Innovations in Powder Metallurgy at the PM2010 World Congress: Powder Compaction

Italy
October 26, 2010

Recent advances in research on the powder compaction process were presented during the PM2010 World Congress held in Florence, Italy, 10–14 October 2010. The focus was often on improving the homogeneity of the density and reducing the risk of cracks in green powder metallurgy compacts.

Dr Georg Schlieper reviews a selection of papers for ipmd.net that highlight developments in this area.

 

The friction free ejection of powder metallurgy parts

The first, and in the author’s opinion one of the greatest highlights of the PM2010 World Congress, was actually presented before the Opening Plenary Session. The Sunday afternoon before the official opening of the congress is traditionally devoted to open meetings of the European Powder Metallurgy Association (EPMA) working groups, and the European Structural Parts Group (ESPG) open meeting saw what could be the most important innovation in powder compaction technology in many years.

Fig_1_StreconDr Jens Grønbæk of Strecon a/s, Sonderborg, Denmark, presented a novel die design that can release compacts inside the die so that they are set free without any frictional forces.

The advantage is evident for anybody who has ever studied crack formation in powder compaction. Most cracks in green compacts are formed in the ejection phase of the compaction cycle due to friction with the die walls, and the new design can help to avoid many of these cracks.

The principle of the dynamic die design developed by Strecon is to exert a radial pressure on the core of the die before filling with powder. In the experimental set-up used in the preliminary study this was realised with a conical ring (Fig. 1).

The inner die diameter is reduced by the radial pressure by about 0.5 to 0.7%. The pressure is maintained while the powder is compacted and, after removing the upper punch, the die pressure is released and the compact can be removed friction-free.

Fig_2_StreconA preliminary study under the guidance of ESPG covered three simple geometries, a gerotor and two bushings with different wall thicknesses and heights. Samples have been compacted from various ferrous powder blends with varying lubricant content.

Compacting pressures up to 1500 MPa were applied and green densities up to 7.5 g/cm³ and excellent surface qualities were achieved (Figs. 2 and 3). The results were encouraging and tests will be continued.

The dynamic die concept seems to be best Fig_3_Streconsuited for parts exhibiting rotational symmetry such as bushings, valve guides, gears, etc, and perhaps it will always be restricted to relatively simple part geometries. Nevertheless, the idea to release the radial pressure on green compacts before ejecting them from the die is revolutionary. It will certainly find its niche among the various PM processes.

Further topics of the ESPG Open Meeting were the continuation of the EPMA’s questionnaire survey of the European PM industry, an update of recent developments in the REACH legislation, and a lecture from a lawyer about the legal aspects of business agreements between PM parts manufacturers and their customers.


Stress distribution on die cores

The stresses induced in shelf (“shouldered”) dies with a conical shrink fit was studied using FEM analysis by a group of researchers from the University of Naples and the University of Salerno, Italy, with the support of consultant Dr Gian Filippo Bocchini.

The effect of various conical angles on the stress distribution was analysed. The target of this work was to avoid excessive tensile stresses on the die core, as this might lead to catastrophic tool failure.


Simulating the formation of cracks in green compacts

The behaviour of metal powders during die compaction is more and more studied by computer simulation. Several authors presented their approaches to this topic.

J. Hernandez of UPC Barcelona, Spain, predicted the formation of cracks in green compacts during ejection from the die. The ejection of a multilevel component under uncontrolled deflection of the tooling was simulated.

The author claims that his finite element model is able to detect, with acceptable engineering accuracy, sources of possible macroscopic cracks formed during the ejection phase.


Understanding bulk behaviour of powder during compaction

B. Harthong of the University of Grenoble, France, calculated the bulk behaviour of a powder during compaction based on numerical simulations of random packings of discrete particles. He stated that his computer model may give access to powder loading conditions which are impossible to study experimentally.


Improving density homogeneity through compaction optimisation

Table_1Where a homogeneous compact density is achieved in powder compaction, the dimensional accuracy of the finished parts is also improved.

Dr Ilaria Cristofolini of the University of Trento, Italy, presented her work on optimising the compaction of a pulley gear.

Dr Cristofolini investigated the complex interaction between compacting pressure, wall thickness, powder transfer, spring-back, and dimensional change during sintering on the dimensional accuracy of a representative component (Table 1 and Fig. 4).

Fig_4The green compact, which is pressed in a cylindrical die, comes out slightly conical due to the spring-back depending on the part’s wall thickness.

After sintering the conicity is inverted due to the dimensional change of the sintered steel. The lower part of the component grows more than the top.

 

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

Italy
October 26, 2010

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