First and second superalloy workpieces are inertia welded together by rotating the first workpiece to an initial contact speed greater than about 750 surface feet per minute, and frictionally engaging under a weld load the first and second workpieces to effect an inertia weld therebetween.

BACKGROUND OF THE INVENTION 
The present invention relates generally to inertia welding, and, more 
specifically, to inertia welding of superalloys. 
Superalloys have been developed for use in aircraft gas turbine engines in 
order to withstand the hostile, high temperature environment therein while 
enjoying a suitable useful life during operation. A typical superalloy for 
turbine engines is high strength, heat resistant, nickel-based alloy known 
under various commercial designations including Inconel, Waspaloy, 
Hastelloy, and Rene in various alphanumeric designations. These 
superalloys also have corresponding industry designations under the AMS 
specifications. 
These superalloys have specific microstructures associated with effecting 
substantially high material strength at the high, elevated temperatures in 
gas turbine engines subject to the hot combustion gas flow therein. Both 
rotor and stator components in the engine are subject to the hot 
combustion gases under heavy loading and high levels of stress during 
engine operation. The use of superalloys in these parts accommodates the 
hostile environment for effecting a suitable useful life thereof. 
Relatively large superalloy components in an engine are typically stator 
components including for example, combustor, turbine, and compressor 
structural casings and pressure vessels which may have outer diameters in 
the exemplary range of 10-80 inches. Rotor components, such as the blades 
and supporting disks, are correspondingly smaller in diameter. 
The rotor and stator superalloy components must be manufactured for the 
specific size and configurations thereof while maintaining the integrity 
of the superalloy material itself without introducing defects or loss of 
strength therein. 
Furthermore, various components of the engine must be fixedly joined 
together, such as by welding. Typical forms of welding locally melt the 
parent material and are not useful for welding superalloys in view of the 
attendant change in microstructure thereof which significantly reduces 
their high temperature strength capability. Superalloy components are 
therefore typically welded using inertia welding in which the parent 
material is not melted. 
In inertia welding, a first workpiece is rotated to a specific speed and 
then a second workpiece is forced into frictional engagement with the 
first workpiece with frictional heat being generated to weld together the 
two components without melting in the contact region. Inertia welding is a 
forging process which requires elevated forging temperatures for the 
specific material. An upper forging temperature is typically the melting 
temperature for the material which must not be reached in inertia welding 
superalloys in view of the resulting change in microstructure thereof. A 
lower forging temperature is the minimum temperature at which an inertia 
weld will in fact be effected. 
Many conventional materials have wide forging temperature ranges and are 
readily inertia weldable using conventional inertia welding machines. 
However, the available inertia welding range for superalloys is relatively 
small, for example about 200.degree. F. or less for nickel-based 
superalloys, which presents a critical problem in welding superalloys 
since unless the inertia welding is precisely effected, resulting damage 
to the welded components results rendering them useless. Since superalloy 
engine components are substantially expensive, the failure to properly 
inertia weld even one component is quite costly. 
Accordingly, less expensive, and relatively small engine rotor components 
have been successfully inertia welded after the specific process 
parameters have been determined therefor in qualification testing with a 
corresponding expense. 
A typical inertia welding machine includes first and second opposed heads 
to which the first and second workpieces may be fixedly attached in 
opposition to each other. The first head is rotatable and is powered by a 
suitable motor for rotating the head and first workpiece to a precise 
rotational speed. The second head is non-rotatable and simply supports the 
second workpiece. 
The first head includes one or more flywheels to provide the rotary inertia 
for effecting welding of the two workpieces. The second head is axially 
translatable by a powered piston which engages together the first and 
second workpieces under a substantial compressive weld load. The second 
workpiece therefore frictionally engages and brakes the rotating first 
workpiece creating friction heating at the contact area therebetween which 
raises the temperature thereof to effect an inertia weld without melting. 
There are only four control variables in inertia welding. These include the 
workpiece geometry such as size and configuration; the applied weld load 
and corresponding weld stress at the contact area of the two engaged 
workpieces; the initial contact speed of the two workpieces typically 
represented as the surface velocity at the contact area which is based on 
the rotary speed and radius at the contact area; and, lastly, the unit 
energy input at the contact area based on the mass moment of inertia of 
the flywheel typically represented by a flywheel function which is the 
product of the flywheel weight and the square of the radius of gyration. 
