Improving the microstructure and mechanical properties of laser cladded Ni-based alloy coatings by changing their composition: A review

Ni-based alloy coatings prepared by laser cladding has high bonding strength, excellent wear resistance and corrosion resistance. The mechanical properties of coatings can be further improved by changing the composition of alloy powders. This paper reviewed the improvedmicrostructure andmechanical properties of Ni-based composite coatings by hard particles, single element and rare earth elements. The problems that need to be solved for the particle-reinforced nickel-based alloy coatings are pointed out. The prospects of the research are also discussed.

the cladding layer, which can effectively improve the wear resistance of the metal surface. Cr atoms can also dissolve in Ni atoms and act as solid solution strengthening, which can significantly improve the hardness of coatings. Ni based alloy coatings have strong metallurgical bonding, good wear resistance and excellent corrosion resistance. It has broad application prospects in engineering [3,4]. The commonly used methods of preparing Ni-based coatings include laser cladding, plasma spraying, TIG and so on.
Laser cladding, known as a promising method of surface engineering, has been widely applied in modification of surface properties. The high energy laser beam is used as the heat source and the alloy powder as the cladding material. The alloy powder and the matrix surface are melted quickly by the radiation of the high energy density laser. After the laser beam removes away, the molten metal solidified rapidly because of the heat conduction of the matrix, thus the cladding layer is completely metallurgic with the matrix, which aiming to improve the surface properties of matrix. At present, the commonly used laser cladding processes are preset powders method and simultaneous powder feeding method. In the preset powders method, the powder is pre-placed onto the substrate surface homogeneously. The laser spot melts the powders and surface of substrate along the set trajectory. After the laser left, the melt pool solidified into coating. The schematic diagram of the laser cladding with the preset powder method is shown in Figure 1. Simultaneous powder feeding method is to send powder to the molten pool while the laser is radiating. The schematic diagram of laser cladding with simultaneous powder feeding method is shown in Figure 2. The coatings obtained by laser cladding have smaller heat affected zone. The coatings have compact structure, good combination with substrate, low porosity and low content of inclusions [5].
The Ni-based coating prepared by laser cladding has a fine grain, dense microstructure, strong metallurgical bonding between the coating and the matrix, high hardness, excellent wear resistance and corrosion resistance. It has been widely used in aerospace, turbines, automo-  bile mold, piston rods, wearing plates and roller, but in different service environments and engineering components, wear is one of the three main reasons for the failure of engineering materials [6], laser cladding Ni-based coating can be used to repair the worn surface of components, which extends service life and improves mechanical properties. In addition, it is necessary to strengthen the surface, such as for steel, aluminum alloy, etc. However, in conventional NiCrBSi alloy coating, the amount of strengthening phase is less, and the surface properties need to be further improved. Ni-based alloy coatings reinforced by hard particles have high hardness, excellent wear resistance and corrosion resistance, and high temperature oxidation resis-tance. In recent years, many researchers have done a lot of research on the use of strengthening phase particles to improve the performance of Ni based coatings, and explored the application of Ni based coatings in engineering. It is expected to provide theoretical and technical support in engineering applications.

Adding hard particles directly
The direct addition of hard particles is the mechanical mixing of reinforced particles with metal powders, and then the composite materials are prepared by solid state reaction or liquid casting. The advantage is that the phase selection is wide, the preparation is relatively simple, and the particle size is easy to control. The commonly used hard phase particles directly added include: WC, NbC, TiC, TaC and VC. For a more intuitive comparison of base alloy, cladding methods, hard particles and obtained mechanical properties, the hard particles, Table 1 listed the  coatings reinforced by hard particles with direct addition  method. WC particles have the advantages of high melting point and moderate strength and toughness, good compatibility and high bonding strength with Ni-based coatings [8]. Ortiz et al. [9] used to prepare NiCrBSi coatings with different WC addition by laser cladding. It is found that the distribution of WC in the coating is not uniform, and the WC content at the bottom of the coating is higher (as shown in Figure 3). Ma et al. [10] prepared Ni60/WC composite coatings with wide-band laser and circular laser.  