AMERICA\ IASTITUTE OF JIIhISG AND METALLURGICAL ESGINEERS

Technical Pubheahon No. 24x5 Class E, Metals Technology, June 1948

DISCUSSION OF THIS PAPER IS ISVITED Dlscusslon In wrltlng (1 co les) may be sent to the Secre- tary Amencan Instltute of Mlnlng and Xletallurg~cal Engineers, ag West 39tg Street. Sew York 18. N Y Unless speclal arrangement 1s made, dlscusslon of thls paper wlll close Aug I S , 1948 Any dlscusslon offered thereafter should preferably be m the form of a new paper

Sintering in the Presence of a Liquid Phase

BY F V LENEL,* MEMBLR AIME

(New York Meetlng, February. 1948)

Two years ago in Chicago a seminar was held on the theory of sintering of pure metal powders As an introduction to this seminar D r Rhinesl gave an excellent survey of the l~terature on this subject His method of presentation was to summarize the experimental observations on sintering and from them to develop a composite

4 theory of sintering in which he combined all the important contributions to the theory into one organic whole

4 In contrast to the mechanism of sinter~ng of pure metal ponders, wh~ch, of course, always takes place In the solid phase, sintering in the presence of a liquid phase, cannot be treated as one unified mechanism, such as the sintering mecha- nism of pure metals The reason is that there are really several mechanisms de- pending upon the type of alloy system which is sintered and the field of its con- stitutional diagran~ in which the sintering takes place Two mechanisms have been invest~gatcd and nil1 be reviewed in this . paper In the first mechanism the liquid is present during the entire time while the compacts are a t the sintering temperature In other words they are sintered between the liquidus and the solidus of the alloy system and are heterogeneous during the entire sintering cycle The second mech- dnism applies to alloys in which the l iqu~d phase is formed during the sintenng

AIanuscr~pt received at the office of the Institute March 8, 1948

* Department of Metallurgical Engineering, Rensselaer Polytechnic Institute. Troy, N Y

References are a t the end of the uauer

process, but disappears before the sintering process is completed through diffusion and formation of a solid solution These alloys are therefore homogeneous a t the end of the sintering cycle This second mechanism is more complicated because of the two stages of sintering, the liquid and the solid, as they may be called I t is therefore not sur- prising that the few detailed investigations of sintering in the presence of a liquid phase have been concerned with heterogeneous sintered alloys, namely the tungsten-nickel- copper alloys, which is the so-called "heavy alloy," and the cemented carbides The mechanism of sintering of these sys- tems is characterized by the fact that theoretical or near theoretical density is attained during sintering, while simultane- ously a distinct grain growth takes place through solution of the smallest grains of the solid phase in the liquid phase and reprecipitation on the larger grains I n the first part of the survey this sintering mecha- nism which is called for short the heavy alloy mechanism is treated in detall In the second part of the survey experimental ob- servations for the sintenng of homogeneous sintered alloys where the liquid phase d ~ s - appears before the completion of sinter~ng are discussed Much less systematic work has been done on these alloys to which thr commercially important porous bronzes and the iron-nickel-aluminum permanent magnet alloys belong

Emphasis will be laid throughout this survey upon the microstructural and den- sity changes during sintering because the sintering mechanism can usually be de- scribed directlv bv these chanees Other . . . . -

Copyright 1948. by the Amerlcan Instltute of Mlnlng and Metallurgical Enmneers. Inc Prlnted In USA

2 SINTERING IN THE PRESENCE OF A LIQUID PHASE

physical and mechanical properties of the sintered compact will be mentioned only incidentally, not because they are not of great interest and importance, but because their full treatment would unduly lengthen this survey For the same reason hot pressing and impregnation with l~quid metals have not been considered

The first who applied the heavy alloy sintering mechanism in practice were probably Pirani2 and his co-workers who from 1907-1909 made high density tung- sten-nickel alloys as an intermediary step in the production of tungsten wire The first tentative delineation of the mechanism as it applies to cemented carbides was given by Hoyt3 in the 1930 Institute of Metals Divis~on lecture However, the essential features of the process are much easier to recognize 1; the copper-nickel- tungsten alloys and it is therefore quite understandable that we h d the first de- tailed description of the mechanism in a paper by Price, Smithells and Williams4 on the sintering of these alloys Their work is discussed in a paper by Jones6 in which he shows that the conditions for the function- ing of the mechanism stipulated by Price are unnecessarily rigid and that the mecha- nism may be expected to work In a large number of alloy systems The essential features of the mechanism may be de- scribed as follows The constituent powders of the alloy system are intimately mixed and then compacted The compacts are heated under conditions which prevent oxidation, decarburization, and so forth of the constituents to a temperature a t which an appreciable amount of liquid is formed- in other words between the solidus and the liquidus lines of the composition The llquid may be formed either by the melting of one or more of the constituents or by reaction between the constituents of the powder mlxture I n order to make the mechanism effective the constituent which remains

solld must be soluble to some degree in this liquid If this is the case, the liquid will dissolve the smallest particles of this constituent and will reprecipitate them on the larger ones during the smtering process and depending upon the solubilities also dQring the subsequent cooling By this process the grains of the solid constituent will grow and simultaneously the voids in the compact will be eliminated and theoretical or near-theoretical density be reached The final slntered structures will therefore consist of grains of the constituent which stayed solid during sintering im- bedded in a matrix

There are three illustrations of the struc- tures obtained Fig I, taken from the paper by Price, Smithells and Will~ams,' shows the structure of an alloy with 2 pct copper, 5 pct nickel and 93 pct tungsten sintered for I hr a t 14oo'C a t 500 X The large rounded grains are pure single crystals of tungsten which were formed by solution of the smallest of the original tungsten par- ticles in the liquid copper-nickel solution and reprecipitation on the larger particles The matrix around the tungsten grains is a ternary solid solution of nickel, copper, and tungsten Fig 2 from the same paper shows a structure again a t joo X which looks very similar, but which consists of 20 pct silver and 80 pct copper and was produced from a compact of the elemental powders sintered 30 min a t goo°C The large rounded grains in this case are copper which stayed solid during the sintering process, the matrix in which the copper grains are imbedded consists of the copper- silver eutectic with 28% pct copper and 71% pct silver melting a t 779'C The liquid eutectic was formed during the sintering by interaction between the sllver particles and the smallest of the copper particles The large copper grams probably owe their shape again to a solution-repre- cipitatlon process Fig 3 taken from Hoyt's3 paper represents the structure of a ce- mented tungsten carbide containing 13 pct

cobalt a t 2500 X. I t w a s produced f r o m a tungsten carbide-cobalt eutectic which is in ball-milled mixture of cobalt powder and the neighborhood of 13ooOC a small tungsten carbide powder, compacted and amount of liquid of the eutectic composition sintered. The time and temperature of of 34 pct WC forms by interaction between

3 FIG 1-2 PCT COPPER, 5 PCT NICKEL, 93 PCT TUNGSTEN ALLOY SINTERED I HR AT I ~ O O ~ ~

FIG 2-20 PCT SILVER, 80 PCT COPPER ALLOY SINTERED HR AT 9o0°C. FIG 3-CEMENTED TUNGSTEN CARBIDE, I 3 PCT CO.

sintering are not given, but are presumably the cobalt and the smallest tungsten car- in the neighborhood of 134 hr a t 1400°C. bide grains which are in contact with each The rectangular and triangular grains in other. As the temperature rises, more liquid this figure are tungsten carbide which is forms by further solution of tungsten imbedded in a matrix of almost pure cobalt. carbide. According to Takeda's6 consti- This structure is obtained by the following tutional diagram of the pseudo-binary mechanism: a t the temperature of,, the WClCo system, the 13 pct cobalt alloy a t

