[1]Patented Nov. 15, 1938 2 9 1 3 7 , 2 0 0 UNITED STATES PATENT OFFICE 2,137,200 ABRASIVE ARTICLE AND ITS MNUFACTURE John A. Royer, Niagara Fafls, N. Y., assignor to The Carborundum Company, Niagara Faffs, N. Y., a corporation Of Delaware Appflcation June 28, 6 Claims. This invention@relates to. metal bonded abrasive articles and their manufacture, and particularly to the metal bonding of such abrasives as siheon carbide, boron carbide, fused alumiria and r) diamonds. The invention is particwarly applicable to the production of abrasive articles adapted for the cutting of extremely hard materials such as glass, tungsten carbide and other hard carbides. 10 One of the objects of the inverition Is the production of a metallic bond which can be readily molded under Pressure and which can be sintered to a coherent mass under relatively low temperature. Another object . is the production of a metal 1 5 bond which can be sintered with comparatively little shrin age evenv@hen formed under low consoudating pressures, and which permits thesubstantial retention of contour of the original pressed article. A further object of the Inven20 tion is the production of a metallic abrasive articIe wwch wiR cut or grind hard materials such as glass or tungsten carbide with a comparatively small loss of abrasive. These and other objects wiH be apparent or will be hereinafter. pointed 25 out. In the bonding of abrasives with metallic materials, it has been customary tG use compositions which produce either a haxd or a brittle matrix. Most of the materials used for this purio p6se have been characte'rized by relatively high melting points and, as they are comparatively hard or brittle, they have little or no plasticity at room temperature except under extremelk high PressUres. Such materials are difficult to mold t5 and, unless very high consolidating pressures are used, the articles when cold molded and subsequently heated tend to shrink during the coalescence of the metal particles. As a result of tWs shrinkage, the original size and shape of the pressed article is not preserved. The low melting metals, as for example those which melt below 700, C., axe relatively soft, aiid f,Dr this reason they have not been regarded as suitable bonds for abrasive articles used in the grinding of hard materials. In producing abrasive wheels and laps, I have found that sintered aluminum bonds, and @articularly those containing intermetallic compounds as hardening agents, will saiisfactoriISr retain abrasive grains duiing cutting and grinding, and that it is po@sible to form a relatively soft n@dx containing aluminum, mold the niix under a fairly low consolidating pressure, and thereafter produce a hardeiii@ effee t by thi@.reaction of the Now 1937, Serial No. 150,765 (Cl. 51-280) aluminum with other,alloying agents to forin intermetame compounds. If alloying agents capable of forxning interinetallic compounds with the alunitnum are added In proportions beyond those ordinartly used in 5 structural aluminum base aboys, the yeswting product when sintered contains an appreciable proportion of hard constituents which impart wearresistance to the matrix. It Is possible tG obtain a hard matrix of this character even when io the original niix from which the article Is pressed Is very soft and plastic. Fbr example, a product which when sintered will consist almost entirely of a hard metallic compound can be produced from a mix contahiing approximately. 70% of 15 the relatively'soft aluminum pgwder. This high content of alimiinum in the original mix permits the deformation of the powder under pressure so as to form a dense mass In making abrasive @@icles by the process 20. herein described, the abmsive grain of powder Is mixed with powdered metal and the mixture . molded and sintered. Mie axticle can be molded by the simultaneous application of heat and pressure'or it can be cold molded and thereafter Sub- 25 Jected to heat either with or without the application of pressure. As previously pointed out, a mix containing an appreciable proportion of free aluminum possesses the advantage that the aluminu m Is plastic and the mixture can be con- 30 solidated into a dense mass by the application of pressure alone. I believe that the plasticity of the aluminum@'and the density afforded b3r'the deforinat ion of the metal particles under pressur, e may. at, least In part account for the fact that, 36 during sintering or alloying, such mixes are not characte rized by a high shrinkage. When the powdered n-dx consists entirely of particles which - are hard and can not readily be deformed by the pressure used for consolidation, the cold 40 pressed material possesses considerable p6rosity,, and upon heating to effect coalescence, the materW shrinks. The preservation of contour without the application of pressure during sintering, which I have @ound to be characteristle of the 45 aluminu m base alloys of the type herein described, is of particular tmportance in the manufacture of curved laps such as are used fGr the grinding of lenses. With such laps the contour of the fiwshed material must be predetermined ro and accurately maintained. Com mercial aluminum powder Is ordinaxily manufac tured for use in paints, and the metal is frequentl y cbntamiriated with stearic acid or Dther organic materials. Powder of this type -is 55
