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Preparation of tungsten-based alloys by the reaction spray process
Preparation of TU,ngsten-Sased Alloys by the Reaction Spray Process S Haerdtle (Dornier GmbH, Friedrichshafen, Germany) The reaction spray process (RSV - Reaktions-SpruhVerfahren) offers the opportunity to prepare alloyed metallic powders in one step. These powders show improved sintering properties compared to powders prepared by mechanical mixing of different powder components. Improvement is mainly shown in sintering activity, homogeneily.in microstructure and mechanical properties of sintered specimens. The process can be applied to a big variety of different alloys such as oxide dispersion strengthened tungsten and molybdenum, two phase tungsten alloys and high speed steels. The area of application for these alloys is where advanced products require superior quality.
(3) Sintering activity of powders produced by the RSV process is high and therefore the sintering temperature can be much low er compared to conventional powders. (4) Relatively low sintering temp eratures lead to extremely fine grained microstructures in sintered specimens . (5) The improved microstructure leads to increased mechanical properties. Typical for the RSV process are powders consisting of agglomerates of about 20um to 1 OO~m in di amet er. The she of the primary particles is below turn, Powder morphology is shown 'i n Fig. 2 and Fig. 3.
EFFECfS OF THE RSV PROCESS ON W + OXIDE PSEUDO ALLOYS
Powder Characteristics The addition of three different oxides has been investigated. the oxide content varied from 0.5% to 5% by weight. Types of oxides are:
- YZ03 - ZrOz - LazO)
The raw materials used to prepare the solution arc:
RSV PRINCIPLES
Th e waste gas is cooled down in a heat exchanger and cleaned in a scrubber.
Fig 1 shows the schematic drawing of the reaction spray process. An aqueous solution of metal salts containing the salts according to the alloy composition is prepared, This solution is atomized into tho hot reaction chamber by a spraying system. The solvent evaporates within fractions of a second leaving extremely fine salt particles. These salt particles arc reduced to metal powder particles in a counterflow of hydrogen. Separation of powder and gas takes place in a hot gas filter.
I\IATERIAL CHARACfERISTICS (1)
(2)
Duo to the rapid evaporation of the solvent the homogeneous distribution of tho dissolved components is transferred to the powder particles. As long as the metal salts are soluble in water thero is no limitation concerning the ratio of different components to one another.
- Ammoniumrnetatungstate - La(N0 3h x 6 HzO - Y (N0 3 h x 6 HzO - ZrOCl z x 8 HzO Concentration achieved was 300g metal/l solution. The powder obtained shows the typical morphology ofRSV-powder.1I consists of spongy agglomerates of primary particles. Figs. 2 and 3 show \V + 0.5% Y Z03 powder. Fig. 3 shows clearly that the size of the primary particles is far below tum and varies very little. The particle size distribution (Fig. 4) shows that the agglomerate size is mainly below tuum, Tap density of the powderis ca. 1.8 g/crn",
Exhaust gas Pressurised or-_--i) (JEJeet or air Water
lUUUX
FIG. 2 SEAt.micrograph of tv + 0.5 % Y Z0 3 powder
Condensatio n disch arge Cooling water discha rge • Dosing pump Solutio n Hydr ogen
FIG. 1 Schematic drawing of the RSV process
MPR February 1990
FIG. 3 SEAt.micrograph of n' + 0.5% 1'Z03 powder
5000X
133
01110 .65
DORNIER GMBH FOWW
o(----------::;;:-~ -.65
%50
10 -.1
-.15 Ol.---+-_ _
~ I J +
~Ull1~_-+++__.....J
1
.1
10
0
PartIcle size (JIm)
No 1 2 3
FIG. 4 Particle size distribution of tv + 0.5% Y20J
600
400
200
100
Imm
PROBE RW4/14135165 RW6I20145166 B301
6.5 6.9 7.4
600 1000 TemperaturlC Oatun Verouchsnr 6Marl969 8130 9 Mar 1969 8131 13 Mar 1969 8135
1200
1400
Sag Bereich:
1600 Korr
1: 2012.0011550 Ph 1: 2012.0011550 Ph 1: 2012.0011550 Ph
Curve 1: W + 0.5% Y20J Curve 2: W + 0.5% Zr02 Curve 3: W + 0.5% La20J FIG. 5 Shrinkage of tv + 0.5% oxide in hydrogen atmosphere 1500 100
•_ _. - - - -
90
____
~ 80 ~ .>.>- - -
<.J
f!1000
::J
~III
~
~70/·
Co
iii
~
~
E
t:
x 0.5% V,O.