The hardness of coatings was improved Ma et al. [10] Wide-band laser and circular laser Q550 steel WC Ni60 The hardness and wear resistance of wide-band laser coatings was higher than circular laser coating Guo et al. [12] Preheated the substrate 1Cr18Ni 9Ti WC NiCrBSi The hardness of coatings was about 5 times than substrate, and the wear resistance increased with the increase of WC. Zhou et al. [13] Laser The hardness, wear resistance and corrosion resistances of coatings were increased Jiang et al. [23] nano-TiCp C45 mild steel plates TiC Inconel 625 powder The hardness and modulus of coatings were higher than Inconel 625 substrate Meng et al. [27] Different content of B 4 C Ti-6Al-4V B 4 C NiCrBSi The hardness of coatings was three times higher than substrate The wear loss of coatings was measured at different sliding distance with a load of 10 kg, and they found the coatings with hard carbides (WC and M 23 C 6 ) and uniform distributed cored eutectic structure improved the hardness and wear resistance. Tobar et al. [11] studied the effect of WC volume fraction on the formation and properties of coatings, dense and crack-free coatings were obtained with less than 50 wt.% WC, the hardness of coatings were dependent on WC content, which ranged between 600 Hv and 1000 Hv. Guo et al. [12] studied the wear resistance of NiCrBSi and NiCrBSi/WC-Ni coatings with the normal load of 5 N, the 3D non-contact surface mapping of the wear scars is shown in Figure 4, they found the hard WC particles is an effective method to improve the wear resis- tance of Ni-based coatings. Zhou et al. [13] studied the effect of laser spots on the performance of WC-NiCrMo coating. It found that the microstructure and mechanical properties of the coatings prepared by circular spots and oval spots were different. The wear resistance of coatings made by multi-track LIHC was lower than multi-track LC under the same laser processing parameters, but the wear resistance improved 1.4 times as the laser scanning speed increase to 1500 mm/min. Li et al. [14] studied the effect of different WC addition on the microstructure of Ni60A/WC laser cladding layer. It is found that when the addition of WC is 20 wt.%, the main phases of the coating include -(Fe, Ni) dendrites and carbides with intergranular W rich. When the addition of WC is 33 wt.%, a lot of floc eutectic structure is distributed in the coating. When the addition of WC reaches 50 wt.%, the coating is mainly composed of M 6 C, M 23 C 6 and Cr 4 Ni 15 W. Zang et al. [15] used laser cladding technology to prepare Ni60/WC coatings on 20Cr steel substrate to improve the hardness and wear resistance of the surface. The phase of Ni60/WC coating is mainly composed of Fe-Ni, Cr 7 C 3 and WC. The hard phase WC and Cr 7 C 3 can improve the hardness and wear resistance of the coating. The hardness of the Ni60-WC coating is 761Hv (The hardness Ni60A is 607Hv), and the wear resistance is 33% higher than that of the Ni60A coating. Cr 3 C 2 is one of the most widely used hard particles to reinforce metal-based coatings. Cr 3 C 2 reinforced Nibased coating has high hardness and excellent wear resistance, making it very suitable for the preparation of wearresistant coatings [16,17]. Yu et al. [18] prepared Ni-Cr 3 C 2 composite coatings on the surface of Q235 steel by laser cladding. The grain size of the coating was fine, the microstructure was uniform and dense. The average hardness of the coating reached 1000-1200 Hv, the friction coefficient of the coating was about 0.44, and the friction performance was stable. The combination of Cr 3 C 2 hard particles and metal matrix can effectively improve the bearing capacity of the cladding layer, which makes the coating have excellent wear resistance. Ni-Cr 3 C 2 coatings were prepared by Da et al. [19] to improve the surface properties of stainless steel. The Cr 3 C 2 decomposed into Cr and C atoms in the molten pool, and Cr 7 C 3 and Cr 23 C 6 were formed by the combination of Cr and C atoms. The hardness of the Ni-Cr 3 C 2 coating reached 1000-1200 Hv, the hardness of the coating was about 400Hv higher than that of the Ni coating, and the wear resistance increased 1.7 times at the load of 60 N. In addition, Zhang and Lei [20] found that the maximum hardness of Ni+Cr 3 C 2 coating has arrived to 480 HV, and the corrosion resistance of Ni+50% Cr 3 C 2 coating was about 50% higher than the substrate. Zhang and Zhang [21] also compared the mechanical properties of Ni-Cr 3 C 2 and Ni-WC coatings. The hardness of the Ni-Cr 3 C 2 coating is about 300 Hv lower than that of the Ni-WC coating, while the corrosion resistance of Ni-Cr 3 C 2 coating is better than that of the Ni-WC coating.
