An online cooling method for laser additive manufacturing with surface hardness fluctuation ≤ 5HRC

Abstract

An online cooling method for laser additive manufacturing with surface hardness fluctuation ≤ 5HRC, relates to the field of additive manufacturing technology, and comprises the following steps: S1, preheating the cold rolling work roll, the preheating temperature is 300-350℃, and the preheating temperature fluctuation does not exceed ±25℃; S2, coaxial powder feeding; S3, air cooling after preheating; S4, applying cooling gas after air cooling; S5, dividing the cold rolling work roll into multiple sections along its axial direction, and cooling each section with cooling gas, the starting working position of the cladding head corresponds to the first section of the cold rolling work roll, and the cooling gas flow corresponding to each section decreases in sequence as the section is farther from the cladding head; S6, during the laser cladding operation, using a local cooling device to perform targeted cooling on the area with abnormally high temperature on the cold rolling work roll. The present invention can improve the problem of precise cooling and temperature control of different sections and improve the quality of ultra-high-speed laser cladding additive manufacturing of cold rolling work rolls.

Description

Technical Field

[0001] The invention relates to the technical field of additive manufacturing, and in particular to an online cooling method for laser additive manufacturing with surface hardness fluctuation of ≤5HRC. Background Art

[0002] Chinese patent document CN114507854A discloses an online cooling and heat treatment method for heat-treatable aluminum alloys based on laser repair. The method adopts a semi-open structure, and the cladding zone and its heat-affected zone are cooled in real time through an atmosphere maintenance unit with an open top and a circulating water cooling system. During the experiment, argon gas is continuously introduced through a gas pipe connected to an argon gas bottle to maintain a protective atmosphere. However, this method is only for damage repair of aluminum alloys, and has limitations in ultra-high-speed laser cladding additive manufacturing, especially for cold-rolled work rolls with a length of ≥1m. Due to the rapid rotation of the cold-rolled work roll, it is difficult to achieve the precise requirement of controlling the surface hardness fluctuation of the M2 high-speed steel additive layer to ≤5HRC by relying solely on the free floating method of the cooling gas and the semi-open structural design. The key is that the stability of the surface hardness of the additive layer depends on the precise control of the surface temperature of the cold-rolled work roll, which is not taken into account in the above scheme.

[0003] Chinese patent document CN112809183A discloses a method for cooling metal in additive manufacturing, which cools the cladding layer and the heat-affected zone below it in real time by setting a cooling device behind the heat source and following the heat source. The cooling device is composed of a plurality of U-shaped tubes to form a U-shaped cooling channel, the cladding layer is located inside the channel, and the device opening is downwardly covered on the cladding layer to achieve approximately parallel contact with the top cladding layer. In addition, the additive equipment is equipped with a temperature detection device to monitor the temperature of the cladding layer behind the heat source. Although the above scheme can increase the cooling rate of the cladding layer and reduce the depth of the heat-affected zone, it fails to take into account the temperature gradient formed on the surface of the additive layer behind the heat source, that is, the closer to the heat source, the higher the temperature, resulting in the use of only uniform cooling conditions, and the surface hardness fluctuation of the M2 high-speed steel additive layer cannot be effectively controlled to exceed 5HRC. In addition, the precision of the U-shaped tube cooling channel is insufficient, the cooling effect is limited, and it is impossible to ensure that the surface hardness of the M2 high-speed steel additive layer reaches the high standard of 62HRC.

[0004] Chinese patent document CN113059159A discloses an additive manufacturing method for preventing cracks in directionally solidified high-temperature alloys. In this method, a cooling device is arranged in front of the heat source and moves synchronously with the heat source to cool the front section of the molten pool in real time. At the same time, an aluminum foil cooled by liquid nitrogen is attached to the cladding layer to enhance the cooling effect. This scheme aims to promote the growth of directional crystals by increasing the temperature gradient of the solid-liquid interface of the molten pool, thereby improving the directionality of the solidification process and helping to prevent the occurrence of cracks. However, although this scheme achieves the cooling purpose to a certain extent, it does not take into account the precision control during the cooling process, which will cause the surface hardness of the M2 high-speed steel additive layer to fluctuate greatly, exceeding the control range of 5HRC.

