Method of manufacturing an article, a method of diffusion bonding and a vacuum chamber

A stack of titanium sheets are placed between a pair of pressurisable chambers in a vacuum chamber. One of the mating surfaces of each pair of mating surfaces has had a stop off material applied in a desired pattern to prevent diffusion bonding. A pump evacuates the vacuum chamber and heaters heat the stack to evaporate volatile binders from the stop off. When all the binder has been removed the stack is heated and the pressure in the pressurisable chambers is increased to cause a pair of resilient members to apply pressure on the stack to diffusion bond the sheets together. The volatile binder is removed quickly and oxidation of the titanium is prevented during baking out of the binder. The integral structure is then heated and internally pressurised to superplastically form one of the sheets to produce an article of predetermined shape. No welding of the sheets before bonding is required.

The present invention relates to a method of manufacturing an article by 
superplastic forming and diffusion bonding. 
It is known to manufacture metallic articles by superplastic forming and 
diffusion bonding metal workpieces. These metal workpieces include 
elementary metal, metal alloy and metal matrix composites. At least one of 
the metal workpieces must be capable of superplastic extensions. 
In one known process the surfaces of the workpieces to be joined are 
cleaned, and at least one surface of one or more of the workpieces is 
coated in preselected areas with a material to prevent diffusion bonding. 
The workpieces are arranged in a stack and the edges of the workpieces are 
welded together, except where a pipe is welded to the workpieces, to form 
an assembly. The pipe enables a vacuum, or inert gas pressure to be 
applied tot he interior of the assembly. The assembly is placed in an 
autoclave and heated so as to "bake out" the binder from the material to 
prevent diffusion bonding. The assembly is then evacuated and the pipe is 
sealed. The sealed assembly is then placed in a pressure vessel and is 
heated and pressed to diffusion bond the workpieces together to form an 
integral structure. Diffusion bonding occurs when two mating surfaces are 
pressed together under temperature, time and pressure conditions that 
allow atom interchange across the interface. The first pipe is removed and 
a second pipe is fitted to the diffusion bonded assembly at the position 
where the first pipe was located. The integral structure is located 
between appropriately shaped dies and is placed within an autoclave. The 
integral structure and dies are heated and pressurised fluid is supplied 
through the second pipe into the interior of the workpieces to be 
superplastically formed to cause at least one of the integral structure to 
produce an article matching the shape of the dies. 
In the known method the welding of the pipe, for subsequently removing 
vaporised binder and for evacuating the assembly, to the workpieces is 
time consuming. The use of the pipe for removing vaporised binder from the 
assembly is also time consuming. Furthermore the assembly contains air 
when it is heated to vaporise the binder and this results in oxidation of 
the surfaces of the workpieces which are subsequently to be diffusion 
bonded. 
Out UK patent application 9208369.0 filed on Apr. 16, 1992 which claims 
priority from our prior UK patent application 9111954.5 filed on Jun. 4, 
1991, incorporated herein by reference, disclosed a novel method of 
manufacturing an article by superplastic forming and diffusion bonding 
which overcame the above problems. In that patent application, it is 
disclosed that the stack of workpieces is placed in a vacuum chamber, and 
the vacuum chamber is evacuated. The stack is heated while it is within 
the vacuum chamber to evaporate the volatile binder from the stop off 
material, while the vacuum chamber is continuously evacuated to remove the 
volatile binder from between the workpieces and the vacuum chamber. The 
edges of the workpieces are welded together while the stack is within the 
vacuum chamber to form a sealed assembly. The sealed assembly is then 
diffusion bonded to form an integral structure and the integral structure 
is superplastically formed. 
The present invention seeks to provide a novel method of manufacturing an 
article by superplastic forming and diffusion bonding which also does not 
have the above mentioned disadvantages and which is simpler than the 
method disclosed in our prior UK patent application mentioned above. 
