In order to improve the surface performance of ductile iron engine body and solve the cavitation problem caused by cavitation collapse during use, laser cladding and argon arc welding were used to perform additive repair of Co-Cr-W cobalt-based alloy layer on the surface of QT400-18 ductile iron, and the microstructure, hardness, room temperature tensile properties and cavitation resistance of the cladding layer and cladding layer samples were analyzed. The results show that the repair layer is mainly composed of fine, staggered columnar crystals and dendrites. The microstructure of the laser cladding layer is finer and has higher hardness than that of the argon arc welding layer. The tensile strength of the two repair layer samples is close to that of the matrix, but the plasticity decreases. After 8 hours of cavitation, the QT400-18 matrix is ​​severely damaged, the surface of the laser cladding sample and the argon arc welding sample is smoother, and its cavitation resistance is significantly improved. The weight loss of the laser cladding sample and the argon arc welding sample is reduced by 95.32% and 93.58% respectively compared with the ductile iron matrix.

Marine diesel engine bodies are generally made of cast iron or steel[1]. During use, cavitation is easily formed at the outlet of the cooling water chamber. When the cavitation collapses, instantaneous high pressure and high temperature are generated on the surface of the body, causing plastic deformation and eventually forming pitting defects[2]. This defect generally appears on the surface of the body in the form of dense honeycomb holes (Figure 1). When the cavitation is more serious, the maximum depth reaches 5mm. After cavitation occurs, the working performance of the diesel engine will be greatly affected. With the reduction of cooling efficiency and the increase of cooling water temperature[3], the aging of the body will gradually increase. If the occurrence of cavitation is not stopped in time, the wall of the cavitation site will continue to thin or even penetrate, causing the sealing of the diesel engine body to decrease, and the coolant will penetrate into it, seriously affecting the service life of the diesel engine, and even causing serious accidents[4-6]. The main way to optimize the anti-cavitation performance of the traditional engine body is to improve the stiffness of the material, which is generally carried out from the two aspects of body manufacturing process control and product surface treatment. In terms of engine body manufacturing, centrifugal casting can effectively improve the rigidity and hardness of the engine body; in terms of surface treatment, the outer wall of the engine body can be treated by surface nitriding or local surface reinforcement [7-8], or a Co-based or Ni-based alloy layer can be formed on the surface by plasma thermal spraying technology, thereby improving the surface cavitation resistance. However, in recent years, the popularity of high-speed and high-power diesel engines has made the manufacturing conditions of engines more stringent, and traditional manufacturing processes cannot completely solve the engine body cavitation problem. When cavitation damage occurs, the engine body still needs to be replaced to ensure the normal operation of the diesel engine. Replacing the engine body is a very difficult and complicated process. The production and replacement of the new engine body will incur a high price, resulting in huge economic losses. Therefore, it is of great significance to study the repair of diesel engine body cavitation.

Additive manufacturing, as a high-speed, high-quality new material forming technology, has received great attention from governments and research institutions at home and abroad in recent years. Among them, laser cladding technology based on inert gas powder feeding and laser melting and wire feeding welding technology based on tungsten inert gas arc are characterized by high efficiency and high flexibility, and are often used to repair damaged parts, such as repairing gas turbine blades and large annular parts in the aerospace field. Cast iron equipment is widely used in the energy, shipbuilding, automobile and other industries, and the number is very large. The demand for repair technology after damage is extremely high [9]. However, the application of additive manufacturing in repairing cast iron equipment is still relatively small, which deserves in-depth research and promotion. In this paper, laser cladding technology and tungsten inert gas arc welding technology are used to prepare Co-Cr-W cobalt-based alloy layer on the surface of ductile iron, and the effects of these two different processes on the microstructure, hardness of each region, room temperature tensile properties and cavitation resistance of Co-Cr-W alloy and ductile iron are compared and analyzed, aiming to provide data support and technical reference for the use of additive manufacturing technology to repair ductile iron engine bodies.

