The work aims to improve the mechanical properties of additive manufactured IN 625 high-temperature alloy, so as to solve the problem of property decline caused by the defects and element segregation during laser cladding. The ultrasonicvibration assistance was applied in the process of laser cladding of IN 625 coating. The effect of ultrasonic power on phase type and crystal size of IN 625 coating was studied by phase detection and microstructure observation. The effect of ultrasonic power on element segregation was studied by analyzing the content, distribution and form of precipitated phase. The effect of ultra-sonic power on the mechanical properties of the coating was studied by testing the microhardness and high temperature wear resistance. The microstructure of the coating was mainly coarse dendrites with disordered direction before the application of ultrasonic vibration. The phase composition of the coating did not change significantly after the application of ultrasonic vibration, but the subcrystals were closely arranged and the size decreased obviously. After the application of ultrasonic vibration (UV),the size and content of precipitated phase of the coating decreased, and the content of Laves phase decreased greatly after the application of ultrasonic vibration, indicating that ultrasonic vibration could inhibit the segregation of Nb, Mo and other elements. After ultrasonic vibration was applied, the microhardness of the coating increased and the wear rate decreased obviously. The wear mechanism changed from the original complex wear of surface fatigue wear, adhesive wear and abrasive wear to the simple wear of abrasive wear and adhesive wear. The ultrasonic vibration can effectively refine the microstructure of the IN 625 coating, inhibit the precipitation of Laves phase, and improve the microhardness and wear resistance of the coating.

Inconel 625 (IN 625) is a type of nickel-based high-temperature alloy. It has been widely used in the aerospace field due to its high tensile, creep and fracture strength. The alloy has excellent oxidation resistance and thermal fatigue strength and can be used in the service temperature range from low temperature to about 1 000 ℃. Laser cladding is an additive manufacturing method with great development potential, with the advantages of short processing time and convenient processing. Compared with traditional processing technology, laser cladding has higher forming quality, can strengthen and repair the surface of locally damaged parts, extend their service life, and help reduce costs and improve efficiency. However, due to the rapid heating and rapid solidification characteristics of laser cladding, there is a large temperature gradient in the cladding layer, which is prone to thermal stress and residual stress, and even defects such as cracks and pores. Experts and scholars have carried out a lot of research to improve the performance of additively manufactured IN 625. Li et al. used laser melting deposition technology to study the strengthening mechanism of spherical WC and non-spherical WC reinforced IN 625 composite coatings. The study found that the addition of spherical WC can improve the microhardness, wear resistance and corrosion resistance of the substrate. Zhao Xuan et al. found that harmful phases such as Laves often appear during the laser cladding of IN 625, which usually have a negative impact on the mechanical properties of the coating. Yang et al. used laser melting deposition technology to prepare IN 625 coatings with Co/TiAl added on 304SS. Compared with IN 625 coatings, the room temperature hardness and strength of the coatings after adding Co, Ti and Al were significantly improved, and the elongation was greatly reduced. Ultrasonic-assisted laser cladding has outstanding performance in refining microstructure, homogenizing element distribution, reducing concentrated stress, improving microhardness and wear resistance, and has become the focus of researchers. Hu et al. compared the effects of ultrasonic assistance on grain structure and found that after the introduction of ultrasonic assistance, the grain size was significantly reduced, the arrangement was more dense, and the grain structure was mostly a network structure formed by equiaxed crystals. Li et al. found in the study of ultrasonic-assisted laser cladding of 3540Fe-CeO2 that the introduction of ultrasonic vibration did not change the phase composition, but changed the coating structure from coarse cellular crystals and dendrites to fine needle-shaped dendrites and equiaxed crystals.

At present, the research on laser cladding IN 625 focuses more on optimizing process parameters and adding elements for modification, while there are few studies on the use of energy field assistance to improve the forming quality of laser cladding IN 625. In this paper, ultrasonic vibration assistance is used in the process of laser cladding IN 625 high-temperature alloy, and the influence mechanism and action mechanism of ultrasonic-assisted laser cladding coating mechanical properties are explored, in order to provide a new reference direction for further improving the performance of additively manufactured IN 625 high-temperature alloy.