[0005] Chinese patent document CN110883093A discloses an asynchronous cold rolling forming process for aluminum alloy stainless steel composite plates, which involves the step of implementing online cooling on the opposite sides of the composite surface of the aluminum alloy plate, specifically by spraying cooling water. The process is automatically controlled through an online feedback function. When it is detected that the temperature of the aluminum alloy surface exceeds 300°C, the system will automatically increase the amount of water sprayed to prevent the composite plate from softening. Although this cooling method shows good results in controlling the temperature of the composite plate, it has some limitations. The method of directly using spray cooling water will cause quenching on the surface of the M2 high-speed steel additive layer and cause cracking. In addition, the scheme does not take into account the cooling accuracy, and the surface hardness fluctuation of the M2 high-speed steel additive layer cannot be controlled within 5HRC.

[0006] In the ultra-high-speed laser cladding additive manufacturing process of cold-rolled work rolls, especially when the number of M2 high-speed steel additive layers reaches three or more, the traditional online cooling method faces challenges and is difficult to meet the strict standards of surface hardness reaching at least 62HRC (≥62HRC) and hardness fluctuation controlled within 5HRC (≤5HRC). This problem stems from the high-speed rotation of the cold-rolled work roll during manufacturing. The temperature of the molten pool area caused by the laser heat source is the highest, while the temperature away from the molten pool area decreases, forming a significant temperature gradient. In order to ensure the quality of the additive layer, it is necessary to control the temperature of the entire length of the cold-rolled work roll consistently, and a gradient cooling method must be used to adjust the cooling conditions according to the temperature gradient of different sections. However, existing solutions often ignore the importance of such precise temperature control and gradient cooling, resulting in insufficient surface hardness of the additive layer (such as hardness <62HRC) and excessive fluctuation (such as volatility >5HRC), resulting in low-quality additive layers. Summary of the invention

[0007] The purpose of the present invention is to provide an online cooling method for laser additive manufacturing to improve the problem of precise cooling and temperature control in different sections.

[0008] In order to achieve the above object, the present invention adopts the following technical solution: an online cooling method for laser additive manufacturing with surface hardness fluctuation ≤ 5HRC, comprising the following steps:

[0009] S1. Preheat the cold rolling work rolls at a temperature of 300-350°C, with a preheating temperature fluctuation not exceeding ±25°C;

[0010] S2, coaxial powder feeding;

[0011] S3, air cooling after preheating;

[0012] S4, applying cooling gas after air cooling;

[0013] S5, dividing the cold rolling work roll into a plurality of sections along its axis direction, and cooling each section with cooling gas, the starting working position of the cladding head corresponds to the first section of the cold rolling work roll, and the cooling gas flow corresponding to each section decreases as the section is farther from the cladding head;

[0014] S6. During the laser cladding operation, a local cooling device is used to perform targeted cooling on the areas on the cold rolling work roll where the temperature is abnormally high.

[0015] Furthermore, in step S5, when the cladding head moves to different sections along the cold rolling work roll, the corresponding cooling gas flow rate is adjusted in real time according to the distance between each section and the cladding head, and the principle is followed that the cooling gas flow rate decreases accordingly as the section is farther from the cladding head.

[0016] Furthermore, in step S5, when the cladding head performs cladding operation on each section of the cold rolling work roll, at least when the drop in the surface temperature of the cold rolling work roll since the start of the cladding operation has not reached 150°C, the cooling gas of the section is blown out in a vertical upward direction to avoid the surface of the cold rolling work roll, thereby reducing the amount of cooling gas directly blown to the surface of the cold rolling work roll, thereby controlling the cooling rate, preventing the material from entering a quenching or semi-quenching state due to excessive cooling, and reducing the risk of cracking.

[0017] Further, in step S5, cooling of cooling gas is performed on each section of the cold rolling work roll by a global cooling device;

[0018] The global cooling device includes an air outlet duct parallel to the axis of the cold rolling working roll. The cold rolling working roll side of the air outlet duct is provided with multiple continuous or intermittent air outlets and multiple adjustment plates. The number and position of the adjustment plates correspond one-to-one to the number and position of the sections of the cold rolling working roll, and are used to adjust the opening size and direction of the air outlet to control the cooling gas flow and blowing direction of each section.

[0019] The above-mentioned global cooling device can realize gradient cooling, so it can also be called a gradient cooling device.

[0020] Furthermore, in step S2, the gas flow rates of the shielding gas and the powder feeding gas used in the coaxial powder feeding do not exceed 20 L / min.

[0021] Further, in step S3, when the cold rolling work rolls are preheated to their set positions and reach the martensitic transformation temperature, the preheating is stopped and air cooling is performed.

[0022] Furthermore, in step S4, the cooling gas is a mixed gas of argon and air, and the mass fraction of argon is 60-65%.

