With the development of science and technology and the improvement of people’s living standards, titanium is increasingly used in industrial production, aerospace, defense military and daily life, so that the requirements for titanium and titanium alloy c and performance are becoming higher and higher. The smelting of titanium alloy raw materials is undoubtedly the most important and critical part. The quality of its smelting directly affects whether the performance indicators of the finished products that are processed subsequently meet the product requirements, thereby promoting the development of modern smelting technology of titanium alloys, including electronics The development of new technologies such as beam and plasma cold bed furnace has created good conditions and foundations for improving the metallurgical quality and mechanical properties of titanium alloy ingots.
1. Titanium alloy melting method
1.1 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. Normally, the finished ingot should be made by VAR melting. It must be remelted at least twice.
VAR method is used to produce titanium ingots. The processes used by manufacturers around the world are basically similar. The difference lies in the use of different electrode preparation methods and equipment. The electrode preparation can be divided into three categories. One is the continuous pressing of the whole electrode by portion feeding. Excluding 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 cast electrodes.
Technical characteristics and advantages of modern advanced VAR furnace:
(1) Full coaxial power input, that is to say the complete coaxiality of the entire 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, a furnace can produce a variety of ingots and the ingot is large, which can greatly improve productivity;
(6) has good economy.
The “coaxial power supply” method can avoid the magnetic leakage caused by the imbalance of the crucible supply current. Weaken or eliminate the adverse effect of the induced magnetic field on the 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, through the advanced electronic control system and weight sensor to ensure that the length of the arc and the melting rate in the smelting process are constant, thus controlling the condensing process. It can effectively prevent segregation and ensure the intrinsic quality of the ingot.
In addition to the above two characteristics, the modern VAR furnace for titanium smelting has realized the enlargement of the VAR furnace. The modern VAR furnace can melt large ingots with a diameter of 1.5m and a weight of 32t. The vAR method is standard for modern titanium and titanium alloys. Industrial smelting method. There are the following technologies that need to be solved. First, the electrode preparation method. The electrode preparation process is very cumbersome. It is necessary to use an expensive press to compress the sponge titanium, intermediate alloy and return residue into a monolithic electrode or a single small electric wrench The single-piece electrode also needs to be welded into a consumable electrode. At the same time, in order to ensure the uniformity of the composition 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. The former is due to the uneven distribution of impurity elements or alloy elements in the electrode. The solidification occurs when the balance distribution is too late during melting; the latter is due to the occasional introduction of high-density inclusions (HDI) and low-density inclusions in the raw materials or process ( 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 (Jian You NC Method) At present, water-cooled copper electrodes have replaced tungsten-thorium-plated gold cast or graphite electric triggers in the initial stage of the titanium industry, which solves the problem of industrial pollution and thus makes the NC method It has become an important method for smelting titanium and titanium gold, and several tons of NC furnaces have been operated in Europe and America. Water-cooled copper electrodes are divided into two types: one is self-rotating; the other is rotating magnetic field, and its purpose 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, smelting and solidification.
The advantages of NC melting are:
①The process of pressing electrodes and welding electrodes can be omitted;
②The 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, and 100% residue can be added during the smelting process 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.
1.3 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 gold in the aerospace field. In order to eliminate metallurgical inclusions in the rotating parts of titanium alloy aircraft engines , Cold furnace bed smelting 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 condensed shell on the wall of the cold furnace 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 The retention time of the inclusion (LDI) 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.
1.3.1 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 melting 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 a variety of feeding methods, and the recovery of titanium residue is relatively easy, that is, waste materials that cannot be used by other melting methods can be used, and pure titanium ingots can still be produced, which greatly reduces the cost of the product;
(3) Can be directly sampled and analyzed from the liquid metal;
(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) The smelting needs to be carried out under high vacuum conditions, so it is not possible to directly smelt using sponge titanium with higher chloride content;
(2) Alloy elements are volatile and it is difficult to control the chemical composition.
1.3.2 Plasma cold bed melting method (the tube is called 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, and there is no obvious burning loss during the smelting process, which can also eliminate HDI and LDI metallurgical defects.
This method has the ability to provide improved traditional Taiwan metal properties, can realize the smelting of a variety of alloys, is a more economical smelting method than the traditional smelting method.
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:
① The 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 is the same as PCHM in that both HDI and LDI can be eliminated. Generally, the former is more suitable for smelting pure titanium; the latter is more suitable for alloys.
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 .
1.4 Cold-pot melting method (referred to as CCM method)
In the 1980s, American Ferrosilicon Company developed a slag-free induction smelting process and pushed the CCM method to industrial production applications for the production of titanium ingots and titanium precision castings. In recent years, the CCM method has begun to industrialize in some economically developed countries The production scale, the maximum diameter of the ingot is lm, the length is 2m, and its development prospect is 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.
CCM method combines the characteristics of VAR method and crucible induction melting of refractory materials. No refractories are required, and no high-quality ingots with uniform composition and no crucible pollution can be obtained without making electrodes.
Compared with VAR method, 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.
1.5 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.
2. Analysis of different melting methods
The quality of the molten cast titanium ingot has a decisive influence on the structure and performance 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 different parts of the ingot is uniform;
② Are the main impurities (Fe, O, etc.) controlled within an appropriate range;
③ Is there any defects such as inclusions, segregation, pores, cracks, shrinkage holes and sparse orange inside the ingot;
④ Whether the surface of the ingot is smooth and there is no separation, the size of the head cavity shrinkage removal.
Today’s aerospace technology places stricter quality requirements on 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 the comprehensive use of the characteristics of each smelting method to realize the physical metallurgical process of titanium and titanium alloys, so as to obtain high-quality titanium and titanium alloy ingots with excellent performance and continuous reproduction.
3. Outlook
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 will still be the ideal method for smelting titanium and titanium alloys. However, the problems of electrode preparation and ingot cleaning still need to be solved. The NC method is mainly suitable for the recovery and smelting of the returned charge. The EBCHM and PCHM methods can provide higher quality titanium and titanium alloy ingots for aerospace and other fields with their unique advantages. . In the near future, it will definitely 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 they may enter industrial-scale production.