Objective To study the effect of NbC particle addition on the microstructure, hardness and wear resistance of laser cladding NbC/Ni60 composite coating on H13 steel surface. Methods Ni60 alloy powder and NbC carbide powder were ball-milled and mixed, and NbC/Ni60 alloy composite coatings with different NbC contents (mass fractions of 0%, 10%, 20%, and 30%) were prepared on the surface of H13 steel substrate by laser cladding technology. The microstructure and phase of the composite coatings were analyzed by scanning electron microscopy (SEM) and X-ray diffractometer. The cross-sectional microhardness distribution of the composite coatings was studied by microhardness tester. The friction and wear properties of the composite coatings were tested at 400 ℃ in vacuum by high temperature friction and wear testing machine. Results In the laser cladding NbC/Ni60 composite coating, the phases are mainly composed of γ-(Ni, Fe) solid solution, Ni2Si, CrB, Cr23C6, and NbC. The cladding layer is mainly composed of cellular crystals and dendrites. The NbC content has a significant effect on the microstructure and morphology of the composite cladding layer. Adding a small amount of NbC can refine the microstructure of the cladding layer. When the mass fraction of NbC is 20%, a large number of dispersed NbC particles tend to aggregate between dendrites. When the mass fraction of NbC is 30%, the NbC phase in the cladding layer presents blocky and petal-like morphologies. The hardness of the NbC/Ni60 composite coating is significantly higher than that of the H13 steel substrate. With the increase of NbC content, the microhardness of the NbC/Ni60 composite cladding layer gradually increases. The average hardness of the NbC/Ni60 composite cladding layer with a mass fraction of NbC of 30% is as high as 848HV. Under the wear conditions of vacuum 400 ℃, pressure 100 N, speed 100 r/min, and time 7200 s, the wear volume of NbC/Ni60 composite coating with NbC mass fraction of 20% is the smallest, so its high temperature wear resistance is the best. The friction coefficient of NbC/Ni60 composite coating with NbC mass fraction of 10% is the smallest. With the increase of NbC content, the friction coefficient of the composite coating increases instead. Conclusion NbC/Ni60 composite coating has a good metallurgical bonding with H13 steel substrate, which significantly improves the high temperature wear resistance; NbC particle hard phase has a good reinforcement effect, which can significantly improve the hardness and wear resistance of NbC/Ni60 composite coating; coarse NbC phase is not conducive to further improvement of the wear resistance of the composite coating. The wear mechanism of NbC/Ni60 composite coating is mainly abrasive wear and fatigue spalling wear.

H13 steel (4Cr5MoSiVl) is a hot working die steel with high hardenability, high toughness, high high temperature strength and high hot hardness. Its service temperature can be close to 600 °C and is commonly used in hot forging dies, hot extrusion dies and die casting dies [1]. Under medium and high temperatures, the surface of the hot working die cavity is prone to wear and thermal fatigue, and is also affected by erosion and stress corrosion, which reduces the life of the die or causes the die to fail. Surface engineering technology can change the surface chemical composition, phase composition and microstructure of the die steel, improve the surface hardness and strength of the die, and increase the ability of the die to resist wear, deformation and thermal fatigue, thereby effectively improving the service life of the die [2]. Compared with traditional surface modification technologies such as carbonitriding (chemical heat treatment), induction heating quenching (surface heat treatment), and thermal spraying, laser cladding technology uses a high-energy laser beam to quickly melt the surface alloy and powder. The cooling rate during solidification is fast (102~106 ℃/s). The prepared coating has a supersaturated solid solution, ultrafine grain structure and metastable phase structure, as well as high strength, high hardness, and excellent wear and corrosion resistance. In addition, it is metallurgically bonded to the substrate and has the characteristics of large thickness, dense structure, small deformation, and good processing flexibility [3-5]. Therefore, it is widely used in automotive molds, aviation, electronic machinery and other fields [6-7]. The surface strengthening laser cladding coating of mold steel often uses Ni-based self-fluxing alloys with high hardness, good wear resistance and good high-temperature oxidation resistance [8-11]. By adding high-hardness ceramic powders such as WC [12-14], TiC [15-16], and NbC [17-19] to Ni-based alloy powders, composite powders are obtained, and then metal-based ceramic composite cladding coatings are prepared to further improve their hardness and wear resistance. This has become one of the hot topics in mold surface strengthening research. Nb alloying can refine the grains and improve the metal structure, thereby improving its performance [17-18]. NbC has a high melting point (3,600 °C), high hardness (2,400 HV), good chemical stability, good wear resistance, etc. Its density (7.79 g/cm3) is very close to the density of nickel-based and iron-based alloys and their substrates for laser cladding, and is a good reinforcement phase [19-24]. In this paper, a YGA pulsed laser was used to prepare a high-hardness NbC/Ni-based alloy composite coating on the surface of H13 hot working die steel using Ni60+NbC composite powder with different NbC contents as laser cladding powder. The microstructure, microhardness and high-temperature wear resistance of the NbC/Ni60 composite cladding layer were studied.

