ABSTRACT: According to the research content, method and technical route of high performance oxygen-free copper pipe for heat pipe, based on the research of composition control and melting casting technology of high performance oxygen-free copper tube blank for heat pipe, planetary rolling technology of large deformation of copper tube blank, drawing process of 60.2 smooth tube coil for heat pipe and forming process of 80.20.10 grooved tube for heat pipe, high performance smooth and grooved tubes for heat pipe have been successfully realized.

Key Words: Heat Pipe; Melt Casting; rolling; Coil Drawing; forming; Smooth Pipe; grooved pipe Middle Diagram classification number: TG2 document identification number: A article number: 1009-3842(2021)01-0014-05

1. Introduction

The heat pipe is a phase change process in which the medium evaporates at the hot end and condenses at the cold end, which makes the heat transfer rapidly. It has extremely high thermal conductivity, good isothermal property, and the heat transfer area can be changed arbitrarily at the cold and hot ends, can Be long-distance heat transfer, temperature control and other advantages. It is mainly used in the fields of cooling of electrical equipment, cooling of CPU and electronic devices, cooling of semiconductor components and large-scale Integrated Circuit Boards, as well as in the fields of aviation, aerospace, military and other industries, examples include laptops, smartphones, 5g Base station, new energy vehicles and high-powered LED lights. Compared with traditional metal radiators, heat pipe radiators have many advantages, such as compact structure, low fluid loss, easy to control dew point corrosion, low noise and high efficiency. The oxygen-free copper pipe is one of the main materials used to make the heat pipe shell, including the heat pipe smooth pipe and the grooved pipe. Therefore, high-performance oxygen-free copper pipe for heat pipe will become a new growth point of copper pipe industry market in the future, and also a new development direction of copper pipe industry production technology. The production equipment and process flow of oxygen-free copper tubes for heat pipes are basically the same as those for air conditioning and refrigeration. The key points are the control of oxygen content during melting casting, the establishment of planetary rolling process and the design of forming die parameters for the tooth rib of grooved tubes.

The product structure of TP2 copper tubes, such as high efficient heat transfer inner thread copper tubes and smooth copper tubes, is relatively single. The product market is located in the refrigeration industry. The main customers are domestic air-conditioning manufacturers, there is a low peak market season is very obvious, customers are too concentrated, due to the existing product technology mature, resulting in lower profits, long account period of the characteristics. Therefore, focusing on the transformation and upgrading development goal of the processing department, we should focus on promoting the research and development of high-quality projects and new products, and increase the research and development of new products in the fields of new energy vehicles, 5g, aerospace, military industry, electronics and other fields, to promote copper processing products to high-end [1]. We will accelerate the research and development of new high-performance oxygen-free copper tubes for heat pipes, increase the reserves of new types of copper tubes and new technologies, upgrade the technical level of production and the competitiveness of the industry, and increase the market share of the products, promote the industry to the high-end development of value chain.

2. Research content, method and technical route of high performance oxygen-free Copper Tube for heat pipe

By studying the key technologies such as composition control and melting-casting process of copper tube blank for heat pipe, planetary rolling process of oxygen-free Copper Tube with large deformation, spinning process of copper tube for inner groove heat pipe and the effect of annealing process on the microstructure and properties of copper tube for heat pipe, etc. , development of a high performance copper tube for heat pipe. Its technical objective is to be able to continuously produce and ensure product performance to meet the following requirements:

(1) small-scale production of high-performance oxygen-free copper tubes for heat pipes, such as heat pipe smooth tubes 80.2,60.2,50.1, heat pipe grooved tubes 80.200.10,60.200.10,60.300.12, etc. . The composition requirement is C10100(TU0) , the content of oxygen is higher than 10 PPM. A set of technical specifications for the production of high performance copper tubes for heat pipes has been formed. 

(2) the dimensional tolerance, mechanical properties and mechanical properties of heat pipe products conform to GB/T17791-2017 national standard for seamless copper and copper alloy tubes for air-conditioning and refrigeration equipment [2] , and the product in the process of processing into a heat pipe, there is no bending surface orange peel and flat depression and other defects.

Research methods of high-performance oxygen-free copper pipe for heat pipe: adopting process test method, selecting suitable horizontal continuous casting process, oxygen content control method, planetary rolling process, etc. By using the advanced design software of internal thread forming die, the spinning forming die parameters of internal grooved copper tubes are developed, the tooth shape parameters of internal thread copper tubes for heat pipe are measured by 3D metallographic microscope, and the tooth forming quality of internal thread copper tubes for heat pipe is analyzed, this paper analyzes the influence of different forming speed, rotating die ratio and forming die parameters on the forming quality, and the influence of different annealing processes on the microstructure and properties of copper tubes, analyze and optimize the forming process pertinently. The technical route of high-performance oxygen-free copper pipe for heat pipe: fully understand the technical requirements of customers for heat pipe → adjust the technical parameters of melting, casting, rolling and disk drawing → according to theory and practical experience, design and manufacture the die for the drawing process of the heat pipe, apply the die and the core to the drawing process, small-scale, pilot-scale and large-scale production test, check the qualified heat pipe products and send them to the customers for trial validation, form the technical specifications, and solidify the technical parameters. Its production process roadmap is shown in figure 1.

