High-end aluminum alloy components are widely used in aerospace, petrochemical, hydrogen energy storage and transportation, etc., but their surfaces have defects such as poor corrosion resistance and wear resistance, low strength and hardness, grain coarsening and surface orange peel, which in turn limit their high-performance and safe service in extreme environments. Therefore, it is extremely important to improve the surface quality and service performance of aluminum alloy components. Laser cladding is an effective technology for surface modification and repair of aluminum alloy components. By selecting suitable cladding materials to clad the surface of aluminum alloy components with wear-resistant, corrosion-resistant and high-strength coatings, it can effectively solve the problems of poor surface performance and short service life of aluminum alloy components. Due to the special physical and chemical properties of aluminum alloys, such as low melting point, high reflectivity, poor wettability, high dilution rate, and easy occurrence of pores, cracks and other problems, laser cladding on the surface of aluminum alloy components brings challenges. Therefore, in view of the technical difficulties faced by high-end aluminum alloy surface laser cladding high-performance coatings, with the goal of improving the service strength, wear resistance and corrosion resistance of aluminum alloy components, the basic theory, preparation method, coating structure, service performance and application prospects of high-performance coatings on aluminum alloy surface laser cladding are comprehensively analyzed. This study provides a theoretical reference for the preparation of high-performance coatings by laser cladding on the surface of complex aluminum alloy components.
Aluminum alloys are widely used in many fields such as biomedicine, navigation industry and missile shells due to their advantages such as light texture, good plasticity and good thermal conductivity. However, the disadvantages of low hardness, easy oxidation and poor wear resistance of aluminum alloys limit their application areas. The economic losses caused by aluminum alloy corrosion are as high as hundreds of billions of yuan each year. Therefore, research on aluminum alloy surface modification has been favored by many scholars. In recent decades, the high energy and fast speed of the laser beam are used to better metallurgically combine the substrate and the modified material. After the substrate is melted and rapidly solidified, fine grain strengthening and large-angle grain boundaries are usually generated to improve the substrate hardness and wear resistance.
Laser cladding is a process of enhancing or repairing the surface of a substrate, using a laser to deposit different material layers on the surface of the substrate. The presence of the cladding layer can improve the surface performance of the material, and it can also play a great role in the surface repair of the material, and can obtain mixed powder coatings and gradient high-thickness coatings. Therefore, laser cladding technology is widely used in additive forming, dissimilar material connection, crack repair and remanufacturing, etc. However, some high-strength aluminum alloys have poor additive manufacturing performance, which will cause thermal cracks, bubbles, coarse grains and other phenomena during material processing. The shortcomings of aluminum alloys such as low laser beam absorption, high thermal conductivity, high thermal expansion coefficient, pilling and porosity limit their application areas. Aluminum alloy powders will undergo phase segregation, anisotropic grain growth and thermal tearing at high solidification rates.
At present, there are relatively few studies on the preparation of corrosion-resistant and wear-resistant coatings by laser cladding on aluminum alloy surfaces. The evolution and distribution of the coating surface organization are extremely complex. The defects of low melting point of the substrate, high reflectivity, poor wettability, high dilution rate, and easy pores and cracks in the coating during the cladding process have not been completely solved, which brings great challenges to the laser cladding of aluminum alloys. Based on this, in order to comprehensively study the wear-resistant and corrosion-resistant coatings on the surface of aluminum alloys and analyze the key technologies of laser cladding, this paper comprehensively analyzes the research status of aluminum alloy laser cladding coatings, the problems existing in the cladding process, the key technologies of aluminum alloy laser cladding, the laser cladding organization and the performance of laser cladding coatings, etc., to provide theoretical reference for the preparation of high-performance coatings by laser cladding on the surface of complex aluminum alloy components.
