Tool for sealing superplastic tube

A tool for sealing two ends of a tube of superplastic material in preparation for superplastically forming the tube against inside surfaces of a die by gas pressure inside of the tube. The tool has a longitudinal axis that is coincident with the longitudinal axis of the tube when the tool is positioned in the tube, and has two end caps with a cross-sectional shape similar to the cross-sectional shape of the tube ends on a plane perpendicular to the axis. A central connecting tube extends between and connects the two end caps. The end caps are made of a material having a coefficient of thermal expansion that is greater than the coefficient of thermal expansion of the tube, and expand, on heating, into intimate sealing contact with the inside surface of the tube ends. A gas connector in at least one of the end caps, located in the end cap radially outside the connection of the connecting tube to the end cap, connects a gas line from a gas management system for introducing forming gas under pressure into the tube, after the ends thereof are sealed by the end caps and the tube is heated to the thermoplastic forming temperature thereof, for inflating the tube against the inside surfaces of the die. The gas needs to fill and act only in the volume between the connecting tube and the tube instead of the entire volume of the tube, thereby saving forming gas.

BACKGROUND OF THE INVENTION 
Superplastic forming is a process which utilizes the properties of certain 
materials that can be extensively strained at relatively low stress levels 
when heated to an elevated temperature known as the superplastic forming 
temperature. Certain formulations of aluminum, rolled in a certain 
pattern, exhibit superplacticity at superplastic temperatures, as do 
titanium and some titanium alloys, certain stainless steels and some super 
alloy materials. All of these materials have been used to form low 
tolerance parts with little or no residual stress, which would have been 
difficult or impossible to achieve with prior art metal forming processes. 
The forming of tubular structures by superplastic forming in the past has 
been performed by superplastic forming two longnitudial halves of the part 
as separate pieces and welding the two pieces together to make the final 
part. This process can produce a satisfactory part, but it is costly and 
great care must be taken to avoid quality problems, especially if the part 
must be capable of withstanding gas pressure. 
An ideal method of forming tubular parts by superplastic forming would be 
to begin with a tubular blank and to superplastically form the tubular 
blank against inside cavity surfaces in a die having an internal 
configuration like the external shape of the final part. This process 
would eliminate the cost of making the parts in two halves then welding 
the halves together and would result in a seamless part having excellent 
part quality and minimal variation from part to part. 
A conventional superplastic forming process utilizes a sheet of 
superplastic material which is captured around its peripheral edge between 
a die base and a die lid. The sheet is heated to superplastic forming 
temperature in the die and the sheet is then strained into contact with 
the surface of the die cavity by gas pressure introduced under the die 
lid. The tubular analog to the flat sheet superplastic forming process, 
that is, using the forming gas pressure to form a tube of superplastic 
material against internal surfaces in a die cavity, would require that the 
tube be sealed around the peripheral edges of both ends of the tube to 
establish a pressure zone inside the tube for straining the tube material 
outward into contact with the inside surfaces of the die cavity. The 
sealing of the tube in a superplastic forming die can be complicated and 
unreliable because of the various factors involved in superplastic 
forming, including the very high temperatures at which certain materials 
become superplastic and the high pressure of the forming gas required to 
strain the material, even at a superplastic temperature. Thus, there has 
long been an unfulfilled need in the art to provide a simple, inexpensive 
and reliable method and an apparatus for sealing the ends of a 
superplastic tube in a superplastic forming die for superplastic forming 
of the tube. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an improved 
method for superplastic forming of tubular structures. Another object of 
the invention is to provide an improved method for end sealing of tubular 
blanks of superplastic material in a die for superplastic forming of 
tubular structures. Yet another object of the invention is to provide a 
tool for sealing the ends of a tubular blank of a superplastic material in 
preparation for superplastic forming of the blank to form a tubular part. 
Still another object of the invention is to provide a tool for sealing the 
ends of a tubular blank in a superplastic forming die, which tool can be 
removed after forming and reused many times to make additional parts. A 
still further object of the invention is to provide a superplastically 
formed part, made from a tubular blank having ends which were sealed to 
contain the forming gas pressure introduced to form the tubular blank 
against the inside surface of a superplastic forming die. 
These and other objects of the invention are attained in two embodiments of 
a method of sealing the ends of a tubular blank of superplastic material 
against escape of forming gas introduced into the interior of the tube. 
