脉冲频率对定向凝固TiAl基合金微观组织与性能的影响

Effects of Pulse Frequency on Microstructure and Properties of Directionally Solidified TiAl-based Alloys

将脉冲电流作用于定向凝固Ti-45.5Al-4Cr-2.5Nb(摩尔分数,%,简称TiAl基合金)合金凝固过程中,研究了不同脉冲频率对TiAl基合金凝固组织与性能的影响。结果表明,脉冲电流密度为52.9 mA/mm~2时,随着脉冲频率增大,由于电流的集肤效应和焦耳热效应,使侧向散热减小,枝晶平均偏离角减小。但随着脉冲频率进一步提高,洛伦兹力增大,电磁搅拌使枝晶碎断或重熔,晶粒偏离角反而增大。晶粒平均宽度随脉冲频率增大先增大后减小,脉冲电流频率较小时,电流的焦耳热效应及电磁搅拌促使枝晶重熔或破碎,使熔区长度增大,温度梯度减小,反而导致晶粒平均宽度增大。随着脉冲频率继续增大,洛伦兹力增强,强烈的电磁搅拌使晶粒细化,晶粒平均宽度减小。与母合金铸锭相比,脉冲电流作用下TiAl合金抗拉强度最高提高了70.7%,伸长率提高了129.5%。

Pulse current was applied to the solidification process of directionally solidified Ti-45.5 Al-4 Cr-2.5 Nb alloy. The effects of different pulse frequencies on the solidification microstructure and properties of TiAl-based alloy were investigated. The results indicate that when the pulse current density is 52.9 mA/mm~2, the lateral heat dissipation and the average deviation angle of dendrites are decreased with the increase of pulse frequency due to the skin effect and Joule heat effect of current. However, with the further increase of pulse frequency, the Lorentz force is increased, and the branch crystal is broken or remelted by electromagnetic stirring, resulting in the increase of grain deviation angle. The average width of grain is firstly increased and then reduced with the increase of pulse frequency. When the pulse current frequency is lower, the dendrites are remelted or broken due to the Joule heat effect of current and electromagnetic stirring. The length of melting zone is increased and temperature gradient is decreased, leading to the increase of average grain width instead. With the continuous increase of pulse frequency, Lorentz force is increased, and strong electromagnetic stirring can refine grains, resulting in the average width reduction of grains. Compared with the master alloy ingot, the tensile strength of TiAl alloy is increased by 70.7% under pulse current, and the elongation is increased by 129.5%.