Current Status of Research on Titanium Alloy Fusion Technology

Titanium is a highly active metal that can react with a variety of elements during the smelting process, including refractory materials based on various metal oxides. Therefore, the smelting of titanium alloy must be carried out in high vacuum or under the protection of inert gas (mainly argon atmosphere). At the same time, smelting under vacuum conditions can also remove some impurities and improve the purity of the titanium alloy. Although vacuum smelting equipment is expensive, the process is complicated and the production cost is high, vacuum smelting is the only feasible way to produce high-quality titanium ingots, and there is no other way. The use of vacuum smelting to smelt alloys has many advantages. First, it eliminates the pollution of harmful gases and refractory materials in the atmosphere, ensures the purity of the smelted alloy, and provides better thermodynamic and kinetic conditions for the purification of the alloy; The melting and casting method of sequential solidification of the alloy improves the removal ability of refractory impurities and significantly improves the as-cast structure of the alloy; at the same time, the melting and solidification of the alloy are carried out in the crucible, and the alloy can be heat-sealed to feed the alloy, which greatly reduces The removal rate of the alloy riser improves the yield of the alloy. 1. Melting characteristics of titanium alloy The uniformity of the composition and solidification structure of the titanium alloy and the content of impurity elements in the alloy will have a greater impact on the properties of the titanium alloy. Therefore, in order to make the performance of titanium alloy excellent and stable, it is necessary to carry out higher requirements on the melting of titanium alloy. Melting titanium alloy has the following characteristics:

• The content of each alloy component is relatively high;
• The physical parameters of various alloying elements are quite different;
• Each alloying element requires higher reaction heat during the dissolution process;
• Very high sensitivity to interstitial elements, especially C, N, and O;
• The alloy has high purity requirements and low composition tolerance;
• The alloy performance is greatly affected by the structure;
• The alloy has a high melting point. Such complex and diverse characteristics add to the acquisition of excellent titanium alloy melts and high-quality ingots.

Characteristics of existing smelting technology

In recent years, with the advancement of science and technology and the actual production of high-quality castings, there have been smelting reactive metals (titanium, zirconium, hafnium) and refractory alloys (tantalum, niobium, tungsten, molybdenum) Many methods, such as vacuum induction water-cooled copper crucible shell furnace smelting (ISM), plasma smelting, vacuum arc smelting (VAR), electron beam smelting (EBM) and cold bed smelting (CHM), etc. Some people are also studying the use of magnetic fields The technology allows the alloy to float without contact with the crucible wall to smelt reactive metals. And vacuum technology has been applied in all smelting methods.

1. Induction Skull Melting

The ISM method mainly uses the eddy current heat generated in the alloy to be melted by the skin effect of the induced current to heat the alloy and finally melt the alloy. The advantages of the ISM approach are many:

• The alloy can be added with alloying elements at any time during melting, which provides a greater degree of freedom for alloying. At the same time, the electromagnetic stirring action ensures the uniform distribution of the alloy composition and the temperature of the molten pool, and improves the smelting efficiency.
• High alloy yield. It is convenient to control the adding time, order and conditions of alloying elements during smelting, which creates conditions for the effective utilization of high vapor pressure alloying elements and the complete melting of refractory alloying elements. This not only ensures the accuracy of the alloy composition, but also greatly saves the composition.
• Reduce the final gas content in the alloy. The self-stirring effect in the smelting process will bring the dissolved bubbles deep in the crucible to the liquid surface, and finally eliminate the melt.
• Higher melting efficiency. It takes less time to melt a furnace of alloy than a vacuum consumable consumable shell furnace, and its cost is lower than that of a vacuum consumable electrode arc condensing furnace.

2. Consumable Arc Melting

The VAR method uses a low-voltage, high-current arc in a vacuum furnace as a heat source to heat and smelt the alloy. The rod-shaped electrode made of the smelted alloy is gradually consumed during the smelting process, and then dropped into a water-cooled crystallizer to solidify into an ingot. . The advantages of the VAR method are:

• The alloy smelting uses water-cooled copper crucibles, so it is not polluted by molds and oxide refractory materials. At the same time, the smelting is carried out under inert atmosphere or vacuum conditions, which reduces the gas content in the alloy;
• It can be produced. Large-tonnage, large-size alloy ingots;
• Since the water-cooled crucible is also a crystallizer for alloy solidification, rapid cooling of the bottom and edges of the crucible will cause the alloy to crystallize directionally, and ultimately eliminate common casting defects such as pores and central shrinkage.

