Understanding powder behaviour during preparation, transfer and filling of dies is important in powder metallurgy as each step influences the quality of the final component. For high productivity and quality during manufacture, it is essential that process and powder characteristics are compatible.
In this investigation of the bulk, dynamic flow and shear properties of four different materials, we show that die filling effectiveness correlates well with many powder properties. Thus the availability of accurate information about the flow and bulk properties of powders allows reliable prediction of die filling efficiency, removing any need for the more usual ‘trial and error’ approach.
Components made from metal powders are manufactured in distinct stages:
- die filling - powder drops from a feed shoe into the die cavity
- powder transfer - powder is transferred within the die and through a series of tool motions to produce a compact approaching the final shape
- compaction - powder in the die is compressed to form a green body
- ejection - after compaction, powder is ejected from the die
- sintering - following compaction, the green body is sintered in a reducing atmosphere
- component is sized or machined in order to ensure the maintenance of dimensional tolerance
The compaction, ejection and sintering stages in particular have a significant impact on the properties of the finished components. A knowledge of the behaviour of powders in preparation, die filling and powder transfer is important, since their packing structure and density distribution may influence later stages, and affect the integrity of the final components.
Powder Flowability and Die Filling
Various experimental techniques have been used to measure powder flowability in relation to die filling. These include:
- Hausner ratio and Carr index (poured and tapped bulk density)
- Hall flowmeter and Flodex flowmeter (mass flow rate or time required to discharge through an orifice)
- Angle of repose; and
- shear cell, which measures the yield strength of a consolidated bulk solid.
While all may be useful in specific process environments, none predicts the behaviour of a powder during die filling.
Factors Influencing Powder Behaviour
Powder behaviour is complex. It is influenced by a combination of physical properties and the characteristics of the processing equipment, and powder flowability cannot be expressed adequately as a single value or index. Die filling is a dynamic process, so any powder characterisation methods used should closely reflect the real industrial situation.
Case Study - Die Filling Behaviour of Tungsten, Aluminium and Glass Beads
Here we examine the die filling behaviour (die filling ratio) of tungsten, aluminium, and two kinds of glass beads of different nominal size. Bulk, dynamic flow and shear properties were characterized using the FT4 Powder Rheometer.®
Particle Size and Morphology
Particle size distribution for each material was determined using a Mastersizer 2000 (Malvern Instruments, Malvern, UK). Particle morphology was characterised with a JEOL 6340F Scanning Electron Microscope (SEM). The results are shown in table I.
Table 1. General powder properties.
||GL Glass beads
||GS Glass beads
||Granules Aluminium Powder
Obtaining repeatable data requires powders to be in a homogeneous packing state. When a powder arrives for testing, it has a unique history that has been influenced by factors such as consolidation, segregation, aeration, caking, or vibration. The ‘conditioning’ needed to remove this history involves gentle displacement of the whole powder sample, loosening and slightly aerating it into a homogenised and reproducible state.
All samples for dynamic, bulk and shear testing were conditioned and ‘normalised’ before measurement, using the FT4 Powder Rheometer.
Please click here if you would like more information on the instrument used in this article or a quote
Measuring Powder Flowability
FT4 Powder Rheometer is a universal powder tester. Accessories such as blades, pistons and shear heads can be rotated and simultaneously moved axially into a powder sample whilst axial and rotational force are measured. A number of control modes are available on both axes, including velocity, force and torque. Standard dynamic tests, aeration testing and shear testing are automated with no operator involvement apart from sample preparation.
Dynamic testing was performed using a 48 mm diameter blade and a 160 ml powder sample contained in a 50 mm bore, borosilicate test vessel (figures 1a and 1b). An automated, 18 segment, 48 mm diameter rotational shear cell accessory (figure 1c) was used for all shear testing, using 85 ml sample.
Figure 1a. Downwards testing mode showing bulldozing action along the entire blade length.
Figure 1b. Upwards testing – shearing with minimal consolidation.
Figure 1c. Shear cell above sample vessel.
Die Filling Rig
A model die filling rig was designed to mimic commercial die filling processes (figure 2). It consists of a stationary die and a motorised unit which drives the shoe at a steady velocity from 50 to 300 mm/s. In this study, the feed shoe was cylindrical with a fixed volume of 160 ml and diameter of 50 mm. The cylinder die had a 10 ml volume and 25 mm diameter.
Figure 2. Schematic drawing for die filling process.
Experiments were carried out in air at shoe velocities of 50 mm/s to 250 mm/s. 160ml samples were conditioned on the FT4 Powder Rheometer and then carefully transferred to the rig for translation over the die. The mass transferred into the die was then measured to determine the filling ratio. This was repeated three times, each time using a newly conditioned sample of powder.
Results and Discussion
Table 2 summarises the most important material properties, which are further discussed in this section...
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This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.
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