HVOF is an important method of thermal spray coating, most of which are used for precision workpieces. When grinding HVOF coatings, it should not only consider the high surface accuracy requirements but also consider the feature of high hardness. Therefore, how to choose a diamond wheel for grinding HVOF coatings relates the grinding quality, grinding precision, and grinding efficiency.
Introduction to HOVF:
HVOF is an extension of high-velocity oxygen-fuel spraying. It is a supersonic coating developed to better meet the needs of customers and enterprises. This treatment is used to improve or restore the surface performance or size of the parts. The molten or semi-molten material is sprayed onto the surface by high temperature and high-velocity air and creates a dense coating with a high finish.
- Energy-saving raw materials can greatly reduce costs.
- Technical improvement can improve spray performance.
- Improve electrical performance.
- Allow the parts to work at high or low temperature.
- Enable parts to work in a harsh chemical environment.
- Improve efficiency.
- Better improve the life of the spray machine.
Process principle of high-velocity oxygen-fuel spraying:
It is basically the same as other spraying methods. Heat the sprayed material first and then coat the surface of the parts by the airflow, obtaining better performance.
In the process of high-velocity oxygen-fuel spraying, mix the oxygen and gas, and fired in the combustion chamber and generates the airflow, and allow the high-pressure airflow to accelerate through the nozzle.
Put the powder in the heated air stream and accelerate to the surface of the part, resulting in a thin layer of stacked foil coating.
Grinding is the only practical finishing method in the post-processing. Since the bonding between the coating grains mainly depends on mechanical inlay bonding and contains certain pores, the heat transfer is slow from the viewpoint of grinding, and the grinding method of the conventional dense material is not suitable for the grinding of the thermal spray material.
If the grinding pressure is too big or the speed is too fast, it may cause the particles on the surface of the coating to transfer or be transferred, which may cause the particles inside the coating to fall off and even cause the entire coating to peel off from the substrate.
Factors that determine the HVOF grinding process include coating type, workpiece shape, finish, tolerance and etc.
General speaking, the finer the thermal spray powder and the lower the coating porosity is. The more uniform the coating, the better the finish after grinding.
Factors when choosing HVOF grinding wheels are type, hardness, workpiece size and shape, grinding amount, surface finish, grinder type, etc. The principles are as follows:
- Choose the sharpest grinding wheel possible. This grinding wheel has fast cutting speed and is not easy to overheat. The degree of the sharpness of the wheel is related to the type and size of the abrasive used in the grinding wheel.
Silicon carbide and diamond grinding wheels are usually used for grinding HOVF coatings. This is because silicon carbide abrasive grains have new sharp cutting edges when they are broken during grinding, while diamond grains have good wear resistance which can grind various tungsten carbide coatings.
The service life and grinding efficiency of the diamond grinding wheel are much higher than that of silicon carbide grinding wheel. (There are also reports on the finishing of HVOF thermal spray coating by CBN grinding wheels.)
When the abrasive grains size is large, it has better impact resistance and higher removal rate, and can quickly remove excess coating.
When the abrasive grains size is small, it has not only small surface area, but also the cutting edge is sharp, and it is more likely to fall into the inside of the coating than the coarse grain size, thereby obtaining higher finish.
Usually, the coarse grinding grain size for HOVF coating is 125-150#, and the fine grinding grains is 380-400#.
- The internal structure and hardness level of the selected grinding wheel should be able to meet the grinding requirements.
The internal structure of the grinding wheel refers to the spacing between the individual abrasive grains inside the grinding wheel. The porous structure grinding wheel has a better grinding effect, which is caused by the holding debris gaps between the particles.
The different hardness of the grinding wheel will also affect the grinding effect of the thermal spray coatings. The harder grinding wheel has a longer life than that of the soft grinding wheel. When the grinding stress is small, the contact area is large and the grinding speed is high, it is recommended to use the softer grinding wheels.
When the grinding stress is large, the finish requirement is high, the contact area is small, and the grinding wheel is narrow, it is recommended harder grinding wheels.
- Select the most suitable bond types:
There are two kinds of bonding agents for silicon carbide: vitrified bonded and resin bonded. Vitrified bond grinding wheel can withstand the high grinding speed and precise fitting tolerance, and it free from water, acid, oil and temperature change of the effect, but requires that the grinding machine speed is less than the safe operating speed of grinding wheel, usually less than 33m/s. Resin bond grinding wheel can be used for higher grinding speed and higher finish.