2 Research Status of Aluminum Alloy Laser Cladding Coatings
At present, many famous scholars at home and abroad use laser cladding technology to modify the surface of aluminum alloys to improve the wear and corrosion resistance of the substrate. Laser cladding aluminum alloy coatings can be roughly divided into
anti-oxidation cladding layers, corrosion-resistant coatings, laser wear-resistant cladding layers, laser biocladding layers, and laser cladding metal ceramic layers. With the research on laser cladding coatings on aluminum alloys, metal powders such as Cu, Ni, Fe, Co, Al, as well as nickel-based alloys, copper-based alloys and ceramic powders can be used as aluminum alloy surface cladding materials to modify the aluminum alloy surface to improve the substrate hardness, wear resistance, corrosion resistance and oxidation resistance. Li Qi et al. [19] used laser cladding technology to prepare NiCrAl/TiC composite coatings, conducted electrochemical corrosion tests, and analyzed the SEM images after corrosion, as shown in Figure 1. As shown in Figure 1, the surface of the A390 substrate is severely corroded, showing a honeycomb structure and cracks on the surface, and the cladding layer only has inconspicuous pitting. It can be concluded that the presence of cladding coatings on the surface of aluminum alloys can increase the corrosion resistance of the substrate. Zhang Pengfei et al. prepared Ti/TiBCN composite coatings on the surface of 7075 aluminum alloy substrates, and used MFT-R4000 reciprocating friction and wear testing machine to conduct friction tests on the substrate and the prepared coatings, and observed the metallographic microstructure after friction, as shown in Figure 2. As shown in Figure 2, the aluminum alloy substrate has low hardness, and the surface is rough and uneven after the friction test, with deep furrows, which is a typical plowing wear, while the coating surface is less affected, so the laser cladding coating shows strong wear resistance.
JIANG et al. used laser cladding technology to focus ion beam and micro-mechanical arm to prepare the AA7075 aluminum alloy cladding layer in situ for surface repair of aircraft structural materials. The cladding layer was tested by transmission electron microscopy (TEM), as shown in Figure 3. The characterization results show that there are different types of coarse phases at the grain boundaries, and the density of nanoparticles in the heat-affected zone is low, resulting in reduced strength.
3 Problems in laser cladding of aluminum alloys
The surface of aluminum alloys is modified by laser cladding, which greatly improves the performance of aluminum alloys. The absorbance during cladding will affect the quality of the coating. Usually, black has better absorbance, and most laser cladding substrates are also black, but the aluminum alloy substrate is silver-white and has poor absorbance, which affects the quality of the coating. At present, laser cladding with aluminum alloy as the substrate still faces many problems.
(1) Aluminum alloys are not easy to melt. Aluminum alloys have strong electronegativity and the surface is easily oxidized to form intermetallic oxides. The oxides have high hardness and melting point and are not easy to melt. In addition, the negative mixing enthalpy of Al with other elements leads to the formation of brittle oxides, which promotes the initiation of cracks under the action of thermal stress, resulting in reduced coating quality.
(2) The surface of aluminum alloy is easy to reflect light, and the laser transmittance is low. According to the Fresnel equation, the laser reflectivity incident on the surface of aluminum alloy material is expressed as: See formula (1) in the figure
Where E*11 —— reflected laser energy on the radial polarization component (J);
E11—— incident laser energy on the radial polarization component (J);
φ1——incident angle (°);
δ——conductivity of the medium (S/m);
γ——angular frequency of the laser (rad/s).
In aluminum alloy, σ/γε0≥1, and most of the laser is incident vertically on the surface of the base aluminum alloy during the cladding process, then formula (1) can be expressed as see formula (2) in the figure
Where B——surface transmittance of aluminum alloy;
A——absorption rate of aluminum alloy [L/(g/cm)].
During the laser cladding process, the surface of aluminum alloy material can only absorb a small part of the energy, and the utilization rate of laser is low, which seriously affects the cladding effect. The reflectivity of aluminum alloy to CO2 laser with a wavelength of 10.6μm is as high as 96.9%. The quality of aluminum alloy laser cladding layer is related to the cladding material itself, and the parameters set during laser cladding, such as laser power, defocus rate, scanning speed, etc., will affect the coating performance.
