Process Description

All electrical field-activated techniques use pulsed electrical discharge combined with rapid heating and pressure application to achieve fast powder sintering. A schematic of the FAST process is shown in Fig. 1. The equipment consists of a mechanical device capable of uniaxial pressure application and the electrical components to apply the pulsed and steady current. The loose powders are directly loaded into a punch-and-die unit. Graphite die and graphite punches are commonly used. This limits the pressure levels to low values, generally <100 MPa. The graphite confinement provides a reducing component to the sintering environment. Spark discharges may have the ability to clean the powder particle surfaces. For sintering more sensitive powders, the machines are equipped with chambers for vacuum or controlled environment.

Fig. 1 Fast process

The consolidation process consists of two stages: (1) an initial activation through the application of a pulsed voltage and (2) the subsequent heating and densification using direct current (dc). These stages may be applied sequentially or simultaneously. Throughout the process, a uniaxial pressure is applied. The pressure may be constant or changed in the second stage. Sometimes, the initial pressure level is light ("-10 to 15 MPa) and then increased during the heating stage.

The FAST processing parameters are shown in Table 1 and Fig. 2. A typical pulse discharge is achieved by the application of a low voltage ("-30 V) and a 600 to 1000 A current. In PAS machines, an alternating current (ac) rectifier is used to achieve a controlled dc pulsing. The result is a square-wave current with adjustable on and off pulses. The duration of each pulse may be varied between 1 and 300 ms. The total pulsing time is typically 30 s, but may go up to 300 s. In some FAST methods, an ac current is used for powder particle activation stage (Ref 29). In the second stage, when regular sintering takes place, the current is essentially dc at a level that depends on the powder type. The conductive powders are mainly heated due to the Joule effect. For nonconductive powders, heating occurs through heat transfer from the die and plungers. In this case, the die and punches are heated through their own resistance.

Table 1 Typical FAST processing parameters

Parameter

Value

Applied pressure, MPa

10-65

Pulse stage:

Voltage, V

30

Current, A

750

Pulse on-stage duration, ms

30-80

Pulse off-stage duration, ms

30-80

Total pulsing time, s

30-60

Resistance heating:

Current, A

1000-4000

Duration (hold time), min

3-5

Atmosphere

Air or controlled

Total time, min

<10

Time

Time

Fig. 2 Processing parameters in FAST sintering

The sintering time is dependent on the type of powder. To determine sintering time, the PAS machines are equipped with a linear gage that monitors the powder shrinkage and shrinkage rate during densification. The shrinkage decreases continuously while the shrinkage rate is at a maximum and then decreases to essentially zero. The densification is considered complete when the shrinkage rate reaches the zero value. When sintering is completed, the temperature is allowed to ramp down and the compact is removed. Stripping the compact out of the punch-and-die unit is easier for a simple configuration such as a circular or square disk. Generally, the part is cooled in the die because ejection of hot compact is difficult. Cooling rates are usually not controlled, although fans can be used or hot specimens can be ejected from the die and quenched afterwards. The entire operation, from the loading of the die to the ejection of the finished part, may be accomplished in less than 10 min.

Temperature measurements are difficult in all field-assisted sintering techniques. The process is fast, and the steady-state conditions are extremely short. These conditions make the use of conventional pyrometers and thermocouples troublesome. Insertion of a thermocouple in the compact is mechanically difficult. A pyrometer sighting of the compact is possible only for transparent dies. Therefore, die rather than part temperature is generally measured using a pyrometer or a thermocouple. The latter is inserted in the die cavity as close to the heating specimen as possible. Large temperature differences between die and compact have been noticed, particularly in the heating stage. For the sintering step, that is, steady heating, these differences have been measured to be 70 to 100 °C for metal powders (Ref 34). Because current passes through the powders, they reach a higher temperature than the external graphite die. For nonconductive powders that are heated from the dies, powders are at a lower temperature than the die.

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