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Functional design puts the bite into hard and refractory metals
Technical trends
Functional design puts the bite into hard and refractory metals It has long been possible tailor a material's characteristics by altering chemical content. But now hardmetals engineers are using functional design to address new applications too. Some of these were discussed at PM2TEC 2003 in Las Vegas. Ken Brookes was there… n discussing microstructures, functional design is not quite the same as functional gradient, a term that has been around for a few years longer. In coverage of PM2TEC 2003 at Las Vegas in the October edition of Metal Powder Report (Page 34) attention was drawn to a functional design paper by Jeremy Watts of the University of Missouri - Rolla, entitled Development of novel materials for mining applications. It introduced an innovative co-extrusion technique for the production of hardmetals with a fibrous microstructure. This resulted in a composite cemented carbide with distinctively anisotropic properties, having potential applications wider than the suggested tricone or rollercone mining bits. Several other papers, discussed below in their order of presentation, covered different aspects of this interesting development, the investigation of which now involves a number of universities and commercial organisations. In the first of these papers, on Functionally designed PM wear-resistant materials, Zhigang Z Fang of the University of Utah covered a range of possibilities. His functionally designed materials (FDM) survey began with a description of the established DC carbide, a so-called "double cemented" carbide in which pre-sintered granules of WC/6Co
I
20
MPR November 2003
hardmetal, of 2-4µm grain size, are embedded in a pure cobalt matrix. To preserve the "two-level" microstructure,
sintering is usually carried out in the solid state by ultrahigh-pressure technology termed "rapid omnidirectional compaction" (ROC), or by hot pressing. Some new property data was included in the paper. PM materials of a fundamentally similar nature, termed "double dispersion" materials, included agglomerates of highcarbide tool steel embedded in low-carbon steel of higher toughness, and WC/Co agglomerates in a similarly tough steel matrix. Fang tackled the development of honeycomb co-extrusion from a somewhat different viewpoint from that of Watts. Figure 1 illustrates the process for fabricating "fibrous monoliths", in which lengths of extruded single fibre are packed together and put through a further extrusion operation. Figure 2 is a 3D micrograph of a honeycomb produced by these means, in which the cores are of polycrystalline diamond (PCD) and the cell walls WC/16Co. After co-extrusion, the samples were subjected to the conventional high-temperature, high-pressure
Press Powder I + polymer binder I Rod Press
Assemble Press
Powder II + polymer binder II
A rod with core & shell Half shells
Fibre with core & shell Co-extrusion
Figure 1. Schematic illustration of co-extrusion process for fabricating fibrous monolith ceramics.
Figure 2. 3-D micrograph of a honeycomb-structured PCD-WC/Co composite.
0026-0657/03 ©2003 Elsevier Ltd. All rights reserved.
Technical trends consolidation employed in the synthetic diamond industry. Impact chipping resistance of rock drill bits of this material showed substantial improvement over the normal homogeneous material. The cell boundary material deters crack propagation and absorbs fracture energies, while the highhardness cell material provides wear resistance. Cracks that do occur tend to run along the cell boundaries. Thus Fang gave a good summary of present and potential FDMs, especially for rock drilling, covering combinations of different materials, different grain sizes and different geometries.
Batching 1st pass feedstock
1st pass coextrusion
FDM commercial application
metal-powder.net
2nd pass filaments
1st pass filaments into 2nd pass feedstock
2nd pass coextrusion
Figure 3. Schematic of the co-extrusion process.
1000
800
Temperature (˚C)
Sean E Landwehr of the University of Missouri - Rolla described the honeycomb extrusion process and its application to mining bits in a following paper entitled Functionally designed cemented carbides. His version of the simplified flowsheet, developed in conjunction with Smith Tool, is given as Figure 3. The resultant material is a multicellular structure termed a fibrous monolith, with unusual properties. The first step is to mix the powders into a thermoplastic binder system in which the primary constituent ethylene ethyl acrylate (EEA) is modified with heavy mineral oil (HMO) and/or methoxypolyethylene glycol (MPEG). The blends are then formed into the first pass feedstock, consisting of a shell and core. For this investigation the core was a conventional cemented carbide, either Kennametal grade 367 (WC/11Co) or Kennametal grade 369 (WC/14Co). Shell material was cobalt metal, Cerac C-1111. The shell-and-core combination was co-extruded in a 10:1 ratio to 2.4mm diameter filaments, then each was cut to standard lengths and recombined for a second extrusion, to make a sub-millimetre cellular structure. Maintaining unidirectional orientation, the second-pass filaments were formed into billets, from which green test samples were cut. The polymer binder is removed by pyrolysis. As shown in Figure 4, complete binder removal requires long holds at low temperature in an oxidising atmosphere to break down the high-molecular-weight
Air
Ar/H2
600
400
200
0
20
40
60
80
100
120
140
160
Time (h)
Figure 4. Heating schedule for binder removal.