Accordingly the present invention provides a method of manufacturing an 
article by superplastic forming and diffusion bonding at least two metal 
workpieces comprising the steps of 
(a) applying a stop off material to prevent diffusion bonding to 
preselected areas of at least one of the surfaces of at least one of the 
at least two metal workpieces, 
(b) assembling the at least two workpieces into a stack relative to each 
other so that the surfaces are in mating abutment, 
(c) placing the stack in pressing means positioned within a vacuum chamber, 
(d) evacuating the vacuum chamber, 
(e) minimising the pressure applied across the thickness of the at least 
two workpieces, by the pressing means, to allow the flow of volatile 
binders and gases from between the at least two workpieces into the vacuum 
chamber, 
(f) heating the stack while it is within the vacuum chamber to evaporate 
volatile binder from the stop off material, while continuously evacuating 
the vacuum chamber to remove the volatile binder form between the at least 
two workpieces and the vacuum chamber, 
(g) applying heat and increasing the pressure applied across the thickness 
of the at least two workpieces, by the pressing means, to diffusion bond 
the at least two workpieces together in areas other than the preselected 
areas to form an integral structure, 
(h) removing the integral structure form the vacuum chamber, 
(i) heating the integral structure and internally pressurising it to cause 
the preselected areas of at least one of the workpieces to be 
superplastically formed to produce an article of pre-determined shape. 
Preferably the stack is placed between a first selective pressing means and 
a second selective pressing means in the vacuum chamber, the pressure 
applied across the thickness of the at least two workpieces is minimised 
by minimising the pressure applied by the first and second selective 
pressing means, the pressure applied across the thickness of the at least 
two workpieces is increased by increasing the pressure applied by the 
first and second selective pressing means. 
Preferably the first selective pressing means is a first selectively 
pressurisable chamber and the second selective pressing means is a second 
selectively pressurisable chamber. 
Preferably the stack is heated to a temperature between 250.degree. C. and 
350.degree. to evaporate the volatile binder from the stop off material. 
Preferably where the workpieces are made of a titanium alloy, the 
workpieces are heated to a temperature equal to or greater than 
850.degree. C. and the pressure applied is equal to or greater than 
20.times.10.sup.5 Nm.sup.-2 to diffusion bond the workpieces together to 
form an integral structure. 
Preferably the workpieces are heated to a temperature between 900.degree. 
C. and 950.degree. C. and the pressure applied is between 
20.times.10.sup.5 Nm.sup.-2 and 30.times.10.sup.5 Nm.sup.-2. 
Preferably the integral structure is heated to a temperature equal to or 
greater than 850.degree. C. to superplastically form the integral 
structure. 
Preferably the integral structure is heated to a temperature between 
900.degree. C. and 950.degree. C. 
Preferably during the assembling of the at least two workpieces into a 
stack a pipe is arranged to extend from the stack, one end of the pipe is 
adjacent a preselected area of at least one of the surfaces of at least 
one of the at least two metal workpieces. 
The present invention also provides a method of diffusion bonding together 
at least two metal workpieces comprising the steps of 
(a) assembling the at least two metal workpieces into a stack relative to 
each other so that the surfaces of the metal workpieces are in mating 
abutment, 
(b) placing the stack between a first selectively pressurisable chamber and 
a second selectively pressurisable chamber in a vacuum chamber, 
(c) evacuating the vacuum chamber, 
(d) applying heat and increasing the pressure in the first and second 
pressurisable chambers to apply pressure across the thickness of the at 
least two workpieces to diffusion bond the workpieces together. 
Preferably where the workpieces are made of titanium alloy, the workpieces 
are heated to a temperature equal to or greater than 850.degree. C. and 
the pressure applied is equal to or greater than 20.times.10.sup.5 
Nm.sup.-2 to diffusion bond the workpieces together to form an integral 
structure. 
Preferably the workpieces are heated to a temperature between 900.degree. 
C. and 950.degree. C. and the pressure applied is between 
20.times.10.sup.5 Nm.sup.-2 and 30.times.10.sup.5 Nm.sup.-2. 
