=-HECK RW61 0166

Density-Pressure Relationships in Powder Compaction R. W. Heckel

A method is described whereby the relationship of both the "at-pressure" powder compact density and the "zero-pressure" compact density to the applied pressure may be obtained from continuous measurements of punch movement during a single compaction operation. The rate of density increase with applied pressure for the metal powders is found to be proportional to the volume fraction of pores for pressures exceeding a lower limit which varies from 15,000 to 30,000 psi, depending on the powder.

T HE compaction of powdered materials is carried out primarily to increase the density of the mate­rial. If the ultimate goal of the over-all process is the attainment of minimum porosity, compaction is responsible for most of the densification. For ex­ample, loose powdered metals have porosities of about 65 to 75 pct. Axial loading in a die or hydro­static pressure may effectively reduce the porosity of most powders to 20 pct or lower with pressures of 50,000 psi and greater.

The relationship between the density of a powder compact and the pressure needed to achieve that density is most commonly obtained by pressing sev­eral compacts, each at a different pressure. Meas­urement of the densities of the individual compacts when they are removed from the die then provides the density-pressure relationship. However, this method suffers from several disadvantages. Be­cause a large number of compacts and pressing operations are required, it is inherently slow. Fur­thermore, since density measurements are made

R. W. HECKEL, Junior Member AIME, Is Research Metal­lurgist, Engineering Reasearch laboratory, E. I. du Pont de Nemours & Co., Wilmington, Del.

Manuscript submitted September 12, 1960. IMD

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I

after the specimens are removed from the die, data obtained refer only to pressures greater than those necessary to form a coherent compact. The third disadvantage is the inability of this method to pro­vide information about the compact density at the applied pressure in the die.

Because the determination of density-pressure relationships has been a slow, tedious process, in­vestigators have sought to express the compaction behavior of powders by other means. The general terms "compactability" and "compressibility" have been used to rank powders qualitatively according to their compaction characteristics. To restore a quantitative nature to the subject and to prevent confusion, Schwarzkopf1 has proposed that compac­tibility be defined as the minimum pressure needed to produce a given green strength, while compres­sibility should be used to indicate the extent to which the density of a powder is increased by a given pres­sure. Probably the most widely used of the compac­tion parameters is the "compression ratio," which is generally defined as the ratio of the compact density obtained by pressing at a given pressure to the apparent density of the loose powder. 2 However, the description of the compaction behavior of a powder by these parameters provides only limited information about the process because of their ap­plicability to just one specific condition such as a given green strength or a given pressure.

The method of obtaining density-pressure rela­tionships which will be described in the present paper was designed to eliminate the shortcomings of the previous techniques while maintaining equiv­alel)t accuracy and precision.

EXPERIMENTAL TECHNIQUES

The general principle employed makes use of the fact that the linear movement of the punch during a

VOLUME 221, AUGUST 1961-671

~

I

I

..J

LOADING V)

CI

~ C ct ILl IX

ILl CI ct CI

oJ or "P C

Imal

a

I S - - - - - -,- - - - - - - - --to , UNLOADING I

~- - - - -:- - - - - - - - - - - ~ J I

0 0 P

PRESSURE Fig . 1- Schematlc r epresentation of data curves along with pertinent values for calculating densities .

single die compaction may be used to calculate the change in volume of the powder as a function of pressure, if the cross-sectional area of the die is known. The density-pressure relationship over the entire range of pressures used in the compaction operation may then be calculated from a knowledge of the weight of the powder and the volume-pres­sure relationship. (It should be noted that the term "density" as used here refers to the average den­sity of the compact.) Since only the change in the volume of the powder may be obtained as a function of pressure, the data must be referred to a known powder compact volume. Two volumes may be used: zero die volume, which corresponds to the readings taken when the top and bottom punches of the die are in physical contact, or the volume of the compacted specimen after completion of the compaction opera­tion.

Since the linear movement of the punches dur ing the application of pressure to the powder is the algebraic sum of the change in height of the compact and the elastic compressive strains in the punches, the latter changes must be measured exper imentally by a blank pressing operation in order to separate the two effects. These punch-elasticity data are used in the reduction of the linear punch- movement data to allow the calculation of compact de nsity at the applied pressure. This will be referred to as "at-pressure" density. The densities measured after the compacts are removed from the die will be known as "zero-pressure" densities.