Ram Molten glass
Pot die
Sample Figure 5. Schematic of ROC process.
November 2003 MPR
21
Technical trends
Figure 6. Schematic diagram of functionally designed cellular microstructure.
Table 1 Weight per cent of cobalt in boundary and cell, and extent of homogenisation of WC/6Co-WC/16Co at different sintering temperatures. Sintering Hold time Weight % Weight % Degree of temperature °C (minutes) of cobalt in of cobalt homogenisation boundary in cell ∆C/∆Co (%) 1000 60 15.98 6.01 0.3 1100
60
15.91
5.95
0.4
1150
60
15.89
5.96
0.7
1200
60
15.94
5.95
0.1
1230
60
10.21
7.89
76.8
1260
60
9.51
8.01
85
1270
60
7.67
8.24
105.7
1290
60
7.64
8.15
105.1
1300
60
7.63
8.01
103.8
1350
60
7.91
7.82
99.1
13
12.8
Density in g/cc
12.6
12.4
12.2
12
11.8
11.6 0
20
40
60
80
100
120
140
160
180
200
Sinter hold time in minutes
Figure 7. Density versus hold time at 1230°C sintering temperature of WC/6Co-WC/16Co.
22
MPR November 2003
polymer, followed by much higher temperature in a reducing atmosphere of Ar/H2. The final stage of densification is rapid omnidirectional compaction (ROC Figure 5) at 1240ºC and 800MPa. Specimens are encapsulated in glass, which at high temperatures becomes a molten pressure-transmissive fluid that converts axial pressure in the press into isostatic pressure. Mechanical testing in the longitudinal direction showed around 25 per cent increase in fracture toughness for both grades. In fully mixed non-cellular structures with similar cobalt additions, fracture toughness would be expected to increase by only 13-17 per cent. On the other hand, TRS values, whether longitudinal or perpendicular, are below those of monolithic material of the same average composition. Further research was suggested to test the theoretical basis for this effect. Wear resistance results were in some aspects surprising. In spite of the substantial additions of cobalt, reductions in wear resistance were notably less than would be expected in simple mixtures. In the transverse direction the reduction was less than 4 per cent when compared with their monolithic counterparts. The theme of cellular co-extrusion at PM2TEC 2003 revealed another investigation from the enterprising University of Utah, Oladapo Eso's Sintering studies of WC/Co composites with functionally designed microstructures. The author compares cellular composites to typical functional gradient materials, but with gradients in two dimensions rather than one and in a regularly repeating pattern (Figure 6). As demonstrated earlier, these materials have interesting anisotropic properties, but are difficult to sinter by conventional means. Thus the objects of this research were to evaluate the effect of sintering temperature and to develop a vacuum sintering technique that retained the desired cellular microstructure. For this study, compositions of the cell and boundary regions were WC/6Co and WC/16Co respectively, with grain sizes 1.5µm and 3µm. Hexagonal cells were of the order of 100µm across. Co-extruded rods were obtained from Advanced Ceramics Research and cut into samples
metal-powder.net
Technical trends
metal-powder.net
13.4 13.3 13.2 13.4
Density in g/cc
