Spray-forming method of forming metal sheet

A metal composition is formed into a sheet by the method comprising; spray forming the metal composition to form a cylindrical deposit, and wire electric discharge machining the cylindrical deposit in a preselected spiral path parallel to the axis of the cylindrical deposit to form the sheet.

This invention is related to a method of forming metals into sheet, and in 
particular forming sheet from high temperature superalloys that are 
difficult to form into sheet by traditional thermomechanical processing. 
As used herein, the term "sheet" means a body having a small thickness 
dimension in comparison to its length and width dimensions. 
BACKGROUND OF THE INVENTION 
Engineering metals such as aluminum alloys, titanium alloys, steels, and 
superalloys, are processed into sheet by melting the desired composition, 
casting the melt into ingots, hot rolling the ingots to slabs, and 
subsequently rolling the slabs into sheet form. Intermediate and final 
annealing operations can be performed on the rolled sheet to recrystallize 
the microstructure or obtain various properties such as improved 
ductility. In rolling, a squeezing type of deformation is accomplished by 
using two work rolls rotating in opposite directions. The principal 
advantage of rolling lies in its ability to produce desired shapes from 
relatively large pieces of metals at very high speeds in a somewhat 
continuous manner. Slabs are generally rolled at temperatures above the 
recrystallization temperature of the metal, or hot forming range, where 
large reductions in thickness are possible with moderate forming 
pressures. Smaller reductions can be made by cold rolling, forming below 
the temperature the metal will recrystallize, to maintain close thickness 
tolerances. 
The multiple deformation forming processes in sheet rolling can produce a 
preferred crystal orientation or texture in the sheet. Crystals in certain 
orientations are more resistant to deformation than are other crystals. 
These deformation resistant oriented crystals tend to rotate during 
deformation thereby producing a preferred orientation. During 
recrystallization, preferred orientations result from the preferential 
nucleation and growth of grains of certain orientations. 
Superalloys are difficult to deform and easy to crack during deformation. 
Since the superalloys were designed to resist deformation at high 
temperatures, it is not surprising that they are very difficult to hot 
work; the alloys having limited ductility and high flow stress. 
Furthermore, additional alloying elements which improved service qualities 
in the superalloy, usually decrease the ability to work or deform the 
alloy into a desired form. As a result, primary or slab rolling of 
superalloy sheet is usually performed at temperatures near the melting 
point of the alloy on rugged, powerful mills built to withstand the high 
stresses encountered in the working of superalloys, and fast handling is 
mandatory to minimize edge cracking. The superalloys have narrow working 
temperature ranges, and are often rolled in packs or layers, that are 
sometimes encased in a steel envelope, to minimize heat loss to the 
relatively cold rolls upon deformation. The narrow working temperature 
range makes the rolling labor intensive, and many intermediate reheating 
steps are required. Some of the superalloys that are commercially 
available in a sheet form are Hastelloy alloy X, IN-600, IN-718, IN-625, 
Rene, 41, and Waspaloy. 
A combination of properties such as strength, formability, and weldability 
are desired in superalloy sheet, and the desired combination of properties 
dictate many aspects of the extensive thermomechanical processing required 
to form the sheet. However, equipment limitations may prevent performance 
of the required thermomechanical processing so that some desired 
properties may not be obtainable in sheets formed from some of the 
superalloy compositions. Because some sets of properties have not been 
attainable in cast alloy materials, resort is sometimes had to the 
preparation of parts by powder metallurgy techniques. However, one of the 
limitations which attends the use of powder metallurgy techniques in 
preparing moving parts for jet engines is that of the purity of the 
powder. If the powder contains impurities such as a speck of ceramic or 
oxide, the place where that speck occurs in the moving part becomes a 
latent weak spot where a crack may initiate. Some of the superalloy 
compositions that are prepared by powder metallurgy techniques are shown 
below in Table I. 
TABLE 1 
______________________________________ 
Superalloy Compositions In Weight Percent 
Unitemp 
Astroloy Rene95 AF2-1DA IN100 
______________________________________ 
Ni Bal. Bal. Bal. Bal. 
