热处理工艺对GH4720Li合金细晶铸锭组织与热加工性能的影响

Effect of Heat Treatment Parameters on Microstructure and Hot Workability of As-Cast Fine Grain Ingot of GH4720Li Alloy

通过对GH4720Li合金细晶铸锭在不同均匀化热处理温度下组织演变规律与热加工性能的研究,确定了合金的最佳均匀化热处理工艺参数。依据不同参数下的一级与二级均匀化热处理工艺实验,深入研究了细晶铸锭的偏析系数以及共晶相含量与均匀化热处理工艺之间的内在联系,并对不同热处理工艺参数下合金的热加工性能进行了评价。实验结果表明:均匀化热处理温度的升高或保温时间的延长能够有效降低合金中共晶相的含量,改善合金中元素的偏析情况。此外,合理的二级均匀化热处理工艺能够降低均匀化温度与保温时间,从而提高均匀化热处理效率。经二级均匀化缓冷(1140℃保温12 h+1170℃保温10 h,以0.2℃/min冷却至1010℃后,出炉空冷)热处理所获得试样的高温塑性显著优于一级均匀化处理和二级均匀化空冷处理的试样,当热变形参数为1120℃、1 s-1时,其最大压下量为50%,显著优于其它热处理工艺参数。合金在1140和1160℃热变形时动态再结晶形核方式主要以非连续动态再结晶为主,原始晶粒边界为动态再结晶晶粒提供形核位置,且M(C,N)型强化相能够促进合金动态再结晶行为的发生。本研究中GH4720Li合金细晶铸锭的最佳均匀化热处理工艺为:1140℃保温12 h+1170℃保温10 h,以0.2℃/min冷却至1010℃后,出炉空冷。

GH4720Li was Ni-Cr-Co base precipitation strengthened superalloy and widely used for high performance applications such as disks and blades of either aircraft engines or land-based gas turbines attributing to its excellent properties including resistance to creep and fatigue,together with corrosion,fracture and microstructural stability for the intended applications.Compared with the double-melting process(vacuum induction melting(VIM)+electroslag remelting(ESR)or VIM+vacuum arc remelting(VAR),a triple-melting process(VIM+ESR+VAR)can eliminate the segregation coefficient of the alloying elements and reduce the content of impurity elements,while the ingot fabricated by the triple-melting process also exhibited lots of shortcomings,such as the coarse grains,dendritic structure,microstructure defects and high forging temperature.The as-cast fine grain ingot prepared by grain refining casting process can eliminate the microscopic shrinkage,reduce the differences among three crystalline regions and improve the hot workability as a result.However,it was hardly to avoid the microstructure defects by simply improving the casting process attributing to its large number of alloying elements.Therefore,the homogenization treatment was always performed on the superalloy ingot.In this work,the optimized homogenization parameter was identified according to the investigation on the microstructure evolution under various homogenization treatment conditions and hot workability of as-cast fine grain ingot after homogenization treatment.The relationships of one-stage as well as two-stage homogenization treatment parameters and segregation coefficient as well as volume fraction of eutectic phase were investigated indepth.The hot workability of the homogenized samples under various conditions was also analyzed with the help of hot compression tests.Experimental results revealed that the increased homogenization treatment temperature and extended holding time were able to decrease the volume fraction of eutectic phase and segregation coefficient of the alloying element significantly.Hot compression tests by the Gleeble3800 dynamic thermal-mechanical testing machine indicated that the samples suffered two-stage homogenization treatment followed by the slowly cooling rate(1140℃,12 h+1170℃,10 h,0.2℃/min furnace cooling to 1010℃,and then air cooling)exhibited better hot workability(the maximum reduction rate of 50%deformed at 1120℃,1 s-1).Discontinuous dynamic recrystallization was identified as the mainly nucleation mechanism of the alloy,and the recrystallized grains preferred to nucleate at the boundaries of the original grains according to the microstructure observation of hot compressed samples.In additions,the M(C,N)type precipitates were able to promote the occurrence of dynamic recrystallization behavior.Homogenization treatment experiments and microstructure observation suggested that the optimized treatment parameters of the as-cast fine grain ingot was 1140℃,12 h+1170℃,10 h,0.2℃/min furnace cooling to 1010℃,and then by air cooling.