9.4mm diameter and 13.55mm long. They were "debound" or dewaxed (the author used "debinded") at an unstated but probably elevated temperature in an atmosphere of 80 argon and 20 hydrogen per cent. Samples were then vacuum sintered on graphite plates at 1000oC, 1150oC, 1200oC, 1230oC, 1260oC, 1270oC, 1290oC, 1300oC and 1350ºC for 1 hour at a ramp rate of 10K/min. Additional samples were sintered at 1230oC, 1260oC and 1400ºC for 10 minutes, 2 hours and 3 hours at the same ramp rate. This arrangement brackets the WC/Co pseudoeutectic temperature of approximately 1280ºC and thus the transition from solid-state to liquidphase sintering. Tests carried out on the sintered samples, included microstructure (after etching polished surfaces with Murakami's reagent) by optical and SEM methods, specific gravity, and composition of cell and boundary regions (by energy dispersive spectroscopy). As can be seen from Table 1, a substantial degree of homogenisation takes place as soon as sintering temperature approaches the liquidus value of the pseudoeutectic, apparently even before liquid is present. Density increases - and porosity decreases - with hold time (Figures 7 and 8), but homogenisation occurs simultaneously (Tables 2 and 3). These results, backed by metallographic analysis, emphasise that, even during solid-state sintering, the impetus for sintering and densification comes largely from cobalt diffusion and migration, as well as from carbide solution in the cobalt, during which the cellular structure tends to disappear. There remains a residual cellular structure (Figures 9 and 10), represented mainly by variations in carbide grain size, but this has a minimal or even a detrimental effect on overall properties. Because homogenisation is so clearly time-dependent, the author suggested that production of WC/6Co-WC/16Co with graded composition and cellular microstructure could be achieved by presintering at 1230ºC followed by HIPing at a solid-state temperature to eliminate residual porosity. Low-temperature sinterHIP was neither investigated as part of this study, nor suggested as a possible production method.
13 12.9 12.8 12.7 12.6 12.5 12.4 12.3 0
10
30
20
40
50
60
70
Time in minutes
Figure 8. Density versus hold time at 1260°C sintering temperature of WC/6Co-WC/16Co.
Table 2 Weight per cent of cobalt in boundary and cell, and extent of homogenisation of WC/6Co-WC/16Co at 1230°C sintering temperature and different holding times. Sintering Hold time Weight % Weight % Degree of temperature °C (minutes) of cobalt in of cobalt homogenisation boundary in cell ∆C/∆Co (%) 1230 10 9.45 6.81 73.6 1230
60
10.21
7.87
76.6
1230
180
10.85
7.61
67.6
Table 3 Weight per cent of cobalt in boundary and cell, and extent of homogenisation of WC/6Co-WC/16Co at 1260°C sintering temperature and different holding times. Sintering Hold time Weight % Weight % Degree of temperature °C (minutes) of cobalt in of cobalt homogenisation boundary in cell ∆C/∆Co (%) 1260 10 8.81 8.28 94.7 1260
30
7.45
7.15
97
1260
60
9.51
8.01
85
Figure 9. Optical micrograph of WC/ 6Co-WC/16Co after sintering at 1400°C for two hours.
Figure 10. SEM micrograph of WC/ 6Co-WC/16Co after sintering at 1400°C for two hours.
November 2003 MPR
23
Technical trends 120
100
80
60
Figure 12. Microstructure of cross-section of green compact of W-Ni-Fe/Ni with bright cells of W-Ni-Fe and dark grey Ni cell boundaries.*
40
20
0 900
950
1000
1050
1100
1150
1200
1250
1300
1350
looked at the Sintering of fibrous monolith composite based on W-Ni-Fe alloy, a candidate for the replacement of depleted uranium in kinetic penetrators and thus of high potential commercially. Improved directional properties result from the coextruded fibrous structure, very noticeable in the green state (Figure 12), but unfortunately this is destroyed during normal liquid-phase sintering.
1400
Sintering temperature (˚C)
Figure 11. Change in degree of homogenisation ∆C/C' with the sintering temperature.
