In order to explore the effect of dilution rate on the microstructure and hardness of cladding layer under different carbon steel substrates, 316L stainless steel cladding layers with different dilution rates were prepared on the surface of Q235 steel, 35 steel and 45 steel substrates by laser cladding technology. The microstructure and hardness of the cladding layer were characterized by laser confocal microscopy, scanning electron microscopy and Vickers hardness tester. The experimental results show that the dilution rate of the cladding layer increases with the increase of laser power. Under the same laser power, the difference of the substrate will have a certain effect on the dilution rate of the cladding layer. The cladding layer and the substrate are metallurgically bonded. With the increase of dilution rate, the elements of the cladding layer and the nearby substrate tend to be consistent, and the bonding effect is enhanced. At the same time, the increase of C content in the cladding layer produces dispersion strengthening. When the dilution rate of the cladding layer exceeds 30%, the characteristics of the rapid solidification structure gradually disappear, and the cladding layer transforms from austenite to martensite, and the hardness value increases by 250~300 HV. (dilution rate; carbon steel; martensite; laser power; element)

The coating prepared by laser cladding technology can improve the wear resistance, corrosion resistance and oxidation resistance of the material surface [1-3]. Compared with traditional metal surface modification methods, laser cladding technology has a small heat input and can effectively reduce the cracking and deformation tendency of the substrate while forming a good metallurgical bond with the substrate, achieving a good modification effect [4-6].

Carbon steel has good comprehensive mechanical properties and is widely used in machinery, transportation and construction. However, due to its composition, the hardness and corrosion resistance of carbon steel are poor. When facing complex working conditions in actual service, it is easy to wear and corrode and fail, which limits its application in engineering. In response to these problems, the use of laser cladding technology to modify the surface of carbon steel has been effective in improving wear resistance and corrosion resistance [7-9]. Bai Yang et al. [10] prepared a 316L stainless steel cladding layer on the surface of Q235 steel by laser cladding technology and found that the material performance after cladding was much higher than the corrosion resistance of ordinary carbon steel and had good high temperature resistance. Zhang Zhong [11] chose to clad WC/Ni60 composite powder on the surface of 45 steel substrate and obtained fine and uniformly distributed needle-shaped Fe23(C,B)6 phase in the cladding layer, which improved the surface hardness and had excellent wear resistance. Wang Ge et al. [12] prepared a defect-free stainless steel cladding layer on the surface of low carbon steel by laser cladding technology. The hardness of the cladding layer was twice that of the substrate, which improved the shortcomings of low surface hardness and poor wear resistance of low carbon steel. The quality of the cladding layer of laser cladding is affected by many factors, including laser power, scanning speed and powder feeding amount [13-14]. The dilution rate can not only reflect the metallurgical bonding between the cladding layer and the substrate and affect the final forming effect, but also change the coating performance by changing the composition of the cladding layer. It is a key evaluation indicator in laser cladding technology. Dong Dongmei et al. [15] clad a nickel-based composite coating on the surface of 45 steel and found that the hardness of the cladding layer was closely related to the dilution rate. Zhang Yang et al. [16] clad Ni60 on the surface of carbon steel and found that the change in the dilution rate of the cladding layer would affect its composition and phase composition. At present, the research on the dilution rate mainly focuses on exploring the causes of the change in the dilution rate and regulating it [17]. It does not consider that in the actual application process, even materials of the same category, such as carbon steel, have differences in composition and performance. Whether this difference will affect the dilution rate and change the organization and performance of the cladding layer is rarely reported. Therefore, laser cladding technology was used, and Q235 steel, 35 steel and 45 steel widely used in engineering were selected as the substrate, and 316L stainless steel was used as the cladding material. The coating was prepared under different laser powers. By comparing the changes in the hardness of the cladding layer, the effect of the dilution rate on the microstructure and performance of the cladding layer on the surface of different carbon steels was explored.

