Remelting Technology of Thermal Spray Coating and Its Development Status

       Abstract: In order to reduce the porosity and oxide inclusions in the thermal spray coating and improve its bonding strength with the substrate, it is necessary to remelt the coating. This paper reviews various coating remelting processes that have appeared in recent years, including laser remelting, electron beam remelting, argon tungsten arc remelting, flame remelting, induction remelting and integral heating remelting, etc. The principle, characteristics and application are introduced, and the differences of each remelting process are compared.



Key words: thermal spraying; coating; remelting; current situation

 

0 Preface 

 

    The rapid development of modern industry and science and technology has put forward higher and higher requirements for the protective performance of the surface of mechanical parts. Thermal spraying technology is one of the most widely used technologies in the field of surface engineering technology. However, because most of the thermal sprayed layers have a porous layered structure, the bonding strength with the substrate is weak, and it is easy to peel off, and the surface of the coating is uneven and wear-resistant. and poor corrosion resistance, etc. Therefore, in recent years, people have actively explored the coating remelting treatment technology.

    Remelting treatment is to use a high temperature heat source to melt part of the metal in the coating, and the molten metal penetrates into other metal particles, filling the pores between the metal particles, reducing or even disappearing the porosity, and the loose layered structure in the thermal spray coating becomes relatively Uniform dense tissue. Appropriate remelting treatment of the thermal spray coating can also improve the bonding strength between the coating and the substrate, thereby improving the wear resistance and corrosion resistance of the coating [1].

    At present, the remelting technology is mainly divided into laser remelting, electron beam remelting, argon tungsten arc remelting, flame remelting, induction remelting and integral heating remelting.

 

1 Laser remelting

 

    Laser remelting is the use of high-energy laser beams to irradiate the surface of the workpiece to rapidly melt and mix the ceramic or alloy coating, forming a layer of molten metal with a thickness of 10-1 000 μm on the surface of the substrate. After the laser is removed, this layer is The process of rapid solidification of molten metal.

    Although ceramic coating has good thermal stability, high hardness and low manufacturing cost, its melting point is much higher than that of alloy coating, so laser remelting process is widely used for post-processing. Laser remelting treatment of ceramic coatings can eliminate most of the pores and oxide inclusions, enhance corrosion resistance, and achieve metallurgical bonding between the coating and the substrate, so as to improve the bonding strength and enhance the wear resistance of the coating [2] .

    Li Chongguiren[3] prepared Al2O3+TiO2 coating on Ti-6Al-4V substrate by the composite process of plasma arc spraying and laser remelting. The results showed that the main phase of the plasma arc sprayed coating changed from γ-Al2O3 to α-Al2O3 eliminates micro-defects such as pores, micro-cracks and layered stacking inside the coating, and forms a good metallurgical bond with the substrate, the hardness of the remelted coating is increased by 50%, and the crack propagation resistance is increased by 2 times . Laser remelting is also suitable for post-processing of alloy coatings and alloy ceramic coatings. Yin Bin et al. [4] carried out laser remelting on the NiCrB-Si plasma arc sprayed layer on the surface of 1Cr18Ni9Ti stainless steel. Laser remelting eliminated the defects such as holes, cracks and unmelted particles in the plasma arc sprayed layer, and enhanced the coating. Density, the microhardness of the remelted layer is also significantly improved, and the anti-wear performance is about 2 times that of the original. Pan Liping et al[5] prepared a 2 mm thick NiCrBSi-WC12Co coating by supersonic flame spraying, and performed laser remelting to change the original lamellar structure of the coating, and the average friction coefficient was reduced from 0.65 to 0.55 , the mass loss is significantly reduced, and the wear resistance is significantly improved. However, due to the poor thermal shock resistance of the coating material, the rapid heating and cooling process in the laser remelting process makes the coating prone to cracks, and the gas in the molten pool is not easily discharged to form pore defects, so cracks, Voids and spalling are the main problems in the current coating laser remelting [6-7].