[0023] Furthermore, the flow rate of the cooling gas is 25 to 30 L / min.

[0024] Furthermore, the above method further comprises the following steps:

[0025] When the laser cladding operation is completed, continue to apply cooling gas cooling to each section of the cold rolling work roll until the surface temperature fluctuation of the additive layer does not exceed ±25°C and the surface temperature does not exceed 200°C. Then stop cooling with cooling gas and allow it to air cool.

[0026] Furthermore, the above method further comprises the following steps:

[0027] During the laser cladding operation, the thickness of each additive layer on the surface of the cold rolling work roll is controlled not to exceed 0.6 mm.

[0028] Furthermore, the laser cladding operation is performed in a semi-enclosed cooling chamber, the upper end of which is open and movable baffles are arranged around it.

[0029] Preferably, the baffle is transparent glass capable of filtering laser light.

[0030] Preferably, the cooling chamber is provided with a device for online monitoring of the temperature of the molten pool and the additive layer.

[0031] The present invention ensures that the cold rolling work roll obtains a uniform and consistent starting temperature condition before laser cladding by preheating the cold rolling work roll and strictly controlling the preheating temperature range and volatility, laying a foundation for the uniformity and consistency of the subsequent cladding process. In addition, the coaxial powder feeding technology is adopted to achieve the synchronization of material supply and laser cladding process, ensure the continuous and uniform cladding of cladding materials, and improve the cladding efficiency and material utilization rate. Among them, the air cooling step after preheating enables the cold rolling work roll to reach a suitable starting temperature before laser cladding, which helps to improve the cladding quality and the bonding strength between the cladding layer and the substrate. Moreover, after air cooling, cooling gas is applied to preliminarily cool the cold rolling work roll, which effectively controls the substrate temperature before cladding and prevents the material performance from being reduced due to excessive temperature. At the same time, by dividing the cold rolling work roll into multiple sections along the axis direction and adjusting the cooling gas flow rate according to the distance between the section and the cladding head, gradient cooling is achieved, which adapts to the temperature requirements of different sections and improves the accuracy and effectiveness of cooling. In addition, during the laser cladding operation, timely and targeted cooling is performed on the areas where the local temperature is abnormally elevated, effectively avoiding local overheating and ensuring the overall quality and performance of the cladding layer. Combining the above measures, the online cooling method of the present invention can significantly improve the quality of ultra-high-speed laser cladding additive manufacturing of cold-rolled work rolls, ensuring that the surface hardness and hardness volatility of the additive layer meet specific requirements, such as the surface hardness reaches ≥62HRC and the hardness volatility is controlled at ≤5HRC, thereby improving the performance and reliability of the final product. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the structure of the laser cladding additive manufacturing equipment in the embodiment. Figure 1 ;

[0033] Figure 2 This is a schematic diagram of the structure of the laser cladding additive manufacturing equipment in the embodiment. Figure 2 ;

[0034] Figure 3 The structure of the global cooling device in the embodiment is shown in FIG. Figure 1 ;

[0035] Figure 4 is an exploded view of a global cooling device in an embodiment;

[0036] Figure 5 The structure of the global cooling device in the embodiment is shown in FIG. Figure 2 .

[0037] In the figure:

[0038] 1——Cold rolling work roll 2——Cladding head

[0039] 3——Local cooling device 4——Global cooling device

[0040] 4a——air outlet duct 4a1——air outlet

[0041] 4b——Adjustment plate 5——Lathe

[0042] 6——Baffle. DETAILED DESCRIPTION

[0043] In order to facilitate those skilled in the art to better understand the improvements of the present invention relative to the prior art, the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0044] In conventional ultra-high-speed laser cladding additive manufacturing of cold-rolled work rolls, online cooling methods such as coaxial powder feeding and cooling or single-tube or multi-tube side blowing are usually used. These methods are suitable for cooling 1 to 2 layers of cladding. However, with the increase in the number of cladding layers, especially when the number of layers reaches three or more (i.e., the number of layers ≥ 3), the heat accumulation brought by each layer of cladding will cause the cooling efficiency to gradually decrease. This temperature accumulation effect makes it difficult for the high-layer cladding layer to achieve the same cooling effect as the bottom layer (such as the first and second layers). Specifically, as the number of layers increases, the temperature of the cladding layer tends to be higher, while the corresponding cooling rate decreases significantly. In particular, when M2 high-speed steel material is used for additive manufacturing on the surface of the cold-rolled work roll, if the surface hardness of three or more layers of cladding is to be guaranteed to reach at least 62HRC (≥62HRC) while keeping the hardness fluctuation within 5HRC (≤5HRC), the existing cooling methods are no longer applicable, and more advanced cooling technologies are required to meet these stringent process requirements.