[2]2 2,137,200. also quite flaky, and izv pressing, tends to give a laminated structure. Although it is possible to use such a material for sintering, I have found it advantageous to employ aluminum powder of a non-flaky character, that is, substantially developed in three dimensions. Such a material can be manufactured by comminuting the aluminum or the aluminum alloy to be used for sintering by spraying, or by a cutting or nilhing operation. 10 A form of aluminum powder very desirable for sintering, when examlned under the microscope, bas the appearance of irregular shaped particles or chips, which may be more or less elongated in one direction but are not ftaky. The particle size i5 Is preferably less tha@% approxiniately 200 mesh. The material should lie ss free as possible from organic mate4als or o)dde on the surface of the particles, and in the preferred form is not unctuous or adherexit'as7 Is the case with the usual com20 mercial powder., It is possible to obtain aluminum powder in which the particle surfaces are practacally oilde fred. and this material is very desirable for sintering. The exact nature of the invention will be more 25 clearly understood from the following detailed description, considered in connection with the accompanying dmwing. In the drawing: Flgure I shows an abrasive ring or disc of a 3o. type adapted for gri@nding tungsten carbide tools; Mgure 2 is a @ection of the disc shovni In Flgure 1, the section -being taken along the section line U-II; Mgure 3.Ulustrates a method of ibing a num35 ber of flat discs or rings; Flgure 4 fllustrates a metal cup wheel which can be used for the surfacing of refractories, particularly those composed of silicon carbide, which are difflowt to cut by other methods; 40 Plgure 5 is a section of a glaw grinding dise or lap of the type used for grinding lenses; Flgure 6 idustrates the contours obtained ln. sintering.a lens grinding disc, the mixtures used being an aluminum b4se alloy and a copper base 45 alloy respectively; and Flgure 7 shows a section-of a fumace adapted for the production of lens grinding discs under prezswe. A method of niaking an abrasivb wheel of the 50 type shown in PI . gurl-s 1 and 2, can be Illustrated by a specific example, although ft -will be understood that other compositions and methods of molding and sintering can be used. A mixture of for example 10% diamoncle of from 80 to 140 .55 grit, 10% silicon carbide of from 180 to about -600 grit, and 80% of powdered metoj is introduced.into the mold to form the cutting surface 2 L If the metal consists of Ingredients to be alloyed, it is thoroughly niixqd, screened, and 60 niixed with the abrasive. The mix containing the diamonds is acewat6ly leveled off, and a bacldng niixture of 20% stucon carbide and 80% metal powder is. added to the mold to form the backing S. If the material is to be cold molded, 65 the entire mass is then pressed into a ring under a pressure of, for_examole, from 10,000 to 40,000 lbs./sq., in. This ring, after removal from the mold, can be sintered in an atmosphere which is non-ri@active with respect to aluminum at the 70 temperature used, without the application Of further consolidating pressure. It is desirable, however, to apply a slight pressure to the ring during sintering in order to pre*ent warping. Although aluminum melts at 660' C., the mate75 rial can be h.eated to a temperature very closely approximating the melting point, and in some cases even slightly above the melting point, without loss of shape. At this temperature coalescence or anoying of the particles of metal powder into a strong body can be readily effected., 5 It is powible, however, to obtain a strong sintered article without reaching the melting temperature of the aluminum,. and such a product is produced by th6 diffusion of the solid particles of metal into each other sa as to form a coherent 10 mas& After the ring has been sintered, it is. ready for mounting upon a suitable backing so that it can be used as an abmslve wheel or lap. 7be backing 4 indicated in Mgure 2 may be a reversi- 15 ble ihermoplastic resin, or a backing of metal can be used, and the sintered ring soldered to the metal. A convenient method of sintering wheels. of the general type shown in Fligures I and 2 is illus- 20 trated in Mgure 3. In this figure, the wheels 6 are spaced between ceramic bats 7 and a weight 8 Is placed upon the top of the uppermost bat to exert sufficient pressure to prevent warping during sintering. The fumace is heated by a 25 wire wound resistor 9; an inert gas is introduced into the furnace through the pipe I 0 and the excess gas escapes through the pipe I 1: A number of different atmospheres can be used for the- sintering operation, the simplest and 30 most convenient being ordinary lllilminating gas. Helium and argon are also very satisfactory. If desired, the wheels can be sintered under vac-, uum. Mgure 4 shows a section of a cup wheel wl-iieh- 35 has been found suitable for the surfacing of refractories such as silicon carbide or fire clay bricks and shapes. The abrasive layer 13 may consist of diamonds bonded with metal or a mixture of diamonds, sflicon carbide or other abra- 40 sive, and metal. The