Compaction to Higher Density

Many methods have been investigated and developed for pressing metal powders to higher densities. Most of the commercial methods involve high-temperature compaction (such as hot isostatic pressing and powder forging, as described in detail in other articles in this Section). In general, these methods are used to develop fully dense or nearly fully dense P/M products. For example, conditions for full-density iron and steel by HIP are summarized in Table 2.

Table 2 Hot isostatic pressing conditions for full-density iron and steel

Material

Pressure, MPa

Temperature, °C

Time, h

4 r m carbonyl iron

200

805

1

75 .'' ■' m sponge iron

98

1000

1

70 .'' ■' m low-alloy steel

150

800

1

190 .'-'m maraging steel

210

1200

3

100 !:m austenitic stainless steel

160

1150

3

120 !:m martensitic stainless steel

150

1150

3

65 ' m Fe-10Al-5Si

200

1000

1

80 "m tool steel

100

1100

1

165 .'■'m iron superalloy

69

1200

Source: Ref 1

However, other methods besides high-temperature compaction are used to achieve higher densities in green or consolidated form. Some of these miscellaneous methods are described here. These methods are not necessarily full-density methods, and none have reached any significant commercial significance. They are described for general reference. Many experiments also have been reported that attempt to press compacts to higher densities or to produce a more uniform density and stress distribution in pressed compacts. Although none of the techniques developed has led to large-scale industrial use, they are discussed here.

Die Barrel Rotation. Rotating the die barrel while the powder is being pressed using a fine atomized aluminum powder was reported by Hammond and Schwartz (Ref 3). They lubricated the die barrel with a suspension of lithium stearate in didecyl alcohol. Annular compacts, 12.7 mm (2 in.) high, were pressed with outer diameters of 38 mm (1 2 in.) and inner diameters of 25 mm (1 in.). The core rod was stationary, but the die barrel could be rotated during compression.

Compaction with a stationary and a rotating die barrel was compared. While in static compression, 20% of the applied stress was consumed in die wall friction, while only 2% of the stress was consumed when the die barrel was-rotated. In addition, the pressure necessary to eject the compact from the die was reduced to approximately one half of the pressure for compacts pressed with a stationary die barrel. Similar experiments on the effect of die barrel rotation on the density of iron powder compacts were reported by Rutkowski et al. (Ref 4).

Triaxial compression by simultaneous isostatic and uniaxial compression is obtained by applying pressure to the circumference of a cylindrical specimen confined in a flexible envelope while an axial load is superimposed by a vertical piston. With this method, the level of pressure necessary to obtain a given density is less than with isostatic or uniaxial compression alone. For example, to compact an atomized iron powder (Ancor 1000) to a relative density of 85%, uniaxial compression at 540 MPa (78 ksi) or isostatic compression at 415 MPa (60 ksi) is necessary. The same density can be obtained by combining a confining (isostatic) pressure of 83 MPa (12 ksi) with a uniaxial pressure of 470 MPa (68 ksi). The principles involved in this method of compaction have been reviewed by Broese van Groenou (Ref 5).

High-Energy-Rate Compacting. The rate at which pressure is applied in compacting in a hydraulic press is slow. Compacting in certain mechanical presses is somewhat faster. The effects of the rate of pressure application in compacting were studied by Davies and Elwakil (Ref 6). They found that somewhat higher densities can be obtained for a given pressure and for a given energy input when compacts from iron powder are pressed in high-speed presses (petro-forge-presses). They also determined the effects of multiple blows during pressing.

In the fabrication of sheet metal products, techniques were developed that formed sheet metal at rates considerably higher than those obtained in fast-acting mechanical presses. This is the high-energy-rate forming technique, which generally uses explosives (see also Ref 1).

The success of high-energy-rate forming in fabricating sheet metal led to extensive experimental work on high-energy compacting of metal-powders. The most common means to achieve high velocity is by the use of explosives. In one experiment, for example, compaction was done in a rigid die with pressure applied by a projectile propelled by an explosive charge that moves through a barrel (Ref 7). In a similar experiment compressed gas actuated the projectile (Ref 8). This experiment showed that the density of copper powder is not so much a function of projectile velocity, but depends primarily on the kinetic energy of the projectile, which can be varied by proper selection of gas pressure and projectile mass (Ref 8). For copper compacts with a volume of 0.86 cm3 (0.052 in.3), relative densities of >95% were obtained with energies of 150 J (110 ft • lb).

The most widely used method of explosive compacting is shown schematically in Fig. 7. The powder to be compacted is placed in a steel tube, which is closed at each end by steel plugs. The steel tube is surrounded by an explosive that is set off by a detonator located so that on detonation the tube collapses uniformly inward. Experiments by Lennon et al. (Ref 9) showed that density is a function of energy. They developed the equation:

where Dc is the compacted density, Z)T is the full density of the material, AZ) is the difference between full and compacted density, 1''and 7 are constants, and E is the net energy absorbed by the powder. The highest relative densities obtained for iron, nickel, copper, and aluminum powders and the corresponding net energies absorbed per unit volume of compact in their experiments were:

Powder

Density(a), %

Net energy, J/cm3

Iron

98.1

261

Nickel

98.1

556

Copper

98.5

285

Aluminum

99.0

182

Percent of theoretical density

Percent of theoretical density

This method of explosive compacting is not necessarily confined to cylindrical compacts. Cones and hollow cylinders have also been explosively compacted by this technique (Ref 10).

