Constant energy rate forming

An improved method for hot working materials previously considered to be difficult or impossible to hot work is described. The method consists of varying the deformation conditions in order to achieve an essentially constant energy input, to the material being worked, on a time basis.

DESCRIPTION 
Technical Field 
This invention relates to methods for hot working difficult to work 
metallic materials. 
Background Art 
The art of hot working materials is an ancient one. Only in recent years 
has the demand for high performance articles led to substantial studies of 
hot deformation processes and resultant improvements. Any hot deformation 
process may be described as deformation or strain as a result of applied 
load or stress. Hot working processes in which the deformation is 
preformed at a constant rate are well-known. In like fashion, processes in 
which the deformation rate is varied in order to produce a constant true 
strain rate (by taking into account the change in workpiece geometry) are 
known. 
It is also known in the prior art to obtain improved forging or hot working 
results through the use of material pretreatment processes to produce 
enhanced ductility as described in U.S. Pat. No. 3,519,503. The present 
invention has obtained enhanced hot workability without the requirement of 
a pretreatment process. 
Disclosure of Invention 
It is an object of this invention to provide a process for the hot working 
of materials under conditions of constant energy input. 
Yet another object is to provide a hot working process which can form 
materials without cracking and failure encountered in these materials in 
the process of the prior art. 
Another object of the invention is to achieve equivalent deformation in the 
materials to those obtained in the prior art but at lower stress levels 
and without time penalties. 
These objects are achieved by hot deforming metallic materials under 
conditions of constant energy input. Measurements of stress and strain are 
made on a regular basis during the hot deformation and from these 
measurements the area under the stress strain curve (which is proportional 
to the energy input to the workpiece) is calculated. The subsequent 
portion of the deformation process is controlled based on the prior 
history and is maintained at a constant energy input rate. Use of a 
constant energy input rate alleviates cracking in hard to work materials. 
Other features and advantages will be apparent from the specification and 
claims and from the accompanying drawings which illustrate an embodiment 
of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
The present invention relates to the hot working or hot deformation of 
various hard to work materials. Hot working is performed at an elevated 
temperature, a temperature usually above the recrystallization temperature 
of the material. Thus, recrystallization occurs continuously as the 
process proceeds so that workhardening does not occur to any significant 
extent. The process of the invention has particular utility with respect 
to nickel base superalloys. Such alloys contains a substantial (20-70%) 
volume fraction of the gamma prime phase distributed as particles in the 
gamma prime matrix. The presence of such a large amount of the second 
phase inhibits recrystallization and the recrystallization temperature is 
usually found to be the same as the gamma prime solvus temperature which 
is the temperature above which the gamma prime phase goes into solution in 
the matrix. 
Consistent with the performance of the process at an elevated temperature, 
it is necessary that hot dies be used in order to eliminate significant 
cooling of the workpiece. While the die temperature need not be exactly 
that of the workpiece, it should be reasonably close, i.e., within 100 and 
preferrably within 50.degree. F. of the sample temperature. It is also 
desirable to perform the process in a nonoxidizing atmosphere in order to 
eliminate oxidation which can have a detrimental effect on the success of 
the process. 
The invention may be broadly understood through consideration of FIG. 1 
which is representative of a stress-strain curve for metallic materials. 
It is comprised of an initial steep linear portion (the elastic portion) 
followed by a subsequent portion of diminished slope and linearity (the 
plastic portion). In the past many material forming operations have been 
performed at a constant displacement rate or strain rate. Both constant 
engineering strain rate and constant true strain rate processing have been 
utilized. Constant engineering strain rate processing is also referred to 
as constant displacement rate processing. 
The essence of the present invention is the performance of hot working 
operations under isothermal conditions and in a manner such that, as the 
stress-strain condition of the material moves along the curve the area 
swept out under the curve is essentially constant with respect to time. 
That is, A.sub.n =A.sub.n+1. As is known to those skilled in the art, the 
area under the stress strain curve is reflective to the amount of energy 
imparted to the material. Thus the present invention comprises the hot 
working of materials in such a way that the energy input to the workpiece 
is essentially constant with respect to time. 
Through the present invention, it is possible to hot work materials which 
previously could not be consistently hot worked. These materials may be 
generally described as the superalloys (Fe, Ni and Co based) titanium 
alloys, tool steels and refractory metals. The method of the present 
invention is also generally applicable to all types of hot working 
processes including those which are tensile, compressive, shear and 
torsion based. The method has particular applicability to the compressive 
hot working ("upset" forming) of superalloys and will be explained with 
reference thereto. The method is an isothermal one, performed using dies 
heated to essentially the working temperature of the alloy and a preform 
or billet heated essentially the same temperature. 
FIG. 2 shows, in schematic form, stress-strain curves plotted using 
different parameters. The symbol .tau. denotes the true stress which is 
determined by dividing the force applied by the instantaneous cross 
sectional area of the workpiece. The symbol .sigma. denotes the 
engineering stress which is determined by dividing the applied stress by 
the initial area of the specimen. At low levels of deformation true stress 
and engineering stress are essentially equivalent because the sample area 
changes only slightly. In a similar manner, .epsilon. denotes the true 
strain which is the integral of dl/l and is equivalent to ln (l/l.sub.o) 
and e is the engineering strain which is calculated based on the change in 
length divided by the original length. FIG. 2 shows the curves resulting 
from the plotting of the same data in different forms. The curve marked 
E=K' represents the invention where E, the energy input rate is held 
constant. 