[0045] Existing online cooling technologies face the challenge of cooling multi-layer (≥3 layers) M2 high-speed steel materials in the ultra-high-speed laser cladding additive manufacturing of cold rolling work rolls. Although these methods can provide basic cooling effects, when it is necessary to achieve high standards of surface hardness not less than 62HRC (≥62HRC) and hardness fluctuation controlled within 5HRC (≤5HRC), existing technologies are difficult to meet the requirements. This problem stems from the high-speed rotation of the cold rolling work roll during the manufacturing process and the high temperature generated by the laser heat source in the cladding area, which leads to a significant temperature gradient in the same horizontal direction. The formation of this temperature gradient is due to the fact that the temperature of the molten pool area under laser irradiation is the highest, and the temperature gradually decreases with the distance from the molten pool. In order to ensure the quality of the additive layer, uniform and consistent cooling control must be achieved for the additive layer of the entire cold rolling work roll, and existing methods have not yet been able to provide such precise cooling effects. In the ultra-high-speed laser cladding additive manufacturing process of cold rolling work rolls, a key factor that is often overlooked is the temperature gradient along the length of the roll body. Unlike single cooling conditions, gradient cooling at different locations along the roller body is crucial to controlling the microstructure and macroscopic properties of the additive layer. Existing online cooling methods generally fail to provide this kind of regionalized precise temperature control, thus ignoring the need for cooling that matches the temperature gradient along the roller body. This oversight results in the surface hardness of the M2 high-speed steel additive layer being lower than 62HRC (<62HRC) and the hardness fluctuation exceeding the specification requirement of 5HRC (>5HRC), which ultimately affects the overall quality and performance of the additive layer.

[0046] Under the premise that the number of additive layers of M2 high-speed steel manufactured by ultra-high-speed laser cladding of cold-rolled work roll is ≥3 and the thickness of additive layers is ≥1mm, the surface hardness of M2 high-speed steel manufactured by additive manufacturing should be ≥62HRC and the surface hardness fluctuation should be ≤5HRC. The following steps and technical parameters can be used to achieve this:

[0047] ① In the ultra-high-speed laser cladding additive manufacturing process of the cold-rolled working roll 1, preheating is a key step to ensure quality. During the preheating stage, the surface temperature of the cold-rolled working roll 1 must be strictly controlled at 300-350°C, and the temperature fluctuation must be controlled within ±25°C (≤±25°C). If the preheating temperature fluctuation exceeds this range, the surface hardness of each additive layer will fluctuate by more than 5HRC (>5HRC). Even if online cooling is implemented subsequently, it will be difficult to reduce the hardness fluctuation to within 5HRC (≤5HRC). In addition, if the preheating step is omitted, it will not only affect the uniformity of the surface hardness of the manufacturing layer, but may also directly cause cracking of the additive layer. Therefore, preheating is a prerequisite for the effectiveness of the subsequent online cooling step and cannot be ignored.

[0048] ② In the ultra-high-speed laser cladding additive manufacturing process, a three-way coaxial powder feeding method is used, in which argon (purity of 99.9%) is used for both the shielding gas and the powder feeding gas. The gas flow rate is controlled at 10-13 L / min (be careful not to exceed 20 L / min) to ensure that the surface roughness of the cladding layer reaches an appropriate level. If the shielding gas flow rate exceeds 20 L / min, the surface roughness of the cladding layer may increase to Ra ≥ 50μm, thereby affecting the uniformity of the surface hardness after online cooling, causing the hardness fluctuation to exceed the 5HRC standard.

[0049] In a coaxial powder feeding system, the gas used to protect the cladding process (shielding gas) and the gas used to transport the powder to the cladding area (powder delivery gas) are the same gas, and in this particular case, they are both argon. Therefore, when adjusting the flow rate of the shielding gas, the flow rate of the powder delivery gas is also adjusted at the same time, because they are the same gas source. This means that for a coaxial powder feeding system, controlling the size of the shielding gas flow directly affects the efficiency of the powder delivery and the protection effect of the cladding area. In other words, there is no separate powder delivery gas flow control, because the shielding gas assumes the dual role of powder delivery and protection at the same time. Therefore, when it comes to adjusting the gas flow rate, it actually refers to adjusting the flow rate of argon gas, and this flow value determines both the powder delivery capacity and the protection effect of the cladding area.

