Technology and technical prospect of smelting titanium alloy materials such as titanium rod and titanium alloy rod -2

1.3 Cold hearth melting method (abbreviated as CHM method)

The metallurgical inclusion defects of titanium and titanium alloy ingots caused by raw material pollution and abnormal smelting process have always affected the application of Grade 5 Titanium Bar and titanium alloy in the aerospace field. In order to eliminate the titanium alloy aircraft engine rotating parts Metallurgical inclusions, cold hearth smelting technology came into being. The biggest feature of the CHM method is the separation of melting, refining and solidification processes, that is, the molten charge enters the hearth for melting first, then enters the refining area of the cold hearth for refining, and finally solidifies into ingots in the crystallization area.

The significant advantage of CHM technology is that it can form a condensate shell on the wall of the cold hearth, 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 (HDI) The extended residence time of inclusion (LDI) particles in high temperature liquid can ensure the complete dissolution of LDI, thereby effectively removing inclusion defects. That is to say. The purification mechanism of cold hearth smelting can be divided into specific gravity separation and melting separation.

1.3.1 Electron beam cold hearth melting method (abbreviated as EBCHM method) Electron beam melting (abbreviated as EB) is a technological process in which the energy of high-speed electrons is used to make the material itself generate heat for smelting and refining. The EB furnace with cold hearth is called EBCHM. The EBCHM method has excellent functions that the traditional smelting method does not have:

(1) Effectively remove high-density inclusions (HDI) and titanium nitride such as tantalum, molybdenum, tungsten, and tungsten carbide. Titanium oxide and other low-density inclusions (LDI);

(2) A variety of feeding methods can be accepted, and the recovery of titanium residues 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 products;

(3) It can be directly sampled and analyzed from molten metal;

(4) It can produce special-shaped ingots, reduce production processes, reduce raw material consumption, and increase yield; the EBCHM method also has the following disadvantages:

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

(2) Alloying elements are volatile and difficult to control the chemical composition.

1.3.2 Plasma cold bed melting method (called PCHM method)

The PCHM method uses the plasma arc generated by the ionization of inert gas as a heat source, and can complete melting in a wide pressure range from low vacuum to near atmospheric pressure. The salient feature of this method is that it can guarantee alloy components with different vapor pressures, and there is no obvious burning loss during the smelting process, and it can also eliminate metallurgical defects of HDI and LDI.

This method has the ability to improve the properties of traditional Taiwanese metals, and can realize the melting of diversified alloys. It is an economical melting method compared with traditional melting methods.

With this method of smelting, for titanium and titanium alloys, an ideal ingot can be obtained by one smelting.

The advantages of the modern PCHM method are:

①Low equipment investment, easy to operate, safe and reliable;

② Raw materials of different types and forms can be used, and the recovery rate of residual materials is high;

③Ensure the chemical composition of diversified alloys;

④ Realized the recovery and reuse of expensive inert gas, reducing production costs. The disadvantage of the PCHM method is that the electrical efficiency is low.

EBCHM and PCHM are similar 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 (smelting speed, temperature distribution during smelting and solidification, composition changes during smelting, removal of insoluble inclusions, etc.) and quality .

1.4 Cold crucible melting method (referred to as CCM method)

In the 1980s, the American Ferrosilicon Company developed a slag-free induction smelting process and pushed the CCM method to industrial production for the production of titanium ingots and titanium precision castings. In recent years, in some economically developed countries, the CCM method has begun to enter industrialization The scale of production, the maximum diameter of the ingot is 1 m, the length is 2m, and its development prospect is impressive.

The melting process of the CCM method is carried out in a metal crucible composed of mutually non-conductive water-cooled arc 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 magnetic field generated Stirring makes the chemical composition and temperature consistent, which improves product quality.

The CCM method combines the characteristics of the VAR method and the crucible induction melting of refractory materials. It does not require refractory materials and does not need to make electrodes to obtain high-quality ingots with uniform composition and no crucible pollution.

Compared with the VAR method, the CCM method has the advantages of low equipment cost and easy operation, but the technology is still in the development stage at present.

1.5 Electroslag smelting method (referred to as ESR method)

The ESR method converts electrical energy into heat energy by utilizing the collision of charged particles when current passes through conductive electroslag. That is, the charge is melted and refined by the heat energy generated by the resistance of the slag. The ESR method uses consumable electrodes to carry out electroslag smelting in inactive slag (CaF2), which can be directly melted 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 ingot;

(2) Axial crystallization of ingot, compact and uniform structure;

(3) Extremely high-precision electrode weighing system and melting rate control system;

(4) The equipment is simple and easy to operate. The disadvantage is that the pollution of the ingot by the slag cannot be removed.

2. Analysis of different smelting methods

The quality of cast titanium ingots has a decisive influence on the microstructure and properties of 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;

②Whether the main impurities (Fe, O, etc.) are controlled within an appropriate range;

③Whether there are defects such as inclusions, segregation, pores, cracks, shrinkage cavities and sparse oranges inside the ingot;

④ Whether the surface of the ingot is smooth, without gaps, and the size of the head shrinkage cavity removal.

Today's aerospace technology puts forward stricter quality requirements for Grade 5 Titanium Bar 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 a 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 continuously reproducible high-quality titanium and titanium alloy ingots with excellent performance.

3. Outlook

From an economic point of view, as the main production method, the VAR method will continue to provide high-quality titanium materials for aviation and non-aviation fields, and will still be an 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 Grade 5 Titanium Bar and titanium for aerospace and other fields with their unique advantages. alloy ingot. In the near future, it will definitely become an important part of the titanium standard smelting process. The CCM method and the ESR method still need to be further improved and perfected, and it is possible to enter industrial scale production.