The TiC reinforced metal matrix coating is studied mostly, because TiC presents high melting point, a high stability and hardness. Wilson and Shin [22] prepared the Inconel 690 coating enhanced by TiC particles by laser direct deposition. The hardness and wear resistance were improved with the TiC addition, the volume loss rate decreased 42% with the content of 40 vol.% TiC at the load of 2.269 kg. Jiang et al. [23] deposited 5 wt.% nanoscale TiC/Inconel 625 coating by laser assisted lumber manufacturing. It found that the hardness and modulus of elasticity of the coating increased by 10.33% and 12.39%, respectively. Shen et al. [24] used TiC particles to enhance the Inconel 625 coating. The TEM images are shown in Figure 5, the coating was mainly composed of (Ni-Cr) solid solution matrix, TiC, and in-situ formed MC (M=Nb, Ti, Mo). Besides, a transition zone with a width of 3-5 µm existed between the matrix and the TiC particles. Xu et al. [25] used pure TiC particles to enhance Inconel 625 coatings to improve the surface properties of 316L stainless steel. The results showed that the distribution of TiC in the coating is very uniform. The hardness of the coating strengthened by TiC was 330 Hv (220 Hv without TiC), and the tensile strength was 824 MPa, which is much better than that of the coating without TiC addition. Besides, the corrosion resistance of the coating also increased with TiC addition. In addition, some researchers added Al 2 O 3 [26], B 4 C [27], SiO 2 [28] to Ni matrix powder to improve hardness, wear resistance and corrosion resistance of the coating.

In-situ synthesis of hard phase particles
The in-situ synthesis method is to add elements, then the elements react to form reinforced phase in the process of laser cladding, aiming to enhance the performance of the coating. This provides enough time to match the interface between the phase and the matrix metal, so the bonding strength and compatibility between the reinforced phase and matrix were significantly improved, and the interface was pure. Thus, the mechanical properties of materials have also been correspondingly improved. In addition, the distribution characteristics of the reinforced phase can also be effectively controlled, such as size, morphology and distribution uniformity. Because of these advantages, in situ synthesis is widely used in laser cladding. The carbides and boride reinforcing phases in the coating are mostly prepared by this method [29]. Du et al. [30] used (Nb+C)/Ni60 powder to prepare NbC particle reinforced Ni60 alloy composite coating by laser cladding on the surface of 45 steel. There were no cracks, pores and other defects in the coating. The main phases in the coatings consist of -Ni, M 23 C 6 , NbC and CrB. NbC was generated by in-situ reaction during laser cladding. The morphology of NbC mainly consisted of fine granular, irregular block and petal-shaped. The microhardness of the Ni60 alloy coating reinforced by NbC particles reached 1000 Hv 0.2 , which is about 38% higher than that of pure Ni60 coating. Niu et al. [31] used (Ni60+Nb 2 O 5 +C) mixed powder as the cladding material. The Ni-based composite coatings reinforced by NbC were prepared on the surface of A3 steel. The main phases of the Ni60/NbC coating include: -(Ni,Fe), carbides and NbC particles. The average hardness of the Ni60/NbC coating reached about 1200 HV, and the wear resistance is 1.5 times higher than that of the Ni60 coating at the load of 300 N.
Cao et al. [32] studied the mechanism of the influence of the morphology of NbC with different Nb addition. The results showed that the morphology of NbC changed from granular to petaloid with the increase of Nb addition. The alloy powder will melt under the action of laser radiation. At higher temperatures, Nb and C combine to form NbC particles. As the temperature decreases, the molten pool begins to solidify, and a large number of NbC particles are engulfed by the liquid-solid interface. Due to the low diffusion rate of Nb and C atoms in the solid phase, the growth rate of NbC in the subsequent cooling process is significantly reduced, resulting in a smaller size (hundreds of nanometers), so most of NbC in the coating is granular. NbC particles not engulfed by the interface remain in the liquid phase and grow rapidly until the end of solidification. Due to the longer growth period of these NbC particles, the particles reached a larger size and eventually distributed in the matrix as petals (5 µm) or polyhedrons (2 µm). Figure 6 shows the formation sketches of two typical NbC petals. NbC has a face-centered cubic structure. Due to the lower interfacial energy of the {100} crystal plane family, the growth rate of NbC in the <100> crystal direction is larger. Therefore, NbC particles can more easily obtain an octahedral structure, as shown in Figure 6a and 6b. The cooling rate of octahedral corners is large, and the The mechanism of NbC growth [32] growth rate of NbC in these regions is greater than the growth rate of the surface. As a result, pits appeared in the center of the surface. As NbC grows further in the <100> crystal direction, these pits are "stretched" (Figure 6c), which makes the cross-section of a single petal form a cross shape, as shown in Figure 6d. If NbC is grown in different crystal directions, the final morphology will be similar to that shown in Figure 6e. For several NbC grains, the growth rate at the corners of the octahedron is large, so that the {010} crystal plane is perpendicular to the <100> direction of the crystal. Also, as shown in Figure 6c 1 , the slower growth rate of NbC on these planes causes pits to appear in the center of the planes.
