Titanium alloy smelting methods are generally divided into: 1. Vacuum consumable arc furnace smelting method; 2. Non-consumable vacuum arc furnace smelting method; 3. Cold hearth smelting method; 4. Cold crucible smelting method; 5. Electroslag smelting five methods.
1. Vacuum consumable arc furnace melting method (referred to as VAR method)
With the development of vacuum technology and the application of computers, the VAR method has quickly become a mature industrial production technology for titanium, and most of today's titanium and its alloy ingots are produced using this method. The salient features of the VAR method are low power consumption, high melting rate and good quality reproducibility. The ingots smelted by the VAR method have good crystallographic structure and uniform chemical composition. Usually, the finished ingot should be obtained by smelting the VAR method. At least two remeltings are required. In the production of titanium ingots by the VAR method, the processes used by manufacturers around the world are basically similar, and the difference lies in the use of different electrode preparation methods and equipment. Electrode preparation can be divided into three categories, one is the use of integral electrodes that are continuously pressed by portions, eliminating the electrode welding process; the other is the pressing of single-piece electrodes, which are tailored and welded into consumable electrodes. And through plasma argon arc welding or vacuum welding to weld into one; the third is to use other smelting methods to prepare cast electrodes.
Technical features and advantages of modern advanced VAR furnaces:
(1) Full coaxial power input, that is to say, complete coaxiality on the height of the entire furnace body, called coaxial power supply’, to reduce the occurrence of segregation;
(2) The electric calibration in the crucible can be fine-tuned in the X axis/Y axis;
(3) It has an accurate electrode weighing system, and the melting rate is automatically controlled to achieve constant-speed melting. Guaranteed melting quality;
(4) Ensure the repeatability and consistency of each smelting;
(5) Flexibility, that is, one furnace can produce a variety of ingot types and the large-scale ingot, which can greatly improve the productivity;
(6) It has good economy. The "coaxial power supply" method can avoid the magnetic bias leakage caused by the unbalanced supply current of the crucible. Attenuate or eliminate the adverse effects of induced magnetic fields on smelting products. And the electrical efficiency is improved, thereby obtaining ingots with stable quality. The purpose of "constant speed smelting" is to improve the quality of the ingot, through the advanced electronic control system and weight sensor to ensure the constant arc length and melting rate during the smelting process, so as to control the solidification process. It can effectively prevent segregation and ensure the inherent quality of the ingot. In addition to the above two characteristics, the modern VAR furnace for titanium smelting also realizes the large-scale VAR furnace. The modern VAR furnace can smelt large ingots with a diameter of 1.5m and a weight of 32t. The vAR method is the standard industrial melting method for modern titanium and titanium alloys. There are the following technologies to be resolved. First, the electrode preparation method. The electrode preparation process is very complicated. It is necessary to use an expensive press to press the sponge titanium, the intermediate alloy and the return residual material into an integral electrode or a single small electric trigger. A single electrode also needs to be welded into a consumable electrode. At the same time, in order to ensure the uniformity of the consumable electrode composition, it is also necessary to configure the corresponding facilities such as cloth, weighing and mixing. Second, there are occasional metallurgical defects such as segregation. Such as composition segregation and solidification segregation. www.lh-ti.com introduced that the former is due to the uneven distribution of impurity elements or alloying elements in the electrode. The latter is caused by the occasional introduction of high-density inclusions (HDI) and low-density inclusions (LDI) into the raw materials or the process, and these inclusions cannot be completely dissolved during the smelting process. Lead to the generation of metallurgical defects such as extremely harmful inclusions.
2. Non-consumable vacuum arc furnace smelting method (Jian Mi NC method)
At present, the water-cooled copper electrode has replaced the tungsten-thorium electroplating or graphite electroplating in the initial stage of the titanium industry, solving the problem of industrial pollution, so that the NC method has become an important method for smelting titanium and titanium gold. NC furnaces are already operating in Europe and America. There are two types of water-cooled copper electrodes: one is self-rotating; the other is rotating magnetic field, the purpose of which is to prevent the arc from burning the electrode. NC furnaces can also be divided into two types: one is to smelt raw materials in a water-cooled copper crucible, and cast into ingots in a water-cooled copper mold; the other is to continuously pour raw materials into a water-cooled copper crucible, smelt and solidify. The advantages of NC smelting are: 1. The process of pressing electrodes and welding electrodes can be omitted; 2. The arc can stay on the material for a long time, thereby improving the homogenization of the ingot composition; 3. The raw materials of different shapes and sizes can be used in During the smelting process, 100% residual material can also be added to realize the recycling of titanium. As a smelting process, the NC method is quite beneficial in terms of improving the recovery rate of residual materials and reducing costs. Usually, NC furnaces and VAR furnaces are used in combination to take full advantage of their respective advantages.
