With the development of science and technology and the […]
With the development of science and technology and the improvement of people’s living standards, titanium is used more and more in industrial production, aerospace, national defense and military industry, and daily life. Therefore, the requirements for the c and performance of titanium and titanium alloys are getting higher and higher. The smelting of titanium alloy raw materials is undoubtedly the most important and most critical link. The quality of its smelting directly affects whether the performance indicators of the subsequent processed finished products meet the product requirements, thus promoting the development of modern smelting technology of titanium alloys, including electronics The development of new technologies such as beam and plasma cooling bed furnaces has created good conditions and foundations for improving the metallurgical quality and mechanical properties of titanium alloy ingots.
1. Titanium alloy smelting method
1.1 Vacuum consumable electric arc furnace smelting 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. Today's titanium and its alloy ingots are mostly produced by this method. The salient features of VAR method are low power consumption, high melting speed and good quality reproducibility. The ingots smelted by VAR method have good crystal structure and uniform chemical composition. Generally, the finished ingot should be made by VAR method smelting. At least two remelts are required.
In the production of titanium ingots by the VAR method, the processes used by manufacturers around the world are 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 the use of integral electrodes that are continuously pressed by portion feeding. Eliminates the electrode welding process: the second is a single piece of electrode pressing, tailored welding into a consumable electrode. It is welded into a whole by plasma argon arc welding or vacuum welding; the third is to use other smelting methods to prepare casting electrodes.
The technical features and advantages of modern and advanced VAR furnaces:
(1) Full coaxial power input, that is to say, the complete coaxiality at 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 direction;
(3) It has an accurate electrode weighing system, and the melting rate is automatically controlled, realizing constant speed melting. Ensure the quality of smelting;
(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 can greatly increase productivity;
(6) has good economic efficiency.
"Coaxial power supply" mode can avoid the magnetic leakage caused by the unbalanced supply current of the crucible. Reduce or eliminate the adverse effect of the induced magnetic field on the smelted product. And improve the electrical efficiency, so as to obtain stable quality ingots.
The purpose of "constant speed smelting" is to improve the quality of ingots. The advanced electronic 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 guarantee the inherent quality of the ingot.
In addition to the above two features, modern VAR furnaces for titanium smelting have also achieved large-scale VAR furnaces. Modern VAR furnaces can melt large ingots with a diameter of 1.5m and a weight of 32t. The vAR method is the standard for modern titanium and titanium alloys. Industrial smelting method. There are also the following technologies to be solved. First, electrode preparation method. The electrode preparation process is very cumbersome. 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 plate The 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 component segregation and solidification segregation. The former is caused by the uneven distribution of impurity elements or alloying elements in the electrode. It is produced by solidification before the equilibrium distribution during smelting; the latter is caused by the occasional introduction of high-density inclusions (HDI) and low-density inclusions (HDI) and low-density inclusions ( LDI), these inclusions cannot be completely dissolved during the smelting process, resulting in metallurgical defects such as extremely harmful inclusions.
1.2 Non-consumable vacuum arc furnace smelting method (Jane NC method) At present, the water-cooled copper electrode has replaced the tungsten-thorium metal cast or graphite electrical switch in the initial stage of the titanium industry, and solved the problem of industrial pollution, thus making the NC method It has become an important method for smelting titanium and titanium alloys, and several-ton NC furnaces are already in operation 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, the purpose of which is to prevent the arc from burning the electrode. The NC furnace 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 put raw materials in a water-cooled copper crucible, smelt and solidify.
The advantages of NC method smelting are:
①The pressing electrode and welding electrode process can be omitted;
②The arc can stay on the material for a long time, thereby improving the uniformity of the ingot composition;
③Raw materials of different shapes and sizes can be used, and 100% residual material can be added during the smelting process to realize the recycling of titanium. As a primary smelting, the NC method is quite advantageous in terms of improving the recovery rate of residual materials and reducing the cost. Usually, NC furnace and VAR furnace are used in combination to give full play to their respective advantages.
1.3 Cold hearth smelting 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 the metallurgical inclusions in the rotating parts of titanium alloy aircraft engines , The cold hearth smelting technology came into being. The biggest feature of the CHM method is the separation of the melting, refining and solidification processes, that is, the melted charge enters the Ling hearth and melts first, then enters the refining zone of the cold hearth for refining, and finally solidifies into an ingot in the crystallization zone.
The significant advantage of CHM technology is that a condensed shell can be formed on the wall of the cold hearth, and its "viscosity zone" can capture high-density inclusions (HDI) such as WC, Mo, and Ta. At the same time, in the refining zone, low density The retention time of the inclusion (LDI) particles in the high-temperature liquid is prolonged, which can ensure the complete dissolution of the LDI, thereby effectively removing the inclusion defects. In other words, the purification mechanism of cold hearth smelting can be divided into two types: specific gravity separation and melting separation.
