What Is Electron Beam Melting?

Electron beam melting refers to a vacuum melting method that converts the kinetic energy of high-speed electron beam current into thermal energy as a heat source for metal melting under high vacuum. Referred to as EBM. This smelting method has the characteristics of high smelting temperature, adjustable furnace power and heating speed, and good product quality, but also has problems such as low metal yield, large specific power consumption, and smelting under high vacuum conditions. Electron beam smelting is not only used for the melting and refining of steel and rare metals, but also widely used in welding and ceramic material casting. ^[1]

In 1905, Siemens (Siemens) and Haisko (Germany) successfully smelted tantalum with electron beam for the first time. The purity and processing performance of the remelted ingot were better than those of the vacuum arc furnace. But at that time, the development level of vacuum technology in the world was still very low, which affected the development of electron beam melting technology. It wasn't until the 1950s that the To-moscai company in the United States developed the electron beam smelting to an industrial production scale, which attracted the attention of countries around the world. Several industrial developed countries have successively carried out the research and development of electron beam furnaces, among which the United States and Germany have developed the fastest. In this way, electron beam melting has also developed into a new special metallurgical technology. China began research and trial production of electron beam melting furnaces in 1958. By the 1960s, it had the scale of industrial production.

Under high vacuum conditions, the cathode is heated by the high-voltage electric field and emits electrons. The electrons are collected into a beam. The electron beam moves toward the anode at a very high speed under the action of the acceleration voltage. After passing through the anode, it is focused on the coil. Under the action of the deflection coil, it accurately bombards the bottom ingots and materials in the crystallizer, so that the bottom ingots are melted to form a molten pool, and the materials are continuously melted and dripped into the molten pool to achieve the melting process. Beam melting principle. Figure 1 is a schematic diagram of the electron beam melting principle. The acceleration voltage of the electron beam furnace is generally used at about 30,000 volts, and the maximum X-ray loss caused does not exceed 0.5%, and the loss of secondary emission electrons will be less. Therefore, the energy of the electron beam is almost completely converted from electrical energy to kinetic energy, and then from kinetic energy to thermal energy. The motion speed V (km / s) of the electron can be determined by the following formula:

Where W is the acceleration voltage.

The electron beam melting process is characterized by melting in a high vacuum environment (the melting vacuum is generally 10 to 10Pa). The temperature and distribution of the molten pool can be controlled during melting, and the maintenance time of the molten pool can be adjusted within a wide range. ; Melting is carried out in a water-cooled copper crucible (crystallizer), which can effectively prevent metal liquid from being contaminated by refractory materials. Therefore, it can be said that electron beam melting provides an indispensable refining means for some metal materials, especially refractory metals.

There are three basic metallurgical reactions in the electron beam melting process: (1) degassing. Electron beam melting can remove hydrogen in most metals, and the removal of hydrogen is easy. Generally, it is basically completed before the furnace charge is melted away: due to the high vacuum, the temperature of the molten pool and the time in the liquid can be controlled, and the nitrogen removal effect Also high. (2) Volatilization of metal impurities. At the electron beam melting temperature, any metal impurities higher than the base metal vapor pressure will be volatile and removed to varying degrees. (3) Remove non-metallic inclusions. Oxide and nitride inclusions may be decomposed under the electron beam melting temperature and vacuum degree, and [O] and [N] may be removed; [O] may also be removed by the carbon-oxygen reaction; in addition, the spindle is bottom-to-bottom The sequential solidification characteristics are also conducive to the floating of non-metallic inclusions.

A typical electron beam melting furnace is generally composed of 6 parts:

(1) Electron gun. The electron gun is the heart of an electron beam melting furnace. It includes a gun head (commonly composed of a filament, a cathode, an anode, etc.), a focusing coil, and a deflection coil. Electron guns can be divided into axial guns (or Pierce guns), non-self-accelerated ring guns, self-accelerated ring guns, and transverse guns according to their structural forms. 2. The number of electron guns is single, double and multiple. (2) Feeding system. If the raw material is a prefabricated consumable electrode, a vertical or horizontal mechanical feeding method is generally used: if the raw material is crumbs, blocks or granules, a feeding silo method is used. (3) Ingot system. Including crystallizer, ingot pulling mechanism and ingot discharging mechanism. (4) Vacuum system. Including vacuum units, vacuum chambers, vacuum pipes and valves and vacuum measurement systems. (5) Power system. Including main power supply (electron gun power supply), control power supply and operation power supply. (6) Cooling system. Including all cooling water and pipeline valves.

According to the type of electron gun used, it can be divided into three types: ring gun, horizontal gun and axial gun electron beam melting furnace. According to its purpose, it can be divided into melting furnaces, regional refining furnaces, electron beam shell furnaces, and Four types of multi-purpose electron beam melting furnaces. ^[2]

There are mainly melting power, melting speed, specific electric energy, vacuum degree, and air leakage rate, such as the number of melting times, the ratio of the consumable electrode to the crucible diameter, the cooling rate, and the ingot cooling system, etc. You must also pay attention to reasonable choices. Among the parameters, melting power, melting speed and specific electric energy are the most important factors affecting the quality of the ingot. The control of these three parameters should be changed throughout the smelting process. In the early stage of melting, the vacuum is relatively low, the furnace charge and the crucible are at normal temperature, the molten pool has not yet formed, the melting power should be lower, and the melting rate should be slower; at the end of the melting, in order to eliminate shrinkage holes on the ingot top, the melting power and melting rate There must be a gradual descent process to complete the deflation. The length of the shrinkage time is related to the size of the ingot and the type of molten metal; and during most of the normal melting period in the middle of the melting, the melting power and melting rate should be kept stable.

Electron beam smelting of metals, like other methods of smelting metals, should be routinely tested and analyzed for basic physical and chemical properties, chemical composition, impurity content, and as-cast structure. Generally, the electron beam smelting metal has high purity and good as-cast structure, so it has high mechanical properties, especially high plasticity, toughness and isotropic coefficient. It should be noted that during the electron beam melting process, due to the high temperature of the molten pool, the high degree of superheat, and the long time that the metal is in the liquid state, the columnar crystals develop during the solidification of the ingot, which adversely affects the blooming. When formulating process parameters, consideration should be given to preventing the problem of excessive growth of columnar crystals. In addition, electron beam melting ingots are also prone to some surface metallurgical defects, such as surface transverse cracks, cold insulation, and uneven surfaces. These should be solved by optimizing process parameters and improving the level of operating technology. ^[3]