1 Experiment

1.1 Materials
The substrate is H13 fine steel produced by Kunshan Yukun Metal Materials Co., Ltd. Considering that the heat accumulation caused by multiple overlaps will cause bending and other effects on the substrate, the substrate size is selected as 100 mm×80 mm×25 mm. Before the experiment, the substrate was polished with a grinding wheel, cleaned with alcohol and dried to ensure that the surface was smooth and free of pollutants. The cladding material is IN 625 nickel-based spherical powder produced by Jiangsu Wellari, with a diameter of 45~105 μm and chemical composition as shown in Table 1.

1.2 Coating preparation
The experiment uses an ultrasonic-assisted laser cladding system, as shown in Figure 1. The laser is the RFL-C3300W laser produced by Raycus Laser, with a maximum power of 3.3 kW, a spot diameter of 2.5 mm, and a focal length of 10 mm. The motion unit uses an ABB robot and a two-degree-of-freedom rotating platform to achieve laser cladding of different paths of the substrate. During the laser cladding process, a coaxial powder feeding device was used, argon was used as the feeding gas and protective gas, and the gas flow rate was set to 18 L/min.
This experiment used an ultrasonic vibration device produced in China with a maximum power of 1 000 W, an input power of 220 V, and an operating frequency of 10~22 kHz. In order to avoid the substrate from being vibrated and offset during the ultrasonic operation, the workbench and the substrate were threaded, that is, the ultrasonic loading method was direct contact.
According to the previous laser cladding research of IN 625, the optimal cladding process parameters are as follows: laser power of 1 500 W, scanning speed of 300 mm/min, and conveying speed of 5 g/min. A multi-pass overlap rate of 40% was selected for ultrasonic-assisted laser cladding experiments, and ultrasonic powers of 0, 400, 600, and 800 W were taken for control experiments.

1.3 Sample processing and detection
1) Phase analysis. The prepared cladding layer was cut into a 10 mm × 10 mm × 10 mm cubic observation sample. The observation surface of the observation sample was ground and polished by MTP-200 metallographic polishing machine, pretreated with 400~2000 mesh sandpaper, and then polished with 15000# diamond paste. The sample phase was scanned and detected by X-ray diffractometer (Bruker D8-Advanced), and the scanning angle was set to 20°~100°. The surface of the sample was cleaned by ultrasonic cleaning machine, and then corroded with aqua regia (concentrated HCL+concentrated HNO3, volume ratio of 3:1) until the microstructure texture could be displayed, and the microstructure of the cubic observation sample was observed. The microstructure of the sample was observed by field emission electron microscopy (FSEM, Quanta 250), and the element content was scanned by energy dispersive spectroscopy (EDS) detector to characterize the element distribution.
2) Hardness test. The microhardness of the coating was measured using a Vickers microhardness tester (HVS-1000) with a static pressure of 3 N and a holding time of 20 s. To compare the microhardness of different areas of the coating, a point was taken every 200 µm from top to bottom for testing. A total of 3 measurements were performed, with a lateral spacing of 50 µm between the measurement points, and the average value of the 3 measurements was taken as the microhardness of the area.
3) High-temperature wear resistance test. The model of the high-temperature friction and wear test bench is HT-1000. Si3N4 was selected as the grinding ball with a diameter of 5 mm. A sample with a size of 15 mm×15 mm×10 mm was prepared for high-temperature wear resistance test. Before the high-temperature test, the coating surface was polished with a metallographic polisher, and the surface was cleaned with anhydrous ethanol and dried to remove contaminants. The parameters of the high-temperature wear resistance test are as follows: load of 9 N, test time of 20 min, temperature of 600 ℃, and friction radius of 4 mm. After the high-temperature wear test, the DSX1000 depth-of-field digital microscope was used to perform three-dimensional scanning imaging of the sample cladding profile and wear morphology. The average values ​​of the three cross-sectional areas of the wear groove of each sample were calculated to obtain the wear volume, and then the wear rate of the coating was calculated.

