With the increasing requirements for the surface service performance of parts in the field of high-end manufacturing, in addition to the direct strengthening process of the material surface, the coating is usually prepared to improve its surface performance. The high-hardness and wear-resistant coating formed by laser cladding is one of the effective means of coating strengthening at present. It forms a thin layer of metallurgical bonding with the substrate, and has strong resistance to indentation and mechanical friction locally. Laser cladding has become a current research hotspot and is gradually being promoted and applied internationally due to its high energy density, high deposition efficiency, controllable preparation thickness, low thermal deformation, rapid cooling, low dilution rate and high alloying degree. The article first summarizes the strengthening mechanism of laser cladding to improve the hardness of materials, including grain refinement, lattice distortion and phase change, but due to the limitations of laser cladding technology itself, the coating is prone to defects; then summarizes other surface strengthening technologies, such as ultrasonic rolling technology, heat treatment, etc., to form a composite strengthening technology of laser cladding technology, which can further improve the surface hardness of materials and achieve better performance.
In the field of high-end manufacturing, the surface performance of parts and components directly affects the performance and service life of the products. The wear of equipment parts often causes additional time and economic cost losses. In order to improve the surface performance of mechanical parts, surface strengthening treatment is usually used to improve the surface performance and service life of the material. In addition to directly strengthening the surface of the material, coatings are usually prepared on the surface. For example, in metallurgy, petrochemical and other fields, thermal spraying is usually performed on the surface of the material to increase the service life. However, due to the obvious shortcomings of thermal spraying technology, it is impossible to effectively improve the surface mechanical properties of the material. In addition, when special coatings are prepared by magnetron sputtering, electroplating and chemical vapor deposition, they often have shortcomings such as limited thickness and poor adhesion.
Laser cladding technology is an emerging metal surface treatment technology, which is often used to prepare high-hardness and wear-resistant coatings with strong bonding and few defects. As a chip-free and efficient three-dimensional forming technology, laser cladding has been successfully applied to the green manufacturing and remanufacturing of some high-end engineering parts.
1 Principle of process enhancement
Laser cladding is to melt a thin layer of the substrate surface at the same time by laser irradiation in different feeding methods on the substrate surface to be clad, and then form a surface coating with extremely low dilution and metallurgical bonding with the substrate after rapid solidification. The characteristics of this technology are high power density, high deposition efficiency, low thermal deformation, rapid cooling, low dilution rate and high metallurgical properties. The principle of laser cladding is shown in Figure 1 [1], and the actual picture is shown in Figure 2.
During the laser cladding process, the laser beam will be concentrated on a small area on the surface of the material, causing it to heat up rapidly to the melting point of the material or higher. At high temperature, the substrate and the cladding material form a molten pool, and the atoms and molecules of the material begin to move in the molten pool and can be rearranged into different crystal structures or chemical compositions. When the laser beam moves to the next area, the previously melted area will cool rapidly to form a new organizational structure, so it is suitable for improving the hardness of the material. The coating preparation process and laser cladding enhancement principle are shown in Figure 3.
The mechanism by which laser cladding technology can improve the hardness of materials includes the following four aspects.
1.1 Grain refinement
Since laser cladding is a special, transient solidification process with a high melting and solidification rate, the growth of grains is inhibited, which promotes grain refinement and improves the hardness of the coating. When preparing FeCoCrNi alloy laser cladding layers, Zhu Zhengxing et al. [2] found that due to the fast cooling rate, the surface grains did not have time to expand, which led to their refinement and the hardness of the material was improved. In order to improve the microhardness and wear resistance of TC4 alloy, Ren Z Y et al. [3] used laser cladding to prepare NbMoTaWTi high entropy alloy coating on the surface of TC4 alloy. The results showed that the microhardness was 71.4% higher than that of the substrate. The reason is that the rapid melting and cooling during the laser cladding process prevented the grains from growing rapidly, resulting in grain refinement and a significant improvement in hardness.
