- Email: [email protected]
The use of hardness in the study of compaction behaviour and die loading
Pouder Technology.17 (1977) 253 - 267 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
The Use of Hardness in the Study of Compaction
A. KANDEIL Faculty
S. CRITCHLEY (Received
Behaviour and Die Loading
and M- C. DE MALfIERBE
of Engineering.
Department
253
Carleton
Liniuersity, Ottawa,
Ont. KIS
5B6
(Canada)
and hf. DOKAINISK
of Mecharzical Engineering.
dlcdfaster
June 1, 19’76; in revised form February
Linirvzrsity. Hamilton.
Ont. (Canada)
1’7. 197’7)
SUMBIARY An esperimental relationship between density, hardness and compacting pressure is obtained for the isostatic compaction of Alcoa grade 1202 atom&d aluminum powder_ These results are used to evaluate the use of hardness measurement in the determination of density contours within compacts and pressure distributions at the compact-die interface. Density contours and pressure distributions are presented for closed die compacts; the results are in general agreement with those reported in the literature for more complex techniques. The technique is shown to be suitable for use in many situations_
INTRODUCTION
Two primary areas of interest of current research in powder metallurgy are the density distribution in green compacts and die loading during compaction. Green density distribution is the controlling factor on shrinkage during sintering, and on the homogeneity of the final product [l] _ It is therefore important to be able to control and measure the green density within samples selected from the production floor. In the design of comples multi-component die sets, a means of detection of areas of high loading is obviously important_ Direct measurement of pressure distributions has been limited to the use of specially constructed cyiindricai dies having extensive iustrumentation [2,3]. A number of experimental techniques have &en used in the study of density distribution. Sectioning and density determination by
direct mass and volume measurements is a difficuit process, especiaily with small products [4] _ The observation of the distortion of easily detected heterogeneities, usually in the form of a grid or network of Iead inserted before compaction, is limited to laboratory use and is not suitable for quality control use. It is also difficult to predict or eliminate the effects of the presence of the network [ 53 _ The use of porosimetry is expanding but is at present limited to a few materials [S] _ A further method involves the measurement of hardness at selected points in the compact [7] _ If a relationship esists and is known between hardness and compacting pressure, the density and compacting pressure may be obtained. Kuczynski [7] obtained such a relationship by measuring the hardness and density of thin die pressed samples- In the present work this approach has been modified to use hardness correlations obtained by isostatic compaction, and estended to cover the prediction of pressures eserted on the die set. Isostatic compacts were used because of their higher uniformity when compared to any die compacts_ The increase in hardness which accompanies powder densification may, in a simphfication, be attributed to two main factors. The first is densification hardening, and is directly attributable to the increase in density of the material to he tested. The second factor is the strain hardening exhibited by some materials under compaction. This second factor will introduce systematic differences between compacts produced in different equipment. The separation of these two effects is the subject of current research. Because of the absence of die wall friction and the associated shearing, isostatic compacts can be assumed
to exhibit the minimum amount of strain hardening of any compaction process. The isostatic results are used in this case to produce the correlation curves between pressure_ hardness and density. These results are used in conjunction with hardness measurements to predict density and pressure distributions for the cylindrical die compaction of aluminum.
TABLE 1 Chemical and screen analysis of atomised
Screen
ar.Jysis
(LJ.S.Std.)
PROCEDURE
The isostatic compaction unit used in this work was an _%utoclave Engineer’s press, with a masimum working pressure of 4000 bar_ Nine samples were produced over the available range of pressure using preweighed amounts of Alcoa aluminum powder grade 1202, with the specifications given in Table l_ The pressures were determined using a bourdon gauge. X latex mould was used to contain the powder during compaction_ The mould had a 100 mm
internal length, 25 mm i-d_ and 2-5 mm wall thickness. The green compacts were carefully machined to right cylinders, nominally 20 ml= in diameter and 20 mm in length- These samples were weighed and measured to de’;ermine their densities. The die compacts were produced in the floating die arrangement shown in Fig. 1, a preweighed amount of powder being carefully loaded into the compaction space for each of the szrnples produced. A lubricant was not used_ Microhardness measurements were carried out on all the samples using a Tukon testing machine and a load of 1 kg, providing Vickers hardness numbers, often referred to as diamond pyramid harness numbers. This test employs a square-based diamcr,d pyramid as the indenting tool. The angle between opposite faces of the pyramid is 136”. There are two features of this method which are essentially different and advantageous compared with the Brine11 method. First, there is geometric similarity between impressions; this similarity is independent of indenter load- Secondly, the small size of the indentation is useful when surveying a specimen which has a variation in hardness through the cross-section- For all measurements a load of 1 kg was used. It is interesting to note that the smallest indentation for the
(FL)
Chemical
analysis
(%)
-pica1 range
Mesh
,-
Al A1203 Fe Si Other metallics,
trace
50
10-20 30 - 30 15 - 25 30-40
50 + 100 -100 +- 200 -200 A- 325 - 325 Particle size .4verage particIe diameter. pm
-
ESPERIMEXT_4L
aluminum
powder
I
I.
