US 3689987 A
This invention is concerned with the provision of an improved method of making metal articles in which the metal is prepared in a form suitable for spraying and is then sprayed on to a target in such a way that most of the droplets of the spray can be individually identified after the sprayed metal has solidified. In other words the spraying is carried out under such conditions that the majority of the droplets retain their identity.
Description (OCR text may contain errors)
United States Patent Teague 1 Sept 12,1972
M n e U 0 T C E m M do G M w m mam n M m m S Emw DE 0L n w Hm m mum .9. MAP. m w m m Welwyn OTHER PUBLICATIONS  Filed: April 7, 1969 The Article Metal Spraying by T. Henry Turner and N. F. Budgen, Copyright Charles Gn'fl'in & Co. Ltd.
21 Appl. No.: 822,815
, 1926, Scientific Lib 30 Foreign Application Priority Data April 5, 1968 rary Call No. TS,65,T8, pages 88- 93.
Great 16,63 9/68 Primary Examiner-Edward G. Whitby Attorney-Burns, Doane, Swecker & Mathis  US. Cl.
ABSTRACT is concerned with the provision of an sprayingay that y can be individually other words the spraying is carried out under such m w h d m w e n .m w m m Sm H m m w u b 1% i.m m w m m e eae e mmmm m m f m 0 e W. mV. m t a n m.m f 0 dm f 0 3 mwkm e mP P .m f me W a mm.& m e wwmmfm mmm m .ltmmnm 6 PR6 m m 1 5 m nm h .9 e um R n no.
99 W MAN 1 1 32 identity. 2,947,623 8/1960 Lincoln ...............;.......75/165 15 Claims, 2 Drawing Figures 2,998,922 9/1961 Gibson;..,........1l7/93.1 PF X' 1 METHOD OF MAKING METAL ARTICLES BACKGROUND OF THE INVENTION Metal articles are usually fabricated from ingots which have been cast from baths of molten metals or alloys. When the metals or alloy components are mutually insoluble in the molten condition, or where the melting points of there materials are so high that fusion, in crucibles made from conventional materials, is not possible, it is usual to resort to powder metallurgy. Where powder metallurgy is used, finely divided powders of the metals and/or alloys are mixed together, pressed to form a compact a partly coherent ingot and, thereafter, the compact is consolidated by sintering and working. Powder metallurgy'is, however, an expensive process. The powdered metals and alloys generally cost significantly more than the massive metals and the operations of pressing, sintering and working are not only time consuming but also necessitate the use of expensive and frequently complicated equipment. For these reasons; the majority of ingots made by the metallurgical industry are made using conventional melting and casting techniques, even though it is generally considered that ingots so produced leave much to be desired.
Ingots made by conventional melting and casting techniques suffer from a number of defects including coring, gross or massive segregation, inverse segregation, impurity segregation, large grain size, shrinkage defects and gas unsoundness.
When wishing to make an ingot which does not in-- clude the defects enumerated above, it is usual to produce ingots by powder metallurgy. However, as previously indicated, the relatively high price of metals and alloys in powder form renders the cost of ingots made by powder metallurgy very high and uneconomic.
According to this invention, a method of making a metal article ,comprises spraying the metal in droplet form on to a target under such conditions that the droplets form a cohernt deposit on the target and that a major proportion of the droplets can be individually identified after solidification. In order-to promote rapid solidification of the droplets the target is preferably cooled. Alternatively, the target may possess a high thermal capacity, but for some applications of this technique better results may be obtained by using targets of poorly conducting material such as graphite or other non-metallic materials. We have found that the method of the invention is especially suitable for making metallic articles from platinum and other platinum group metals.
An examination of micro-photographs taken of an article made in accordance with the invention showed that the majority of the droplets retained their identityand that metallurgical changes were restricted in linear dimension to the diameters of the droplets.
The method of the present invention has a number of advantages:
1. Owing to the relatively rapid cooling of the sprayed metal, any segregation effects are finely distributed within the article.
2. The basic grain size is restricted to the volume of individual droplets. The droplets flatten-out" on impact with the target or previously solidified droplets so that the article or ingot so formed develops a preferred orientation in one direction before any working is carried out.