1. Test materials and methods
1.1 Test materials
Co-Cr-W cobalt-based alloy powder (powder particle size of 53-150μm) prepared by gas atomization method and cobalt-based alloy welding wire of the same grade (welding wire diameter of 2mm) were used as materials for additive repair layer; QT400-18 ductile iron was used as matrix material. The chemical compositions of cobalt-based alloy and
QT400-18 ductile iron are shown in Tables 1 and 2, respectively.

1.2 Test methods
Laser cladding and argon arc welding were used to add material to fill slots on ductile iron slotted plates. Microstructure and room temperature tensile specimens were prepared by wire cutting sampling to study their microstructure and room temperature mechanical properties. Then, multi-pass forming was performed on the surface of ductile iron substrate to study its pitting resistance. The sampling method and sample size of room temperature tensile specimens are shown in Figure 2 (a); the area of ​​the laser cladding repair layer and the argon arc welding repair layer are both 20 mm×20 mm, and the pitting sample is taken in the middle. The sampling method and sample size are shown in Figure 2 (b).

1.3 Experimental equipment Precision laser cladding equipment is used for laser cladding. The laser model is Trudisk4002, and the maximum power is 4000 W. The laser cladding parameters are: laser power 1700~2100W, powder feeding amount 6~8 g/min, powder feeding gas flow rate and shielding gas flow rate are both 15L/min, and the spot diameter is 4.5 mm. The AC/DC dual-purpose welding machine is used to carry out argon arc welding additive, the equipment model is MW-3000Job, and the argon arc welding process parameters are tungsten electrode diameter 3.2 mm, current 60~80A, and gas flow rate 8~12 L/min. The XOQS-1000 ultrasonic cavitation tester was used to conduct the cavitation test, and the experimental time was 8 h.

2 Experimental results and analysis
2.1 Microstructure analysis
The cross section of the sample was taken as the observation surface. Its macrostructure consists of two parts: the substrate and the additive repair layer. The additive repair layer is trapezoidal. No cracks, pores and other defects visible to the naked eye were found in the substrate and the additive repair layer. Figure 3 shows the microstructure of the Co-Cr-W alloy laser cladding layer. A good metallurgical bond was formed between the substrate and the cladding layer, and there were no obvious large-sized pore defects in the cladding structure. The high cooling rate of laser cladding has an inhibitory effect on the growth of grains. The grains solidify rapidly before they have time to grow. Therefore, the cladding layer is mainly composed of fine, staggered columnar crystals and dendrites. The grains grow crosswise and upward along the temperature gradient direction, and have a certain directionality in a small range, but the overall directionality is weak [10]. This is because during the laser cladding process, the laser power density is concentrated and the heat source is continuously moving, which makes the temperature gradient of the molten pool uneven. The matrix structure of ductile iron is spherical or flocculent graphite distributed on the ferrite matrix. After the heat affected zone is remelted, part of the graphite diffuses to a certain extent and is no longer spherical. Figure 4 shows the microstructure of the Co-Cr-W alloy argon arc welding layer. It can be seen that the fusion effect between the layers is good, and there are no obvious large-sized pore defects in the welding structure. Similar to the laser cladding process, most of the weld layer structure is staggered columnar crystals and dendrites, as shown in Figure 4 (a). Due to the larger heat input of argon arc welding, the driving force for grain growth is greater, the grain growth rate and growth time are far greater than those of the laser cladding process, and its structure size is much larger than the cladding structure, as shown in Figure 4 (b). Similar to the laser cladding layer, after remelting, the graphite in the heat-affected zone near the bonding line diffuses to a certain extent and appears scattered, while the matrix morphology away from the heat-affected zone is still ferrite matrix + spheroidal graphite.