Density, g/cm 3 Apparent 1.3 Tapped 1.5
“3 - 9s
i: 1 *
99-4 0.3 0.15 0.05 0.0
11-9
I
110
Fig_ 1_ Schematic of the closed die compaction apparatus. 1 composite disc, 2 guide sleeves, 3 copper shims, -2 tungsten carbide piston, 5 tungsten carbide die set with shrink-fit steel SUppOrt ring, 6 compaction chamber, 7 tungsten carbide closure, 8 support plate for removaI of die assembly. 9 load cell, 10 movable platten of Vecor press, 11 fixed platten of Vecor press.
samples studied covered about 200 particles_ To prepare the samples for hardness measurements, they were first abraded on a series of 220,320,400 and 600 grit wet silicon carbide papers, and polished using 6 and 1 pm diamond pastes successively_ Five indentations were used to evaluate the hardness of each of the isostatic compacts. The hardness variation on the top and bottom surfaces of the die compacts was determined from a line of sixteen indentations across each sample. The distance between adjacent indentations was 1.5 mm_ The hardness values used are the mean values from symmetrically placed indentations. To examine the hardness variation within the specimens, they were cut into half-
255
cylinders and the surface polished as previously described_ Five rows of measurements each of sixteen indentations separated by l-5 mm were made on each sample. The rows were spaced at 2-5 mm intervals_ The reported values are mean results of symmetrically placed indentationsRESULTS AND DISCUSSION The experimental pressure-density data for isostatically compacted aluminum poxvder are shown in Fig_ 2. The densities are expressed as relative densities, related to a theoretical density, pa, of 2.7 g/cm3_ Figure 3 illustrates the variation of hardness with compacting pressure_ The curve eshibits marked linearity in two distinct regions which correspond to the second and third stages of compaction_ -4 possible explanation for this linearity is the following. During compaction of a metal poxvder, a non-linear load displacement characteristic results from the combined effects of the alter
a
0
IO00
-
2ooo cw=*cTty(i
44
FRESSURE
lo
altered microstructure geometry and the strain hardening of the materiai as a result of plastic flow. Upon release of the pressure, however, unloading will occur elastically and therefore linearlv. If pressure were reapplied, linear behaviour would persist until the pressure reached that originally applied, at which point yielding of the strain-hardened particles would again occur. Thus, the pressure to which a compact was pressed corresponds to a yield stress for that compact. Since a linear relationship can be observed between yield stress and hardness for many metals, such a relation between compacting pressure and hardness could also be espected. The two regions are attributable to the differing mechanisms both during compaction and during indentation, corresponding to the reduction in relative particle movement, on reaching the third stage of compaction_ Hardness and density correlation is presented in Fig. 4. The hardness readings for the top and bottom of the die compacts were used Mth Fig. 3 to produce ‘tie pressure distributions sholvn in Fig. 5 for a mean compacting
20
Ill
D
- 8.x
Fig. 2. Ln [l/(1 - D)] OS. pressure for isostatically compacted aluminum powder.
-S-
Fig. 4. Density-hardness correlation compacted aluminum powder.
