As an emerging surface treatment technology, laser cladding technology will inevitably produce some defects during the cladding process. The appearance and expansion of these defects restrict the further development of laser cladding technology. This paper introduces the core mechanism of crack and pore formation during laser cladding, and summarizes a variety of measures to suppress defects, including optimization of process parameters, substrate preheating and post-treatment, element addition, and application of auxiliary processes, etc., providing a theoretical reference for the further development and application of laser cladding technology in the future.

Laser cladding technology is a surface modification and processing technology [1-3]. A high-energy laser beam is used to form a molten pool on a metal substrate, and the cladding powder is placed on the substrate through different addition methods for melting. After rapid solidification, the cladding material forms a metallurgical bond with the substrate [4]. Compared with other surface modification technologies such as electroplating, thermal spraying, electrospark machining and laser cutting, laser cladding has the advantages of small thermal deformation of the substrate, narrow heat-affected zone, and fast cooling speed of the molten pool [5], and its application scenarios are very wide. In the field of aerospace, due to the harsh working environment, the blades of aircraft engines are often subjected to various impacts. In order to obtain high-quality coatings and repair the casting defects of aircraft engine blades, Kong Fanli et al. [6] performed laser cladding repair on aircraft engine blades by changing the content of Y2O3. It was found that when the content of Y2O3 was 1.5wt%, the repair effect was the best. In tunnel excavation, the TBM cutting machine is in direct contact with the rock, resulting in huge wear. In order to improve the wear resistance of the cutter ring and extend the service life through surface strengthening treatment, Hu et al. [7] performed laser cladding on the cutter ring of 5Cr5MoSiV1 tunnel boring machine after heat treatment. The study found that the wear resistance of the coating after cladding was 7 times that of 5Cr5MoSiV1, which greatly improved the wear resistance of the cutter ring. In the rolling mill industry, Li Yun et al. [8] used laser cladding technology to prepare Fe-based coatings and found that while repairing the rolling mill, it could also extend its service life. In terms of machine tool remanufacturing, the key is to remanufacture the damaged spindle. Han Yuyong et al. [9] conducted a laser cladding experiment on a 45 steel lathe spindle in service (Figure 1). By comparing the surface quality of the repair layer of plasma spraying and oxyacetylene flame spraying, they found that laser cladding is the most suitable method for lathe spindle remanufacturing. In addition, laser cladding technology is widely used in the automotive industry and mold industry, such as automobile engine valve production, rolling mill guide plate production, etc.

Correspondingly, there are extensive studies on the basic theory, equipment development, process optimization, and industrial application of laser cladding technology. Among many studies, cladding quality has always been an important issue of concern to researchers. Macroscopic quality evaluation is one of the criteria for judging the quality of the cladding layer. Macroscopic quality means that the cladding layer should have a smooth surface and no defects such as cracks and pores on the surface [10]. Therefore, the study of cladding defects has always attracted extensive attention from researchers at home and abroad. There are many factors that affect the entire cladding process. The unique rapid heating and solidification characteristics of laser cladding often inevitably produce certain defects. Studying the formation mechanism and suppression measures of defects is of great significance to optimizing laser cladding technology. Common cladding defects include cracks, pores, unfused, solid inclusions, etc. This article focuses on analyzing the formation mechanism of common cladding defects such as cracks and pores, and summarizes measures to suppress defects from different perspectives. Provide a certain theoretical reference for the future research and application of laser cladding technology.

1 Laser cladding crack formation mechanism and suppression measures

1.1 Crack formation mechanism

Laser cladding is a process of rapid heating and rapid cooling. Both heating and cooling may cause cracks [11]. In addition, the performance of the substrate material and the cladding material is very different, which directly leads to residual stress in the cladding layer and between the cladding layer and the substrate. When the residual stress is greater than the tensile strength of the cladding layer, stress concentration is likely to occur, resulting in cracking of the cladding layer [12] (see Figure 2). Residual stress can be divided into thermal stress, structural stress and constraint stress [13]. Thermal stress is the main factor in the formation of cracks [14]. The calculation formula is: See formula (1) in the figure

Wherein, ∆ is thermal stress; E1 is the elastic modulus of the cladding layer; E2 is the elastic modulus of the substrate; ΔТ is the difference between the processing temperature and the room temperature; μ is the Poisson’s ratio; α1 and α2 are the thermal expansion coefficients of the cladding layer and the substrate layer respectively; h2 is the substrate thickness.

