The geometry or shape of the parts has a certain influence on the spraying (welding) effect. For example, when the coating is sprayed on the cylindrical surface, it tends to shrink and hold tightly when the coating is cooled; The cooling of inner surface coating tends to separate from the workpiece, and the simple surface spraying powder is easy to deposit; It is difficult to deposit powder on some tortuous parts of complex surface. Therefore, it is of practical significance to study and excavate the spray (welding) characteristics of various types of parts and correctly use the spray (welding) process to ensure the spray quality.
1. The key points of spraying process for shaft surface are as follows:
(1) Before spraying, the surface preparation shall be strictly controlled, and the purification treatment and roughening treatment shall be carried out according to the process requirements. The treated surface shall not contact with moisture or be touched by hand.
(2) Strictly control the spraying specification, and control the temperature rise of the workpiece not to exceed 250 ℃. When the coating is thick and the workpiece is small, the continuous spraying temperature rise may exceed 250 ℃, intermittent spraying method should be used to reduce the temperature, but the residence time should not be too long, otherwise the coating will be polluted, resulting in poor interlayer bonding.
(3) After finishing the transition angle and edge, and the journal with hole or keyway, the hole should be plugged and the edge of keyway should be polished into a fillet, and the sprayed edge should be noted.
(4) The preheating temperature of the matrix on the cylindrical surface should be taken as the lower limit, and the matrix temperature should be controlled in the spraying process, so as to reduce the shrinkage force of the matrix.
2. Key points of plane spraying process:
(1) Pay attention to repair edges and corners; (2) preheating should be uniform, and both ends of large parts can be preheated.
(2) When spraying, powder should be sprayed from the surrounding edge, and then led to the plane. When spraying powder for many times, the direction of the last spraying should be perpendicular to that of the previous spraying.
(3) In order to prevent the coating edge from warping, the edge of the workpiece can be grooved before spraying, and the stress can be reduced by sandblasting roughening.
3. The key points of coating process for seam surface are as follows:
(1) the cylindrical part of the nozzle should be sprayed before the plane part.
(2) the spray direction of the spray gun is 45 degrees along the circumference of the workpiece, which may make the powder deposition rate low, but it can spray clear edges and corners. During spraying operation, pay attention not to spray the spray gun perpendicular to the corner of the seam, which will cause loose coating structure or ash inclusion at the corner.
(3) in case of burr and edge warping during spraying, it shall be removed in time before spraying.
4. Key points of spraying process for parts with local serious damage:
(1) polish the local damage, spray or repair with other methods, and then spray the whole plane.
(2) for deep and wide local defects, electric welding can be used to repair them first, and then comprehensive spraying can be carried out.
(3) for local defects with cracks, the starting and ending points of cracks should be found first, and the crack stopping holes should be drilled, beveling should be made, all cracks should be cleaned up, then the defective parts should be repaired by spraying or welding, and finally the surface to be sprayed should be sprayed.
5. Key points of large equipment guide rail process:
① When repairing the machine tool guide rail, it is difficult to remove the oil by heating the whole machine tool guide rail evenly, and it is also difficult to remove all the deep oil stains. If the heating temperature is high, the machine tool guide rail is easy to deform. Therefore, the following operations can be carried out.
② The temperature of the workpiece is about 80-100 ℃, and the oil stains are wiped off with the baking process. The process is repeated several times, and then the high-power spray gun is used for rapid spraying.
③ Solvent cleaning or brush plating is used, then flame baking and gunning are used.
④ Depending on the specific situation of the rail strain, the rail repair glue or casting defect repair machine welding process can also be used to complete.
6. Key points of external surface spraying process:
In the same way as the shaft parts, the coating shrinkage force makes the coating hoop the substrate, which is conducive to the improvement of the bonding strength. The lower limit of the thermal temperature should be taken. In the spraying process, the temperature of the workpiece should exceed 250 ℃. It must also be noted that for the parts with different shapes, volumes and spraying parts, there should be different requirements for the temperature control of the workpiece. When spraying the cylindrical parts of small parts, the workpiece temperature should not be too high, and when cooling, the temperature of the workpiece should not be too high, The large shrinkage stress of the workpiece will cause the coating to fall off or cause large deformation.
7. Key points of inner hole spraying process:
① The shrinkage force of the spray layer during cooling tends to make the coating separate from the substrate, which is not conducive to the improvement of the strength
② When the inner hole is shallow and large, the cylindrical spray gun can be used to spray with a certain spray angle. For the spraying of the inner part, the cylindrical spray gun is mostly used, but the "covering effect" caused by the use of the thread before spraying should be paid attention to to to prevent the coating from loosening.
③ For deep hole spraying (the diameter of the hole is greater than 200 mm), it is necessary to use the extended tube inner hole spray gun, but the hole depth should not exceed 2 times of the diameter, otherwise it will have a great impact on the spraying quality. In order to prevent the temperature of the extended tube inner hole spray gun from being too high, air cooling with pressure of 0.4-0.6 MPa should be applied.
④ The rotation speed of the workpiece should be a little faster than that of spraying the outer circle to prevent the coating from overheating. The rotation direction of the workpiece should make the spraying face rotate downward to avoid the scattered powder into the surface to be sprayed. The flame should be slightly higher than the center line of the workpiece.
⑤ In the spraying process, the temperature of workpiece should be controlled not higher than 250 ℃, and intermittent spraying can be used for thicker coating.
⑥ Due to the shrinkage stress of the substrate, the surface of the inner hole of the spraying parts tends to make the substrate close to the coating. If the preheating temperature of the workpiece is insufficient and the temperature of the workpiece is low, the coating will fall off due to the large shrinkage stress difference between the coating and the substrate. Therefore, the preheating temperature can be taken as the upper limit, and the preheating temperature of the substrate can be appropriately increased when the deformation conditions of the workpiece permit, so as to increase the shrinkage force of the substrate.
⑦ Two step spray welding is adopted, which is similar to spray welding before remelting. The spray welding points of various parts can be determined according to the specific situation and the spray welding process requirements.
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