30
OPH
40
for isostatically
PRESSURE EAR
01 0
IO
20
30 HAROWSS - O.!H.
Fig_ 3. Powder compacting pressure us hardness For isostatically compacted aluminum_
10
I 5
0
s
10
RAOIALDISTANCE-mm Fig. 5. Pressure distribution on the upper and lower faces of a closed die aluminum compact_
pressure of 3520 bar. The two curves for the upper and lower faces of the compact e_xh%bit the results of die wall friction and particle movement_ The top curve indicates a hardness and pressure increase towards the outside of the compact, while the bottom face exhibits the reverse effect. Integration of the pressure distribution gives an indicated mean pressure of 3440 bar and 2050 bar on the upper and lower faces respectively_ Because of the strainhardening effects in die compaction, it was expected that the integrated values would be lower than the values measured by the load cell. Since this difference is small, little shearing must occur between the powder and the compacting piston Frictional forces between the compact and the die wall can be estimated in this case to be close to 40% of the applied load. This high value is attributable to bonding which occurred between the carbide dies and the aluminum powder, especially at higher pressures_ The density variation within the die compact is illustrated by Fig. 6, the maximum densities being predicted at the top outside perimeter close to the piston-cylinder interface, and in the lower central zone of the compact These results agree with the results reported and explained in detail elsewhere [S] _ The accuracy of the method is limited by two factors, the hardness measurement itself, and the assumption that hardness and density are related by a single value function_ In the
Fig_ 6_ Density
variation in the cross-section
case of the isostatic compact which is uniform, the inherent scatter of the hardness readings on such a statistical material as a green compact may be reduced (in this case to within 1%) by repeated tests and the use of mean values. This is evident in the lack of scatter in the hardness-pressure curve produced in this manner. However, when the technique is to be used in the case of a complex part it may not be possible to repeat the hardness measurements to reduce the error bounds to a predetermined value. However, the technique is still valuable to provide qualitative assessments of loadings and densities in these cases. The assumption of a single value densityhardness relationship depends upon a uniform compaction process during which, as mentioned earlier, the effects of strain hardening will be systematic. Such is the case in isostatic compaction_ In die compaction there are areas of loading which approsimate to isostatic, Le. away from the die walls in the centre of the compact, and areas such as the areas of high density close to the piston-die interface which are subjected to large shear strains. Comparing the two areas, it is easy to see that for a given hardness value the highly sheared material may be less dense than the isostatically compacted material. To evaluate these differences, the mean density of the compact was predicted by integration of the compact density distribution and also obtained by direct measurement and weighing_ The direct measured
of a die compact.
value was 94% and the integrated density was 97% From this small difference it is reasonable to assume that the predicted vahres are sufficiently accurate to evaluate most density distribution problems_ If increased accuracy is required, steps must be taken to remove the effects of strain hardening from both the calibration curves and the final compact The results indicate that the method predicts loads sufficiently well for use in most design situations_ Since the effects of strain hardening have not yet been fully separated, any absolute figure of accuracy is somewhat speculative; however, a conservative estimate for the case of aluminum would be +5% on both pressure and absolute dl:nsity values. Resolution and repeatability between values produced under similar conditions (ix_ in the same die set under similar loads) can be shown to be in the order of al-2%
COxcLUSION
The use of hardness measurements for the prediction of pressure and density within compacted materials has been shown to be feasible_ The method is presently best suited to the qualitative analysis of density distribution, quantitative work being restricted in accuracy except for comparative situations_ Pressure distributions on die sets may be measured satisfactorily, with accuracies as good as
any presently used methods. More work is required to quantitatively assess: (1) the effects of strain hardening in the densification process, (2) the use of the methods for other porous materials, and (3) the densification hardening of nonstrain-hardening powders REFERENCES J_ McDermott. Powdered metals technolo~, Noyes Data Corporation, New Jersey, 19i4. I. &I. Fedorchenko. R. A. Kovynev and 0. F. Polukhin. Use of pin sensors for studying the pressures exerted on the working parts of die sets, Poroshk. MetalI., 91 (19’70) 26. G. hI_ Zhdanovich, Pressure and density distribution in the single and double-ended pressing of axially symmetric compacts, Poroshk hletall.. '76 (1969) 24. A_ Y_ Kandeil, On the compaction of metal powders with particular reference to densification hardening. M_ Eng. Thesis. McAIaster Univ_. Hamilton. 1975. R. Kamm. 31. Steinberg and J. Wulff, Lead grid study of metal powder compaction, Met. Technol., (1949) 694. H. K. Palmer and R. C. Rowe. X study of the compaction behaviour and pore structure of polymer compacts using mercury porosimetry, Powder Technol.. 10 (1974) 225 - 930. G. C. Kcczynski and I. Zaplatynskj, Density distribution in metal powder compacts, J. Met., (1956). R. A_ Federchenkc. R. A. Kovynev et al., _A photoelastic technique for examining contact stresses in powder compaction, Poroshk Metall., 11 (196s) 850.