From the formula, it can be concluded that when α1 ≥ α2 When α1 < α2, the cladding layer will be subjected to tensile stress, resulting in increased crack sensitivity; when α1 < α2, the crack sensitivity decreases.

Huang Zhe et al. [18] performed variable parameter laser cladding to prepare GH3536 coating, and cracks appeared. After analysis, it was found that there were many tiny holes on the austenite grain boundaries in the laser cladding layer. Holes with close distances were prone to form cracks, and tiny cracks in the same area and in the same direction would connect and expand to form micro cracks, as shown in Figure 3 (a). In addition, due to the unique rapid solidification characteristics of laser cladding, different dendrite structures produce stress concentration at the intersection during the solidification process, thereby generating crack sources, as shown in Figure 3 (b). When the cracks expand along the dendrite growth direction, inter-dendritic cracks will be formed, as shown in Figure 3 (c); some cracks will expand around the dendrites, forming circumgranular cracks, as shown in Figure 3 (d). Feng et al. [19] used laser cladding to prepare four Fe alloys with high hardness and crack sensitivity. The crack resistance test of the Ni60 coating was carried out. It was found that under laser scanning, the metal on both sides of the Y-shaped groove expanded thermally. When the temperature dropped, the cladding metal contracted accordingly. However, since the shrinkage of the substrate was restricted, constrained stress was generated, and then cracks occurred in the coating. Yu Ting et al. [20] used NiCrBSi self-fluxing alloy powder for cladding on a medium-carbon steel plate and found that even if the thermal expansion coefficients of the substrate and the coating were reduced, the cracking of the Ni60 coating could not be avoided. Further research found that the Ni60 nickel-based coating contains a large amount of primary hard phases that are subjected to microscopic compressive stress, while the substrate is subjected to microscopic tensile stress; in addition, the molten pool is subjected to macroscopic tensile stress during the solidification shrinkage process. Under the combined action of the two, the coating is prone to cracks. Therefore, the main reason for the generation of cracks is related to the residual stress inside the material after laser cladding. When this residual stress is greater than the tensile strength of the cladding layer, stress concentration is easily generated at pores, inclusions, tips, etc., resulting in cracking of the cladding layer [21].

Cracks can be divided into interface cracks, cladding layer cracks and overlap zone cracks according to the location of the cracks [22] (see Figure 4). The most common interface cracks are mainly formed at the interface between the cladding layer and the substrate [23]; cladding layer cracks refer to cracks caused by solidification during the cladding process; overlap zone cracks mainly occur near the overlap zone. Among them, interface cracks and cladding layer cracks can be attributed to the large difference in thermal expansion coefficients between the substrate material and the cladding material, and the uneven distribution of temperature and stress; overlap zone cracks are caused by the unreasonable overlap rate in the process parameters, which generates crack sources and is caused by various stresses.

