Smelting method of titanium alloy
Titanium alloy smelting methods are generally divided into: 1. Vacuum consumable electric arc furnace smelting;2. Non-consumable vacuum arc furnace smelting;3. Cold hearth smelting;4.Cold crucible smelting; 5. Five methods of electroslag smelting.
1. Vacuum consumable electric arc furnace smelting (VAR method for short)
With the development of vacuum technology and the application of computer, VAR method has quickly become a mature industrial production technology of titanium. Today's titanium and its alloy ingots are mostly produced by this method.VAR method is characterized by low power consumption, high melting speed and good quality reproducibility. The ingots melted by VAR method have good crystal structure and uniform chemical composition.In general, finished ingots should be smelted by VAR method.At least two remelting.In the production of titanium ingot by VAR method, the process used by manufacturers in different countries is basically similar, but the difference lies in the use of different electrode preparation methods and equipment.Electrode preparation can be divided into three categories. One is to use the whole electrode which is continuously pressed by parts, excluding electrode welding process; the other is to use the single electrode which is pressed and welded into consumable electrode.And through plasma argon arc welding or vacuum welding into a whole;Third, the casting electrode was prepared by other smelting methods.
Modern advanced VAR furnace technical features and advantages:
(1) full coaxial power input, that is to say, the whole height of the furnace on the complete coaxiality, coaxial power supply, reduce the generation of segregation phenomenon;
(2) the electrical correction in the crucible can be fine-tuned in the X-axis/Y-axis upward;
(3) with accurate electrode weighing system, melting rate is automatically controlled and constant melting speed is achieved.The quality of smelting is guaranteed.
(4) ensure the repeatability and consistency of each melting;
(5) flexibility, that is, one furnace can produce a variety of ingots and large cast ingots, can greatly improve productivity;
(6) good economy.The "coaxial power supply" method can avoid the magnetic deviation leakage caused by the imbalance of crucible supply current.Reduce or eliminate the adverse effect of induction magnetic field on smelting products.It also improves the electrical efficiency, so as to obtain stable quality ingots.The purpose of "constant speed melting" is to improve the quality of the ingots. The advanced electronic control system and weight sensor ensure the constant length of arc and melting rate during the melting process, thus controlling the coagulation process.It can effectively prevent segregation and guarantee the inner quality of ingot.In addition to the above two characteristics, modern VAR furnace for titanium smelting has also realized the large size of VAR furnace. The melting diameter of modern VAR furnace is 1.5m, 32t large ingots.VAR method is modern titanium and titanium alloy standard industrial smelting process.The following technologies need to be addressed.First, electrode preparation method.The preparation of electrode is very complicated.Need to use expensive press to sponge titanium, intermediate alloy and return residual material pressed into a whole electrode or single piece of small electric switch.The single electrode also needs to be welded into a consumable electrode.At the same time, in order to ensure the uniformity of consumable electrode components, it is necessary to configure the corresponding facilities such as cloth, weighing material and mixing material.Second, there are occasional metallurgical defects such as segregation.Such as component segregation and coagulation segregation.The former is described as due to the uneven distribution of impurity or alloy elements in the electrode.Solidification occurs without equilibrium distribution in smelting;The latter is due to the occasional introduction of high-density inclusions (HDI) and low-density inclusions (LDI) into the raw material or process. These inclusions cannot be completely dissolved in the smelting process, resulting in the occurrence of hazardous inclusions and other metallurgical defects.
2. Non-consumable vacuum arc furnace smelting (Jane NC method)
At present, water-cooled copper electrode has replaced the tungsten-thorium platform gold or graphite platform gold in the initial stage of the titanium industry, solving the problem of industrial pollution, thus making the NC method an important method for smelting titanium and titanium platform gold. NC furnaces of several tons have been running in Europe and America.There are two types of water-cooled copper electrodes: one is self-rotating;The other is a rotating magnetic field, which is designed to prevent the arc from burning the electrode.NC furnace can also be divided into two types: one is in water-cooled copper crucible smelting raw materials, in water-cooled copper mold casting into ingots;The other is continuous casting of raw materials into a water-cooled copper crucible for smelting and solidification.The advantages of NC melting are: (1) can eliminate the pressing electrode and welding electrode process;(2) can make the arc stay on the material for a long time, so as to improve the ingot composition uniformity degree;(3) different shapes and sizes of raw materials can be used, in the smelting process can also add 100% residual material, to achieve the recycling of titanium.As a primary smelting, NC method is quite advantageous in terms of improving the recovery rate of residual materials and reducing the cost.Generally, NC furnace and VAR furnace link station are used to give full play to their advantages.