The disadvantages of the VAR method are:

• The preparation of the electrode is difficult. It is necessary to ensure that the electrode is straight, with sufficient strength and conductivity, and at the same time, to ensure that the alloying elements are reasonably distributed in the electrode;
• The ingot is a columnar crystal structure, so it is not conducive to the follow-up Pressure processing billet;
• If the process is not properly controlled during the smelting process, metallurgical defects such as segregation and solidification will occur. Composition segregation may be caused by uneven distribution of alloying elements in the electrode and electrode drop during smelting; solidification segregation is caused by unclean raw materials or improper electrode preparation technology, which will eventually lead to the introduction of low-density inclusions (LDI) and high-density in the alloy Inclusions (HDI) defects, the above-mentioned inclusions cannot be completely removed during the vacuum arc smelting process, and can only be eliminated by advanced cooling bed technology.

3. Vacuum non-consumable arc melting (Non-Consumable Arc Melting)

The biggest difference between non-consumable electrode arc smelting and consumable electrode arc smelting is the electrode. Because W and graphite have a higher melting point and good conductivity, the early electrode materials generally used tungsten rods or graphite rods. Non-consumable arc smelting in a water-cooled copper crucible is the earliest method of smelting titanium. Practical applications show that electrodes made of these two materials will cause pollution to the molten alloy. The two water-cooled copper electrodes developed now solve the problem of electrode contamination to the alloy, because the trace amount of copper in the alloy is within the allowable range. Now there are two types of water-cooled copper electrodes: one is a self-rotating non-consumable electrode, and the other is a rotating magnetic field, called a rotating arc (or Durarc rotating) electrode. The purpose is to make the arc point rotate on the electrode. In order to avoid local overheating and burning of the electrode head to contaminate the molten alloy.

4. Electron Beam Melting

EBM uses the electron flow emitted from the surface of the heated cathode under vacuum to produce high-speed motion under the action of a high-voltage electric field, and through focusing and deflection, the high-speed electron flow is accurately shot to the anode. The electrons will undergo energy conversion during the high-speed motion. Its high-speed kinetic energy is converted into heat energy and is finally absorbed by the anode, which melts the anode’s high melting point metal. Since the anode metal is melted by the bombardment of electrons, the electron beam furnace is also called the electron bombardment furnace. Compared with other vacuum melting methods, it has the following main advantages:

• Since the smelting is carried out under high vacuum, it is ensured that the impurities with higher gas or vapor pressure can be removed at the smelting temperature, and a high purification effect can be obtained;
• The smelting speed and heating speed can be Adjust within a larger range. The retention time of the molten metal material in the liquid state can be controlled within a wide range, which is conducive to the complete reaction of carbon and oxygen in the liquid state, so that impurities with low diffusivity can diffuse to the surface of the melt and participate in the evaporation;
• power The density is high, the surface temperature of the molten pool is high and can be adjusted. At the same time, the scanning of the electron beam has a stirring effect on the molten metal;
• The molten metal is solidified in the water-cooled copper crucible to form an ingot, so the molten metal will not be contaminated by refractory materials.
• It can get very high temperature, can melt any refractory metal, and can also melt non-metal.

In addition to the above advantages, electron beam melting has the following disadvantages:

• When smelting alloys, the added elements are easy to volatilize, and the composition and uniformity of the alloys are not easy to control;
• The structure of the electron beam furnace is more complicated, requiring a DC high-voltage power supply, and the operating cost is high;
• When the electron beam furnace is smelting It is necessary to take special protective measures for the human body to avoid X-rays harmful to the human body during work.

5. Plasma Arc Melting

Plasma smelting is the use of high-temperature, fast, and pure plasma energy to melt reactive metals or alloys. The power of plasma arc smelting can be made larger. Generally, the temperature of plasma arc core can reach 24000-26000K (much higher than the temperature of free arc: 5000-6000K), and the speed can reach 100-500m/s. The advantages of this method are high melting temperature, concentrated energy, fast melting speed, very small loss of alloy elements, good product purity, and good quality.

6. Cold Hearth Melting

Cold bed smelting technology is an advanced smelting technology that only began to develop in the 1980s. According to different heat sources, cooling bed smelting can be divided into electron beam cooling bed smelting and plasma beam cooling bed smelting. The advantage of cold bed smelting is that it can improve the uniformity of the ingot composition and effectively eliminate various inclusions in the titanium alloy, including high-density inclusions (HDI) and low-density inclusions (LDI).

7. Magnetic Suspension Induction Melting

Magnetic levitation smelting technology is also a new smelting technology developed in the 1980s. The so-called levitation smelting means that the magnetic field generated by the crucible during the smelting process generates a force that pushes the furnace charge toward the center, so that the charge does not contact the crucible wall and achieves levitation smelting.

