Introduction

IN POWDER CONSOLIDATION, the imposition of an electric current is known to enhance sintering kinetics and therefore high densities are achieved at lower temperatures or in shorter times as compared to conventional sintering methods. This way, full or near-full densification may be realized with minimal undesirable microstructural changes. A shorter processing time usually results in productivity gains. In addition, there is less sensitivity to initial powder preparation, such as no need for sintering aids and subsequent debinding steps. Most of the time, air sintering is appropriate; that is the need for controlled environment is eliminated. Finally, interparticle bonding is improved. As a consequence, sintered parts of higher quality may be expected.

Numerous processes have been developed that apply an external current to assist powder consolidation. The initial idea to use resistance sintering of metal powders started with Taylor in 1933 in hot pressing of cemented carbides (Ref 1). Cremer patented a field-assisted sintering method in 1944 (Ref 2). He used a 60 Hz current for 1 to 2 cycles at a current density of 62 kA/cm2 under 70 to 140 MPa pressure to sinter copper, brass, bronze, and aluminum. In late 1950s, resistance sintering under pressure was applied to metal powders by Lenel using equipment similar to spot welding machines (Ref 3). Presently, resistive hot pressing is commonly used and consists of a low-voltage (5 to 40 V), high-amperage (up to 25 kA) current passing through the powders with a simultaneous pressure application. The electrically conductive powders are contained in an internally insulated die. The passage of the electric current provides the resistive heating of the powders by Joule effect; that is, the electric power is used as the heating source for the consolidating compact. Therefore, the processing times are extremely short, on the order of seconds. Pressure levels are low to moderate (15 to 340 MPa).

In another case of field-assisted sintering, large electric currents induce magnetic forces that contribute to powder compaction due to a pinch effect. By concentrating the pulse in the compaction zone, the magnetic field forces may reach high values, up to an equivalent to 5 GPa pressure (Ref 4). Examples of such consolidation methods are dynamic magnetic compaction (DMC) (Ref 5) and indirect high-energy, high-rate processing (HEHR) (Ref 6, 7).

In field-activated sintering, an initial activation of powder particles is achieved by the application of electrical discharges on either conductive or nonconductive materials. (It is also sometimes called electroconductive sintering for conductive materials.) These discharges are produced by pulsed electric power. The electrical discharge per se does not density powders and, therefore, additional energy is required to increase the final density. This extra energy may be mechanical, as an applied pressure, and/or thermal energy. The electrical discharge only activates the sintering mechanism (i.e., neck formation) for densification by these other means.

In one of these methods, known as electrical discharge compaction (EDC), the electrical energy is suddenly released by discharging a capacitor bank through the powder preform contained in an insulating container (Ref 8, 9, 10, 11, 12, and 13). Usually, there is only one discharge. Repeating up to three discharges increases activation and thus contributes to final density. No further density improvement is observed beyond 4 to 5 discharges. The discharge period is on the order of hundreds of /'s at a high voltage (up to 30 kV) and current density (—10 kA/cm2). After discharge, a high amperage current is passed through the metal powder column for Joule heating. In this stage, an external pressure may also be applied.

Neck formation between powder particles takes place in the discharge stage. The experimental measurement of the powder column electrical resistance has shown an initial rapid drop in the discharge step. The reduction of initial high resistivity of powder column was first attributed to the neck formation between metal powder particles (Ref 8, 11). Later, Okazaki and coworkers demonstrated that the resistance drop is caused by oxide removal rather than only neck buildup. To verify this, they documented oxide film disruption on nickel powders by electron microscopy studies (Ref 8, 10). Some quantitative oxygen measurements in initially heavily oxidized powder and final EDC consolidated part indicated that most of the nickel oxide was dissociated by the sudden heat generated by discharge. This explosive disruption of surface oxides on metal powders due to electric discharge contributes to neck formation. The actual pore elimination and powder consolidation to high densities is achieved by Joule heating and pressure application in the second stage of the process. The method has been applied to compact a variety of metal powders in England (Ref 11, 12, 13), Japan (Ref 14), and the United States (Ref 8, 9, 10). In many cases, electrical discharge compaction was not the final fabrication step. It was aimed to produce at a high-density compact capable of withstanding subsequent processing such as forging or rotary swaging (Ref 8, 11, 12). The initial EDC compacts may be obtained as bars with the same or stepped various cross sections, L-shaped, or hexagonal (Ref 12). More details about EDC may be found in Ref 8.

