The properties of Ni-base superalloy castings are closely related to the uniformity of their as-cast microstructure,and different solidification methods have serious effect on microstructural uniformity.In this paper,the influences of high rate solidification(HRS) process(with or without superheating) and liquid metal cooling(LMC) process on the microstructure of DZ125 superalloy were investigated.Blade-shape castings were solidified at rates of 40 μm·s-1 to 110 μm·s-1 using HRS process and a comparative experiment was carried out at a rate of 70 μm·s-1 by LMC process.The optical microscope(OM),scanning electron microscope(SEM) were used to observe the microstructure and the grain size was analyzed using electron back scattered diffraction(EBSD) technique.Results show that for the castings by either HRS or LMC process,the primary dendrite arm spacing and size of γ' precipitates decrease with increasing the withdrawal rate;the dendrites and γ' precipitates at the upper section of the blade are coarser than those in the middle,especially for the HRS castings without high superheating technique.When the withdrawal rate is 70 μm·s-1,the castings by HRS with high superheating technique have the smallest PDAS with fine γ' precipitates;while the size distribution of γ' precipitates is more homogenous in LMC castings,and the number of larger grains in LMC castings is smaller than that in the HRS castings.Moreover,high superheating technique yields smaller grains in the castings.Both the LMC method and HRS with high superheating technique can be used to prepare castings with reduced maximum grain size.
Ge BingmingLiu LinZhang JunLi YafengFu HengzhiLiu Xiaofu
The dendrite morphologies and spacings of directionally solidified DZ125 superalloy were investigated under high thermal gradient about 500 K/cm. The results reveal that, with increasing cooling rate, both the spacings of primary and secondary dendrite arms decrease, and the dendrite morphologies transit from coarse to superfine dendrite. The secondary dendrite arms trend to be refined and be well developed, and the tertiary dendrite will occur. The predictions of the Kurz/Fisher model and the Hunt/Lu model accord basically with the experimental data for primary dendrite arm spacing. The regression equation of the primary dendrite arm spacings λ1 and the cooling rate Vc is λ1=0.013Vc-0.32. The regression equation of the secondary dendrite arm spacing λ2 and the cooling rate Vc is λ2=0.00258Vc-0.31, which gives good agreement with the Feurer/Wunderlin model.
ZHANG Weiguo LIU Lin ZHAO Xinbao HUANG Taiwen YU Zhuhuan QU Min FU Hengzhi
The primary dendrite morphology and spacing of DZ125 superalloy have been observed during directional solidification under high thermal gradient about 500 K/cm.The results reveal that the primary dendrite arm spacing decreases from 94μm to 35.8μm with the increase of directional solidification cooling rate from 2.525 K/s to 36.4 K/s.The regression equation of the primary dendrite arm spacingsλ 1 versus cooling rate is λ 1 =0.013(GV)-0.32.The predictions of Kurz/Fisher model and Hunt/Lu model accord reasonably well with the experimental data.The influence of directional solidi fication rate under variable thermal gradient on the primary dendrite arm spacing has also been investigated.
Zhang Weiguo Liu Lin Huang Taiwen Zhao Xinbao Qu Min Yu Zhuhuan Fu Hengzhi