3. Study on technology of high performance oxygen-free Copper Tube for heat pipe 3.1 composition control of high performance oxygen-free copper tube for heat pipe and melting and casting technology 3.1.1 key technical difficulties (1) melting temperature control

Melting temperature is an important parameter. In a certain temperature range, the higher the temperature, the lower the viscosity, the better the fluidity and the better the filling ability. However, the defects of shrinkage, porosity and porosity are easy to appear due to high temperature, heavy gettering and oxidation of liquid metal. The temperature is too low, will hinder the copper liquid in the non-metallic impurity to float, is disadvantageous to the copper liquid purification. At the same time, the viscosity of liquid copper increases with the temperature decreasing, and the fluidity of liquid copper decreases. Therefore, it is very important to select the appropriate melting temperature. The normal setting temperature of melting furnace is 1175 °C, and that of holding furnace is 1160 °C.

(2) control of Oxygen (O) content. High-performance oxygen-free copper tubes for heat pipes are required to be C10100(TU0) , of which the oxygen content is required to be higher than 10 PPM. The oxygen content is higher than the standard, because the oxygen content is related to the heat transfer efficiency, the lower the oxygen content is, the higher the heat transfer efficiency is. Reasons for high oxygen content: Melting furnace and casting furnace are in the atmosphere of melting, oxygen in the air will go into the copper melt. The solution: After feeding the melting furnace, when the copper liquid level reaches the standard level, adding a high purity graphite powder deoxidizer to the liquid level or covering the copper melt surface with a thick layer of wood carbon; At the same time, a deoxidizer is added to the liquid surface of the holding furnace. The cover thickness of charcoal is 100 ~ 200 mm, the copper solution is forbidden to be exposed, and the new charcoal is changed at intervals, that is, the chemical equation is Cu2O + C Cu + CO2. (3) control of Phosphorus (P) content.

The maximum solubility of phosphorus in copper (714 °C eutectic temperature) is 1.75% , and it is almost zero at room temperature. In TP2 copper alloy tube, the addition of phosphorus can reduce the electrical conductivity and thermal conductivity of the tube, and the thermal conductivity is the key performance of the tube. In addition, the higher phosphorus content leads to the higher sintering and bending Surface roughness of the heat pipe, which affects the surface quality of the heat pipe. Therefore, the phosphorus content of the heat pipe must be controlled below 10 PPM. The reason of high phosphorus content: Because the oxygen-free tube of heat pipe is melting TP2 copper alloy (P content 150 ~ 400ppm) in furnace, there are more TP2 copper alloy melt in the furnace cavity of melting furnace and casting furnace, at the same time, there is a large amount of residual TP2 copper alloy melt in the furnace wall and its gap. The solution to the problem: first, the melting furnace, casting furnace stripped off the surface of the melt carbon coating, the surface of the melt exposed to air smelting, so that the oxygen in the air and the phosphorus in the surface of the melt continuous reaction, resulting in phosphorus oxide scum, in order to achieve a very good phosphorus removal effect, that is, diffusion oxidation burning phosphorus, the chemical equation is p + O2 = P2O5.

3.1.2 test results

1) the results of controlling the content of P and O. Phosphorus (P) content in melting furnace: 2.8ー15.4 PPM; casting furnace: 6.2ー19.8 PPM; casting billet: 8.5ー11.2 PPM. The content of oxygen (o) in melting furnace is 1.1ー6.1 PPM, in casting furnace is 3.1ー7.3 PPM, in billet is 3.0ー9.3 PPM. Oxygen Content Control Ah below 10 PPM, to meet the heat pipe oxygen content of oxygen-free copper pipe technical requirements.

(2) microstructure of billet. The microstructure of the billet is fine and uniform, which can meet the requirement of rolling process of high performance oxygen-free copper tube for heat pipe, as shown in Fig. 2. 3.2 Planetary Rolling Process 3.2.1 Planetary Rolling Force analysis of high performance oxygen-free copper tube blank for heat pipe after milling process, the blank of oxygen-free Copper Tube is rolled by three-roll planetary rolling. The total deformation of the workpiece is the accumulation of the deformation caused by rolling the workpiece with three rollers in turn. At each instant, the contact between the roll and the workpiece is not linear, but a narrow strip, as shown in figure 3.