backing 14 can be of resin, metal or any suitable material. Sintered aluniinijm base aboy bonds and particularly those containing intermetallic compounds, have been found very satisfactory for wheels of this type- 45 in the case of metal bonded diamond wheels, and particularly those used for cutting or lapping glass, It Is possible to use a relatively soft sintered bond prgviding there is sufficient abrasive to make the surface of the material wear- 50 resistant. ,In such cases, the bond is more or less resilient and during grinding or lapping the abrasive acts much as the hard particles in a bearing metal. -The wear Is taken almost entirc-ly by the hard particles embedded In the re- r,5 silient matrix bnd the metal, eien though soft, is not wom away. I have found -it to advantage to include with the diamonds a certain PrOPOrtion of other abrasives such as silicon carbide, boron carbide or fused all'-in- 1-n- o--rd-e-r Lto in- (o crease the wear-resistant properties of the wheel. This additional abrasive mey be some*hat flner in grit size than the diamonds, although with' he finer grit diamond wheels this Is not always necessary. The additional abrasive when @Us- 65 tributed throughout the metal matrix stiffens the metal and makes it very resistant to wear or abrasion. Thus, even in cases when the additioniil abrasive does no cutting whatever (as when the wheell is used for cutting tungsten car- 70 bide, which is practically as hard as the additional abrasive itself), it reduces the wheel loss, wwch under ordinary conditions is due at -least in part to the "undercutting" or tearing out of the metpa m?@trix surrounding the diamonds. 75
[3]2,187,200 3 The addition of materials such as silicon carbide, bbron carbide or fused alumina, quartz or glass to the mix makes possible the use of. a fairly low percentage of diamonds to -do the cutting, with very little wear of the surrounding matrix. The action of the softer abrasive in making the matrix re@istant to wear is of special lmportatice in the cutting cyf glass, silicon carbide or other hard materials which readily chip and form 10 detritus which has an abrasive action upon the metal of the wheel. A section of a wheel which can be used i6r the grinding of lenses, in which the bond can be sintered aluminum or a-sintered aluminum alloy, i5 is shown in Figure 5. Such a wheel can be made by the method described in,connection with the production of wheels of the 'type shown in Mgures 1 and 2.' The layer IS containing the diamonds is first- iritroduced into the mold and lev20 eled off, and the mix to form the backing 17 then added. The backing layer may consist either of powdered metal or a mixture of powdered metal and an abrasive or filler cheaper than diamonds. The addition of an equivalent 25 quantity of abrasive to the backing tends to give the same shrinkage to the backing as to the surface layer containing -the diamonds, and minimizes warping. ngure 6 shows diagrammatically the contour 30 obtained with the tylx of wheel shown In Figure 5, with the use of sintered aluminum base alloy and a copper-tin alloy respectively. The contour 20 (representing approximately that of the article as originally pressed) 'was obtained with a pbw35 dered mixture of 5% diamonds, 10% silicon carbide and 76.5% powdered aluminum and 8.5% copper, and a bacldng of the same composition except that the diamonds were rpplaced by an equivalent quantity of silicon carbide. The con,i tour 21 represented by dotted lines was obtained from a composition containing 5 % diamohds, fO% silicon carbide, 76.5% copper and 8.5% tin,. with a similar backing in which the diamonds were replaced by silicon carbide. In contrast 45 with the contour obtained with,the aluminum alloy, the contour obtained-with the copper alloy departs considera@bly from that of the original pressed article. Figure 7 shows diagrammatically a method of sintering a lens grinding cuse having a grinding surface of metal bonded diamonds,. In which pressure is applied during the sintering process. The furnace chamber 23 is heated by the wire wound resistance element 24, and the mold 25 is placed upon a support 26 resting upon the bottom of the furnace. Pressure is applied by means of the screw Jack 27. The mold plunger consists of a cylinder 28 which is bored to receive the pin 29. This pin also extends through the bottom or Go curved portion of the mold. In assenibling the mold, the pin and the oiiter ring 30 are placed In position with respect to the curved portion 25, the powdered mix 31 Is introduced Into the mold around the pin 29, and the cylindrical plunger 63 28 is then inserted so as to fit Into the ring and at the same time surround the upper portion of the central pin. 'The mold parts are preferably made of graphite or carbon, but, can be made of heat resistant metal. The mix can be compressed during the heating process from loose powd er, but it is desirable to preforin the article by cold pressing before the conibined application of pressure and heat. a'its latter procedure results In a very dense article. 