钛合金VAR过程中自然对流下的宏观偏析行为模拟

Numerical simulation of macrosegregation caused by buoyancy driven flow during VAR process for titanium alloy

摘要:

宏观偏析是钛合金铸锭熔炼中面临的一种重要缺陷，严重的宏观偏析将使铸锭在后续热加工过程中产生?斑，从而严重影响产品性能。因此理解宏观偏析的形成机理，进而通过工艺调控来减轻宏观偏析，是钛合金铸锭生产中需要解决的关键问题。本文采用合金凝固的连续介质模型，模拟了钛合金熔炼过程中温度场、流场、溶质场的演化行为，模拟所得溶质元素分布规律与已有实验观测结果一致。通过对模拟结果中宏观偏析形成过程的详细分析以及不同流动情况下偏析程度的对比，揭示了热浮力和溶质浮力引起的自然对流在宏观偏析形成中的作用机制，发现抑制自然对流，可最大程度的减轻铸锭宏观偏析。此外模拟结果还表明，当熔池内流动情况一定时，合金元素在固/液两相中的平衡分配系数决定了该元素的宏观分布规律和偏析程度。

Abstract:

Macrosegregation is a typical defect occurred during the vacuum arc remelting (VAR) of titanium alloy ingot. In the subsequent heat treatment serious segregation of special elements would lead to the so called beta-fleck, which will significantly reduce the property of product. Reducing the incidence of macrosegregtion requires a thorough understanding of the formation mechanism of such defect. In this paper a continuum model for alloy solidification has been used to simulate the temperature evolution, solute distribution and liquid flow. The segregation patterns obtained in simulations are consistent with the observations in experiments. The development of segregation in the conditions with different thermal and solute buoyancy forces were analyzed. It was shown that the inhibition of flow can minimize the macrosegregation to the greatest extent. It was also found that the solute partition coefficient determines the segregation pattern and severity in the same fluid flow conditions.