The precise inertia welding process parameters for various high strength 
turbine alloys have been developed over years at substantial cost. Since 
turbine rotor components are critical to overall engine performance, 
reliability, and life, absolute compliance with proven process parameters 
is required to ensure effective inertia welding without incipient melting 
which would alter the required microstructure of the materials and 
correspondingly decrease their strength rendering them unusable in the 
engine. For example, typical critical rotating engine components include 
fan, compressor, turbine, and shaft components formed of nickel-based 
superalloys such as Inconel 718, Rene 95, and Rene 88. 
The historically proven process parameters for these superalloys include a 
welding stress within the range of 25,000-70,000 psi; a unit energy input 
within the range of about 25,000-90,000 ft.-lb/sq. in.; and an initial 
contact speed measured in surface feet per minute in the preferred range 
of about 400-550 SFM, and not exceeding 750 SFM to prevent weld defects 
including incipient melting. 
A typical inertia welding machine is limited in size, and therefore cannot 
accommodate many of the large components found in gas turbine engines. The 
operational size limit may relate to one or more of several process 
parameters such as energy (including flywheel limitations), physical 
external dimensions of the workpieces being welded, cross sectional area 
of the welded surfaces, machine speed (in terms of rotational speed and/or 
surface velocity), contact pressure, and others. 
Accordingly, inertia welders are typically used for welding the relatively 
smaller rotor components as opposed to the larger stator components. The 
stator components must, therefore, be otherwise joined together which is 
typically effected using other manufacturing processes such as large 
diameter one-piece investment castings; seamless one-piece large diameter 
rolled forgings; or fabrications using other types of welding processes. 
However, these manufacturing processes have one or more disadvantages when 
used to produce large stator components of a gas turbine engine including 
time consumption; expense; higher defect levels; and difficulty in 
precisely controlling the inertia welding parameters. Furthermore, some 
high strength materials for large turbine stator components cannot be 
fabricated with conventional processes. For example, the nickel-based 
superalloy known as Waspaloy cannot be investment cast, and is not easily 
weldable to the levels of quality and integrity required for gas turbine 
use. 
Accordingly, it is desired to provide an improved inertia welding process 
for the fabrication of large, superalloy stator components of a gas 
turbine engine using commercially available inertia welding equipment. 
BRIEF SUMMARY OF THE INVENTION 
First and second superalloy workpieces are inertia welded together by 
rotating the first workpiece to an initial contact speed greater than 
about 750 surface feet per minute, and frictionally engaging under a weld 
load the first and second workpieces to effect an inertia weld 
therebetween.

DETAILED DESCRIPTION OF THE INVENTION 
Illustrated in FIG. 1 is an inertia welding machine 10 for inertia welding 
together first and second workpieces or parts 12, 14 which may have any 
suitable configuration. In the exemplary embodiment illustrated, the 
workpieces are annular members which are inertia welded together for use 
as combustor, turbine, or compressor structural casings or pressure 
vessels in an aircraft gas turbine engine. The workpieces are coaxially 
aligned with each other and have opposing weld preps or contact areas 16 
at which inertia welding is effected. The weld preps 16 have an average 
diameter D and a weld prep thickness T. 
The machine 10 includes a first rotary head 18 to which is suitably fixedly 
attached the first workpiece 12, and a second head 20 to which the second 
workpiece 14 is suitably fixedly attached. The first head 18 is 
operatively joined to a suitable motor 22, such as a hydraulic motor, for 
being rotated during operation at a suitable rotary speed expressed in 
revolutions per minute (RPM). Suitably attached to the first head are one 
or more annular flywheels 24 which are selectively used for controlling 
the rotary mass moment of inertia of the first head for effecting inertia 
welding energy. 