The elements added to the cladding material cause microscopic grain refinement in the coating, which improves the hardness of the coating. Zhou Fang et al. [4] added Si element when preparing MoFeCrTiW high entropy alloy coating on Q235 steel surface by laser cladding technology. The results showed that with the increase of Si element content, the eutectic structure increased, the dendrite morphology changed from cellular dendrite to fine columnar dendrite and equiaxed dendrite, and the hardness and wear resistance of the coating were improved. The microstructure of the coating under different Si content is shown in Figure 4. Hao Wenjun et al. [5] studied the effect of Si content on the microstructure and properties of CoCrFeNi high entropy alloy laser cladding coating and found that because the atomic radius of Si element is relatively small,
the addition of Si element can make the coating grains fine and uniform, and can also improve the density of the coating microstructure, increase the difficulty of sliding between grain boundaries, and improve the hardness of the coating.
1.2 Lattice distortion
The distortion of the lattice improves the hardness of the material. During the plastic deformation process of the laser cladding layer, a large number of lattice defects such as dislocations and vacancies will be introduced, causing some elements to be out of equilibrium, thus causing lattice distortion. After lattice distortion, the internal energy of the material will often increase, thereby generating greater microstress, inhibiting dislocation slip, and improving the strength and hardness of the material. Since high entropy alloys are composed of multiple elements, the size, structure and bonding energy of each element atom are significantly different. Among them, element atoms with different atomic size, bond type and lattice potential energy may be randomly distributed on the lattice points, resulting in serious lattice distortion in the crystal structure. Therefore, this phenomenon is more obvious in high entropy alloys.
Ma Rucheng et al. [6] used laser cladding technology to prepare FeCrSixNiCoC coatings with different Si contents and found that the lattice constant of each coating phase decreased, causing lattice distortion, and improving the strength and hardness of the material. Shi Y et al. [7] found that the large lattice in the coating was strained and distorted, resulting in an increase in the microhardness of the coating when studying the laser cladding AlCrFeNiCuCo high entropy alloy coating on the aluminum surface. Ni C et al. [8] used laser cladding technology to synthesize Al0.5FeCuNiCoCr high entropy alloy coating with aluminum as the substrate and found that atoms of different sizes increased the lattice strain in the coating, and because the atomic radius of the Al element is larger than that of other elements, the distortion of the lattice crystal is increased, which improves the hardness of the material. When studying the wear resistance of titanium alloy surface, Zhan Siwei et al. [9] prepared TiZrHfCrMoW high entropy alloy coating on the surface of TC4 substrate. The results showed that due to the high mixing enthalpy and atomic radius difference of multiple principal elements and the severe distortion of the lattice, the solid solution strengthening effect was improved, and the average hardness of the coating surface was increased by about 60% compared with the substrate.
1.3 Phase change
The change of the coating phase increases the hardness of the coating. Laser cladding, a high-temperature processing technology, can heat the material surface to a high temperature in a very short time, causing it to partially or completely melt, and form different organizational structures through rapid cooling. At this high temperature, the atoms and molecules of the material begin to become dynamic and can be rearranged into a more stable or more optimized crystal structure, causing the phase change of the material. Gao Yulong et al. [10] prepared CoCrNiMnTix high entropy alloy laser cladding coating on Q235 steel surface and found that with the increase of Ti content in the coating, the coating phase changed from single FCC phase to FCC+Laves phase. The XRD spectrum is shown in Figure 5. The hardness of the coating was improved. Ma Shizhong et al. [11] prepared CoCrFeNiWx high entropy alloy coating on 45# steel surface and found that with the increase of W content in the alloy, the cladding coating phase changed from single FCC phase to FCC phase+μ phase (Fe7W6), and the microhardness was improved. Zuo Runyan et al. [12] used laser cladding technology to prepare CoCrFeNiTix high entropy alloy coating on 45# steel surface and found that with the increase of Ti content, the cladding layer changed from single face-centered cubic (FCC) phase to a mixture of face-centered cubic and body-centered cubic (BCC) phases, which improved the hardness of the cladding layer. Yu Liying et al. [13] found that when studying the effect of Al content on the microstructure and properties of FeCoCrNi alloy, the addition of Al changed the AlCoCrFeNi alloy coating from a single FCC structure to a FCC+BCC mixed structure. Since the BCC structure is much harder than the FCC structure, the coating hardness is greatly improved.
