1148~35 FUSABLE, SELF-FLUXING ALLOY POWDERS
This invention pertains to spray-and-fuse self-fluxing alloy metal powders and more particularly to a method eliminating voids in the fused powder metal coating. Spray-and-fuse, self-fluxing metal powders are well known in the art and can be deposited on a base metal ob~ect by any spray process such as flame spray and plasma spray, for example, and t-hen the de-posited coating can be simultaneously or subsequently fused to provide a con-tinuous surface coating. A dense coating results with the powder particles metallurgically bonded to the base metal. The fused metal surface coating can impart wear resist~nce, corrosion resistance, oxidation resistance, high room temperature and hot hardness, and similar desirable surface properties to the base metal. Such alloy metal powders can be used to repair or build up worn, damaged, or improperly machined parts as well as to provide protec-tion to new parts. The metal powders commonly used are nickel or cobalt al-loys such as described in United States Patent Nos. 2,875,043; 2,936,229;
and 3,305,326. The most common method of providing metal powders is to gas or water atomize molten metal such as suggested in United States 2,956,304.
The atomized metal powder, however, results in a wide distribution of par-ticle sizes which requires screening to provide the ma~ority of the particles between about 43 microns and 110 microns. Smaller particles overheat and larger particles are too hard to soften. A certain degree of porosity is normal in the fused coatings made from these metal powders. However, too often they exhibit large voids or discontinuities which are detrimental to the performance of the fused coating. Such voids or porosity in the fused coating can be attributed to many factors, including porosity indigenous to water ato~ized metal powders, gaps between the sprayed particles which in-completely close during fusing, etc. For instance, United States Patent No.
2,936,229 recognizes an inherent deficiency in spray-weld alloys wherein ,,~p .
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~1~8~35 isolated pores appear in fused alloy coatings and becomes even more apparent in subsequent grinding and surface polishing operations. Said patent sug-gests overcoming the problem by the inclusion of a very minor amount of aluminum in the alloy. Thus, surface pores or voids in nickel alloy finished parts are often encountered in alloy metal powder spraying and fusing where-by the finished parts often must be discarded due to defects.
Applicants have now found that a critical range of alloy metal powder with an upper size limit of about 88 microns can be advantageously utilized in spray-and-fusable alloy powders. Elimination of the conventional larger alloy particles above 88 microns up to 105 microns (170 to 150 Tyler mesh) surprisingly reduces to a negligible level the large pores and overall por~sityof the fused alloy coating. A substantial improvement can be real-ized by crushing the atomized particles to provide crushed alloy particles having predominantly a particle size distribution between 43 and 88 microns which is particularly suitable for pressurized air or gas powder feed spray-ing system such as a Wall Colmonoy Spraywelder manufactured by Wall Colmonoy Corp. These and other advantages will become more apparent by referring to the detailed description of the invention.
Briefly, the present invention pertains to providing a powdered nickel or nickel-chromium alloy having at least 95% by weight of the alloy particles below about 88 microns, spray applying the alloy particles to a metal substrate, and fusing the alloy particles to provide a substantially porosity-free fused alloy surface coating.
Thus, according to the present invention, there is provided in a process for spraying under pressurized metal powder feeding and fusing pow-dered aluminum-free alloy of nickel or cobalt containing minor amounts of boron and silicon, the improvement comprisine:
pro~iding an alloy powder consisting of alloy particles wherein at least ..
-1~48~35 95% by weight of said alloy particles are between about 10 and 88 microns, 0 to 5% by weight of said alloy particles are between ô8 and 105 microns, at least about 70% by weight of the alloy particles are crushed alloy particles.
Preferably, the powdered alloy of this invention comprises at least 95% by weight alloy metal particles below a particle size of about 88 mic-rons, a maximum of 30% below 43 miçrons, with the balance, if any, up to 5%
being between 88 to 105 microns. The spray-welding or fusable alloy par-ticles are nickel alloy or nickel-chromium alloy or cobalt alloy particles.
The alloys are self-fluxing alloys and contain minor amounts of alloyed boron and silicon to impart fluxing properties to the substrate surface and alloy during the fusing process as well as facilitate spraying of the metal powder.