2 Results and discussion

2.1 Coating phase
The phase diffraction curve of the IN 625 high-temperature alloy coating is shown in Figure 2, where the phase content is reflected in the diffraction peak intensity. The phase composition of each coating is roughly the same. Ultrasonic vibration is used as an auxiliary external field and does not change the elemental composition during the molten pool solidification process. It can be seen from the diffraction spectrum that the coating is mainly composed of γ-Ni solid solution phase and Cr23C6, and most of the solute atoms such as Cr and Fe are dissolved in the γ-Ni matrix. Except for γ-Ni and Cr23C6, the Laves phase [16] that often appears in the IN 625 coating has no obvious diffraction peak, which may be due to its small size and low content. It can be seen from the literature that in IN 625 powder, the affinity between Cr and C is strong. These alloy elements and C form Cr23C6 hard precipitation phase. The smaller hard phase is dispersed between dendrites and γ-Ni solid solution, which plays a role in dispersion strengthening and improving wear resistance. In addition, the area of ​​the non-main peak is calculated and it is found that its area gradually increases with the increase of ultrasonic power. The diffraction peak area represents the content of the phase, indicating that with the application of ultrasound, the distribution of γ-Ni is more uniform, and with the increase of ultrasonic power, the organization has a better uniform distribution effect.

2.2 Coating microstructure
In order to explore the effect of ultrasonic vibration power on the microstructure of IN 625 high-temperature coating, other process parameters are kept unchanged and analyzed under a single variable. The FSEM images of the coatings under different ultrasonic powers are shown in Figure 3a. It can be seen that when there is no ultrasonic vibration, the dendrites are coarse and the subcrystals are unevenly distributed; after the ultrasonic vibration is applied, the crystals are evenly distributed and the size is reduced; compared with the coating without ultrasonic power of 600 W, the crystal size of the coating is significantly reduced, the subcrystal structure in the dendrite is arranged more finely and tightly, and the distribution is more uniform. In order to facilitate the comparison of the changes in crystal size before and after the application of ultrasound, the Image J software is used to count the subcell diameters in Figure 3a, and the statistical results are shown in Figure 3b. It can be seen that the average diameter of the subcells without ultrasound is 6.1 μm, and the subcell size decreases significantly after the application of ultrasound. When the ultrasonic power is 600 W, the grain refinement is most obvious, and the subcell diameter at this time is only 2.6 μm. It can be seen from the ultrasonic cavitation effect that during the laser cladding process, due to refractory alloy elements, air and other reasons, cavitation bubbles exist in the molten pool and exist around the growth space when the crystal nucleates. Under the combined effect of positive and negative high-frequency sound pressure, the cavitation bubbles collapse, causing local supercooling and generating high-pressure microjets. When these microjets are ejected to the growing crystals or dendrite arms around them, the solid crystal network built by the primary crystals will be broken to form broken crystals. These broken crystals are dispersed to various areas of the coating under the effects of the acoustic flow effect, mechanical effect and melt convection of the ultrasonic energy field, becoming new nucleation points to continue growing. It can be seen from the solidification theory that the organizational morphology of the metal melt is affected by the supercooling degree C0 of the surrounding environment. Increasing the supercooling degree, reducing the temperature gradient G and increasing the crystallization rate v can increase the degree of supercooling, promote the transformation of crystals from larger cellular crystals and columnar crystals to smaller dendrites or equiaxed crystals, and promote the refinement and uniform distribution of the subcrystal structure inside the dendrite. When laser cladding is just started, the molten pool absorbs insufficient heat energy, resulting in a large temperature gradient in the molten pool. Therefore, v is small and the G/v value is large in the initial stage of cladding. As the laser energy accumulates, the temperature distribution of the molten pool tends to be uniform, the temperature gradient gradually decreases, the solidification rate accelerates, but the crystal growth rate slows down, and the crystal does not have time to grow completely, resulting in a reduction in size. When an ultrasonic external field is applied, the ultrasonic thermal effect increases the thermal energy in the molten pool. The size of ultrasonic thermal energy is related to the power. Therefore, to a certain extent, as the ultrasonic power increases, the size of the coating microstructure gradually decreases and the grain structure is refined. However, as the ultrasonic power continues to increase, the grain refinement effect becomes worse. This may be because although increasing the ultrasonic power increases the sound pressure level along the axis, the higher the ultrasonic power, the more intense the cavitation effect, the more bubbles are generated, and the local large-scale aggregation of bubbles will form a cavitation area with low density, resulting in a sharp attenuation of the energy of the ultrasonic wave after passing through this area, which weakens the effect of the ultrasound. That is, too high ultrasonic power will enhance the shielding effect of bubbles on sound waves.
Crystal nucleation theory can also explain the mechanism of ultrasound promoting grain refinement. The nucleation rate is affected by the embryo crystal size r, the critical nucleation size rk, and the free energy ΔG in the molten pool. For crystals, the nucleation conditions can only be met when the energy required for nucleation is lower than the free energy in the molten pool and r>rk. The specific relationship is shown in equations (1) to (3).
Where: σ is the Gibbs-Thomson effect; ΔG is the free energy in the molten pool; ΔGc is the critical nucleation energy; f(θ) is the nucleation factor between 0 and 1; GS is the free energy per unit volume of the solid phase; GL is the free energy per unit volume of the liquid phase; ΔHS is the enthalpy change; ΔSS is the enthalpy change; T is the thermodynamic temperature; ΔVS is the volume change; ΔP is the input work.
After adding ultrasonic vibration, the input work ΔP in equation (3) increases, the free energy ΔG increases, and the critical nucleation size rk and the critical nucleation energy ΔGc decrease. Therefore, after the introduction of ultrasound, nucleation is accelerated, grain density is increased, crystal volume is reduced, crystal spacing is reduced, and the organization is refined. The SEM images of the coating organization before and after the application of ultrasound are shown in Figure 4. It can be found that after the application of ultrasound, the dendrite size is reduced. Compared with the dendrite spacing d2 (4.5 μm) without the application of ultrasound, the dendrite spacing d1 (3.2 μm) is significantly reduced after the application of 600 W ultrasound. In addition, it can be seen from the literature that the acoustic streaming effect of ultrasound has an inhibitory effect on the growth of dendrite arms and also plays a role in refining grains.