90
10
(W) + L
n iro nt ht
50
50
tun
pe
60
nt
rce
40
rce 30
Fe )
(Ni)
20
Fe ,δ
80
Metal matrix at equilibrium: 32.5W-60.1Ni-7.4Fe
ten
40
60
gs
we ig
70
70
Cell boundary: pure Ni
L
(α
90
Metal matrix measured: 38.28W-55.16Ni-6.56Fe
80
20 30 Fe7We
pe
MPR November 2003
Overall: 83.6W-14.6Ni-1.8Fe
W
Cell: 93W-5Ni-2Fe
ht
24
enterprising investigators of Utah University and Advanced Ceramics Manufacturing, with further variants on their versatile honeycombed functional microstructure. On this occasion Peng Fan of the university
ig We
In his introduction to this interesting paper, the author mentioned parallel work on composites with PCD (polycrystalline diamond) in the nominally 100µm cells and WC/11Co in the coextruded boundary walls. It may (or may not) be significant that no mention was made in the paper of this material, though it could equally be a possible candidate for conventional vacuum sintering - or of vacuum presintering followed by HIPing. This will perhaps form the subject of a future contribution. In looking at the author's results as presented, especially the diagram (Figure 11) showing changes in the degree of homogenisation, readers will hardly avoid noticing the sudden rise at about 1200ºC, conventionally regarded as well below the temperature where liquidphase sintering commences. However, we are not given details of impurity levels in what were presumably commercial rather than especially high-purity starting materials, and the impurities may be critical at the start of densification. Could the impurities present create a highly localised ternary, quaternary or quinternary system having a eutectic temperature as low as 1200ºC? And could the interparticle surface reactions be sufficiently exothermic to maintain liquidphase sintering on a microscopic or submicroscopic scale, even where the measured average temperature of the compact is lower? Or are these idle thoughts on matters already thoroughly investigated? Even in the session on tungsten heavy alloys, one could not avoid the
10
(γFe) Fe
10
20
30
40
50
60
70
80
90
Ni
Weight percent nickel
Figure 13. W-Ni-Fe phase diagram at 1465oC showing starting compositions for cell, cell boundary and overall material and the measured and equilibrium compositions for metal matrix.
Figure 14a & b. Microstructures of samples after solid-state sintering at 1460°C for four hours with bright cells and dark grey cell boundaries.
metal-powder.net
Technical trends W
Overall composition of the composite: 83.6W-14.6Ni-1.8Fe
90
10
80
20
(W) + L
iro n nt ht
50
70
ten
40
60
gs
tun
50
Metal matrix measured: 39.18W-54.17Ni-6.65Fe
nt
rce
60
rce
40
pe
pe
ht
we ig
70
ig We
30
Fe7We
30
Metal matrix at equilibrium: 36.0W-57.0Ni-7.0Fe
20
80
L 90
Fe
(αFe, δFe) (γFe) 10
20
10
30
40
50
60
70
80
90
Ni
Weight percent nickel
Figure 15. W-Ni-Fe phase diagram at 1500°C showing measured and equilibrium compositions for metal matrix and the overall composition of the composite.
Initially, cells are of tungsten-base heavy alloy, with composition 93W/5Ni/2Fe wt per cent. Walls are of pure nickel, but their thickness (not quoted in the paper) must be minimised because of their effect in lowering average density, a vital factor in the intended application. Round discs 8mm thick were cut from the green extruded rod for
alternative two-step process, after the deoxidation stage, specimens were first sintered at 1460ºC for 4 hours, then again at 1480oC or 1490ºC for a period before cooling. Essentially, sintering at 1460ºC is in the solid state (Figure 13), which maintains the honeycomb structure but leaves porosity (Figure 14a and b). HIPing removes the porosity but is expensive. At 1480ºC and above, liquid-phase sintering takes place (Figure 15), with minimal residual porosity but eventually complete loss of the cellular structure through diffusion (Figure 16a and b). Whilst the researchers appear to have resolved the technological problem, it remains to be seen whether the improvement in properties will make the material sufficiently competitive with depleted uranium.