The Use of Hardness in the Study of Compaction
A. KANDEIL Faculty
S. CRITCHLEY (Received
Behaviour and Die Loading
and M- C. DE MALfIERBE
of Engineering.
Department
253
Carleton
Liniuersity, Ottawa,
Ont. KIS
5B6
(Canada)
and hf. DOKAINISK
of Mecharzical Engineering.
dlcdfaster
June 1, 19’76; in revised form February
Linirvzrsity. Hamilton.
Ont. (Canada)
1’7. 197’7)
SUMBIARY An esperimental relationship between density, hardness and compacting pressure is obtained for the isostatic compaction of Alcoa grade 1202 atom&d aluminum powder_ These results are used to evaluate the use of hardness measurement in the determination of density contours within compacts and pressure distributions at the compact-die interface. Density contours and pressure distributions are presented for closed die compacts; the results are in general agreement with those reported in the literature for more complex techniques. The technique is shown to be suitable for use in many situations_
INTRODUCTION
Two primary areas of interest of current research in powder metallurgy are the density distribution in green compacts and die loading during compaction. Green density distribution is the controlling factor on shrinkage during sintering, and on the homogeneity of the final product [l] _ It is therefore important to be able to control and measure the green density within samples selected from the production floor. In the design of comples multi-component die sets, a means of detection of areas of high loading is obviously important_ Direct measurement of pressure distributions has been limited to the use of specially constructed cyiindricai dies having extensive iustrumentation [2,3]. A number of experimental techniques have &en used in the study of density distribution. Sectioning and density determination by
direct mass and volume measurements is a difficuit process, especiaily with small products [4] _ The observation of the distortion of easily detected heterogeneities, usually in the form of a grid or network of Iead inserted before compaction, is limited to laboratory use and is not suitable for quality control use. It is also difficult to predict or eliminate the effects of the presence of the network [ 53 _ The use of porosimetry is expanding but is at present limited to a few materials [S] _ A further method involves the measurement of hardness at selected points in the compact [7] _ If a relationship esists and is known between hardness and compacting pressure, the density and compacting pressure may be obtained. Kuczynski [7] obtained such a relationship by measuring the hardness and density of thin die pressed samples- In the present work this approach has been modified to use hardness correlations obtained by isostatic compaction, and estended to cover the prediction of pressures eserted on the die set. Isostatic compacts were used because of their higher uniformity when compared to any die compacts_ The increase in hardness which accompanies powder densification may, in a simphfication, be attributed to two main factors. The first is densification hardening, and is directly attributable to the increase in density of the material to he tested. The second factor is the strain hardening exhibited by some materials under compaction. This second factor will introduce systematic differences between compacts produced in different equipment. The separation of these two effects is the subject of current research. Because of the absence of die wall friction and the associated shearing, isostatic compacts can be assumed
to exhibit the minimum amount of strain hardening of any compaction process. The isostatic results are used in this case to produce the correlation curves between pressure_ hardness and density. These results are used in conjunction with hardness measurements to predict density and pressure distributions for the cylindrical die compaction of aluminum.
TABLE 1 Chemical and screen analysis of atomised
Screen
ar.Jysis
(LJ.S.Std.)
PROCEDURE
The isostatic compaction unit used in this work was an _%utoclave Engineer’s press, with a masimum working pressure of 4000 bar_ Nine samples were produced over the available range of pressure using preweighed amounts of Alcoa aluminum powder grade 1202, with the specifications given in Table l_ The pressures were determined using a bourdon gauge. X latex mould was used to contain the powder during compaction_ The mould had a 100 mm
internal length, 25 mm i-d_ and 2-5 mm wall thickness. The green compacts were carefully machined to right cylinders, nominally 20 ml= in diameter and 20 mm in length- These samples were weighed and measured to de’;ermine their densities. The die compacts were produced in the floating die arrangement shown in Fig. 1, a preweighed amount of powder being carefully loaded into the compaction space for each of the szrnples produced. A lubricant was not used_ Microhardness measurements were carried out on all the samples using a Tukon testing machine and a load of 1 kg, providing Vickers hardness numbers, often referred to as diamond pyramid harness numbers. This test employs a square-based diamcr,d pyramid as the indenting tool. The angle between opposite faces of the pyramid is 136”. There are two features of this method which are essentially different and advantageous compared with the Brine11 method. First, there is geometric similarity between impressions; this similarity is independent of indenter load- Secondly, the small size of the indentation is useful when surveying a specimen which has a variation in hardness through the cross-section- For all measurements a load of 1 kg was used. It is interesting to note that the smallest indentation for the
(FL)
Chemical
analysis
(%)
-pica1 range
Mesh
,-
Al A1203 Fe Si Other metallics,
trace
50
10-20 30 - 30 15 - 25 30-40
50 + 100 -100 +- 200 -200 A- 325 - 325 Particle size .4verage particIe diameter. pm
-
ESPERIMEXT_4L
aluminum
powder
I
I.