The result of the present invention is that a given amount of strain can be 
imparted at a lower average and lower maximum stress level than those 
levels required in the prior art. Intuitively, it can be appreciated that 
a material will likely crack upon exceeding a particular applied stress 
and a process able to achieve the same strain at a lower maximum stress 
level will likely alleviate cracking. It may be true that one could 
achieve the same result by reducing the strain rate to a very low value 
and in fact this process is employed in certain circumstances and is known 
as creep forming. For various technical reasons, such a process may not 
always produce a desirable result and in any event such a process takes an 
excessive amount of time. The present invention produces the desired 
deformation in a short period of time without cracking. 
While the inventive process can be performed in a variety of ways, these 
methods all rely on the use of the prior stress-strain-time history of the 
workpiece to predicting what the future stress-strain-time history should 
be to arrive at the desired constant energy input rate. However, with 
reference to FIG. 1, it is apparent that while A.sub.3 can be used to 
predict A.sub.4 or A.sub.5, A.sub.1 is not predictive of A.sub.2 because 
of the substantial change in the stress-strain curve shape which occurs 
during A.sub.1. For this reason, the initial part (i.e., about 0.2%-10% 
strain) of the invention process is performed under conditions of constant 
strain rate (either engineering strain rate or true strain rate). However, 
FIG. 2 illustrates that this is not a serious compromise because at low 
strain values, all the curves coincide. 
A more detailed explanation of the invention may be derived from FIG. 3 and 
the discussion which follows. FIG. 3 shows a part of the stress strain 
curve, which sets forth the stress and strain in the material during the 
deformation process. It will be appreciated that a time scale may also be 
marked on the curve. With specific reference to FIG. 3, the stress and 
strain conditions .sigma. and e are shown over different time intervals t. 
Thus for example over time t.sub.i, the stress in the material increases, 
from .sigma..sub.i to .sigma..sub.i+1 while the strain increases from 
e.sub.i to e.sub.i+1 thus the area under the curve can be calculated to a 
sufficient degree of accuracy as: 
##EQU1## 
This area can be computed only at the completion of time interval t.sub.i 
and cannot therefore be used to modify the immediately subsequent time 
interval t.sub.i+1. Instead, the information derived during t.sub.i is 
used to vary the deformation rate during t.sub.i+2 so that the area 
A.sub.i+2 =A.sub.i. 
It will be appreciated that only variable in the process is the 
displacement rate D of the ram or die which is deforming the material. 
Consideration of the righthand portion of the FIG. 3 shows that varying 
the displacement rate D produces two effects. The most obvious effect is 
to decrease the amount of strain achieved in a particular time interval. 
This is depicted in the shifting of the vertical lines in the figure. 
However, it is also known that changing the strain rate of deformation 
changes the stress or resistance to deformation of the material. The 
strain rate sensitivity is denoted as m which is derived as follows: 
##EQU2## 
m is a material property which varys only slightly with temperature and 
strain. For the case of fine grain (.about.10.sup.2 micron grain size) 
nickel base superalloys m is about 0.6.+-.0.1, while the coarser grain 
size nickel base superalloys (.about.10.sup.3 micron grain size), m is 
about 0.25.+-.0.1. The relationship between D and e is known and thus 
knowing D and m can be predicted. Using a digital computer then, for the 
time period t.sub.i+2, D can be varied such at A.sub.i+2 =A.sub.i, thus 
resulting in a constant energy input rate to the workpiece. 
We believe that substantial benefits will result if the energy input is 
held constant within .+-.10% of a nominal value and preferrably within 
.+-.5% of a nominal value. 
It is well-known that for a stress-strain curve such as those shown in FIG. 
1, if a material is loaded into the plastic region and then unloaded, upon 
unloading the material condition will return to a zero stress condition 
along a line parallel to the original elestic portion of the curve. In 
other words even when a material has been severely plastically deformed it 
still contains a predictable amount of recoverable elastic energy. We 
believe that in the present invention the constant energy input should, in 
theory, actually be a constant plastic energy input. This would mean that 
the dotted lines separating the various areas in FIG. 1 should actually be 
sloped towards the origin parallel to the initial elastic portion of the 
stress strain curve. However, this is of only theoretical importance since 
for any reasonable amount of total strain the elastic component will be 
insignificant. 
FIGS. 4A, 4B and 4C show three alternative pieces of apparatus for working 
metal according to the present invention. FIG. 4A shows a hot working 
apparatus consisting of a base and frame member 1 having a cavity 2 
therein which contains workpiece 3. Moveable ram 4 is arranged so that it 
can deform the workpiece upon application of force F provided by actuating 
means. The actuating means may be for example, hydraulic means or any 
mechanical means. In FIG. 4A the ram assembly 4 has attached thereto load 
sensing means, for example, a strain gage for sensing the applied load, 
and displacement sensing means 6 which may be, for example, a linear 
voltage displacement transformer, for sensing the position of the ram 4. 
