With Ni60A+WC powder as cladding material, ZrB2+ZrC in-situ reinforced composite coating was prepared on the surface of zirconium alloy by laser cladding technology. The influence of laser power on the forming, microstructure, reinforcement phase and microhardness of the cladding layer was studied. The results show that the cladding layer has good appearance and no defects such as pores and cracks are found. With the increase of laser power, the width and height of the cladding layer increase significantly. The reinforcement phase of the cladding layer is mainly fine α-Zr dendrites, granular and blocky ZrC and Zr(B,C)2. The layered eutectic structure of ZrB2+ZrC is generated inside the unmelted WC particles. At high temperature, the formation Gibbs free energy of ZrB2 and ZrC is close, and the layered eutectic structure of the two is easy to form. With the increase of laser power, the growth of secondary dendrites in the cladding layer is inhibited, and the granular reinforcement phase gradually transforms into the block reinforcement phase. The microhardness of the cladding layer increases significantly with the increase of laser power. When the laser power is 2.8 kW, the microhardness value of the cladding layer is the highest (about 720 HV0.5).

Zirconium and its alloys are widely used in the fields of energy, power, chemical equipment and nuclear energy due to their advantages such as low thermal neutron absorption cross section, excellent corrosion resistance, good mechanical properties and processing performance. They are considered to be ideal materials for nuclear reactor cladding tubes and cooling channels. However, due to the low surface hardness and poor wear resistance of zirconium alloy, it is severely affected by friction and erosion in the service environment and is prone to wear failure. Zirconium alloy as cladding tube generally has severe outer wall wear. After the outer wall fails and ruptures, it is easy to cause nuclear fuel leakage and cause nuclear fuel disaster accidents. Therefore, modifying and strengthening the surface of zirconium alloy to improve surface hardness, wear resistance and corrosion resistance is the key to prolonging the service life of zirconium alloy components.

In recent years, researchers have explored and studied a variety of technologies for surface modification and strengthening of zirconium alloys, such as vapor deposition, electrochemical deposition, surface nitriding, shot peening, laser surface treatment, etc. However, existing studies have shown that the above technologies have limitations such as poor process stability, low deposition efficiency, and low bonding strength between the coating and the substrate. Laser surface treatment technology can greatly improve the hardness, wear resistance, and corrosion resistance of the zirconium alloy surface. Currently, there are mainly laser surface treatment technologies such as laser remelting, laser alloying, and laser cladding. Laser cladding uses a high-energy-density laser beam as a heat source and can efficiently prepare a surface coating that forms a metallurgical bond with the zirconium alloy substrate. Kim et al. prepared a Cr coating with a thickness of about 100 μm on the surface of a zirconium alloy by laser cladding technology, which effectively improved the corrosion resistance of the zirconium alloy surface. Some scholars have also prepared a nickel-based composite coating on the surface of a zirconium alloy by laser cladding technology, but the microstructural evolution mechanism of the nickel-based composite coating on the surface of the zirconium alloy and the influence mechanism of laser power are still lacking in-depth research.

In the laser cladding process, the interaction mechanism of multiple components in the high-temperature molten pool is relatively complex. It involves not only the melting of alloy powder, but also the melting of base material and its interaction with alloy powder. The thermodynamic mechanism of the formation of in-situ reinforcement phase is not clear. In view of the above problems, in-situ reinforcement composite coating was prepared on the surface of zirconium alloy by laser cladding technology. The influence of laser power on the formation, microstructure, reinforcement phase characteristics and microhardness of the cladding layer was analyzed, which provides an experimental and theoretical basis for the research on laser cladding modification of zirconium alloy surface.

1. Test materials and methods

The test selected zirconium alloy (R60702) as the laser cladding base material, with a size of 79mm×45mm×9 mm. The alloy powder is a mixed powder of Ni60A+35%WC. The powder microstructure is shown in Figure 1. The composition of Ni60A alloy powder is shown in Table 1. The powder particle size is 50-150μm. Before the test, the alloy powder of a certain ratio was put into a ball mill and stirred for 150-200 min to obtain a uniform mixed powder. The mixed alloy powder was placed in a vacuum drying oven and dried for 20 h for standby use.

The laser cladding test instrument selected in the test is IPGYLS-6000-S2T, with a maximum output power of 6 kW. The processing route trajectory is controlled by a robot (ABB IRB 2600-20/1.62). The laser cladding test adopted a coaxial powder feeding mode, a rectangular spot with a size of 5 mm×5 mm, pure argon gas protection, argon purity greater than 99.99%, and a gas flow rate of 15 L/min. The powder feeding rate during synchronous powder feeding was 20 g/min. Different laser powers were set to perform single-pass cladding on the zirconium alloy test plate. The process parameters are shown in Table 2.

The cladding sample was cut perpendicular to the cladding direction by wire cutting. After grinding and polishing, it was corroded with a mixed corrosion solution (HF∶HCl∶HNO3=1∶3∶6). The JSM-6480 scanning electron microscope with an energy dispersive spectrometer (EDS) was used to analyze the microstructure and element distribution of the cladding layer; the microhardness distribution was measured by a fully automatic microhardness tester with a load of 5 N, a loading time of 10 s, and a step length of 100 μm.