The present invention also provides a vacuum chamber for use in processing 
at least two metal workpieces for diffusion bonding comprising pump means 
arranged to evacuate the vacuum chamber, heater means arranged to heat a 
stack of at least two metal workpieces placed int eh vacuum chamber, a 
first pressure containing member and a first resilient member define a 
first selectively pressurisable chamber, a second pressure containing 
member and a second resilient member define a second selectively 
pressurisable chamber, the first and second resilient members are spaced 
from, and confront, each other to allow a stack of at least two metal 
workpieces to be positioned between the first and second resilient 
members, means to selectively pressurise the first and second chambers to 
cause the first and second resilient members to apply a diffusion bonding 
pressure on the stack to diffusion bond the mating surfaces of the metal 
workpieces. 
Preferably the means to selectively pressurise the first and second 
chambers is arranged to minimise the pressure in the first and second 
chambers such that the first and second resilient members press the stack 
of workpieces together with minimum pressure to allow the pump to remove 
volatile binders, evaporated by the heater means, from between the at 
least two metal workpieces and the vacuum chamber. 
Preferably the first pressure containing member has ribs to maximise 
rigidity. 
Preferably the ribs extend into the first pressurisable chamber. 
Preferably the second pressure containing member has ribs to increase 
rigidity. 
Preferably the ribs extend into the second pressurisable chamber. 
Preferably the volume of the first pressurisable chamber is minimised to 
minimise damage to the vacuum chamber in the even of a leak from the said 
chamber. 
Preferably the volume of the second pressurisable chamber is minimised to 
minimise damage to the vacuum chamber in the event of a leak from the said 
chamber. 
Preferably the first or second resilient member has a coating to prevent 
adhesion between the said first or second resilient member and any stack 
of at least two metal workpieces. 
Preferably the first or second resilient member is metallic. 
The present invention also provides a vacuum chamber for use in processing 
at least two metal workpieces for diffusion bonding comprising pump means 
arranged to evacuate the vacuum chamber, heater means arranged to heat a 
stack of at least two metal workpieces placed in the vacuum chamber, a 
pressure containing member and resilient member define a selectively 
pressurisable chamber, means to selectively pressurise the pressurisable 
chamber to cause the resilient member to apply a diffusion bonding 
pressure on the stack to diffusion bond the mating surfaces of the metal 
workpieces.

In FIG. 3, three sheets of titanium allow 50, 52 and 54 are assembled into 
a stack 48. 
Prior to assembling the sheets 50, 52 and 54 into the stack 48, the mating 
surfaces 56, 58, 60 and 62 of the sheets 50, 52 and 54 are prepared for 
diffusion bonding by chemical cleaning. One of the mating surfaces 56 and 
58 in this example, mating surface 58, has had a stop off material 
applied, and one of the mating surfaces 60 and 62, in this example mating 
surface 62, has had a stop off material applied. The stop off may comprise 
powdered yttria in a binder and binder e.g. the stop off known as "stopyt 
62A" which is sold by an American company named GTE Service Corporation of 
100 Endicott Street, Danvers, MA01923, USA. Other suitable stop off 
materials may be used. 
The stop off material is applied in desired patterns 64 and 66, shown as 
the shaded areas in FIG. 3, by the silk screen printing process in this 
example, but other suitable methods may be used. The desired patterns 64 
and 66 of stop off material prevent diffusion bonding between preselected 
areas of the sheets 50, 52 and 54. In this example the stop off is applied 
in straight lines, but it may be applied as dots or other suitable 
patterns depending on the particular article to be manufactured. The three 
sheets of titanium alloy 50, 52 and 54 are then assembled into the stack 
48. The sheet 50 has a pair of dowel holes 68 which are axially aligned 
with corresponding dowel holes 70 in sheet 52 and with corresponding dowel 
holes 72 in sheet 54 to ensure the correct positional relationship between 
the three sheets 50, 52 and 54 in the stack 48. The sheets 50, 52 and 54 
are maintained in this positional relationship by a pair of dowels (not 
shown) which are inserted in the axially aligned dowel holes 68, 70 and 
72. 