2.3 Element segregation
In order to explore the effect of ultrasonic-assisted laser cladding on the element distribution of IN 625 coating, the difference in the element content of the precipitated phase of the coating under the action of ultrasound of different powers was studied, and the point energy spectrum of the precipitated phase was measured to explore the effect of ultrasonic assistance on the element segregation and element distribution of the coating.
The scanning electron microscope image of the middle cross-section of the laser cladding IN 625 high-temperature alloy coating sample is shown in Figure 5, where the dark gray area is the dendrite phase and the bright white area is the interdendritic segregation phase. During the sample preparation process, the corrosive liquid corrodes the matrix phase, leaving the more corrosion-resistant Laves phase. The elements such as Nb, Mo, and Cr in the Laves phase have stronger reflection ability for electrons, so they appear bright white under the scanning electron microscope. As can be seen from Figure 5, with the increase of ultrasonic power, the number of precipitated phases decreases and the distribution becomes more uniform. When the ultrasonic power is increased to 800 W, there is no obvious bright white precipitate phase in the SEM image.
The EDS point selection positions are shown in Figure 6, and the element contents at the bottom of each group of coatings are shown in Table 2, where points 1 to 3 are located in the non-ultrasonic sample, points 4 to 6 are located in the ultrasonic sample, points 1 and 4 are located in the precipitate phase, points 2 and 5 are located in the γ phase, and points 3 and 6 are located in the unknown area. It can be seen that the mass fraction of the Nb element in point 1 is about 12%, and the mass fraction of the Nb element in point 2 is 3.72%. It can be judged that the precipitate phase at the bottom of the coating is mainly the Laves phase. It can also be seen from Table 2 that the content of the Mo element in the Laves precipitate phase is 2 to 3 times that of the matrix phase. Therefore, the Laves phase is mainly caused by the segregation of the Nb and Mo elements, which is consistent with the conclusion of the literature. In addition, after applying ultrasound, the content of Nb and Mo elements in point 4 is significantly reduced, almost consistent with the matrix, indicating that after applying ultrasound, the segregation of Nb and Mo elements is suppressed. The significant reduction in the content of Nb and Mo elements also indicates that applying ultrasound can reduce the precipitation of Laves phase.
The schematic diagram of the binary processing of ultrasonic-assisted laser cladding IN 625 coating is shown in Figure 7. In order to further study the changes in the precipitation phase of IN 625 coating before and after the introduction of ultrasound, the content of the precipitation phase of the coating before and after the application of ultrasound is analyzed, and the threshold processing and calculation are performed by Photoshop according to the method shown in Figure 7. The coating cross section is divided into 3 areas along the vertical laser cladding direction, 5 SEM images are taken in each area, and the content of the precipitated phase is calculated using the Photoshop binarization tool, and the average value of the 5 groups of statistical data is taken as the average precipitation amount. After PS threshold processing, the Laves phase and MC-type carbides in the coating are displayed as white dots, and the black area is the γ matrix phase. Data collection is performed based on the image color difference, that is, the pixel blocks occupied by the two colors in the whole image are calculated respectively. The ratio of the two numbers is the mass fraction of the precipitated phase in the figure. The processing results are shown in Figure 8.
