GB2330419A

GB2330419A - Method and apparatus for characterising a powder

Info

Publication number: GB2330419A
Application number: GB9820706A
Authority: GB
Inventors: David Michael Mauri Guyoncourt
Original assignee: AEA Technology PLC
Current assignee: Ricardo AEA Ltd
Priority date: 1997-10-03
Filing date: 1998-09-23
Publication date: 1999-04-21
Anticipated expiration: 2018-09-23
Also published as: GB2330419B; GB9820706D0; GB9720910D0

Abstract

The behaviour of a powder during compression may be characterised by its yield surface. By subjecting a powder compact 16 to an axial load while enclosing the powder 16 in an elastomeric sleeve 14 close-fitting within a bore of a metal ring (12) or die, the powder compact 16 can be made to experience a range of different radial and axial pressures. The metal ring 12 may also be subjected to the axial load, and be of a deformable metal. Alternatively the metal ring may be substantially rigid and act merely as a die. Such tests, which may use a range of deformable rings 12 or sleeves 14 differing in their dimensions, enable several points on the yield surface to be determined.

Description

Method and Apparatus for Characterising a Powder The present invention relates to a method and to an apparatus for characterising powders.

The compaction of powders in a die is a stage in the processing of many materials. It is important to minimise density variations within and between such compacts, particularly for high performance engineering ceramics. However, friction between the powder and the walls of the die may cause the pressure to vary within the compact depending on the shape and size of compact and on the friction. The behaviour of the powder in the die can be modelled by computer software, but such models require dependable, accurate data on the behaviour of the powder when compressed for example relating to friction characteristics, elastic moduli, and the yield surface.

The yield surface is particularly significant, but adequate measurements require the application of orthogonal stresses independently of each other.

According to the present invention there is provided a testing apparatus suitable for characterising powders, the apparatus comprising a metal ring defining a cylindrical bore, an elastomeric or plastic tubular sleeve close-fitting within the bore defining a cylindrical sample chamber, and means to apply axial loads at least on the chamber and the sleeve.

The tubular sleeve undergoes deformation comparatively readily, so that if the sample chamber is filled with a powder under test and the powder is subjected to an axial load, the radial pressure exerted on the powder by the ring via the sleeve is substantially uniform. The powder in the sample chamber may be loose or pre-compacted.

The invention also provides a method of characterising a powder, using such a ring and tubular sleeve, filling the sample chamber with the powder, and subjecting the powder to an axial load.

If the ring is also subjected to the axial load, the radial stress depends upon the yield strength of the metal ring, the internal and external diameters of the ring, and on the friction between the end surfaces of the ring and the faces of the platens applying the load.

Pressures of several times the yield strength of the metal can be generated, and different radial pressures can be attained by using metal rings of different wall thicknesses, or of different materials. If the ring does not experience the axial load, but merely acts as a rigid die, then the radial pressure depends on the bulk modulus of the sleeve material and on its change of volume.

Different radial stresses can be obtained by using sleeves of different wall thickness.

Preferably, the sample chamber is provided with end plates of a low friction material such as polytetrafluoroethylene (PTFE). The radial pressure may be determined by calculation, or a pressure sensor may be provided within the tubular sleeve or in a recess in the inner wall of the ring. Pressure sensors may also be provided within the platens to measure the axial load, or within the end plates of the sample chamber.

Means such as ultrasonic transducers may also be provided to enable the density of the powder compact to be measured during the application of the axial load; the density may be calculated from measured values of the diameter of the compact during compression, and the diameter may be measured ultrasonically.

The invention will now be further and more particularly described with reference to, and as shown in, the accompanying drawings in which: Figure 1 shows as a graph of shear stress against hydrostatic pressure a typical yield surface of a compacted powder; Figure 2 shows a longitudinal sectional view of an apparatus for characterising a powder; and Figure 3 shows a longitudinal sectional view of an alternative apparatus for characterising a powder.

If a powder is subjected to an external force there is a characteristic resistance to relative movement between particles. The yielding of the powder depends on both the hydrostatic pressure and the shear stress (which may be in any direction). The yield surface, that is those values of pressure and shear stress at which particle movement occurs, is typically as shown graphically in Figure 1, to which reference is made, consisting of a tensile failure surface, T, at negative pressure, a shear failure surface, S, and a series of cap surfaces representing yielding at progressively higher density, p, moving to higher values of hydrostatic pressure.

Referring now to Figure 2 there is shown a sectional view of an apparatus 10 for characterising a powder, by enabling the positions of at least some points of the yield surface to be determined. The apparatus 10 comprises a deformable metal ring 12 defining an axial hole in which a close-fitting tubular sleeve 14 of elastomeric material locates. The bore of the sleeve 14 is filled with a powder 16 to be tested or with a partlycompacted powder sample. The ring 12, sleeve 14 and powder 16 stand on a rigid steel platen 18 and another rigid steel platen 20 rests on their top end. A hydraulic press (not shown) is used to push the platens 18 and 20 together.

During compression the volume of the elastomeric sleeve 14 is decreased, subjecting the powder 16 to a radial compressive stress. The radial stress depends on the yield strength of the metal of the ring 12, its internal and external diameters, the friction between the ring 12 and the platens 18 and 20, and on the bulk modulus of the elastomer of the sleeve 14. The deformation of the ring 12 is not uniform along its length, but the elastomeric sleeve 14 makes the radial stress more uniform along its length. The magnitude of the radial stress can be changed by using rings 12 of different materials, wall thickness, or heights, or sleeves 14 of different elastomeric materials.

