In order to improve the laser cladding quality of hydraulic support cylinder, the characteristics of high-speed laser cladding and conventional laser cladding coating formation were comprehensively analyzed through process practice, and the cylinder base material, alloy powder and the applicability relationship between the two were discussed. The results show that the particle size of the alloy powder matched with high-speed and conventional laser cladding is 100-300 mesh and 300-500 mesh respectively. In order to reduce the cladding splash and improve the uniformity of powder feeding, the particle size distribution range is further adjusted to 90-250 mesh and 270-400 mesh; there is no obvious difference in the applicability of the alloy powder for the common materials of the cylinder base material 27SiMn steel, 30CrMnSiA steel and 40Cr steel, but the dilution rate and overlap defects require strict control of process parameters; the use of argon gas to protect the molten pool can obtain a better cladding macroscopic surface than nitrogen.
1 Introduction
The laser cladding of the arc surface of the hydraulic support cylinder adopts shaft rotary machine tools, and the high-speed and conventional speed are different from the rotation speed controlled by the machine tool. Industrial laser cladding is divided into two types according to the scanning speed: high-speed and normal-speed. The scanning line speed of high-speed cladding can reach 100-300 mm/s, which is 30 times that of normal-speed laser cladding. In normal-speed laser cladding, the laser energy is preferentially absorbed by the substrate. In high-speed laser cladding, the cladding powder preferentially absorbs the laser energy and reaches a molten state before reaching the substrate surface. In high-speed laser cladding, the laser beam melts the powder to a limited extent, and only 20% of the laser energy remains after passing through the powder beam, so that the substrate surface moving at a higher speed participates in the molten pool at a small depth. Li Liqun, Lou Liyan, et al. [1, 2] used ultra-high-speed laser cladding to obtain a low-dilution metal coating with good microstructure and performance. Tan Tai Fanliang et al. [3] achieved industrial application of high-speed laser cladding. By controlling the high-speed movement of the cylinder substrate, the effective cladding rate of the parent material surface can reach 25-200 m/min, significantly improving the processing speed. Through the analysis of the current status and development of ultra-high-speed laser cladding technology by industry insiders [4, 5], compared with the stability of traditional laser cladding, high-speed laser cladding has many industrial application problems, but it is still developing in a positive direction.
Based on the application practice of high-speed laser cladding and conventional laser cladding in hydraulic support cylinders, the process characteristics are analyzed in combination with the conventional and high-speed laser cladding mechanisms, the synergistic effect of alloy powder materials and parent materials on cladding quality is explored, and the influence of molten pool temperature, dilution and overlap on forming quality is discussed. The gas protection of laser cladding molten pool is analyzed and verified, hoping to provide certain technical references and inspirations for the production and application of laser cladding in hydraulic support cylinder industry.
2 Experimental materials and methods
The experimental substrate is the common parent material of hydraulic support cylinder. The cladding powder is coarse powder for conventional speed and fine powder for high speed. The laser used for normal laser cladding is a 10kW high-power semiconductor fiber-coupled laser, and the high-speed cladding uses a 4kW fiber laser. The process parameters are implemented in the production process.
The 10mm×10mm×10mm sample of the cladding area was cut by wire cutting. After sandpaper grinding and polishing, the hardness gradient was tested using an HVS-1000A digital microhardness tester; the metallographic sample was corroded with 4% nitric acid alcohol solution, and the microstructure and structure of the cladding layer were observed under an Axio Lab.A 1 metallographic microscope. The Zeiss EVO10 scanning electron microscope was used for micro-area composition detection. A 60mm×40mm sample was cut for a neutral salt spray test (test temperature 35℃±2℃, salt spray deposition rate 2mL/80cm²·h, cycle 96h).
3 Results discussion
3.1 Differences in alloy powders
In order to ensure the hardness and corrosion resistance of the laser cladding layer on the surface of the hydraulic support cylinder, the physical properties and composition of the alloy powder used need to be adjusted according to the characteristics of the forming process. The alloy powder used for laser cladding of the surface of the hydraulic support cylinder is mainly an iron-based series, with the main elements being C, Si, Mn, Cr, Ni and Mo, and a certain amount of strong carbide-forming elements such as Nb and V are added. The hardness of the coating is adjusted by adjusting the content of alloy elements, and the hardness, crack sensitivity and retained austenite content of the cladding layer are improved by adding other elements, thereby improving the wear resistance and toughness of the formed coating. The main chemical composition and melting point of the alloy powder are shown in Table 1. Taking the powder used for conventional laser cladding as a reference, the content of Cr and Ni in the high-speed powder, which contributes greatly to corrosion resistance and hardness, is increased. At the same time, rolling is used as an auxiliary process for high-speed cladding to improve the hardness of the cladding surface. For alloy powders of the same purpose, the surface hardness (480HV) formed by high-speed cladding is lower than that of conventional laser cladding (520HV).