debinding (in 80/20 Ar/H2 atmosphere in a retort furnace) and sintering tests. Sintering cycles were divided into two categories. In the so-called one-step process, samples were heated to 1000ºC, held for 1 hour in H2 to remove surface oxidation, then heated to sintering temperature (1460oC -1500ºC), held for one to four hours and furnace cooled. In the
Figure 16a & b. Microstructures of samples after liquid-phase sintering with W particles (dark grey) embedded in Ni-Fe matrix (bright).*
Figure 17a and b. Microstructures of sample after two-step sintering at 1460°C for four hours plus 1480°C for 30 minutes with W particles (dark grey) embedded in Ni/Fe matrix (bright).*
* The quality of the four images above and Figure 12 is reduced by heavy pixellation
metal-powder.net
November 2003 MPR
25
Functional design puts the bite into hard and refractory metals It has long been possible tailor a material's characteristics by altering chemical content. But now hardmetals engineers are using functional design to address new applications too. Some of these were discussed at PM2TEC 2003 in Las Vegas. Ken Brookes was there… n discussing microstructures, functional design is not quite the same as functional gradient, a term that has been around for a few years longer. In coverage of PM2TEC 2003 at Las Vegas in the October edition of Metal Powder Report (Page 34) attention was drawn to a functional design paper by Jeremy Watts of the University of Missouri - Rolla, entitled Development of novel materials for mining applications. It introduced an innovative co-extrusion technique for the production of hardmetals with a fibrous microstructure. This resulted in a composite cemented carbide with distinctively anisotropic properties, having potential applications wider than the suggested tricone or rollercone mining bits. Several other papers, discussed below in their order of presentation, covered different aspects of this interesting development, the investigation of which now involves a number of universities and commercial organisations. In the first of these papers, on Functionally designed PM wear-resistant materials, Zhigang Z Fang of the University of Utah covered a range of possibilities. His functionally designed materials (FDM) survey began with a description of the established DC carbide, a so-called "double cemented" carbide in which pre-sintered granules of WC/6Co
I
20
MPR November 2003
hardmetal, of 2-4µm grain size, are embedded in a pure cobalt matrix. To preserve the "two-level" microstructure,
sintering is usually carried out in the solid state by ultrahigh-pressure technology termed "rapid omnidirectional compaction" (ROC), or by hot pressing. Some new property data was included in the paper. PM materials of a fundamentally similar nature, termed "double dispersion" materials, included agglomerates of highcarbide tool steel embedded in low-carbon steel of higher toughness, and WC/Co agglomerates in a similarly tough steel matrix. Fang tackled the development of honeycomb co-extrusion from a somewhat different viewpoint from that of Watts. Figure 1 illustrates the process for fabricating "fibrous monoliths", in which lengths of extruded single fibre are packed together and put through a further extrusion operation. Figure 2 is a 3D micrograph of a honeycomb produced by these means, in which the cores are of polycrystalline diamond (PCD) and the cell walls WC/16Co. After co-extrusion, the samples were subjected to the conventional high-temperature, high-pressure
Press Powder I + polymer binder I Rod Press
Assemble Press
Powder II + polymer binder II
A rod with core & shell Half shells
Fibre with core & shell Co-extrusion
Figure 1. Schematic illustration of co-extrusion process for fabricating fibrous monolith ceramics.
Figure 2. 3-D micrograph of a honeycomb-structured PCD-WC/Co composite.
0026-0657/03 ©2003 Elsevier Ltd. All rights reserved.
Technical trends consolidation employed in the synthetic diamond industry. Impact chipping resistance of rock drill bits of this material showed substantial improvement over the normal homogeneous material. The cell boundary material deters crack propagation and absorbs fracture energies, while the highhardness cell material provides wear resistance. Cracks that do occur tend to run along the cell boundaries. Thus Fang gave a good summary of present and potential FDMs, especially for rock drilling, covering combinations of different materials, different grain sizes and different geometries.
Batching 1st pass feedstock
1st pass coextrusion
FDM commercial application
metal-powder.net
2nd pass filaments
1st pass filaments into 2nd pass feedstock
2nd pass coextrusion
Figure 3. Schematic of the co-extrusion process.