1.2 Crack suppression measures

1.2.1 Process parameter optimization

The process parameters of laser cladding have a direct impact on the quality of the cladding layer. By optimizing the laser cladding process parameters, the temperature gradient during the cladding process can be controlled, thereby inhibiting the generation of cracks. Common process parameters that can be optimized include laser power, scanning speed, powder feeding rate, etc. Zhao Shuguo et al. studied the laser cladding CBN film layer on the surface of TC11 and found that the crack rate first decreased and then increased with the increase of laser power or scanning speed. When the laser power reached 1.8 kW or the scanning speed was 4 mm/s, the crack rate reached the lowest; while with the increase of powder feeding rate, the cracks first increased and then decreased. When the scanning speed was 3 mm/s and the powder feeding rate was 1 r/s, the crack rate was the lowest. Yao Fangping et al. [26] used simulation software to numerically simulate the temperature field and stress field under different laser powers to find the optimal laser power for preparing Ni-based coating on the surface of H13 steel. The results showed that when the laser power was 1.4 kW, the cladding layer had good quality and there were no obvious cracks or other defects in the cross section. Khorram et al. [27] used the laser cladding CBN film layer on Inconel steel to prepare Ni-based coating on the surface of H13 steel. Amdry 997 powder was used for laser cladding on 713 high-temperature alloy, and a crack-free coating with a scanning speed of 140 mm/min was obtained.

In general, by precisely controlling the laser cladding process parameters, effective control of cracks can be achieved, thereby improving the stability and reliability of the laser cladding process and providing high-quality cladding layers for various application fields. However, different materials and application scenarios may require different parameter optimization strategies, so it is necessary to go through multiple tests to find the best process parameters, and design and adjust the process according to the specific situation.

1.2.2 Auxiliary process treatment

Auxiliary process treatment usually includes substrate preheating and laser remelting. Substrate preheating is to preheat the substrate to reduce the temperature difference between it and the cladding layer, thereby reducing thermal stress; laser remelting is to perform secondary melting and solidification of the molten pool to heal cracks and reduce defects. Wang Ran et al. used mixed Al2O3-ZrO2 powder to perform laser cladding on titanium alloy (Ti-6Al-4V), using different temperatures for substrate preheating (see Figure 5) After comparative analysis, it was concluded that when the preheating temperature reached 200-300°C, the residual stress of the cladding layer was reduced most significantly, which significantly improved the crack sensitivity and inhibited the occurrence of cracks. Wang et al. [29] used different temperatures to preheat the substrate and prepared Fe-based amorphous coating on the surface of H13 steel. The results showed that too high or too low preheating temperature could not inhibit the occurrence of cladding cracks. When the preheating temperature reached 250°C, the cladding cracks disappeared, and the cladding layer quality was the best at this time. Wang Huizhao et al. [30] used Al2O3-ZrO2 (8% Y2O3) mixed powder to clad TC4 The alloy surface was then laser remelted for post-treatment of the cladding layer. It was found that the residual stress of the remelted coating was significantly reduced, and the fracture toughness was improved, thereby improving the sensitivity of cracks. In summary, the essence of substrate preheating is to reduce the temperature gradient, and the purpose of laser remelting post-treatment is to reduce or even eliminate the residual stress inside the organization. As auxiliary process treatment methods, the two have a significant effect on inhibiting cracks in laser cladding technology. It emphasizes the importance of appropriate temperature control and post-treatment steps in practical applications to reduce crack sensitivity and improve the quality of cladding coatings.

1.2.3 Doping element method

Rare earth elements such as cerium (Ce) and yttrium (Y) have relatively active chemical properties. Adding an appropriate amount of rare earth elements and their oxides to the cladding powder can refine the grains, improve the fluidity of the melt, reduce the surface tension of the molten pool, improve the surface roughness of the cladding layer, optimize the organization and improve the performance.

Zhang Xinjian et al. [32] studied the effect of different Y2O3 contents on Ni60 The influence of CeO2 on the coating structure and performance, the experiment found that active yttrium ions as a purifying agent can remove harmful impurities such as sulfur and phosphorus generated during the cladding process, thereby obtaining a crack-free coating. When the Y2O3 content reaches 1.0wt%, the cladding layer quality is the best. Chen Shungao et al. [33] used a laser cladding multi-pass overlap process to prepare Ni60 coatings containing different CeO2 contents on 45 steel in order to study the effect of CeO2 on the performance of the cladding layer. It was found that adding suitable rare earth oxides can inhibit the generation of coating cracks. When the CeO2 mass fraction is 0.4%, the coating quality is the best, and the cracks are the least. Niu Liyuan et al. [34] added different contents of rare earth oxide CeO2 to Co-based alloy powder, and used laser cladding technology to prepare Co-based alloy coatings on the surface of 316L stainless steel substrates. Through research and comparison, it was found that after adding CeO2, the macro cracks on the cladding surface disappeared in large numbers. With the increase of rare earth oxide content, the surface smoothness of the cladding layer improved; but when the CeO2 content is When the content of WC is 2.5%, a few shrinkage cavities appear on the surface of the cladding layer. Therefore, adding an appropriate amount of rare earth elements and rare earth oxides can purify the molten pool, improve the quality of the cladding layer, and inhibit the generation of cracks.