1.4 The effect of cladding materials on coatings
The type and proportion of powder are key factors affecting the performance of coatings. Therefore, in the experiment, it is very important to reasonably design the formula of cladding powder. In addition, since the reinforcement method of the coating is closely related to its reinforcement effect, it is very important to design the powder according to the reinforcement effect. Among them, second phase strengthening, fine grain strengthening, solid solution strengthening, microstructure optimization, amorphization and microstructure optimization are effective ways to improve the hardness and wear resistance of the coating. The effects of common additive elements on the surface properties of cladding coatings are shown in Table 1.
Al (can refine grains, form oxide layers, and improve the hardness, wear resistance, oxidation resistance, and toughness of the coating); Si (adding an appropriate amount can improve the hardness and wear resistance of the coating by affecting the lattice); Mn (adding an appropriate amount can have the effect of solid solution strengthening and improving hardness; it can improve the coating morphology); Ti (improve the hardness, wear resistance, and high temperature resistance of the coating); C (significantly improve the hardness, strength, wear resistance, and ductility of the high entropy alloy coating); B (can reduce grain boundary energy, refine dendrite structure, and improve coating wear resistance); Nb (adding an appropriate amount can improve wear resistance); WC (refine grains, improve coating hardness and wear resistance); Cr (improve coating hardness, oxidation resistance, and corrosion resistance).
Since the hardness of a material reflects the ability of the material to resist material pressing into the surface, the higher the hardness, the shallower the depth of material pressing into the material surface, and the smaller the wear volume generated by cutting, that is, the smaller the wear, the higher the wear resistance[21]. Therefore, hardness can be used to measure the wear resistance of metal materials, and when hardness increases, its wear resistance will also increase.
2 Composite strengthening
Due to the rapid heating and cooling solidification characteristics of the molten pool during laser cladding, there is a large temperature gradient in different height directions when the cladding layer is formed by moving upward along the solid-liquid interface at the bottom of the molten pool, and the composition and mass fraction percentage of each element in the alloy powder are different from those of the substrate, which makes the thermophysical parameters between the two very different. The cladding layer often has certain defects, which greatly weakens the mechanical properties of the cladding layer. In order to further improve the mechanical properties such as hardness of the coating, it can be combined with other surface strengthening technologies to improve the hardness of the material by using a composite strengthening method.
2.1 Ultrasonic rolling composite strengthening
Ultrasonic rolling surface strengthening technology is a new type of material surface treatment process. By combining high-frequency ultrasonic vibration with static pressure to reciprocately roll the surface of the workpiece, the finishing effect of “cutting peaks and filling valleys” is achieved, and a deeper surface nano-hardening layer and beneficial residual stress are obtained [22-23]. The principle of ultrasonic rolling processing is shown in Figure 6.
The principle of ultrasonic rolling strengthening is to refine the microstructure, reduce surface defects, and form a deep nano-gradient hardened layer and residual compressive stress zone on its surface, thereby significantly improving the comprehensive properties of the material such as hardness, fatigue resistance, wear resistance and corrosion resistance.
The combination of laser cladding and ultrasonic rolling can effectively solve the problems of poor surface quality caused by laser cladding technology, and can refine the grains on the surface of the cladding coating, thereby significantly improving the mechanical properties of the material and reducing the roughness. The principle of laser cladding-ultrasonic rolling composite strengthening is shown in Figure 7 [25].
