Titanium alloy smelting methods are generally divided into: 1. Vacuum consumable arc furnace melting method; 2. Non-consumable vacuum arc furnace melting method; 3. Cold hearth melting method; 4. Cold pot melting method; 5. Electroslag melting Five methods.
1. Vacuum consumable arc furnace melting method (abbreviated as VAR method)
With the development of vacuum technology and the application of computers, the VAR method quickly became a mature industrial production technology of titanium. Most of today’s titanium and its alloy ingots are produced using this method. The remarkable characteristics of the VAR method are low power consumption, high melting speed and good quality reproducibility. The ingots melted by the VAR method have a good crystalline structure and a uniform chemical composition. Generally, the finished ingot should be made by VAR melting. It must be remelted at least twice. To produce titanium ingots by the VAR method, the processes used by manufacturers around the world are basically similar, the difference is that different electrode preparation methods and equipment are used. Electrode preparation can be divided into three major categories. One is the use of integrally pressed integral electrodes for continuous pressing, which eliminates the electrode welding process. The second is the pressing of a single piece of electrode and tailor welding to form a consumable electrode. And it is welded by plasma argon arc welding or vacuum welding; the third is to use other melting methods to prepare casting electrodes.
Technical characteristics and advantages of modern advanced VAR furnace:
(1) Full coaxial power input, that is to say the complete coaxiality of the whole furnace height, called coaxial power supply ‘, to reduce the occurrence of segregation;
(2) The electric calibration in the crucible can be finely adjusted in the X axis / Y axis;
(3) has an accurate electrode weighing system, the smelting rate is automatically controlled, and constant speed smelting is achieved. Ensure the smelting quality;
(4) Ensure the repeatability and consistency of each smelting;
(5) Flexibility, that is, one furnace can produce a variety of ingots and large-scale ingots, which can greatly improve productivity;
(6) has good economy. The “coaxial power supply” method can avoid magnetic leakage caused by the imbalance of the crucible supply current. Reduce or eliminate the adverse effects of induced magnetic fields on smelting products. And improve the electrical efficiency, thereby obtaining stable quality ingots. The purpose of “constant speed smelting” is to improve the quality of the ingot. The advanced electric control system and weight sensor ensure the constant arc length and melting rate during the smelting process, thereby controlling the condensing process. It can effectively prevent segregation and ensure the intrinsic quality of the ingot. In addition to the above two characteristics, modern VAR furnaces for titanium smelting have also realized the enlargement of VAR furnaces. Modern VAR furnaces can melt large ingots with a diameter of 1.5 m and a weight of 32 t. The vAR method is the modern industrial melting method for modern titanium and titanium alloys. The following technologies need to be solved. First, the electrode preparation method. The electrode preparation process is very cumbersome. It is necessary to use expensive presses to compress the sponge titanium, intermediate alloy and return residue into a whole electrode or a single small electric wrench. Monolithic electrodes also need to be welded into consumable electrodes. At the same time, in order to ensure the uniformity of the components of the consumable electrode, it is also necessary to configure 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.titanium-machining.com introduces that the former is due to the uneven distribution of impurity elements or alloy elements in the electrode. Solidification occurs when smelting is too late for equilibrium distribution; the latter is due to the occasional introduction of high-density inclusions (HDI) and low-density inclusions (LDI) in the raw material or process, which cannot be completely dissolved during the smelting process Causes metallurgical defects such as extremely harmful inclusions.
2. Non-consumable vacuum arc furnace melting method (Jian You NC method)
At present, the water-cooled copper electrode has replaced the tungsten-thorium Taiwan gold cast iron or graphite electric trigger of the initial stage of the titanium industry to solve the problem of industrial pollution, thus making the NC method an important method for smelting titanium and titanium Taiwan gold NC furnaces have been operating in Europe and America. Water-cooled copper electrodes are divided into two types: 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 feed raw materials in a water-cooled copper crucible to melt 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 longer time, thereby improving the uniformity of the ingot composition; ③ raw materials of different shapes and sizes can be used, During the smelting process, 100% residue can also be added to achieve the recycling of titanium. The NC method as a one-time smelting is quite advantageous from the viewpoint of improving the recovery rate of residues and reducing costs. Usually, NC furnace and VAR furnace are used together to give full play to their respective advantages.