1.2.4 Other processes

Wu Chengmeng et al. [35] used Ni60+35%WC powder as the coating material, and placed 316 stainless steel mesh as a high-temperature relaxation softening zone on the surface of 45 steel for cladding (see Figure 6). By comparing and observing the crack generation of 316 stainless steel mesh, it was found that the high-temperature relaxation softening zone can absorb a certain residual stress on the coating surface and effectively inhibit the generation and expansion of coating cracks. Qi et al. [36] prepared a Co-based cladding layer on a 42CrMo substrate and added a magnetic field during the cladding process. The study found that in magnetic-assisted cladding processing, a small magnetic field can produce a large magnetostrictive effect. When the magnetic induction intensity reaches 20 mT, the magnetostrictive effect can be effectively inhibited. The magnetostrictive effect is the largest when the magnetostrictive effect is applied. It can reduce the difference between the thermal expansion coefficient and elastic modulus between the substrate and the cladding material, thereby reducing thermal stress. Liu Hongxi et al. [37] used a mechanical vibration-assisted process to prepare a Fe-Cr-Si-B-C alloy coating on the surface of 45 steel. By comparing the cladding layers with and without mechanical vibration, it was found that when the amplitude A = 0.28 mm and f = 200 Hz, the number of cracks per unit length under the mechanical vibration-assisted process was only 0.06 line/mm; while the number of cracks without mechanical vibration reached 0.32 line/mm; in addition, it was found that the residual stress between dendrites was reduced under mechanical vibration, and the cracking sensitivity was significantly reduced. Therefore, the mechanical vibration-assisted process can significantly inhibit the generation of cracks.

In summary, the generation of cracks is the result of the combined effects of several stresses on the coating during the cladding process. Through continuous exploration, researchers have found that measures such as optimizing process parameters, preheating and remelting the substrate, adding rare earth elements and oxides, and auxiliary process treatment can inhibit the generation and expansion of cracks to varying degrees. In addition, the generation of cracks can be suppressed by applying high-frequency micro-forging assistance, ultrasonic vibration technology, numerical simulation, finite element simulation, etc.

2 Causes of pore formation in laser cladding and measures to suppress it

2.1 Causes of pore formation

Pore is another common defect in the laser cladding process. The formation of pores can also affect the quality of the cladding layer and its corrosion and wear resistance [38]. The causes of pore formation are very complex. After years of research, researchers at home and abroad generally divide pores into three types, namely, pores, keyholes and unfused pores [39], as shown in Figures 7 and 8. Porosity is relatively common. If the metal powder is damp or undergoes a chemical reaction at high temperature, it is easy to generate gas during the cladding process. If it cannot escape in time, pores will be generated; in addition, when the airflow is too large, turbulence will be generated around the molten pool to draw air in. Due to the extremely fast cooling rate, the gas will solidify and crystallize before it can escape, thus forming pores [40]. The main factor affecting pore formation is the aspect ratio of the molten pool, followed by the convection intensity and convection time of the molten pool [41]. Yan et al. [42] used NiCuFeBSi Alloy powder was laser clad on HT250 and 45 steel respectively, and it was found that 45 steel produced fewer pores. After analysis, it was found that HT250 contained graphite and had a higher carbon content than 45 steel. C combined with oxygen in the air to form CO. In addition, when the temperature reached above 1673 K, SiO2 in cast iron and C underwent an oxidation-reduction reaction, producing more CO gas in the molten pool, which prevented the gas from escaping from the molten pool and formed pores. Shen Zehui et al. [43] prepared nickel-based coatings with different WS2 contents on the surface of titanium alloy (see Figure 9) and found that as the WS2 content increased, the pores of the cladding layer increased. This is because WS2 decomposes at high temperatures, and S atoms combine with oxygen to form SO2 gas. The gas in the molten pool has no time to be discharged, which will produce pore defects.