In response to the problems of low hardness and poor wear resistance of CoCrFeMnNi high entropy alloy, Liu Hao et al. [26] used laser cladding to prepare CoCrFeMnNi high entropy alloy strengthened with Ti and Mo elements on 45# steel substrate, and used ultrasonic rolling technology to treat the surface of the coating. By comparing the microstructure and mechanical properties before and after rolling, the effect of ultrasonic rolling on the coating was analyzed. The results showed that in addition to the reduction of coating roughness after ultrasonic rolling, the microhardness and surface residual stress increased. Ji Haowen et al. [27] used ultrasonic rolling technology to improve the friction and wear properties and corrosion resistance of GH5188 laser cladding coating. The coating surface was treated with ultrasonic rolling. The microstructure morphology of the coating before and after treatment is shown in Figure 8. The results show that after ultrasonic rolling, the coating surface achieved a mirror effect. Compared with the case without rolling, its roughness decreased by 58%; a nanocrystalline layer with a thickness of 18 μm was prepared; compared with the H13 substrate, the surface microhardness of the coating without rolling increased by 21% and the wear resistance increased by 69%. The surface microhardness of the coating after ultrasonic rolling increased by 70% and the wear resistance increased by 81%. This shows that ultrasonic rolling has a significant effect on improving the performance of the coating. Zheng Kaikui et al. [28] used laser cladding technology to clad an iron-based alloy coating on the surface of H13 die steel as the substrate, and then used ultrasonic rolling technology to treat the coating surface. The effects of ultrasonic rolling process parameters on the microstructure and quality of the coating were studied. The results showed that the microstructure of the coating was significantly refined after ultrasonic rolling treatment, and it was found that with the increase of the reduction, static pressure, rolling rate and rolling times, the surface hardness first increased and then decreased; with the increase of the reduction, static pressure and rolling rate, the surface residual stress first increased and then decreased. Shen XH et al. [29] used laser cladding technology to prepare two medium entropy alloy coatings. After cladding, the two prepared coatings were strengthened by ultrasonic rolling technology, and the coating samples before and after rolling were tested. The results showed that the roughness values of the two treated samples were reduced by 88.7% and 87.6%, the porosity was reduced by 63.8% and 73.4%, and the microhardness values were increased by 41.7% and 32.7%, respectively. In addition, the mechanical properties and wear resistance of the two treated coating samples are better than those of the corresponding untreated samples.
2.2 Heat treatment composite strengthening
Heat treatment refers to a metal heat processing process in which the material is heated, kept warm and cooled in the solid state to obtain the expected structure and performance[30]. Annealing heat treatment is often used to treat the laser cladding layer. By heat treating amorphous materials, the free volume of the material can be reduced, the distance between atoms can be reduced and the bonding strength between atoms can be increased; at the same time, during heat treatment, due to the nucleation and growth of nanocrystals, the formed nanocrystals have an inhibitory effect on the growth of shear bands. Therefore, the higher the heat treatment temperature, the higher the hardness of the material. Table 2 is the experimental conclusion drawn from the laser cladding and heat treatment composite strengthening test.
However, when the heat treatment temperature continues to increase, the crystallinity continues to increase, the nanocrystals continue to aggregate and grow, the size of the crystalline phase continues to expand, and the resistance to the shear band at the interface between the crystalline phase and the amorphous phase becomes smaller and smaller, so that the microhardness decreases after reaching the maximum value. Liu Xueyou et al. [35] prepared CoCrFeNiB0.5 high entropy alloy coating on the surface of 45# steel by laser cladding method, and analyzed the test results at different annealing temperatures. The results showed that under 700℃ annealing, the interdendritic structure has a growth trend and the interdendritic structure expands; under 900℃ annealing, some small dendrites have crystal arm fracture, dissolution, irregular growth and coarsening; under 1100℃ annealing, the dendritic structure gradually disappears, forming a large number of granular and spherical organizational morphologies, and the average hardness of the alloy coating also decreases. The hardness change is shown in Figure 9. CoCrFeNiSi0.2 -800 ℃ annealing and heat preservation for 30 min – microhardness is significantly improved, friction factors are reduced, and wear resistance is improved.
CoCrFeNiW0.6 -1 000 ℃ annealing and heat preservation for 2 h – promotes the formation of μ phase, significantly improves hardness, and improves friction and wear performance.
MoFeCrTiWNb2.5 (Al2O3) 0.5 – 750 ℃ annealing for 6 h – hardness and wear resistance are significantly improved.
CrFeMoNbTiW -1 000 ℃ annealing for 10 h – microhardness is increased by 72.5%, and wear resistance is gradually improved in the range of 800~100 ℃.