The motor and first head are suitably attached to a supporting frame 26 at 
one end thereof, and the second head 20 is carried by a suitable truck or 
carriage 28 on the opposite end of the frame 26. The second head 20 is not 
rotatable on the truck 28, and the truck 28 is operatively joined to a 
hydraulic piston 30 which is configured for translating the truck 28 
horizontally atop the frame 26 for engaging the first and second 
workpieces at the weld preps 16 under a specific weld load or force F, 
expressed in pounds for example. 
The inertia welding machine 10 illustrated in FIG. 1 is conventional in 
configuration and basic operation, and in an exemplary embodiment is 
commercially available from the Manufacturing Technology Inc. (MTI) 
company, of South Bend, Ind., under Model 800. 
In inertia welding, the first head 18 and attached first workpiece 12 are 
accelerated to a predetermined rotary speed, and then the piston 30 is 
actuated to drive the truck 28 and attached second workpiece 14 in 
frictional engagement with the first part at the weld prep 16 under a 
predetermined weld load F. Upon application of the weld load, the motor 22 
is disconnected from the first head 18, which in the case of a hydraulic 
motor is accomplished by simply interrupting the hydraulic pressure, and 
the inertia of the flywheels 24 imparts energy at the engaging weld preps 
16 which undergo frictional heating as the second workpiece 14 
frictionally brakes the rotating first workpiece 12. 
The friction generated at the weld preps 16 locally increases the 
temperature of the two workpieces to a temperature below the melting 
temperature of the workpieces, yet sufficiently high for effecting a 
forged, friction weld 32 therebetween as shown in the exemplary flowchart 
of FIG. 2. 
FIG. 2 also shows the basic process steps in accordance with a preferred 
embodiment of the present invention for effecting the inertia welding of 
the two workpieces using the machine 10 illustrated in FIG. 1. 
There are four control parameters in effecting an inertia weld. 
Fundamentally, the geometry, including size and configuration, of the 
workpieces 12, 14 is one parameter since the inertia weld is effected at 
the corresponding annular weld preps 16. As indicated above, part size is 
a factor both for being physically supported in a given type of inertia 
welding machine, and for the amount of energy required to effect the 
inertia welding thereof which is dependent on available flywheel inertia. 
A second control parameter is the weld load F which is exerted over the 
weld preps 16 to effect a corresponding weld stress or load per unit area 
in compression, typically represented in pounds per square inch (psi). 
A third control parameter is the unit energy input effected at the weld 
preps for inertia welding thereat which is typically represented by 
foot-pounds over the contact area in square inches. The unit energy is 
effected by the mass moment of inertia of the rotating first head 18 and 
the attached flywheels 24 which may be adjusted in a finite number of 
increments by the additional or subtraction of individual flywheels 24. 
And, the fourth control parameter is the initial contact speed of the 
rotating first workpiece 12 at its weld prep 16 typically expressed in 
surface feet per minute (SFM) which is the product of the circumferential 
length at the weld prep 16 and the rotary speed expressed in RPM. 
In the exemplary embodiment illustrated in FIGS. 1 and 2, the two 
workpieces 12, 14 are formed of a high strength, heat resistant superalloy 
material for use in various hot section components of an aircraft gas 
turbine engine. A typical turbine superalloy is nickel-based and has an 
exemplary forging temperature range of about 200.degree. F. below the 
melting temperature thereof. 
For example, the superalloy material of the workpieces 12, 14 may be 
nickel-based and include those commercially available under the trademarks 
Inconel, Waspaloy, Hastelloy, and Rene which have various alloy 
designations such as Inconel 718, Rene 95, and Rene 88, all of which have 
corresponding AMS specifications which are conventionally known. 
As indicated above, the inertia welding machine 10 has been commercially 
used in this country over many years for inertia welding relatively small 
gas turbine engine rotor components formed of nickel-based superalloys. 
However, inertia welding of larger gas turbine engine hot section stator 
components formed of nickel-based superalloys has not been possible 
because of the operational size limit of the inertia welding machine for 
imparting sufficient energy to effect a suitable inertia weld. The design 
and manufacture of larger inertia welding machines to overcome the 
operational size limit is currently constrained by various technological 
limitations as a result of which the present invention was made. 
