With the development and progress of modern science and technology, higher performance indexes are put forward for military equipment, industrial production equipment and scientific research instruments. Moreover, the performance requirements for the components of these instruments are getting higher and higher. In order to meet the special use requirements, some precious metals such as titanium, nickel, cobalt, tungsten and zirconium are often used to manufacture parts, and casting, forging and welding are difficult and uneconomical. Using thermal spraying technology to spray a special coating on the surface of ordinary materials to create a special working surface can achieve the purpose of saving materials and energy.
Thermal spraying technology is a method and technology that uses a heat source to heat the sprayed material to a molten or semi-molten state, and spray it on the pretreated substrate surface at a certain speed, and form a coating through the deposition of tiny particles, thus giving the substrate surface special functions. It is an important part of surface engineering technology. Using different coating materials and different process methods, functional coatings with wear resistance, corrosion resistance, high temperature oxidation resistance, thermal barrier, electromagnetic shielding and insulation can be prepared on the surface of the substrate.
The commonly used thermal spraying methods include flame spraying, arc spraying and plasma spraying, and the coating materials include metal, metal alloy, ceramics, cermets, plastics and other composite materials. Thermal spraying method and technology have been widely used in aerospace, metallurgy, energy, military equipment, petrochemical industry, machinery manufacturing, transportation, biological engineering and other national economic fields.
At present, all kinds of functional coatings made by thermal spraying technology are mostly used for the protection, strengthening and repair of parts, and there are few reports on making parts directly by thermal spraying technology. This paper introduces a method and technology of manufacturing sliding bearing by thermal spraying. Because of the special technology, it can also be used as an exploration of combining thermal spraying technology with material accumulation technology to make special parts.
2 traditional sliding bearing manufacturing method
Sliding bearing is a common mechanical part, which consists of rotating journal and fixed bearing seat. The inner surface of the bearing seat in contact with the journal is called bearing bush or bearing liner. Because the shaft often bears a certain load and transmits motion and power in use, the commonly used materials are all kinds of steel and cast iron, which are relatively wear-resistant; The common materials of bearing bush or bearing liner are bearing alloy, bronze, aluminum-based alloy and zinc-based alloy. Among them, bearing alloys (mainly alloys of tin, lead, antimony or other metals) are widely used under heavy load and high speed because of their good wear resistance, high plasticity, good running-in performance, good thermal conductivity, good adhesion resistance and good oil adsorption, but bearing alloys are relatively weak and expensive, so centrifugal casting is generally used.
3. Technological method of manufacturing sliding bearing liner by thermal spraying technology.
At present, there are many materials and documents introducing the repair of sliding bearings by thermal spraying technology. Volkswagen also has an invention patent that plasma spraying is used to make wear-resistant aluminum bronze coating on the inner hole of connecting rod of internal combustion engine. However, due to the limitations of existing thermal spraying equipment and operation methods, various functional coatings prepared by thermal spraying technology are mostly limited to the outer surface and larger inner hole surface of parts. Once the hole diameter is smaller than φ35 or the hole depth is larger, it is difficult to construct. If the cavity surface of the part is special, it cannot be constructed.
This paper introduces a process method of manufacturing sliding bearing bushing by thermal spraying technology, which is used to manufacture the following sliding bearing parts. As shown in Figure 1 Figure A, the diameter of the inner hole is φ20, the length is 30, the inner hole surface is the working face, and the bearing alloy is 3mm thick. The alloy layer of this part is generally cast by centrifugal casting. Because the aperture is too small, thermal spraying has no inner hole spraying to complete this task.
In order to solve the problem of thermal spraying coating on the inner hole surface of the above parts, the author puts forward a new thermal spraying construction method. A new tool is made. The main structure consists of mandrel, nut and baffle. Low melting point alloy with melting point of 183℃ is uniformly deposited on the surface of mandrel 1. Finally, a refractory coating is coated on the surface of the low melting point alloy. The refractory coating is made of special binder and refractory material with high temperature resistance. When spraying, the cutter can be clamped on the turntable for thermal spraying, and the cutter is preheated to about 100℃, and then the required bearing alloy coating is sprayed by arc spraying equipment. After reaching the designed coating thickness, the coating surface should be cleaned to remove floating ash, and supporting materials such as steel and stainless steel should be sprayed on the coating surface to make it reach the required diameter size. Finally, the nuts and baffles at both ends are removed, heated to the melting temperature of the low-melting alloy, and the mandrel is taken out to obtain the required sliding bearing blank with the inner hole surface of the bearing alloy. In order to increase the bonding force between the bearing alloy coating and the supporting material, the alloy coating can be coarsened and sandblasted after spraying, and then the supporting material is sprayed.
This kind of sliding bearing liner made by thermal spraying technology is amorphous bearing alloy, which has higher hardness and certain porosity than the base material, and can accommodate some grease, and its service performance is better than that of casting method. In addition, because the thermal spraying technology different from the casting method is used to make the bearing alloy lining, the thickness of the bearing alloy can be appropriately reduced in the design of parts to save precious metals.
4 related issues that need attention
When using thermal spraying technology to manufacture the above sliding bearings and similar parts, besides observing the safety regulations of thermal spraying operation, the following related issues should also be considered.
(1) Appropriate coating material and spraying process should be selected according to the working conditions of the working face. For example, the bearing alloy is sprayed on the inner surface of bearing parts by arc spraying method introduced in this paper. In order to prevent excessive oxidation of alloy materials during spraying, nitrogen can be used as spraying medium gas.
(2) If the coating surface requires precision and roughness, it is necessary to obtain the required technical requirements through mechanical processing. The coating thickness and tooling size should be reasonably designed, and the processing capacity should be reserved to ensure the working thickness of the coating. For example, when the sliding bearing introduced in this paper is supplemented, the inner hole of the blank should be used as the rough benchmark, and the outer cylinder should be processed first; Then, the inner hole is machined based on the outer cylinder to ensure the uniform thickness of the remaining coating.
(3) Bonding force and residual stress between two coatings. Because the supporting coating is sprayed on the working coating by arc spraying after spraying, the supporting coating is thick, and if the combination between the two coatings is not good, the working coating may peel off in the supplementary processing stage or the use stage. Therefore, the thermal expansion coefficient and bonding force of the two materials should be fully considered. If necessary, a transition coating can be sprayed between them, and a composite coating and a gradient coating can be designed to increase the bonding strength. In the coating design stage, it is necessary to fully consider whether the two coatings can be fully combined, and carry out sufficient process experiments in practical application. In addition, the residual stress should be reduced by controlling the temperature before and during spraying, and the heat treatment process to eliminate internal stress can be arranged in the later stage if necessary.
(4) Workpieces and fixtures are easy to disassemble and assemble, and fixtures can be reused. Because the coating material is in molten and semi-molten state during spraying, it is sprayed on the workpiece in the shape of a cone column driven by high-speed airflow, and some coating material will inevitably stick to the surface of the tool. Considering that tools and workpieces are easy to disassemble and reuse, release agent should be coated on the surface of tools before spraying.
Taking the manufacture of sliding bearing liner as an example, this paper introduces a process method combining thermal spraying technology with material accumulation technology. According to this idea, other complex parts and other functional coatings can also be manufactured, such as hydraulic valve bodies with wear-resistant and corrosion-resistant coatings in inner holes and spiral cooling channels. It must be admitted that this method is not suitable for mass production of products at present, and can only be used for small batch production of single pieces with special performance requirements and special structures.
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