Thus far , consideration has been given to die com­paction only. However, the method is adaptable to hydrostatic compaction by suitable measurements of volume decrease as a function of hydraulic pres­sure. Corrections for the compressibility of the hydraulic fluid and elastic deformation of the hydrau­lic cylinders would be necessary.

672-VOLUME 221,AUGUST 1961

"

Die compaction of powdered materials may be carried out under one of many possible experimental conditions . In particular, a tensile bar die of 1.00:. sq in. cross-sectional area with cavity dimensions specified by the Metal Powder Association3 and having the die body supported by springs so that the die behaved in a double-action manner was used in the present investigation. The chOice of die was arbitrary ; any die of interest could have been used. Die lubrication was carried out by painting the body and punches with a 3 pct solution of stearic acid in carbon tetrachloride. The die was cleaned after each compaction operation. One-half cubic inch (0.50 in. depth) apparent volume 2 of powder was used for the compacts, which averaged about 0.20 cu in. in volume after pressing to 100,000 psi. The loading was accomplished on a 120,000-lb capacity Baldwin tension-test machine. A loading rate of 6000 psi per min was used throughout. The measurement of the 'amount of punch movement in the die was car­ried out with the dial gage and lever assembly hav- , ing a precision of 0.0001 in. The base of the as­sembly was fixed to the table of the testing machine and the lever contacted the crosshead. As the table moved upward, preSSing the punches of the die into the die body, the relative movement of the cross­head and table, identically equal to the amount of punch movement, was indicated on the dial gage.

ANALYSIS OF THE EXPERIMENTAL DATA

If the scale on the dial gage is chosen so that the dial-gage readings decrease as the load is applied to the die, the experimental data, when plotted, take the form shown schematically in Fig. 1 by the curves RS and ST. These curves represent the data taken on the application and removal of the pressure dur­ing the pressing operation, respectively.

The mathematical reduction of the experimental data into curves relating the average denSity of the powder compact and the applied pressure requires the following information: a) The weight of the pow­der in the die, b) the cross-sectional area of the die cavity, c) the elastic changes which take place in the die itself as a function of the applied pressure (if the at-pressure densities are to be calculated), and d) either the dial-gage reading at zero die volume without an applied pressure or the thickness of the powder compact when removed from the die. The weight of the powder and the dimensions of the die cavity may be measured directly, the die elasticity may be measured by a blank run (without powder), the zero-volume gage reading may be taken before placing the powder in the die, and the compact thick­ness may be measured after the preSSing operation when it is removed from the die. Because of the extremely small difference in cross -sectional area between the die cavity and the compact when it is removed fron the cavity, the areas of the two may be assumed to be equal without affecting the accu­racy of the analysis.

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The r eduction of the dial-gage readings vs ap­plied-pressure data to give average compact d 'n­sity at ze?'o p?'esslwe vs pressure may be obtai neo through the use of Fig. 1. The relationship betw('cn zero-pressure density and the applied pressure in­dicates the compact density, measured after re­moval from the die, which may be obtained by pres­sing at the given pressure. In terms of Fig. 1, by loading the die to the point S, a density character­istic of the point T is obtained. Thus, we may con­sider the curve S T to be a curve of constant zero­pressure density. The zero-pressure density of the compact may be calculated for any pressure P up to P max ' assuming that, if we would have unloaded the die after having reached, for instance, point M (pressure P), we would have proceeded along curve MN, such that

x -Uo = lp -up [1]

and, by vertical displacement, curves MN and OT can be brought into coincidence. The average zero­pressure compact density, PR" may be calculated

o from the general expression

Pp = WI Vp o 0 [2]

where W is the weight of the powder in the compact and VPo is the calculated compact volume which would be measured experimentally if the pressure were dropped from the value P to zero; i.e., the volume of the compact at point N.

Actually, the volume of the compact at point N may be express,ed as

VPo = ex -uo + to)·A

Substituting from Eq. [1],

VPo = (lp -up + to)·A

Thus, from Eq. [2],

W

If a calculation based on a zero-volume reading, a, is desired, the substitution

to = Uo - a [4] may be made in Eq. [3]. In general, experimental data and calculations based on to will be made more easily and will have greater inherent accuracy, especially in the more important high-pressure range, since actual measurement of the compact establishes the denSity value at P max' However, for materials whose compacting characteristics are desired, but whose poor interparticle coheSion, even at the maximum pressure, does not permit their re­moval from the die intact, the density calculations must necessarily be based on a zero-volume meas­urement.