2.2 Microhardness analysis
The microhardness test of the samples prepared by the two processes was carried out, and the test direction was perpendicular to the fusion line. The results are shown in Figure 5. It can be seen that the average microhardness of the laser cladding area of ​​Co-Cr-W cobalt-based alloy is higher than 520 HV, and the highest is close to 600 HV, while the average microhardness of the argon arc welding area is higher than 420 HV, which is lower than the hardness of the laser cladding layer. This is because the laser cladding layer has a finer and denser structure, and there are more grain boundaries in contact with the indenter under the same pressure during the hardness test, which hinders plastic deformation. Since the matrix QT400-18 is ferrite cast iron, it is relatively soft, and the average microhardness is below 230HV.
The microhardness of the heat-affected zone ranges from 638 to 822 HV, which is much greater than the hardness of the matrix and the additive zone. On the one hand, because the carbon solubility of the ferrite matrix of ductile iron is relatively weak, during the additive repair process, the high-temperature molten cobalt-based alloy causes a part of the ferrite matrix near the bonding line to remelt. According to the change of temperature gradient, a molten pool, a solid phase transformation zone and a non-phase transformation zone are formed from the bonding line to the matrix in sequence. The high-temperature liquid metal in the molten pool has the ability to dissolve graphite, and austenitization occurs in the solid phase transformation zone, which also greatly improves the carbon solubility of the cast iron matrix. However, due to the rapid cooling rate, the C element dissolved in the matrix has no time to precipitate, so that the matrix in the solid phase transformation zone forms a supersaturated solid solution, that is, high-carbon martensite [11], which causes the hardness of the heat-affected zone of ductile iron to increase. On the other hand, the molten cobalt-based alloy and ductile iron undergo a certain degree of dilution when combined. The C element in the ductile iron enters the cobalt-based alloy, which increases the carbides in the cobalt-based alloy at the joint. During the cooling process, the carbides precipitate and are distributed between the grains and on the matrix, greatly increasing the hardness and strength near the joint line between the cobalt-based alloy and the ductile iron [12]. These two factors together cause the hardness of the joint in the heat-affected zone to increase suddenly.

2.3 Mechanical properties analysis
The room temperature tensile properties of the filled test plates were tested. The fracture positions of the laser cladding specimens and the argon arc welding tensile specimens were both in the matrix position, and the fracture position of the QT400-18 tensile specimen was in the middle of the specimen, as shown in Figure 6. The tensile properties results are shown in Table 3.
As can be seen from Table 3, the tensile strength of all specimens is close to that of the matrix. The average tensile strengths of the laser cladding specimens (1-1, 1-2), the argon arc welding specimens (2-1, 2-2) and the matrix are 419, 409, and 412 MPa, respectively, with a deviation of 0.7% to 1.7%. It can be considered that both additive Co-Cr-W alloys can form a good metallurgical bond with QT400-18 ductile iron. However, although the fracture position of the specimen is in the matrix, the plasticity has been greatly reduced. Compared with the average elongation of the matrix, the average elongation of the slotted specimen decreased by 69.53%, and the plasticity decreased. Combined with the hardness analysis in Section 2.2, it can be seen that this is because high-carbon martensite is formed in the heat-affected zone of ductile iron during remelting. The structure and composition of high-carbon martensite will greatly increase the hardness near the fusion line. The specific reasons are as follows:
(1) High-carbon martensite is a lamellar hard and brittle phase. Although it has high strength and hardness and maintains the original tensile strength of QT400-18, the existence of its twin substructure reduces the effective slip system, making it difficult for high-carbon martensite to undergo plastic deformation.
(2) From the perspective of composition, high-carbon martensite is a supersaturated solid solution with a high carbon content and has a large lattice distortion. The cooling rate of argon arc welding and laser cladding is relatively fast, and there is still a large quenching stress during the air cooling process, which reduces the plasticity.