60
o 5% V,O.
50 40
Vacuum 5
10 limeh
15
5
FIG. 8 SEM-micrograph of sinteted tv + 0.5% La20J
134
20
FIG. 7 Density vs sintering time for different oxide contents
2 .eSK X 2SKU WD: 21M" $ : e0000 P :00 01 0 2eU" - - -- - - - -- . .
The powder has been compacted uniaxially applying a pressure of 100 Nzrnrn" to 400 N/mm 2 • The green density achieved varie s from 40% to 65% of theoretical density. Shrinkage was inv estigated in hydrogen atmosphere up to a temperature of1600C. Fig. 5 shows shrinkage curves of W containing 0.5% oxide . The type of oxide influences the course of curve but not the final density. The shrinkage of tungsten containing Y20 J rosp, La.O, mainly takes place between 1200C and 1550C(curves 1 and 3). With Zr02 dispersed in tungsten shrinkage starts at ca. 1000C (curve 2). Sntering was carried out in vacuum at a temperature of 1600C (Fig. 6). Maximum density achieved was 96% of theoretical density depending on oxide content and sintering time . Fig. 7 shows the relation
15
limeh
FIG. 6 Sintering program
Powder Processing
10
2000X
l0 ,8 KX 25KU 2UI'l -
-
-
-
W O : 2 1 M~
S :00000 P : 000 12
-
FIG. 9 SEM-micrograph of siniered tv + 0.5% La20J
between density and sintering time at a temperature of '1600C for tung sten plus 0.5% YzOJ resp. 5% YzOJ. Figs. 8 and 9 show SEM micrographs of tungsten plus 0.5% La.OJ' The remaining porosity is ca. 2%. the oxide particles are locat ed at the grain boundaries. The size of the oxide particles is in the range of 0.5J1m to 111m. the particles arc evenly distributed. Grain growth at . elevated temperatures is hindered by the more even particle distribution of the oxide particles. Therefore high temperature strength should be improved with these alloys processed via the RSV process.
SOME MORE APPLICATIONS FOR THE RSVPROCESS The RSV process is also applied for preparing
5UUUX
- Hard metal powder - High speed steel - Tungsten heavy alloy - Molybdenum-based alloys Improvement in properties of sintercd specimens is alre ady shown for HM. HSS and W heavy alloys .
CONCLUSION The reaction spray process shows advantages in preparing alloyed powders wherever homogeneity in distribution of a variety of different components is required. The homogeneity in powder particles can be transferred to sintercd specimens leading to improved mechanical properties.
MPR February 1990
(3) Sintering activity of powders produced by the RSV process is high and therefore the sintering temperature can be much low er compared to conventional powders. (4) Relatively low sintering temp eratures lead to extremely fine grained microstructures in sintered specimens . (5) The improved microstructure leads to increased mechanical properties. Typical for the RSV process are powders consisting of agglomerates of about 20um to 1 OO~m in di amet er. The she of the primary particles is below turn, Powder morphology is shown 'i n Fig. 2 and Fig. 3.
EFFECfS OF THE RSV PROCESS ON W + OXIDE PSEUDO ALLOYS
Powder Characteristics The addition of three different oxides has been investigated. the oxide content varied from 0.5% to 5% by weight. Types of oxides are:
- YZ03 - ZrOz - LazO)
The raw materials used to prepare the solution arc:
RSV PRINCIPLES
Th e waste gas is cooled down in a heat exchanger and cleaned in a scrubber.
Fig 1 shows the schematic drawing of the reaction spray process. An aqueous solution of metal salts containing the salts according to the alloy composition is prepared, This solution is atomized into tho hot reaction chamber by a spraying system. The solvent evaporates within fractions of a second leaving extremely fine salt particles. These salt particles arc reduced to metal powder particles in a counterflow of hydrogen. Separation of powder and gas takes place in a hot gas filter.
I\IATERIAL CHARACfERISTICS (1)
(2)
Duo to the rapid evaporation of the solvent the homogeneous distribution of tho dissolved components is transferred to the powder particles. As long as the metal salts are soluble in water thero is no limitation concerning the ratio of different components to one another.