One method includes welding an end cap on each end of the tubular blank, 
and providing a gas inlet tube in at least one of the end caps. After 
superplastic forming the tubular blank to produce the tubular part, the 
two end portions of the tube, including the end caps, are severed from the 
tubular part to produce two open ends of the part. A second embodiment of 
the method utilizes a reusable tool having end caps which fit snugly in 
the tubular blank. The end caps have a coefficient of thermal expansion 
greater than the coefficient of thermal expansion of the tubular blank, so 
when the blank and the installed tool are heated in the die, the end caps 
expand more than the tubular blank to produce a sealing interference fit 
between the end caps and the blank. A connection is provided in at least 
one end cap to enable the interior of the tubular blank to be pressurized 
with forming gas for forming the tubular blank against the inside surfaces 
of the die for superplastically forming the tubular part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to the drawings, wherein like reference characters designate 
identical or corresponding characters, and more particularly to FIG. 1 
thereof, a tubular blank 20 is shown which will be welded into the 
integral tubular assembly shown in FIG. 2 and formed in the die shown in 
FIG. 3 to produce a formed tubular structure shown in FIG. 4 which is then 
trimmed to produce the tubular part having a pull-out shown in FIG. 5. The 
tubular blank 20 is a seamless or welded tube of titanium alloy containing 
titanium, aluminum and vanadium, but instead it could be other 
commercially useful alloys of titanium such as titanium 15-3-3-3. Two end 
caps 22 and 24 are welded onto the ends of the tube 20 to produce a sealed 
interior volume 26 within the tube 20 and between the two ends 22 and 24. 
The end caps 22 and 24 will usually be the same material as the tube 20, 
but need not be since they do not need to be superplastic for the process 
to work as described herein. A gas pipe connection 28 is inserted in a 
hole 30 drilled through the center of the end cap 22 and is welded into 
place to form a gas tight connection between the gas pipe connection 28 
and the end cap 22. The welded assembly 32, shown in FIG. 2, is completely 
gas tight except for the opening into the enclosed volume 26 through the 
end 34 of the gas pipe 28. 
If desired for gas purging of air, a pipe similar to the gas pipe 28 may be 
provided in the cap 24 for connection to a purge line. This would provide 
a cross channel flow path for a purging air out of the enclosed volume 26 
to minimize formations of oxide or alpha case on the inside walls of the 
tube 20 during superplastic forming of the welded assembly 32. However, 
the preferred embodiment does not utilize a purge line because, after 
forming, the part is treated in an acid etch solution to remove the alpha 
case that forms on the outside surface of the part, so purging the inside 
would merely waste time and forming gas since the inside surface is etched 
at the same time as the outside surface anyway. 
As shown in FIG. 3, the welded assembly 32 is inserted in a cavity 36 in a 
die base 38 and die lid 40 having a corresponding cavity 42 is placed over 
the die base 38 using alignment posts 44 and alignment plugs 46 to 
position the lid 40 accurately on the base 38. As understood by those 
skilled in the art, the die base 38 and the die lid 40 are normally held 
in a press having heated platen so that the die lid 40 is lowered onto the 
die base 38 when the die is to be closed by lowering the upper platen of 
the press (not shown). The usual practice is for the die 38-40 to be 
heated to a temperature at or about the superplastic forming temperature 
of the blank 20 before the welded assembly 32 is inserted in the cavity 
36-42. After closing the die lid 40 on the base 38, the welded assembly 32 
quickly reaches superplastic forming temperature and is ready to be 
expanded by forming gas pressure to assume the shape of the die cavity 
36-42. 
The connection tube 28 projects out beyond the outer edge of the die 38-40 
through a hole drilled through the die wall 47 at the parting line of the 
die. A gas line 48 is connected to the gas connection tube 28 and leads to 
a gas management system 49 such as that disclosed in U.S. patent 
application Ser. No. 08/138,282 filed on Oct. 15, 1993 entitled "Gas 
Control for Superplastic Forming", the disclosure which is incorporated 
herein by reference. This gas management system controls the flow of 
forming gas, normally argon, under pressure into the interior of the 
welded assembly 32 through the gas line 48 and the connection pipe 28 to 
apply gas pressure against the interior walls of the tubular blank 20. The 
pressure of the forming gas against the inside walls of the tubular blank 
20 at superplastic forming temperature strains the walls outward against 
the inside surfaces of the cavity 36-42 and so that the blank 20 assumes 
the shape of a cavity 36-42 in the die. In the case of the part 
illustrated in FIG. 5, the tube is provided with a central pull-out 50 to 
serve as a T connection for a cylindrical duct. 
After forming, the gas pressure in the formed structure 52 shown in FIG. 4 
is reduced to atmospheric pressure and the die lid 40 is raised off of the 
die base 38. The formed structured 52 cools quickly when exposed to the 
air and can be removed from the cavity 36 with handling equipment or 
protective gloves. When the structure 52 is cooled to room temperature, 
the end caps 22 and 24 are severed as indicated in FIG. 5, and a disc 54 
is cut off the end of the pull-out 50 to produce a cylindrical duct with a 
cylindrical pull-out 50 to function as a T connection in a cylindrical 
duct network. 