For example, the present invention now allows the use of a conventional 
inertia welding machine for inertia welding relatively large gas turbine 
engine hot section stator components such as the two workpieces 12, 14 
made from nickel-based superalloys. The stator components represented by 
the workpieces 12, 14 have an average diameter D at the weld preps in a 
range from about 10 inches to about 80 inches. 
In one example, the workpieces 12, 14 are portions of either a combustor 
casing or low pressure turbine casing which are joined together by inertia 
welding in accordance with the present invention. In an exemplary 
combustor casing size, the workpieces have an average diameter at the weld 
preps of about 42 inches, and the low pressure turbine casing example has 
an average diameter at the weld preps of about 72 inches. The specific 
nickel-based superalloy for these exemplary casings is Waspaloy. 
In view of the relatively large diameter of the stator components, a 
suitably large inertia welding machine is required for the inertia welding 
thereof. The Model 800 inertia welder identified above has the physical 
size capability, but lacks sufficient flywheel weight to effect inertia 
welding under the historically proven process parameters. Since the large 
stator workpieces have a different configuration and greater energy 
requirement than those previously welded in the machine, corresponding 
inertia welding parameters are required. Although the size of the 
components is now given, the remaining three inertia welding parameters, 
including the weld load, initial contact speed, and unit energy input, 
must be determined. 
The weld load is independent of the other two parameters and is established 
by calculating the contact area at the beginning of the weld cycle. For 
the larger of the two stator components, i.e., the low pressure turbine 
casing having an average diameter at the weld prep 16 of about 72 inches, 
and a weld prep thickness T of about 300 mils, the contact area is 
conventionally determined to be 67.9 square inches (sq. in.). 
The historically proven weld stress for nickel-based superalloys previously 
used for turbine rotors is in the range of about 25,000-70,000 psi. The 
weld stress varies, based on experience, as a function of the wall 
thickness and the particular superalloy being welded. 
Assuming an acceptable weld stress at the minimum of the range, a required 
weld load of 1,698,000 lbs. is calculated as the product of the stress and 
contact area. The Model 800 inertia welding machine has a weld load 
capability of 4,500,000 lbs., with the calculated weld load being well 
within this capability. 
The initial contact speed and unit energy input are functions of the 
specific alloy being welded and the weld prep contact area as historically 
proven. They are related as follows: 
EQU E=WK.sup.2 RPM.sup.2 /5873A Eq.(1) 
EQU RPM=12SFM/.pi.D Eq.(2) 
Where E is the unit energy input measured in ft.-lb./sq. in.; and WK.sup.2 
represents a flywheel function or parameter, with W being the flywheel 
weight and K being the radius of gyration. The initial contact rotary 
speed is measured in revolutions per minute (RPM); the initial 
circumferential contact speed is measured in surface feet per minute (SFM) 
at the average diameter D; and the weld prep contact area A has been 
determined above. 
Historically, the initial contact speed and unit energy input have 
controlled the size of the parts which could be processed using inertia 
welding. For superalloys, the unit energy input is limited to the 
conventional range of about 25,000 to about 90,000 ft.-lb./sq. in. 
Furthermore, the initial contact speed SFM for superalloys has been limited 
to the conventional range of about 400-550 SFM and has not exceeded 750 
SFM to prevent poor quality inertia welds due, for example, to microscopic 
incipient melting along the grain boundaries of the superalloy in the weld 
joint. 
Since the inertia welding of larger workpieces necessarily requires a 
corresponding increase in energy for effecting the inertia weld, energy 
may be increased by increasing the initial contact speed and the flywheel 
moment of inertia. However, this cannot be done randomly in view of the 
interrelationship of the inertia welding control parameters since an 
undesirable inertia weld permanently damages the workpieces and wastes 
substantial money. 
Equation (1) may be rearranged for determining the required flywheel size 
as follows: 
EQU WK.sup.2 =E5873A/RPM.sup.2 Eq.(3) 
For the specific example above using the maximum initial contact speed of 
750 SFM and an exemplary unit energy input of 50,000 ft.-lb./sq. in., 
Equation (3) requires 12,587,000 lb.-ft..sup.2. The maximum flywheel 
parameter or inertia available from the Model 800 machine is 1,000,000 for 
the basic machine and 2,000,000 for a specially configured larger version 
thereof. In either case, the machines do not have the capability to 
provide a sufficient amount of flywheel inertia for effecting inertia 
welding under conventional practice. By adjusting the various parameters 
in Equation (3) in accordance with conventional practice, the flywheel 
inertia may be reduced, yet is still substantially greater than the 
capability of the Model 800 machine. 
