钛合金的五种熔炼方法

钛合金的五种熔炼方法

The melting methods of titanium alloys are generally divided into: 1. Vacuum consumable arc furnace melting method; 2. Non consumable vacuum arc furnace melting method; 3. Cold furnace bed melting method; 4. Cold crucible melting method; 5. Five methods of electric slag melting.

1. Vacuum consumable arc furnace melting method (VAR method for short)

With the development of vacuum technology and the application of computers, VAR method quickly became a mature industrial production technology for titanium. 今天, the vast majority of titanium and its alloy ingots are produced using this method. The significant characteristics of VAR method are low power consumption, high melting speed, and good quality reproducibility. The ingots melted by VAR method have good crystalline structure and uniform chemical composition. Usually, finished ingots should be melted using VAR method and undergo at least two remelting processes. The VAR method is used to produce titanium ingots, and the processes used by manufacturers around the world are basically similar, with the difference being the use of different electrode preparation methods and equipment. Electrode preparation can be divided into three categories: first, using integral electrodes that are continuously pressed by adding materials in batches, excluding electrode welding processes; second, single electrode pressing and splicing into consumable electrodes. And welded together by plasma argon arc welding or vacuum welding; The third is to use other melting methods to prepare casting electrodes.

The technical characteristics and advantages of modern advanced VAR furnaces:

(1) Full coaxial power input, which refers to the complete coaxiality of the entire furnace height, is called coaxial power supply, reducing the occurrence of segregation phenomenon;

(2) The electric calibration inside the crucible can be fine tuned in the X-axis/Y-axis direction;

(3) Equipped with a precise electrode weighing system, the melting rate is automatically controlled, achieving constant melting speed and ensuring melting quality;

(4) Ensure the repeatability and consistency of each melting process;

(5) Flexibility refers to the ability of one furnace to produce multiple ingot types and the large-scale production of ingots, which can significantly improve productivity;

(6) Has good economic viability. The “coaxial power supply” method can avoid magnetic leakage caused by unbalanced current supply to the crucible, weaken or eliminate the adverse effects of induced magnetic fields on melted products, and improve electrical efficiency, thereby obtaining stable quality ingots. The purpose of “constant speed melting” is to improve the quality of ingots, by using advanced electronic control systems and weight sensors to ensure constant arc length and melting rate during the melting process, thereby controlling the solidification 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 melting have also achieved the large-scale production of VAR furnaces. Modern VAR furnaces can melt large ingots with a diameter of 1.5m and a weight of 32t.

VAR method is an industrial melting method that is a modern standard for titanium and titanium alloys. But there are still the following technologies that need to be addressed:

Firstly, the electrode preparation method. The electrode preparation process is very complicated, requiring the use of expensive presses to compress sponge titanium, intermediate alloys, and returned residual materials into integral electrodes or single small electric plates. Single electrodes also need to be welded into consumable electrodes. 同时, in order to ensure the uniformity of consumable electrode components, corresponding facilities such as fabric, weighing, and mixing need to be configured.

Secondly, occasional metallurgical defects such as segregation, such as compositional segregation and solidification segregation.

The former is caused by the uneven distribution of impurity elements or alloy elements in the electrode, which solidifies before reaching equilibrium distribution during melting; The latter is due to the occasional introduction of high-density inclusions (HDI) and low-density inclusions (LDI) into the raw materials or process, which cannot be completely dissolved during the melting process, resulting in the production of highly hazardous metallurgical defects such as inclusions.

2. Non consumable vacuum arc furnace melting method (simplified as NC method)

At present, water-cooled copper electrodes have replaced tungsten thorium electrode or graphite electrode in the initial stage of titanium industry, solving the problem of industrial pollution and making NC method an important method for melting titanium and titanium alloy. Several ton NC furnaces have been operating in Europe and America.

There are two types of water-cooled copper electrodes: one is self rotating; Another type is a rotating magnetic field, which aims to prevent electrode burnout caused by electric arcs.

NC furnaces can also be divided into two types: one is to melt raw materials in a water-cooled copper crucible and cast them into ingots in a water-cooled copper mold; Another method is to continuously feed raw materials into a water-cooled copper crucible for melting and solidification.

The advantages of NC melting method are: ① It can eliminate the processes of pressing electrodes and welding electrodes; ② Can make the arc stay on the material for a longer period of time, thereby improving the uniformity of the ingot composition; ③ Different shapes and sizes of raw materials can be used, 和 100% residual materials can be added during the melting process to achieve the recycling of titanium.

The NC method, as a smelting process, is quite advantageous in terms of improving the recovery rate of residual materials and reducing costs. Usually, NC furnaces and VAR furnaces are used in conjunction to fully leverage their respective advantages.

