What are the process steps for melting titanium ingots using vacuum self-consuming electrode arc furnaces?

The vacuum self-consuming electrode arc furnace, also known as VAR (vacuum arc remelting), involves blending sponge titanium and its alloy materials in a certain chemical composition ratio, mixing them evenly, and then pressing them into different shaped blocks. These blocks are welded into electrodes using a vacuum plasma welding box. The resulting blocks are then subjected to secondary melting under vacuum or inert gas protection to form ingots. Special-purpose ingots require three melting processes. Industrial-scale ingots typically weigh between 3 and 6 tons, while large ingots can reach up to 15 tons.

The melting operation of the vacuum self-consuming arc furnace is carried out in five steps: electrode welding, arc ignition, normal melting, topping off, and cooling.

(1) Electrode welding. Electrode welding should achieve straight welding, firm connection, and the weld seam should be able to conduct the working current. The specific operation of electrode welding: First, insert the consumable electrode and the auxiliary electrode into the furnace, and adjust them to be at the center of the crucible for furnace assembly. After pre-vacuuming, conduct arc welding in a vacuum environment. The welding process can be completed instantly, causing the consumable electrode and the auxiliary electrode to be welded together. The remelting electrode is mostly welded in the furnace.

(2) Arc ignition. The method for initiating the arc is to directly place some arc initiation agents (such as sponge titanium) on the bottom crystallizer, and keep the distance between the arc initiation agent and the consumable electrode tip generally at 20 to 30 mm. To achieve a smooth arc initiation, the no-load voltage (i.e., open-circuit voltage) must be raised to 70V. Under the no-load voltage, through the instantaneous contact between the consumable electrode and the arc initiation agent, an arc discharge occurs, thereby achieving stable arc combustion, creating a certain amount of metal pool, and creating conditions for transitioning to normal smelting. In actual production, it is required that the arc initiation period be as short as possible and a metal pool be formed quickly to reduce the impact of the arc on the bottom of the crystallizer.

(3) Normal Melting. After arc ignition, gradually increase the melting current and quickly switch to normal melting. The accuracy of the operation during the melting period directly affects the quality of the melted product. Once the molten pool covers the bottom of the crucible, quickly raise the current to the set value required by the process and carry out normal melting. At the same time, control other process parameters such as voltage, vacuum degree, and melting rate. Once the melting current is determined, whether the melting is normal depends on the length of the arc. If the arc is too long, the heat is not concentrated, resulting in a sluggish metal molten pool, floating impurity films on the surface, increasing the degree of metal contamination; if the arc is too short, it will cause frequent short circuits of the arc, resulting in a sharp change in the molten pool temperature, and also cause severe splashing; when the arc length is normal, the molten pool is very clear and active, the molten liquid fluctuates slowly and pushes the impurity film towards the crystallizer wall.

(4) Cap formation, also known as filling the shrinkage cavity. The purpose of cap formation is to reduce the shrinkage and porosity at the top of the ingot, decrease the cutting length of the titanium ingot, and increase the yield of the ingot. After entering the cap formation stage from normal melting, the current of the hot cap formation gradually decreases, reducing from 1/3 of the normal melting current, and finally reaching 1/10; the hot cap formation time generally accounts for 1/4 to 1/3 of the total melting time. To determine the optimal start time of cap formation, the electrode stroke or average rapid melting method is generally used to calculate and determine the reserved electrode quantity.

(5) Cooling. The ingot is cooled to a temperature below 400°C under vacuum or inert gas protection and then taken out of the mold. Generally speaking, the cooling effect with inert gas protection is better than that with vacuum cooling.

The melted titanium ingot is processed by peeling to form the titanium optical ingot.