A large specific surface area is conducive to heat absorption, so the alloy powder used for high-speed cladding has a smaller particle size and a larger proportion of fine powder, as shown in Figure 1. High-speed and normal-speed laser cladding powders are divided into two types: coarse-grained range of 100-270 mesh and fine-grained range of 300-500 mesh. The physical properties of high-speed and normal-speed laser cladding powders are shown in Table 2. Among them, high-speed powders have greater fluidity and bulk density.
Limited by the progress of industrial application of high-speed cladding technology, the development of high-speed cladding powders is relatively slow. Although 3D printing has formed a considerable application scale, due to the added value of cylinder laser processing and the low cost of iron-based powders, most large-scale powder suppliers in China do not provide high-speed cladding cylinder iron-based powders, and there are few laser cladding fine powders used in hydraulic support cylinders. In order to improve the heat absorption capacity of alloy powder, high-speed powder reduces the powder particle size, but with the output process of pneumatic powder feeding, fine powder dust is serious and powder utilization rate is low. Therefore, with the advancement of application, the particle size of high-speed cladding powder is optimized from the original 300-500 mesh to 270-400 mesh, and the particle size of ordinary speed powder is optimized from 100-300 mesh to 90-250 mesh. On the one hand, the powder particle size range is narrowed to reduce the proportion of fine powder particles; on the other hand, the particle size is adjusted to coarse powder as a whole to improve the uniformity of powder particle size, thereby optimizing the uniformity of powder feeding.
Figure 2a shows the particle size distribution of ordinary speed powder with a size of 90-250 mesh after optimization, where the median diameter D50 is at 106.1μm (about 150 mesh), coarse powder above 90 mesh accounts for about 20%, and fine powder below 250 mesh accounts for about 6%.
Figure 2b shows the particle size distribution of 270-400 mesh alloy powder for high-speed cladding conventional powder and high-speed powder fine powder, where the diameter D50 is at 39.19 μm (about 383 mesh), coarse powder above 270 mesh accounts for about 39.48%, and fine powder below 400 mesh accounts for about 46.7%, of which 90% of the particles have a particle size of ≤62.26 μm.
3.2 Characteristic analysis of the parent material of the oil cylinder
The thermal stability of the material is closely related to the thermal expansion coefficient. The smaller the thermal expansion coefficient, the better its thermal stability. The thermal expansion coefficient of most metal materials is inversely proportional to the binding energy [6, 7]. The linear expansion coefficient and melting point of carbon steel and nickel-chromium steel are shown in Table 3. Common parent materials for hydraulic support oil cylinders include 27SiMn steel, 30CrMnSiA steel and 40Cr steel. The carbon equivalent and welding cold crack sensitivity and hot crack sensitivity are calculated according to the empirical formula recommended by the International Welding Association. The calculation results of the three show that they all have a certain tendency to hot cracking and cold cracking. Among them, 40Cr steel is prone to segregation during melting and crystallization, so it is more sensitive to crystallization cracks, and post-weld cracks must also be prevented. Combined with Table 1, it can be seen that the linear expansion coefficient of alloy powder forming materials is greater than that of carbon steel, which provides a guarantee for the formation of high-quality upper cladding with metallurgical bonding. Production practice has proved that the applicability of common parent materials for oil cylinders to upper cladding powder is not significantly different.
3.3 Factors affecting high-speed and normal-speed laser cladding process
(1) Cladding process parameters
1) Figure 3 shows a high-speed laser cladding sample. The curve trend of typical elements Mo, Fe, and Cr is used to determine the size of the dilution layer. Through metallographic size measurement and micro-area composition detection, the dilution rate is
55/782=7%. Figure 3b shows the interlayer situation of the cladding layer of the same material. The blue curve representing the Ni element and the red curve representing the Cr element have local convexity, which is related to the secondary energy received at the overlap [8].
The molten pool contains the upper alloy powder and the parent material composition. A suitable molten pool can not only control the function optimization of the cladding metal, but also reduce the influence of the laser cladding energy on the parent material, thereby reducing the influence of the parent material on the cladding layer. The dynamic molten pool temperature under the synergistic effect of parameters such as laser power, focal length, scanning line speed, powder feeding rate and overlap rate has a decisive influence on the dilution rate. When the molten pool temperature is low, it is easy to cause oxide inclusions to form in the near-fusion zone. Under the low energy irradiation of high-speed cladding (power 4kW, scanning line speed 200mm/s), there are many pores and slag inclusions at the interface between the same materials (see Figure 3c).
