钛合金三次真空自耗电弧熔炼过程中的宏观偏析传递行为

Evolution of Macrosegregation During Three-Stage Vacuum Arc Remelting of Titanium Alloys

宏观偏析是钛合金真空自耗电弧熔炼铸锭中的一种典型凝固缺陷,且在后续加工过程中难以消除,对铸锭性能具有重要影响。实际钛合金铸锭生产中通常采用三次真空自耗熔炼工艺,以减少夹杂、提高成分均匀性,然而当前对于宏观偏析在多次熔炼中的传递规律仍缺乏清晰理解。本工作通过数值模拟方法对钛合金三次真空自耗熔炼过程中熔池内的液相流动及溶质偏析行为进行分析,发现前次铸锭的径向成分不均匀会在对流作用下消除,对下一次熔炼铸锭的宏观偏析无影响;而前次铸锭沿轴向的成分不均匀会传递给下一次熔炼的铸锭,这一传递效果在熔池深度增大时由于对流作用而被削弱。此外,模拟结果还表明,如果三次熔炼中始终保持铸锭正置作电极,铸锭宏观偏析最重;至少一次将铸锭反置作电极,可显著减轻宏观偏析。对TC4合金采用真实熔炼工艺参数进行模拟,获得的Al元素及V元素偏析规律与实验观测结果基本吻合。

Macrosegregation is a typical solidification defect formed during vacuum arc remelting(VAR)process.This defect adversely affects the property of ingots as the defect sustains even in the subsequent heat treatment process.In the industrial production of titanium alloys,VAR is repeated thrice to eliminate inclusions and improve the homogenization of composition.However,the evolution of macrosegregation during the different stages of the triple VAR process remains unclear.In this study,the melt flow behavior and macrosegregation of titanium ingots in the multistage VAR process are examined via solidification simulations,considering both buoyancy and electromagnetic force.The results show that the strong fluid flow in the upper part of melting pool eliminates nonuniform concentration along the radial direction of the electrode.In contrast,the nonuniform concentration along the axial direction can be inherited in the sequential ingot.However,with the increase in the depth of melt pool,the sustained melt flow from the bottom to upside can reduce the axial macrosegregation delivery.In addition,the use of the previous ingot directly as the electrode for the subsequent remelting process results in severe macrosegregation.However,turning the previous ingot upside-down at least once during the three-stage VAR process can substantially reduce the macrosegregation.Overall,the simulated macrosegregation of Al and V elements in TC4 ingot agree well with that observed in experiment.