Density, g/cm 3 Apparent 1.3 Tapped 1.5
“3 - 9s
i: 1 *
99-4 0.3 0.15 0.05 0.0
11-9
I
110
Fig_ 1_ Schematic of the closed die compaction apparatus. 1 composite disc, 2 guide sleeves, 3 copper shims, -2 tungsten carbide piston, 5 tungsten carbide die set with shrink-fit steel SUppOrt ring, 6 compaction chamber, 7 tungsten carbide closure, 8 support plate for removaI of die assembly. 9 load cell, 10 movable platten of Vecor press, 11 fixed platten of Vecor press.
samples studied covered about 200 particles_ To prepare the samples for hardness measurements, they were first abraded on a series of 220,320,400 and 600 grit wet silicon carbide papers, and polished using 6 and 1 pm diamond pastes successively_ Five indentations were used to evaluate the hardness of each of the isostatic compacts. The hardness variation on the top and bottom surfaces of the die compacts was determined from a line of sixteen indentations across each sample. The distance between adjacent indentations was 1.5 mm_ The hardness values used are the mean values from symmetrically placed indentations. To examine the hardness variation within the specimens, they were cut into half-
255
cylinders and the surface polished as previously described_ Five rows of measurements each of sixteen indentations separated by l-5 mm were made on each sample. The rows were spaced at 2-5 mm intervals_ The reported values are mean results of symmetrically placed indentationsRESULTS AND DISCUSSION The experimental pressure-density data for isostatically compacted aluminum poxvder are shown in Fig_ 2. The densities are expressed as relative densities, related to a theoretical density, pa, of 2.7 g/cm3_ Figure 3 illustrates the variation of hardness with compacting pressure_ The curve eshibits marked linearity in two distinct regions which correspond to the second and third stages of compaction_ -4 possible explanation for this linearity is the following. During compaction of a metal poxvder, a non-linear load displacement characteristic results from the combined effects of the alter
a
0
IO00
-
2ooo cw=*cTty(i
44
FRESSURE
lo
altered microstructure geometry and the strain hardening of the materiai as a result of plastic flow. Upon release of the pressure, however, unloading will occur elastically and therefore linearlv. If pressure were reapplied, linear behaviour would persist until the pressure reached that originally applied, at which point yielding of the strain-hardened particles would again occur. Thus, the pressure to which a compact was pressed corresponds to a yield stress for that compact. Since a linear relationship can be observed between yield stress and hardness for many metals, such a relation between compacting pressure and hardness could also be espected. The two regions are attributable to the differing mechanisms both during compaction and during indentation, corresponding to the reduction in relative particle movement, on reaching the third stage of compaction_ Hardness and density correlation is presented in Fig. 4. The hardness readings for the top and bottom of the die compacts were used Mth Fig. 3 to produce ‘tie pressure distributions sholvn in Fig. 5 for a mean compacting
20
Ill
D
- 8.x
Fig. 2. Ln [l/(1 - D)] OS. pressure for isostatically compacted aluminum powder.
-S-
Fig. 4. Density-hardness correlation compacted aluminum powder.