At-pressure densities may be obtained from punch­movement data as a function of the applied pressure by modifying Eq. [3] to take into account the axial elastic compressive strains in the punches. In terms of the notation used previously, the at-pres-

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10r--------------------------c~~-r~~ F. N. w

9

--<I F, ······ ·10 HI

- --0 ell -- - Jf/ ...

10' '''USU''I. , I I.

6

i~ /" ),.

10'

- 18

8

16

14

8 6

12

10

4

3 6

Fig . 2- Zero-pressure denSity-pressure relationships for -200 +250 mesh electrolytic iron. -140 +200 mesh chem­ically preCipitated copper. -250 mesh nickel. and - 15~ tungsten powders .

sure density, Pp, may be shown to be:

W p = ~-------,--;---;-p (l p + e p - Uo + to) . A

[5]

where ep is the elastic strain in the punches at the applied pressure, P. It should be noted that Eq. [5] differs from Eq. [3] in the substitution of (ep - uo) for up. If the at-pressure density calculation is to be based on a zero-volume reading, a, Eq. [4] may be substituted into Eq. [5].

EXPERIMENTAL RESULTS

To test the accuracy of Eqs. [3] and [5J in the calculation of density-pressure curves, compacts were pressed from an electrolytic iron powder and a chemically precipitated copper powder at pres­sures of 10,000, 25,000, 50,000, 75,000, 100,000, and 120,000 psi. The data obtained from each preSSing was used to calculate both zero-pressure and at­pressure densities at lower pressures using Eqs. [3] and [5 J. The 90 pct confidence intervals, de­termined from a "t" distribution,· on zero-pres­sure densities at pressures in the range investigated averaged about ± 0.05 g per ce. Comparisons of den­sities of compacts pressed at 10,000, 25,000, 50,000, 75,000, 100,000 psi with densities calculated from data obtained by compacting at higher pressures gave an average error of about 0.06 g per cc. It may therefore be concluded that Eq. [3] is valid for the powders used and for the range of pressures em­ployed in the present investigation. Further, it is probable that this equation would also apply for all powders having pressing characteristics similar to those of the iron and copper powders used.

The results of calculations of at-pressure den­sities for both the iron and copper powders did not yield as good reproducibility as those for zero­pressure values because of the difficulty in obtain­ing values of Uo in Eq. [5]. The double-action fea­ture of the die caused the unloading curve to level out at about 200 lb, the force of the supporting springs. This prevented the zero-pressure data

VOLUME 221, AUGUST 1961-673

.; o ..... e '" .; ~ C/)

z w o

2.5..------------------,

1.0

0 .5 GRAPHITE

o ~-------------~--------------~ 103 104

PRESSURE. p". I.

Fig. 3-At-pressurc and zero-pressure density-pressure relationships for artificial graphite powder.

point from being obtained directly and necessitated an extrapolation to obtain uo. The at-pressure den­sities (p p) for both iron and copper powders aver­aged slightly greater than the zero-pressure den­sities (PPo)' but the dispersion of the data, as in­dicated by 90 pct confidence intervals, averaged about twice as large as that for the zero-pressure values. Relative at-pressure densities· could not

-Relative density Is defined as the ratio or the density or the compoct (p) to thot or the met III without porosity.

be obtained because an unequivocal measurement of I the change in density of iron and copper free of voids

under the state of stress imposed by the die could not be made.

Fig. 2 shows the zero-pressure relative density­pressure curves for iron, copper, nickel, and tung­sten powders. The maximum applied pressure for the copper, nickel, and tungsten powders was re­stricted to avoid extraction difficulties which were encountered with the iron powder.