1 Sample preparation and test method

1.1 Sample preparation

The test substrate materials were Q235 steel, 35 steel and 45 steel, and the sample size was 110 mm×100 mm×20 mm. The cladding powder was 316L stainless steel powder, the morphology is shown in Figure 1, and the powder particle size is φ75~φ150 μm. Before laser cladding, the oxide on the substrate surface was first removed by sandblasting, and then the surface particles and oil were wiped with alcohol and blown dry. Before cladding, the stainless steel powder was placed in a vacuum drying oven at 120 ℃ for 4 h. Laser cladding uses a homemade five-axis LC600 laser cladding equipment equipped with a semiconductor laser and a synchronous powder feeding laser head. In order to study the effect of dilution rate on the microstructure and performance of the cladding layer of different substrates, the control variable method is used to set 4 sets of cladding process parameters for each substrate as shown in Table 1. Each parameter is clad in a single pass, and the morphology of the cladding layer is shown in Figure 2. 1.2 Experimental method After obtaining the cladding layer samples with different dilution rates, a cutting machine is used to cut samples with a size of 10 mm×10 mm×20 mm, and the samples are polished and etched with ferric acid solution for 2 s. The surface is then rinsed with alcohol and blown dry. The macroscopic morphology and microstructure of the samples under different substrates and cladding parameters were observed using a laser confocal microscope, and the composition of the cladding layer was detected using a scanning electron microscope equipped with an energy dispersive spectrometer (EDS). The microhardness of the cladding layer was tested by a micro-Vickers hardness tester, with an applied load of 2.94 N and a holding time of 15 s.

2 Experimental results and discussion
2.1 Dilution rate of cladding layer
During the laser cladding process, the laser scans the substrate to melt the surface to form a molten pool. The molten powder enters the molten pool and undergoes metallurgical bonding with the substrate to form a cladding layer. The composition of the cladding layer is determined by the substrate and the cladding powder. The proportion of the substrate alloy in the cladding layer is defined as the dilution rate. Figure 3 shows the cross-sectional morphology of the cladding layer, which is mainly composed of the cladding layer, heat-affected zone and substrate. The dilution rate is the ratio of the substrate melting area to the cladding area, which can be approximately expressed as [9]: (1) Where η is the dilution rate, %; h is the height of the substrate melting part, mm; and H is the total height of the cladding layer, mm. There are many factors that affect the dilution rate, such as the properties of the substrate material, laser power, scanning rate and powder feeding amount. Among them, the laser heat input can affect the heat absorption of the powder and the residence time of the molten pool, and has the most significant effect on the dilution rate [11]. Therefore, this experiment adopts the control variable method, that is, only changing the laser power for the same substrate to obtain different dilution rates. The cross-sectional morphology of the cladding layers on the surfaces of Q235 steel, 35 steel and 45 steel by different processes is shown in Figure 4. There are no pores and crack defects inside the cladding layers of the 12 groups of samples, indicating that the cladding layer has good forming quality. The dilution rate of the cladding layer of each sample is shown in Figure 4. For the same substrate, under the same scanning speed and powder feeding conditions, the dilution rate increases with the increase of laser power. This is because the greater the laser power, the greater the heat input. At this time, the heat absorbed by the substrate increases, and the melting part increases accordingly, which increases the dilution rate of the cladding layer. A longitudinal comparison of different substrates found that except for the 1700 W laser power, the difference in the dilution rate of the cladding layer was within 10%. At this time, the properties of the three carbon steels had little effect on the dilution rate; at the 1700 W laser power, the dilution rate of the cladding layer on the surface of Q235 steel was significantly larger, indicating that the energy absorption efficiency of Q235 steel increased more significantly when the laser power increased, which may be related to the fact that Q235 steel contains more impurity elements. 2.2 Effect of dilution rate on microstructure of cladding layer
When the dilution rate of cladding layer is less than 30%, the cladding layer structures on the surfaces of the three carbon steels are similar, with the characteristics of austenite rapid solidification structure that changes with the depth of cladding layer. From the bottom to the top of the cladding layer, there are planar crystals, coarse cellular crystals and dendrites, fine dendrites and equiaxed crystals, as shown in Figure 5. The formation of this structure is related to the temperature gradient G at the solid-liquid interface front and the solidification rate ν. According to the metal solidification theory, G/ν determines the morphology of the crystal structure, and Gν determines the size of the crystal structure [13]. In the early stage of solidification, the liquid phase in contact with the substrate solidifies first, and the substrate temperature is much lower than that of the liquid phase. Therefore, the liquid phase will have a large degree of undercooling during solidification, and the solidification front has a large temperature gradient. At the same time, at the beginning of solidification, the interface solidification rate ν is small, and the G/ν is large at this time. The solid-liquid interface will advance in the form of a plane crystal, as shown in Figure 5 (a); as the solid-liquid interface reaches the edge of the plane crystal, the formation of the bottom structure hinders the heat conduction of the middle structure of the cladding layer. At the same time, it is difficult for the middle liquid phase to exchange heat with the outside world. Therefore, the temperature gradient is small and the heat dissipation is slow. At this time, the solidification rate increases, G/ν decreases, and the liquid phase at the front of the solid-liquid interface will generate coarse columnar crystals and dendrites, as shown in Figure As shown in Figure 5(b), as the solid-liquid interface continues to advance to the top of the cladding layer, G/ν drops sharply, reaching the conditions for the formation of dendrites and equiaxed crystals. At this time, the heat conduction direction is not only perpendicular to the solid-liquid interface and pointing to the substrate, but also heat conduction with the air. Therefore, the cooling speed during the solidification process is fast, and fine equiaxed crystals and dendrites will be generated, as shown in Figure 5(c).