.3. Due to the entrapment of adsorbed or dissolved gas at the interface between solidified droplets within the ingot the grain boundaries of material produced in this way show a surprising resistance to migration. Rapid grain growth upon annealing is not so apparent as it would be with material from conventionally cast ingots.
4. Due to its stable boundaries the sprayed material is rather harder and stronger than conventional material both at room and. elevated temperatures. This improvement is achieved without sacrifice of the other proper ties such as electrical conductivity, temperature coefficient of resistance, and resistance to corrosion.
5. Shrinkage defects and gas unsoundness in ing-ots can-be largely eliminated.
The invention will now be described with reference to four examples,
EXAMPLE 1 Two 15 ounce ingots of an alloy containing 49 percent of platinum, 50 percent of gold and 1 percent of rhodium were prepared by melting in alumina crucibles and casting in a heavy copper mould to ensure rapid solidification. One ingot was cold forged, rolled down to square rod and finally drawn to wire 1 mm in diameter. This wire was then put into a flame spraying equipment which incorporated an oxy-acetylene torch and was sprayed at high speed into a heavy cold mould to produce an ingot with a cross section three-fourths inch wide and one-half inch thick, which was comparable to that originally cast. The other ingot was retained for purposes of comparison. The following comparative test results were then obtained on materials from the flame sprayed and conventionally cast ingots:
Conventionally Flame Cast Sprayed Density (grms/cc at 20C) 19.94 17.14 Mean Hardness H, [82 182 Max Hardness from mean 27 31 R.M.S. hardness variation 22.4 25.1 Grain size mm.
See figure 1 See figure 2 Mean size of Platinum Dendritcs.
After homogenization both ingots were rolled to sheet from which the following characteristics were ob- Two 15 ounce ingots of pure platinum were produced by melting in alumina crucibles and casting into heavy copper ingot moulds to ensure rapid cooling. One ingot was reduced to 1 mm wire, the other being retained for reference purposes.
The wire was sprayed at an impact velocity higher than 200 ft/sec. to produce an ingot comparable in shape and size to the original. Comparison of the cast and flame sprayed ingots produced the following results:
V Cast lngot Flame Sprayed lngot Density grm/cc 21.45 20.2 Hardness V.P.N. 40.45 62 Sheet obtained by rolling down the two ingots was annealed and the following results obtained:
(tons per sq.in)
Stress rupture tests on sheet specimens showed that conventionally produced material lasted for one-half to 1 hour at 1,400 C in air under an applied tensile stress of 700 p.s.i. Material obtained from the flame sprayed ingots lasted for 25-60 hours when tested under the same conditions.
It was found that specimens of wire from the flame sprayed ingots lost weight in air less rapidly than the conventionally produced platinum wires. At l,400 C in a convected air stream, the flame sprayed wires lost platinum at the rate of 2.21Xl' gm.per sq.cm per hour, whereas the corresponding figure for the conventional wire was approximately 5 percent higher.
EXAMPLE 3 A ounce batch of high purity thermocouple grade platinum produced from a conventional cast ingot was obtained in the form of wire one-sixteenth inch in diameter, half of which was sprayed to produce an ingot approximately 3 inches long, three-fourths inch wide and one-half inch deep. The flame sprayed ingot, and the remaining portion of the original material were reduced to 0.040 inch diameter wire. The high temperature strength and thermoelectric properties of the two wires were then compared, with the following results:
Wire from: tf Cast lngot Flame sprayed ingot Life at l400C under an applied stress of 700 p.s.i. (hr).
E.M.F. generated against Platinum 27' at the gold point (pV).
The sprayed wire had therefore a greatly improved resistance to stress at high temperatures than the wire from a conventionally cast ingot. It was also found to be more resistant to grain growth at high temperatures. The increased negative thermal E.M.F. with respect to pure platinum provides a very sensitive index of purity. In general the presence of impurities in platinum renders it thermo-electrically positive with respect to the pure metal. Platinum 27 is the standard thermometer grade platinum, made by the US. National Bureau of Standards, against which all other grades of thermocouple platinum are compared on an International basis. The original wire from the cast ingot was therefore purer than Platinum 27, and this purity was still further improved by the operation of spraying. This purification is obviously associated with the removal of dissolved base metals in the form of insoluble oxides.