The output from the strain sensing means and the displacement sensing 
means are proportional to the force applied and displacement of the ram. 
Such signals may be combined in an integrating controller 7 which produces 
an output signal which is representative of the area under stress strain 
curve. This signal is then used to control the subsequent motion of the 
actuator. In operation the actuator is programmed initially to follow a 
path of constant strain rate deformation (it will be recalled that the 
difference between constant strain rate deformation (whether engineering 
or true strain rate) and constant energy rate deformation is initially 
insignificant, for example for strain levels less than about 10%). The 
integrating controller 7 thereafter operates on a predetermined schedule, 
i.e., constant time period or constant strain period and controls 
subsequent ram displacement based on the prior energy input history to 
achieve the desired subsequent constant energy input rate. The apparatus 
described in FIG. 4A is the most precise and flexible apparatus for 
performing the invention process, however particularly for the repetitive 
or production forming of parts, a less complex apparatus may be used such 
as that in FIG. 4B. In FIG. 4B the mechanical aspects of the apparatus are 
similar to that of FIG. 4A, however the signal for the actuator is 
provided by a preprogrammed controller. The information necessary to 
program the controller could be derived from the apparatus shown in FIG. 
4A. The program might run strictly as a function of time or it could 
alternatively operate in response to a displacement signal from a 
displacement sensing means 6 such as that previously described. FIG. 4C is 
yet another alternative. Again the mechanical apparatus are essentially 
unchanged. In FIG. 4C the principle of operation is to directly monitor 
the energy input to be constant with respect to time. The implementation 
of this embodiment is somewhat more complex however, and requires that the 
power input to run the apparatus without a workpiece be known and this 
power input be subtracted from the actual observed input to provide a 
useful indication of the energy actually used to deform the workpiece. 
EXAMPLE 1 
Specimens of a modified IN 100 composition (nominal composition 12% Cr, 18% 
Co, 3.2% Mo, 4.3% Ti, 5% Al, 0.8% V balance nickel) were produced from a 
billet which had been produced from powder by hot isostatic pressing. The 
specimens were rectangular parallelepipeds with a 2:1 aspect ratio and a 
square cross section. These specimens were reduced 60% in height at 
several temperatures. 
Deformation was accomplished in a hydraulic testing apparatus controlled by 
a programmable controller. Heated ceramic dies were used and a vacuum 
atmosphere was maintained. The controller was programmed to operate in 
accordance with the following scheme: 
a. An initial displacement schedule that would achieve a condition of 
essentially contant strain rate deformation was calulated. A strain rate 
of 0.17%/sec or 10%/min was selected. 
b. Forming was performed according to this schedule until 1% strain was 
achieved. Subsequent deformation was performed in increments of constant 
strain (2% strain per increment), and the time to achieve the constant 
strain increment was the variable. Each time interval was computed as 
follows: 
##EQU3## 
where P.sub.i+1 is determined by extrapolation from P.sub.i-1 and P.sub.i 
and where P.sub.i=1 is the load measured at center of the strain increment 
(i.e., at 2%, 4% etc. strain) 
so that 
##EQU4## 
which in turn means that 
EQU (.sigma.e).sub.i+1 =(.sigma.e).sub.i 
implying that 
##EQU5## 
FIG. 5 shows true stress - true strain curves for identical samples 
deformed, at 1070.degree. C., under conditions of constant energy input 
and constant displacement rate. The energy input rate was 0.038 MPA/sec. 
It is apparent that by employing conditions of constant energy input the 
reduction was achieved without exceeding a stress of about 20 ksi while 
under conditions of constant displacement rate a maximum stress of about 
29 ksi was required. 
The nickel base superalloys may be divided into three categories based on 
gamma prime content. The "lean" alloys contain less than about 30%, by 
volume, of the gamma prime phase. These alloys generally quite workable 
and are worked below the gamma prime solvus. The intermediate alloys 
contain from about 30 to about 50%, by volume, of the gamma prime phase. 
These alloys are quite difficult to work and are again worked below the 
gamma prime solvus temperature. Finally, the "rich" alloys contain greater 
than about 50% by volume, of the gamma prime phase and are generally 
considered to be unworkable except in the super plastic condition which is 
achieved through powder metallurgy procedures. However, the present 
invention can be used to hot work cast rich (non superplastic) superalloys 
above their gamma prime solvus. The invention process will generally be 
performed in accord with the preceding guidelines when applied to nickel 
base superalloys. 
Preliminary indications are that superior results can be obtained, in the 
case of the "rich" alloys, by initially forming above the gamma prime 
solvus and during the forming process decreasing the temperature to below 
the solvus temperature. This provides formability in combination with a 
desirable fine final grain size. 
It should be understood that the invention is not limited to the particular 
embodiments shown and described herein, but that various changes and 
modifications may be made without departing from the spirit and scope of 
this novel concept as defined by the following claims.