The apparatus 10 may include a pressure sensor (not shown) embedded in the sleeve 14, so the radial stress can be continuously monitored during the compression.

The apparatus 10 may also include means to measure the average density of the powder compact 16 - for example by measuring the diameter of the powder compact 16 ultrasonically - so that in the course of compression a series of cap surface points can be determined at different densities. The apparatus may also include a plastic or elastomeric disc (not shown) to fit onto the top of the powder 16 and incorporating a pressure sensor to enable the axial force on the powder 16 to be measured.

Tests have been carried out with the apparatus 10, though without the sensors discussed above. After each compression the dimensions of the powder compact 16 and of the ring 12 were measured, and the radial pressure calculated from the inferred volume change of the sleeve 14 and the bulk modulus of the elastomer. The axial pressure was estimated from the density of the powder compact by a comparison with the results of die compaction tests. The following Table 1 shows the metal of which each ring 12 was made, the initial values of the height H, and the inner and outer diameters D1 and D2 of the rings 12, and the initial diameter d of the powder compact 16.

Table 1

Ring Ring r DimensionsXmm Number Material H D1 D2 d 1 Cu-5%Sn 12 16 32 10 2 Cu-5%Sn 12 16 32 12 3 Al-4tMg 16 16 29 10 4 Al-4%Mg 12 16 29 10 5 Cu-5%Sn 16 16 32 10 6 Cu-5%Sn 12 16 32 10 7 Cu-5%Sn 12 16 32 12 8 Al-4%Mq 12 16 29 10 9 99.8%Al 12 16 24 12 The following Table 2 shows the results of tests carried out on an iron alloy powder using the rings 12 described in Table 1, including the calculated values of the radial and axial stresses Pr and Pz.

Table2

Ring Relative Density of Diameter Elastomer Pr/MPa Pz/MPa Number Powder Compact Increase Volume of Change Compact before I after % % % % 1 81.1 90.7 ~ 4.9 6.7 91 541 2 80.0 91.6 6.0 17.8 336 580 3 80.6 84.0 8.2 16.2 165 363 4 81.2 89.7 5.1 9.1 - 122 500 5 71.9 83.0 1.9 7.0 71 343 6 71.4 88.8 0.5 14.2 194 474 7 72.0 88.2 0.8 10.2 192 456 8 71.9 85.4 2.6 7.3 98 389 9 71.8 75.1 10.5 0.9 17 244 t will be appreciated that these experimental measurements enable nine points to be plotted on the graph of the yield surface for this particular powder.

Referring now to Figure 3 there is shown an alternative apparatus 30 for characterising a powder.

The apparatus 30 comprises a thick walled steel die 32 which is cylindrical and defines a cylindrical bore in which are two close-fitting steel punches 34 and 35.

Between the punches 34 and 35 is an elastomeric sleeve 36 defining a cylindrical bore which is filled with a powder 38 under test (or which encloses a partly-compacted powder sample). A hydraulic press (not shown) is used to push the punches 34 and 35 together.

During compression the die 32 opposes radial expansion of the powder compact 38, and the elastomeric sleeve 36 ensures the radial pressure on the compact 38 is substantially uniform. The press incorporates means to monitor the force on the punches 34 and 35 and their displacement. After compression has been performed the dimensions of the powder compact 38 are measured. Hence the volume change of the elastomeric sleeve 36 can be calculated, and hence the radial stress on the compact 38 determined; the axial stress on the compact 38 can be determined from the stress in the sleeve 36 and the force on the punches 34 and 35.

Pressure sensors (not shown) may be embedded in the steel die 32 in recesses communicating with the bore so the pressure in the sleeve 36 can be continuously monitored. In this case the values of density, radial stress and axial stress can be determined throughout the compression process, enabling many more points on the yield surface to be determined. By using elastomeric sleeves 36 of different wall thicknesses, the relationship between the axial and radial stresses can be altered. Furthermore the steel die 32 may also be provided with a tungsten carbide liner (not shown) for the bore, fitting tightly into the bore, and with the elastomeric sleeve 36 locating within the liner. If pressure sensors are provided in the steel die 32, they may be calibrated by applying hydrostatic pressures to the bore of the liner.

Claims

Claims 1. A testing apparatus suitable for characterising powders, the apparatus comprising a metal ring defining a cylindrical bore, an elastomeric or plastic tubular sleeve close-fitting within the bore defining a cylindrical sample chamber, and means to apply axial loads at least on the chamber and the sleeve.
2. A testing apparatus as claimed in claim 1 wherein the sample chamber is also provided with end plates of a low friction material.
3. A testing apparatus as claimed in claim 2 wherein a pressure sensor is provided within an end plate of the sample chamber.
4. A testing apparatus as claimed in any one of the preceding claims wherein a pressure sensor is provided within the tubular sleeve.
5. A testing apparatus as claimed in any one of the preceding claims wherein a pressure sensor is provided within the metal ring, adjacent to the bore of the ring.
6. A testing apparatus as claimed in any one of the preceding claims also including means to enable the density of the powder compact within the sample chamber to be measured during application of an axial load.
7. A testing apparatus suitable for characterising powders, substantially as hereinbefore described with reference to, and as shown in, Figure 1 and Figure 2 or Figure 3 of the accompanying drawings.
8. A method of characterising a powder, wherein the powder is used to fill a sample chamber of a testing apparatus as claimed in any one of the preceding claims, and the powder is then subjected to an axial load.
9. A method as claimed in claim 8, wherein the powder is pre-compacted before being placed in the sample chamber.