Keyholes are pores formed by the collapse of the molten pool caused by laser impact, which prevents the air or protective gas in the molten pool from escaping. They generally occur in laser cladding of aluminum alloys [46]. Park et al. [47] used different energy inputs to laser deposit pure V powder on Ti-6Al-4V plates and found that in single-layer scanning, the evaporation temperature of vanadium oxide was lower than that of pure vanadium, resulting in a large number of pores formed inside the vanadium oxide. The pores were spherical in shape, indicating that the pore type was keyholes. Unfused pores (see Figure 7) refer to pores formed when the metal powder was not completely melted during the laser cladding process. Li Deying et al. [48] prepared Fe-Al-Si composite coatings on the surface of Q235 steel and compared them with Fe-Al coatings. It was found that the Fe-Al coatings had pore defects, which were caused by the incomplete fusion of iron powder and aluminum powder and the generation of some gas.

2.2 Porosity suppression measures

2.2.1 Process optimization

Yang Xing et al. [49] used different laser powers to prepare FeCoNi CrMo high entropy alloy cladding layers and found that when the laser power was 1.0~1.6kW, the cladding layer had no defects such as pores. Li Hao et al. [50] prepared Cu-10Pb-10Sn cladding layers on the surface of Q235 steel to study the effect of laser power on the porosity of the cladding layer. The study found that with the increase of laser power, the porosity of the cladding layer would first decrease and then increase. The selection of appropriate laser power parameters can play a role in suppressing the generation of pores. The porosity was lowest when the laser power was 1000 W. Shen Hao et al. [41] studied the effect of laser power and scanning speed on NiCo CrAlYSi cladding layers and found that when the laser power was constant, the porosity decreased with the increase of scanning speed. When the laser power and scanning speed were considered comprehensively, when the laser energy input ratio reached 36 J/mm2, the porosity decreased. When the porosity of the cladding layer surface is the lowest, the cladding layer quality is the best. Therefore, optimizing the laser cladding process parameters is the simplest and most intuitive measure to suppress porosity defects.

In many studies that suppress pore formation by optimizing process parameters, the optimization of laser power is crucial to reducing porosity. By precisely controlling the laser power, the generation of pores can be effectively reduced and the quality of the cladding layer can be improved.

2.2.2 Adding element method

Li Qiang et al. [51] added an appropriate amount of In2O3 to the nickel-based tungsten carbide composite alloy powder to prepare a cladding layer without pore defects on the surface of A3 steel by laser cladding technology. After analysis, it was found that an appropriate amount of In2O3 can inhibit the decomposition of WC, thereby reducing the generation of CO or CO2 gas formed by the combination of C and O, thereby inhibiting the generation of pores. When the In2O3 content is 2.0%, a cladding layer without pore defects can be obtained. Wang Huiping et al. [52] added 0.6% Y2O3 to prepare TiC composite coating has the lowest porosity and the best cladding quality. Therefore, adding some rare earth elements and their oxides can make the coating structure more uniform and purify impurities in the grain boundaries. In addition, adding some metal elements that are prone to oxidation reactions can also inhibit the generation of pores. For example, Yamaguchi et al. [53] studied the effect of aluminum addition on the porosity of WC-Co cladding microspheres. WC-Co alloy powder was laser clad on the surface of AISI 304 stainless steel. Before cladding, aluminum was locally added to the substrate surface using laser alloying technology. The results showed that aluminum and its oxides can inhibit the generation of pores. This is because aluminum has a strong affinity for oxygen. Compared with carbon oxidation, aluminum oxidation occurs first, that is, aluminum oxidation can eliminate free oxygen atoms in the melt, prevent the generation of CO gas, and eliminate decarburization.