 

2 Electron beam remelting

 

    Electron beam remelting is the use of high-energy electron beams to bombard the surface of the workpiece, converting part of the kinetic energy into heat energy, so that the temperature of the coating instantly reaches above the melting point and melts, thereby refining the structure and improving the surface properties of the material. Since the electron beam remelting is carried out under vacuum conditions, it can effectively prevent the coating from being polluted by harmful gases such as hydrogen, oxygen, and nitrogen during remelting, and is beneficial to the degassing and purification of the coating. Therefore, it is especially suitable for chemical activity. Remelting treatment of high magnesium, aluminum and other substrate surface coatings.

    Weisenburger et al. [8] carried out electron beam remelting of CoNiCrAlY coating. After remelting treatment, the surface roughness of the flame sprayed layer decreased from 62 μm to 3.7 μm, and the surface roughness of the flame sprayed layer decreased from 47 μm to 8.0 μm. The pores in the thickness range of 30–40 μm on the coating surface were observed. Yu Bin et al. [9] prepared a silicide coating on the surface of Nb521 niobium alloy, and carried out electron beam remelting. The study showed that the ceramic grain size and surface roughness of the coating surface decreased after remelting treatment. The sintering effect is enhanced, the density is increased, and the oxidation resistance and thermal shock resistance of the silicide layer are improved.

 

3 Tungsten argon arc remelting

 

    Tungsten argon arc remelting is a method of remelting using a tungsten rod as an electrode under the protection of argon gas. During the remelting process, argon gas is ejected from the nozzle of the welding torch, creating an inert gas shield around the arc to prevent the detrimental effects of the air on the electrode and coating. Due to the excellent protective effect of argon gas during remelting of tungsten argon arc, the metallurgical reaction in the remelting zone is simple and easy to control, so a high-quality remelting layer can be obtained; Stable combustion, easy adjustment of heat input, especially suitable for remelting coatings on thin sheets and heat-sensitive materials. In addition, argon tungsten arc remelting operation is simple and the cost is low.

    Du Xiaodong et al. [10] prepared WC particle reinforced steel-based composite coating on 45 steel substrate. After tungsten argon arc remelting, the density of the coating was improved, the microstructure was fine and uniform, and it formed a good metallurgical relationship with the substrate. Combined, the cladding layer is composed of F, P and fine WC particles evenly distributed on it, and the structure is significantly improved. Chen Guishan et al. [11] prepared the alloy coating by arc spraying method, and then obtained the wear-resistant coating with metallurgical bonding by argon tungsten arc remelting. The surface hardness of the coating was increased to HV1200, which is 6～60% of the 20G substrate. 7 times, it is metallurgical bonding with the matrix, and the high temperature erosion resistance is 2.5 to 4 times that of the 20G steel matrix.

 

4 Flame Remelting

 

    Flame remelting is a method in which the spray coating is heated with a flame such as oxyacetylene, melted and then cooled and solidified to improve the performance of the spray coating. Compared with other remelting methods, flame remelting heat source temperature is lower, so it is suitable for remelting alloy coatings.

Dong Xiaoqiang et al. [12] prepared a NiCrBSi +w(WC) 15% coating by plasma arc spraying, and carried out flame remelting treatment. After remelting, the microstructure of the coating was significantly improved, with cracks, pores, unmelted particles Reduced, the grains are refined, and a hard phase is formed. After remelting, the hardness of the coating reaches HV800, and the wear performance is significantly improved. The flame remelting equipment is simple, easy to operate, and easy to operate on the construction site. However, because it is difficult to precisely control the heating depth and achieve uniform heating, the speed of the flame gun and the distance from the workpiece must be strictly controlled.

 

5 Induction Remelting

 

    Induction remelting is to use the alternating magnetic field in the induction coil to generate eddy currents in the workpiece. The skin effect of the eddy currents allows the heat to concentrate on the surface of the workpiece to melt the coating, thereby improving the structure and performance of the coating and the bonding between the coating and the substrate. Combined with the conditions, the induction remelting process is not only fast, but also has less influence on the matrix structure and mechanical properties.