2 Experimental results and discussion

2.1 Effect of laser power on cladding layer morphology
The surface quality and morphology of the cladding layer are greatly affected by the laser power. As shown in Figure 2, when the laser power is 2.0 kW, the laser power is low, and the low heat input fails to fully melt the alloy powder. There is incompletely melted alloy powder around the cladding layer, and the cladding layer is poorly formed. With the increase of laser power, the larger heat input fully melts the alloy powder, accelerates the flow of the molten pool, and the cladding layer is better formed.

By measuring the cross-sectional forming characteristic parameters of the cladding layer, the effect of laser power on the width and height of the cladding layer is shown in Figure 3. When the laser power is 2.0 kW, the width of the cladding layer is about 5.6 mm, and the height of the cladding layer is about 1.44 mm. With the increase of laser power, the width and height of the cladding layer increase significantly. When the laser power is 2.8 kW, the width of the cladding layer is 5.95 mm, and the height of the cladding layer increases to 1.76 mm. The increase in the width and height of the cladding layer is mainly due to the increase in the amount of zirconium alloy parent material melted by the larger laser power. A large amount of parent material participates in the molten pool reaction, forming a wider and higher cladding layer.

2.2 Effect of laser power on the microstructure of the cladding layer
The zirconium alloy parent material melts into the molten pool and interacts with the powder alloy elements to form various forms of in-situ reinforcement phases. The microstructure characteristics of the cladding layer with different laser powers are shown in Figure 4. There are many fine dendrites distributed in the cladding layer, mainly distributed in the upper and lower parts of the cladding layer. During the laser cladding process, the substrate transfers heat quickly. When the laser beam leaves the molten pool, the alloy at the bottom will solidify quickly to generate dendrites. In the upper part of the cladding layer, the fluidity of the protective gas causes a more significant effect of convective heat dissipation. Therefore, the molten pool solidifies into fine dendrites without obvious direction under the dual effects of convective heat dissipation and substrate heat conduction. There are also fine granular reinforcement phases distributed around the dendrites, with a size of less than 5μm, as shown in Figure 4 (a). The dendrites and granular reinforcement phases were subjected to energy spectrum analysis, and the results are shown in Table 3. The main component of the dendrites is Zr, and its phase is speculated to be α-Zr, while the surrounding granular reinforcement phases are mainly ZrC formed in situ, mainly because WC decomposes in the high-temperature molten pool, and C combines with Zr to form ZrC. There are more eutectic structures at the bottom of the cladding layer, as shown in Figure 4 (c). The eutectic structure is mainly distributed at the grain boundary. According to the results of energy spectrum analysis, the eutectic structure is mainly composed of ZrC and ZrB2.

When the laser power increases to 2.5 kW, the number and size of α-Zr dendrites do not increase, the growth of secondary dendrites is inhibited, and the number of granular ZrC increases significantly, and gradually transforms from granular to blocky reinforcement phase. The increase in the size and number of ZrC reinforcement phase consumes a large amount of Zr atoms, which inhibits the growth of α-Zr dendrites during the molten pool solidification process. As the laser power continues to increase to 2.8 kW, the number of blocky and granular reinforcement phases increases, and the secondary dendrites become finer and more numerous, as shown in Figure 4 (e). The energy spectrum analysis of the reinforcement phase shows that the blocky reinforcement phase is Zr (B, C) 2 based on the atomic percentage. This is mainly due to the large affinity between Zr and B atoms in the molten pool under high heat input, and a large amount of ZrB2 is generated at high temperature. The radius size of C atoms is similar to that of B atoms. C can replace B in ZrB2 clusters at high temperatures to form Zr (B, C) 2.

The change in laser power did not eliminate the α-Zr dendrites in the cladding layer. In order to study the distribution characteristics of elements around the dendrites, the dendrites were line scanned and analyzed. The results are shown in Figure 5. The Zr content is high at the secondary dendrites, and Zr element enrichment occurs. The distribution characteristics of Zr element and C element are similar, with low content at the dendrite trunk and enrichment at the secondary dendrites. The distribution of Ni element is opposite to that of Zr and Cr. Ni element is mainly distributed in the matrix of the cladding layer, forming a composite cladding layer of γ-Ni matrix and in-situ reinforcement phase. W element mainly comes from the decomposition of WC particles, and its distribution characteristics do not form a certain corresponding relationship with C. It also proves that at a laser power of 2.0 kW, the WC in the middle and upper parts of the cladding layer is decomposed more completely, which can provide sufficient C atoms for in-situ synthesis of ZrC.