2.2.3 Laser remelting

Wu et al. [54] studied the effect of different remelting powers on Stellite 6/WC composite coating forming quality, it was found that the laser remelting process can reduce defects such as pores, and the quality of the cladding layer is best when the remelting power is 1.4 ~1.5 kW. As the laser remelting power increases, pores will appear at the bottom of the cladding layer. This is because excessive laser energy will dissolve the WC particle powder, and the free carbon atoms will undergo oxidation reaction to form CO or CO2. When the gas does not have time to escape, pores will form. Zhang Qiang et al. [55] used 316L stainless steel powder with SiC particles added as the cladding material, and performed a repair method of cladding and remelting on the automobile transmission shaft material. As shown in Figure 10, the experiment showed that remelting can reduce the porosity of the cladding layer surface. Even if new pores are generated every time remelting, the porosity of the previous cladding layer will be reduced. Therefore, the porosity of the cladding layer decreases from the bottom to the top, thereby playing a role in inhibiting the generation of pores.

2.2.4 Auxiliary process

Hu Yong et al. [56] used electromagnetic assisted processing to apply a steady-state magnetic field on both sides of the substrate during the cladding process, and applied a stable direct current at both ends. The directional Lorentz force obtained by this electromagnetic composite field was used to suppress the generation of pores. The study found that when the Lorentz force is downward, as the magnetic field strength increases, the fluidity of the pores is better, and a pore-free cladding layer can be obtained when the magnetic field strength is 0.6 T. Yu Benhai et al. [57] used a homemade electromagnetic stirring device to assist laser cladding to prepare WC-Co alloy coatings. When the magnetic field rotates, Lorentz force is generated, causing the gas in the molten pool to move upward until it escapes, thereby suppressing the generation of pores. In addition to using electromagnetic composite fields to reduce the porosity of the cladding layer, Xu Lifeng et al. [58] also used pulse current enhanced laser cladding technology to reduce porosity, as shown in Figure 11.

In summary, pores are the most easily generated defect in the pore formation mechanism. There are many measures to inhibit the formation of pores, the most common of which are process optimization, element addition, and auxiliary application of electromagnetic composite field. In addition, unlike traditional laser cladding, laser induction hybrid rapid cladding (LIHRC), pulse current enhanced laser cladding, etc. can also produce coatings without pore defects.

3 Summary and Outlook

As a new type of surface modification technology, laser cladding has a wide range of engineering applications, but the complexity of its forming process and the characteristics of rapid heating and solidification will inevitably cause various defects in the cladding layer. The main mechanism of crack formation involves factors such as the difference in various properties of the substrate and the cladding material, and the stress concentration generated during the solidification process; the main reason for pore formation is that the gas in the air during the rapid solidification of the molten pool or the gas formed during the cladding process has no time to escape. In order to inhibit the cracks and pore defects generated during laser cladding, the formation and expansion of common defects can be suppressed to a certain extent by optimizing the laser cladding process parameters, adding elements to the cladding material, preheating and post-treatment of the substrate, and applying auxiliary processes.

Laser cladding technology is an excellent surface modification technology, but the problems of molten pool control and defect control have not been completely solved. In the future, advanced sensing technology and artificial intelligence algorithms can be used to achieve real-time monitoring and automatic control of the laser cladding process; by establishing a multi-scale numerical model, the temperature and thermal stress in the laser cladding process can be more accurately predicted; larger-sized laser cladding equipment can be developed and production efficiency can be improved to meet the needs of large-scale industrial applications. In short, laser cladding technology has great potential in the field of surface treatment, but it still needs to continuously overcome application difficulties. Future research will focus on multiple aspects such as process, control and production efficiency. With the in-depth study of basic theory and engineering application, it is expected to obtain a higher quality cladding layer.