4 SINTERING IN TIIE. PRESEXCE OF A LIQUID PHASE.

i400°C would consist a t equilibrium of about 79 pct solid tungsten carbide grains and 21 pct of the liquid phase of 38 pct tungsten carbide and 62 pct cobalt Prefer- ential solution of the smallest tungsten carbide grains in the liquid and repre- cipitation on the larger grains is responsible for the formation of the regular tungsten carbide grains, in this case prisms rather than spheres as for the tungsten-copper- nickel alloy This precipitation continues during cooling until the eutectic tempera- ture is reached I n freezing the eutectic deposits its tungsten carbide content on the neighboring grains rather than appear- ing in a typical lamellar eutectic structure, because the space between tungsten-car- bide grains in which the eutectic freezes is, as Hoyt3 explains, seldom more than 2

microns wide Almost pure cobalt is there- fore left as a binder

I t is interesting to note that a sintering mechanism apparently very similar to the heavy alloy mechanism accounts for the slnter~ng of some ceramic materials Roll- finke7 has drawn an analogy between the sintering of sillimanite and the sinterlng of cemented carbides I n either case through a reaction between solid constituents a eutectic is formed which acts as a liquid cementing material between the remaining solid particles However, neither the cera- mists nor any of the powder metallurgists have been able to explain exactly why the materials in question shrink during sinter- ing It is known, of course, that surface tension and interfacial tenslon forces are responable for the disappearance of the pores, just as surface tension forces alone m the case of sintering pure metals, but it has not been possible to describe the course of action through which these forces bring about densification and the way in which the solution precipitation phenomenon is connected with the action of the forces

I n this respect the situation is somewhat simllar to that in which Rhinesl found

himself when he wrote in his survey of the sintering of pure metals "the movement of metal (which is the cause of shrinkage) is presumed to be accomplished through the action of plastic flow or of surface diffusion or of both acting cooperatively under the influence of surface tension as the major driving force " Since Rhines wrote his survey, A J Shaler8.9 in his work on the kinetics of the sintering process has shown that pure metals shrink through a process of viscous flow or lattice diffusion rather than surface diffusion or plastic flow On the basis of Frenkel'slo work on the viscous flow of crystalline bodies under the action of surface tension Shaler was able to derive mathematically the rate a t which the densification of porous bodies of a given pore size should proceed He showed how this rate would be modified through the pressure of entrapped gas of a given pres- sure and proved his theory by experiments on the sinterlng of loose copper powder under controlled test conditions In the discuss~on of the heavy alloy mechanism a close examination will be made of the existing data on the rate a t which densifica- tion during sintering proceeds in the hope that some day these shrinkage rates may be amenable to mathematical treatment simi- lar to the one Shaler has given for the rate of densification of pure metal powders

I n order to galn a more complete under- standing of the heavy alloy mechanism the salient features of the s~ntering process are discussed in the following order (I) The mixlng of the powders (2) The compact- ing of the powders (3) The sintering atmosphere (4) The effects of impurities (5) The temperature of sintering and the formation of a eutectic liquid (6) The difference in melting point and the solu- bility of the components in each other (7) The particle size of the original powder and the grain size of the final product (8) The composition, that is, the ratio of molten and solid phases during sintenng and the shape and structure of the pre-

clpltated grams (9) The effect of slnterlng time, that is, the rate of slntering In this discussion not only the fundamental In- vestigations ment~oned above, but also a number of later papers whlch throw light on one or the other feature of the s~nterlng process are referred to

Mzxzng of the Powders

As is well known the ingredients of cemented carblde powder mlxtures are always ballmilled together The ball mllllng has two purposes, first, i t coats the mdi- vidual grams of tungsten carbide w ~ t h cobalt and thus br~ngs them lnto intlmate contact, and secondly, it decreases the slze of the particles by grlnding The Brltlsh commercial practice26 In the manufacture of heavy alloy provides only for thorough mlxing in the dry state, while Kieffer and HotopI2 recommend wet grlnd~ng of the tungsten-copper-nickel powder mixture in order to Insure very even distribution of the components and thereby h~gh density In the slntered alloy Kurtz13 in t h ~ s country also reported ball mill~ng of the powders and attributes his success In obtaining high denslty from powders which are coarse compared with Price's powder In part to the ball milling operation

Compactzng the Powders

Price and co-workers4 show that the compacting pressure has very llttle Influ- ence upon the final denslty of the sintered compacts, as compacts which are molded to a lower green density shrlnk correspond- lngly more durlng slntering Meyer and Ellender" confrm thls behav~or for ce- mented carbides a t least m those cases where the partlcle slze of the tungsten carbide powder is fine enough and the slnterlng temperature hlgh enough so that near theoretlcal denslty IS reached durlng sintering

Tize Sznlerzng Atmosphere

Both heavy alloy and cemented carbldes are sintered in hydrogen In the case of heavy alloy KelleyI4 has emphasized the Importance of uslng pure dry hydrogen because of the affinlty of oxygen to tung- sten Kelley believes that the activlty between tungsten, copper, and nlckel will be increased If the partlal pressures of water vapor and oxygen are kept low In the case of cemented carbldes Hoyt3 has graphically portrayed the decarburlzation and subsequent oxidat~on of the tungsten carblde when cemented carbldes are s~ntered in hydrogen contain~ng water vapor Sykes16 also shows structures con- talnlng W2C Instead of WC which were obtained by decarburization and reports that cemented carbldes are usually placed in carbon tubes or boxes for sinterlng to protect the alloy against loss m carbon Clark,16 who In his work on the constitution of tungsten-alumlnum alloysproducedstruc- tures u ith 90 pct W, 10 pct A1 from powders very s~mllar to the heavy alloy structure, slntered In vacuum because alloys richer than 60 pct In tungsten would not shrlnk when sintered In hydrogen, presumably because of the slow el~mlnatlon of alr entrapped during pressing

Effects of Impurztzes

The importance of pure raw mater~als when near theoretlcal density 1s to be at- tamed during sintering is generally em- phasized As Hoyt3 expla~ns, lmpurlties such as surface films of very small magni- tude or tenacious ox~de films or even a massive lmpurlty mlght prevent the wet- tlng of the liquld cement and thus introduce a locallty of low cohesion between l~quid and solld Both Hoyt3 and Sykes16 show microstructures of cemented carbldes which are porous because of the ~nfluence of ~mpurltles

6 SINTERING IN THE PRESENCE OF A LIQUID PHASE

The Temperature of Sznterzng and the For- matzon of a Eutectzc Lzquzd

In contrast to the sintering of pure metal powders there does exist a mlnimum smter- ing temperature for the heavy alloy sintering mechanism, this is the tempera- ture when sufficient liquid is formed so that the solution-reprecipitation process can take place Price and co-workers4 give a very illuminating series of density data and micrographs of tungsten-copper-nickel alloys slntered at increasing temperatures from 950 to 1400°C where quite suddenly between 1300 and 1350°C the density jumps from 72 to 90 pct of theoretical and the characteristic microstructure of large tungsten grains in a matrix appears While in heavy alloy the liquid is formed by the melting of the copper and nickel solution, in cemented carbides it is pro- duced by the eutectic reaction between tungsten carbide and cobalt Hoyt3 made the formation of such a eutectic quite probable, it was definitely established by the experiments of Uryman and Kelley17 who showed that the liquid squeezed out in hot-pressing cemented carbides contained up to 20 pct tungsten and had a cored dendritic structure interlaced with a eutectic network According to Sykes18 and to Takeda6 the temperature of the stable WC-Co eutectic lies between 1270 and 128o"C, while Sandford and Trentlg report a eutectic temperature of approximately 1320°C Under certain conditions of cool- ing a metastable ternary eutectic of a cobalt rich solid solution, a cobalt-tungsten double carbide and graphite may be formed which melts at 1225"C, however the cobalt-tungsten double carbide is not found in cemented carbides uith up to 55 pct cobalt unless the carbon content of the tungsten carbide is below that required for the compound WCl9 and may there- fore be disregarded in this discussion HoytJ made a thermal analysis of a 13 pct cobalt tungsten carbide alloy and found thermal

arrests at 1355°C in heating and 1320°C in cooling 1355°C is probably the point where a fairly rapid formation of liquid occurs and will, according to Meyer and Ellender," depend not only upon the tem- perature of the eutectic, but also upon the rate of reactlon between the cobalt and the tungsten carbide which m turn is influenced by the rate of heating, the percentage of binder and the fineness and distribution of the tungsten carbide Meyer and Ellender" have reported high hardness, which prob- ably means high density for an 8 pct cobalt alloy sintered a t only 12oo"C However this alloy contalned 4 pct iron and I pct chromium as impurities which may account for the satisfactory hardness a t the low sintenng temperature At excessive sinterlng temperatures too much liquid phase wlll be formed and a decrease in density and hardness wlll be observed as was shown by Meyer and Ellender" and by Sykes l6