As shown in Figure 6, as the dilution rate of the cladding layer increases, the characteristics of the rapid solidification structure disappear, and the cladding layer structure changes from austenite to martensite. When the dilution rate of the cladding layer is 36%, martensite appears inside the columnar crystals, and the number is small, as shown in Figure 6 (c); when the dilution rate increases to 58%, the columnar crystals in the cladding layer disappear and all change into martensite structure, as shown in Figure 6 (a). The influence of the dilution rate of the cladding layer on the structure has two main mechanisms: First, as the dilution rate increases, the matrix elements entering the cladding layer after melting increase, while diluting the Cr and Ni elements in the austenitic stainless steel, the C content of the cladding layer is increased, resulting in the cladding layer being unable to maintain a stable austenite structure, and the increase in C content reduces the martensite transformation temperature, creating conditions for the formation of martensite; the other is that a large dilution rate corresponds to a higher heat input, which means that the heat of the molten pool is higher, and the cooling rate of the cladding layer is reduced, resulting in the inability to form a rapid solidification structure. As shown in Figure 7, when the dilution rate exceeds 30%, martensite structure also appears in the surface cladding layer of 35 steel and 45 steel. Unlike Q235 steel, at a laser power of 2700 W, the surface cladding layer of 35 steel and 45 steel can still maintain the characteristics of rapid solidification structure, and martensite only appears inside the columnar crystal, as shown in Figure 7 (a) and (e). This is because at the same laser power, the cooling rate of the 35 steel and 45 steel substrates is greater than that of the Q235 steel substrate, and the cladding layer can still maintain the characteristics of rapid solidification structure. If the laser power continues to increase, theoretically, complete martensitic transformation can also occur.