1 Experiment

The substrate is H13 steel (4Gr5MoSiV1), and its wire cutting size is 50 mmx40 mmx20 mm. The chemical composition is shown in Table 1, and the structure is tempered troostite and carbide. The surface of the substrate was polished, cleaned and dried in sequence using 400~1200 sandpaper. The cladding layer powder is a mixed powder of NbC+Ni60. Among them, the particle size of NbC powder is 1~3 μm. The main chemical composition of Ni60 powder is shown in Table 1, and its particle size is 50~150 μm. In the experiment, NbC powder was added to Ni60 alloy powder at a mass fraction of 0%, 10%, 20%, and 30%, respectively. The powder was mixed by ball milling at a speed of 200 r/min, a ball-to-material ratio of 5:1, and a ball milling time of 6 h. After ball milling, a relatively uniform and irregularly shaped mixed powder was obtained. Before laser cladding, the mixed powder was pre-coated on the surface of the H13 steel substrate with an organic binder. The pre-coated layer thickness was about 300~400 μm and was preheated (dried at 200 °C) for use. The laser cladding samples were heated (insulated) to reduce cracks that may be caused by rapid cooling and rapid heating.

Laser cladding uses CDQS-UHS-700B Nd:YAG solid industrial laser equipment, the numerical control system is PA8000NT CNC, the laser wavelength of the YAG laser is 1.06 μm, and the laser cladding principle and the schematic diagram of the coating sample are shown in Figure 1. The parameters of the laser cladding process are: laser current is 250 A, frequency is 20 Hz, pulse width is 2.5 ms, spot diameter is 1 mm, and scanning speed is 100 mm/min. During the experiment, argon gas was used as the protective gas with a flow rate of 10 L/min. The Phenom XL desktop scanning electron microscope was used to observe and analyze the structure and surface morphology of the cladding layer, and the MiniFlex 600 X-ray phase diffractometer (Cu target, tube voltage is 40 kV, tube current is 15 mA, scanning range is 25°~100°, scanning speed is 4 (°)/min) was used to analyze the phase. The microhardness was tested using a HXD-1000TM microhardness tester (loading load of 0.49 N, loading time of 20 s). The wear characteristics of different samples were tested at a high temperature of 400 °C using a MG-2000 high-speed high-temperature friction and wear testing machine. The friction and wear diagram is shown in Figure 2. The wear conditions were set as follows: vacuum degree was less than 1 mPa, loading pressure was 100 N, speed was 100 r/min, time was 7 200 s, and wear stroke was 138 m; the wear sample size was φ4 mm x 15 mm, and the grinding disc was quenched and tempered 45# steel.

2 Results and analysis

2.1 Phase analysis of laser cladding layer

The NbC/Ni60 composite cladding layers with NbC mass fractions of 10%, 20%, and 30% are recorded as 10%NbC/Ni60, 20%NbC/Ni60, and 30%NbC/Ni60, respectively, and their X-ray diffraction patterns are shown in Figure 3. As shown in Figure 3, the Ni60 cladding layer mainly contains γ-(Ni, Fe) solid solution, Ni2Si, Fe3C, Cr23C6, and CrB; after adding NbC, the NbC phase is added to the cladding layer, and the phases of the NbC/Ni60 composite cladding layer are mainly composed of γ-(Ni,Fe) solid solution, Ni2Si, Fe3C, Cr23C6, CrB, and NbC; in the cladding layer, Ni and Fe form a γ-(Ni, Fe) solid solution, and metallurgical chemical reactions occur between the coating elements under the action of laser to form Ni2Si, Fe3C, and Cr7C3 as metastable phases after rapid solidification, and NbC as an added hard phase. After laser cladding, the uniform distribution of multiple hard phases (such as carbides) on the coating surface will be beneficial to improving the hardness and wear resistance of the composite cladding layer.