In those alloy systems where the liquid phase is formed by a eutectlc react~on, this reaction must take place a t the points of contact of two dissimilar components ac- cording to Jones In the copper-silver sys- tem, for example, the eutectic must be formed a t the points of contact between particles of elemental copper and silver, the sintenng mechanism is not expected to work successfully uith a powdered alloy of copper and silver A certain negatlve proof of this prediction may be found in the work of Hensel and LarsenZ0 on sintering copper- silver alloy powder compacts w ~ t h 739 and 10 pct copper a t 765 and 82s°C, that is, below and above the eutectic temperature of 779°C No difference in the microstruc- ture for the two slnteAng temperatures was found, the structure showed little grain growth and the original shape of the pow- dcr particles was very pronounced For compacts made of a mixture of elemental powders of the 10 pct copper composition

sintered at 825°C one would expect to find the heavy alloy sintering mechanism

F v LENEL TP 2415 7

The Dzfference zn Meltzng Pozltt altd the Solubzlzty of the Compolteltts z l t Each Other

Price, Smithells and Williams in their first paper4 on sintered tungsten-nickel-cop- per alloys stipulated the following condi- tions for the functioning of the heavy alloy mechanism (I) an appreciable dif- ference in melting points of the components,

k (2) the high melting point metal should be soluble in the lower melting point metal, but (3) the lower melting point metal should be insoluble or have a very small solubility in the high melting point metal Later work on sintering in the presence of a liquid phase has definitely shown that the author's first and third conditions, ap- preciable difference in melting point and insolubility of the loner melting point

L constituent in the high melting constituent are not necessary for the functioning of the mechanism In order to prove the latter

1 point, Price, Smithells and iVilliams4 slntered compacts of 4 pct silver, 6 pct cobalt, and 90 pct tungsten for one hour a t 15ooOC In spite of the similarity of silver and copper on the one hand and nickel and cobalt on the other very little shrinkage took place and the final density a a s only 61 pct of theoretical They attributed this result to the immiscibility of silver and cobalt in the liquid state and the solubility of cobalt in tungsten In contrast to Price and his co-workers, KurtzI3 achieved 9934 pct of theoretical density in tungsten- cobalt-silver alloys Kurtz did not report the exact composition and sintering condi- tions of his alloys, the microstructure he shows differs considerably from that of the tungsten-copper-nickel alloy, as the tung- sten grains are irregular in shape rather than rounded and the liquid constituent does not form a continuous phase surround- lng the tungsten grains, but seems to be accumulating in little lakes

I n his discussion to the paper by Price and co-workers, W P SykesZ1 pointed out that even one of the examples cited by the

authors, namely the iron-copper system, does not follow their rule Price and co- workers showed that an 80 pct iron 20 pct copper alloy sintered 3 hr at 1400°C showed 93 pct of theoretical density and the typical structure of rounded grains of iron in a matrix of copper saturated with iron despite the fact that 9 pct copper is soluble in iron at 1400°C which is obviously not a very small solubility Direct micro- graphic evidence of the solution of the copper in the outer rim of the solid iron particles during sintering is shown in heat- tinted high magnification micrographs of 25 pct copper-iron compacts by Korthcott and Leadbeater 22

Sykes calls attention to the tungsten- iron, tungsten-cobalt, molybdenum-cobalt, molybdenum-iron, and molybdenum-nickel systems All of them show the characteris- tic structure of large rounded grains in a matrix in certain composition ranges, as Sykes proved by a number of micrographs IIoaever, in all of them one or more inter- mediate phases occur formed by a peritectic reaction between tungsten or molybdenum and the liquid phase, and all of them exhib~t appreciable solubility of the second phase in tungsten or molybdenum at the peri- tectic temperature

Jones5 in his discussion of the paper by Price and co-workers went even further According to him the only condition neces- sary for the functioning of the mechanism is that there must be present at the time of sintering solid particles of a certain fineness and a liquid which is partially capable of dissolving them I n the copper-silver system, for example, the melting points of the two metals are little more than loo°C apart, the temperature of sintering, goo°C, is lower than either of the melting points, and both metals have appreciable solid solubility in each other On the basis of his stipulation Jones predlcts that the heavy alloy mechanism should be falrly common and should be found in sinterlng any of the

8 SINTERING IN THE PRESENCE OF A LIQUID PHASE

binary systems a t the temperature shown in Table I

TABLE I-Sznterzng Temperature of Varzous Alloys

Alloy I Per Cent A 1 Per Cent B

-.

Compos~t~on Per Cent -

I

Jones himself verified his prediction on the first alloy system In the table, the 70 pct silver-30 pct lead alloy sintered a t 4o0°C for 3 hr He observed an increase In density from 50 pct of theoretical for the green compact to 95 pct of theoretical for the sintered compact I n contrast to these results on a sdver-lead alloy are B a l ~ h i n ' s ~ ~ data on a 75 pct copper 25 pct lead alloy He investigated the dimensional changes in sintering compacts of the mixed powder a t 550 and 8m°C after compacting to den- sltles from 40 to 80 pct of theoretical Nelther the atmosphere nor the time of sintering is given in the paper Balshin found that the compacts pressed to low green densities would shrink, those pressed to high densities would grow in sintering, but in no case anywhere near theoretical density was reached, although Balshin reports a rounded shape of the copper grains which may indicate a t least a be- ginning of the heavy alloy mechanism Balshin attributes the lack of shrinkage of copper-lead compacts as compared to those of pure copper to the fact that the particles of copper are almost completely isolated

because of the distribution of the liquld lead in the interstices between the copper particles The solubility of copper in lead a t 800°C, which according to Hansen'sZ5 diagram amounts to about 4 pct is not so much lower than the solubility oi silver in lead a t 400°C of about 7 pct According to Jones' stipulation one would therefore expect the heavy alloy mechanism in sintering a 75 pct copper 25 pct lead alloy a t 8o0°C In view of Balshin's incomplete description of his experiments it is impossi- ble to asslgn reasons for his obtaining low sintered densities

An extreme case among the alloys in the table mould be the 50-50 copper-nickel alloy sintered a t 137s°C, because these metals form a complete series of solid solution At equilibrium this system would conslst of about >$ of a liquid with 40 pct nickel and 60 pct copper and about N of a solid with 55 pct nickel and 45 pct copper Whether such a system mould still keep its shape may be somewhat questionable, but on the other hand complete densification may possibly be obtained before equilib- rium is reached No data have been pub- lished on the s~ntering of copper-nickel alloys between the liquldus and the solidus However, Jones' prediction for another solid solution alloy, the 90 pct copper 10 pct tin alloy sintered a t 925OC has been proven by Prlce, Wdliams and Garrard26 who reported the typical heavy alloy structure of large rounded grains in a continuous matrix