The rolling process is feed forward Rotary Rolling, that is, during the rolling process, a point moves to e point, b point moves to f point, so as to achieve the effect of reducing rolling. But at each instant, the circumferential ring-shaped micro-element rolling workpiece on the roller mushroom head, as shown in Fig. 3, can be regarded as a cylindrical roller rolling workpiece. The locus of this locus can be regarded as the locus of the point on the roll under the assumption that there is no slip at the contact point between the workpiece and the roll. As shown in figure 4.

In the process of Planetary Rolling, the rolling deformation is mainly reducing deformation. The deformation of copper tube blank rolled by three-roll Planetary Rolling is very large, but the rolling Torque is very small. Because the power of the motor is proportional to the rolling Torque, the energy consumption of planetary rolling is very low compared with extrusion. The large deformation of metal can be realized by the rotation and revolution of the rolling mill roll, and the twisting motion of copper tube can be controlled by modifying the auxiliary driving speed to provide a certain amount of propulsion, the tube is subjected to strong three-dimensional compressive stress in the deformation zone. According to the theory of force analysis in rolling process, the equation for calculating rolling force is obtained as follows.

3.2.2 the experimental results show that the rolling force of oxygen-free copper billet is greater than that of TP2 billet in the three-roll planetary rolling process, it is necessary to reduce the rolling speed of oxygen-free copper billet in order to reduce the load of rolling mill, otherwise the wall thickness is difficult to control. Due to the characteristic of oxygen-free copper, the roll is easy to stick to copper and slip when rolling oxygen-free copper billet. The final determination of oxygen-free copper heat pipe rolling process as shown in Table 1.

3.3 drawing process of 60.2 smooth coil for heat pipe 

3.3.160.2 smooth coil drawing process

The drawing process and production pass of high performance oxygen-free Copper Tube for heat pipe were designed according to the principle of tube hardening. In addition, the requirements of ovality for heat pipe are very high. Therefore, the last drawing pass is adjusted to the internal thread forming machine, and the drawing process of 60.2 smooth pipe coil for heat pipe is finally determined as shown in Table 2.

3.3.2 test results (1) performance test results. External Diameter (6 ± 0.03) : 5.98 mm, wall thickness (0.2 ± 0.02) : 0.203 mm, tensile strength (≥260) : 360 MPA, elongation (5)% , vickers hardness (90-110) : 106.8 HV, copper content (≥99.97%) : 99.992% , oxygen content (≤10 PPM) : 6.5 PPM. All the performance indicators meet the technical standards of customers.

3.4 grooved tube forming process for heat pipe 80.200.10 grooved tube forming process 3.4.180.200.10 grooved tube forming process according to customer's technical requirements, the forming process and die of the grooved tube were optimized and calculated by the method of backward pushing and the software of internal thread design. According to the internal thread pipe specifications, such as d 0 s 0 h 0(thick tooth height of bottom wall of outer diameter) , the parameters of steel ball, spinning ring, thread core, reducing die and tube blank are calculated from the fixed diameter die and back to frontAfter spinning and pre-determined diameter 9.52 by the forming machine, and then re-sizing and drawing to 8 by the forming machine, the 80.200.10-110 grooved tubes for heat pipes meeting the technical standards of customers were successfully trial-produced, thus the final forming process of 80.200.10 grooved tube is determined as shown in Table 3.

3.4.2 experimental results (1) results of various performance tests. External Diameter (8 ± 0.03) : 8.01 mm, wall thickness (0.2 ± 0.02) : 0.202 mm, tensile strength (≥260) : 340 MPA, vickers hardness (90-110) : 102.7, copper content (≥99.97%) : 99.99% , oxygen content (≤10 PPM) : 7.5 PPM, tooth height (0.20 ± 0.2) : 0.216 mm, tooth number (110) : 110, Helix Angle (0 ± 2 °) : 0 ° , tooth Apex Angle (5 ~ 25 °) : 15 ~ 20 ° .

4. Conclusion 

through the research and development of high-performance oxygen-free copper tubes for heat pipes, the melting-casting process of oxygen-free copper tubes for heat pipes was successfully developed by using the existing TP2 melting-casting furnace equipment, the oxygen-free copper casting blank of high quality for heat pipe with qualified composition and microstructure was obtained, and the technical specifications for producing 60.20 smooth heat pipe and 80.200.10-110 grooved heat pipe with high performance were formed. Small batch production of heat pipe 60.2 smooth pipe and 80.200.10 grooved pipe was successfully realized, and the oxygen content was controlled to be less than 10ppm, the performance indexes of the products, such as dimensional tolerance, mechanical properties and mechanical properties, fully meet the national standard of GB/T17791-2017 seamless copper and copper alloy tubes for air-conditioning and refrigeration equipment and the requirements of customers. 

Source: Chinanews.com, author: Tang Xiaoshui, Li Hua