2.3 Effect of dilution rate on element distribution of cladding layer The macroscopic distribution of elements is an important indicator affecting the performance of the cladding layer, which can reflect the metallurgical bonding process between the cladding layer and the substrate [14]. In order to gain a deeper understanding of the distribution characteristics of each component element during the laser cladding process, EDS tests were performed on three carbon steel cladding layers with different dilution rates. The test results show that the cladding layers on different substrates have similar element distribution and diffusion characteristics. Since the martensitic transformation of the Q235 steel cladding layer is the most complete, a specific analysis is given below. Figure 8 shows the element scanning results of the Q235 steel cladding layer at different dilution rates. As shown in Figure 8 (a), when the dilution rate is 10%, various elements segregate in the cladding depth direction. The stainless steel and the alloy elements in the Q235 steel matrix diffuse mutually. Cr, Ni, Si and Mo in the stainless steel diffuse across the original interface to the Q235 steel matrix, and Fe in the Q235 steel diffuses across the original interface to the stainless steel layer. At the same time, an obvious mutual diffusion zone is formed at the interface, indicating that the bonding mode between the substrate and the cladding layer is diffusion-type metallurgical bonding [15]. As the dilution rate increases, the difference in element distribution between the cladding layer and the nearby substrate gradually decreases, as shown in Figure 8 (b) to (d). This is because as the dilution rate increases, the molten pool insulation time is extended, and the element diffusion between the cladding layer and the substrate is more sufficient. Among them, the atomic radius of Fe, Cr, and Ni is close, and the diffusion mode is substitution diffusion, and the diffusion rate is slow, while Mo and Si have great differences in properties with the Q235 steel substrate, and the diffusion is relatively easy. Therefore, when the dilution rate is 58%, the distribution of these two elements between the cladding layer and the substrate is basically the same, as shown in Figure 8 (d). In order to quantitatively analyze the changes in the three main elements of Fe, Cr, and Ni in stainless steel, the element content of the cladding layer of the Q235 steel substrate at different dilution rates was detected and statistically analyzed, as shown in Table 2. As shown in Table 2, the Fe content in the cladding layer gradually increases with the increase of the dilution rate, while the Cr and Ni content gradually decrease with the increase of the dilution rate, indicating that the melting of the matrix elements changes the composition of the cladding layer and creates the conditions for the formation of martensite.