[0050] ③ In the process of ultra-high-speed laser cladding additive manufacturing of M2 high-speed steel on the cold-rolled working roll, when the temperature of the surface of the cold-rolled working roll 1 at a position 250 mm away from the cladding head in the horizontal direction reaches the martensite transformation point of 600°C, the preheating step is immediately stopped and air cooling treatment is performed for 45 to 55 seconds.

[0051] ④After completing the air cooling process of 45 to 55 seconds, online cooling is carried out immediately. At this time, it is not advisable to use conventional argon with a purity of 99.9% as the cooling medium, because this may cause quenching and cracking. A mixture of argon and air should be used as the cooling gas, in which the mass fraction of argon should be strictly controlled within the range of 60-65%. This mixed ratio of cooling gas can effectively avoid quenching cracking while ensuring that the cooling effect meets the process requirements.

[0052] ⑤ In order to ensure that the entire surface of the cold rolling work roll 1 can be cooled online, this embodiment designs a set of slender air outlet cooling devices parallel to the axis of the roll (equivalent to the global cooling device 4), which can be seen in Figure 1-5 In addition, in order to achieve precise cooling of the cladding area, this embodiment also designs a single-tube cooling device (equivalent to the local cooling device 3) that can move synchronously with the cladding head 2. The single-tube cooling device is usually located next to the cladding head 2. It can be independently controlled and its position adjusted to achieve local cooling of the cladding area. Figure 1 , 2The single-tube cooling device is fixed beside the cladding head 2, but in actual application, it can be installed on other movable devices as needed to achieve independent movement. In the additive manufacturing process, the closer the surface of the additive layer is to the cladding head 2, the higher the temperature, forming an obvious temperature gradient distribution. Therefore, the blowing flow of the slender air outlet cooling device cannot maintain consistency, but a gradient cooling gas flow control is used to achieve uniform cooling. To this end, the slender air outlet cooling device designed in this embodiment is divided into five sections (correspondingly, the cold-rolled working roll 1 is also divided into five sections along its axial direction, which correspond one to one), and the gas flow rate and the size of the air outlet 4a1 of each section can be adjusted independently to adapt to the temperature changes along the roller surface. In the section closest to the cladding head 2 (for example, the first section), due to the temperature peak (the highest temperature), the opening width of the air outlet 4a1 is set to the maximum, that is, 5mm. As the distance from the cladding head 2 increases, the opening width of the air outlet 4a1 of the subsequent sections decreases successively, which are 4.5mm, 4mm, 3.5mm and 3mm respectively, as can be seen in Figure 5 This method of adjusting the size of the air outlet 4a1 according to the gradient ensures that the entire area from the cladding head 2 to the roller end can be properly cooled, thereby maintaining the temperature uniformity of the additive layer and helping to achieve the goal of controlling the surface hardness fluctuation within 5HRC (≤5HRC).

[0053] Specifically, if Figure 3-5 As shown, the elongated air outlet cooling device includes an air outlet duct 4a parallel to the axis of the cold rolling work roll 1, and the cold rolling work roll side of the air outlet duct 4a is provided with a plurality of continuous or intermittent air outlets 4a1 and a plurality of adjustment plates 4b, the number and position of the adjustment plates 4b correspond to the number and position of the sections of the cold rolling work roll 1, and the opening size and direction of each air outlet 4a1 can be independently adjusted as needed to control the cooling gas flow and blowing direction of each section. In this embodiment, the air outlet 4a1 provided on one side of the air outlet duct 4a is a continuous strip air outlet structure, and the strip air outlet is provided with 5 adjustment plates 4b, and the shape of the adjustment plates 4b can be arc-shaped. These adjustment plates 4b are arranged closely and can be independently rotated and adjusted. By rotating the adjustment plate 4b, a series of openings with variable sizes and directions can be formed on the strip air outlet, thereby corresponding to the cooling requirements of different sections on the cold rolling work roll 1.

[0054] The elongated air outlet cooling device shown in Figures 3-5 is only an exemplary structure. In actual applications, the appropriate configuration of the air outlet 4a1 and the adjustment plate 4b can be selected according to specific needs.