This concludes the comments on the solid solubility of the liquid constituent in the solid one Much more critical is the solubility of the solid constituent in the liquid First, it should be remembered that solubility in the liquid, but not solid solubil~ty of the two constituents below the freezing point of the liquid, is required As a matter of fact, the excellent properties of tungsten carbide-cobalt alloys as tool materials depend according to Takeda6 upon the fact that the solid solubility of

tungsten carbide in cobalt is very small so that almost pure cobalt forms the binder in cobalt-tungsten carb~de alloys In the case of nickel-tungsten carbide alloys the binder will contain tungsten carbide and giaphite besides the nickel, in the case of iron-tungsten carbide alloys ~t will contain a brittle tungsten Iron double carbide Takeda contends that these types of

F binders will make iron-tungsten carbide and nickel-tungsten carbide unsatisfactory as tool materials Tha t the cobalt in the sintered cemented carbides is really almost pure was already shown by Hoyt3 on the basls of measurements of magnetic induc- t ~ o n and electromotive force

How great the solubility of the solid constituent in the liquid has to be in order to make the heavy alloy mechanism opera- , ble 1s still an open question If the solubility is infinitely small, a solution and reprecipi- tation of the solid particles IS, of course,

C impossible However, if the liquid com-

pletely wets the solld particles and if there is enough liquid present to completely envelop the sol~d particles, one would expect complete densification even without any solubility Complete wetting means that the surface tension of the solid must be larger than the sum of the surface tension of the liquid and the interfac~al tension between solid and liquid This interfacial tension is very dependent upon the purlty of the metals Thin layers of adsorbed or chemically combined impuri- ties may therefore change the wetting characteristics radlcallp and may also have a strong Influence upon the sintering mechanism Price and co-workers4 were unable to obtain dense compacts of tung- sten and copper by their method of sinter- ing and a t t r ~ b u t e i t definitely to the insolubility of tungsten m copper As is well known, tungsten-copper, tungsten-silver, and molybdenum-s~lver are widely used for contact materials and welding elec- trodes Kieffer and Hotop2' who describe the German practice in producing these

combinations state that because of the insolubility of tungsten In copper and the very small solubility-a few tenths of a percent-of tungsten in s~lver above I~ooOC, these two types of materials can- not be produced by sintering a compact of the mixed powders above the melting point of copper or silver Instead, the compacts are sintered below the melting points and later on repressed hot, or impregnation methods using l~quid copper and silver, respectively, are used, these methods are outslde the scope of this survey Molyb- denum-s~lver contacts can, however, be sintered above the melting points of sdver, because several per cent of molybdenum are soluble In silver a t 14w'C As would be expected the solution of the very fine molybdenum particles in the liquid silver a t 14ooOC is accompanied by a considerable shrinkage of the compact hlolybdenum ir not soluble in solid silver, therefore during cooling the dissolved molybdenum is deposited upon the larger molybdenum grains, a phenomenon very similar to the one observed in cemented carbides

Very little has been published about the American practice in producing contact materials A micrograph a t 750 X of a dense tungsten-copper compound is shown In an article by Hensel, Larsen and Snazy 28

The structure shows grains of tungsten, some of them irregular in shape, but most of them rounded of varying apparent diameter from about I to about 6 microns in a matrix of copper The sue of the tungsten part~cles is therefore considerably smaller than in Price's micrographs and the ratlo of t u n ~ s t e n to base metal is also con- siderably lower The authors note that the structure is completely free from poroslty and has the appearance of an alloy in which a secondary phase has been precipitated

The Partzcle Sgze of the Orzgand Powder a ~ ~ d the Graan Saze of the Fznal Product

One of the important features of the heavy alloy sintering mechanism is the

I 0 SIILTE.RING IN THE PRESENCE OF A LIQUID P W S E

grain growth of the solid particles by solu- tion of the finest particles in the liquid and reprecipitation on the larger ones A casual observation of the micrographs in the paper by Price and co-workers will prove this growth immediately Before sintering, over 80 pct of the tungsten particles had diameters smaller than 5 microns, after sintering the average grain size of the particles is 25 microns I n the micrographs of commercial cemented carbides this grain growth is not so directly evident The reason is that the composition and the time and temperature of sintering are chosen to avoid excessive grain growth, because the finer the tungsten carbide grains in the final product are, the higher is its density and strength (cf the data and microstructures, Fig 3 and 4 in Sykes'16 paper), and the better its properties as a tool material Tha t considerable grain growth can take place also in cemented carbides has been proven repeatedly

Wyman and Kelley17 showed it par- t~cularly in alloys ui th a fairly high binder content SykesI5 illustrated it by micro-

graphs of the increasing grain size of a 6 pct cobalt alloy when the slntering temperature is increased from 1425 to I 500 and I 575OC Kieffer and Hotop29 show side by side the structure of a cemented carbide after 2 hr sintering and after 2 0 0 times 2 hr sintering a t 15rnOC Accordingly, there seems to be hardly any doubt that the solution and precipitation process is an essential part in the sintering of all cemented carbides Grain size in t h ~ s discussion refers, of course, to the size of the individual par- ticles of tungsten carbide The grain size of the cobalt, on the other hand, in which the tungsten carbide particles are imbedded is generally very large Back reflection X ray diffraction patterns taken by Sandford and TrentI9 indicate that the gram slze is of the order of I mm and that most of the cobalt is In the cubic form

In order to explain the process of grain growth Price, Smithells and Williams4

point to the well established fact that for very small crystals the interfacial tension between crystals and their saturated solu- tion is greater than for larger crystals and that for this reason these very small crys- tals have a higher solubility than larger ones 30 The authors cite experiments by H ~ l e t t ~ ~ on the solubil~ty of gypsum crystals in water according to which crystals o 6 microns in diam have an 18 pct higher solubility than crystals 4 microns in diam which have normal solubility To support their theory, Price and co-workers made sintering experiments with copper- nickel-tungsten compacts from three grades of tungsten powder Compacts made from a fine tungsten powder (98 pct finer than I

micron) reach theoretical density in one hour at 14rn0C, those made from a medium powder with 13 pct finer than I micron, 67 pct finer than 2 microns, reach only 99 pct of theoretical density in one hour a t 14ooOC and have to be sintered 3 hr a t 15rnOC in order to reach full theoretical density, compacts made from a "coarse" tungsten powder with only I pct of par- ticles finer than I micron and 61 pct of particles coarser than 5 microns reach only 97 pct of theoretical density, even when they are sintered 3 hr a t 15ooOC Meyer and Ellender" report analogous results for the hardness of cemented carb~des with 8 pct cobalt Compacts from coarse powder with only 0 8 pct particles finer than I

micron and 91 pct coarser than 3 microns reach a maxlmum hardness of only Rock- well A 75 uhen sintered 3.i h r a t I~ooOC, because the material contains a consider- able number of pores which rn testing on the Rockwell machine cause a low readlng However, compacts from a powder with 2 2 pct of particles finer than I micron reach a hardness of Rockwell A 89 after % hr s~ntering a t 15ooOC KurtzI3 reached 99% pct of theoretical density in slntering for about 1% hr a t 1350 to 14rn°C tungsten- copper-nickel compacts from a ball-milled powder with only some 13 pct of particles

