The failure of the mold is mostly caused on the surface. The nickel-based high-temperature alloy coating is laser clad on the surface of H13 mold steel to repair the worn surface and extend the service life of the mold. The phase composition, element distribution and laser cladding process parameters of the cladding layer on the macroscopic morphology and microstructure of the nickel-based coating on the surface of the mold steel were analyzed by XRD, OM, SEM and EDS. The results show that the surface of the cladding layer is smooth, without macroscopic cracks, pores and unmelted phenomena. With the increase of scanning speed, the growth morphology of the cladding layer changes, the chrysanthemum-shaped eutectic structure disappears, the interface structure changes from cellular crystals to plane crystals, and the bottom structure of the cladding layer is composed of a large number of columnar dendrites and a small number of cellular crystals. The cladding layer is composed of a matrix phase and a strengthening phase. -(F,Ni) is the matrix phase of the cladding layer, and the strengthening phases are mainly Ni,Si2, Ni.Si,BI, CrzC. And CrB, etc. The hardness of the cladding layer is high, with an average hardness of 565HV0.2, which is 2.1 times that of the substrate.

Hot working die steel is a steel material used to manufacture dies used in the hot working process of various metal and alloy materials. It is mainly used to produce hammer forging dies, hot extrusion dies and die casting dies. Its working environment is in contact with metal or metal liquid above the recrystallization temperature. Therefore, compared with ordinary die steel, it has higher requirements for high temperature hardness, thermal fatigue resistance and thermal wear resistance in terms of mechanical properties. Due to the harsh working environment of hot working dies, the working surface of the die is often subjected to large cyclic stamping loads, temperature differences, friction and wear, etc., which seriously reduces the service life of the die. The failure of the die is mostly caused on the surface. Repairing the worn surface or improving the surface structure and performance can effectively extend the service life of the die, reduce manufacturing costs, and reduce economic losses. Domestic and foreign scholars have studied laser cladding Ni-based coatings. Zhao Xueyang et al. [14] studied the laser cladding Ni-based composite coating on the substrate surface and obtained the optimal component addition amount of ceramic particles, lubricating phase, and rare earth element oxides in the composite coating. Hang Wenxian et al. [5] found that after adding WC and RE, the cladding layer structure became more uniform and the wear resistance was improved; Li Fuquan et al. [ studied the influence of WC content on the change of cladding layer structure. With the increase of WC mass fraction, the carbide in the cladding layer continued to increase. However, there are few studies on the influence of laser energy density on interface organization and element diffusion. Therefore, this paper obtains nickel-based coating by laser cladding nickel-based self-fluxing alloy powder on the surface of H13 die steel; and analyzes the macroscopic morphology, hardness distribution and phase composition of the nickel-based cladding layer; and studies the influence of different laser energy densities on the microstructure of the coating and the distribution of elements at the interface.

1 Experimental materials and methods

1.1 Experimental materials and coating preparation

H13 steel (hot working die steel 4Cr5MoSiV1) is used as the substrate for laser cladding experiment. The sample size is 90mmx45mmx10mm. The sample is in the annealed state when it leaves the factory. Its chemical composition (mass fraction, %) is: 0.42C, 0.89Si, 0.3Mn, 5Cr, 1.27Mo, 0.88V, 0.16Ni, 0.021P, 0.008S, and the balance is Fe. Before the laser cladding experiment, the substrate surface needs to be cleaned and the cladding powder needs to be dried. All samples are ground by a grinder to remove surface oxides before cladding, and then the surface oil is cleaned by acetone ultrasonic cleaning. Finally, the cladding samples and cladding powder are placed in a vacuum drying oven for 4 hours. The cladding layer material is nickel-based self-fluxing alloy powder (Ni50A), the powder particle size is -150 mesh to 300 mesh, and its chemical composition (mass fraction, %) is: 0.48C, 3.87Si, 10.47Cr, 2.35B, 3.05Fe, and the rest is Ni.

The laser cladding equipment consists of a 1.5kW fiber-coupled semiconductor laser (spot diameter 5mm), a lateral synchronous powder feeder, a water cooler, a protective gas path, and an xy-axis mobile platform built by the research team. Argon is used as the powder feeding gas and protective gas in the laser cladding process. The sample numbers and their corresponding process parameters are as follows: Sample 1 (laser power 1050W, scanning speed 1.5mm/s, powder feeding rate 6g/min, gas flow rate 5L/min); Sample 2 (laser power 1050W, scanning speed 2.5mm/s, powder feeding rate 6g/min, gas flow rate 5L/min).

1.2 Material characterization and performance test method

Select the part with stable cladding layer, cut the metallographic sample along the cross section of the cladding layer by wire cutting, then polish the cross section step by step with metallographic sandpaper, and finally polish it mechanically with a polishing agent with a particle size of 1μm for 25~35min. Aqua regia prepared with hydrochloric acid and nitric acid (ratio HCI:HNO3=3:1) is used as the metallographic etchant, and etched at room temperature for 15~20s. After the metallographic sample is etched, it is rinsed with water and blown dry. The microstructure is observed using a metallographic microscope. The organization of the cladding layer was analyzed by a tungsten filament scanning electron microscope (SEM) and an energy spectrum analyzer built into the scanning electron microscope; the physical phase of the cladding layer was analyzed by a D/MAX-RB12KW X-ray diffractometer; the hardness was tested by a HVS-1000Z microhardness tester with a load of 200g for 20s.