Heat treatment can improve the organizational structure of the coating. Reasonable heat treatment can refine the organization, generate the second phase and precipitate intermetallic compounds. Therefore, in-depth understanding of the mechanism of heat treatment is of great significance to improving the performance of the coating.
2.3 Other composite strengthening methods
Friction stir treatment technology can cause severe plastic deformation on the surface of the material, thereby achieving crack closure, grain refinement and other characteristics[36]. Combining friction stir treatment technology with laser cladding technology is an effective means of reducing defects such as pores and cracks in the cladding layer. With the in-depth study of the principle of friction stir technology, it will provide a more feasible direction for controlling the defects of laser cladding layers. After the laser cladding process is completed, the cladding layer is post-processed by friction stir treatment technology. The large amount of friction heat generated by the close contact between the high-speed rotating stirring tool and the cladding layer is used to soften the cladding layer[37]. Under the stirring action, the thermoplasticity and plastic flow of the cladding layer material are achieved[38]. Therefore, the defects such as pores and cracks in the cladding layer can be reduced or even eliminated without re-melting the cladding layer[36]. At the same time, the high-speed stirring tool causes severe plastic deformation in the stirring deformation zone, refines the microstructure in this area, and thus improves the mechanical properties of the cladding layer[39]. The process is shown in Figure 10.
Yang G et al. [38] treated the WE43 alloy powder cladding layer with a stir friction technique and found that the porosity decreased from 1.4% to 0.04% from the outermost deposition zone (DZ) of the cladding layer to the central deformation zone (NZ) of the stir friction technique. At the same time, the microhardness of the cladding layer was also improved. Xie S et al. [40] treated the nickel-chromium cladding layer with a stir friction technique to inhibit the generation of microcracks in the cladding layer. The plastic flow of the cladding material caused the cracks in the cladding layer to close after treatment. Compared with the untreated original cladding layer, the wear resistance of the cladding layer after treatment was improved to a certain extent.
Surface post-treatment methods of laser cladding layers also include laser impact, laser milling, laser remelting, etc. Table 3 shows other commonly used composite strengthening methods of laser cladding technology.
Cold rolling – FeCrCuMnNi – Strength and hardness are significantly improved.
Laser remelting – IN718 alloy – average hardness increased from 250.3 HV to 276.9 HV.
Ultrasonic impact – CrCoNi Zhongshang alloy – coating microhardness reached 374.1 HV0.2, 122% higher than the unimpacted coating.
Alternating magnetic field – (Fe-Cr-Si-B-C) – Fe-based self-soluble alloy mechanical properties significantly improved
3 Conclusion
As a commonly used metal surface treatment technology, laser cladding technology aims to improve the surface properties of the material such as hardness by preparing a coating to cause microscopic changes such as grain refinement, lattice distortion, and phase change inside the workpiece. Due to the technical characteristics of laser cladding, certain defects often occur, resulting in the surface properties of the cladding layer failing to meet some application requirements. Therefore, the laser cladding strengthening mechanism has gradually developed from a single process to a composite process, so that the performance of the coating can be further improved to meet the use requirements. The existing research is summarized as follows:
(1) Ultrasonic rolling composite strengthening treatment can further improve the performance of the cladding layer. Thanks to the ultrasonic rolling strengthening technology, the surface of the alloy can be smoothed and the microstructure becomes finer by combining high-frequency ultrasonic vibration with static pressure.
(2) Heat treatment is often used for post-treatment of coatings with amorphous materials as cladding materials. It can improve the surface performance of the coating by changing the chemical composition and microstructure of the surface and interior of the material by reducing the free volume of the material, reducing the distance between atoms, and increasing the bonding strength between atoms.
(3) Laser cladding technology has been widely used in aerospace, automobile manufacturing, medical and other fields. With the increasing requirements for the surface quality of the cladding layer, higher requirements are also placed on the comprehensive equipment performance of laser cladding and the corresponding composite process. In the future, a process parameter matching mechanism suitable for various working conditions can be established to meet the needs and applications of surface strengthening performance under different conditions.