The high elastic moduli of most metals render the difference between at-pressure and zero-pres­sure densities quite small. However, graphite, hav­ing a modulus of about 1 x 108 psi, should exhibit a sizeable difference. Fig. 3 shows both at-pressure and zero-pressure density-pressure relationships for an artificial graphite powder. The lack of reli­able data on the density of void-free graphite neces­sitated the expression of density as g per cc. It should be noted that a 14 pet decrease in density comes about by releasing the applied pressure from 75,000 psi. This is almost twice the change in den­sity which should occur on the basis of elastic changes in the graphite particles if a modulus of 1 x 108 psi is assumed correct for these conditions. This information would seem to indicate that the

674-VOLUME 221, AUGUST 1961

-I~ .I

3.0..----------------:---,

2.0

--0 FI ·····6 HI _.-{J Cu

--..q W

o ':-0--'--2'-...I.--4'-...I.--e'-...I.-....J"L-..I...-...J10L-..I...-..... 12'--"--..J14.104

PRESSURE. ",".1.

Fig. 4-1n (l/l-D) vs pressure for -200 +250 mosh elec­trolytic Iron. -140 +200 mesh chomlcally preolpitated cepper. -250 mesh nickel and .... 15/-1 tungsten powders.

volume of the specimen increased upon release of the pressure as a result of both elastic expansion of the graphite particles and sliding of particles over each other, thus bringing about a condition of poorer packing.

DISCUSSION

If the compaction of powders is considered to be analogous to a first-order chemical reaction, the pores being the reactant and densification of .the bulk of the product, the "kinetics" of the process may be described by a proportionality between the change in density with pressure and the pore fraction.

dD dP

or

(I-D)

dD - = K (I-V) dP

where:

D is the relative density,

P is the pressure,

I-D is the pore fraction,

and K is a proportionality constant.

[6]

In order to reduce this to a more 'usable expression:

dD (I-D) = KdP

D dV p j(I_D)=KjdP

Do 0

where Do is the relative density of the loose powder at zero pressure. Therefore:

or

In (I-Do) -In (I-D) = KP

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1 ! #

Tabl. I. Values of Constanh A and K from Eq. [11] for Various Metal Powders Along with the Lower Limit of Linearity of the D,ata

Powder

Iron (electrolytic -200 + 250 mesh)

Copper (chemically precipi­tated -140 +200 mesh)

Nickel (-325 mesh) Tungsten (- IS/I)

A

0.79

0.71 0.85 0.60

In (l~D) = liP + In (l~n) a

1.73 x 10-'

1.65 X 10-' 1.22 x 10-' 8.18 x 10-6

Lower Limit,

psi

25,000

30,000 25,000 15,000

[7]

The validity of this analysis was checked by plotting In (Ill-D) vs P for the metal powders already dis­cussed. The results are shown in Fig. 4. Although the data do not lie on a straight line over the entire pressure scale, linearity exists over 65 to 80 pct of the pressure range studied and would seem to in­dicate that extrapolation of the values to even higher pressures would be justified. The nonlinearity in the early stage of compaction is probably due to the ef­fect of rearrangement processes in the powder and the general behavior of the powder as individual particles rather than a coherent mass.

It is postUlated that in Eq. [8] the constant A which is always somewhat larger than [In (1/l~Do)], represents the degree of packing achieved at low pressures as a result of rearrangement processes before appreCiable amounts of interparticle bonding take place. It is also postulated that the constant K the slope of the linear region, gives a measure of the ability of the compact to density by plastic defor­mation.

The values of A and K from Eq. [8] for the metal powders are given in Table I along with the lower limit of linearity of the data.

The ability to express the compaction behavior of powders in terms of the constants A and K has sev­eral advantages. The mathematical expreSSion of the density-pressure relationship of a given powder necessarily permits the analytical determination of the density values in the range of pressures investi­gated and also permits extrapolation to pressures in excess of those available experimentally. Most im­portant, though, is the fact that the constants quan­titatively describe the compaction behavior of a given powder. The evaluation of the constants A and K in terms of powder properties and the manner of compaction is currently underway at this Laboratory. The ultimate goal of this research will be the quan­titative determination of the density-pressure curves from basic data on powders and the compaction I

process.