Cr 15 13 12 10 
Co 17 8 10 15 
Mo 5.25 3.5 2.75 3 
W 3.5 6.5 
Nb 3.5 4.6 5.5 
Ta 1.5 
Al 4 3.5 4.6 5.5 
Ti 3.5 2.5 2.8 4.7 
C 0.06 0.06 0.04 0.05 
B 0.03 0.01 0.02 0.014 
Zr 0.05 0.06 
V 0.09 
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It is an object of this invention to provide a simplified method of forming 
metal into sheet. 
It is another object of this invention to provide a method for forming 
sheets from metal compositions that are difficult to deform. 
It is another object of this invention to provide a method for forming 
sheet having a substantially isotropic and fine crystal or grain structure 
.

DETAILED DESCRIPTION OF THE INVENTION 
Sheet formed by the method of this invention has a uniform crystalline 
structure free of macrosegregation, and has a very fine grain size of 
random or isotropic orientation. Highly alloyed metal compositions that 
are difficult to ingot cast due to macrosegregation of the alloying 
elements can be formed into sheet by the method of this invention. Because 
extensive deformation of the metal in multiple rolling steps is not 
required in the method, the ductility and high temperature strength of the 
metal are not limitations to forming the sheet or to forming a desired 
thickness of the sheet. 
Although the method of this invention can be practiced on any of the 
engineering metals such as steel, stainless steel, or iron, it is 
especially useful as a method for forming sheet from the very high 
strength and difficult to deform metals such as the superalloys or 
titanium alloys. Therefore, the method of this invention is advantageously 
performed on the superalloy composition described above, and especially 
those in Table 1, or the tri-nickel aluminide base superalloys, for 
example, in U.S. Pat. Nos. 4,613,386, 4,606,888, 4,609,528, 4,650,519, 
4,661,156, 4,676,829, or the titanium alloys, including the intermetallic 
compounds of the titanium aluminides, for example, those disclosed in U.S. 
Pat. Nos. 4,253,873, 4,087,292, 4,292,077, 2,880,087, 3,411,901, 
4,716,020, 4,770,726, 3,203,794, 4,294,615, and in pending application 
Ser. No. 07/325,738. The above list of metal compositions is not meant to 
be a complete list, but is merely an example of the metal compositions 
that can be formed into sheet by the method of this invention. 
In the method of this invention, a cylindrical deposit is formed by a spray 
forming process, such as plasma spraying of powders, or preferably, by 
spraying gas atomized molten metal. Some of the methods for plasma 
spraying metal powders are described, for example, in U.S. Pat. Nos. 
4,689,468 and 4,838,337, both incorporated herein by reference. Spraying 
of gas atomized molten metal is described in "The Spray Forming of 
Superalloys", H. C. Fiedler et al., Journal of Metals, Vol. 39, No.8, 
August, 1987, pp. 28-33, and U.S. Pat. No. 3,826,301, both incorporated 
herein by reference. Spraying of gas atomized molten metal is preferred 
because of the lower level of interstitial atoms such as nitrogen or 
oxygen, and the greater spray forming capacity that are found with the use 
of this process. 
A preferred method of spraying gas atomized molten metal is shown in FIG. 
1. Referring to FIG. 1, a crucible 10 is provided as a molten metal 
dispensing crucible. The crucible is used to hold a body of liquid metal 
and to dispense it as a stream to an atomization zone located beneath the 
crucible. The crucible is preferably a segmented crucible made up of a 
number of water cooled metal segments which fit around and form the walls 
of the crucible. Each segment is electrically isolated from its adjoining 
segments and each segment is individually water cooled. A benefit of the 
water cooling is to permit a skull of the metal of the melt to form on the 
inside of the crucible and contain the molten metal. The skull avoids the 
need for a ceramic crucible to contain the melt so that introduction of 
ceramic particles into the melt from spalling or cracking of the ceramic 
crucible is minimized. The melt is kept at its operating temperature by 
the action of a set of induction coils 14. The segmented character of such 
a crucible permits the electric flux to penetrate the crucible 12 to act 
on the liquid metal content of the crucible 16. However, crucible can be 
formed as a single piece from a high temperature ceramic compatible with 
the melt composition, for example, the superalloys can be melted in 
alumina, zirconia or magnesia crucibles. It has recently been found that 
titanium can be melted in a calcia crucible, "Melting and Precision 
Casting of Pure Titanium Using Calcia," T. Degawa, et al., Proceedings of 
the Sixth World Conference on Titanium, Societe Francaise de Metallurgie, 
France, 1988, pp. 707-713. 