The advantages of magnetic levitation induction smelting are:

• The charge and the crucible have no contact, and are completely melted in the magnetic levitation state, achieving pollution-free smelting, and obtaining high-purity, non-inclusion metal ingots;
• The shape of the charge is freely selected, and it is not necessary to press the electrode.
• It is easy to control the alloy composition, and the electromagnetic stirring effect can ensure the uniformity of the alloy composition;
• The melting efficiency is high and the operation is convenient;
• It can work in any atmosphere and pressure;
• The material can be fully processed Overheating, it can melt high melting point metals (above 2000℃) and can maintain high temperature for a long time. With strong stirring, it will help to dissolve the refractory components in the charge. It can eliminate harmful inclusions in aviation titanium alloy.

Current status of research on ceramic crucible vacuum induction smelting of titanium alloys

High-temperature titanium alloys have low fluidity at pouring temperature, so the melt must be kept at a certain degree of overheating during smelting to prevent cold barriers and insufficient pouring defects during the pouring process. At present, the commonly used methods for smelting high-temperature titanium alloys include vacuum consumable arc smelting (VAR) and vacuum induction shell smelting (ISM). Compared with VAR and ISM methods, the use of ceramic crucible vacuum induction smelting of titanium alloys is easy to control Advantages of alloy composition, increased superheat and cost.

The selection of ceramic crucible materials must meet the following conditions:

• The crucible material must be able to withstand high temperatures, and the melting point must be above the melting point of Ti and most of the alloying elements;
• It should not react with the alloy melt during the smelting process ;
• Must be able to withstand heat shock. Due to the high chemical activity of the alloy melt, the final overheat treatment must be very rapid.

This process will result in a higher thermal gradient in the parts of the crucible that are in contact with the melt and those that are not in contact with the melt. According to the above principles and literature reports, some metal oxides such as Al2O3, ZrO2, CaO and Y2O3 are suitable as crucible materials to smelt titanium alloys. Researchers used Al2O3 crucible vacuum induction smelting of Ti-48Al-2Cr alloy. The results showed that the alloy ingot structure obtained under various conditions contained α2 and γ photo layers and Al2O3 particles. The appearance of particles may be attributed to the mechanical interaction between the melt and the crucible. At the same time, no titanium oxide compound is found in the structure to reduce the possibility of chemical reaction between the melt and the crucible. The volume fraction of Al2O3 particles decreases with the increase of cooling rate, and increases with the increase of the holding time of the melt in the crucible. The study also found that the purity of the crucible has an important influence on the production of Al2O3 particles. The higher the purity of the crucible, the fewer Al2O3 particles produced. Researchers used a ZrO2 crucible with Y2O3 coating to study the effect of overheating parameters (temperature and holding time) on the interface reaction between Ti-48Al alloy and crucible.

The results showed that the optimized melting process for casting Ti-48Al alloy using vacuum induction melting technology The parameters are the overheating temperature of 1550℃ and the holding time of 60s. It is feasible to use Y2O3 as the surface material of the crucible to smelt TiAl alloy. Overheating temperature and holding time are important factors affecting the interface reaction between alloy and crucible and directly affect the distribution of interface alloying elements and microhardness. Researchers successfully melted Ti-48Al alloy using CaO crucible vacuum induction melting technology. The results show that when the superheating temperature is 1550℃ and 1600℃, the oxygen content of the alloy is 0.08wt.% and 0.10wt.%, respectively. The excessively high oxygen content limits its application under critical conditions. When the superheating temperature reaches 1600℃, the maximum α-type reaction layer thickness and surface microhardness of the alloy reach 50μm and 326HV, respectively. When the researchers used different crucibles to smelt Ti-46Al alloy under the same conditions, they found that the Y2O3 crucible had the largest number of oxide inclusions, the CaO crucible was the least, and the ZrO2 crucible was the second. The study also found that the oxygen content has an important effect on the room temperature tensile properties of the alloy. The oxygen content of Ti-46Al alloy can reach 0.4wt.% when the Y2O3 crucible is used to smelt the Ti-46Al alloy, which can effectively improve the strength of the alloy. Researchers studied the contamination of Ti-46Al-8Nb alloy by Y2O3 crucible during directional solidification. The results showed that the non-metallic particles formed during directional solidification were Y2O3. The volume fraction of Y2O3 particles increased with the increase of reaction time and melt temperature. Increase. By calculation, the activation energy for the formation of Y2O3 particles is 421.8k J/mol, and the time index is 0.55. The morphology and distribution of Y2O3 particles indicate that they are formed by the eutectic reaction during the solidification of the alloy.