2.4 Cavitation test
Considering that the solution containing acid radicals and hydroxide ions is corrosive, the electrochemical reaction between them and metals will promote cavitation, and cavitation will cause corrosion to intensify. Therefore, in order to eliminate the interference of corrosive solutions, deionized distilled water was used for cavitation test. The cavitation test results of ductile iron substrate, laser cladding sample and argon arc welding sample are shown in Figure 7. As can be seen from Figure 7 (a), the cavitation weight loss of substrate, laser cladding sample and argon arc welding sample increases with time, and the weight loss of cavitation for 8 h is 51.1, 2.39 and 3.28 mg respectively. That is, under the same cavitation time, the cavitation weight loss of pure ductile iron sample is the largest and the cavitation resistance is the worst. As shown in Figure 7 (b), the weight loss rate of ductile iron increases with time. When the cavitation is 4 to 6 hours, the cavitation intensifies; it basically reaches stability at 6 hours, and the weight loss rate of ductile iron is about 6.1 mg/h. This is because cavitation leaves defects such as cavitation pinholes and cavitation pits on the surface of ductile iron, which in turn produces a large stress concentration and accelerates the occurrence of cavitation. The weight loss rate of laser cladding samples and argon arc welding samples increases first and then decreases with time, and basically reaches stability at 4 hours. The weight loss rates of laser cladding samples and argon arc welding samples in the cavitation stability period are about 0.41 and 0.51 mg/h, respectively. As cavitation progresses, the weight loss rate of cavitation reaches a stable state and slowly decreases.

Combined with the results of the 8-hour cavitation test, from the perspective of weight loss, it can be seen that the laser cladding layer reduces the mass loss of ductile iron by 95.32%, while the argon arc welding layer can reduce the mass loss by 93.58%; from the perspective of weight loss rate, it can be seen that the laser cladding layer reduces the mass loss of ductile iron by 93.27%, while the argon arc welding layer can reduce the mass loss by 91.64%. In summary, cobalt-based alloy surfacing and cladding on ductile iron substrate can greatly improve the ability to resist cavitation.

Figure 8 shows the macroscopic morphology of three samples after 8 hours of cavitation. It was observed that the central area of ​​the QT400-18 sample surface became very rough, showing obvious diffuse reflection under illumination, and the cavitation part had no obvious reflection, indicating that the central area of ​​the sample suffered severe cavitation damage; while there was almost no cavitation damage on the outermost part of the sample surface, and there were a few bright silver areas that were not damaged by cavitation on the second outer part, because cavitation itself is an uneven form of damage. The laser cladding sample and the argon arc welding sample still showed a good mirror state as a whole, showing mirror reflection under illumination, indicating that the cavitation damage suffered by the sample was relatively light.

SEM observation of the three samples after cavitation was performed, and the morphology is shown in Figure 9. It can be seen that the substrate surface obviously suffered severe material peeling, a large amount of ferrite was missing, resulting in the surface being no longer intact, and loose spherical graphite fell off to form holes.

After the QT400-18 substrate was peeled off by cavitation material, the peeling surface mainly presented a cleavage surface. High-frequency water vibration generates a large number of cavitation bubbles. When the cavitation bubbles collapse, instantaneous high-temperature and high-pressure impact is generated, forming the earliest cavitation cracks on the surface crystals. The cracks preferentially extend along the cleavage plane and grain boundary. QT400-18 is a ferritic ductile iron. The room temperature structure is mainly dispersed spheroidal graphite and ferrite matrix. The characteristics of ferrite are low hardness and low strength, but excellent plasticity and ductility [13]. Low hardness and low strength are the main reasons for the poor resistance to cavitation. It is very easy to be peeled off during the cavitation process, causing the surface to be damaged. The stress concentration of the rough surface increases, making the subsequent cavitation more severe, and the holes formed after the graphite falls off also aggravate the expansion of the cavitation cracks, resulting in increasing weight loss and weight loss rate.