Wu [33] used pure Ti and pure graphite to in-situ synthesize TiC to enhance the Ni-based coating. The average microhardness of the coating reached 1250 Hv 0.2 and the wear resistance has also been greatly improved at the load of 0.91 kg. Chao [34] used Ta 2 O 5 and graphite to synthesize TaC particles in composite coatings to improve the surface properties of the coating. In addition, some researchers produced in-situ TiB [35], VC [36], TiB 2 [37] and TiN [38] particles to improve the performance of coating.

Adding a single element to improve the performance of coating
Nb can refine the grains of coatings significantly, and can also reduce the defects such as holes and cracks. The NbC particles can be generated by in-situ reaction with the carbon atoms in the molten pool in the process of laser cladding. NbC presents a high melting point (3600 ∘ C), high free energy of formation (140.7 kJ/mol), high chemical stability, high stability and hardness. NbC possess a similar density (7.79 g/cm 3 [39]) as that of Ni-based coatings, which is beneficial to the distribution uniformity of carbide particles in coatings. Wu et al. [40] produced Ni60A coatings with different Nb addition. Nb and C reacted to form NbC particles to refine the carbide particles and improve the wear resistance of the coatings. When the addition of Nb was 6 wt.%, the NbC phase was found in the coating. Compared with Nb-free coatings, the average hardness decreased by about 300 Hv 0.2 , while the wear resistance increased by nearly 4 times. Cheng et al. [41] prepared Ni-Nb composite alloy coatings by laser cladding on 42CrMo substrate by adding Nb elements with different mass fractions in Ni-based coatings. When the amount of niobium added is 10 wt.%, The microstructure of the coating is significantly refined and denser, and the hardness of the coating reaches the highest, which is twice that of the nickel-based alloy coating without niobium. Hu et al. [42] investigated the effect of Ti addition on crack and hardness of Inconel 625 coating. With the addition of Ti, the tendency to cracks becomes smaller. The addition of Ti promoted the formation of the ( + Laves) eutectic phase, and caused the Laves phase to change from a point or short rod morphology to a layered and reticulated shape. When the amount of Ti added is 5 wt.%, The microhardness of the coating is increased by 100 Hv compared with that without Ti. Ma [43] added different mass fraction of Ti to the Ni60/WC composite coating to explore the effect of Ti on the microstructure and mechan-  [43] ical properties of the Ni60/WC coating, and analyzed the hard phase particles synthesized in situ. The microstructure of Ni60-20WC composite coating and Ni60-20WC-2Ti coating is shown in Figure 7 and 8. The results showed that the main ceramic particles in the Ni60/WC composite coating were Cr 5 B 3 and M 23 C 6 (M is Cr and W). After Ti addition, a large number of netted TiC appeared in the coating. Besides, the distribution of bulk Cr 5 B 3 particles became more homogeneous, and the morphology changed from star shape to uniform square shape. Figure 9 shows the hardness distribution map of Ni60-20WC coating and Ni60-20WC-2Ti coating. It is found that the hardness distribution of coatings was more uniform with Ti addition, and the wear resistance increased by 2.6 times at the load of 100 N.
Yang et al. [44] prepared Al+ (Ti+B4C) composite coatings on the surface of AZ91D magnesium alloy, and the effects of Al on the microstructure and mechanical properties of the coatings were studied. In the process of laser cladding, Al 3 Mg 2 , Al 12 Mg 17 , Al 3 Ti and TiC particles were produced by in-situ synthesis. The schematic diagram of compounds is shown in Figure 10. The hardness of the coating reached 348 Hv, which is about 5-6 times higher than that of AZ91D. After adding Al, the friction coefficient of the coating decreased, while the wear resistance and corrosion resistance of the coating increased.