F V LENEL-TP 2415 I I

finer than 2 microns All of these tests cer- tainly prove, as C h a ~ t o n ~ ~ pointed out in the discuss~on of the paper by Price and co- workers that a large amount of shrinkage and presumably gram growth may take place even if only a small percentage of the powder is less than one mlcron in diameter If only particles less than I micron came into play in connection with the solution preclpitatlon process, one would expect very llttle grain growth to take place .4nother phenomenon whlch is not ex- plained by the d~fferential solubility of very small grains 1s the considerable grain growth upon prolonged sintering Price and co- workers4 showed that a copper-nickel- tungsten compact sintered one hour a t 1400°C wlll have an apparent slze of the tungsten grains of 10-50 microns, whlle after an additional 5 hr of sintering a t 1400°C the grain size will be three to four times as large Jonesz3 in his discussion of the paper asked q u ~ t e rightly whether this growth from the "size of golf balls to that of rugby footballs" also represents continued precip~tation f rom the supersaturated liquid phase Similar grain growth In compacts of 25 pct copper 75 pct iron slntered a t I roo°C for J$ hr and 4 hr respect~vely is shown by Northcott and Leadbeater z2 These authors believe that the grain groh th may be only partly due to solut~on and reprecipitation and partly to slmple coalescence of the Iron particles

In discussing the sintering of copper- sllver compacts Jones6 asserts that it is possible to proceed If all the particles are coarser than I mlcron but belleves ~t neces- sary that some of the particles should be finer than I micron a t the temperature of sintenng, a condition which may be brought about by the diminution In size of the particles during the eutectlc formlng process I t should be noted that the critical particle size for the dlfferentlal solubility of tungsten particles in a copper-nickel matrix a t r400°C 1s not known I t may be much larger or smaller than the one

micron for the solubil~ty of gypsum in water a t room temperature I t cannot be calculated because the interfacial tens~on between tungsten and the copper-nickel liquid would have to be known for such a calculation I n general, it may be said that the physlcs of the solution precipitation process are not yet completely understoocl and that further experiinents as well as a theoretical approach may be necessary to solve the problem

The Composztzon, thut zs, the Rutzo of Molten clrrd Solzd Phases dzcrzitg Szntertrzg

and the Shupe and Structure of the Preczpztated Gruztts

Price, Smithells and Williams4 set thc limit of the tungsten to copper-nickel ratlo In their alloys a t 95 5 At higher ratios they reported lower density than for alloys lower in tungsten, although theoretically the dens~ty should be higher Kurtz13 reported on tungsten alloys with 9935 pct of tungsten and an unspecified W pct of alloylng ingred~ents which were sintered between 1500 and r650°C to a densit] of 19 2 g per cc which is 99% pct of the value for fully swaged and drawn tungsten Microstructures of these alloys s h o ~ equiaxed grains and look almost like thosc of pure tungsten except that the grain boundaries in which the alloylng ingredients are concentrated are a little w~der With less than I pct of l~quid constituent present, it IS

not surprising that the rounded g r a m which both Price and co-workers and Kurtz show for 90 pct tungsten alloys could not be formed Grain sizes from 2 0 0 grains to 18,ooo grains per sq mm are illustrated in Kurtz's micrographs of the 99% pct tungsten alloy, but nothlng is reported on how these different grain sizes are obtained

Commercial cemented carbides contain between 3 and 20 pct cobalt or other blnder material Ko investigations on the struc- ture or denslty of cemented carbides w ~ t i lower binder content than 3 pct were found Alloys with higher binder content up to

I 2 SINTERING IN THE PRESE

55 pct cobalt have, according to Wyman and Kelley,17 the same structure as those with lower cobalt except for the ~ncrease In the slze of the tungsten carbide grains Dawih13' and Dawihl and Hlnnueber36 investigated the tungsten carbide skeleton which IS formed during the sintering of cemented carbides by determining the transverse strength of the carbides before and after dissolving the cobalt wlth hydro- chloric acid After thls treatment the tung- sten carbide skeleton contains less than o 04 pct of cobalt Its transverse strength is 43 pct of the orlginal strength for a 3 pct cobalt alloy, 27 pct of the onglnal strength for a 6 pct cobalt alloy and no strength for an 11 pct cobalt alloy This indicates that during the normal one hour sinterlng the tungsten carbide particles of cemented carbides with up to 10 pct cobalt grow to- gether and form a skeleton This skeleton formation 1s a function of tlme, since the I I pct cobalt material showed 10 pct of the onglnal transverse strength after the hydro- chloric acid treatment when sintered for 120 hr, but crumbled after the treatment when sintered for only one hour Measure- ments of the coefficient of expansion of cemented carbldes confirm the skeleton formation On the other hand, Sandford and TrentlQ report that molten zinc will attack the cobalt in such a way that when the zinc-cobalt alloy formed is dissolved away, the carblde is left in the form of a powder containing very few aggregates On the basis of Dawihl's work S k a ~ p y ~ ~ suggests dividing the cemented carbldes Into three groups, those with less than 3 pct cobalt where the amount of binder is insuf- ficlent to form a strong and impact-resist- ing skeleton during the sintering process, those with 3 t6 8 pct cobalt where maximum toughness is ach~eved by the formation of a fairly complete skeleton and those with higher cobalt content where the toughness is due to the cobalt binder rather than the tungsten carbide The cemented carbides seem to be the only group of alloys in

.NCE OF A LIQULD PHASE

which such a skeleton is formed When Prlce, Smithells and Wllliams4 treated their alloy wlth aqua regia for 24 hr, all of the copper-nickel phase was dissolved and rounded grams which proved to be slngle crystals of pure tungsten were left unat- tached as a resldue

The shape, prlsmatic or spherical in which the solid particles appear In the microstructure of the sintered alloy seems to depend upon the crystallographlc system in which the metal or compound crystal- lues Tungsten carbide which is hexagonal appears in the shape of prisms, whlle not only all the cubic metals, such as tungsten, molybdenum, copper, iron and silver, but also the cubic carbides, like tantalum carbide and the mlxed tantalum-titanium carbides (d Kelley's3' and E n g l e ' ~ ~ ~ micrographs) appear in spherical form I t would be interesting to see whether the 90 pct tin-10 pct lead alloy sintered a t 190°C which Jones6 suggests would also show prlsmatic crystals of the tetragonal tin In connection with the complex ce- mented carbides brief reference may be made to the literature on the structure and the relationship between structure and properties of these alloys, no detalled discussion seems necessary since the phenomena involved In sintering complex carbides do not differ greatly from those for the simpler tungsten carbide-cobalt alloys 16,34,36,39-43

The Eject of Sznterzng Tzme, that zs, the Rate o f Sznterzng

The published data on the effect of sintering time or more specifically the rate at which densification takes place are unfortunately very sparse Price, Smithells and Wllllams4 in their paper on tungsten- nickel-copper alloys present a table, in which shrinkage and density of an alloy with 2 pct copper, 5 pct nickel and 93 pct tungsten sintered a t 14ooOC is tabulated vs sintering time Their data are shown in Fig 4 in the form of a graph I t will be

F V LENEL-TP 2415 13

seen that half of the density increase from decreases again With a slntering tempera- the green density of 10 5 to the theoretical ture of 1350°C the density Increases to denslty of 17 8 has been gained in less than 14 j or about 97 pct In 45 mln , stays con- I mln , whlle ~t takes 6 hr for thls particular stant for sintering t~mes up to 2 hr and

-0 l/2 1 2 3 4 5 6

TIME (HOURS1

FIG 4-DENSITY YS SlNTP IUNG TIWE AT 1400°C FOR 2 PCT CU, j PCT NI, 93 PCT W ALLOY

alloy to reach theoretical density KO quantitative data are given on the relation- ship between sintering tlme and tempera- ture I t may be assumed that the rate will Increase rapidly with increasing tempera- ture, although practical difficulties with distortion are encountered a t temperatures too far above the crltical Besides raising the temperature, increasing the percentage of very fine tungsten particles and lower- lng the refractory-base metal ratio will speed up the reaction 26 Graphs for density vs slntering tlme of a commercial cemented carbide with 6)/4 pct cobalt sintered a t 1350, 1400, and 14jo0C are shown by Burden." When the alloy is slntered a t r400°C i t reaches a density of 14 7 g per cc which is about 98 pct of theoretical In 30 min and does not exceed this density even when sintered for 4 hr When sintered a t 1450°C it ieaches theoretical denslty withln tenths of a percent In about 155 hr, but for longer sintering tlmes ~ t s density

increases agaln slowly to 14 7 for 4 hr sinter- ing time The purpose of Burden's graphs was to illustrate the control of the com- merclal sintering operation and it is doubt- ful whether his data should be used for calculations of the slntering mechanism