A sustained activation of the powder particle surface may be achieved by the application of multiple power pulses that produce repeated electrical discharges. This activated consolidation has been pioneered in the spark-sintering technique, which was developed in the 1960s (Ref 15, 16, 17). This technique comprises two steps. The first step consists of the application of an alternating current of 500 to 1000 Hz frequency at a low pressure for 15 to 30 s. The spark discharges generated in this step activate powder particle surfaces. The next stage proceeds with resistance heating of the powders under the same or increased pressure. An increase of pressure in the second step provides the best results. All tools--die and plungers--are made of high-density graphite. Extensive experimental spark-sintering work was performed on various materials such as aluminum, beryllium, copper, nickel, iron, titanium, and their alloys; superalloys; refractory metals; carbides; borides; oxides; and beryllides in the late 1960s and early 1970s (Ref 18, 19, 20, and 21). Bars up to 250 mm (— 10 in.) in diameter and complex near-net-shape parts have been processed. Multilevel components that involve sinter bonding of various parts were also fabricated by spark sintering. In the latter case, the main attraction was the solid metallurgical bonding achieved while minimizing side effects such as grain coarsening or other microstructural alterations involved in joining materials. The process was advanced to an industrial scale using a semiautomatic system in the commercial production of beryllium and titanium parts (Ref 18, 19). Mechanical properties were comparable to those of wrought metals, particularly when postconsolidation forging was applied. The main difficulty reported was the temperature measurement and control. Other problems were related to the mechanical strength of the graphite die, powder reaction with graphite, tooling cost for complex parts, and in some cases, low productivity. Frustration due to the lack of understanding of the physics of electrical activation often surfaced.

A similar method termed impulse resistance sintering was used in the mid 1970s (Ref 22). The main efforts were directed toward achieving balanced heating and densification by the control of heating rate and distribution and of heat losses. A large difference in temperature between the specimen center and the die wall due to the cooling of the edges by the die was documented. In some cases (e.g., for conductive ceramics such as TiB2), the die temperature was only 50 to 60% of that of the densifying specimen. A mushroom-shaped punch was used to minimize heat losses at the water-cooled contact with the copper electrodes (small cross section) and thus to provide the highest temperatures at the powder-punch interface (large cross section).

In the last decade, the benefits of a pulse discharge have been recognized more widely, and numerous field-activated sintering techniques have been developed. Such techniques are instrumented pulse electrodischarge consolidation, spark/plasma sintering, or plasma-activated sintering developed in Japan (Ref 23, 24, 25, 26, 27, and 28), resistance/spark sintering under pressure in Korea (Ref 29), pulsed electrical discharge with pressure application in Russia (Ref 30, 31),

HEHR processing in the United States (Ref 6, 7) and plasma sintering in Brazil (Ref 32). All these methods are similar concepts to pulse discharge and subsequent resistance sintering. The pulsed electrodischarge stage may be built in as an added option for powder activation in other nonfield sintering techniques such as the piston-cylinder high-pressure method (Ref 33). This article addresses only pulsed-electrical-discharge activated-pressure sintering that is generally called field-activated sintering technique (FAST). Because an in situ plasma generation may be possible among discharged particles, plasma-activated sintering (PAS) is another common name given to such field-activated densification. A distinction has to be made in the latter case to differentiate PAS from plasma sintering techniques where the specimen is "bathed" in an external plasma environment such as in microwave sintering.

0 0

Post a comment