2.2 Microstructure of laser cladding layer

The cross-sectional microstructures of two regions of NbC/Ni60 composite cladding layers with different NbC contents are shown in Figure 4. As shown in Figure 4, the thickness of the NbC/Ni60 composite cladding layer is about 250~300 μm, and it has a good metallurgical bonding interface with the substrate. The Ni60 cladding layer is mainly a fine dendrite structure formed by rapid solidification, with a dendrite spacing of 2~5 μm. After adding a small amount of NbC (10%), the structure of the NbC/Ni60 composite cladding layer is refined, and the dendrite spacing is 1~3 μm. In the 10%NbC/Ni60 composite cladding layer, the particle size of NbC particles is 0.5~3 μm. With the increase of NbC content, when the mass fraction of NbC is 20% and 30%, the composite cladding layer mainly presents block-shaped, granular, petal-shaped, star-shaped and other morphologies. NbC particles gradually aggregate and grow, and some flakes are as long as 6 μm. The results of regional energy spectrum analysis (see Table 2) show that the dendrites contain a large amount of Ni, Fe, and Cr elements, belonging to the γ-(Ni,Fe) type solid solution. The content of Ni and Fe between dendrites decreases, and the content of Cr, Si and other elements increases significantly, and the aggregation of Ni2Si, Fe3C, Cr23C6 and other phases occurs. After the addition of NbC, Nb elements exist in the dendrites (point C). There are also other elements (such as Ni, Si, Cr, Fe, B, etc.) in the NbC phase. The reason is that part of the NbC phase grows during solidification, and Ni2Si, Cr23C6 and other phases precipitate or aggregate on the NbC surface. The reasons for the microstructure refinement of the NbC/Ni60 laser cladding layer are analyzed. On the one hand, during the laser cladding process, part of the NbC is dissolved and decomposed in the molten pool, releasing Nb and C elements, resulting in composition fluctuations or composition supercooling, which can inhibit the growth of the crystal nucleus; on the other hand, NbC, as a heterogeneous crystal core, plays a refining role. With the increase of NbC content, a large number of unmelted NbC dispersed white small particles also hinder the growth of dendrites.

2.3 Microhardness of laser cladding layer

The effect of different NbC contents on the microhardness of the cross section of the NbC/Ni60 cladding layer is shown in Figure 5. As shown in Figure 5, the microhardness of the cladding layer is much greater than that of the quenched and tempered H13 steel. After adding NbC, the microhardness of the cladding layer surface increases with the increase of NbC content. When the mass fraction of NbC is 0%, 10%, 20%, and 30%, the average hardness of the NbC/Ni60 composite cladding layer is 635HV, 690HV, 753HV, and 848HV, respectively. After rapid melting and solidification, the dendrite structure of the laser cladding layer is significantly refined. The addition of NbC improves the hardness of the composite cladding layer. On the one hand, the addition of NbC makes the dendrite grains smaller, and other intermetallic compounds (such as carbides) are refined, which plays a role in refining grains; NbC decomposes in the molten pool, and some Nb and C elements are dissolved into the matrix structure, which plays a role in solid solution strengthening; the hardness of NbC particles (2400HV) is extremely high, and they reprecipitate or grow after melting during laser cladding, which has a high second phase strengthening effect. NbC is dispersed in the cladding layer, which produces a good strengthening effect. The microhardness of the cladding layer gradually decreases with the increase of the distance from the surface. The hardness of the cladding layer decreases significantly after passing through the bonding area between the cladding layer and the matrix interface (300~400 μm from the surface).

2.4 Friction and wear properties of laser cladding layers

When NbC/Ni60 composite cladding layers with different NbC contents were tested at 400 °C, the friction coefficient-time curve and wear loss results are shown in Figure 6. As shown in Figure 6a, during the wear test, the friction coefficient of the cladding layer fluctuated with the extension of time, but was generally stable; the friction coefficient of the Ni60 cladding layer was higher, with an average friction coefficient of 0.82; the friction coefficient of the 10%NbC/Ni60 composite cladding layer was the smallest, with an average friction coefficient of 0.69; the initial addition of NbC had the effect of reducing the friction coefficient, and the friction coefficient increased with the increase of NbC content. As shown in Figure 6b, the wear loss of the Ni60 cladding layer is only 18 mg, which is significantly lower than the wear loss of H13 steel hardened by heat treatment (quenching and tempering) of the matrix (34 mg); when the mass fraction of NbC is 10%, 20%, and 30%, the average wear loss of the cladding layer is 5, 2.3, and 3.7 mg, respectively. The wear resistance of H13 steel is significantly improved by laser cladding. The addition of NbC to the cladding layer Ni60 significantly reduces its wear loss and improves wear resistance. With the increase of NbC content, the wear loss of the cladding layer shows a trend of first decreasing and then increasing; when the mass fraction of NbC is 20%, the wear loss is the smallest and the wear resistance is better. The addition of NbC is conducive to fine grain strengthening, solid solution strengthening, and second phase strengthening, thereby further improving hardness and wear resistance. When the mass fraction of NbC is 30%, the average hardness of the cladding layer is the highest, but its wear volume increases and its wear resistance decreases. The reason is that the coarse NbC particles in the cladding layer are easy to produce large internal stress, resulting in excessive surface brittleness.