3. Cold hearth smelting (CHM)
The metallurgical inclusion defects of titanium and titanium sheeran ingots caused by pollution of raw materials and abnormal smelting process have been affecting the application of titanium and titanium sheeran in the field of aerospace.In order to eliminate metallurgical inclusions in rotating parts of titanium alloy aircraft engine, the technology of cold furnace bed smelting came into being.The most prominent characteristic of CHM method is the separation of melting, refining and solidification processes, that is, after the melted charge enters the ling furnace bed, it is melted first, then entered the refining area of the cold furnace bed for refining, and finally solidified into ingots in the crystallization area.The obvious advantage of CHM technology is that it can form a coagulation shell in the wall of the cold furnace bed, 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 (LDI) particles can stay longer in the high-temperature liquid, which can ensure the complete dissolution of LDI and effectively remove inclusion defects.That is to say.The purification mechanism of cold hearth smelting can be divided into specific gravity separation and melting separation.
3.1 electron beam cold furnace bed smelting (EBCHM) electron beam smelting (EB) is a process that USES the energy of high-speed electrons to generate heat in the material itself to conduct smelting and refining.EB furnaces with a cold hearth are called EBCHM.EBCHM method has excellent functions that traditional smelting method does not have:
(1) effectively remove tantalum, molybdenum, tungsten, tungsten carbide and other high-density inclusions (HDI) and titanium nitride.Low density inclusion (LDI) of titanium oxide;
(2) it can accept a variety of feeding methods, and the recovery of titanium residue is relatively easy, that is, it can use wastes that cannot be used by other smelting methods, and still make pure titanium ingot, which can greatly reduce the cost of the product;
(3) can be directly sampled from the metal liquid analysis;
(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 the condition of high vacuum, so the titanium sponge with high chloride content cannot be directly melted;
(2) alloy elements are volatile and difficult to control the chemical composition.
3.2 plasma cold and envy bed smelting (cylinder PCHM method)
Using plasma arc generated by ionization of inert gas as heat source, PCHM method can complete smelting in a wide range of pressure from low vacuum to near atmosphere.The characteristic of this method is that it can guarantee the composition of alloy with different vapor pressure, and there is no obvious difference in the melting process.For titanium and titanium alloy, the ideal ingot can be obtained by the method.The advantages of modern PCHM method lie in:
(1) low equipment investment, easy to operate, safe and reliable;
(2) can use different types and forms of raw materials, high recovery of residues;
(3) to ensure the diversification of the chemical composition of the alloy;
(4) the realization of expensive inert gas recycling and reuse, reduce the production cost.The disadvantage of PCHM method is low electrical efficiency.EBCHM is similar to PCHM in that both HDI and LDI can be eliminated.Generally, the former is more suitable for pure titanium smelting.For alloys, the latter is more appropriate.Similar to VAR method, the above two methods realize a wide range of automatic process control, including process parameters (melting speed, temperature distribution during melting and solidification, composition change during melting, removal degree of insoluble inclusions, etc.) and quality.
4. Cold crucible melting (CCM method for short)
In the 1980s, the American ferrosilicon corporation developed the slag free induction smelting process, which promoted the CCM process to industrial production and application for the production of titanium ingots and titanium precision castings.In recent years, in some economically developed countries, CCM method has started to step into industrial production scale, with the largest diameter of cast ingots being l m and 2m in length. Its development prospect is remarkable.The CCM melting process is conducted in a metal crucible with water-cooled arc-shaped blocks or copper tubes that do not conduct electricity to each other. The greatest advantage of this combination is that the gap between each two blocks is an enhanced magnetic field, which generates strong agitation to make the chemical composition consistent with the temperature, thus improving the product quality.CCM method has the characteristics of both VAR method and crucible induction melting of refractory materials. It does not need refractory materials and does not need to make electrodes to obtain high-quality ingots with uniform composition in primary melting and no crucible pollution.Compared with VAR method, CCM method has the advantages of low equipment cost and simple operation, but it is still in the development stage at present.
5.Electroslag smelting (ESR)
The ESR method converts electrical energy into thermal energy by means of the collision of charged particles when electric current passes through conductive electroslag.The furnace charge is melted and refined with the heat generated by slag resistance.ESR method USES consumable electrode to conduct electroslag smelting in inactivated slag (CaF2). It can be directly fused 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 ESR furnace ensures the repeatability of the best quality ingots;
(2) axial crystallization of the ingot, dense and uniform structure;
(3) electrode weighing system and melting rate control system with extremely high accuracy;
(4) the equipment is simple and easy to operate.The disadvantage is that it can not discharge the pollution of slag on the cast ingots.