Chromium can be added to provide better corrosion and oxidation resistance to nickel alloys and also can be combinedwith carbon and boron to improve wear and abrasion resistance. Copper and molybdenum also can be alloyed to im-prove fusing of the alloy powder and permit buildup of thicker fused alloy coatings on the metal substrates. For purposes of the invention, the alloy preferably contains by weight about 70% to 90% nickel, 1% to 6% silicon, 1%
to 4% boron, 0 to 20% chromium, 0 to 6% iron, and 0 to 2% carbon. Useful nickel and nickel-chromium alloys are illustrated in the following Table I.
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1148~)35 Representative chemical compositions of a cobalt-based self-flux-ing alloys suitable for use in the present invention are gi~en below:
TABLE II
Component Weight-%
Co Balance Ni lô.0-21.0 Cr 18.0-20.0 Fe 1.0- 4.0 Si 3.2- 3.8 B 2.8- 3.2 C 0.5- o.8 W 5.0- 8.o Rockwell Hardness (RC) 55-61 The alloy powders can be produced by a variety of means such as gas or liquid atomization, grinding, and vibratory or rotary or gyratory crushers.
Preferably, the alloy powders are produced by liquid or gas atomization such as described in United States Patent No. 2,g56,304 and water ato~ization is quite often preferred. The particle size of atomized powder ordinarily is regulated by the apparatus but nevertheless, produces a range of particle sizes. Atomized particles between about 43 and 88 microns are screened for use in accordance with the process of this invention. Larger oversized par-ticles above 88 microns can be crushed in accordance with a preferred aspect of the invention wherein the ma~ority of alloy particles utilized in the flame-spray process step are crushed atomized particles which are subsequent-ly screened to provide a particle size range between 43 and 88 microns of crushed alloy particles. Preferably, the powdered alloy contains at least about 70% by weight crushed alloy particles and most preferably, between about 90% and 100% by weight crushed particles. Crushed or irregularly : .
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shaped alloy powder has been found to yield improved deposition efficiencies and lower porosities over atomized or other uncrus~led powder in conventional spray equipment with pressurized gas powder feed. Though reduced porosity in the fused coating can be so~ewhat achieved by use of crushed powder, t~e un-e~pected results of the invention are that gross voids still found in ~used coatings made from crushed powder can be substantially eliminated by main-taining at least 95~ of the crushed particle si~e distribution between about 10 and 88 micronæ and the remainder no larger than 105 microns. Although up to about 30% by weight of the particles can be uncrushed particles, pref-erably substantially all of the particles are crushed in accordance with thisinvention.
The alloy powder initially can be produced by a conventional atom-ization technique, shotting technique or the like. Preferably, atomization is employed. In atomization processes using water, spherical porosity even up to production of essentially hollow spheres is found in the res~tin~
atomi7ed powder. It is ~elieved that the gas evolu-tion due to the water atomization accounts for this spherical porosity. Crushing oP said atomized powder appears to substantially eliminate such indigenous spherical porosity as such porosity acts as fracture sites during the crushing operation. Even with powders produced by conventional shotting techniques, crushing of the powder and restriction of particle size distribution as defined in this in-vention substantially eliminates gross voids in the fused coating. A variety of conventional attrition or grinding mills can be e~ployed for crushing the alloy powder. Suitable apparatus includes ball mills9 rod mills, vibratory crushers, gyratory crushers, rotary crushers, some of which having multi planar motion for additional effective attrition~ and like apparatus.
The basic spray-and-fuse, self-fluxing metal powder is conventional in composition such as those metal powders found for example, in the follow-~148~35 ing United States Patents: 2,875,043; 2,936,229; and 3,305,326. Typically, a variety of other components are added to the basic nickel or cobalt matrix metal for providing a variety of special properties. Additions of silicon and boron are responsible for advantageous fluxing characteristics by form-ing low melting point glasses. Silicon and boron also lower the melting point of the alloy to facilitate spraying and fusing operations by forming lower melting point eutectic phases. Chromium is added to provide greater corrosion and oxidation resistance to the matrix. Chromium also combines with boron and carbon to ~orm the hard precipitates responsible for wear and abrasion resistance. Copper and molybdenum can be added to the matrixme~ lfo~
decreasing the fluidity during fusing of the applied metal powder and permit buildup of thicker coatings on the base metal to which the alloy powder is applied. On occasion, it can be advantageous to add aluminum to the matrix metal as a deoxidant and/or for obtaining a self-fusing alloy metal powder.