CONCLUSIONS

Since the curves in Fig. 4 exhibit some curvature at low pressures, Eq. [7] does not describe the process quantitatively. The replacement of the term [In (l/l-Do)] in Eq. [7] with a constant, A, gives an expression, Eq. [8], which is quantitatively valid except at the lowest pressures. 1) Both zero-pressure and at-pressure density- '

pressure curves for a given powder may be deter­[8] mined from punch-movement data for a single pres-In (I!D) = KP + A

To see if the transition from nonlinear to linear sing operation. behavior in Fig. 4 can be associated with bonding, 2) Density-pressure data indicate that the rate of a -80 + 140 mesh alumina powder (which densifies change of density with pressure at any pressure is by crushing) was compacted in the same manner as proportional to the pore fraction in the compact at the metals. In this case there was little interparti- that pressure. cle bonding and the plot of In (l/l ..... D) vs P did not 3) There is little difference between zero-pres-have a linear region over the pressure range studied sure and at-pressure densities for iron, copper; (0 to 75,000 psi). All of the densification came about nickel, or tungsten powders~ since they are char-as a result of crushing and rearrangement of the acterized by relatively high elastic moduli. On the individual particles. This is shown by the fact that other hand, there is a large decrease in density of 65 pct of the alumina particles after pressing were graphite compacts upon r'eleasing the applied less than 140 mesh, the minimum size of the orig- pressure. This may be accounted for both by elastic inal powder, and 38 pct were less than 250 mesh. expansion of the compact and sliding of particles as From this information it is concluded that the the pressure is removed. curved region in a plot of In (i/l-D) vs Pis asso- 4) Density-pressure curves may be described by ciated with densification which takes place by a mech- two parameters. It is postulated that one is related anism of individual particle movement in the absence to low pressure densification by interparticle mo-of interparticle bonding. It was also found that the tion and the other is a measure of the ability of the transition from curved to linear behavior in the compact to densify by plastic deformation after ap-powders whose compaction curves are shown in Fig. preciable interparticle bonding has taken place. 4 corresponded closely with the minimum pressure necessary to form a coherent compact. Therefore, it is concluded that the densification which is rep-resented by the linear region of a plot of In (l/l-D) vs P occurs by plastic deformation of the compact after an appreciable amount of interparticle bonding has taken place.

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REFERENCES

'P. Schwarzkop(: Powder Melal/grll, Mlcmlllan, Ne .. York, 1947. 'Metal Powder A •• ociatlon, Standord 4-45, 1945. 'Metal Powder A •• ociatlon, Standard 10-50, 1953. "P. G. Hoel: InlrmiMel;on 10 Malla .... 'jea/ SIGII.,;e., John WlIey and Sona,

Ne .. York, 1948.

VOLUME 221, AUGUST 1961-675

I

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I

HECK RW6 2 0173

A Normalized Density -Pressure

Curve for Powder Compaction

R. W. Heckel

IT has been shown previouslyl,Z that the effect of applied pressure on the density of a metal powder may be expressed by a relationship of the form:

In 1~D=KP+ln1_~o +B [1]

1.0

0. 9

0 0.8

;;. 'Vi

O. 7 c: CI.I

0 Q) > :;::; 0.6 ...,

Qj 0:=

0. 5

0.4

.~ '1-... -.... _d ,#If' ~ •

...... ' l":~ ----_" ,..f, t' . __ ,--.. . ~ ..... """ ...

,,~ : ... . ~ ." , ..... '.JO' . .. , ~I , .. "1/ ' 1 . , ,. ,

" i /I~ /

'! ./ ,I .I./- ' I . I

. ..:.(/ i ./ ( "

:'

0. 0 0.5 1.0 1..5 2.0

Reduced Pressure. -3P (To

Fig. 1-Normalized density-pressure curve.

2. 5

R. W. HECKEL, Junior Member AIME, is Research Metal­lurgist, E.!. du Pont de Nemours & Co., Inc., Experimental Station, Engineering Research laboratory, Wilmington, Del.

Mdnuscript submitted February 21, 1962. IMD

vOLUME 224, OCTOBER 1962-1073

1-

, .

Table I.

Description of the various powders whose density-pressure curves are normalized In Fig. 1.

Compaction Parameters

Powder Partlc Ie Size Source A

Iron" Iron· Iron· Iron" Iron" Iron b

-40 . 80 Mesh -80 +140 Mesh

-140 +200 Mesh -200 +250 Mesh

Charies Hardy Co. Charles Hardy Co. Charles Hardy Co. Charles Hardy Co.

