The smelting current is an important parameter of vacuum consumable smelting. It is related to the titanium material being smelted, the diameter of the ingot, the diameter ratio of the crucible to the electrode, the pressure in the furnace, the polarity, the furnace structure and the type of power supply. The size of the smelting current, in addition to determining the melting rate of the metal and the temperature of the molten pool, also directly affects the shape, volume and depth of the molten pool. The greater the current, the greater the metal melting rate and the better the surface quality of the ingot. At the same time, with the increase of the current, the increase in the depth of the metal pool causes the ingot structure to deteriorate-the granular crystal is thick, the radial development, the degree of looseness and segregation increase. If the melting current is small, the melting rate is low, the metal bath is shallow, the columnar crystals are fine and axially developed, which is beneficial to obtain a cast titanium ingot with a small degree of looseness, a small degree of component segregation, and a dense crystal structure. In the charge melting stage, the heat is not only used for the melting of the charge, but also for the reduction reaction that is carried out simultaneously with the melting, so the energy consumed in this stage accounts for about 2/3 of the total energy of the entire melting process. For example, only about 80% of the total carbon is added to the charge, and the remaining 20% of the carbon is added during the slagging stage. This kind of charge with less carbon has a faster melting speed and a lower melt temperature. And, it is possible to increase the volume of the molten pool, which can increase the furnace output. The operation method of distributing all of the carbon in the charge will prematurely produce a melt containing a higher amount of low-valent titanium, which is detrimental to the smelting process. In this method of operation, the slagging stage relies on the residual carbon in the melt for reduction. If there is too much carbon in the charge, there may not be a slagging stage, and sometimes it may be diluted after adding titanium concentrate to get out of the furnace . Only a part of the carbon is added to the furnace charge, and the rest of the carbon should be added during the slagging stage for slagging reduction. The supplementary carbon floats around the electrode on the surface of the melt and has an arc stabilizing effect. If low-grade acid-soluble titanium slag is produced, slag formation is not necessary.
A Melting power
In vacuum consumable arc melting, the power used for metal melting only accounts for 30% to 50% of the total input power, and 50% to 70% of the power is lost in various forms. These include metal volatilization loss, electrode heat loss, metal surface heat loss and liquid metal heat conduction loss. The total input power required for smelting can be calculated according to the following empirical formula:
Ptotal = Pmelting / = IV
Where Ptotal——total input power required for melting, kW h;
P melting-the power required for metal melting, kw. h;
——Thermal efficiency, generally 30% ～ 50%;
I——Arc current, kA;
V——Arc voltage, V.
B Melting speed
The speed of vacuum consumable melting is related to the current smelting, as shown in Figure 8-30. Under the premise of ensuring the quality of ingots, increasing the melting speed is conducive to increasing production efficiency. The main method to increase the smelting speed is to increase the voltage and current. Generally, it is better to increase the current.
C Crucible ratio
The ratio of electrode diameter to crucible diameter is referred to as crucible ratio, which is one of the important parameters that affect the quality of ingot and safe production. For titanium, the crucible ratio is generally between 0.625 and 0.88. The relationship between crucible diameter and minimum gap during vacuum consumable melting is shown in Figure 8-31
There is currently a tendency to use large-section electrodes. The advantage of the large-section electrode is that the thermal energy of the arc is evenly distributed on the entire surface of the molten pool, making the metal molten pool flat, increasing the temperature gradient of the solid-liquid two-phase zone of the molten pool, and conducive to obtaining high quality with small composition segregation and high density Ingot.
D Melting vacuum
The melting vacuum of titanium and titanium alloys is generally 0.1 to 1.0 Pa.
E Stir current
The rotation of the molten metal pool has both good and bad effects on the quality of the ingot. Appropriate stirring current can refine grains and reduce the degree of crystal segregation. The stirring current is usually determined according to the alloy being melted
The size and frequency of. Generally speaking, for the titanium alloy with positive segregation characteristic components, in the secondary remelting, a lower frequency and a smaller stirring current are selected.