1000
800
Temperature (˚C)
Sean E Landwehr of the University of Missouri - Rolla described the honeycomb extrusion process and its application to mining bits in a following paper entitled Functionally designed cemented carbides. His version of the simplified flowsheet, developed in conjunction with Smith Tool, is given as Figure 3. The resultant material is a multicellular structure termed a fibrous monolith, with unusual properties. The first step is to mix the powders into a thermoplastic binder system in which the primary constituent ethylene ethyl acrylate (EEA) is modified with heavy mineral oil (HMO) and/or methoxypolyethylene glycol (MPEG). The blends are then formed into the first pass feedstock, consisting of a shell and core. For this investigation the core was a conventional cemented carbide, either Kennametal grade 367 (WC/11Co) or Kennametal grade 369 (WC/14Co). Shell material was cobalt metal, Cerac C-1111. The shell-and-core combination was co-extruded in a 10:1 ratio to 2.4mm diameter filaments, then each was cut to standard lengths and recombined for a second extrusion, to make a sub-millimetre cellular structure. Maintaining unidirectional orientation, the second-pass filaments were formed into billets, from which green test samples were cut. The polymer binder is removed by pyrolysis. As shown in Figure 4, complete binder removal requires long holds at low temperature in an oxidising atmosphere to break down the high-molecular-weight
Air
Ar/H2
600
400
200
0
20
40
60
80
100
120
140
160
Time (h)
Figure 4. Heating schedule for binder removal.
Ram Molten glass
Pot die
Sample Figure 5. Schematic of ROC process.
November 2003 MPR
21
Technical trends
Figure 6. Schematic diagram of functionally designed cellular microstructure.
Table 1 Weight per cent of cobalt in boundary and cell, and extent of homogenisation of WC/6Co-WC/16Co at different sintering temperatures. Sintering Hold time Weight % Weight % Degree of temperature °C (minutes) of cobalt in of cobalt homogenisation boundary in cell ∆C/∆Co (%) 1000 60 15.98 6.01 0.3 1100
60
15.91
5.95
0.4
1150
60
15.89
5.96
0.7
1200
60
15.94
5.95
0.1
1230
60
10.21
7.89
76.8
1260
60
9.51
8.01
85
1270
60
7.67
8.24
105.7
1290
60
7.64
8.15
105.1
1300
60
7.63
8.01
103.8
1350
60
7.91
7.82
99.1
13
12.8
Density in g/cc
12.6
12.4
12.2
12
11.8
11.6 0
20
40
60
80
100
120
140
160
180
200
Sinter hold time in minutes
Figure 7. Density versus hold time at 1230°C sintering temperature of WC/6Co-WC/16Co.
22
MPR November 2003
polymer, followed by much higher temperature in a reducing atmosphere of Ar/H2. The final stage of densification is rapid omnidirectional compaction (ROC Figure 5) at 1240ºC and 800MPa. Specimens are encapsulated in glass, which at high temperatures becomes a molten pressure-transmissive fluid that converts axial pressure in the press into isostatic pressure. Mechanical testing in the longitudinal direction showed around 25 per cent increase in fracture toughness for both grades. In fully mixed non-cellular structures with similar cobalt additions, fracture toughness would be expected to increase by only 13-17 per cent. On the other hand, TRS values, whether longitudinal or perpendicular, are below those of monolithic material of the same average composition. Further research was suggested to test the theoretical basis for this effect. Wear resistance results were in some aspects surprising. In spite of the substantial additions of cobalt, reductions in wear resistance were notably less than would be expected in simple mixtures. In the transverse direction the reduction was less than 4 per cent when compared with their monolithic counterparts. The theme of cellular co-extrusion at PM2TEC 2003 revealed another investigation from the enterprising University of Utah, Oladapo Eso's Sintering studies of WC/Co composites with functionally designed microstructures. The author compares cellular composites to typical functional gradient materials, but with gradients in two dimensions rather than one and in a regularly repeating pattern (Figure 6). As demonstrated earlier, these materials have interesting anisotropic properties, but are difficult to sinter by conventional means. Thus the objects of this research were to evaluate the effect of sintering temperature and to develop a vacuum sintering technique that retained the desired cellular microstructure. For this study, compositions of the cell and boundary regions were WC/6Co and WC/16Co respectively, with grain sizes 1.5µm and 3µm. Hexagonal cells were of the order of 100µm across. Co-extruded rods were obtained from Advanced Ceramics Research and cut into samples