    Zhang Hongxia et al. [13] prepared a Ni-based self-fluxing alloy coating on the surface of carbon steel, which was remelted by an intermediate frequency induction remelting process. After remelting, the Ni-based coating formed a good metallurgical bond with the substrate. There is an obvious diffusion transfer band between the substrate and the Ni coating substrate, and there are abundant hard phases distributed in the Ni coating substrate, which significantly improves the hardness and wear resistance of the coating.

 

6 Overall heating and remelting

 

    The overall heating and remelting is usually in a neutral or reducing atmosphere, heating the spray coating to a certain temperature in the liquid-solid phase range, at this time the coating is semi-solid, the material becomes dense and molten, so the metallurgical reaction This resulted in the deposition of a large amount of hard phases in the form of the coating mixture, which remained on the coating after cooling, providing excellent friction and wear resistance [14].

    Wang Shaoyun [15] and others studied the effect of in-furnace remelting process on the contact fatigue properties of NiCrBSi supersonic plasma arc sprayed coatings. The structure is dense, the micro-porosity is less, the interface between the coating and the substrate is well bonded, and the contact fatigue life is significantly improved.

 

7 Differences between various remelting processes

 

    It can be seen from the foregoing that remelting treatment shows great development potential in improving the structure and performance of the thermal spray coating, but in the specific implementation, it should be correctly selected according to the different characteristics and application scope of various remelting processes. The differences between various remelting processes are mainly reflected in the following aspects:

    (1) Applicable coating types Laser remelting and electron beam remelting both use heat sources with high energy density and are basically not limited by the types of coating materials. Induction remelting utilizes the skin effect, so the remelting effect depends on the relative permeability and resistivity of the coating. Flame remelting and bulk heating remelting are suitable for alloy coatings with lower melting points.

    (2) Applicable to different types of workpieces Laser remelting can be used to treat workpieces with complex surface shapes, which can selectively partially melt the surface coating of the workpiece, and can realize automatic production, but laser remelting can usually only process sheet metal, not Suitable for handling thick and large pieces. In electron beam remelting, a vacuum chamber must be provided, so the size of the workpiece is limited by the size of the vacuum chamber, and it is suitable for small samples or workpieces requiring precise remelting area. Gas tungsten arc remelting is difficult to remelt coatings on low melting point and evaporable metals (eg lead, tin, zinc). Flame remelting heats the entire workpiece to a higher temperature and is not suitable for thicker and larger workpieces. Induction remelting is limited by the shape and size of the induction coil and is only suitable for relatively small cylindrical workpieces.

    (3) Different effects on the workpiece: Laser remelting, electron beam remelting, and tungsten argon arc remelting have small heating area, concentrated energy, small heat-affected zone, and small deformation of workpiece after treatment. Induction remelting is an annular heat source, and the effect of temperature difference is relatively small, which can avoid cracks, peeling or excessive melting of the coating material in the remelting layer. The overall heating and remelting needs to put the whole workpiece into the heating furnace, and the substrate is greatly affected by the heat.

    (4) Laser remelting and electron beam remelting are not suitable for operation on the construction site due to the complicated operation and maintenance of the equipment and the large size of the equipment, and the damage to the human eye caused by the laser in the laser remelting Therefore, there must be reliable and safe protection facilities. Flame remelting and argon tungsten arc remelting are easy to operate, simple equipment, high degree of automation, suitable for on-site operation, low cost, and meet the needs of large-scale production. Induction remelting has fast heating speed, high power density, and easy control of time, temperature and heating depth. The overall heating and remelting is very suitable for mass production, and the operation is simple and easy.

 

8 Conclusion and Outlook

 

    In conclusion, the remelting treatment can effectively improve the structure and properties of the thermal spray coating, thereby significantly improving the service life of the parts. Today, when green manufacturing, low-carbon economy and sustainable development have increasingly become the development consensus of all countries in the world, the remelting technology of thermal spray coating will be more and more widely used. 

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