[0055] In the conventional ultra-high-speed laser cladding process, the opening widths of the five air outlets 4a1 can be kept fixed to provide a stable cooling effect. However, in order to further improve the cooling efficiency and adapt to the cooling requirements of different sections, the opening widths of the air outlets 4a1 of the corresponding sections can be dynamically adjusted according to the real-time position of the laser cladding head 2 on the cold-rolled work roll 1. Specifically, when the cladding head 2 moves to different sections along the cold-rolled work roll 1, the corresponding cooling gas flow rate can be adjusted in real time according to the relative distance between each section and the cladding head 2. This adjustment follows a principle: as the distance between the section and the cladding head 2 increases, the cooling gas flow rate of the section is reduced accordingly to maintain the temperature uniformity and cooling efficiency of the entire surface of the cold-rolled work roll 1.

[0056] The following is a brief introduction to the equipment for realizing cladding additive manufacturing. Figure 1 , 2 The air outlet duct 4a is usually connected to a supply device for conveying cooling gas thereto (not shown in the figure). The cold rolling work roll 1 is usually fixed on the three-jaw chuck and the top of the lathe 5. A set of slender air outlet cooling devices are arranged on both sides of the cold rolling work roll 1. The air outlet duct 4a can be installed on a bracket, which can be fixed on the lathe 5 or the robot arm. The laser cladding head 2 can be installed on another robot arm through another bracket. The single-tube cooling device (local cooling device 3) can be installed next to the laser cladding head 2 through an adjustable connecting block. In addition, the equipment for realizing cladding additive manufacturing includes but is not limited to the above-mentioned methods. In actual application, appropriate devices and configurations can be selected according to specific needs.

[0057] ⑥ During the ultra-high-speed laser cladding process, due to the continuous movement of the laser cladding head 2, a constantly changing temperature gradient will be formed on the surface of the entire cold-rolled work roll 1. It is necessary to control the surface temperature of the additive layer ≥50mm away from the cladding head 2 through online cooling treatment, so that its maximum temperature and minimum temperature are within the range of ±25℃. If the temperature in a certain area is too high (for example, the temperature rises abnormally and exceeds the range of ±25℃), the existing air outlet 4a1 may be difficult to achieve precise local cooling of the area due to its large coverage. In order to solve this problem, the single-tube cooling device can be quickly moved to the high-temperature area. The device cools the area specifically through its side-blowing cooling port, thereby ensuring that the surface temperature of the entire cold-rolled work roll 1 is maintained within the range of ±25℃. It is worth noting that a common misunderstanding is that some peers may mistakenly control the surface temperature of the additive layer within 50mm (≤50mm) from the cladding head to be consistent with other areas. This practice will instead cause the M2 high-speed steel additive layer to undergo quenching and crack. The correct approach is to focus on controlling the surface temperature of the additive layer more than 50 mm (≥50 mm) from the cladding head to avoid the risk of quenching cracking. In this way, the uniformity and stability of the surface temperature of the cold rolling work roll 1 during the additive manufacturing process can be ensured, thereby ensuring the quality and performance of the final product.

[0058] ⑦ In the ultra-high-speed laser cladding additive manufacturing process of the cold-rolled work roll 1, the direction of the cooling port (i.e., the air outlet / air outlet 4a1) has a significant effect on the cooling effect. Generally speaking, when the cooling port is vertically downward, the cooling effect is optimal; when the cooling port is vertically upward, the cooling effect is relatively poor. In order to effectively control the transition process of the molten pool from semi-solidification to full solidification, this embodiment adopts a strategy of dynamically adjusting the direction of the cooling port.

[0059] In the ultra-high-speed laser cladding additive manufacturing process of the cold-rolled work roll 1, the cooling port is usually directly facing the cold-rolled work roll 1 or the surface of the additive layer for cooling. However, in the initial stage of additive manufacturing, that is, before the surface temperature of the additive layer drops by 150°C, this embodiment adopts a different strategy: the cooling port direction is adjusted to be vertically upward. This adjustment is intended to slow down the cooling rate, prevent the material from entering a quenching or semi-quenching state due to excessive cooling, and reduce the risk of material cracking. When it is monitored that the surface temperature of the additive layer drops by 150°C or more (≥150°C), different cooling port directions can be selected to achieve effective cooling control. Although the cooling purpose can still be achieved to a certain extent by adjusting the cooling port direction to be vertically upward or directly toward the surface of the additive layer, this method is different in efficiency compared to the vertically downward cooling direction. Considering the cooling efficiency and production rate, the preferred method is to adjust the cooling port direction to be vertically downward, which can not only speed up the cooling process, but also ensure that the material forms a uniform microstructure during the cooling process, thereby improving the overall quality and performance of the final product.

