Electrolyte-plasma processing: how it works

Electrolyte-plasma processing scheme

By electrolytic-plasma processing, a workpiece is an anode, a cathode is an operating cell, and positive potential of a power supply is led to a workpiece.

Electrolytic-plasma mode

Depending on the voltage applied by passing of electrical current through the aqueous electrolyte solution, various modes of electrical processes are observed near the anode.

The first mode is usual electrolysis, by which metal ions' transfer takes place and, depending upon the electrolyte composition and electrode material, gas emission is observed. All this is described by classical electrochemistry.

With electrode voltage increasing to 60–70V, transient or switching mode is set. A steam-plasma cover is generated periodically around the active electrode (anode) with frequency of about 100 Hz, which leads to the current cutoff for 10-4s.

At a voltage of more than 200V around the anode a stable steam-plasma cover, characterized by small oscillation with U=const, is formed. In this voltage period ( 200-350V ) the process of electrolytic-plasma treatment takes place. Across all the treated surface pulsed electrical discharges occur. Joint exposure of a workpiece to chemically active medium and electrical discharges leads to simultaneous effect of polishing and product surface cleaning.


Plasma treatment (polishing) procedure for semiautomated line

Перед полированием изделие может быть подготовлено путём абразивной механической шлифовки (например, с помощью пескоструйной, барабанной или вибрационной установки). Это позволит сократить время электролитно-плазменного полирования при наличии заусенцев и других дефектов изделия.
Before polishing a product may be prepared by means of mechanical abrasive grinding (e.g., in sandblasting, round type or a vibratory installation). This will reduce the time for electrolytic-plasma polishing in case of burrs and other defects being in a product.

Plasma polishing includes 7 main stages:

  1. Preparing a workpiece for processing
  2. Manual loading on a hanger
  3. Automated operations:
    1. energizing the pulled hanger
    2. gradual immersion in an electrolyte
    3. holding for 2–5 minutes
    4. lifting the hanger with workpieces
    5. de-energizing
  4. Washing in warm water
  5. Warm air drying
  6. Manual unloading
  7. Control

A workpiece is installed on a special hanger with a reliable electrical contact provided. Operating voltage is then applied, and the part is slowly immersing into the preheated electrolyte.

By polishing process, the electrolyte temperature is maintained at 60–900C through cooled electrolyte being pumped from a preparatory bath in the working one. After being treated for 2-5 minutes, the product is lifted from the bath and the voltage is cut off. Then the washing of the product in warm water and warm air drying are carried out. The quality of the processed surface, the occurrence of burrs and sharp edges are checked for.



Performance specification

For stainless steels and copper alloys treatment 3–5% aqueous solutions of ammonium sulfate and ammonium chloride are used. When processing other metals and alloys, aqueous solutions with salts' concentration not higher than 10 % are used. The average duration of polishing is 2-5 minutes, and that of deburring is 5-20 seconds.

Time: to 5 min.

Current density: 0,20,6 A/cm2

Temperature: 6090 °C

Voltage: 200350 V

Output speed to 32 mcm/min.

Solution acidity: 48 pH

Salt content in an electrolyte: 0,510%

The roughness value attainable to Ra 0,01 micron

In the course of numerous projects' realization, polishing technology was optimized.The product's size and shape, the occurrence of holes and cavities, the product's arrangement on the hanger, the initial state of the surface, the material composition and that of electrolyte were taken into account. Such optimization allowed to obtain a high surface quality with minimal expenditure of energy.

In literature, to explain the effect of polishing electrophysical model was accepted, ie it is assumed that roughness smoothing takes place owing to the microdischarges applied to the surface protuberances. As for the electrochemical processes, they are considered to be less important. However, the practice of polishing various metals showed that the process is very dependent on the electrolyte composition. Moreover, to produce the polishing effect, for each metal type a certain (special) electrolyte is used. For polishing low-carbon and lean alloy steels, copper alloys, brass, chrome and other metals and alloys special solutions' compositions were worked out.

Thus, it was found that polishing process has a pronounced electrophysical and electrochemical nature. To reduce the power consumption surface shielding by different insulating materials is applied (teflon shields are the most effective ones).



Processing with jet electrolyte flows

Electrolytic-plasma treatment effectively meets the challenge of metal products' finishing. However, there is a number of unresolved technical issues, restricting the application of electrolytic-plasma polishing in working environment:

  1. Restriction of a workpiece size conditioned by:
    1. restricted overall dimensions of a working bath;
    2. amount of current being able to exceed permissible limits;
  2. long-length workpieces processing (tapes, tubes, wire , etc.);
  3. It is unable to process the inner surface of a hollow article or the surface in deep holes or cavities.

The use of jet flows of electrolyte appears to be effective way to solve these problems and gives an opportunity to expand significantly the technological spectrum of electrolytic-plasma exposure and the range of work pieces.

Installation diagram of electrolytic-plasma polishing of a large capacity workpiece (sheet) with electrolyte jet. 1 — operating cell; 2 — electric motor;  3 — centrifugal submerged pump;  4 — nozzle guide;   5 — electrolyte jet; 6 — workpiece and programmable mobility mechanism; 7 — DC source; 8 — electrolyte.

Installation diagram of electrolytic-plasma polishing of workpieces with electrolyte jet. 1 - operating cell; 2 - force pump; 3 - electric motor; 4 - flexible hose; 5 - nut; 6 - slow-speed reversible motor; 7 - quench; 8 - workpiece; 9 - screw.

 

 

Installation diagram of electrolytic-plasma polishing of long-length workpieces (tubes and tapes) with electrolyte jet. 1 - storage tank; 2 - operating cell; 3 - pump; 4 - electric motor; 5 - fabric nozzle; 6 - electric motor.

Installation diagram of electrolytic-plasma polishing of intricate-profile workpiece by intensification of metathetical processes by means of a submerged electrolyte jet. 1 - operating cell; 2 - intricate-profile workpiece; 3 - mehanoaktivator; 4 - electric motor; 5 - hoisting mechanism.