When ultrasound is not applied, the mass fraction of the bottom precipitated phase is 4.70%~6.14%; as solidification proceeds, the thermal gradient difference in the middle of the cladding layer decreases, the microstructure morphology also changes, the columnar dendrites disappear, and turn into smaller dendrites, the grain morphology decreases, resulting in the compression of the intergranular spacing, the segregation elements in the middle of the coating, and the local enrichment content decreases, so the content of the segregated precipitate phase decreases to 3.82%~4.71%. At the top of the coating farthest from the interface, since it is in direct contact with the air, there are better heat exchange conditions. In addition, the synergistic effect of the inert protective gas improves the solidification efficiency of this area, increases the solidification rate, shortens the segregation time of alloy elements such as Nb, and further reduces the content of the top precipitation phase. The mass fraction of the top precipitation phase is 2.88%~3.31%. After applying ultrasound, the content of the precipitation phase in the coating decreases significantly. When the applied ultrasonic power is 600 W, the mass fraction of the segregation phase in the top, middle and bottom regions of the coating decreases by 58%, 23% and 63% respectively. The ultrasonic sound flow effect acts in the molten pool, increases the molten flow velocity in the center of the molten pool, promotes the uniform distribution of elements in the cladding layer, avoids the local enrichment of Mo and Nb elements, and thus inhibits the segregation of Laves and other phases.
Four experimental groups with different ultrasonic powers were set up, and four elements, Fe, Cr, Nb, and Mo, were selected as tracer objects to explore the effect of ultrasonic power on the element distribution in the laser cladding IN 625 high-temperature alloy coating. The results of energy spectrum scanning show that in the non-ultrasonic group, each tracer element is enriched, among which the content of Fe element is the highest at the matrix junction, and it is speculated that there are more Fe elements in H13. During cladding, due to the different surface tensions of the two liquids, under the action of the tension gradient, the matrix elements move from the “high concentration” side to the “low concentration” side, and diffusion occurs. Except for the special phenomenon of Fe element at the metallurgical junction, Fe and Cr elements are still enriched from top to bottom in the coating cross section, and the color is bright.
After the introduction of ultrasound, the “rich and poor” phenomenon is effectively improved, and the highlight areas of each element are more evenly distributed. When ultrasound is not applied, alloying elements such as Nb and Mo that are prone to produce harmful phases such as Laves phases show obvious enrichment phenomena. After ultrasound is applied, the enrichment phenomenon of Nb and Mo elements is greatly improved, which also confirms the experimental results of the decrease in the content of precipitated phases mentioned above. With the increase of ultrasonic power, the original large block-like enriched area is transformed into a sheet-like enriched area, and the element identification color in the coating is more uniform from top to bottom. After ultrasound is applied, the convection speed in the molten pool is accelerated under the acoustic streaming effect, thereby accelerating the diffusion speed of the alloying elements. Therefore, under the action of ultrasonic “stirring”, the movement time of the elements is prolonged, and the elements are distributed more evenly in the coating. Under conventional laser cladding technology, how to suppress the segregation of harmful phases has always been a difficult problem for laser cladding. Ultrasonic-assisted laser cladding can solve this problem, that is, through the ultrasonic effect, the uniform distribution of strengthening elements is promoted and the scale and number of segregated phases are reduced.
The effect of ultrasound on the distribution of precipitated phases in the coating is shown in Figure 10. The original precipitation phase is distributed in the intergranular state in a chain shape, while the precipitation phase of the coating prepared with ultrasonic assistance is dispersed in the dendrites in a granular state. From the morphological point of view, the original rhombus precipitation phase becomes an ellipsoidal precipitation phase with a smaller area, and the distribution spacing increases (as shown in Figure 11). This further confirms that ultrasonic vibration can homogenize the distribution of precipitation phases and reduce the size of precipitation phases, thereby inhibiting the segregation of elements.

