The melting methods of titanium alloy 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. Five methods of electroslag smelting.
1. Vacuum consumable arc furnace melting method (VAR method)
With the development of vacuum technology and the application of computer, VAR method has quickly become a mature industrial production technology of titanium. The notable characteristics of VAR method are low power consumption, high melting speed and good quality reproducibility. Normally, the finished ingots should be melted by VAR method and remelted at least twice. VAR method is used to produce titanium ingot. The process used by manufacturers around the world is basically similar. The difference is that different electrode preparation methods and equipment are used. Electrode preparation can be divided into three categories. The first is to adopt the integral electrode which is continuously pressed by feeding parts, excluding electrode welding process; the second is to press a single electrode and weld it into a consumable electrode. And through plasma argon arc welding or vacuum welding into one; Third, the casting electrode was prepared by other smelting methods.
Technical features and advantages of modern advanced VAR furnace:
(1) all coaxial power input, that is to say, the whole height of the furnace on the coaxial power supply, called coaxial power supply, reduce segregation phenomenon;
(2) the electrical calibration in the crucible can be fine-tuned in the X /Y axis;
(3) with accurate electrode weighing system, melting rate is automatically controlled, constant speed melting is realized, and melting quality is guaranteed;
(4) ensure the repeatability and consistency of each melting;
(5) flexibility, that is, one furnace can produce a variety of ingot types and large scale ingot casting, which can greatly improve productivity;
(6) good economy. The "coaxial power supply" mode can avoid the magnetic leakage caused by the unbalance of crucible supply current. Reduce or eliminate the adverse effects of induced magnetic fields on smelted products. And improve the electrical efficiency, so as to obtain stable quality ingot. The aim of "constant speed melting" is to improve the quality of the ingot by ensuring the constant arc length and melting rate in the melting process through advanced electronic control system and weight sensor, 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 realized the large-scale VAR furnace. 5m, 32t large ingot.
The VAR method is the standard industrial melting method for modern titanium and titanium alloys. But the following technologies need to be addressed:
First, electrode preparation method; The electrode preparation process is very complicated, which requires an expensive press to press titanium sponge, intermediate alloy and returned residual material into a whole electrode or a single small electric wrench. A single electrode also needs to be welded into a consumable electrode. At the same time, in order to ensure the uniformity of the composition of consumable electrode, it needs to be equipped with cloth, weighing material, mixing material and other corresponding facilities.
Second, there are occasional metallurgical defects such as segregation. Such as composition segregation and coagulation segregation.
The former is caused by non-uniform distribution of impurity elements or alloy elements in the electrode and solidification without time balance distribution during melting. 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 metallurgical defects such as extremely harmful inclusions.
2. Non-consumable vacuum arc furnace melting method (Jane you NC method)
Currently, water-cooled copper electrode has replaced the initial phase of tungsten titanium industry thorium platform gold electric cast or graphite electric wrench, solved the problem of industrial pollution, which makes NC method has become an important method of melting titanium and titanium gold, a few tons of NC furnace has been in Europe and the United States.
Water-cooled copper electrodes are divided into two types: one is self-rotating; The other is a rotating magnetic field, which aims to prevent the arc from burning the electrode.
NC furnace can also be divided into two kinds: one is in water-cooled copper crucible melting raw materials, in water-cooled copper mold cast into ingots; The other is continuous casting of raw materials, melting and solidification in a water-cooled copper crucible.
The advantages of NC melting method are: (1) can save 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 uniformity of the ingot composition; (3) can use different shapes and sizes of raw materials, in the smelting process can also add 100% residue, titanium recycling.
As a primary smelting, NC method is quite advantageous from the point of view of improving residue recovery and reducing cost. Usually, NC furnace and VAR furnace unit used to give full play to their respective advantages.
3. Cold hearth melting method (CHM method for short)
The metallurgical inclusion defects of titanium and titanium alloy ingots caused by raw material pollution and abnormal smelting process have always influenced the application of titanium and titanium alloy in aerospace field. In order to eliminate the metallurgical inclusions in the rotating parts of titanium alloy aircraft engine, the cold hearth melting technology was developed.