metal-powder.net
Technical trends
metal-powder.net
13.4 13.3 13.2 13.4
Density in g/cc
9.4mm diameter and 13.55mm long. They were "debound" or dewaxed (the author used "debinded") at an unstated but probably elevated temperature in an atmosphere of 80 argon and 20 hydrogen per cent. Samples were then vacuum sintered on graphite plates at 1000oC, 1150oC, 1200oC, 1230oC, 1260oC, 1270oC, 1290oC, 1300oC and 1350ºC for 1 hour at a ramp rate of 10K/min. Additional samples were sintered at 1230oC, 1260oC and 1400ºC for 10 minutes, 2 hours and 3 hours at the same ramp rate. This arrangement brackets the WC/Co pseudoeutectic temperature of approximately 1280ºC and thus the transition from solid-state to liquidphase sintering. Tests carried out on the sintered samples, included microstructure (after etching polished surfaces with Murakami's reagent) by optical and SEM methods, specific gravity, and composition of cell and boundary regions (by energy dispersive spectroscopy). As can be seen from Table 1, a substantial degree of homogenisation takes place as soon as sintering temperature approaches the liquidus value of the pseudoeutectic, apparently even before liquid is present. Density increases - and porosity decreases - with hold time (Figures 7 and 8), but homogenisation occurs simultaneously (Tables 2 and 3). These results, backed by metallographic analysis, emphasise that, even during solid-state sintering, the impetus for sintering and densification comes largely from cobalt diffusion and migration, as well as from carbide solution in the cobalt, during which the cellular structure tends to disappear. There remains a residual cellular structure (Figures 9 and 10), represented mainly by variations in carbide grain size, but this has a minimal or even a detrimental effect on overall properties. Because homogenisation is so clearly time-dependent, the author suggested that production of WC/6Co-WC/16Co with graded composition and cellular microstructure could be achieved by presintering at 1230ºC followed by HIPing at a solid-state temperature to eliminate residual porosity. Low-temperature sinterHIP was neither investigated as part of this study, nor suggested as a possible production method.
13 12.9 12.8 12.7 12.6 12.5 12.4 12.3 0
10
30
20
40
50
60
70
Time in minutes
Figure 8. Density versus hold time at 1260°C sintering temperature of WC/6Co-WC/16Co.
Table 2 Weight per cent of cobalt in boundary and cell, and extent of homogenisation of WC/6Co-WC/16Co at 1230°C sintering temperature and different holding times. Sintering Hold time Weight % Weight % Degree of temperature °C (minutes) of cobalt in of cobalt homogenisation boundary in cell ∆C/∆Co (%) 1230 10 9.45 6.81 73.6 1230
60
10.21
7.87
76.6
1230
180
10.85
7.61
67.6
Table 3 Weight per cent of cobalt in boundary and cell, and extent of homogenisation of WC/6Co-WC/16Co at 1260°C sintering temperature and different holding times. Sintering Hold time Weight % Weight % Degree of temperature °C (minutes) of cobalt in of cobalt homogenisation boundary in cell ∆C/∆Co (%) 1260 10 8.81 8.28 94.7 1260
30
7.45
7.15
97
1260
60
9.51
8.01
85
Figure 9. Optical micrograph of WC/ 6Co-WC/16Co after sintering at 1400°C for two hours.
Figure 10. SEM micrograph of WC/ 6Co-WC/16Co after sintering at 1400°C for two hours.
November 2003 MPR
23
Technical trends 120
100
80
60
Figure 12. Microstructure of cross-section of green compact of W-Ni-Fe/Ni with bright cells of W-Ni-Fe and dark grey Ni cell boundaries.*
40
20
0 900
950
1000
1050
1100
1150
1200
1250
1300
1350
looked at the Sintering of fibrous monolith composite based on W-Ni-Fe alloy, a candidate for the replacement of depleted uranium in kinetic penetrators and thus of high potential commercially. Improved directional properties result from the coextruded fibrous structure, very noticeable in the green state (Figure 12), but unfortunately this is destroyed during normal liquid-phase sintering.
1400
Sintering temperature (˚C)
Figure 11. Change in degree of homogenisation ∆C/C' with the sintering temperature.