The stack 48 is then placed in a vacuum chamber, which will be more fully 
described later in this specification. The stack 48 is placed between a 
pair of pressurisable chambers positioned within the vacuum chamber. The 
vacuum chamber is evacuated and the pressure in the pressurisable chambers 
is reduced, preferably they are evacuated, and then the stack 48 is heated 
to a temperature between 250.degree. C. and 350.degree. C. to evaporate 
the binder from the stop off material and which has enabled the stop off 
to be spread through the silk screen. The pressure in the pressurisable 
chambers is reduced, preferably to a vacuum, before the stack is heated so 
that the pressurisable chambers do not apply a load on the stack 48 during 
the baking out of the binder. During the baking out of the binder, the 
vacuum chamber is continuously evacuated to remove the binder from between 
the sheets and from the vacuum chamber. The volatile binders are allowed 
to be removed from between the sheets, throughout the full periphery of 
the stack, along all edges of the sheets, because the pressurisable 
chambers are not clamping the edges of the sheets together. After the 
binder has been removed, which is determined either by monitoring the 
binder levels in the gas extracted from the vacuum chamber or by 
maintaining the vacuum chamber at the temperature between 250.degree. C. 
and 350.degree. C. for a predetermined time the stack 48 is diffusion 
bonded together to produce an integral structure. 
The heating of the un-welded/un-bonded stack of sheets in the continuously 
evacuated vacuum chamber enables the volatile binders to be removed from 
the stack throughout the full periphery of the stack, along all edges of 
the sheets. This allows the volatile binders to be removed much quicker 
than in the known prior art and as quickly as in our UK patent application 
GB9111954.5 and GB9208369.0. This too dispenses with the need to fit a 
pipe to the stack and hence there is a time saving. Furthermore by heating 
the un-welded/un-bonded stack of sheets int eh continuously evacuated 
vacuum chamber, there is no air present in the vacuum chamber to oxidise 
the surfaces of the sheets. 
The temperature in the vacuum chamber is increased such that the stack of 
sheets is heated to a temperature greater than 850.degree. C. Preferably 
the stack of sheets is heated to a temperature between 900.degree. C. and 
950.degree. C. The pressure in the pair of pressurisable chambers is 
increased such that the pressurisable chambers apply a pressure greater 
than 20 atmospheres, 294 lbs per square inch (20.26.times.10.sup.5 
Nm.sup.-2). Preferably the pressure applied by the pressurisable chambers 
is between 294 lbs per square inch (20.26.times.10.sup.5 Nm.sup.-2) and 
441 lbs per square inch (30.39.times.10.sup.5 Nm.sup.-2). The temperature 
and pressure is held constant for a predetermined time. For example if the 
stack of sheets is heated to 925.degree. C. and the pressure applied by 
the pressurisable chambers is 300 lbs per square inch, the temperature and 
pressure are held constant for 2 hours. The temperature and pressure are 
then reduced to ambient, diffusion bonding having been achieved and the 
stack of sheets, which is then an integral structure, is removed from the 
vacuum chamber. 
A pipe is fitted to the integral structure, and argon is introduced into 
the area, within the integral structure, containing the stop off in order 
to break the adhesive grip which the diffusion bonding pressure has 
brought about. The argon is carefully introduced to those areas which 
contain the stop off, and the argon seeps through the stop off and 
eventually reaches the opposing end of the integral structure. The argon 
may initially be caused to travel between one pair of workpieces and on 
reaching the opposite end return to the inlet end between another pair of 
workpieces. In any event, the argon must travel the whole length of the 
interior of the integral structure such as to break the adhesive grip 
between the stop off and the sheets brought about during the diffusion 
bonding step. 
This step is carried out at room temperature because the metal is elastic 
at room temperature and the minimal extension which occurs does not go 
beyond the elastic limit. Consequently, the integral structure regains it 
s shape when pressure is removed at the end of the stop. If this step is 
attempted whilst the integral structure is at the common diffusion bonding 
and superplastic forming temperature, there is a serious risk of 
progressive plastic deformation lengthwise of the integral structure, 
rather than simultaneous deformation over the whole structure. In such 
circumstances, rupturing of the integral structure frequently occurs. 
The integral structure is placed between appropriately shaped split dies, 
and the whole is positioned within an autoclave which is then evacuated so 
as to avoid contamination of the titanium integral structure. 
The dies and integral structure are again heated to a temperature greater 
than 850.degree. C., preferably between 900.degree. C. and 950.degree. C. 
In this example, the dies and integral structure are heated to 925.degree. 