The biggest feature of CHM method is the separation of melting, refining and solidification process. That is to say, after the molten burden enters the ling furnace bed, it is melted first, then enters the refining area of the cold furnace bed for refining, and finally solidifies into ingots in the crystallization area. The significant advantage of CHM technology is that it can form solidification shells in the wall of cold furnace bed, and its "viscous zone" can capture high-density inclusions (HDI) such as WC, Mo, Ta, etc. Meanwhile, in the refining zone, the detention time of low-density inclusions (LDI) particles in high-temperature liquid is prolonged, which can ensure the complete dissolution of LDI, so as to effectively remove inclusion defects. That is to say. The purification mechanism of cold hearth melting can be divided into specific gravity separation and melting separation.
3.1 electron beam cold hearth melting (EBCHM) electron beam melting (EB) is a process that USES the energy of high-speed electrons to generate heat in the material itself for smelting and refining. EB furnace with cold hearth is called EBCHM. EBCHM has excellent functions that traditional smelting does not:
(1) effectively remove tantalum, molybdenum, tungsten, tungsten carbide and other high-density inclusions (HDI) and titanium nitride. Titanium oxide and other low-density inclusion (LDI);
(2) can accept a variety of feeding methods, titanium residue recovery is relatively easy, that is, can use other smelting methods can not use the waste, still can produce pure titanium ingots, greatly reduce the cost of products;
(3) it can be directly sampled from the metal liquid for analysis and analysis;
(4) can produce shaped billet, reduce production process, reduce raw material consumption, improve the yield;
EBCHM still has the following disadvantages:
(1) smelting needs to be carried out under high vacuum conditions, so titanium sponge with high chloride content cannot be smelted directly;
(2) the alloy elements are volatile and difficult to control the chemical composition.
3.2 plasma cold jealousy bed melting method (tube PCHM method)
The PCHM method USES the plasma arc generated by the ionization of inert gases as the heat source, and can be used to complete the melting in a wide pressure range from low vacuum to near atmospheric pressure. The characteristic of this method is that it can guarantee the composition of the alloy with different vapor pressure, and there is no obvious ability to improve the properties of the traditional alloy in the smelting process. For titanium and titanium alloys, ideal ingot can be obtained by one smelting. The advantages of modern PCHM method are:
(1) equipment investment is low, easy to operate, safe and reliable;
(2) can use different kinds and forms of raw materials, residue recovery rate is high;
(3) to ensure the diversity of alloy chemical composition;
(4) the realization of expensive inert gas recycling, reducing production costs.
The disadvantage of PCHM method is low electrical efficiency. EBCHM and PCHM have in common the ability to eliminate HDI and LDI. The former is more suitable for smelting pure titanium. For alloys, the latter is more suitable. Like VAR method, the above two methods can achieve a wide range of automatic process control, including process parameters (melting speed, distribution of temperature during melting and solidification, change of composition during melting, removal degree of insoluble inclusions, etc.) and quality.
4. Cold pot melting method (CCM method for short)
In the 1980s, the American ferrosilicon company developed the slageless induction smelting process and introduced the CCM method to industrial production, which was used to produce titanium ingot and titanium precision castings. In recent years, in some economically developed countries, CCM method has begun to step into the industrial production scale, with the maximum diameter of ingot l m and length of 2m, and its development prospect is remarkable. The CCM melting process is carried out in a metal crucible composed of water-cooled arc-shaped blocks or copper pipes which 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 stirring generated by the magnetic field makes the chemical composition and temperature consistent, thus improving the product quality. CCM method combines the characteristics of VAR method and crucible induction melting of refractory materials, which can obtain high quality ingots with uniform composition and no crucible pollution without refractory materials and without making electrodes. Compared with VAR method, CCM method has the advantages of low equipment cost and simple operation, but at present, this technology is still in the development stage.
5. Electroslag smelting (ESR)
ESR converts electrical energy into heat energy by means of the collision of charged particles when a current passes through a conductive electroslag. The charge is melted and refined by the heat energy generated by the slag resistance. ESR method USES the consumable electrode for electroslag smelting in the inactive slag (CaF2), which can be directly fused and cast into ingot billets of the same shape with good surface quality, and is suitable for direct processing in the next process. The advantages of this law are:
(1) the complete coaxiality of ESR furnace ensures the repeatability of ingot casting with the best quality;
(2) axial crystallization of ingot, dense and uniform structure;
(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 ingot cannot be polluted by the slag.