In contrast, the protective effect of the cobalt-based alloy layer prevents large-scale material loss on the surface of the additive repair sample, leaving only the high-frequency vibration ripples of cavitation, which are parallel or strip-shaped. According to the research of Guo Xuming et al. [14-18], the main phases of Co-Cr-W cobalt-based alloy are γ-Co and Cr23C6. Among them, γ-Co is the matrix phase, and Cr23C6 is the M23C6 type carbide dispersed in the matrix and grain boundaries. The two main phases have an improvement effect on the cavitation resistance of the alloy layer, which is mainly reflected in:
(1) The anti-cavitation effect of γ-Co. On the one hand, the collapse of cavitation cavitation bubbles will produce instantaneous high pressure, which will continuously act on the matrix surface, causing plastic deformation, and can induce the martensitic phase transformation of Co-Cr-W alloy, so that the γ-Co phase will transform into the ε-Co phase. The transformation process can disperse the impact energy of the collapse of cavitation cavitation bubbles and no longer concentrate, thus reducing the probability of cavitation. On the other hand, the stacking fault energy of the γ-Co phase is low, and deformation dislocations are easy to occur. When the impact energy of cavitation bubbles continuously acts on the surface of the alloy layer, it will cause plastic deformation on the surface, causing intergranular slip and dislocation intersection, and then produce work hardening, increase the surface hardness of the alloy, and thus greatly enhance the ability to resist deformation.
(2) Anti-cavitation effect of Cr23C6. Dispersed Cr23C6 has a strengthening effect on the cobalt-based alloy matrix. The collapse of cavitation bubbles triggers plastic deformation on the surface of the cobalt-based alloy layer. During the dislocation slip process of the grains, they encounter dispersed Cr23C6 particles, which will form a pileup around them. As the deformation continues to increase, the dislocation pileup around the Cr23C6 particles also continues to increase, and the subsequent dislocation slip becomes more and more difficult, which gradually deepens the hardening degree and improves the cavitation resistance. Combined with the cavitation weight loss rate curve, it is not difficult to find that the weight loss rate changes less and less with time, which proves that the cobalt-based alloy layer has a strong anti-cavitation performance.
From the comparison of different processes, it is found that the cavitation resistance of the laser cladding layer is better than that of the argon arc welding layer. It is not difficult to know from the above microstructure and hardness analysis that the grain size of the laser cladding is smaller than that of the argon arc welding structure, the microhardness is higher than that of the argon arc welding structure, and it has stronger density and grain boundary bonding; and the large number of grain boundaries at the same time disperse the stress concentration at the grain boundaries, so the laser cladding layer has better cavitation resistance. Laser cladding process equipment is mainly divided into machine tool type and robot type equipment, which have a large footprint, more electrical circuits and gas circuits, and the installation equipment and process debugging are relatively complicated; while manual argon arc welding equipment has a small footprint, simple installation, and is portable. In order to ensure the best cavitation resistance of the repair layer, the laser cladding process can be used first. Considering that the alloy layer formed by argon arc welding is also very resistant to cavitation, and the process has higher flexibility, argon arc welding can be used as an auxiliary means when repairing cavitation damage to ensure that the repair process is smooth and the repair results achieve the expected effect.

3 Conclusions
(1) The laser cladding layer and the cladding layer have similar microstructures, both of which are staggered columnar crystals and dendrites. The laser cladding layer has finer microstructure and higher hardness. The QT400-18 ductile iron matrix is ​​composed of spherical or flocculent graphite distributed on the ferrite matrix. After remelting, the graphite in the heat affected zone near the bonding line diffuses to a certain extent, forming high-hardness high-carbon martensite. The carbon content at the junction of the cobalt-based alloy layer and the ductile iron increases, which together cause the hardness of the junction to be high and the plasticity to decrease.
(2) Co-Cr W alloy cladding and cladding on the QT400-18 ductile iron matrix can greatly improve the cavitation resistance. After cavitation for 8 hours, the weight loss of the laser cladding layer and the argon arc cladding layer was reduced by 95.32% and 93.58% respectively compared with the ductile iron matrix.
(3) The Co Cr-W alloy layer formed by laser cladding on the QT400-18 substrate has better cavitation resistance and can be used first. Considering that the alloy layer formed by argon arc welding also has good cavitation resistance and this process has higher flexibility, it can be used as an auxiliary means to repair cavitation damage.