[0060] This method of dynamically adjusting the direction of the cooling port according to temperature changes aims to balance the relationship between cooling efficiency and material properties, ensuring that the additive layer can obtain good cooling effect during the solidification process while avoiding cracking or other defects caused by excessive cooling speed, thereby improving the overall quality and performance of the final product.

[0061] In the initial stage of additive manufacturing, in order to reduce the cooling gas flow directly blown to the cold-rolled working roll 1, this embodiment adopts a simple method: adjust the direction of the cooling port to vertically upward. This method effectively controls the flow rate by changing the blowing direction of the gas without changing the opening size of the cooling port. In addition, we can also achieve the same cooling effect by adjusting the opening size of the cooling port or directly adjusting the gas flow rate. The reason for choosing to adjust the direction of the cooling port is that it provides a flexible way to achieve flow control without changing the existing equipment configuration, which helps to optimize the cooling process while maintaining equipment stability.

[0062] It should be noted that in the additive manufacturing process, the so-called starting stage refers to the stage when the surface temperature of the additive layer begins to drop and the drop has not yet reached 150°C. This stage is determined according to the specific position of the cladding head 2 on the cold rolling work roll 1. Specifically, when the cladding head 2 starts cladding in each section of the cold rolling work roll 1, the starting stage of the section starts from this time. For example, when the cladding head 2 starts cladding in the first section, the starting stage of the first section begins; similarly, when the cladding head 2 moves to the second section, the starting stage of the second section begins. Similarly, when the entire cold rolling work roll 1 is divided into five sections, each section has its own independent starting stage, and there are a total of five starting stages. This division ensures that the cooling process can be precisely controlled according to the real-time position of the cladding head 2.

[0063] ⑧ Since the surface hardness of the M2 high-speed steel additive layer needs to reach 62HRC, the overall cooling gas flow rate of the slender air outlet 4a1 needs to be controlled at 25-30 L / min.

[0064] ⑨ In order to increase cooling efficiency and save cooling gas, this embodiment designs a set of semi-enclosed cooling chamber structure. Figure 1 As shown, the top of the structure remains open, while movable baffles 6 are used around to facilitate quick loading and unloading and replacement of the cladding work roll. The baffle 6 uses transparent glass that can filter laser light to facilitate observation of the laser additive manufacturing process and status of the surface of the cold rolling work roll 1. In addition, devices for online monitoring of the temperature of the molten pool and the additive layer during the laser additive process can be installed on the side walls of the semi-enclosed cooling chamber. The positions of these monitoring devices are fixed to ensure that the spacing of each observation remains unchanged, thereby improving the accuracy of online monitoring.

[0065] ⑩ When the ultra-high-speed laser cladding additive manufacturing of the M2 high-speed steel additive layer is completed, the online cooling is adjusted to achieve the surface temperature fluctuation of the additive layer ≤±25℃ and the surface temperature ≤200℃, then the online cooling is stopped and the additive layer is air-cooled to improve the toughness of the additive layer. If it continues to cool to room temperature, the M2 high-speed steel additive layer will become excessively brittle, thereby reducing its service life and failing to meet the performance requirements of the cold rolling work roll 1.

[0066] In fact, if you want to achieve the surface hardness of additively manufactured M2 high-speed steel ≥ 62HRC, you must make the surface hardness of additively manufactured M2 high-speed steel ≥ 60HRC without preheating and online cooling. To this end, you need to select process parameters that can achieve this hardness index. Otherwise, even with preheating and online cooling, it is difficult to achieve the above indicators.

[0067] ⑪ To ensure that the surface hardness of the additively manufactured M2 high-speed steel reaches at least 62HRC (≥62HRC) and the hardness fluctuation is controlled within 5HRC (≤5HRC), the thickness of each stacked layer must be controlled to not exceed 0.6mm (≤0.6mm). Accurate control of layer thickness is essential to obtain a uniform microstructure and excellent mechanical properties. If the thickness of a single stacked layer exceeds this limit, the brittleness of the additive layer will increase and may cause the fluctuation of the surface hardness to exceed the technical requirements, thus affecting the performance and quality of the final product.

[0068] In summary, the present invention aims at the cold-rolled work roll ultra-high-speed laser cladding additive manufacturing of M2 high-speed steel, under the premise that the number of additive layers is ≥3 and the thickness of the additive layer is ≥1mm, to achieve the surface hardness of the additively manufactured M2 high-speed steel ≥62HRC and the surface hardness fluctuation is ≤5HRC. First, a suitable preheating temperature of the cold-rolled work roll 1 is adopted, and then a slender air outlet cooling device and a semi-enclosed cooling chamber are designed to control the cooling effect, and a suitable online cooling gas flow rate and gradient cooling treatment method are selected to ensure that the surface additive layer of the entire cold-rolled work roll 1 can be accurately controlled by the cooling gas of the slender outlet, thereby achieving the above invention objectives.

