Methods of field-assisted consolidation of powder materials are related to high-performance techniques of sintering, which are presently intensively developing. Their main principles consist in the joint action of a powerful electrical discharge and of mechanical pressure on the processed material. The main advantage of these methods is in the opportunity to closely control within wide ranges, the powder processing parameters like amplitude, duration, and form of the pulses of the electric current; the magnitude of these ranges is determined by the capacity of the experimental equipment. Another benefit is these methods’ capability to obtain ceramic and metallic powder specimens, such as freestanding components or powder coatings, of high quality at low prime cost. The additional undeniable advantage of these techniques is the short or ultra-short process duration resulting in the increased productivity and, frequently, in the dramatically improved structure of the manufactured components.
Methods of electro-consolidation can be sub-divided into two main groups: the ones using low- and moderate voltage current sources (voltages of the order of tens of volts and electric current of the order of thousands of amperes), the known techniques, such as spark-plasma sintering and resistance sintering belong to this group; and the ones using high-voltage power supplies (high voltage capacitor discharge through a powder sample, the amplitude of the voltage-tens of kilovolts, the amplitude of the current pulse - hundreds of thousands of amperes) - the related techniques include electric discharge sintering (or high voltage electric discharge consolidation - HVEDC). The distinctive features of HVEDC methods are: high rate of heating, low integral temperature of consolidation, ultra-short duration of the process of consolidation. Full advantages of the methods of HVEDC can be realized under optimal process parameters.
Optimal parameters of the high voltage electric discharge consolidation strongly depend on the initial properties of consolidated powders, in particular, on the conductivity of ceramic or metal powders and alloys under applied mechanical pressure. The conductivity of ceramic or metal powders is directly dependent on the size distribution of powder particles. Thereby, the analysis of the influence of the particle size and particle shape of a powder on the HVEDC process outcomes is of considerable importance.
The HVEDC apparatus consists basically of: a charging unit, a bank of capacitors and trigatron switch, an electrical discharge ignition system. The capacitor bank consists of thirty 200 μF capacitors that can store up to 6 kV. HVEDC uses the pulse current generated from the capacitor bank to quickly heat a powder column subjected to constant pressure during the process. In this process the powder is poured into an electrically non-conducting ceramic die. In technological unit the ceramic die is plugged at the bottom end and the top end with molybdenum electrode-punches and an external pressure of up to 400 MPa is applied to the powder by air-operated press. A high voltage capacitor bank is discharged through the powder column. The discharge current is measured by a toroidal Rogowsky coil bent around the powder column. HVEDC method uses the passage of the pulse electric current to provide the resistive heating of the powder by the Joule effect. Joule heating occurs at the inter-particle contact to instantaneously weld powder particles, resulting in densification. The achieved powder compact density as a result of HVEDC process depends on the applied external air-operated pressure, the magnitude and waveform of the pulse current that depends on RLC – parameters of the electrical discharge circuit.
The choice within the present study of zirconium alloy powders as of the investigated processed materials is related to the usefulness and variety of these materials systems’ applications. Zirconium alloys have been widely used in a variety of applications including neurosurgery and electronics due to their high corrosion-resistant properties. Zirconium weakly absorbs slow neutrons, which favors its application in the field of nuclear technology, especially in the construction of nuclear reactors.
In that study zirconium alloy powders (Zr, Zr+1% Nb) with particles of spherical and flake shapes and also, with particles of irregular shape, have been sintered by field-assisted consolidation process.
We have investigated the electric conductivity of the powder and it was found that the electric conductivity and shrinkage increase monotonically during 1–2 min after the application of pressure. We have applied pressure for 2 min before the measurements of conductivity. The density of the consolidated samples is increased to a maximum value depending on the applied pressure with an increase in the amplitude of the current pulse. Density measurements after the high voltage electric discharge consolidation process were performed using the Archimedes principle in distilled water.
With a further increase in the amplitude of the current pulse, density of the consolidated samples is significantly reduced. The surfaces and cross sections of the consolidated powder compacts were polished for microstructural observation by optical microscopy.
Earlier obtained research results indicate that the most important factors, which determine the success of the HVEDC process are the current density amplitude in the powder specimen and the applied pressure.
We examined the effect of the amplitude of the current pulse on the density of the consolidated samples of zirconium alloy powders (Zr, Zr+1% Nb). It should be noted that the different particle morphology required different pressure levels for obtaining the same levels of the densification. For different applied pressures the influence of the current pulse amplitude on the final density of the consolidated zirconium powder alloy samples has been investigated.