The diamond grinding wheel usually uses resin bond, because the vitrified bond diamond grinding wheel has a high manufacturing process, and the large-sized grinding wheel is often used in the grinding thermal spray process. Vitrified bond is difficult to reach the dimensional standard, so the resin bond is usually used.
Example of grinding HOVF coatings:
For KN-75 sprayed WC HOVF coatings, the recommended grinding process is as follows:
- Precision and high-quality grinding equipment
- Water-soluble cooling media to rinse
- The resin bond diamond grinding wheel is used. When the outer diameter of the workpiece is less than 50mm, the grinding wheel of φ500 is selected. When the outer diameter of the workpiece is larger than 50mm, the grinding wheel of φ750 is selected.
4 Grinding in two steps:
The first step: rough grinding, the grinding parameters are: abrasive grain size 125-150 mesh; grinding wheel RPM is 25-30m / s; workpiece RPM is 0.3m / s; grinding depth <0.01mm; moving speed 0.2-0.3 m/min, longitudinal feed amount 0.025-0.05mm
The second step:fine grinding, the grinding parameters are: abrasive grain size 380-400 mesh, grinding wheel RPM 25-30m/s; workpiece RPM 0.5m/s; grinding depth less <0.005mm, moving speed 0.05-0.1m/s, the longitudinal feed rate is 0.025-0.05mm.
The condition of the grinding surface of the grinding wheel changes during the grinding process. After a period of using, the following 2 situations occur. The abrasive grains on the surface of the grinding wheel are worn away so that the height of the abrasive grain is equal to that of the bond.
The surface of the grinding wheel is filled by grinding material. Both of 2 conditions can weaken the wear of the grinding wheel, resulting in more friction than grinding. In this case, the grinding wheel should be dressed or replaced with a new one.
When the grinding wheel is dressed with diamond tools, the traverse speed of the tool through the surface of the grinding wheel affects the final cutting action. The rapid traverse can sharpen the grinding wheel surface, so that the grinding grain is sharpened again, thereby improving the grinding ability of the grinding wheel.
In contrast, when the traverse is slower, the grinding wheel surface is closed, making the abrasive grains passivated and the grinding wheel is hardened. For the thermal spray coating, the slow dressing method is not recommended. Therefore, it is very important to keep the sharpness of the grinding wheel when using a diamond grinding wheel for HOVF coatings, which is good for obtaining a high surface finish.
Recommended grinding method for HVOF coatings:
Wet grinding is usually recommended for grinding HVOF. If there are applied appropriate protection measures, dry grinding can also be used. However, the advantages of wet grinding are much greater than dry grinding.
When wet grinding, the harder grinding wheel can be used without increasing the incidence of hot crack, minimizing surface particle detachment, and obtaining the better surface finish. The grinding wheel will not be blocked quickly, and the truing times is also little.
In addition, wet grinding also helps to wash away debris residue. The filtration of the grinding fluid and the concentration also has an effect on the surface finish.
Principles of grinding HVOF coatings:
As long as it is carefully handled during the grinding process, HOVF coatings surface with a good finish can be obtained. The following are the factors to consider when determining the process of grinding HVOF coatings.
1 Softer, a free-grinding grinding wheel greatly reduces the chance of polishing and abrasive removal.
2 Keep the grinding wheel surface clean and sharp
3 Adopt the correct grinding wheel dressing process
4 Select a coarse-grained grinding wheel for rough grinding. Fine-grained grinding wheels are used for fine grinding. If you try to use a coarse-grained grinding wheel to obtain a good surface finish, it may cause the abrasive grains to escape, contaminate or burn.
5 Use light grinding HVOF coatings are usually thinner, and excessive grinding pressure can cause delamination of the surface of the coating or surface grains detachment.
6 The final grinding process should be non-sparking grinding, otherwise, the surface of the grinding wheel will be passivated or glazed.
7 Always keep the coating under pressure and down-cut the substrate to minimize delamination and grains detachment.
8 Optimization treatment of coating grinding process
Grinding parameter changes have a big effect on the grinding speed and finish. HOVF coating finish depends to a large extent on the grinding process. When there is a problem of grinding wheel, adjust the wheel speed, feed rate, workpiece speed, and dressing process.