EXAMPLE 4 Some tantalum sheets 4 inches wide, 12 inches long and one-half inch thick were anodically cleaned in a 10 percent solution of hydrofluoric acid, rapidly washed in distilled water, and dried. All exposed surfaces were then cleaned abrasively by subjecting them to a blast of alumina granules, and platinum was sprayed on to produce a uniform surface coating approximately 0.05 inch thick, The composite sheet was reduced in thickness by a ratio of 50 percent by cold rolling before being vacuum annealed for one-half hour at 700 C. Cold rolling was then continued until a platinum clad strip 4 inches wide, 6 feet long, and approximately 0.10 inch in total thickness was obtained.
From this strip platinum clad tantalum electrodes were obtained. These electrodes were used as insoluble anodes in some specialized electro-deposition processes and for the cathodic protection of marine steelwork.
The invention also includes articles when made in accordance with the method described above. The articles may be in sheet, rod or wire form and the metal may be sprayed using arc, flame or plasma spraying techniques. Such articles have high corrosion resistance.
Metallic materials made in accordance with the invention may be used for spinerettes and other apparatus for use in the glass, glass fiber and synthetic fiber fields. In such instances, the fine grain size of the metallic material assists in the resistance of apparatus to erosion by molten glass and molten synthetic fibers.
The metallic materials made by the method of the invention are eminently suitable for catalytic purposes where a state of high surface activity is assisted at the surface of the material by a fine stable grain structure. For example, metallic material made by the method of the invention may be used in the manufacture of platinum alloygauzes used for the catalytic oxidation of ammonia to nitric acid, igniter tubes for gas turbine engines and heater wires used for igniting coal and natural gas burners.
Other uses of metallic materials made in accordance with the invention include permanent magnets, electrical contacts, thermocouples and resistance thermometers, electrical heater wires and elements.
In some cases it may be useful to use a mask, displaced an appropriate distance from the target, so that a well defined deposit is built up or produced on the target and complicating effects from gases that may be trapped in a concave mould forming the target are reduced.
In the accompanying drawings FIG. 1 illustrates the microstructure of conventionally cast Au-Rh-Pt Alloy while FIG. 2 illustrates the microstructure of spray deposited ingot of Au-Rh-Pt Alloy prepared in accordance with the present invention.
1. A method of making metal article comprising spraying a metal in droplet form onto a cold .target under such conditions that the droplets form a coherent deposit on the target and that a major proportion of the droplets can be individually identified after solidification, said target being cooled at least during spraying.
2. A method of making a metal article comprising the steps of preparing a sheet of tantalum, cleaning the sheet, washing and drying the sheet and then spraying platinum onto the cold sheet of tantalum under such conditions that the droplets form a coherent deposit on the target and that a major proportion of the droplets can be individually identified after solidification.
3. A method of making a metal article comprising spraying an alloy containing 50 percent gold, 49 percent platinum and 1 percent rodium onto a cold target under such conditions that the droplets form a coherent deposit on the target and that a major proportion of the droplets can be individually identified after solidification.
4. A method of making a metal article comprising spraying a metal in droplet form onto a cold stationary target which is cooled during spraying or is of such thermal capacity, or both, so that solidification of the droplets into a coherent deposit on the target is promoted, removing the deposit from the target and subjecting the solidified deposit to mechanical working so as to densify the deposit.
5. A method according to claim 1 wherein the target is made of material of high thermal capacity.
6. A method according to claim 1 wherein the metal to 1 be sprayed is selected from the metals of the Platinum group.
7. A method according to claim 1 wherein the spraying is carried out by flame spraying technique.
8. A method according to claim 1 wherein the spraying is carried out by plasma spraying technique.
9. A method according to claim 1 wherein the metal to be sprayed is platinum.
10. A method according to claim 1 including the further steps of removing the coherent deposit of metal from the target and cold working the coherent deposit removed from the target.
11. A method according to claim 10 including the further step of annealing the coherent deposit after cold working.
12. A method according to claim 6 including the further steps of removing the coherent deposit of metal from the target and cold working the coherent deposit removed from the target.
13. A method according to claim 12 including the further step of annealing the coherent deposit after ld k m rii c thod according to claim 2 including the further step of cold rolling the composite platinum-tantalum sheet to approximately 50 percent of its original thickness.
15. A method according to claim 2 including the further steps of cold rolling and then annealing the composite platinum-tantalum sheet.