Vacuum self-consumption arc melting (VAR) is one of the most commonly used melting methods for titanium alloys, and the process can be divided into three stages: arc start to establish the molten pool period, the normal melting period and shrinkage period. If the ingot is melted without shrinkage, the shrinkage is concentrated in 1/2 of the depth of the molten pool, while the depth of the molten pool in normal melting > crucible diameter!1.
Titanium alloy ingot head shrinkage is due to the end of the melting instantaneous power failure so that the surface of the melt pool before the internal solidification, residual gas is not discharged in a timely manner and the formation of gas holes, these holes that is, shrinkage holes. Ingot in the subsequent processing, with shrinkage hole part needs to be removed, the removed part is the risers, generally according to the disposal of residual material. In order to improve the rate of material, usually in the last melting of the late supplementary shrinkage operation, that is, step by step to reduce the current so that the depth of the molten pool from the bottom to the top gradually reduced. At the end of the fill shrinkage, the surface of the melt pool is in the liquid state, the electrode is basically not consumed, there is no new solution, but the depth of the melt pool has been reduced, the impurity elements and gases upward, making the shrinkage holes become smaller and shallower. The longer the time to make up the shrinkage, the shallower the risers. Therefore, in industrial production, in order to improve the success rate of titanium alloy ingots, generally extend the shrinkage time, but may lead to the following problems: (1) shrinkage into shrinkage, increase the difficulty of ultrasonic flaw detection of ingot risers, increasing the risk of ingot defects to the forging products inherited; (2) for some titanium alloy ingots containing Fe, Mo, Zr and other elements prone to segregation, the head of the head is prone to micro-segregation, and even unqualified composition; (3)) the situation; (4) the ingots will be more shallow, the more shallow the riser is. (iii) the head of the ingot grain size is significantly larger than other parts of the same process after forging, forgings produced by the head of the ingot and other parts of the ingot production of forgings in the organisation, there are differences in performance.
In accordance with the characteristics of the cooling process, the shrinkage process is divided into three stages: the starting period, the cooling period and the baking period. In the starting period of shrinkage, the size of the melting current before shrinkage determines the depth of the melt pool, so the melting current is the key control parameter. In the cooling period, based on the current and melting speed in the stable melting period, the melting speed is reduced by decreasing the current step by step to reduce the metal liquid entering the molten pool, so that the molten pool will gradually and steadily become shallower and shrink at a faster but suitable speed, thus reducing the depth of the risers and the size of the shrinkage holes in a rapid and reliable manner. With the cooling down of shrinkage, a certain amount of electrode reserved for shrinkage is gradually melted. The cooling period is divided into fast current reduction and stepped current reduction according to the different current reduction speeds. The duration of fast current reduction is shorter, while the duration of stepped current reduction is longer, and the reduction speed of stepped current reduction is consistent, so the research focuses on the duration of stepped current reduction. Baking period is by maintaining a small current, so that the surface of the molten pool is in a non-solidified state, impurity elements and gases floating, the molten pool continues to become shallow, further reducing the riser depth and shrinkage hole size; this stage of the current remains unchanged, and the focus of the study is on the duration of the baking.
Based on the above analysis, TC11 titanium alloy ingot with more alloying elements and commonly used in the industry was selected as the object to study the influence of three key parameters, namely, melting current, step-down current duration and baking duration, on the risen crater depth, the distribution of the head components and grain size, with a view to improving the existing process and providing reference for the formulation of subsequent shrinkage filling process for other ingot castings.
Conclusion
(1) The riser depth, head composition deviation and average grain size of TC11 titanium alloy ingot increase with the increase of VAR melting current.
(2)With the prolongation of cooling duration, the ingot riser depth decreases, and there is a certain linear relationship between the two:H=-1.81+205.93.The prolongation of cooling duration does not have much effect on the ingot head composition and average grain size.
(3)With the extension of baking duration, the depth of ingot riser decreases, but it will lead to the deviation of ingot head composition and the increase of average grain size. (4) Before VAR melting and shrinkage, using smaller melting current, extending the cooling period and controlling the baking duration, we can obtain TC11 titanium alloy ingots with small riser depth, uniform composition and better grain size.