- Ammoniumrnetatungstate - La(N0 3h x 6 HzO - Y (N0 3 h x 6 HzO - ZrOCl z x 8 HzO Concentration achieved was 300g metal/l solution. The powder obtained shows the typical morphology ofRSV-powder.1I consists of spongy agglomerates of primary particles. Figs. 2 and 3 show \V + 0.5% Y Z03 powder. Fig. 3 shows clearly that the size of the primary particles is far below tum and varies very little. The particle size distribution (Fig. 4) shows that the agglomerate size is mainly below tuum, Tap density of the powderis ca. 1.8 g/crn",
Exhaust gas Pressurised or-_--i) (JEJeet or air Water
lUUUX
FIG. 2 SEAt.micrograph of tv + 0.5 % Y Z0 3 powder
Condensatio n disch arge Cooling water discha rge • Dosing pump Solutio n Hydr ogen
FIG. 1 Schematic drawing of the RSV process
MPR February 1990
FIG. 3 SEAt.micrograph of n' + 0.5% 1'Z03 powder
5000X
133
01110 .65
DORNIER GMBH FOWW
o(----------::;;:-~ -.65
%50
10 -.1
-.15 Ol.---+-_ _
~ I J +
~Ull1~_-+++__.....J
1
.1
10
0
PartIcle size (JIm)
No 1 2 3
FIG. 4 Particle size distribution of tv + 0.5% Y20J
600
400
200
100
Imm
PROBE RW4/14135165 RW6I20145166 B301
6.5 6.9 7.4
600 1000 TemperaturlC Oatun Verouchsnr 6Marl969 8130 9 Mar 1969 8131 13 Mar 1969 8135
1200
1400
Sag Bereich:
1600 Korr
1: 2012.0011550 Ph 1: 2012.0011550 Ph 1: 2012.0011550 Ph
Curve 1: W + 0.5% Y20J Curve 2: W + 0.5% Zr02 Curve 3: W + 0.5% La20J FIG. 5 Shrinkage of tv + 0.5% oxide in hydrogen atmosphere 1500 100
•_ _. - - - -
90
____
~ 80 ~ .>.>- - -
<.J
f!1000
::J
~III
~
~70/·
Co
iii
~
~
E
t:
x 0.5% V,O.
60
o 5% V,O.
50 40
Vacuum 5
10 limeh
15
5
FIG. 8 SEM-micrograph of sinteted tv + 0.5% La20J
134
20
FIG. 7 Density vs sintering time for different oxide contents
2 .eSK X 2SKU WD: 21M" $ : e0000 P :00 01 0 2eU" - - -- - - - -- . .
The powder has been compacted uniaxially applying a pressure of 100 Nzrnrn" to 400 N/mm 2 • The green density achieved varie s from 40% to 65% of theoretical density. Shrinkage was inv estigated in hydrogen atmosphere up to a temperature of1600C. Fig. 5 shows shrinkage curves of W containing 0.5% oxide . The type of oxide influences the course of curve but not the final density. The shrinkage of tungsten containing Y20 J rosp, La.O, mainly takes place between 1200C and 1550C(curves 1 and 3). With Zr02 dispersed in tungsten shrinkage starts at ca. 1000C (curve 2). Sntering was carried out in vacuum at a temperature of 1600C (Fig. 6). Maximum density achieved was 96% of theoretical density depending on oxide content and sintering time . Fig. 7 shows the relation
15
limeh
FIG. 6 Sintering program
Powder Processing
10
2000X
l0 ,8 KX 25KU 2UI'l -
-
-
-
W O : 2 1 M~
S :00000 P : 000 12
-
FIG. 9 SEM-micrograph of siniered tv + 0.5% La20J
between density and sintering time at a temperature of '1600C for tung sten plus 0.5% YzOJ resp. 5% YzOJ. Figs. 8 and 9 show SEM micrographs of tungsten plus 0.5% La.OJ' The remaining porosity is ca. 2%. the oxide particles are locat ed at the grain boundaries. The size of the oxide particles is in the range of 0.5J1m to 111m. the particles arc evenly distributed. Grain growth at . elevated temperatures is hindered by the more even particle distribution of the oxide particles. Therefore high temperature strength should be improved with these alloys processed via the RSV process.
SOME MORE APPLICATIONS FOR THE RSVPROCESS The RSV process is also applied for preparing
5UUUX
- Hard metal powder - High speed steel - Tungsten heavy alloy - Molybdenum-based alloys Improvement in properties of sintercd specimens is alre ady shown for HM. HSS and W heavy alloys .
CONCLUSION The reaction spray process shows advantages in preparing alloyed powders wherever homogeneity in distribution of a variety of different components is required. The homogeneity in powder particles can be transferred to sintercd specimens leading to improved mechanical properties.
MPR February 1990