Special Processing of Thermal Spray Coatings Electrochemical Grinding

Electrochemical grinding is a method that combines electrochemical action and mechanical grinding for removing metal surface.

1. Process features of electrochemical grinding thermal spray coating

（1）High productivity

Electrochemical grinding can increase grinding parameters, especially radial grinding stock (grinding depth). The productivity is 3-5 times higher than that of general grinding according to time per piece.

(2) Good processing quality

In electrochemical grinding, the grinding wheel is mainly to remove the oxide film on the coating surface. So it can decrease the thermal stress, residual stress, and deformation of thermal spray coating surface caused by grinding heat and grinding force. It does not crack and burns on the machined surface. The dimension accuracy and surface roughness can reach IT7 and Ra 1.25 respectively. Thermal spray coating parts by electrochemical grinding are proved to be qualified and the effect is good.

(3) Small wear loss and good economy of grinding wheel

Grinding wheel is mainly to remove oxide film by electrochemical grinding. Copper-based diamond conductive wheel grinds hard-to machine thermal spray coatings. The ratio of metal removal and wheel wear is about 400(inner holes grinding).

(4) Strong adaptability and wide processing range

Electrochemical grinding can process all kinds of difficult-to-machine conductive thermal spray coatings and all types of surfaces (such as gear surface).

In short, electrochemical grinding is a processing method of high productivity, good processing quality and economic. It is a new processing method for processing difficult-to-machine thermal spray coatings.

2. Selection of electrolyte for electrochemical grinding

The electrolyte is an important factor that affects productivity and processing quality in electrochemical grinding because it directly affects the anode electrochemical reaction.

The rate of anodic reaction and degrees of chemical dissolution of reaction products for different components of the electrolyte is different, according to the theory of electrochemical reaction. Electrochemical grinding requires not only oxidation but also passivation. In order to obtain high productivity and good surface quality, sodium nitrite or disodium hydrogen phosphate is usually used as the main components of a water solution (its pH value is 8-9). In addition, another important requirement for electrolyte is not to rust machine and no harm to human body.

The selection of electrolyte should consider the following 5 requirements:

(1) The electrolyte can make metal surface formed to passivation film that is compact structure and strong adhesion. Have good dimensional accuracy and surface roughness.

(2) Good conductivity to achieve high productivity

(3) No dust machine and fixtures

(4) It is harmless to the human body

(5) It has a good economic effect, low price, rich source and is not easy to consume in processing.

It is difficult to meet the above five requirements at the same time. In actual production, the best electrolyte for electrochemical grinding should be selected according to the technical requirements of different products and different materials.

Experiments show that it is difficult to process the thermal spray coating by electrochemical grinding. The electrolyte with sodium hydrogen phosphate as the main component has better grinding performance.

The processing efficiency of this electrolyte is high. This is mainly because disodium hydrogen phosphate in aqueous solution is ionized, and then the solution is weak alkalinity (pH is 8-9), besides the conduction and oxidation of sodium nitrite and potassium nitrate. It is good for the dissolution of oxides and accelerates the electrolysis process, thus obtaining high current efficiency and improving the processing efficiency. The electrolytes for hard-to-machine thermal spray coatings by electrochemical grinding are recommended as follows: disodium hydrogen phosphate 3%~5%; sodium nitrite 2%; potassium nitrate 2%; water 91%~93%.

The pH value is 8-9 for the electrolyte of the above components. It is not a dust machine.

3. Equipment for electrochemical grinding

The grinding machine for electrochemical grinding is professional electrochemical machine or modified by conventional grinding machine or lathe. DC power supply for electrochemical grinding requires adjustable voltage (5-20V). The Max.working current can be adjusted from 10-1000A according to the processing area and required productivity. As long as power is allowed, it can be usually used with DC power supply equipment for electrochemical processing.

Small centrifugal pumps are generally used for circulating pumps supplying electrolytes, but the best ones are acid and corrosion resistance. There should also be devices for filtering and precipitating electrolyte impurities. In the process of electrolysis, harmful gases, such as carbon monoxide, sometimes occur. Therefore, it is better to have forced air extraction device or neutralization device on the machine, at least operate the machine in the place where the air is more circulating.

The electrolyte is usually injected through a tube and a flat nozzle. The nozzle is connected to the tube and injects electrolyte into the working area. The inner grinding head is composed of a high speed grinding wheel shaft and a three-phase alternating current machine. The radial swing difference of the high speed grinding wheel shaft is less than 0.01 mm and the rotational speed is 5000-6000 r/min. The power of three-phase AC motor is 0.75 kW and the speed is 2800 r/min. It is installed on the slide board of the lathe and insulated with the slide board. The axis line of the grinding wheel is the same height as the center of the lathe spindle. The modification of lathe spindle mainly insulates chuck and spindle and installs an anode brush at the end of the spindle, which connects with the output positive of DC power box.

DC power supply can use silicon rectifier DC power supply, output voltage 0 ~ 30V, output current 0 ~ 200A. It has stepless voltage regulation and over-voltage and over-current protection device. Grinding machine cooling pump can be used for electrolyte pump. The electrolyte can be sprayed directly on the parts through the rubber hose passing the spindle hole by the cooling tank of a machine, and then flowed back to the electrolyte tank by the return pipe.

4. Process parameters of electrochemical grinding

(1) Process parameters of grinding wheel

Diamond copper conductive grinding wheel, graphite grinding wheel and silver infiltration grinding wheel can be used. Grinding wheel speed υ=15~20 m/s, axial feed rate ƒa=0.5~l.0 m/min( internal and external grinding);ƒa=10~15 m/min( surface grinding); workpiece speed υw=10~20 m/min; radial feed rate ƒr =0.05~0.15 mm/DST.

Voltage and current specifications

The optimum current density and voltage can be obtained from the current efficiency feature curve in the above electrolyte test. Voltage for rough processing is 8-12V, and current density 20-30A/cm2 and the voltage for finishing is 6~8V, and the current density is l0~15 A/cm2. When the above parameters are applied, if blackening occurs on the grinding surface, the voltage and radial feed rate should be reduced, and the axial feed rate should be increased.

Conclusion: In addition to electrochemical grinding, with the rapid development of diamond sanding abrasive belt, especially the new resin diamond sanding abrasive belt can grind thermal spray coatings, and it can achieve a good effect.