A second embodiment of the invention utilizes a reusable tool in the form 
of a spool shown in FIG. 6. The spool 60 includes an end cap 62 welded to 
one end of a connecting tube 64 and second end cap 66 welded to the other 
end of the connecting tube 64. The end cap 62 has an axial hole 68 
extending completely through the end cap and communicating from the left 
hand edge surface through to the right hand edge surface of the end cap 
62. The outside diameters of the end caps 62 and 66 are equal and are just 
slightly less than the internal diameter of a tubular blank 70 of 
superplastic material such as the titanium alloy used in the tubular blank 
20 shown in FIG. 1. The spool 60 slides with a snug fit into the tubular 
blank 70 and the assembly is placed in a heated split die having a die 
base 72 and a die lid 74. The die is closed in the same manner as the die 
in FIG. 3, and the heat in the die raises the temperature of the assembled 
spool 60 and tubular blank 70 to the superplastic forming temperature of 
the blank 70. 
Before forming gas can be introduced into the cylindrical annular space 76 
between the connecting tube 64 and the tubular blank 70, the ends of the 
tubular blank 70 must be sealed against escape of the pressurized forming 
gas. The sealing of the tubular blank 70 is accomplished by differential 
expansion of the end caps 62 and 66 relative to the expansion of the 
tubular blank 70. The die base 72 and die lid 74 are both made of a high 
temperature tool steel such as ESCO 49C. Likewise, the end caps 62 and 64 
and the connecting tube 64 are also made of ESCO 49C tool steel. The 
diameter of the circular openings 76 and 78 of the cavity 80 in the die 
72-74 at the superplastic forming temperature of the tubular blank 70 is 
larger than the external diameter of the tubular blank 70 at room 
temperature but smaller than the external diameter of the tubular blank 70 
at superplastic forming temperature, so the assembly of the tubular blank 
70 and the spool 60 may be placed in the cavity 80 of the die 72-74, with 
the ends of the blank 70 containing the end caps 62 and 66 in the circular 
openings 76 and 78, and the die lid 74 closed on the die base 72. However, 
the external diameter of the end caps 62 and 66 at room temperature is 
such that, on expansion of the end caps 62 and 66 as the spool 60 
equalizes in temperature with the die 72-74 after closing, the annular 
space between the end caps 62 and 66 reduces to less than the thickness of 
the tubular blank 70. As a consequence, an interference fit is created in 
the annular space between the end caps 62 and 66 and their respective 
circular openings 76 and 78. 
Because the assembled tubular blank 70 and the spool 60 is cool when it is 
installed in the die 72-74, it fits into the circular openings 76 and 78 
without interference and the die lid 74 can be closed and clamped securely 
on the die base 72 by the press in which the die halves are installed. 
Because of the configuration of the assembled tubular blank 70 and the 
spool 60 inside the tubular blank 70, the tubular blank 70 heats first and 
expands, followed by the heating of the spool 60. The coefficient of 
thermal expansion of the ESCO 49C, about 11.1.times.10.sup.-6 
in/in/.degree.F. at 1650.degree. F., is greater than the coefficient of 
thermal expansion of the titanium alloy used in the blank 70, which is 
about 6.2 .times.10.sup.-6 in/in/.degree.F. at 1650.degree. F. Therefore 
the end caps 62 and 66 expand greater than the tubular blank 70. The 
dimensions of the circular openings 76 and 78 in the die cavity 780 and 
the external diameter of the end caps 62 and 66 is selected so that the 
annular space between the end caps 62 and 66 and the corresponding 
circular openings 76 and 78 is smaller than the thickness of the tubular 
blank 70. When the end caps 62 and 66 finally reach their full operating 
temperature which is the temperature of the superplastic forming 
temperature of the blank 70, the blank 70 has already reached superplastic 
forming temperature and the overlapping dimensions causes the superplastic 
material of the tubular blank 70 to be forced into a sealing surface 
profile cut into the die around the circular openings 76 and 78. The 
flowing of the superplastic material into the seal profiles facilitates 
the sealing of the interface between the blank 70 and the circular 
openings 76 and 78, and between the blank 70 and the end caps 62 and 66, 
and also prevents development of excessive stresses in the die 72-74 which 
could possible occur otherwise. 
Forming gas introduced under pressure from a gas management system 80 like 
the gas management system 49 used in the embodiment of FIG. 3, strains the 
tubular blank 70 as illustrated in FIG. 8 into contact with the interior 
surfaces of die 72-74. 
After the tubular blank 70 has been formed against the inside surfaces of 
the inside cavity 80, the gas management system 84 reduces the forming gas 
pressure to atmospheric and the die lid 74 is opened by raising the upper 
platen of the press. The formed blank 70 cools quickly when exposed to air 
at room temperature and the formed blank and the spool 60 can be lifted 
out of the cavity 80. The contraction of the end caps 62 and 66 is greater 
than the contraction of the end portions of the blank 70 because of 
dirrerential coefficients of thermal expansion, enabling the spool 60 to 
slide axially out of the formed blank 70. The formed blank 70 is trimmed 
and cleaned to produce the final part. 
Obviously, numerous modifications and variations of the preferred 
embodiments disclosed here and will become apparent to those skilled in 
the art upon reading this disclosure and examining the drawings. 
Accordingly, it is expressly to be understood that these modifications and 
variations, and the equivents thereof, may be practiced while remaining in 
the spirit and scope of the invention as defined in the following claims.