0.67 0.73 0.68 0.73 0.77 0.70 0.68 0.92 0.98 0.87 0.72 0.90 0.62

1.86 x 10-' 1. 91 x 10-' 1.99 x 10-' 1. 91 x 10-' 1.81 x 10-' 1.90 X 10-' 1.08 x 10-' 1.38 x 10-' 1.56 x 10-' 1.48 x 10-' 3.81 x 10-' 3.00 x 10-' 0.76 x 10-'

SAE 4630 Steel Spherical Nickel Spherical Nickel Annealed Nickel Spherical Copper Spherical Copper Tungsten

-250 Mesh -140 +200 Mesh

-80 Mesh -140 +200 Mesh

Ch aries Hardy Co. George Cohen Co. Vanadium Alloy Steel Co. Linde Co.

-250 Mesh Linde Co. -250 Mesh

-140 +200 Mesh Charles Hardy Co. Linde Co.

-250 Mesh Linde Co. - 151' Charles Hardy Co.

·Star Grade (Electrolytic) bSlntrex (E lectrolytic)

where: D is the relative density. of the compact

+Relative density Is defined as the ratio of the density of the com­pact to that of the metal without porosity.

Do is the relative apparent density of the powder P is the applied pressure, and K and B are constants

Eq. [1] is valid above the pressure necessary to achieve interparticle bonding, which takes place within the range from 5000 to 25,000 psi depending on the powder. Experimental observationsl! have shown K to be related to the nominal yield strength of the powdered metal by the following equation:

or

K = 2.08 x 10-e + 0.320 ~ <70

K ;;_1_ 3<70

[2]

where <70 is the nominal yield strength of the metal. In addition, it was found that B is a function of the size and shape of the powder particles; B de­creases as the particle size decreases and as the shape of the particles becomes more spherical. For spherical particles, B is approximately zero.

In a survey of the effect of various powder and processing variables on the density-pressure re­lationships of powders,3 it was found that increases in Do were almost always accompanied by decreases in B. In general, the sum:

In _1_+ B =A 1-Do

was approximately constant, the median value being about 0.75, with 86 pct of the values falling in the range of 0.60 to 0.90. In comparison to the wide spread in values of K (7.6 x 10-e psCl for a tungsten powder to 3.8 x to-5 psi-l for a copper powder), the value of A remains relatively constant. ~qs. [1] and [2] , and the relative insensitivity of

the parameter A to powder and processing variables; make it possible to normalize density-pressure

1074-VOLUME 224, OCTOBER 1962

data for different powders to a common set of axes by plotting relative density, D, vs reduced pressure, P/3<70' Fig. 1 shows such a plot for the powders iden­tified in Table I. The solid curve represents a least­squares fit of the data; the vertical bars give the 95 pct confidence limits on the line. Also shown in Fig. 1 are the approximate tolerance bands (dashed lines) for future data points obtained under the same experimental conditions which were used to obtain the original data. These bands have a 95 pet probability of including 90 pct of all future data points.

All of the density data were calculated from measurements of die punch movement as a function of applied pressure on the die. This method has been described in detail previously.l The data in Fig. 1 were obtained by using a tensile bar die with a cavity specified by the Metal Powder Association. 4 The pressing was carried out at 6000 psi per min on a Baldwin tension test machine. The data were taken over the range of pressures from zero to 100,000 psi. The values for the nominal yield strength of copper, iron, nickel, SAE 4630 steel, and tungsten were taken to be 10,000, 20,000, 50,000, and 100,000 psi, respectively.&-7

Fig. 1 may be used to approximate denslty­pressure data for powders undergoing die com­paction where the yield strength of the metal is known. The figure is limited to die compaction only. However, it should be possible, at least in theory, to construct a similar diagram for hydrostatic compaction if sufficient density-pressure data are obtained.

JR. W. Heckel: Trans. Met. Soc. MME. 1961. vol. 221. p. 671. 'R. W. Heckel: Trans. Met. Soc. A/ME. i961. vol. 221. p. 1001. ·R. W. Heckel: Progress in Powder Metallurgy. Proceedings of

Seventeenth Annual Technical Meeting of lite Metal Powder Industries Federation, 1961.

'Metal Powder Association. Standard 10-50 1953. • • C. J. Smlthells: Meta/$ Reference Book. Intersclence Publishers. New York, 1949.

'Metol& Handbook. ASM. Cleveland. 1948. 'V. D. Barth: Phyalcal and Mechanical Properties of Tungsten and

Tungsten-Base Alloys, D.M.I.C •• Report No. 127. Battelle Memorial In­stitute, M.rch 15. 1960.

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