In accordance with the present invention, the energy Equation (1) may be 
rearranged as follows: 
EQU RPM.sup.2 =E5873A/WK.sup.2 Eq.(4) 
And, the rotary speed Equation (2) may be arranged as follows: 
EQU SFM=RPM.pi.D/12 Eq.(5) 
For an exemplary unit energy input E of 50,000 ft-lb/sq. in. and the 
contact area of 67.9 sq. in. of the above example, a required rotary speed 
from Equation (4) of 99.8 RPM is calculated which is within the 
operational limit of the specific welding machine with all the flywheels 
24 installed thereon for obtaining the maximum flywheel inertia of 
2,000,000 WK.sup.2. Correspondingly, the initial contact speed Equation 
(5) requires 1880 SFM, which is substantially greater than the 400-750 SFM 
range according to conventional practice. 
In another configuration of the workpieces 12, 14 having an increased 
thickness T of about 500 mils, and using the minimum unit energy input and 
maximum flywheel inertia, a rotary speed of about 128.9 RPM may be 
calculated, which corresponds with an initial contact speed of 2400 SFM, 
which is even greater yet than the conventional limit. 
In accordance with the present invention based on the insight obtained from 
Equations (4) and (5), and confirmed by sub-scale development testing, an 
improved method for inertia welding together the first and second 
superalloy workpieces 12, 14 has been discovered in which the first 
workpiece 12 may be rotated to an initial contact speed greater than about 
750 SFM, and then the second workpiece 14 frictionally engages the rotated 
first workpiece under a weld load to effect the inertia weld 32 
therebetween. This superspeed or supraspeed inertia welding of superalloys 
is possible by correspondingly adjusting the flywheel parameter. 
This discovery extends the inertia welding operating parameters in a new 
combination for now allowing inertia welding of substantially larger 
superalloy workpieces, not heretofore possible, without incipient melting 
or other weld defects. The conventional welding machine 10 may be used for 
imparting a unit energy input at the inertia weld in the proven range of 
about 25,000 to about 90,000 ft.-lb./sq. in., and a weld load F within the 
capability of the conventional machine may be used as defined by the 
product of the contact area of the workpieces at the weld preps 16 and a 
weld stress within the proven range of about 25,000 to about 70,000 psi. 
The first workpiece 12 is rotated to an initial contact speed SFM which is 
a function of a product of the unit energy input E and the contact area A 
divided by the flywheel parameter WK.sup.2 as represented by Equation (4) 
including the constant 5873 therein. 
Sub-scale development testing of the improved inertia welding process has 
demonstrated that the initial contact speed may be as high as about 3000 
SFM for effecting acceptable welds in nickel-based superalloys. This 
significant improvement in inertia welding of superalloys now allows the 
inertia welding of large gas turbine engine stator components which was 
previously not possible in a conventional inertia welding machine. Turbine 
hot section components having average diameters at the weld preps 16 
ranging from 10 inches to as high as about 80 inches may now be inertia 
welded with success. 
The improved inertia welding process extends the capability of conventional 
inertia welding machines beyond the flywheel inertia capabilities thereof 
by instead preferentially increasing the initial contact speed for 
effecting the required unit energy input at the inertia weld. The improved 
process is accurately controlled to consistently produce high quality and 
high integrity inertia welds in superalloys not before possible. The 
improved process may now be applied to large superalloy turbine stator 
components for providing a substantial reduction in cost and manufacturing 
cycle time therefor. 
While there have been described herein what are considered to be preferred 
and exemplary embodiments of the present invention, other modifications of 
the invention shall be apparent to those skilled in the art from the 
teachings herein, and it is, therefore, desired to be secured in the 
appended claims all such modifications as fall within the true spirit and 
scope of the invention.