90
10
(W) + L
n iro nt ht
50
50
tun
pe
60
nt
rce
40
rce 30
Fe )
(Ni)
20
Fe ,δ
80
Metal matrix at equilibrium: 32.5W-60.1Ni-7.4Fe
ten
40
60
gs
we ig
70
70
Cell boundary: pure Ni
L
(α
90
Metal matrix measured: 38.28W-55.16Ni-6.56Fe
80
20 30 Fe7We
pe
MPR November 2003
Overall: 83.6W-14.6Ni-1.8Fe
W
Cell: 93W-5Ni-2Fe
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enterprising investigators of Utah University and Advanced Ceramics Manufacturing, with further variants on their versatile honeycombed functional microstructure. On this occasion Peng Fan of the university
ig We
In his introduction to this interesting paper, the author mentioned parallel work on composites with PCD (polycrystalline diamond) in the nominally 100µm cells and WC/11Co in the coextruded boundary walls. It may (or may not) be significant that no mention was made in the paper of this material, though it could equally be a possible candidate for conventional vacuum sintering - or of vacuum presintering followed by HIPing. This will perhaps form the subject of a future contribution. In looking at the author's results as presented, especially the diagram (Figure 11) showing changes in the degree of homogenisation, readers will hardly avoid noticing the sudden rise at about 1200ºC, conventionally regarded as well below the temperature where liquidphase sintering commences. However, we are not given details of impurity levels in what were presumably commercial rather than especially high-purity starting materials, and the impurities may be critical at the start of densification. Could the impurities present create a highly localised ternary, quaternary or quinternary system having a eutectic temperature as low as 1200ºC? And could the interparticle surface reactions be sufficiently exothermic to maintain liquidphase sintering on a microscopic or submicroscopic scale, even where the measured average temperature of the compact is lower? Or are these idle thoughts on matters already thoroughly investigated? Even in the session on tungsten heavy alloys, one could not avoid the
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(γFe) Fe
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Weight percent nickel
Figure 13. W-Ni-Fe phase diagram at 1465oC showing starting compositions for cell, cell boundary and overall material and the measured and equilibrium compositions for metal matrix.
Figure 14a & b. Microstructures of samples after solid-state sintering at 1460°C for four hours with bright cells and dark grey cell boundaries.
metal-powder.net
Technical trends W
Overall composition of the composite: 83.6W-14.6Ni-1.8Fe
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(W) + L
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Metal matrix measured: 39.18W-54.17Ni-6.65Fe
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Fe7We
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Metal matrix at equilibrium: 36.0W-57.0Ni-7.0Fe
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(αFe, δFe) (γFe) 10
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Weight percent nickel
Figure 15. W-Ni-Fe phase diagram at 1500°C showing measured and equilibrium compositions for metal matrix and the overall composition of the composite.
Initially, cells are of tungsten-base heavy alloy, with composition 93W/5Ni/2Fe wt per cent. Walls are of pure nickel, but their thickness (not quoted in the paper) must be minimised because of their effect in lowering average density, a vital factor in the intended application. Round discs 8mm thick were cut from the green extruded rod for
alternative two-step process, after the deoxidation stage, specimens were first sintered at 1460ºC for 4 hours, then again at 1480oC or 1490ºC for a period before cooling. Essentially, sintering at 1460ºC is in the solid state (Figure 13), which maintains the honeycomb structure but leaves porosity (Figure 14a and b). HIPing removes the porosity but is expensive. At 1480ºC and above, liquid-phase sintering takes place (Figure 15), with minimal residual porosity but eventually complete loss of the cellular structure through diffusion (Figure 16a and b). Whilst the researchers appear to have resolved the technological problem, it remains to be seen whether the improvement in properties will make the material sufficiently competitive with depleted uranium.
debinding (in 80/20 Ar/H2 atmosphere in a retort furnace) and sintering tests. Sintering cycles were divided into two categories. In the so-called one-step process, samples were heated to 1000ºC, held for 1 hour in H2 to remove surface oxidation, then heated to sintering temperature (1460oC -1500ºC), held for one to four hours and furnace cooled. In the
Figure 16a & b. Microstructures of samples after liquid-phase sintering with W particles (dark grey) embedded in Ni-Fe matrix (bright).*
Figure 17a and b. Microstructures of sample after two-step sintering at 1460°C for four hours plus 1480°C for 30 minutes with W particles (dark grey) embedded in Ni/Fe matrix (bright).*
* The quality of the four images above and Figure 12 is reduced by heavy pixellation
metal-powder.net
November 2003 MPR
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