2.4 Microhardness and wear resistance of coating
The hardness distribution results of ultrasonic-assisted laser cladding IN 625 high-temperature alloy coating are shown in Figure 12. When ultrasonic vibration is not applied, the microhardness of IN 625 coating is 312.21HV~339.98HV, and the average hardness is 325.54HV. When the ultrasonic power is 400 W, the average microhardness of the coating is 344.71HV, which is about 6% higher than the hardness of the sample without ultrasonic treatment; when the ultrasonic power is 600 W, the average microhardness of the coating is 379.94HV, which is 16.8% higher than the cladding sample without ultrasonic treatment; when the ultrasonic power is further increased to 800 W, the average microhardness of the coating is 372.39HV, and the microhardness does not continue to increase, but decreases. From the above, it can be seen that ultrasonic vibration is beneficial to improve the microhardness of the coating. With the change of ultrasonic power, the hardness of the coating increases first and then decreases, but the microhardness level when it decreases is still higher than the microhardness of the cladding sample without ultrasonic action.
After the introduction of ultrasound, the sparse structure phenomenon of the coating surface disappears, and the surface microhardness distribution is more uniform. Combined with the changes in the microstructure of the coating after applying ultrasound, it can be seen that applying ultrasonic vibration during the laser cladding process is beneficial to accelerate the flow rate of the melt and make the temperature of the molten pool uniform, thereby reducing the cooling rate of different positions in the molten pool. At the same time, the ultrasonic cavitation effect causes the temperature and pressure inside and outside the cavitation bubble to change, and the cavitation bubble absorbs heat and expands and bursts, resulting in the formation of high-speed impact flow to break the surrounding dendrites while increasing the surrounding supercooling. Under the combined effect of the two, the number of grain nucleation points increases and the coating structure is refined, which is beneficial to improve the coating’s resistance to plastic deformation, so the hardness is improved. It can be seen from Section 2.2 that when the ultrasonic power increases to 800 W, the shielding effect of the bubble on the ultrasound is enhanced, and the structure refinement effect becomes worse, which is also the main reason for the reduction in hardness.
From the friction and wear test, it can be seen that after applying ultrasonic vibration, when the ultrasonic power is 400, 600, and 800 W, the average friction coefficient is 0.427 2, 0.396 7, and 0.379 9, respectively. Compared with the average friction coefficient of 0.464 4 without ultrasonic vibration, it decreased by 8%, 14% and 18% respectively. The wear volume was measured by a depth of field digital microscope, and the modeling results are shown in Figure 13a. It can be seen that the coating thickness is about 1.3 mm, the heat affected zone thickness is about 0.2 mm, and the wear scar depth is about 32 μm, so the friction and wear experiment did not wear through the substrate, but only occurred on the coating. The wear volume is calculated based on the three-dimensional morphology. Three equidistant sections are selected as a set of data on the three-dimensional morphology. The average value of the three sets of data sets is obtained, and the wear volume is calculated by the formula V=SL, where V is the wear volume, S is the wear area, and L is the total wear length. Finally, the wear rate of the coating is calculated, and the calculation results are shown in Figure 13b. The wear rate of the coating without ultrasound is 4.83×10‘−5
mm’3/(N·m). After ultrasound is applied, the wear rate of the coating is significantly reduced. Among them, when the ultrasonic power is 600 W, the wear rate of the coating is the lowest, only 3.87×10’−5mm’3/(N·m).
The morphology after wear is scanned by electron microscope, and the results are shown in Figure 14. It can be seen that the wear type of IN 625 nickel-based high-temperature alloy coating is micro-motion wear, which is a composite wear mechanism including surface fatigue wear, adhesive wear, and abrasive wear. When ultrasonic vibration is applied, the pit phenomenon is reduced and the color of the adhesion area becomes lighter, which proves that the degree of fatigue wear and adhesive wear is weakened. In addition, it is found that after ultrasound is applied, the scratches change from deeper straight groove scratches to shallower plow groove scratches. It can be seen that compared with the wear morphology without ultrasound, increasing the ultrasonic power can reduce the area of ​​wear debris accumulation. The scratch depth in the grinding area becomes shallower, which proves that the degree of abrasive wear is reduced. As shown in Figure 14, with the addition of ultrasound and the increase of ultrasonic power, the wear surface morphology of the sample gradually becomes smoother, and the degree of large-area peeling is reduced. In addition, white dots appear in Figures 14a~d, which are surface precipitation phases that appear during high-temperature grinding. The precipitation phase will lead to aggravated abrasive wear. The more precipitation phases, the more serious the abrasive wear. Comparing the wear SEM images, it can be seen that when ultrasonic vibration is not applied, the scratches formed by abrasive wear are deeper and more obvious. In summary, after applying ultrasound, the wear type of the coating changes from the original complex wear to simple wear dominated by abrasive wear and adhesive wear. With the increase of ultrasonic power, the degree of abrasive wear and adhesive wear is further reduced.
It can also be found from Figure 14 that an oxide layer appears in the wear morphology without ultrasonic vibration and with 400 W ultrasonic vibration, that is, static oxidation occurs, and the size of the oxidation area reflects the degree of oxidation. When the coating surface is squeezed by the grinding ball, it peels off. Then, as the temperature decreases, the oxidation area increases. The larger the oxidation area, the more serious the static oxidation. The static oxidation degree of Figure 14b~d is significantly better than that of Figure 14a. This is because ultrasonic vibration can reduce the diffusion of air into the coating by reducing the bubbles or pores in the molten pool, so that the coating has good oxidation resistance. When the ultrasonic power increases to 600 W, the static oxidation layer is transformed into an attached oxidation layer. This is because under high temperature and friction conditions, the oxides formed by the oxidation of the metal alloy adhere to the friction surface. This attached oxide avoids direct contact between the grinding ball and the coating surface, and improves the surface wear resistance of the coating. From the point scanning results, it can be seen that the attached oxides are mainly FeO and Cr2O3, among which the melting point of FeO is 1360℃ and the melting point of Cr2O3 is 1450℃. These two oxides can well protect the friction surface and improve the wear resistance of the coating. When the ultrasonic power increases to 800 W, the amount of attached oxides decreases. At the same time, as can be seen from Section 2.2, the crystal size is larger than that at 600 W, so the coating wear resistance is lower than that at 600 W ultrasonic power.