75 For most purposes, it Is desirable to forni a som ewhat harder matrix than that afforded by the usual aluminum base alloys and for this purpos e metals which form hard intermeta]Hc compou nds with aluminum can be Introduced into the mix. For this purpose, the following mixture of 5 met al powders can be used: Aluminum-copper, 10% to 20% copper. Alunilnum-nickel, 5% to 20% nickel. AluniJnum-iron, 5% to 15% iron. Aluminum-magnesiu'm,'10% to 25% mag- 10 nesium. In ' the above compositions the percentages are give n merely as examples and It will be understoo d that the hardness of the matrix will vary 15 with the amount of the addition agent used. T ernary mixtu!resof metal powder can also be use d in which an intermetallic compound is for med from other metals than the aluminum. Exa mples of such mixtures are aluminiini, mag- 20 nesi um and silicon, in which the magnesium and silic on react to forrn a compound Mg2$i, which har dens the aluminum matrix. When a very hard matrix is desired, the alu--: min um can be combined with other metals to for m a composition containing a relatiiely high pro portion of an intermetallic compound. Examp les of such compounds are CUA12, MnAb, Fe AI3, NiAI3 (or NiAl). Similar compounds with cob alt and magnesium exist; the exact composi- ,,.1 tion of these compounds is not known with certaint y, but the aoproximate formulae are M93M2, M9 2Al3 ' and C03All3. The compositions required to produce varying percentages of these com-@ Oou nds can be easily deduced from stoichi6metric 1 - ielations involved in the formula of the compou nd. Sope of the Qompounds, as for example, the copper compound, are quite brittle, and k Is - desirable to retain some free aluminum In the alloy composition so as to niake the material 4,) resistant to impact or shock. Examples of satisfactory coinpositions are., Aluminum, 20 to 40% nickel. Aluniinum, 20 to 40% cobalt. Aluminum, 15 to 35% iron. In sintering compositions containing intermetallic compounds from mixtures of powdered metals, it is possible to produce an entirely different structure from that obtained in a cast alloy of tliez same composition. In a cast alloy, crystal growth is free to take place in all directions, and the compound niay often be in the form of a continuous brittle network, or in the form of large dendrites. In many cast. alloys, the network of compound renders the material very brittle bef6re the composition of the niass of alloy has become such that the w,hole mass is composed of an intermetallic compound. In alloys made from sintered powders, the intermetame (;o compound is usually intermingled with the,pure metal (or a ductile solid -solution In which the pure metal is the principal ingredient), and the structure,is broken uf to a greater extent than with a cast niaterial.,, This structure makes Pos- C,5 sible 9, high degree df hardness without encountering. the extreme brittleness charaeteristic of some of the pure compounds. In, using metal powder, it Is also possible to get a much more uniform distribution of the abra- 70 sive than can be obtained in a cast alloy mixture. In making a powdered mix the abrasive can be mixed with pure aluminum powder and the desired hardening agent In the proper proportions, or a mix can be made, by using a certaln 75
[4]4 2,137,200 propbrtion of previously alloyed material. With the, harder alloys, especially with coml)gsitions contairiiiig very high percentages of high melting materials such as iron, cobalt aiid nickel, it is desirable to form at least a part of the mixture from material which has been previously@ alloyed. For this purpose a certain proportion of the pure intermetallic compound can be added to the mix. These compounds can be readily crushed and It Is possible to make up alloys approximating the compound compositions, crush them to powder and then mix them with additional aluminum or with aluminum and a further quantity of compound-forming Ingredient. In addition to the abrasive articles illustrated and described, the sintered aluminum and aluminum aroy bonds herein described are adapted for peripheral grinding and cut off wheels for the grinding and cutting of glass, silicon carbide, ter'O ra cotta, and all varieties of refractory and abrasive materials, as@ well as tungsten carbide and other extremely hard alloys. Although the above description- has been primarily concerned with wheels containing diamonds, the bonds are applicable to the production of abrasive articles containing boron carbide ei.ther with or without an abrasive of a lower degree of hardness, and also to the bonding of silicon carbide and fused alunitna. Abrasive laps @o containing boron carbide and silicon carbide have been found satisfactory for the lapping and cutting of glass, the surfacing of refractory articles and otber similar operations. While the cutting rate is somewhat lower than that of the wheels containing diamonds,. the greatly reduced cost makes such wheels commercially practicable. By the term "IntermetaUle compound" is meant an alloy ingredient consisting of two or more metals in which the metals form a chethical com:0 pound. characterized by homogeneity, a deflnite atomic ratio of each metal to each of the other metals, expressible In smar Integers, and chemical and physical prgperties different from any one of the constituent metals. By the term "aluminum base alloy" is meant an alloy of aluminum. and one or more other met- 5 als in which alloy the aluminurii Is predominant. I