.Further quantitative data upon these relationships would undoubtedly help the understanding of the heavy alloy sintering mechanlsm

ALLOYS WHICH ARE HOMOGENEOUS AS SINTERED

Very few lnvestlgations of the sinterlng mechanlsm of alloys which become homo- geneous du r~ng the sintering process exlst Kleffer and Hotop have speculated in a general way on the mechanlsm Surface films and surface irregularities are sald to be eliminated by the llquid phase, so that the rounded powder particles are better able to move The llquid due to its surface tension has a tendency to take

14 SINTERING IN THE PRESE :NCE OF A LIQUID PHASE

up as little space as possible and will there- fore move the solld particles into positions of closest packing One should therefore find a sudden increase in shrinkage when the liquid phase is formed Once the more or less homogeneous solid solution alloy IS formed, the sinterlng mechanism should closely resemble the slnterlng of pure metals

Copper-lzn Alloys

The largest number of experimental investigations have been made on the cop- per-tin system Thls is the commercially most Important alloy in the group of alloy systems which become homogeneous during sintering However, in spite of the billlons of selflubricatmg bearings, filters and friction materials which have been pro- duced from copper and tin powders, there is much yet to be learned about the sinter- ing mechanism The alloys which has been most extensively studled is one consisting of 90 pct copper, 10 pct tin, usually with an addition of graphite As mentioned above, Price, Wllliams and GarrardZ6 sintered an alloy of this composition at 925°C At this temperature the alloy is heterogeneous consisting of a solid phase higher in copper and a liquid phase hlgher in tin Generally, however, this alloy is sintered a t a tempera- ture sllghtly below its solidus of 838°C for a sufficiently long time, 15 to 60 rnin , so that a fairly homogeneous alpha bronze is formed by diffusion Thiq structure is therefore solld a t the end of the sinterlng period The question arlses how to get from the discrete particles of copper and tin in the green compact to the homoge- neous alpha bronze solid solution in the finlsh sintered compact Two routes have been suggested and experimental evidence has been adduced for both The two routes are the same up to the tin melting point A certain amount of diffusion between solid tin and solld copper wlll take place, but unless the compact 1s heated very slowly or is held a considerable time below the melt- ing point of tin, this solid diffusion will not be extensive When the tin melts, the two

routes diverge According to Jones" the melting tin surrounds each particle of cop- per with a thin film or as Drapeaud6 describes the phenomenon "the molten or liquid tin appears to be absorbed into the copper structure leaving spaces or voids where the tin was originally When the tln 1s absorbed, in the solld copper struc- ture, a diffusion and alloying tends to occur " According to these suggestions the molten tin is drawn by capillary forces Into the voids in and between the copper parti- cles forming a sort of cementing medium, whereupon inter-diffusion of tin and copper, that is, diffusion of the tin Into the copper and a t the same tlme solution of the copper in the liquid tin-rich phase, follows Evi- dence of thls sintering mechanism may be seen in the colored micrographs of H E Halld7 which were Interpreted by Sauer-

1

wald Hall sintered a compact of 84 6 pct copper, 9 4 pct tin, and 6 pct graphite compressed at 60,000 psl at 810°C and ,

took the micrographs before sintering and after I, 3, 7, 15, and 30 rnin of sintering Hall's Flg 3 showing the specimen heated for 3 mm at 810°C which 1s interpreted in Sauerwald's Fig 19-2 suggests that it was taken shortly after the tin-rich liquid phase had finished spreading through the inter- stices in and between the copper particles In the center of several of the areas from which the tin-rich liquld seems to have spread voids are recognizable which accord- ing to Drapeau's interpretation may be the locations of the original tin particles This spreading of the tin-rich liquid phase may be seen even more graphically in colored movlng pictures of the diffusion of copper- tin-graphite compacts which were taken through a microscope a t the laboratories of the Metals Disintegrating Co under condi- tions very similar to the ones for Hall's micrographs

The second route is suggested by H L Waindg who investigated the sintering of compacts of 88 2 pct copper, 9 8 pct tin, and 2 pct graphite Wain used compacting pressures of 5, 10, IS, 20, 25, 40, 70, and

~oo,ooo psi, sintering temperatures of 300, 500, 700, 800, and 830°C and sintering times of 30, 60, 240, and 480 min Only a few micrographs are shown in the paper, but the author obviously has taken many more and on the basis of these micro- graphs, particularly those of samples sinteied a t 300 and 5o0°C describes the slntering process as follows "When the

r tin particles melt, they rapidly dissolve sufficient copper to form the higher melting point q solid solution-the compound Cu3Sn with additional tin in solid solution accord- ing to Hoyt's60 diagram-which is solid up to temperatures above 6m°C Thus the tin rich areas a t this stage consist of a solid shell of q possibly containing a molten core of higher tin content depending on the size of the original tin particles This core gradu-

C ally absorbs copper by solid diffusion through the q shell The formation of a relatively high melting point envelope

. ) explains the lack of evidence of any flowing of molten tin or tin rich solution into the pores and cavities of the copper matrix On further heating to 5o0°C the diffusion trend is for tin to diffuse out into the copper matrix, thus one gets a bronze fringes around the tin areas which a t this stage are duplex q plus 6 With continued heating a t 5o0°C the a bronze area increases In extent a t the expense first of q then of 6 "

Wain himself emphasizes that the mecha- nism he describes resembles much more the sintering by diffusion of two solid consti- tuents since the molten phase soon after i t has appeared is capsuled off and most of the diffusion is taking place in the solid phase He is well aware that his mecha- nism could be modified by changes in the rate of heating Neither his nor Hall's data are sufficient to decide whether such differences in the rate of heating may be responsible for the difference in mechanism I t is even possible that the conclusions which Wain drew on the basis of micro- graphs of the relativity slowly heated samples sintered a t 300 and 500' C may not hold for the samples sintered a t higher

temperatures which obviously were heated faster To this reviewer it appears that other factors also may modify the mecha- nlsm The compacting pressure may influ- ence the size of the inter- and intraparticle cavities and thereby the effects of the capillary forces which cause the liquid tin to spread, the size of the tin particles may determine whether there is sufficient liquid phase present a t each center of melting to permit spreading and so on I t seems quite possible that either route of sintering or a combination of both routes may be fol- lowed depending upon the experimental conditions and it must be left to further experimental work to decide what these condit~ons are I t may be ment~oned that the existence of the q and 6 phases in com- pacts sintered at 4m°C was also proved by Ishikawa61 by means of X ray diffrac- tion These intermediate copper-tin phases would, of course, appear in the diffusion sequence regardless of whether the liq- uid phase spreads or becomes rapidly capsulated

The last stages of the sintenng process, that is the homogenization of the alpha- bronze have been made the subject of a special study by Carter and M e t ~ a l f e , ~ ~ who investigated the homogeneity of 89 pct copper, 10 pct tin, I pct graphite compacts sintered between 700 and goo°C by X raj diffraction methods They find a differ- ence in structure between compacts sin- tered above and below 798OC, the peritectic temperature which is the solidus line of the copper-tin system from 12 to 2 2 pct tin Compacts sintered below this temperature are quite inhomogeneous with the com- position of the alpha solid solution varying within a wide range up to 10 pct and some second phase delta also present The extent of the inhomogeneity a t the low sintering temperature depends somewhat on sinter- ing t~me , compacting pressure, fineness of the tin powder and the amount of graphite present Compacts sintered above 798OC, on the other hand, show only a small varia- tion in the composition of the alpha solid