3. Cold hearth melting method (referred to as CHM method)
The metallurgical inclusion defects of titanium and titanium gold ingots caused by the contamination of raw materials and the abnormal melting process have always affected the application of titanium and titanium platform gold 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 melting, refining and solidification processes, that is, the melted charge enters the Ling hearth and then melts, then enters the refining area of ​​the cold hearth for refining, and finally solidifies into ingots in the crystallization area. The obvious advantage of CHM technology is that it can form a shell on the wall of the cold hearth bed. 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 (LDI) The residence time of the particles in the high-temperature liquid is extended, which can ensure the complete dissolution of LDI, thereby effectively removing the inclusion defects. That is to say. The purification mechanism of cold hearth smelting can be divided into two types: specific gravity separation and melting separation.
3.1 Electron beam cold hearth smelting method (abbreviated as EBCHM method) Electron beam smelting (abbreviated as EB) is a process that uses the energy of high-speed electrons to make the material itself generate heat for smelting and refining. The EB furnace with a cold hearth is called EBCHM. The EBCHM method has excellent functions not available in the traditional smelting method:
(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 multiple feeding methods, and it is easier to recover titanium residues, that is, it can use waste materials that cannot be used by other smelting methods, and can still produce pure titanium ingots, which greatly reduces the cost of the product;
(3) Can be directly sampled and analyzed from the molten metal;
(4) It can produce special-shaped ingots, reduce the production process, reduce the consumption of raw materials, and improve the yield;
EBCHM method also has the following disadvantages:
(1) The smelting needs to be carried out under high vacuum conditions, so it is not possible to directly smelt using sponge titanium with high chloride content;
(2) Alloy elements are volatile and it is difficult to control the chemical composition.
3.2 Plasma cold-bed melting method (PCHM method)
The PCHM method uses a plasma arc generated by inert gas ionization as a heat source, and can be smelted in a wide pressure range from low vacuum to near atmospheric pressure. The remarkable feature of this method is that it can ensure the alloy components of different vapor pressures. There is no obvious in the smelting process. This method has the ability to provide improved traditional Taiwan metal properties, can realize the smelting of multiple alloys, and is a more economical method than traditional smelting Smelting method. Using this method of smelting, for titanium and titanium alloys, an ideal ingot can be obtained in one smelting. www.titanium-machining.com The advantages of the modern PCHM method are:
① Equipment investment is low, easy to operate, safe and reliable;
②Different types and forms of raw materials can be used, and the recovery rate of residual materials is high;
③ To ensure the chemical composition of multiple alloys;
④ Achieve expensive inert gas recovery and reuse, reducing production costs. The disadvantage of the PCHM method is its low electrical efficiency. EBCHM and PCHM are similar in that they 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 electrically achieve a wide range of process automation control, including process parameters (smelting speed, temperature distribution during melting and solidification, changes in composition during melting, removal of insoluble inclusions, etc.) and quality .
4. Cold pot melting method (referred to as CCM method)
In the 1980s, 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 precision castings of titanium. In recent years, in some economically developed countries, the CCM method has begun to scale up industrial production. The maximum diameter of the ingot is 1 m and the length is 2 m, and its development prospects are remarkable. The smelting process of the CCM method is carried out in a metal crucible with 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. Stirring makes the chemical composition and temperature consistent, thereby improving product quality. The CCM method combines the characteristics of the VAR method and the induction melting of refractory crucibles. It does not require refractory materials and does not require electrodes to obtain a high-quality ingot with a uniform smelting composition and no crucible pollution. Compared with the VAR method, the CCM method has the advantages of low equipment cost and easy operation, but from the current point of view, the technology is still in the development stage.
5. Electroslag melting method (referred to as ESR method)
The ESR method utilizes the collision of charged particles when current passes through conductive electroslag, and converts electrical energy into thermal energy. That is, the heat energy generated by the slag resistance melts and refines the charge. The ESR method uses consumable electrodes to perform electroslag smelting in inactive slag (CaF2). It 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 ingot;
(2) The ingot is axially crystallized and the structure is dense and uniform;
(3) Electrode weighing system and melting rate control system with extremely high precision;
(4) The equipment is simple and easy to operate. The disadvantage is that the slag cannot be used to pollute the ingot.