 
5. The curved sections are flexible, as further described below, to 
accommodate the opening and closing of the dies. The curved and straight 
sections are joined at fittings 76 into a continuous coil or helix 
structure, producing a magnetic field schematically illustrated by field 
lines 80 in FIG. 4. By applying a time varying current to coil 70, a time 
varying magnetic field 80 is created that heats the metal workpiece via 
eddy current heating. Each straight section 72 and curved section 74 
preferably comprises a copper tube having an interior longitudinal passage 
through which a cooling fluid such as water may be pumped to cool the 
tubing sections themselves. 
FIG. 5 illustrates a preferred construction for curved section 74. The 
curved section comprises a pair of fittings 76, each of which contains a 
relatively small diameter section 92 dimensioned so as to fit snugly 
within straight section 72, and a larger diameter flange 94. Passages 96 
extend through the fittings, including the flange. A pair of flexible 
copper strips 98 are joined between flanges 94, such as by brazing. 
Finally, a flexible jacket 100 is secured over the copper strips between 
the flanges to contain the cooling fluid. The jacket preferably comprises 
a flexible, non-conducting material capable of holding (for example) 80 
psi at 140.degree. F. A suitable design utilizes four copper strips 
approximately five inches long, 0.75 inches wide, and 0.03 inches thick. 
Such a connection can accommodate three inches of travel between the dies. 
One commercial vendor through which a suitable design can be obtained is 
Flex-Cable. 
The frequency at which the coil driver drives the coil depends upon the 
nature of the workpiece. Current penetration of copper at 3 KHz is 
approximately 0.06 inches, while the penetration at 10 KHz is 
approximately 0.03 inches. The shape of the coil used to create the 
induction heating has a significant effect upon the magnetic field 
uniformity. This in turn is important because temperature uniformity is 
directly affected by the uniformity of the magnetic field. Uniform heating 
in turn means that different portions of the workpiece will reach the 
appropriate temperature for superplastic forming at the same time. 
Solenoidal type coils provide such a uniform field, and are therefore 
preferred. Greater field uniformity is seen by a part which is symmetric 
about the centerline of the coil. The additions of variations, such as 
series/parallel coil combinations, variable turn spacings and distances 
between the part and the coil can be established by standard electrical 
calculations. 
Some advantages of the invention are illustrated by the graphs shown in 
FIGS. 6 and 7. FIG. 6 illustrates a typical SPF cycle using the prior art 
technique of resistance heating of the metallic workpiece and dies. The 
dies are heated to superplastic forming temperatures starting at time 0. 
Because of the large thermal mass of the thermally conductive metallic 
dies used in the prior art, the superplastic forming temperature (here 
1650.degree. F.) is not attained until about 23 hours later. The workpiece 
is then loaded into the dies and the superplastic forming operation is 
carried out, for 30 minutes in this example. 
Because of the large times that would be necessary for the metallic dies to 
cool, the workpiece is removed from the dies while the dies are still at 
operating temperature. Because the workpiece is still at superplastic 
forming temperatures, the part must be very carefully unloaded to minimize 
bending or distortion of the hot part. Even careful removal can result in 
some distortion of the formed part which must then be further processed in 
order to obtain proper part tolerances. This increases the costs and 
complexities in manufacturing. 
After removing the workpiece from the dies, a new workpiece may be inserted 
and superplasticly formed. Upon completion of the production cycle, the 
metallic dies are then cooled. Because of the large thermal mass of the 
thermally conductive metallic dies, this cooling process takes a 
substantial amount of time. During the heating and cooling of the metallic 
dies used in the prior art, no superplastic forming can take place; 
therefore, the facility has a large amount of downtime where it is not 
producing parts. 
FIG. 7 provides a comparable graph for the technique of the present 
invention. Using the invention, a workpiece temperature of 1650.degree. F. 
may be reached in about 20 minutes, and the workpiece may be formed in 
about 30 minutes and cooled to 1200.degree. F. in about 15 minutes. Thus 
the total cycle time is only slightly greater than the one hour. This much 
shorter time is due to the fact that the workpiece has a much lower 
thermal mass than the metallic dies used in the prior art. The reduced 
cycle times of the present invention reduce the downtime involved in a 
part run. 
Because of the short amount of time required to heat and cool the dies used 
in the present invention, the workpiece temperature can be reduced to 
approximately 1200.degree. F., without adding significant downtime, before 
removing the workpiece from the dies. This allows the workpiece to cool to 
a temperature below the superplastic forming temperature, which reduces 
the risk of part distortion or damage during the unloading operation. 
While the invention has been described in connection with the forming of a 
single sheet workpiece, it would readily be apparent to those skilled in 
the art that the process could readily be extended to workpieces that 
comprise multiple sheets. Generally, in such processes, the gas pressure 
is applied between pairs of sheets to produce multi-layer structures such 
as rib-stiffened or truss core fuselage structures, stability critical 
aerosuffaces, beaded shear webs, inlet structures, and other complex shape 
assemblies. For all such structures, the use of the SPF processes of the 
present invention will significantly reduce process steps, part counts, 
and fasteners, resulting in decreased cost, higher reliability, and 
reduced weight. 
Non-metallic materials which are electrically conductive can also be used 
with the apparatus of the present invention. These materials include 
composites that contain an electrically conductive material, e.g., 
graphite fibers. The induction heating process of the present invention 
induces currents in the electrically conductive fibers within the 
composite material which result in heating of the fibers and subsequent 
heating of the entire workpiece. 
It will also be apparent to those skilled in the art that the workpiece 
itself need not comprise a metal or other electrically conductive 
material. For example, if the workpiece is not conductive, it may be 
placed in a contact with or bonded to a conductive heating plate that 
absorbs heat from the inductive field, and transfers it to the workpiece 
material.