冷却速度对GH4169合金凝固过程微观偏析及糊状区稳定性的影响

Effect of cooling rate on microsegregation behaviors and mush zone stability of GH4169 alloy during solidification process

通过不同冷却速度下的凝固重熔试验,结合SEM微观分析及电子探针成分分析,研究GH4169合金凝固过程微观偏析的演化规律,分析凝固前沿液相区的密度变化规律,并提出基于相对Rayleigh数的凝固前沿糊状区的稳定性判据。结果表明:冷却速度低于0.05℃/s时,液相中的Nb含量随出炉温度的下降很快升高到一个较高值,在较低温度出炉冷却时,Nb含量迅速升高到14%以上,可达合金名义成分中Nb元素含量的3倍;提高冷却速度至0.1℃/s,高温出炉时,Nb含量升高很慢,当出炉温度低于1 310℃时,Nb含量迅速升高到10%以上;进一步提高冷却速度,残余液相中Nb元素含量变化不大,与合金的名义成分接近。当冷却速度较高时(大于0.1℃/s),GH4169合金凝固过程凝固前沿糊状区的稳定性较好,相对Rayleigh数较低;而当冷却速度较低时,即低于0.02℃/s时,凝固前沿糊状区稳定性判据Rv值比较高,而且在1 330℃附近出炉冷却时,存在一个峰值区,最大峰值接近7.5×10-13。说明当凝固过程冷却速度较低时,元素的微观偏析使得GH4169合金凝固前沿糊状区的稳定性降低,可能导致微观偏析向宏观偏析转变。

The microsegregation behaviors of GH4169 alloy during its solidification process were investigated by remelting tests, SEM and EMPA analysis. Furthermore, the density variation for the solidification front of this alloy was analyzed and a stability criterion based on relative Rayleigh number model was put forward. The results show that, when the cooling rate is lower than 0.05 ℃/s, Nb content in the residual liquid phase increases with decreasing the pouring temperature, it can reach 14%, three times of the designed Nb content for this alloy. When the cooling rate increases to 0.1 ℃/s and the pouring temperature is lower than 1 310 ℃, Nb content in the residual liquid phase reaches 10%promptly, while when the pouring temperature is higher than 1 310 ℃, Nb content in the residual liquid phase varies slowly. With the increase of cooling rate during the solidification process, Nb content in the residual liquid phase can decrease to the designed value gradually. At the same time, a heavier microsegregation of Nb element may occur at lower cooling rate in the mush zone for GH4169 during its solidifying process, resulting in the bigger relative Rayleigh number and the worse stability of mush zone, which may increase the probability of freckle formation.