A stream 18 of liquid metal pours from the bottom spout 20 of crucible 10 
passing into an atomization zone 21 where it is acted on by a jet or jets 
of atomizing gas emitted from nozzle 22, the gas being delivered to the 
nozzle 22 from a source which is not shown. Suitable atomizing gases are 
inert gases such as argon or nitrogen. 
The result of the atomization of the liquid stream 18 by the atomizing gas 
in zone 21 is the production of a cone 24 of droplets of liquid metal. The 
formation of such a cone is consistent with the practice of the art of 
spray forming. The droplets impact on a receiving surface 26 which is 
shown in the form of an annular band formed on the hollow mandrel 28 to 
form a cylindrical deposit 30. The mandrel 28 is given both a rotary and a 
reciprocating motion, indicated by the arrows, by drive means which are 
not shown. Such motion causes a uniform layer to form on the receiving 
surface 26. The rotary and reciprocating motion of the mandrel are 
controlled to provide a cylindrical deposit 30 having a desired width 32 
and cross-section or wall thickness. 
Preferably, mandrel 28 is a cylinder having a surface in the form of a 
screen having screen openings which permit at least partial passage of 
metal spray droplets or liquid particles therethrough. Such mandrels are 
described in copending application Ser. No. 07/328,212, filed Mar. 24, 
1989, incorporated herein by reference. 
After spraying is complete, mandrel 28 is removed from cylindrical deposit 
30, for example, by wire electric discharge machining. Preferably, uneven 
end portions 34 are also removed in a like manner from cylindrical deposit 
30 to define the desired width 32. At the interface between the mandrel 28 
and deposit 30, and in the uneven end portions of the deposit there is 
generally formed a higher amount of porosity, voids, or other gas atomized 
molten metal spray forming defects as compared to the central portion of 
deposit 30 within width 32. Preferably, in removing mandrel 30 and uneven 
end portions 34, the excess porosity, voids, or other spray formed defects 
are removed. Electrical discharge wire machining is a process that is 
similar in configuration to band sawing, except with electric discharge 
wire machining, the saw is a wire electrode 36 of small diameter. Cutting 
occurs based upon the erosion effect of electric sparks occurring between 
two electrodes, the electrodes being the wire and the cylindrical deposit. 
Electric discharge wire machining is described, for example, in "Tool and 
Manufacturing Engineers Handbook", Fourth Edition, Vol. 1, 1983, pp. 14-42 
to 14-61, incorporated herein by reference. 
Referring to FIG. 2, cylindrical deposit 30 is electric discharge wire cut, 
or machined, in a spiral path 34 that is parallel to the cylinder axis. An 
electrical discharge wire machine, not shown, has means for feeding the 
cutting wire, movement of the workpiece, and a power supply to provide the 
spark energy for the cut. An X-Y translation table, not shown, is mounted 
on the machine base for holding and positioning the deposit 30. Movement 
of the X-Y translation table is controlled by a programmed computer. A 
suitable wire electric discharge machine is the Mitsubishi DWC 110, 
Mitsubishi Electric Corporation, Japan. 