C. Argon is introduced into the interior of the integral structure between 
the adjacent sheets, so as to force the sheets apart in the areas which 
have stop off and to force the parted portions of the outer sheets into 
the respective dies. 
The magnitude of the movement of at least one of the sheets during 
deformation, is such as to require superplastic extension to occur. The 
term "superplastic" is a standard term in the metal forming art and will 
not be described herein. 
In order to achieve superplastic forming without rupturing the thinning 
metal the argon is introduced in a series of constant volume pulses, at a 
pre-calculated rate which will achieve a desired strain rate, as is taught 
at PP 615-623 in the book "The Science, Technology and Applications of 
Titanium" edited by R I Jaffe and N E Promisel, published by Pergamon 
Press in 1970, which is hereby incorporated by reference. The method 
ensures that the metal is subjected to that strain rate which will achieve 
the maximum permissible speed of extension at any given point in the 
procedure. 
On completion of superplastic forming, the inert argon atmosphere and the 
gas pressure within the integral structure is maintained whilst the 
structure is cooled to room temperature. The integral structure is then 
removed from the autoclave and the piping removed. This integral structure 
may be the finished article, or some final machining of the integral 
structure may be required to produce the finished article. 
It may be possible to arrange a pipe in the stack during the stacking of 
the metal workpieces such that the pipe extends from the stack, and one 
end of the pipe is adjacent to the preselected area of one of the 
surfaces, of one of the metal workpieces, which prevents diffusion 
bonding. During the diffusion bonding step, the pipe is diffusion bonded 
into the integral structure and the pipe is subsequently used to introduce 
argon into the integral structure to break the adhesive grip, and 
subsequently to superplastically form the integral structure. 
The advantage of this method over the method disclosed in our prior patent 
application GB9111954.5 and GB9208369.0 is that the requirement for 
welding the edges of the stack of sheets to form a sealed assembly before 
diffusion bonding is obviated, and this reduces the processing time. It is 
also not necessary to move the stack of workpieces between the baking out 
step and the diffusion bonding steps, and hence there is no possibility of 
damage to the stop off whilst it is brittle. 
A vacuum chamber 10, for use in processing the metal sheets for 
superplastic forming and diffusion bonding in the method described above 
is shown in FIGS. 1 and 2. The vacuum chamber 10 comprises a pressure 
vessel 12 which is designed for use with a range of pressures from a 
vacuum to atmospheric pressure. A pump 14 is connected to the interior of 
the vacuum chamber 10 via a pipe 16 in order to evacuate the vacuum 
chamber 10. 
A first pressurisable assembly 18 and a second pressurisable assembly 28 
are located in the vacuum chamber 10. The first pressurisable assembly 18 
comprises a first rigid platen member 20 and a first resilient member 22 
which define a first pressurisable chamber 24. The second pressurisable 
assembly 28 comprises a second rigid platen member 30 and a second 
resilient member 32 which define a second pressurisable chamber 34. The 
first rigid platen member 20 is provided in this example, with a number of 
parallel ribs 26 which extend into the first pressurised chamber 24 and 
which increase the rigidity of the first rigid platen member 20. The 
second rigid platen member 30 is also provided with a number of parallel 
ribs 36 which extend into the second pressurisable chambers 34 and which 
increase the rigidity of the second rigid platen member 30. If the platen 
members 20 and 30 are sufficiently rigid, the ribs may not be required. 
The first and second rigid platen members, 20 and 30 respectively, are 
constructed to withstand the pressures achieved during the diffusion 
bonding step. The first and second rigid platen members 20 and 30 
respectively are arranged to transmit the loads achieved during diffusion 
bonding to a restraining frame (not shown). 
The first and second resilient members 22 and 32 respectively, are 
preferably thin membranes of metal which are coated to prevent adhesion to 
the sheets 50, 52 and 54 of the stack 48. The first and second resilient 
members 22 and 32 are preformed to fit, approximately, the exterior shape 
of the outer workpieces 50 and 54 of the stack 48. 
The periphery of each of the first and second resilient members is arranged 
to overlap onto the solid, rigid, edges of the first and second rigid 
platen members respectively. 