3. Cold furnace bed melting method (referred to as CHM method)

The metallurgical inclusion defects in titanium and titanium alloy ingots caused by pollution of raw materials and abnormal melting processes have been affecting the application of titanium and titanium alloy in the aerospace field. In order to eliminate metallurgical inclusions in rotating parts of titanium alloy aircraft engines, cold hearth melting technology has been introduced.

The biggest feature of the CHM method is the separation of the melting, refining, and solidification processes. That is, the melted furnace material enters the Ling furnace bed for melting first, then enters the refining zone of the cold furnace bed for refining, and finally solidifies into ingots in the crystallization zone. The significant advantage of CHM technology is that it can form a condensed shell on the wall of the cold furnace bed, and its “viscous zone” can capture high-density inclusions (HDI) such as WC, Mo, Ta, ETC. 同时, in the precision zone, the residence time of low-density inclusions (LDI) particles in high-temperature liquid is prolonged, which can ensure the complete dissolution of LDI and effectively remove inclusion defects. That is to say, the purification mechanism of cold furnace bed melting can be divided into two types: specific gravity separation and melting separation.

3.1 Electron Beam Cold Bed 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 melting and refining. The EB furnace with a cold furnace bed is called EBCHM. The EBCHM method has excellent functions that traditional melting methods do not possess:

(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) Multiple feeding methods are acceptable, and the recovery of titanium residue is relatively easy. Even if waste materials that cannot be used by other smelting methods can be used, pure titanium ingots can still be produced, greatly reducing the cost of the product;

(3) It can be directly sampled, analyzed, and tested from liquid metal;

(4) Can produce irregular shaped ingots, reduce production processes, lower raw material consumption, and improve product yield;

The EBCHM method also has the following drawbacks:

(1) Melting needs to be carried out under high vacuum conditions, so sponge titanium with high chloride content cannot be directly melted;

(2) Alloy elements are volatile and difficult to control their chemical composition.

3.2 Plasma Cold Bed Melting Method (PCHM Tube Method)

The PCHM method uses the plasma arc generated by inert gas ionization as a heat source, and can complete melting in a wide pressure range from low vacuum to near atmospheric pressure. The significant feature of this method is that it can ensure the composition of alloys with different vapor pressures, and there is no obvious difference during the melting process. This method has the ability to provide improved traditional table metal properties and can achieve the melting of diversified alloys. It is an economical melting method compared to traditional melting methods. By using this method for melting, ideal ingots can be obtained in one melting process for titanium and titanium alloys. The advantages of modern PCHM method are:

① Low equipment investment, easy operation, safe and reliable;

② Different types and forms of raw materials can be used, with high residual material recovery rate;

③ Ensure the chemical composition of diversified alloys;

④ The expensive recycling and reuse of inert gases have been achieved, reducing production costs.

The disadvantage of PCHM method is low electrical efficiency. The similarity between EBCHM and PCHM lies in their ability to eliminate HDI and LDI. The former is generally more suitable for melting pure titanium; For alloys, the latter is more suitable. Like the VAR method, the above two methods achieve large-scale process automation control, including process parameters (melting speed, temperature distribution during melting and solidification processes, changes in composition during melting, degree of removal of insoluble inclusions, ETC。) and quality.

4. Cold crucible melting method (CCM method for short)

In the 1980s, the American ferrosilicon company developed slag free induction melting technology, pushing the CCM method to industrial production applications for the production of titanium ingots and precision castings. In recent years, in some economically developed countries, the CCM method has begun to enter the industrial production scale, with the maximum diameter of ingots being 1m and the length being 2m, and its development prospects are remarkable. The CCM smelting process is carried out in a metal crucible composed of non-conductive water-cooled arc-shaped blocks or copper tubes. 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, thereby improving product quality. The CCM method combines the characteristics of VAR method and induction melting of refractory materials in crucibles. It does not require refractory materials or electrodes to obtain high-quality ingots with uniform composition and no crucible contamination in a single melting process. Compared with VAR method, CCM method has the advantages of low equipment cost and easy operation, but from the current perspective, this technology is still in the development stage.

5. Electroslag Melting Method (ESR Method for short)

The ESR method converts electrical energy into thermal energy by utilizing the collision of charged particles when current passes through conductive slag. The heat generated by the resistance of the slag is used to melt and refine the furnace material. The ESR method uses consumable electrodes for electroslag melting in non active slag (CaF2), which can be directly melted into ingots of the same shape and has good surface quality, suitable for direct processing in the next process. The advantages of this law are:

(1) The complete coaxiality of the ESR furnace ensures the repeatability of the best quality ingots;

(2) Axial crystallization of ingots, with dense and uniform structure;

(3) A highly accurate electrode weighing system and melting rate control system;

(4) The equipment is simple and easy to operate. The disadvantage is that it cannot discharge the pollution of slag on the ingot.