1.3.1 Electron beam cold hearth smelting method (EBCHM method for short) Electron beam smelting (EB for short) is a process that uses the energy of high-speed electrons to generate heat from the material itself 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 tantalum, molybdenum, tungsten, tungsten carbide and other high-density inclusions (HDI) and titanium nitride. Low-density inclusions such as titanium oxide (LDI);
(2) It can accept a variety of feeding methods, and the recovery of titanium residue is relatively easy, that is, waste materials that cannot be used by other smelting methods can be used, and pure titanium ingots can still be produced, which greatly reduces the cost of the product;
(3) It can be directly sampled and analyzed from the molten metal;
(4) It can produce special-shaped ingots, reducing the production process, reducing the consumption of raw materials, and increasing the yield; the EBCHM method also has the following shortcomings:
(1) The smelting needs to be carried out under high vacuum conditions, so it is not possible to directly smelt titanium sponge with higher chloride content;
(2) Alloying elements are volatile, and it is difficult to control the chemical composition.
1.3.2 Plasma cooling bed smelting method (called PCHM method in short)
PCHM method uses the plasma arc generated by the ionization of inert gas as the heat source, and can complete the smelting 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 with different vapor pressures, without obvious burning loss during the smelting process, and can also eliminate the metallurgical defects of HDI and LDI.
This method has the ability to improve the metal properties of traditional Taiwan, and can realize the smelting of diversified alloys. It is a smelting method that is more economical than traditional smelting methods.
Using this method of smelting, for titanium and titanium alloys, an ideal ingot can be obtained in one smelting.
The advantages of the modern PCHM method are:
①Equipment 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;
③Ensure the chemical composition of diversified alloys;
④ The expensive inert gas is recovered and reused, and the production cost is reduced. The disadvantage of the PCHM method is low electrical efficiency.
EBCHM and PCHM are the same in that both can eliminate HDI and LDI. Generally, the former is more suitable for smelting pure titanium; while 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 .
1.4 Cold crucible melting method (abbreviated as CCM method)
In the 1980s, the American Ferrosilicon Company developed a slag-free induction smelting process and promoted 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 be industrialized The scale of production, the largest diameter of the ingot is lm, the length is 2m, and its development prospect is eye-catching.
The CCM melting process is carried out in a metal crucible composed 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 every two blocks is an enhanced magnetic field, which generates a strong magnetic field. Stirring makes the chemical composition and temperature consistent, thereby improving product quality.
The CCM method has the characteristics of both the VAR method and the induction melting of refractory materials in the crucible. It does not require refractory materials and does not need to make electrodes to obtain high-quality ingots with uniform composition and no crucible contamination.
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.
1.5 Electroslag smelting method (abbreviated as ESR method)
The ESR method uses the collision of charged particles when the current passes through the conductive electroslag to convert electrical energy into heat. That is, the charge is melted and refined by the heat energy generated by the slag resistance. The ESR method uses consumable electrodes for electroslag smelting in inactive slag (CaF2), which can be directly fused and 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 crystallizes axially, 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 contamination of the ingot by the molten slag cannot be discharged.
2. Analysis of different smelting methods
The quality of cast titanium ingots has a decisive influence on the structure and properties of the subsequent cold and hot processed materials. The quality of titanium and titanium alloy ingots is mainly measured from the following aspects:
① Whether the chemical composition of the different parts of the ingot is uniform;
②Whether the main impurities (Fe, O, etc.) are controlled in an appropriate range;
③ Whether there are defects such as inclusions, segregation, pores, cracks, shrinkage holes and sparse orange in the ingot;
④ Whether the surface of the ingot is smooth, there is no gap, and the amount of shrinkage on the head is removed.
Today's aerospace technology puts forward stricter quality requirements for titanium and titanium alloy ingots. In addition to strictly controlling the quality of the production process, multiple smelting should be used, at least one of which is carried out in vacuum to obtain high-quality ingots. This requires that the characteristics of each smelting method must be comprehensively utilized to realize the physical metallurgical process of titanium and titanium alloys, so as to obtain excellent performance and continuous reproducible high-quality titanium and titanium alloy ingots.
From an economic point of view, the VAR method as the main production method will continue to provide high-quality titanium materials for the aerospace and non-aerospace fields, and it will still be an ideal method for smelting titanium and titanium alloys. However, it is necessary to solve the problems of electrode preparation and ingot cleaning. The NC method is mainly suitable for the recycling and smelting of the return charge. With their unique advantages, the EBCHM and PCHM methods can provide titanium and titanium alloy ingots with higher quality requirements for aerospace and other fields. . In the near future, it will become an important part of the standard titanium smelting process. The CCM method and ESR method still need to be further improved and perfected, and it is possible to enter industrial scale production.