2) Figure 4 shows the overlap area of the non-optimal process of conventional laser cladding when multiple overlaps are overlapped (power 10kW, scanning line speed 8mm/s). The overlap rate calculation result is 13%. There is a difference in Cr element composition between the two overlap areas, and a convexity phenomenon appears at the interface between the two layers. This is due to the decomposition of the intergranular passivation film caused by the secondary molten pool. The decomposed Cr is preferentially combined by C to form carbides, which are locally enriched and precipitated, correspondingly resulting in local Cr deficiency in this area and weak points in corrosion resistance.
Affected by the laser beam energy during the latter cladding, the local heat treatment of the previous overlap area has significantly coarse grains, and the presence of impurities such as oxide scale and spheroidized materials on the surface of the previous cladding layer leads to some defects such as pores and slag inclusions in the overlap area. After a certain thickness of the cladding material is removed by mechanical processing to obtain the finished surface, there is a uniform overlap area on it, which becomes a weak area of corrosion protection [9, 10]. Preliminary verification shows that there is no significant difference in the average Cr content under multiple processes, but elements such as Cr and Mo that contribute to corrosion resistance are all concentrated at the grain boundaries, and the higher the energy density of laser cladding, the greater the degree of element concentration [11].
Figure 5 shows the rust morphology and intergranular corrosion morphology of the cladding surface. The macroscopic manifestation is uniformly spaced rust distributed along the ring, and the microscopic manifestation is intergranular loosening. Overlap defects and local Cr depletion cannot be avoided. The only way is to adjust the process parameters to obtain appropriate laser energy density and laser-powder matching, improve the forming quality of a single pass, reduce the degree of spheroidization and oxidation, and thus reduce the defect rate of the overlap of multiple cladding passes [6].
(2) Auxiliary scheme of protective gas The molten pool of laser cladding is exposed to the air. Common types of pores in the cladding layer are CO pores, hydrogen pores, and nitrogen pores. The metal phase in the molten pool undergoes redox reactions. For example, FeO + C = Fe + CO. If CO gas does not have time to escape, pores will be formed, and oxides of Fe, Al, etc. will form slag inclusions. Gas and powder around the molten pool, as well as moisture on the surface of the workpiece are the main causes of hydrogen pores. Nitrogen mainly comes from air. If nitrogen is used as a protective gas, its source will be increased. If these gases do not have time to escape, pores will be formed in the solidified cladding layer, and their shape is white and round. Argon (relative molecular weight of argon is 39.95, air is 28.8, nitrogen is 28, and oxygen is 32) gravity exhaust method can be used to reduce the air and moisture around the molten pool, so as to achieve a certain molten pool protection effect.
Figure 6 shows the surface of the cladding with argon protection added to the side of the conventional laser cladding. Its color is light silver white and there is no obvious oxide scale. The surface of the cladding without argon is smoky yellow, and some oxide scales can be removed after cooling.
4 Conclusion
By comprehensively analyzing the forming characteristics of high-speed laser cladding and ordinary laser cladding coatings, discussing the cylinder base material, alloy powder and the applicable relationship between the two, the following conclusions are drawn.
1) The difference between high-speed and ordinary laser cladding processes is mainly reflected in the different requirements of the melting method for alloy powder and the difference in forming performance. In order to achieve the laser cladding performance and quality improvement of the hydraulic support cylinder surface, it is necessary to optimize and compensate the composition and particle size of the alloy powder.
2) The commonly used materials of the hydraulic support cylinder base material 27SiMn steel, 30CrMnSiA steel, and 40Cr steel are analyzed. The carbon equivalent and weldability of the alloy powder used for ordinary and high-speed laser cladding are better than those of the base material, and the linear expansion coefficient of the alloy powder forming material is greater than that of carbon steel, which provides a guarantee for the formation of a high-quality upper cladding layer with metallurgical bonding.
3) The molten pool temperature is the decisive factor for the forming quality. The dilution rate, slag inclusion, etc. are all controlled by the molten pool temperature, which also determines the characterization of the forming hardness and corrosion resistance. In addition, the overlap problems caused by unsuitable processes increase. Therefore, process control is the key to ensure the uniformity of the cladding structure.
4) The use of gas protection for the laser cladding molten pool has positive quality benefits, and the use of argon can obtain a better macroscopic surface than nitrogen.