Material is removed by spark erosion as a wire electrode 36 passes through 
cylindrical deposit 30. The wire electrode 36 moves vertically over 
sapphire or diamond wire guide spools 38, one above and one below the 
cylindrical deposit 30. A pulsating direct current sufficient for spark 
erosion cutting is supplied to wire electrode 36 operatively connected to 
a power supply, not shown. Electrode wire 36 is used once and then 
discarded because the wire becomes misshapened after one pass through the 
cylindrical deposit. Nozzles 42 provide a steady stream of deionized water 
or other fluid to cool the cylindrical deposit and electrode wire, and to 
flush the cut area. 
The cylindrical deposit 30 is mounted on the X-Y translation table for 
movement in the plane normal to the electrode wire 36. A programmed 
computer controls the X-Y translation table so that the cylindrical 
deposit 30, is rotated in the direction of arrow 44, and fed into 
electrode wire 36 in the spiral cutting path 34. Spiral path 34 is 
controlled so that there is a preselected spacing between each spiral to 
provide a desired sheet thickness. For example, spiral path 34 has the 
same spacing between spirals through the entire wall thickness of the 
cylindrical deposit 30 to form a sheet having the same thickness, or the 
spacing between spirals can be increased or decreased to form preselected 
lengths of various sheet thicknesses. The cross-section or wall thickness 
of deposit 30 limits the number of spirals that can be cut from the 
deposit, and determines the linear feet of sheet that can be cut from 
deposit 30. 
After wire electric discharge machining the sheet will have approximately 
the curvature of the cylindrical deposit. If a flat sheet is required, 
cold rolling, or cold rolling combined with annealing steps can be 
performed to flatten the sheet and achieve other desired properties such 
as increased strength or ductility in the sheet. The electric discharge 
wire machining can also leave a surface oxide on the sheet which can be 
removed by a reducing atmosphere in the annealing step, or by grit 
blasting or shot peening of the surface. Such grit blasting or shot 
peening can also be performed to improve fatigue properties of the sheet. 
EXAMPLE 1 
A charge of about 20 kilograms of a nickel based superalloy comprised of in 
weight percent; about 18% cobalt, 16% chromium, 5% molybdenum, 5% 
tungsten, 2.5% aluminum, 3% titanium, 3% niobium, 0.05% zirconium, 0.01% 
boron, 0.075% carbon, and the balance nickel was melted in a magnesia 
crucible. The molten metal was poured from a nozzle in the bottom of the 
crucible having a 6 millimeter bore, and gas atomized molten metal spray 
formed onto a cylindrical mandrel approximately 17.5 centimeters in 
diameter. Nitrogen was used as the atomizing gas impinging on the molten 
metal stream pouring from the nozzle to form the molten metal spray. The 
spray formed cylindrical deposit had a width of about 15 centimeters and a 
cross-section, or wall thickness, of about 3 centimeters. A center portion 
of about 4.7 centimeters was removed from the central portion of the 
deposit width. 
The center portion was then positioned on the computer controlled X-Y 
translation table of a wire electric discharge machine so that the 
cylinder axis was parallel to the cutting electrode wire in the machine. A 
0.020 inch brass wire was used as the electrode wire, with cutting 
proceeding from the inside diameter to the outside diameter of the 
cylindrical deposit. The mandrel was removed from the inside diameter of 
the center portion, and the computer was programmed so that a spiral path 
was cut into the cylindrical deposit with a spacing of about 0.877 
millimeters between each spiral. Cutting proceeded at a rate of about 3.3 
linear centimeters per hour and was terminated after two revolutions were 
cut from the cylindrical deposit to form a sheet having a length of about 
122 centimeters and width of about 4.7 centimeters. If cutting had 
continued through the entire thickness of the central portion, the length 
would have measured to about 853 centimeters. The resulting sheet measured 
about 0.965 millimeters in thickness uniformly across the width of the 
sheet. 
The grain size of the sheet was determined by standard metallographic 
techniques according to ASTM E 112, Annual Book of ASTM Standards, 
Philadelphia, Pa. A sample of the sheet was viewed in the thickness 
dimension at the center and near each edge of the sheet width, and found 
to have a substantially equiaxed uniform size 8 grain size across the 
width of the sheet.