16 SINTERING IN THE PRESENCE OF A LIQUID PHASE

solution From these observations the authors conclude that complete homo- genization of the compacts can be accom- plished only by diffusion between a tin-rich liquid and the alpha phase which coexist above 798"C, unless the sintering time is abnormally long Carter and Metcalfe's results d~sagree with those of Ishikawasl and Wain,49 who claim that fairly homo- geneous alpha bronzes can be obtained by sintering for 3 to 4 hr a t temperatures of 650 to 7o0°C

A number of investigators have studied the dimensional changes dunng sintenng of porous bronzes Ishikawa, s1 Koehring, 63

and D r a p e a ~ ~ ~ determined the influence of the particle sue of copper and of tin, D r a p e a ~ ~ ~ the influence of the compressi- bility of the copper powder, K ~ e h r i n g , ~ ~ G ~ e t z e l , ~ ~ Wain49 and Lennoxs5 the influ- ence of compacting pressure, K o e h ~ i n g , ~ ~ Drapeau,46 Wain,49 Carter and M e t ~ a l f e , ~ ~ and Lennoxss the influence of sintenng temperature and sintering tlme, Goetzels4 and K o e h r ~ n g ~ ~ the influence of variations in composition, Koehringb3 the influence of the rate of heatlng The results of these investigations are of great practical im- portance for the control of dimensional changes In the sintering of porous bearings However, they can throw comparatively little light upon the sintering mechanism of these alloys Rhinesl and Shaler8t9 have shown that the sintering mechanism of pure metals accounts only for shrinkage of compacts and that any observed growth must be attributed to the influence of gases during sintering This may not hold true for sintering in the presence of a liquid phase Duwez and Martens,66 for example, belleve that the formation of intermetall~c compounds dunng sintering may be accompanied by growth However, from all the evidence it seems certain that the evolution of gases is a major cause for dimensional changes dunng sintering of copper-tin bronzes and will more or less obscure the influence of forces directly

connected with the sintering mechanism Koehnngb3 has discussed in detail the sources and the effects of the gases Wain49 shows a micrograph of a compact sintered a t 830°C having the typical major porosity of samples sintered near the recrystalliza- tion temperature where the rate of gas evolution is particularly high The forma- tion of this major porosity is, of course, of great technical ~mportance, because upon ~t is based the interconnected porosity which serves as the oil reservoir of the self- lubricating beanngs K ~ e h r i n g ' s , ~ ~ Goet- zel'ss4 and Wain's49 observation that the sintered density of copper-tin compacts is largely independent of the compacting pressure at medium and high pressures may be explained by the counteracting effects of gas evolution and surface tension forces At lower compacting pressures the green density will be low, the gases which are released during sintering can escape easily and the surface tension forces which tend < . to shrink the compact can become active At higher pressures, the green density will be high, the released gases will be entrapped and will expand the compact overcom- pensating the effect of the surface tension forces I ~ h i k a w a , ~ ~ K0ehring5~ and Dra- peaud6 found that compacts made of fine powders will in general shrink more or grow less than those made of coarse pow- der Thls fact must be due to the faster rate of homogenization where the particles are small, as was observed by Ishlkawa

Copper-zznr Alloys

Practically all commercial slntered brass products are made from alloy powders which are s~ntered without the presence of a l~quid phase The sintenng of mlxed powders was investigated by Owen and PickupK7 who determined the rate of change in composition of the alpha phase on the surface of the copper particles in loose powder m~xtures of copper and zlnc sintered for times up to 200 hr at temperatures from 450 to 600°C Measur-

ing the lattice parameter by precision X ray diffract~on they were able to derive quantitative relationships between the rate of homogen~zation, the diameter of the powder particles, and the temperature As expected the homogenization proceeds more rapidly the finer the powder and the higher the temperature Goetzelbs reports the structure of 85 pct copper 15 pct zinc compacts slntered for one hour a t 600°C ' remains obscure and nonhomogeneous After slnter~ng for one hour a t 8o0°C the alpha solid solutlon is predominant The Increase in growth wlth rislng zlnc content In copper-zinc compacts IS due, according to Goetzel, to the large surface of the zlnc constituent which gives off a large amount of zlnc vapor a t the sintering temperature

i and to the evolut~on of gases such as oxy- gen which would comb~ne wlth hydrogen to form water vapor Lennoxb5 investigated the relationship between dens~ty and * compacting pressure in sintered compacts of 70 pct copper and 30 pct zlnc

Iron-copper Alloys

Whether iron-copper compacts should be treated in the section on heterogeneous or homogeneous s~ntered alloys depends upon the copper content Up to approxl- mately 9 pct of copper are soluble in gamma iron a t temperatures from I IWOC up Alloys with higher copper content are therefore heterogeneous a t the slnter~ng temperature Microstructures of iron-cop- per alloys with 15, 20, and 25 pct copper and data on poros~ty for alloys with from 5 to 35 pct copper sintered a t I IW and I I 20°C are shown by il'orthcott and Lead- beaterzz and by Chadwick, Broadfield, and Pugh Alloys w ~ t h 15 pct copper already show rounded lron grains while the typical heavy alloy structure is fully developed in the 2 0 and 25 pct copper alloys The porosity decreases In general with increas- Ing copper content and becomes smallest for 25 pct copper For 75 pct iron 25 pct copper compacts made with an Iron

powder specially prepared by reduction a t 650°C of scale from mild steel turnings Chadwick and co-workers find pract~cally theoretical density regardless of compact- ing pressure after one hour sintering, a behavior typlcal for the heavy alloy mechanism On the other hand, electrolytlc lron powder Investigated by these authors as well as the powders tested by Northcott and Leadbeater st111 show considerable porosity and a dependence of porosity upon compact~ng pressure In compacts of this analysls For certain electrolyt~c and oxlde- reduced powders, but not for others, Northcott and Leadbeater find a s l~ght growth or a t any rate less shrinkage than for straight lron in the 5 and 10 pct copper range, a phenomenon also observed by this reviewer rn hlch has as yet found no explana- tlon These observat~ons clearly lnd~cate that the method of producing an iron powder may have considerable influence upon ~ t s sinter~ng behavior not only when s~ntered by itself, but also ahen s~ntered In the presence of l~quld copper SqulrefiD and Kelleyfil ~nvest~gated the mechan~cal propert~es of alloys w ~ t h 5 and 10 pct copper wh~ch are cons~derably higher than those of straight lron IJowever, then data do not p e r m ~ t any conclusions upon the s~nterlng mechan~sm of t h e ~ r alloys

Iron-nzckel-alumzltum Alloys

The ~ron-n~ckel-aluminum alloys whlch are used for permanent magnets of the Aln~co type also belong to the group of alloys in which the liquid phase disappears during the sinter~ng process Both Howe62 and K~effer and Hotop63 show microstructures of sintered iron-nickel-aluminum alloys which have the typical appearance of a homogeneous solid solution alloy having equiaxed grains The particles wh~ch are precip~tated during cooling and give the alloy ~ t s magnehc hardness are apparently too small to be resolved, in contrast to the copper-tin and copper-zinc alloys these alloys reach 98 to 99 pct of theoret~cal

r 8 SINTERING IN THE PRESENCE OF A LIQUID PHASE.

