Fig 5 Density distribution in compacts

Even compacts with only one level in the direction of pressing show variation in density in the pressing direction due to the friction between the die wall and the powder. This friction is reduced as much as possible by lubricants. Usually, lubricants are added to the metal powder in powder form. Lubricants reduce friction between the powder and the side wall, as well as between individual powder particles. They also may be applied as a thin coating to the walls of rigid dies. In large-scale production, this die wall lubrication is not commonly used.

Mathematical relationships and modeling of pressure-density relationships in powder compaction have important practical value, especially with greater emphasis on concurrent engineering. Modeling of compaction is discussed in the article "Mechanical Behavior of Metal Powders and Powder Compaction Modeling" in this Volume.

Cold pressing in rigid dies is the most commonly used compaction process, and the concept of die pressing is straightforward. Powder is poured into a die cavity, a movable punch seals the die cavity, and a load is then applied via the advancing punch. In the most simple case there is only one moving punch, and the die is stationary. However, a density gradient in the compact occurs as a consequence of die wall friction, with the highest density being next to the punch face. A floating die table reduces the density gradient by moving the die to offset the friction effect. The powder is densified from both top and bottom planes, and the middle plane has the lowest density. As more features are added to the compact, additional punches are required to produce an acceptable green compact.

Typical green densities for die compacted parts are 75 to 85% of full density. Green density is related almost exponentially to the applied load. At low densities, a small increase in load causes a major increase in density, while at high density levels, a large increase in applied load is required to get a small increase in density. The required compaction pressure to achieve a desired level of density is a function of the following:

• The powder shape (i.e., sponge, flake, water atomized, reduced)

• Particle size and size distribution

• Powder chemistry (i.e., prealloyed, blended master alloy)

• Lubrication practice

For steel powders, commonly used pressures are in the 400 MPa (30 tsi) range to achieve green densities from 80 to 85% of full density. The use of "low compressibility" powders, which truly do require less pressure to achieve the same density as prealloyed powders, allows either larger parts to be made on the same pressing equipment, or parts with higher green density to be made at the same compaction pressure. More information on compressibility is provided in the article "Compressibility and Compactibility" in this Volume.

Cold pressing in rigid dies has advantages of dimensional control due to the well defined cavity, high compaction pressures due to mechanical or hydraulic pressing equipment, process repeatability due to mechanization and improved powder consistency on a batch-to-batch basis, and high rates of production. Limitations of the process include size restrictions due to press capacity, height-to-diameter limitations due to die wall friction, ejection cracking problems with compacts pressed from powders with poor green strength, and the natural limitations of re-entrant angles and undercuts. Creative toolmakers circumvent or overcome these limitations daily.

Warm compaction is a rather recent development in production die pressing. In warm compaction, a plasticizer is added to the powder whereby the application of moderate temperatures causes the plasticizer to soften or melt and friction is substantially reduced. Friction here refers to friction between the powder and the rigid die wall and to friction between powder particles. With this process, significantly higher green densities can be achieved, even >92% of full density. This technology opens new applications to pressed and sintered parts because of the achievable density and the improved mechanical and physical properties. In addition, higher green strengths allow machining of green compacts.

Usually warm compaction involves the use of a polymer addition that helps bond particles together. The polymer-coated powder is more costly than typical die-compaction grades, unless a simple lubricant is admixed with the powder. Various common stearates or other lubricants work, including, Teflon (E.I., duPont de Nemours & Co., Inc., Wilmington, DE). Depending on which polymer is selected for coating the powder, ejection forces can be highly variable. Close temperature control is necessary, since product uniformity suffers if the polymer is too hot. In tests with various powders, the green density usually increases by 0.15 g/cm3 over room-temperature compaction. After cooling to room temperature, the warm-compacted powder is stronger because chilling the polymer adds strength to the compact. However, there is no evidence of greater strength during ejection, which means that green cracks from ejection stresses are not reduced by warm compaction. Consequently, a hold-down pressure is required during ejection to avoid cracking.

Heating of the die and punches requires modifications to the compaction press, and a heater is required in the powder feed mechanism. Both microwave and hot-oil heaters are available for heating the powder. A typical temperature for the powder and tooling is ^-150 °C (300 °F), and compaction pressures are usually in the range of 700 MPa (50 tsi) for steels. The major role of warm compaction is to lower the pressure required for attaining densities of >7.0 g/cm3 of ferrous P/M compacts.

Hot Pressing. The production of large billets can be accomplished by compacting powder in heated dies. The use of elevated temperatures and long dwell times allows densities of >95% of full density to be achieved at compaction pressures that are one third to one half those needed for cold pressing to lower density levels. Full density is usually not achieved, and 3 to 5% porosity remains in the billet. This porosity somewhat limits the use of hot pressed billets because the properties are reduced from those of fully dense material. For this reason hot pressed billets are often used as stock for upset forging, closed die forging, and other deformation processes that can eliminate this residual porosity. Some metals, such as beryllium, are routinely processed by hot pressing, and acceptable performance levels for many applications are achieved.

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