Due to the high density of WC particles, under the convection of the high-temperature molten pool of laser cladding, WC particles are located at the bottom of the molten pool. Incompletely decomposed WC particles are found at the bottom of the cladding layer, showing an irregular shape, as shown in Figure 6. There are densely distributed blocky reinforcement phases around the WC particles, which are mainly formed by the in-situ formation of ZrC blocky reinforcement phases by C and Zr formed by the decomposition of WC. It should be noted that there is a layered eutectic structure inside the unmelted WC particles, as shown in Figure 6 (b). Through EDS composition analysis, it is inferred that the eutectic structure is ZrB2+ZrC. The melting point of WC is about 2750℃[16]. WC particles with a size of less than 100μm are generally melted and decomposed in the high-temperature molten pool or even during the powder feeding process, and are relatively evenly distributed in the cladding layer with the liquid convection, while larger WC particles are deposited at the bottom and decomposed to varying degrees. WC particles are subjected to liquid convection in the high-temperature molten pool. The edges of the particles melt first, decompose into W atoms and C atoms, and diffuse. The high-temperature liquid phase in the molten pool also diffuses Zr atoms and B atoms along the grain boundaries that melt and decompose first. The atomic diffusion channel is shown by the arrows in Figure 6 (b).

In order to analyze the thermodynamic stability of WC during laser cladding and the thermodynamic mechanism of the in-situ generation of the enhanced phases ZrC and ZrB2, the following reactions were thermodynamically calculated: W+C=WC, Zr+C=ZrC, Zr+2B=ZrB2. The reaction Gibbs free energy changes of the three phases of WC, ZrC, and ZrB2 are shown in Figure 7. The solidification points of ZrC and ZrB2 are 3445 and 3245℃ respectively. Although both are higher than the melting point of WC, it can be seen from Figure 7 that ΔG0 (ZrB2) < ΔG0 (ZrC) < ΔG0 (WC). At high temperature, the formation Gibbs free energy of ZrB2 and ZrC is close, and it is easy to form a layered eutectic structure of the two. As the temperature decreases, the difference in Gibbs free energy between the three becomes more obvious, indicating that ZrB2 and ZrC are easier to form during the solidification of the molten pool. Under the diffusion kinetics of Zr atoms and B atoms, the transformation of WC to the layered eutectic structure of ZrB2+ZrC is thermodynamically feasible.

2.3 Effect of laser power on microhardness of cladding layer
The distribution law of microhardness of cladding layer is shown in Figure 8. From the zirconium alloy substrate to the transition zone and then to the cladding layer, the microhardness shows a trend of gradual increase. The microhardness of the zirconium alloy substrate is about 200 HV0.5. There is unmelted WC and in-situ generated ZrB2+ZrC eutectic structure near the transition zone, which leads to a significant increase in microhardness. The microhardness of the cladding layer is significantly higher than that of the zirconium alloy substrate, which also shows that the in-situ preparation of the enhanced coating by laser cladding on the surface of the zirconium alloy substrate is beneficial to improve the hardness of the zirconium alloy surface. The microhardness of the cladding layer varies greatly, which is mainly related to the morphological size and distribution of the reinforcement phase. The microhardness peak points are mostly in the reinforcement phase aggregation area.

From the three microhardness distribution curves, it can be seen that with the increase of laser power, the microhardness of the cladding layer increases significantly. When the laser power is 2.0 kW, the microhardness of the cladding layer is about 350 HV0.5, and the microhardness fluctuates greatly, mainly due to the uneven size and distribution of the reinforcement phase. As the laser power increases to 2.8 kW, the microhardness of the cladding layer is the highest, with the maximum value of about 720HV0.5, and the microhardness of the cladding layer is relatively uniform. Without WC addition, the cladding layer generates more Zr-Ni brittle phases (NiZr, NiZr2, etc.), resulting in excessively high microhardness values ​​that easily cause cracking. The addition of WC inhibits the reaction of Zr and Ni to generate brittle Zr-Ni intermetallic compounds, and the microhardness of the cladding layer is controlled within a reasonable range, effectively inhibiting the cracking of the cladding layer.

3 Conclusions
(1) The ZrB2+ZrC in-situ reinforced composite coating was prepared by laser cladding Ni60A+WC on the surface of zirconium alloy. The cladding layer has a good appearance and a dense structure. No defects such as pores and cracks were found. The cladding layer formed a good metallurgical bond with the zirconium alloy substrate. With the increase of laser power, the powder melted more fully and thoroughly, and the width and height of the cladding layer increased significantly.
(2) The reinforcement phase of the cladding layer is mainly fine α-Zr dendrites, granular and blocky ZrC and Zr(B,C)2. Zr and C have a strong affinity, and the distribution characteristics of the two elements are similar. A layered eutectic structure of ZrB2+ZrC is generated inside the unmelted WC particles. With the increase of laser power, the growth of secondary dendrites in the cladding layer is inhibited, and the granular reinforcement phase gradually transforms into the block reinforcement phase.
(3) The microhardness of the cladding layer is significantly higher than that of the zirconium alloy substrate. With the increase of laser power, the microhardness of the cladding layer increases significantly. When the laser power is 2.8 kW, the microhardness of the cladding layer is the highest (about 720 HV0.5). The addition of WC inhibits the reaction of Zr and Ni to form brittle Zr-Ni intermetallic compounds. The microhardness of the cladding layer is controlled within a reasonable range, which effectively inhibits the cracking of the cladding layer.