2 Experimental results and analysis

2.1 Macroscopic morphology

Figure 1 shows the morphology of the cladding layer after slag removal. It can be seen that the cladding layer has good slag removal properties, the cladding layer in the stable cladding stage is relatively flat, and there are fewer splashing particles on both sides. The surface of the cladding layer is smooth and has a small waviness. There are no macroscopic cracks, pores, and unmelted phenomena on the surface of the cladding layer. There is a small amount of gray-green chromium oxide slag that has not fallen off locally.

2.2 Physical phase of the cladding layer

Figure 2 shows the XRD spectrum of the Ni50A cladding layer. By comparing with the PDF standard powder diffraction card, it is found that the main phase of the Ni50A cladding layer is γ-(Fe, Ni) as the matrix phase, and the type of the strengthening phase is further determined. The strengthening phase is mainly Ni3Si2, Ni6Si2B1, Cr23C6. And CrB, etc. The γ-(Fe, Ni) matrix phase is the toughness phase of the cladding layer. Cr23C6 has a high hardness but weak bonding strength, poor stability, and a large size. It is not easy to maintain a fine dispersed phase.

2.3 Microstructure of the cladding layer

Analysis of the structure of the junction between the cladding layer and the matrix: According to the solidification theory, the morphology of the solidification structure is determined by the solidification rate R and the temperature gradient G during the solidification process. In the early stage of solidification, the G/R value is large, the maximum temperature gradient direction in the cladding layer is perpendicular to the bonding interface, there is basically no component supercooling at the solid-liquid interface, and the solidification rate is small, so the plane crystal 7 is first formed at the interface. When the G/R value decreases, the ratio of the temperature gradient to the solidification rate is small and insufficient to form a planar crystal. At this time, the structure will grow in the form of cellular crystals. When the temperature gradient is further reduced and the solidification rate is further increased, the growth mode of the cladding layer will change from cellular crystals to dendrites and equiaxed crystals.

The fusion zone of sample 1 appears as a white bright band under the optical microscope. The white bright band structure in Figure 3 (a) is not a planar crystal, but a cellular columnar crystal. In the process of interface structure formation in Figure 3 (a), the ratio of the temperature gradient to the solidification rate during the laser cladding process is small and insufficient to form a planar crystal; the interface structure grows in the form of cellular columnar crystals, and there are semi-molten austenite grain boundaries in the matrix at the interface. After rapid heating and rapid cooling by laser cladding, the original austenite grain boundaries in the matrix structure are relatively clear, and lamellar martensite is distributed on the gray-white matrix. The cellular columnar crystals grow along the semi-molten austenite grain boundaries and are well combined with the matrix. The dendrites and granular structures at the bottom of the cladding layer grow along the cellular columnar crystals. As the G/R value further decreases, dendrites gradually appear above the white bright band, and a large number of lamellar eutectic structures and granular structures are distributed at the bottom of the cladding layer, with a small amount of columnar dendrites locally.

When the scanning speed is increased to 2.5 mm/s, the growth morphology of the cladding layer changes, the chrysanthemum-shaped eutectic structure disappears, the interface structure growth characteristics are plane crystal shape, and the bottom of the cladding layer is a large number of columnar dendrites and cellular crystals, which grow vertically to the interface. With the increase of scanning speed, the laser energy density decreases, the cooling rate increases, and the G/R value increases, so the interface structure growth morphology of the cladding layer changes.

The microstructure of the upper cladding layer of sample 1 is shown in Figure 4, and the energy spectrum analysis results of the corresponding points in Figure 4 are shown in Table 1. From the scanning analysis results of point 1, the main element of the block structure corresponding to point 1 is Cr, the atomic percentage of Cr is 78%, and the atomic percentage of C and Cr is close to 7:1; combined with the XRD analysis results, it can be seen that the block structure is the carbide M2C6 of Cr. From the scanning analysis results of point 2, the main element of the block structure corresponding to point 2 is Cr, the atomic percentage of Cr is 59.033%, and the atomic percentage of C and Cr is close to 4:1; combined with the XRD analysis results, it can be seen that the block structure is the carbide of Cr element Cr23C6. Combined with the energy spectrum analysis, it can be seen that the structure of point 3 is mainly composed of Si and Ni elements. Unlike the structure at point 1, the main element at point 3 is Ni, and the atomic fraction of Ni is 66.187%; combined with the XRD analysis results, it can be seen that the structure at point 3 is a eutectic structure composed of -(Fe,Ni) solid solution and carbide. Point 4 is the matrix structure of the cladding layer. Combined with the energy spectrum analysis, it can be seen that the structure of point 4 is mainly composed of Fe and Ni elements, and the atomic fraction of Ni is 64.033%; combined with the XRD analysis results, it can be seen that the structure at point 4 is -(Fe,Ni) solid solution, which is the matrix structure of the cladding layer.