30
OPH
40
for isostatically
PRESSURE EAR
01 0
IO
20
30 HAROWSS - O.!H.
Fig_ 3. Powder compacting pressure us hardness For isostatically compacted aluminum_
10
I 5
0
s
10
RAOIALDISTANCE-mm Fig. 5. Pressure distribution on the upper and lower faces of a closed die aluminum compact_
pressure of 3520 bar. The two curves for the upper and lower faces of the compact e_xh%bit the results of die wall friction and particle movement_ The top curve indicates a hardness and pressure increase towards the outside of the compact, while the bottom face exhibits the reverse effect. Integration of the pressure distribution gives an indicated mean pressure of 3440 bar and 2050 bar on the upper and lower faces respectively_ Because of the strainhardening effects in die compaction, it was expected that the integrated values would be lower than the values measured by the load cell. Since this difference is small, little shearing must occur between the powder and the compacting piston Frictional forces between the compact and the die wall can be estimated in this case to be close to 40% of the applied load. This high value is attributable to bonding which occurred between the carbide dies and the aluminum powder, especially at higher pressures_ The density variation within the die compact is illustrated by Fig. 6, the maximum densities being predicted at the top outside perimeter close to the piston-cylinder interface, and in the lower central zone of the compact These results agree with the results reported and explained in detail elsewhere [S] _ The accuracy of the method is limited by two factors, the hardness measurement itself, and the assumption that hardness and density are related by a single value function_ In the
Fig_ 6_ Density
variation in the cross-section
case of the isostatic compact which is uniform, the inherent scatter of the hardness readings on such a statistical material as a green compact may be reduced (in this case to within 1%) by repeated tests and the use of mean values. This is evident in the lack of scatter in the hardness-pressure curve produced in this manner. However, when the technique is to be used in the case of a complex part it may not be possible to repeat the hardness measurements to reduce the error bounds to a predetermined value. However, the technique is still valuable to provide qualitative assessments of loadings and densities in these cases. The assumption of a single value densityhardness relationship depends upon a uniform compaction process during which, as mentioned earlier, the effects of strain hardening will be systematic. Such is the case in isostatic compaction_ In die compaction there are areas of loading which approsimate to isostatic, Le. away from the die walls in the centre of the compact, and areas such as the areas of high density close to the piston-die interface which are subjected to large shear strains. Comparing the two areas, it is easy to see that for a given hardness value the highly sheared material may be less dense than the isostatically compacted material. To evaluate these differences, the mean density of the compact was predicted by integration of the compact density distribution and also obtained by direct measurement and weighing_ The direct measured
of a die compact.
value was 94% and the integrated density was 97% From this small difference it is reasonable to assume that the predicted vahres are sufficiently accurate to evaluate most density distribution problems_ If increased accuracy is required, steps must be taken to remove the effects of strain hardening from both the calibration curves and the final compact The results indicate that the method predicts loads sufficiently well for use in most design situations_ Since the effects of strain hardening have not yet been fully separated, any absolute figure of accuracy is somewhat speculative; however, a conservative estimate for the case of aluminum would be +5% on both pressure and absolute dl:nsity values. Resolution and repeatability between values produced under similar conditions (ix_ in the same die set under similar loads) can be shown to be in the order of al-2%
COxcLUSION
The use of hardness measurements for the prediction of pressure and density within compacted materials has been shown to be feasible_ The method is presently best suited to the qualitative analysis of density distribution, quantitative work being restricted in accuracy except for comparative situations_ Pressure distributions on die sets may be measured satisfactorily, with accuracies as good as
any presently used methods. More work is required to quantitatively assess: (1) the effects of strain hardening in the densification process, (2) the use of the methods for other porous materials, and (3) the densification hardening of nonstrain-hardening powders REFERENCES J_ McDermott. Powdered metals technolo~, Noyes Data Corporation, New Jersey, 19i4. I. &I. Fedorchenko. R. A. Kovynev and 0. F. Polukhin. Use of pin sensors for studying the pressures exerted on the working parts of die sets, Poroshk. MetalI., 91 (19’70) 26. G. hI_ Zhdanovich, Pressure and density distribution in the single and double-ended pressing of axially symmetric compacts, Poroshk hletall.. '76 (1969) 24. A_ Y_ Kandeil, On the compaction of metal powders with particular reference to densification hardening. M_ Eng. Thesis. McAIaster Univ_. Hamilton. 1975. R. Kamm. 31. Steinberg and J. Wulff, Lead grid study of metal powder compaction, Met. Technol., (1949) 694. H. K. Palmer and R. C. Rowe. X study of the compaction behaviour and pore structure of polymer compacts using mercury porosimetry, Powder Technol.. 10 (1974) 225 - 930. G. C. Kcczynski and I. Zaplatynskj, Density distribution in metal powder compacts, J. Met., (1956). R. A_ Federchenkc. R. A. Kovynev et al., _A photoelastic technique for examining contact stresses in powder compaction, Poroshk Metall., 11 (196s) 850.