From a macroscopic perspective, ultrasonic vibration accelerates the circulation/convection speed inside the molten pool through mechanical and acoustic flow effects, improves heat transfer efficiency, reduces the surface tension of the molten pool, reduces the internal temperature gradient, and accelerates the liquid phase solidification rate, which is conducive to forming a dense coating structure and improving the resistance to plastic deformation. From a microscopic perspective, under the ultrasonic cavitation effect, the cavitation bubble is shocked and ruptured and a high-speed microjet is generated to break the solid dendrite network, and the broken crystals form new crystal centers to continue to grow, while the ultrasonic thermal effect increases the supercooling of the coating, accelerates the nucleation rate of the crystal, refines the grain size, plays a role in fine grain strengthening, and is conducive to improving the wear resistance of the coating.

3 Conclusions

The IN 625 coating was prepared on the H13 substrate by ultrasonic vibration assisted laser cladding. The content, distribution mode and precipitation morphology of the precipitated phase were studied by phase detection and microstructure observation, and the microhardness and high temperature wear resistance of the coating were measured. The following conclusions were drawn:
1) Under different powers, the types of phases of the IN 625 coating after ultrasonic vibration assisted laser cladding did not change significantly. The IN 625 coating was mainly composed of fcc γ-Ni phase and Cr23C6. Due to the low Laves content, no obvious Laves diffraction peak appeared. After applying ultrasound, the structure of the coating was significantly refined. When the ultrasonic power was 600 W, the subgrain structure size in the dendrite was the smallest and the distribution was the most uniform and compact.
2) After the introduction of ultrasound, the acoustic streaming effect and cavitation effect of ultrasound reduced the enrichment of elements, inhibited the segregation of elements such as Nb and Mo in the coating, and reduced the content of Laves phase in the coating. From the morphological point of view, after applying ultrasound, the precipitated phase changes from chain distribution to granular dispersed distribution, the distribution spacing expands and the size of the precipitated phase decreases significantly.
3) With the increase of ultrasonic power, the hardness of the coating increases first and then decreases, but the microhardness during the decrease is still significantly higher than the microhardness of the cladding sample without ultrasonic action. After applying ultrasound, the degree of adhesive wear and abrasive wear of the coating is significantly reduced, and the friction coefficient and wear rate of the coating are reduced. When the ultrasonic power is 600 W, the wear rate of the coating reaches the lowest, and the wear resistance of the coating is greatly improved.