3. Cold hearth melting method (referred to as CHM method)
The metallurgical inclusion defects of titanium and titanium alloy ingots caused by the pollution of raw materials and abnormal smelting process have always affected the application of titanium and titanium alloy in the aerospace field. In order to eliminate metallurgical inclusions in the rotating parts of titanium alloy aircraft engines, cold hearth melting technology came into being. The biggest feature of the CHM method is the separation of the melting, refining and solidification processes, that is, the molten charge is first melted after entering the ling hearth, and then enters the refining zone of the cold hearth for refining, and finally solidifies into ingots in the crystallization zone. The significant advantage of CHM technology is that a condensate crust can be formed on the bed wall of the cold hearth, and its "viscous zone" can capture high-density inclusions (HDI) such as WC, Mo, Ta, etc. At the same time, in the refining zone, low-density inclusions The extended residence time of (LDI) particles in the high temperature liquid can ensure the complete dissolution of LDI, thereby effectively removing inclusion defects. That is to say. The purification mechanism of cold hearth smelting can be divided into two types: gravity separation and melting separation.
3.1 Electron Beam Cold Hearth Melting Method (EBCHM for short) Electron beam melting (EB for short) is a process in which the energy of high-speed electrons is used to make the material itself generate heat for smelting and refining. An EB furnace with a cold hearth is called EBCHM. The EBCHM method has excellent functions that the traditional smelting method does not have:
(1) Effectively remove high density inclusions (HDI) such as tantalum, molybdenum, tungsten, tungsten carbide and titanium nitride. Low density inclusions (LDI) such as titanium oxide;
(2) It can accept a variety of feeding methods, and the recovery of titanium residues is relatively easy, that is, scraps that cannot be used by other smelting methods can be used, and pure titanium ingots can still be obtained, which greatly reduces the cost of the product;
(3) It can be directly sampled from the metal liquid for analysis and testing;
(4) It can produce special-shaped ingots, reduce the production process, reduce the consumption of raw materials, and improve the yield;
The EBCHM method also has the following disadvantages:
(1) Smelting needs to be carried out under high vacuum conditions, so sponge titanium with high chloride content cannot be used for direct smelting;
(2) Alloy elements are volatile and difficult to control the chemical composition.
3.2 Plasma cold bed smelting method (called PCHM method)
The PCHM method uses the plasma arc generated by the ionization of the inert gas as the heat source, and can complete the smelting in a wide pressure range from low vacuum to near atmospheric pressure. The notable feature of this method is that it can ensure alloy components with different vapor pressures, and there is no obvious difference in the smelting process. This method has the ability to improve the traditional metal properties, and can realize the smelting of diversified alloys. It is a more economical method than traditional smelting methods. smelting method. Using this method for smelting, for titanium and titanium alloys, ideal ingots can be obtained in one smelting. The advantages of the modern PCHM method are:
①Equipment investment is low, easy to operate, safe and reliable;
② Different types and forms of raw materials can be used, and the recovery rate of residual materials is high;
③ Ensure the chemical composition of diversified alloys;
④ Realize expensive inert gas recovery and reuse, reducing production cost. The disadvantage of the PCHM method is the low electrical efficiency. EBCHM and PCHM are similar in that both can eliminate HDI and LDI. Generally, the former is more suitable for smelting pure titanium; for alloys, the latter is more suitable. Like the VAR method, the above two methods realize a wide range of process automation control, including process parameters (melting speed, temperature distribution during smelting and solidification, changes in composition during smelting, degree of removal of insoluble inclusions, etc.) and quality. .
4. Cold crucible melting method (CCM method for short)
In the 1980s, the American Ferrosilicon Company developed the slag-free induction melting process and pushed the CCM method to industrial production applications for the production of titanium ingots and titanium precision castings. In recent years, in some economically developed countries, the CCM method has begun to enter the scale of industrial production. The maximum diameter of the ingot is 1 m and the length is 2 m, and its development prospect is eye-catching. The smelting process of CCM method is carried out in a metal crucible which is a combination of water-cooled arc blocks or copper tubes that are not conductive to each other. The biggest advantage of this combination is that the gap between each two blocks is an enhanced magnetic field, and the strong magnetic field is generated. Agitation aligns chemical composition and temperature, which improves product quality. The CCM method combines the characteristics of the VAR method and the crucible induction melting of refractory materials. It does not require refractory materials or electrodes to obtain high-quality ingots with uniform composition and no crucible pollution. Compared with the VAR method, the CCM method has the advantages of low equipment cost and simple operation, but from the current point of view, the technology is still in the development stage.
5. Electroslag smelting method (referred to as ESR method)
The ESR method converts electrical energy into heat energy by using the collision of charged particles when an electric current passes through the conductive electroslag. That is, the heat energy generated by the slag resistance is used to melt and refine the charge. ESR method uses consumable electrodes for electroslag smelting in inactive slag (CaF2), which can be directly cast into ingots of the same shape, and has good surface quality, which is suitable for direct processing in the next process. The advantages of this method are:
(1) The complete coaxiality of the ESR furnace ensures the repeatability of the best quality ingots;
(2) The ingot is crystallized in the axial direction, and the structure is dense and uniform;
(3) Electrode weighing system and smelting rate control system with extremely high precision;
(4) The equipment is simple and the operation is convenient. The disadvantage is that the contamination of the ingot by the slag cannot be drained.
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Nicole
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