Innovations in Powder Metallurgy at the PM2010 World Congress: Advances in Powder Production and Characterisation – Part 1

namely
November 3, 2010

Our review of the PM2010 World Congress in Florence continues with a report on topics related to powder production and characterisation.

In the first part of this review, Dr Georg Schlieper focuses on a selection of papers for ipmd.net that highlight developments in this area.

New water atomised iron powders with high green strength

Fig_1

The market for iron powders with low apparent density and high green strength has been dominated in the past by sponge iron powder from the Swedish powder manufacturer Höganäs AB. This dominance in the market is now being tackled from two sides, namely by Hoeganaes Corp., USA, and Rio Tinto Metal Powders, Canada.

The two competitors have independently developed water atomised iron powders with high green strength. This is achieved by creating particle shapes similar to those of sponge iron powder.

Without disclosing any details of the manufacturing routes, two separate papers by P. Sokolowski of Hoeganaes Corp. and François Chagnon of Rio Tinto Metal Powders presented the characteristics of the new iron powder grades and compared them to those of sponge iron powder. They had to admit that the green strength was still about 25% lower than that of sponge iron, but it is significantly higher than that of common water atomised iron powders.

The new powder grades are reported to exhibit fewer non-metallic inclusions than sponge iron. The sintered strength of PM alloys made from these powders is comparable, however the dimensional change is somewhat different from sponge iron powder.


Measuring the anisotropy of the green strength

Fig-2_1

Dr Alexander Tausend of the University of Aachen, Germany applied a new method to characterise the green strength of metal powders. Originally developed for characterising concrete, the so-called Brazilian Disc Test is capable of measuring anisotropies in the strength of brittle materials. 

Test specimens were simple cylindrical discs of roughly 30mm diameter and 6mm height. However, these discs were not pressed to this shape, but machined in the green state from larger cylindrical slugs. The axis of the test specimens was perpendicular to the axis of the slugs. Two samples were machined out of each slug Fig_3(Fig. 2). The direction of compaction of the original slug was marked on each test sample with a felt-tip pen.

The specimens were placed under radial pressure between two punches for testing as shown in Fig. 3. The load on the punches induces a compressive stress at the outer surface and a transverse tensile stress along the diameter with a maximum at the centre of the specimen.

When the pressure in the compressive test was applied not in the direction of compaction, but under an angle of between 0° and 90°, the load to fracture was different. This is an indication that the green strength depends on the direction of loading, i.e. it is anisotropic.

Fig_4

The tested specimens should look as shown in Fig. 4. The arrows indicate where the pressure had been applied in the test and the black line indicates the direction of compaction. This specific sample was tested at an angle of 22.5°.

Test results for different types of iron powder are displayed in Fig. 5. They include water atomised grades (ABC) and sponge iron powders (SC and MH). All samples had been compacted at a pressure of 600 MPa. In this graph, 0° indicates the direction of compaction and 90° is perpendicular to the direction of compaction. All powders exhibit a distinct anisotropy of the green strength. The strength is highest when the load is applied in the direction of compaction and lowest perpendicular to this direction. In all tests the green strength in the direction of compaction was two to six times higher than in the perpendicular direction.

Fig_5Dr Tausend proposed an explanation of these experimental results based on a model of the microscopic structure of green powder compacts. He presumed that the shape of powder particles, which initially are irregular, is deformed during compaction into a shape resembling a lentil. Then the length of a crack in the vertical direction, parallel to the direction of compaction, should be longer and would therefore consume more energy than a horizontal crack perpendicular to the direction of compaction. Diagonal cracks take an intermediate position (Fig. 6).

Fig_6

This model can at least qualitatively explain the anisotropy of the green strength, and it also makes clear in a very descriptive manner why horizontal cracks occur much more frequently in green compacts than diagonal or vertical cracks. It also explains that a high green strength in the direction of compaction is not the whole answer to crack problems. More important is the resistance against horizontal cracks, and Fig. 5 shows that in this respect sponge iron powders are not always better than water atomised powders.


The proceedings of the PM2010 World Congress are now available to purchase in printed format or on CD from the EPMA. www.epma.com

namely
November 3, 2010

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