In order to further confirm whether there is element diffusion and metallurgical bonding between the cladding layer and the substrate, line scanning and surface scanning analysis were performed on the interface; combined with the distribution of surface elements, the interface bonding type was analyzed to see whether it was an element diffusion interface.

From the results of element line scanning at the interface, as shown in Figure 5, the main elements in the substrate are Fe and the main element Ni in the replication interface substrate, which show an obvious transition trend near the interface. The interface composition changes in a gradient, and the elements on both sides diffuse with each other, but the diffusion range is small, so it can be judged that the dilution rate is low; the remaining elements do not have obvious diffusion near the interface (such as C, Si, Cr and other elements), and these elements are distributed more evenly in the substrate and cladding layer. Therefore, the element diffusion at the interface is mainly manifested as the mutual diffusion of Fe and Ni. The combination of the substrate and the cladding layer is metallurgical bonding, and there is an element diffusion interface.

Combined with the distribution of surface and line elements, the Si element distribution content in the cladding layer is higher than that in the substrate and the distribution is uniform, with less element segregation. The Cr element distribution content in the cladding layer is higher than that in the substrate, but the distribution is uneven, and there is local element segregation. It can be seen that the Cr element is evenly distributed in the interface and the substrate. The Fe content in the cladding layer is lower than that in the substrate, but the distribution is uneven, and there is a small amount of element segregation. The interface and element transition area can be clearly seen.

Figure 6 shows the interface surface scanning analysis results of sample 1. Figure 6 (a) shows that the Fe content gradually increases from the top (cladding layer) to the bottom (substrate), and the Fe element is evenly distributed in the substrate. As can be seen from Figure 6 (b), the Ni content in the cladding layer is higher than that in the substrate, but the distribution is uneven, and there is a small amount of element segregation; the interface and element transition area can also be clearly seen, and the Ni element in the substrate is evenly distributed.

2.4 Microhardness of the cladding layer

From the top of the cladding layer to the substrate, a point is taken every 0.2 mm. The value of each point is obtained by taking the average of multiple values. These points are selected at the same vertical depth from the surface of the same sample.

The hardness of the cladding layer is relatively large, with a maximum value of about 668HV0.2 and an average hardness of 565HV0.2, which is 2.1 times that of the substrate. The obvious improvement of the hardness of the surface layer of the Ni-based alloy laser cladding of H13 steel is due to the distribution of high-hardness, high-melting-point hard strengthening phases and eutectic structures in the cladding layer. The γ-(Fe,Ni) solid solution, grain boundaries and a large number of precipitation phases in the grains constitute the strengthening skeleton of the cladding layer, which can greatly improve the comprehensive performance of the cladding layer. Under the effect of laser heating, the hardness of the cladding zone and the heat-affected zone is relatively high, ranging from 300 to 691.7HV0.2. The main reason for the sudden change in the hardness of the heat-affected zone is that under the effect of laser heat, the dilution zone structure of H13 steel contains a large number of strengthening phases, and the heat-affected zone structure is needle-shaped and lath-shaped martensite, so the hardness of the heat-affected zone suddenly changes by 8. As the distance from the top of the cladding layer increases, the influence of the heat input of laser cladding on the matrix structure gradually decreases, and the hardness gradually decreases. The surface grains of the cladding layer are fine, and the cladding layer structure gradually coarsens from the top to the bottom, so the hardness gradually decreases, and there are strengthening phases and eutectic structures locally, and the hardness fluctuates.

3 Conclusions

(1) The surface of the cladding layer is flat and smooth, without macro cracks, pores and unmelted phenomena. The cladding layer is composed of matrix phase and strengthening phase. γ-(Fe, Ni) is the matrix phase of the cladding layer, and the strengthening phases are mainly Ni3Si2, Ni6Si2B1, Cr23C6 and CrB. When the scanning speed is 1.5 mm/s, the interface structure of the cladding layer is cellular crystal, and the bottom of the cladding layer is composed of a large number of eutectic structures, cellular crystals and a small number of dendrites.

(2) With the increase of scanning speed, the growth morphology of the cladding layer changes, the chrysanthemum-shaped eutectic structure disappears, the interface structure changes from cellular crystals to planar crystals, and the bottom structure of the cladding layer is composed of a large number of columnar dendrites and a small number of cellular crystals. The element diffusion at the interface is mainly manifested as the mutual diffusion of Fe and Ni. The combination of the matrix and the cladding layer is a metallurgical combination, and there is an element diffusion interface. The cladding layer has a high hardness, with an average hardness of 565HV0.2. From top to bottom, the cladding layer structure gradually coarsens, and there are local strengthening phases and eutectic structures, so the hardness gradually decreases and fluctuates.