The first and second pressurisable assemblies 18 and 28 respectively are 
arranged so that the first and second resilient members 22 and 32 
respectively are spaced from each other and confront each other. The stack 
48, of sheets 50, 52 and 54 is locatable between the first and second 
resilient members 22 and 32 respectively. 
Heating devices 46 are provided within the vacuum chamber 10 to heat the 
stack 48 of sheets 50, 52 and 54, before the sheets 50, 52 and 54 are 
diffusion bonded together, to evaporate the binder from the stop off 
applied to the surfaces of the sheets. The heating devices in this example 
are radiant heaters and reflectors, but other suitable heaters may be 
used. 
A first pressurising pump 38 is connected to the first pressurisable 
chamber 24 via a pipe 40, and a second pressurising pump 42 is connected 
to the second pressurisable chamber 34 via a pipe 44. Alternatively a 
single pressurising pump may be connected to both first and second 
pressurisable chambers 24 and 34. 
The first and second pressuring pumps 38 and 42 are arranged to reduce the 
pressure in, or evacuate, the first and second pressurisable chambers 24 
and 34 respectively before the stack 48 is heated so that the edges of the 
sheets 50, 52 and 54 are not clamped together, to allow the volatile 
binder to be removed from between the sheets 50, 52 and 54. 
The first and second pressurising pumps 38 and 42 are arranged to increase 
the pressure in the first and second pressurisable chambers 24 and 34 
respectively during the diffusion bonding step so that the first and 
second resilient members 22 and 32 respectively exert the diffusion 
bonding pressure on the stack 48 of sheets 50, 52 and 54. The first and 
second resilient members 22 and 32 exert pressure across all the area of 
the stack 48 to be diffusion bonded. 
The first and second pressurising pumps 38 and 42 are arranged to increase 
the pressure in the first and second pressurisable chambers 24 and 34 
respectively so that a pressure of between 294 lbs per square inch and 441 
lbs per square inch is exerted on the stack 48. 
The first and second pressurisable assemblies 18 and 28 are designed such 
that the volumes of the first and second pressurisable chambers 24 and 34 
respectively are minimised, so that the effects of any loss of pressure 
into the vacuum chamber 10 is minimised, there is a minimised possibility 
of rupturing the pressure vessel 12. 
The use of the first and second pressurisable assemblies allows the 
diffusion bonding process to be carried out in a vacuum furnace which has 
a pressure vessel designed for use with a range of pressures from a vacuum 
to atmospheric pressure rather than a HIP furnace which has a pressure 
vessel designed for use with a range of pressures from a vacuum to 441 lbs 
per square inch or more. The vacuum chamber is of simpler construction 
because the requirement for an electron beam welding gun is obviated. 
The method of diffusion bonding and superplastic forming described is best 
suited for the diffusion bonding of a flat stack of workpieces which are 
subsequently subjected to other processes, such as machining and/or 
twisting, before superplastic forming into the final form. The final form 
may be a finished article or some minor machining may be required to 
produce the finished article. The method provides a lower production cost 
method for diffusion bonding. It is also applicable to stacks of 
workpieces which have projections on the workpieces. In particular the 
first and second resilient members are preformed to fit, approximately, 
the exterior shape of the outer workpieces of the stack. For example the 
resilient members may be preformed to fit root shapes on the surface of 
the outer workpieces of the stack, which eventually form a fan blade. The 
resilient members may be preformed to fit clappers/snubbers preformed on 
the surface of the outer workpieces of the stack, which eventually form a 
fan blade. 
Although the description has referred to titanium sheets or titanium 
workpieces the present invention is equally applicable to workpieces of 
other elementary metals, metal alloys and metal matrix composites which 
are diffusion bondable and one of the workpieces must be capable of 
superplastic extension. Aluminum and stainless steel are capable of 
superplastic extension at suitable temperatures and pressures. 
The method is suitable for manufacturing heat exchangers, components for 
turbomachines, for example fan blades, fan duct outlet guide vanes etc. 
Although the description has referred to a stack of three metal sheets it 
is possible to use stacks comprising two metal sheets or stacks comprising 
four or more metal sheets depending upon the particular article to be 
manufactured.