摘要
利用单晶高温合金SX1、SX2分别作为籽晶和铸造材料,进行了LMC工艺下籽晶法控制单晶高温合金晶体取向的浇注试验和数值模拟,并对籽晶和铸件进行了解剖分析.结果表明:当籽晶和铸造材料为同一成分时,可以获得完整的单晶铸件,并且籽晶与单晶铸件取向一致.当籽晶和铸造材料为不同成分时,如果籽晶的液相线高于铸造材料的液相线,则获得完整的单晶铸件,并且籽晶与单晶铸件取向一致;反之,如果铸造材料的液相线高于籽晶的液相线,则会在籽晶重熔界面处产生大量的杂晶形核,并在纵向温度梯度下长大为定向柱晶,无法获得完整的单晶铸件.数值模拟结果表明这可能是由于合金浇注后,在抽拉过程中,上部铸造材料由于液相线较高,在籽晶重熔界面处优先凝固,导致重新形核,形成定向柱晶.因此,在利用籽晶法进行单晶铸件取向控制时应当保证籽晶的液相线等于或者高于铸造材料的液相线,才能获得完整的单晶组织,并精确控制取向.
Abstract
Liquid Metal Cooling(LMC)directionally solidified(DS)experiment and numerical simulation were conducted based on seed method using single-crystal superalloy(SX)SX1 and SX2 as materials.The seeds and castings were cut to observe the microstructure of the dendrites.It was found that the complete SX casting can be achieved with the same orientation between casting and seed when the same materials were used in casting and seed.In case of different composition between seed and casting,the complete SX casting can also be obtained with preferred<001>orientation if the liquidus of seed was higher than that of casting.Otherwise DS structure would be found from stray grain formed at meltback interface of seed,which indicated that SX growth was failed.The numerical simulation results show that casting material with higher liquidus than that of seed can solidify preferentially from meltback interface and DS structure formed subsequently during withdrawing.Therefore,it is necessary for orientation control to ensure that the liquidus line of seed is equal to or higher than that of casting during seed method.
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