Development of Technology for Laser Synthesis of Antifriction Friction Surface of Large-Sized Ship Propeller Shafts
The reliability and efficiency of the operation of transport ships is characterized, among other things, by the trouble-free operation of the propulsion system, which can be ensured only by a set of progressive constructive-technological solutions implemented during the construction of the ship. This is especially true for Arctic navigation ships, to which the customer places increased demands on reliability, operational efficiency and environmental friendliness .
The propulsion systems of modern Arctic navigation ships, in particular, icebreakers, are classified as heavily loaded and usually consist of a powerful multi-shaft powerplant with a unit power of 20 MW and more, and a short rigid shaft line with a minimum number of bearings, while the shafts have a diameter 700 mm and above . On the icebreaker of the project 22220 “Arktika” (Fig. 1a), the shaft diameter is 860 mm, and in the projected icebreaker of the project 10510 “Lider” (Fig. 1b) will be more than 1050 mm.
The existing antifriction technology of forming friction surface of the stern shaft bearing is based on the installation of bronze linings on the propeller shaft with a “hot” fit. Considering the dimensions of the propeller shaft and linings, the risk of jamming of the liner during the nozzle is very high. In addition, with this technology, cladding is a stress concentrator, which reduces the fatigue strength of the propeller shaft and can lead to its breakage.
To replace the traditional technology of installation of expensive bronze linings on the propeller shaft, CSSRT JSC suggested using the technology of powder laser surfacing to create an antifriction layer of bronze on a steel sleeve.
One of the main technological advantages of the laser deposition method in comparison with such traditional methods as electric arc or plasma surfacing is a low heat supply to the base material. This helps to reduce the deformation of the created products. High cooling rate contributes to the formation of the desired fine-grained microstructure. This technology provides a complete metallurgical adhesion of the surfacing layer to the base and low liquefaction (mixing with the base metal) as compared with traditional methods. Thus, one coating layer is enough to completely replace the base material. At the same time, the tolerances in thickness and surface quality are quite acceptable and, depending on the task, require only minimal modifications or immediately comply with technical requirements.
The main task in the development of the technological process of laser powder deposition is to obtain a high-quality coating (without pores), with good adhesion to the base material. It is required to ensure minimal mixing and dissolution of the deposited material with the base. For given speeds of movement and feed rate of surfacing powder, it is possible to determine the optimal spot size and power density at which the maximum deposition rate is ensured.
During the implementation of the laser deposition process, powders are fed, as a rule, using pressure or gas-injecting feeders directly to the laser radiation area. In this case, the powder is mixed with the gas stream, is accelerated by the jet and heats up during the flight from the mixing zone to the sprayed surface, after which it falls on the substrate surface. The working scheme was adopted four-sided coaxial powder feed.
In the course of the experimental work, three characteristic profiles were determined, describing the interaction of the deposited material. At high values of the powder feed rate or insufficient radiation power density, a regular spherical profile is formed on the surface of the product (type 1), which does not allow the layers to overlap correctly. Excessive dissolution and surface deformations of the form of the deposited layer (type 2) are characteristic, On the contrary, at high values of radiation power density and insufficient supply of surfacing powder. Most preferred is an elliptical shape with clear boundaries and optimal fusion with minimal dissolution at the base (type 3). Such a profile is preferable for surfacing with overlapping layers.
During surfacing with overlapping layers, pore formation is possible with the nature of powder transfer to the substrate close to type 1, for cases of high powder flow and thicker weld layers. Thus, by the nature of the formation of single layers, it is possible to predict the appearance of internal porosity during multi-pass surfacing with overlapping rollers.
40HFA and 38X2H2MA steel were used as the base material. To create an antifriction layer, bronze powders RotoTec 19850, RotoTec 19868 and metal powder EuTroLoy 16625G.04 with a fraction size of 53–150 microns were used (see the table).
Development of the technology of laser synthesis (surfacing) was carried out on a robotic laser-deposited laser-developed complex developed by CSSRT JSC (Fig. 2). The complex is built according to a modular principle, which allows for flexible reconfiguration, replacing modules, and also adapting equipment to a specific technological task. The complex is equipped with a 4-kW fibre laser source, a displacement and positioning module (a six-axis manipulator with a two-axis positioner), an optical module, a powder supply module, and a control system that ensures the setting and operation of equipment in manual and automatic mode. The complex is also equipped with a video surveillance system to monitor the process in real time, as well as a protective cabin to meet the requirements of laser and industrial safety.
In the course of research, samples were obtained with weld rollers (Fig. 3) in one or several passes. Sample microsections (Fig. 4) were made on MetaServ 250 grinding and polishing machine (Buehler).
The study of the microstructure and grain size of the surface layer of thin sections was carried out on a Nikon MA200 metallographic microscope, with an increase from 100 to 1000 times, depending on the research methods.
The width of the transition zone between the base metal and the deposited layer was 0.1–0.2 mm. The structure of the alloy obtained by the fusion of the EuTroLoy 16625G.04 powder consists of a nickel solid solution and precipitates of chromium carbides, borides and silicides uniformly distributed in the alloy matrix (Fig. 5.6). The transitional area is characterized by a mixed austenitic-marten structure. The precipitates of carbide, boride, and silicide phases disappear as they approach the base metal.
On samples made using bronze powders, the layer structure consists mainly of copper with a grain size of 5 to 15 microns. The transitional zone of the fusion of bronze and steel base is characterized by the presence of precipitations of a solid solution of copper in the ferritic-bainite structure of the steel matrix.
A test multi-pass surfacing on a shaft billet with a diameter of 218 mm (Fig. 7) was performed to obtain a product with a diameter of 220 mm in the drawing. The powder feed rate is 14 g / min. The width of the transition zone between the base metal and the deposited layer was 0.1–0.2 mm. The average value of the height of the surfacing roller H (thickness of the layer obtained by laser welding in one pass) is 0.6 mm.
It has been established that with multi-pass laser synthesizing it is necessary to clean the weld surface from burr before applying each subsequent layer.
Fabricated samples with anti-friction bronze were tested for abrasive wear according to the Brenelle-Haworth scheme. The studies were carried out as follows: a sample with a deposited layer was pressed to a rotating rubber disk for 10 minutes, feeding quartz sand with a particle size of 200–600 μm into the friction zone. According to the test results of three samples, the average value of the weight loss of the deposited coating was determined.
The test results confirmed the high antifriction properties of the bronze layer. The weight loss of the deposited coating during the abrasive wear tests was no more than 0.0055 g.
Studies of the technology of laser synthesis of antifriction friction surface show the possibility of its successful application for the manufacture of propulsion systems for Arctic navigation ships.