[0069] The above embodiments are preferred implementation schemes of the present invention. In addition, the present invention may also be implemented in other ways. Any obvious replacement without departing from the concept of the present technical solution is within the protection scope of the present invention.

[0070] In order to make it easier for ordinary technicians in the field to understand the improvements of the present invention over the prior art, some drawings and descriptions of the present invention have been simplified, and for the sake of clarity, some other elements are omitted in this application document. Ordinary technicians in the field should realize that these omitted elements may also constitute the content of the present invention.

Claims

1. An online cooling method for laser additive manufacturing with surface hardness fluctuation ≤ 5HRC, characterized in that: The following steps are involved: S1. Preheating the cold rolling work roll (1) to a temperature of 300 to 350°C, with a preheating temperature fluctuation not exceeding ±25°C; S2, coaxial powder feeding; S3, air cooling after preheating; S4, applying cooling gas after air cooling; S5, dividing the cold rolling work roll (1) into a plurality of sections along its axial direction, and cooling each section with cooling gas, wherein the starting working position of the cladding head (2) corresponds to the first section of the cold rolling work roll (1), and the cooling gas flow rate corresponding to each section decreases as the section is farther from the cladding head (2); S6. During the laser cladding operation, the local cooling device (3) is used to perform targeted cooling on the area on the cold rolling work roll (1) where the temperature is abnormally increased; In step S5, when the cladding head (2) moves to different sections along the cold rolling work roll (1), the corresponding cooling gas flow rate is adjusted in real time according to the distance between each section and the cladding head (2), and the principle is followed that the cooling gas flow rate decreases accordingly as the section is farther from the cladding head (2); In step S5, when the cladding head (2) performs a cladding operation on each section of the cold rolling work roll (1), at least when the drop in the surface temperature of the cold rolling work roll (1) since the start of the cladding operation has not reached 150° C., the cooling gas of the section is blown out in a vertically upward direction to reduce the amount of cooling gas directly blown onto the surface of the cold rolling work roll (1); The method further comprises the following steps: When the laser cladding operation is finished, continue to cool each section of the cold rolling work roll (1) with cooling gas until the surface temperature fluctuation of the additive layer does not exceed ±25°C and the surface temperature does not exceed 200°C, then stop cooling with cooling gas and allow the roll to cool in air; The method further comprises the following steps: During the laser cladding operation, the thickness of each layer of the added material on the surface of the cold rolling work roll (1) is controlled to be no more than 0.6 mm.

2. The online cooling method for laser additive manufacturing with surface hardness fluctuation ≤ 5HRC according to claim 1, characterized in that: In step S5, cooling of the cold rolling work roll (1) is performed using a global cooling device (4) to cool each section of the cold rolling work roll (1). The global cooling device (4) comprises an air outlet duct (4a) parallel to the axis of the cold rolling working roll (1), and the cold rolling working roll side of the air outlet duct (4a) is provided with a plurality of continuous or intermittent air outlets (4a1) and a plurality of adjustment plates (4b), the number and position of the adjustment plates (4b) corresponding to the number and position of the sections of the cold rolling working roll (1) one by one, and is used to adjust the opening size and direction of the air outlet (4a1) to control the cooling gas flow and blowing direction of each section.

3. The online cooling method for laser additive manufacturing with surface hardness fluctuation ≤ 5HRC according to claim 1, characterized in that: In step S2, the gas flow rates of the shielding gas and the powder feeding gas used in the coaxial powder feeding do not exceed 20 L / min.

4. The online cooling method for laser additive manufacturing with surface hardness fluctuation ≤ 5HRC according to claim 1, characterized in that: In step S3, when the cold rolling work roll (1) is preheated to its set position and reaches the martensitic transformation temperature, the preheating is stopped and air cooling is performed.

5. The online cooling method for laser additive manufacturing with surface hardness fluctuation ≤ 5HRC according to claim 1, characterized in that: In step S4, the cooling gas is a mixed gas of argon and air, and the mass fraction of argon is 60-65%.

6. The online cooling method for laser additive manufacturing with surface hardness fluctuation ≤ 5HRC according to claim 5, characterized in that: The flow rate of the cooling gas is 25-30 L / min.

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