Ta is a strong carbide forming element, which is easily reacted with C to form tantalum carbide (TaC). TaC has high hardness, high melting point (about 3880 ∘ C) and high chemical stability, and has excellent chemical resistance, thermal shock resistance and corrosion resistance [45]. Yu et al. [46] added Ta element to NiCrBSi powder, and used in-situ TaC particles to improve the mechanical properties of NiCrBSi coating. The results showed that the formation of TaC particles inhibits the formation of M 7 C 3 and M 23 C 6 , the amount of coarse carbide (M 7 C 3 and M 23 C 6 ) decreased, and the grain size is reduced. When the addition of Ta is 5 wt.%, the microhardness of the coating increased by 18%. and the wear resistance also improved. In addi-   [44] tion, the addition of Ta reduced the crack sensitivity of Ni based composite coatings.

Adding rare earth elements
Rare earth elements are a general term for a class of elements and belong to the third group of elements. There are 17 kinds of lanthanide elements, such as lanthanum (La), cerium (Ce), and chemical elements similar to lanthanide. In the electron layer of the rare earth atom, the 4f electron layer is not filled by electrons. This results in a very active chemical property. The role of rare earth in metallurgy mainly includes: purifying the tissue, reducing the defect rate of the coatings, refining the microstructure, promoting the precipitation of the second phase, and forming the reinforced hard phase.
Zhao et al. [47] prepared Ni-based coatings with La 2 O 3 addition on the surface of 30CrMnSiNi2A steel. It is found that La tends to be distributed in dendrites, which limits the secondary growth of dendrites. The microstructure of the coating is more compact, and cracks and holes are not found in the coatings. The hardness of the coating reached 4 times that of the matrix. The friction coefficient was obviously lower than that of the matrix steel, and the wear resistance was nearly 9 times higher than matrix. Wang et al. [48] used laser cladding technology to prepare Ni60 alloy coating with La 2 O 3 , Y 2 O 3 and CeO 2 addition on 6063Al surface. The effects of rare earth elements on the microstructure and mechanical properties of Ni based coatings were studied. It has been found that the addition of rare earth elements results in the formation of stable rare earth compounds in the coating. The coatings with Y 2 O 3 addition appeared Al 4 La. The coatings with Y 2 O 3 addition appear YAl 3 , AlNiY and Ni 17 Y 2 . CeNi 5 and Ce 3 Ni 6 Si2 appear in the coatings with CeO 2 addition. After adding rare earth, the microstructure of the coating was more compact and the hardness increased from 200 Hv to 350 Hv. The wear resistance was significantly improved at the load of 5N with the addition of La 2 O 3 and CeO 2 .

Conclusion
The paper reviewed the research progress of improving the microstructure and mechanical properties of Ni-based composite coatings. The hardness, wear resistance and corrosion resistance of coatings can be effectively improved by many ways, such as in-situ synthesis of hard phase particles, adding hard particles directly, single element or rare earth elements, but there are still many problems that still need to be further studied.
1) The direct addition of hard particles can significantly improve mechanical properties of Ni-based coatings. WC, Cr 3 C 2 and TiC can effectively improve the hardness and wear resistance of coatings, the corrosion resistance of Ni-Cr 3 C 2 coatings is better than that of Ni-WC coatings. However, some researchers found that the particles sometimes are distributed non-uniformly, such as sank or floated in the coatings. 2) The size, morphology and distribution of in-situ NbC, TiC, TaC, TiB, VC, TiB 2 and TiN particles can be improved, resulted in the increase of hardness and wear resistance. In situ NbC consisted of fine granular, irregular block and petal-shaped, and the <100> crystal direction showed a higher growth rate. The wear resistance of Ni60/NbC coatings improved 1.5 times than that of Ni60 coatings.
3) The addition of Nb, Ti, Al, Ta elements has effect of refining grain and microstructure, hard particles can be in-situ synthesized by the single element and C element in the molten pool, which improved the wear resistance of coatings. However, the improvement results of single element on hardness were inconsistent, some researchers found the hardness of coatings decreased with the addition of Nb or Ti elements. 4) Rare earth La 2 O 3 , Y 2 O 3 and CeO 2 have the effect of reducing the defect rate, refining the microstructure, promoting the precipitation of the second phase and hard particles. La 2 O 3 and CeO 2 are more effective to improve the hardness and wear resistance of coatings. 5) Laser cladding is a process of non-equilibrium crystallization. The thermodynamics, phase transition kinetics, interfacial behavior and diffusion behavior involved in heating and solidification are needed to be explored. In addition, the relevant standards of laser cladding are not yet perfect, and this technology has not been widely applied in industrial production.