density during sintering According to Hotopa4 this density can be reached for compacting pressures from 40 to 140,000 psi, while Kalischere6 reports a minimum briquetting pressure of 180,000 PSI in order to reach near theoretical density when sintering 24 hr a t 12ooOC Accord- ing to Garvine6 ~t is British practice to sinter for 2 hr a t 13ooOC which results in a mater~al of 96 pct of theoretical density The aluminum In the alloys is added to the powder mixture in the form of a 50 pct iron-50 pct aluminum alloy in order to mini- mize difficulties with oxidation For the \ame reason it is necessary to sinter in very pure hydrogen Kalischer'sa5 statement that t~tanium hydride additions are neces- sary to slnter the alloys satisfactorily 19

generally disputed The minimum tem- perature for s~ntering in the presence of a llquid phase would be II~oOC, the melting point of the 50/50 ~ron-alummum alloy Temperatures from 1200-13goOC were reported by Hotop64 for the sintering of commerc~al permanent magnets w ~ t h sin- tering times from 1-6 hr Kalischer66 obtalned optimum magnetic properties by sintering 2 0 hr a t 1200°C Howea9 recom- mends presintering of the compacts a t 600°C, whlle according to the German practice the alloys are immediately sin- tered a t the final sintering temperature I t would, of course, be of particular Interest to know why it is so much easier to reach theoretical density in these ~ron-n~ckel- aluminum alloys than in other alloys which become homogeneous during s~n t e r~ng The comparatively large amount of liquid formed during sintering-26 pct in an alloy with 13 pct aluminum-may be partially responsible, but a complete answer to the question has to await a study of the s~nter- ing mechan~sm of these alloys

I F N Rh~nes Seminar on the theory of smterlng Trans AIME (1946) 166, 474

2 M Piran~ The early days of n~ckel- tungsten powder metallurgy Trans Electrochem Soc (1944) 85. 163

3 S L Hoyt Hard metal carb~des and cemented tunesten carb~des Trans AIME (1936) 9

4 G H S Price, C J Smithells, and S V Williams Sintered alloys, part I- copper-nickel-tungsten alloys- s~ntered with a liquid phase present Jnl Inst of Metals (1938) 62, 239

5 W D Jones Manufacture of non-porous allovs from oowders Metal Treatment ( G 9 ) h I3 =

6 S Takeda A metallograph~c study of the action of the cementing material for cemented tungsten carb;de SCI Rep. Tohoku Imp Univ . Honda Anniversary Vol (1936) 864

7 F Rollfinke Powder metallurgy and its relation to ceramics Ztsch Verern Deutscher Ingenreure (1940) 84, 681

8 A J Shaler The lunet~cs of the s~ntering nrocess Sc D Thes~s. Mass Inst of =- ~ ~ ~ - - - ~.

Tech (1947) 9 A J Shaler On the mechan~sm of sinter-

Ing To be pubhshed In Ind and Eng Chem

10 J Frenkel Viscous flow of crystalline bodies under the actlon of surface ten- sion U S S R Jnl of Phys (1945) 9. ( 5 ) . . .

8385 I I 0 Meyer and W Ellender Die S~nterung

von Hartmetalleg~erungen Archrv f d Eisenhuttenwesen (1948) 11, 545

rz R Kleffer and W Hotop Pulvermetal- lurgie und S~nterwerkstoffe 135 (1943) Berlin

13 J Kurtz S~ntered high denslty tungsten and tungsten alloys Proc 2nd annual spnng meetmng. Metal Powder Assoc~a- tion, New York 40 (1946)

14 F C Kelley D~scuss~on to Kurtz, p 51 of ref 13

15 W P Sykes Cemented tungsten carb~de alloys Trans AIME (1938) 128, 76

16 W D Clark The alummum-tungsten equ~librium diagram Jnl Inst of Metals (1940) 66, 271

17 L L Wyman and F C Kelley Cemented tungsten carbide, a study of the act~on of the cementing matenal Trans AIME. (1931) 93, 208

18 W P Svkes Discussion to Wvman and -. ~el ley: p 227 of ref I 7

19 E J Sandford and E M Trent The physical metallurgy of s~ntered car- b~des Spec~al Rep No 38 (1947) 84, Iron and Steel Inst London

zo F R Hensel and E I Larsen Certa~n -~ -

char&teristlcs of silver-base powder metallurgical products Trans AIME (1945) 161, 569

21 W P Sykes D~scuss~on to Price. Sm~thells and W~ll~ams, p 263 of ref 4

22 L Northcott and C J Leadbeater Sintered iron-copper compacts Speclal Rep No 38 (1947) 142. Iron and Steel Inst London

23 W D Jones Discussion to Pr~ce. Smithellz and Williams. p 254 of ref 4

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25 M Hansen Der Aufbau der Zweistof- flegierungen 598 (1936) Berl~n

26 G H S Pr~ce, S V Williams, and G J 0 Garrard Heavy alloy, its production.

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W Dawihl and J Hinnueber Ueber dcn Aufbau der Hartmetallegierungen Kol- loadzlsch (1943) 104, 233

F Skaupy Dispersoidchemische unil verwandte Gesichtspunkte be1 Sinter- hdrtmetallen Kolloidzlsch ( 1 ~ 4 2 ) 98, 92, and (1943) 102, 269

F C Keliey Cemented tantalum car- bide tools Trans ASST (1932) 19, 233

E W Engle Cemented carbides Chap 39. 436. Powder Metallurgy. ed by J Wulff (1942) Cleveland

W Dawihl Zlsch f Metallkunde (1940) 32, 320

P M McKenna Tool ,CIdter~als (Ce- mented Carbides). Chap 40. 454. Powder Metallurnv, ed bv J Wulff . - (1942) Cleveland -- .

G A Meerson. G L Sverev, B Y Osinovskaia Zhurnal Prikladnoi Khamti (1930) 13,-66

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P Schwarzkopf Powder Metallurgy 196-201 and 354-356 (1947) New York

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J E Drapeau S~ntenng of powdered copper-tin mixtures Chap 32, 332. Powder Metallurgy, ed by J Wulff (1942) Cleveland

H E Hall Sintering of copper and tin powder Melds and Alloys (1939) 10,297

F Sauerwald Present status of powder metallurav Mefallwirtschafl ( I O L ~ I ) 20, . . 649, 671--

H L Wain Powder metallurgy, influence of some processing variables on the properties of sintered bronze Report ACA-25. Australian Council for Aero- nautics (1946) XIelbourne

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54 C G Goetzel Some properties of sintered and hotpressed copper tin compacts Trans AIME (1945) 161, 569

55 J W Lennox The production of some non ferrous engineering components by powder metallurgy Special Rep S o 38. p 174, Iron and Steel Inst . 1947. London

50 P Duwez and H E Martens The power metallurgy of porous metals and alloss havlng a controlled porosity T P 2343, iMetuls Tech Aprll 1948

57 E A Owen and L Pickup X-ras stud> of the inter&ffusion of copper and zinc Proc Royal Soc . London Serles A. (1935) 149, 283

58 C G Goetzel Sintered and hotpressed compacts of copper-zinc powder Chap 34, 352, Powder Metallurgy. ed by J Wulff (1942) Cleveland

59 R Chadwick. E R Broadfield, and S F Pugh Observations on the pressing. sintenng. and properties of iron-copper powder mixtures Special Rep No 38. p 151. Iron and Steel Inst (1947) London

60 A Squire The properties of iron-copper compacts Watertown Arsenal Lab Rep WAL No 671/11

61 F C Kelley Properties of sintered iron- copper powder Iron Age (Aug 15, 1946) 158, 57

G H Howe Sintenng of Alnico Iron A R ~ (Jan 11. 1940) 145, 27

R Kieffer and W Hotop p 359 of ref 12

W Hotop Permanent magnets from sintered iron-nlckel-aluminum Slahl und flasen (1941) 61. 1105

P R Kallscher Some expenments in the production of aluminum-nickel-iron alloys by powder metallurgy Trans AIME (1941) 145, 369

S J Garvin Production of sintered per- manent magnets Special Rep No 38. p 67. Iron and Steel Inst 1947, London

F C Kelleq Discussion to P R Kalischer. P 375 of ref 65

R Kieffer and W Hotop p 357 of ref 12 C H Howe Sintered alnico Chapter 48

530. Powder Metallurgy, ed by J Wulff (1942) Cleveland