1. The purpose of roughening the surface before thermal spraying
The sandblasting pretreatment of the surface of the substrate is to use high-hardness abrasive particles to spray the surface of the substrate at a high speed, and to scour, chisel and hammer the surface of the substrate to remove the rust, scale, burr, welding slag, old ceramic coating on the surface of the substrate. layer and other contaminants, and form a substrate surface with considerable roughness, so that the metal substrate exposes the fresh active surface, and at the same time produces purification, roughening and activation effects. In addition, sandblasting also has a certain stress relaxation on the matrix material and has the effect of improving its fatigue strength. In addition, sandblasting has high production efficiency, so it is especially suitable for surface pretreatment and on-site construction of large-area and mass-produced products, so it has become the most commonly used surface pretreatment method in the industry.
2. What are the requirements for the surface roughness of sand blasting by thermal spraying
In order to obtain satisfactory coating bonding strength, thermal spraying has specific requirements on the roughness of the surface of the substrate after sandblasting.
1) The surface roughness (Ra is the arithmetic mean of the peaks and troughs of the sandblasted surface) is used to represent the appropriate surface roughness values for thermal spraying under different circumstances, which are listed in the following table. (Table 1)
Application object
Most spray coats
Thin metal parts
plastic parts
Surface roughnessRa/um
2.5-13
1.3
6
As the surface roughness increases, the bond strength of the coating increases. But after the table Ra > 10μm, this effect is greatly weakened. In order to obtain the best bonding strength of the coating, the corresponding surface roughness dimension should be 3/4 of the diameter of the powder to be sprayed.
2) Represented by the surface morphology of the sandblasted state
According to GB8923-88 "Steel surface rust grade and rust removal grade before painting" standard, the sandblasting and rust removal grade of steel materials during thermal spraying is required to reach the highest level of Sa3.0, that is, the metal matrix is white, and the sandblasted The surface is free of all kinds of obvious oil, grease, dust, rolling, rust, coating, oxide, corrosion products and other foreign substances. At least the sub-advanced Sa2.5 level of near white should be achieved. The appearance and color of the surface of the substrate after sandblasting should be compared with the standard grade pictures or standard grade samples of sandblasted steel surfaces.
3. What is the effect of sand blasting gas pressure on the surface properties of the substrate
The change of blasting gas pressure has the greatest impact on the internal stress of the substrate surface. The use of high-pressure gas to blast the surface of thin plates and long parts often causes distortion of the workpiece, and the surface of soft substrates (such as aluminum, zinc, babbitt, etc.) Inlaid sandblasted abrasive. In addition, with the increase of blasting gas pressure, the surface activity of the substrate increases, the surface roughness value increases, and the blasting efficiency increases. For steel, stainless steel and alloy steel workpieces, the gas pressure value should be ≥ 0.5Mpa. For soft substrate workpieces, the gas pressure value should be ≤ 0.3MPa.
4. What is the effect of sand blowing distance, angle and time on the surface properties of the substrate
The change of the sand blowing distance has the greatest impact on the sand blowing efficiency, followed by the impact on the surface roughness of the machine body. Generally, the surface hardness of the substrate is greater than 45HRC, the sand blowing distance is 100mm~150mmmm, the surface hardness of the matrix is between 25HRC~45HRC, and the sand blowing distance is 150mm~200mm, the surface hardness of the soft substrate is less than 150HB, and the sand blowing distance is 250mm~300mm.
The change of the sand blowing angle mainly affects the roughness of the surface of the substrate. When the sand blowing angle changes from 30° to 75°, the roughness increases with the increase of the sand blowing angle, but the best sand blowing angle is 70°~ 80°.
The change of sand blowing time has a great influence on the activation degree of the surface of the substrate. Generally, the surface activity increases with the increase of the sand blowing time for fixing the workpiece, but when the sand blowing time reaches about 20S, the surface activity basically reaches saturation. Generally, the surface roughness of the substrate reaches Sa3 level, and the sand blowing time is about 5S~10S.
5. How to choose the form of sandblasting abrasive
Sandblasting abrasives must be clean, dry, and angular. It is forbidden to use abrasives used after casting shot blasting for thermal spray sandblasting pretreatment. Abrasives will be crushed to varying degrees in the sandblasting process depending on the type and performance of the abrasives. The fine dust not only affects the blasting efficiency, but also deposits on the pretreated surface of the substrate, which will affect the bonding of the spray coating and pollute the environment. Therefore, when the grinding of the abrasive exceeds 20%, the fine powder should be sieved, and the abrasive should be cleaned and dried before use. It is best to use a mixed abrasive of 50% recycled abrasive + 50% new abrasive for reuse.
6. The principle of jet suction sand blowing machine
The jet-suction type (also known as suction-type) sandblasting machine uses the negative pressure caused by the compressed air flow in the jet-suction chamber of the sandblasting gun, sucks the sand through the sand pipe, and sprays it from the nozzle with the airflow to roughen the surface of the workpiece. processing device. This sandblasting method is simple in equipment and easy to use, but has less sand intake, low jetting speed and low sandblasting efficiency. It is usually used for sandblasting of small-area or thin-walled parts and non-ferrous metals. The structure diagram of the jet-suction sandblasting machine is shown in the figure. During the sandblasting process, the high-speed airflow from the air nozzle forms a negative pressure around, and the abrasive is sucked from the conical hopper at the bottom of the sandblasting box through the sand suction pipe. into the high-speed jet of air. In the high-speed air flow, the abrasive is accelerated and sprayed onto the surface of the workpiece substrate. In the closed sandblasting cabinet, the sand that hits the surface of the workpiece is ejected, collected in the sandblasting cabinet, dropped into the funnel after being screened by the screen, and is recycled and reused. The abrasive can be used continuously until the abrasive is broken and loses the blasting effect. Jet-suction blasters are available in small, manually operated units as well as medium-sized, fully automated systems. This kind of sandblasting machine is suitable for the use of non-metallic abrasives with relatively low density, and is not suitable for the use of metal abrasives with relatively high density. Because of the relatively high density and large particle size of cast iron sand or steel sand, a larger negative pressure suction force is required.