The alloy particles of thiæ invention are particularly suitable for use with pressurized air or gas feed systems such as Colmonoy Spraywelder flame spray equipment marketed by the Wall Colmonoy Company Gases can be uti-lized such as acetylene, hydroeen and similar fuel gases. The alloy metal powder can be sprayed onto the metal substrate and then fused in a furnace or by heat torches or other heat induced means. The resulting fused alloy coat-ings are essentially free of large porosity voids as will become more evident by referring to the following examples.
Comparative measurements of porosity in coatings obtained from pow-ders made according to the present invention and from powders screened to con-ventional particle size distributions were made. The powders used were of several heats of two general nickel-base self-fluxing alloy compositions.
The typical chemical compositions of these two alloys are as follows:
1148~35 Alloy I Alloy II
Ni bal.% bal.%
Cr 13.5~ 12.5%
Fe 4.7% 4.0%
Si 4-3%
B 3.0% 2.3%
a o.6% 0.5%
Twelve powder samples were evaluated. Powders nos. 1 through 5 were normal water atomized powders screened to conventionally used particle size distributions. Powders 6 and 7 were produced by cruæhing water atomized powders larger than 150 mesh and screening to conventionally used particle size distributions. Powders ô through 12 were also produced by crushing +150 mesh water atomlzed powders, but they were screened to particle size distrib-utions where the -150 +170 fraction was progressively minimized.
All powders were of similar ccnventionally used particle size dis-tributions except for the -150 +170 fraction. All met the following specif-ication.
+115 0% max.
+150 0.1~ max.
-325 10-25Z wt.
The -150 + 170 fraction~ oP powder~ nos. 1 through 7 were between 8 and llZ. The -150 + 170 fractions of powders no. 8 through 12 were as fol-lows:
Powder No.Percent -150 + 170 (88-105pm) 8 5.9 9 4.2
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~148/~35 Powder No. Percent ~150 + 170 (8c-105~m) 11 2.7 12 2.3 Each powder was similarly applied onto steel rods (3 inches long by 0.5 in. diameter) by standard oxygen-acetylene flame spraying procedures using a Wall Colmonoy Model H Spray-welder. Spray parameters were as fol-lows:
oxygen pressure/flow 24 psi/45%
acetylene pressure/flow 15 psi/78%
air pressure 22 psi gun tip to specimen dist. 11 inches specimen rotation speed 300 rpm gun traverse speed 100 inches/min.
number passes 16 The bars were each then similarly fused with an oxygen-acetylene welding torch using standard procedures immediately after spraying. The ap-proximate coating thickness was between 0.05 and o.o6 inches. Each coated bar was then sectioned after cooling, and uniformly prepared for metallo-graphic examination. Standard metallographic grindinB and polishing tech-niques were followed. Wet silicon carbide papers down to 600 grit were usedfollowed by polishing with 0.3 pm and 0.05 ~m alumina slurries. Three photo-micrographs at a magnification of 50 times were taken of each coating sample in random locations equally spaced. Porosity counts and measurements were made from the three photographs and averaged for each sample. The total num-ber of pores per mm , the total number of pores greater than 20 ~m in diam-eter per mm , and the total number of pores greater than 40 ~m in diameter per n~ were measured. The results are given in the following Table III.
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~1~8~35 These results show that the crushed powder screened only to the conventional distribution does not necessarily improve the level of porosity.
Where the overall number of pores is lower (as is the case for no. 7) the number of large pores is higher. ~owever, it can be clearly seen that by limiting the -150 +170 mesh size of the crushed powder below 6%, a marked im-provement in both the overall number of pores and the number of large pores is obtained. Below about 5% -150 ~170 mesh size of crushed powder, the num-ber of large pores (> 20 ~m) remains low and there are no very large pores (~ 40 pm) found.
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