Fig. 7 Explosive compacting with powder contained in a steel tube

Vibratory Compacting of Powders. Vibration can be very effective in obtaining higher packed densities in powders. The relative densities of powders vibrated under carefully controlled conditions are much higher than those obtained by simply pouring the powder into the container. Therefore, much lower compaction pressures are required to reach a given density for a vibrated powder than for a poured powder. This is illustrated in Fig. 8 for a carbonyl iron powder. The density of 5.53 g/cm3 (71% relative density), reached by compacting under a pressure of 245 MPa (35 ksi), is due to the plastic deformation of the iron powder particles, while the 5.37 g/cm3 (69% relative density) obtained by vibrating at 167 oscillations per second is due mainly to vibratory packing. Plastic deformation during the simultaneous compacting at 2.4 MPa (0.36 ksi) is minimal. The method of consolidating powders by vibrating and simultaneous compacting is, therefore, primarily applicable to hard powders, such as refractory metal and cemented carbide powders, which can be densified relatively little by pressure application alone.

Fig. 8 Effect of powder vibration on densities of carbonyl iron compacts. Oobtained in static pressing. • Vibratory compacting at a frequency of 233 oscillations per second. Ovibratory compacting at a frequency of 167 oscillations per second

Melt-spray deposition of powders encompasses a wide variety of materials and product forms. It can be used to produce monolithic shapes by build up of the spray deposition, or it can be used to form coatings by deposition of a thin layer. One of the largest applications for melt-spray deposition is welding, including hardfacing and plasma spraying techniques for coatings.

Spray forming is also a method for producing preforms by a buildup of sprayed metal powder. These preforms and billets subsequently can be consolidated into various mill shapes. Of these processes, the Osprey process, developed in Wales by Osprey Metals Ltd., and the controlled spray deposition process are in commercial use. Several other methods are being developed; plasma spray buildup has high commercial potential. Laser techniques, such as laser glazing, also have commercial potential, especially when combined with rapid solidification technologies.

Osprey Process. Facilities for production of preforms made by the Osprey process consist of induction melting equipment and a preform production unit. In the Osprey process, an alloy is melted and subjected to gas atomization under inert conditions (usually nitrogen or argon is used). The atomized droplets are collected in a mold or group of molds, in which final solidification occurs. Molds are normally copper cooled by water. High-temperature ceramics offer other material options for molds. During solidification, welding of particles causes buildup of alloy in the mold. A preform having a density >96%, and normally >99%, of theoretical is generated by this buildup of alloy. The preform then can be consolidated to full density and formed into a mill or near-net shape part.

Alloys that have been processed by the Osprey process include stainless steels, high-speed steels, and nickel-base superalloys, although many materials appear to be compatible with the process. Alloy development has centered on high-alloy ferrous metals, Stellite alloys, superalloys, and composite materials. Because an inert atmosphere is maintained during spraying, oxygen levels similar to conventional ingot metallurgy products are attained, typically 20 to 40 ppm for superalloys. The high preform density ensures that no interconnected porosity is present in the preform, preventing internal oxidation during transfer of the material to subsequent consolidation and forming operations.

The Osprey process is used to produce a wide variety of preform shapes and sizes. Typical preform shapes are tubes, rings, cylinders, disks, or simple billets. Size is dictated by economics, with the melt facility, atomizer, and inert chamber sized for a particular product line. The largest preform size produced weighed 540 kg (1200 lb). Typical deposition rates range from 10 to 90 kg/min (20 to 200 lb/min). More information on the process is described in the article "Spray Forming" in this Volume.

The controlled spray deposition process is similar to the Osprey process in principle, but uses different machinery. Controlled spray deposition uses centrifugal atomization, while the Osprey process involves gas atomization. This process is used for production of mill shapes from high-alloy steel, which utilizes the enhanced workability of P/M workpieces. Highly alloyed metals may suffer from macrosegregation, which reduces the material workability. Elimination of segregation on a macroscale, coupled with a uniform distribution of fine carbides (2 to 3 /'m range for M-2 high-speed steel), results in improved workability for P/M workpieces produced by spray deposition. These billets are processed subsequently into mill shapes and sheet products.

By atomizing liquid metal into droplets 0.5 to 1.5 mm (0.02 to 0.06 in.) in diameter, solidification rates three or more orders of magnitude higher than those of conventional ingot solidification are achieved. Impacting liquid droplets of metal onto a cooled substrate increases the solidification rate, resulting in solidification rates of 10,000 to 1,000,000 °C/s (18,000 to 1,800,000 °F/s). Controlled spray deposition relies on this type of splat solidification to build up a solidified deposit that becomes a workpiece for subsequent deformation processing. As the thickness and temperature of the built-up material increases, the solidification rate decreases, but it remains much higher than that of conventional ingot solidification. Heating prior to hot working can remove any microstructural variations that may exist throughout the thickness of the built-up deposit.

Along with the metallurgical benefits of controlled spray deposition, economic advantages of direct spraying of powder into preform shapes are attained by eliminating sieving, blending, and other powder preparation steps. Also, primary compaction of powder into a green shape is eliminated. Controlled spray deposition proponents claim that these reductions in equipment needs and processing steps allow more efficient utilization of energy, compared to conventional pressing and sintering P/M technology.

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