(3) Pores are easily formed during aluminum alloy cladding. Due to the fast thermal conductivity and high thermal expansion coefficient of aluminum alloy, it deforms greatly when it is heated and melted rapidly under the action of laser beam. During the rapid cooling process, pores are formed because the gas does not have time to escape. When the expansion coefficient and wettability of the cladding material are greatly different from those of the aluminum alloy, cracks are easily generated at the coating interface, affecting the coating quality.
4 Key technologies for aluminum alloy laser cladding
At present, the commonly used laser cladding methods are to pre-set the cladding material coating and then perform laser cladding and synchronize cladding of cladding powder material and laser, as shown in Figure 4. The synchronous powder feeding method is easy to realize automatic control and is suitable for industrial production. It also has the advantages of good cladding quality and high energy absorption rate. Shen Yuwei used a high-power semiconductor laser to laser clad the surface of 5052 aluminum alloy substrate to obtain an Al-Si alloy coating with a surface hardness of more than 900HV. WANG et al. [25] used selective laser melting to prepare particle-reinforced aluminum-based composite materials, combining aluminum matrix and reinforcement. The structural optimization of complex components was applied to mold manufacturing, aerospace industry and automobile industry, as shown in Figure 5.
5 Performance of aluminum alloy laser cladding coatings
The corrosion-resistant and wear-resistant coatings on the surface of aluminum alloys are mainly Ni-based, Co-based, TiC or metal-ceramic composite materials, which have good corrosion resistance and wear resistance. Among them, the ceramic cladding layer has relatively excellent corrosion resistance, wear resistance, good chemical stability and other properties, which can greatly improve the surface hardness and wear resistance of the substrate, but its brittleness is weak. Under the action of high-power lasers, laser cladding technology can be used to form aluminum alloy/ceramic composite coatings with high hardness, strong corrosion resistance and wear resistance, and certain toughness, and further modify the surface of the material. YUE et al. used excimer laser technology to modify the surface of AA7075 aluminum alloy. As shown in Figures 6 and 7, the TEM/electrochemical corrosion test results show that two layers of dense aluminum oxide films are formed in the laser melting zone and the coarse second phase particles are eliminated; the polarization resistance of the laser-treated sample is one order of magnitude higher than that of the untreated sample, and the capacitance is about 6 times lower, which can effectively enhance the corrosion resistance of the substrate.
6 Conclusion
Using advanced laser cladding technology and selecting suitable cladding materials to form aluminum oxide, refine grains, change dislocation density, and modify the surface of aluminum alloys, it can enhance its strength, hardness, corrosion resistance and wear resistance, so that aluminum alloys can be used more widely and more efficiently in industrial production. However, some characteristics of aluminum alloys, such as low melting point, high reflectivity, poor wettability, high dilution rate of cladding layer, and easy appearance of pores and cracks, cannot meet industrial needs.
In view of the problems faced in aluminum alloy laser cladding, the contents that need to be explored at present mainly include the following aspects.
1) Development of laser process parameters and exploration of auxiliary technologies. The settings of process parameters such as laser power, defocus rate, and scanning speed all affect the performance of laser cladding coatings, so it is crucial to establish a window of aluminum alloy cladding process parameters.
2) Design of cladding materials. Combined with the research on other substrate surface cladding materials, a composite aluminum alloy surface cladding material system was explored. By adding an intermediate layer material transition, adding a light absorber to reduce the reflectivity of the laser, and adding a binder to enhance the metallurgical bonding between the substrate and the powder.
3) At present, the research on laser cladding coatings on aluminum alloy surfaces lacks research on the mechanism and regularity of organizational changes. With the development of lasers, more and more researchers have made outstanding contributions to laser cladding on aluminum alloy surfaces. Using laser cladding technology to modify the surface of aluminum alloys, improve surface properties, and perform surface repair will be a development trend.