2.4 Effect of dilution rate on the hardness of the cladding layer

As shown in Figure 9 (a), the change of the hardness of the cladding layer with the dilution rate can be divided into three stages: in the first stage, the dilution rate is below 30%, and the hardness of the cladding layer increases slowly with the dilution rate, with a small change; in the second stage, the dilution rate of the cladding layer exceeds 30%, and the hardness of the cladding layers of the three substrates increases significantly, with an increase of 250~300 HV; in the third stage, the hardness decreases after reaching the maximum value. The hardness change of the cladding layer is affected by many factors. In the first stage, the martensitic transformation has not yet occurred. The increase in the hardness of the cladding layer is mainly due to the composition change caused by the increase in the dilution rate. More C is mixed into the cladding layer, and the increase in C content will promote the formation of carbides and strengthen the cladding layer. The hardness of the cladding layer in the second stage is greatly improved, mainly from the strengthening effect of the martensitic transformation. The dilution rate increases, and the cladding layer gradually changes from austenite to martensite. The more martensite content, the more obvious the hardness increase. When the martensitic transformation is basically completed, the hardness value tends to be stable. In the third stage, as the dilution rate increases, the supersaturation of carbon atoms in martensite increases, which intensifies the difficulty of carbon atom solid solution, thereby slowing down the martensite strengthening effect; and as the dilution rate increases, the substrate temperature rises, and the cooling process produces a tempering effect on the cladding layer, resulting in partial decomposition of martensite. When the martensite solid solution strengthening effect is less than the decomposition effect, the hardness of the cladding layer decreases. For example, the hardness of the cladding layer on the surface of Q235 steel decreases after the complete martensite transformation, while the 35 steel and 45 steel substrates have a higher carbon atom content and are less difficult to solid solution than Q235 steel. The solid solution strengthening effect is greater than the martensite decomposition, so the hardness will still increase, but the increase will decrease. When the dilution rate continues to increase, it is judged that the hardness will decrease. As shown in Figure 9 (b), when the dilution rate of the cladding layer is less than 30%, unlike the common hardness gradient structure [15], the hardness of the heat affected zone of the three carbon steels is higher than the hardness of the cladding layer. Because at this time, the hardness of the cladding layer is mainly determined by the properties of the cladding powder, that is, the powder hardness of austenitic stainless steel, and the substrate undergoes local phase transformation due to the heat effect of the molten pool, generating high-hardness martensite, which leads to the phenomenon that the hardness of the heat-affected zone is higher than that of the cladding layer [18]. As the dilution rate increases, the hardness of the heat-affected zone tends to decrease, which is particularly obvious for the Q235 steel substrate. Combined with the disappearance of the rapid solidification structure of the Q235 steel cladding layer at a dilution rate of 58%, it is speculated that the heat-affected zone of the substrate with a large dilution rate dissipates heat relatively slowly, the cooling rate decreases, the martensite transformation is incomplete and accompanied by grain growth, resulting in a decrease in hardness. The substrate part far away from the cladding layer is less affected by the heat of the molten pool and mainly plays a role in heat conduction, accelerating the cooling of the molten pool and failing to reach the phase transformation temperature. Therefore, the hardness of the three substrates is still determined by the carbon content of the material and is basically not affected by the change in the dilution rate of the cladding layer, as shown in Figure 9 (c). 3 Conclusion
As a surface strengthening technology, laser cladding has great potential in surface modification of carbon steel. Different from the previous method of controlling the dilution rate to ensure the performance of the cladding layer, this experiment proves that at a larger dilution rate, the cladding layer and the substrate can achieve better metallurgical bonding, and the incorporation of matrix elements can also make the cladding layer have better mechanical properties. The main conclusions are as follows.
(1) The cladding layer prepared in the experiment has no pores or cracks inside, and the molding quality is good. The laser power has a significant effect on the dilution rate of the cladding layer. The dilution rate shows an upward trend with the increase of laser power. At the same power, the difference in the substrate will affect the dilution rate of the cladding layer, which is mainly related to the laser absorption efficiency of the substrate.
(2) When the dilution rate is less than 30%, the cladding layers on different substrates have the same microstructure morphology, which is planar crystals, coarse columnar crystals and dendrites, fine dendrites and equiaxed crystals from the bottom to the top of the cladding layer. When the dilution rate exceeds 30%, the microstructure characteristics gradually disappear with the rapid solidification, and the cladding layer transforms from austenite to martensite. The cladding layer and the substrate are metallurgically bonded. As the dilution rate increases, the elements of the cladding layer and the nearby substrate tend to be consistent, and the metallurgical bonding effect is improved. At the same time, the change in the dilution rate causes the elemental composition of the cladding layer to change, creating composition conditions for martensite transformation. (3) The dilution rate of the cladding layer on the surface of Q235 steel, 35 steel and 45 steel is related to the hardness. According to the change process, it can be divided into three stages: In the first stage, the increase of the dilution rate leads to the mixing of more C in the cladding layer, which promotes the formation of carbides and strengthens the cladding layer; in the second stage, the cladding layer undergoes martensitic transformation, the dilution rate increases, and the cladding layer gradually transforms from austenite to martensite, and the hardness is significantly improved, with an increase of 250~300 HV; in the third stage, the difficulty of solid solution of carbon atoms in martensite increases, and the martensite is partially decomposed with the increase of matrix temperature. When the solid solution strengthening effect of martensite is less than the decomposition effect, the hardness of the cladding layer decreases. The difference in the matrix leads to the change of the dilution rate under the same laser power, which causes the microstructure and hardness of the cladding layer on the surface of the three carbon steels to change, indicating that even if the materials are similar, the laser cladding parameters are not universal, and the dilution rate should be used as the basis when exploring the performance. Here, we only studied the microstructure and properties of the cladding layer when the dilution rate changes due to changes in laser power. It is still necessary to explore whether the changes in dilution rate caused by other parameters in laser cladding, such as scanning speed and powder feeding amount, will have the same impact on the cladding layer.