The friction and wear test was carried out at 400 ℃. The wear surface morphology of NbC/Ni60 cladding layers with different NbC contents (0%, 10%, 20%, 30%) is shown in Figure 7, and the energy spectrum analysis of the local area of ​​the wear surface is shown in Table 3. As shown in Figure 7, the surface of the quenched and tempered 45# steel of the grinding wheel is severely worn, with a large amount of peeling and pit areas. Due to the low hardness, plastic deformation and adhesive wear are prone to occur, and the adhesive part is torn, and its wear mechanism is mainly adhesive wear. The surface of the heat-treated (quenched and tempered) H13 steel sample has deep wear furrows, obvious delamination pit areas, and poor wear resistance. Its wear mechanism is mainly abrasive wear and adhesive wear. The surface wear degree of the Ni60 cladding layer is low, with shallow scratches and a small amount of delamination. The scratches on the surface of the NbC/Ni60 cladding layer become shallower and finer. There are flaky wear debris (Ⅰ, Ⅲ, Ⅳ) on the surface of the NbC/Ni60 cladding layer, the main components of which are O, Fe, and Cr, which are mainly the products of adhesive wear and detachment of the grinding disc. Due to the high hardness of NbC, the deformation resistance of the cladding layer is improved by adding NbC, the micro-cutting effect on the wear surface is weakened, and only the furrows caused by abrasive wear are left. With the increase of NbC content, the furrow-shaped scratches become fewer and shallower, and the surface tends to be smoother. There are fine abrasive particles on the surface of the 20%NbC/Ni60 cladding layer. From the energy spectrum analysis (II), it can be seen that it is rich in Nb and C, indicating that NbC falls off during the wear process. The wear mechanism is mainly abrasive wear, and it also shows a trend of changing from a two-body mode to a three-body wear. The addition of NbC can avoid more serious wear of the cladding layer to a certain extent, and play a certain role in reducing wear and resisting wear. When the mass fraction of NbC is 30%, some peeling marks appear on the surface of the cladding layer. This is mainly because the NbC particles on the surface of the 30%NbC/Ni60 cladding layer are coarsened and easily become brittle. Under the action of cyclic repeated shear stress, stress concentration is prone to occur, thereby initiating cracks and causing surface peeling fatigue wear.

3 Conclusions

1) The phases of the laser cladding NbC/Ni60 composite coating are mainly composed of γ-(Ni, Fe) solid solution, Ni2Si, Fe3C, Cr23C6, CrB, and NbC. The addition of NbC significantly refines the microstructure of the cladding layer. When the NbC content is high, NbC gradually aggregates and grows, mainly showing block, granular, petal, star-shaped and other morphologies.

2) The hardness of the laser cladding NbC/Ni60 composite coating is significantly higher than that of the H13 steel substrate. The microhardness of the cladding layer increases with the increase of NbC content, and the average hardness of the 30%NbC/Ni60 composite cladding layer is the highest.

3) Under the friction and wear conditions of vacuum 400 ℃, pressure 100 N, speed 100 r/min, and time 7 200 s, the appropriate addition of NbC has the effect of reducing the friction factor. The friction factor of the 10%NbC/Ni60 composite cladding layer is the smallest; the high temperature wear resistance of the 20%NbC/Ni60 composite coating is the best. The NbC/Ni60 composite coating has excellent wear resistance. The addition of NbC can significantly reduce the wear rate of the cladding layer. Although excessive and coarse NbC particles increase its hardness, its wear rate is reduced. The main wear mechanisms of H13 steel are abrasive wear and adhesive wear. The main wear mechanisms of NbC/Ni60 composite cladding layer are abrasive wear and fatigue spalling.