Refined microstructure and propensity to crack of typical hard-to-deform GH4151 superalloy prepared by electron beam drip melting

Abstract

High-alloyed superalloy castings often exhibit issues such as coarsening of low-melting point phases and severe microsegregation during the preparation process using existing dual or triple melting methods, which can even lead to cracking of the castings. This study employs Electron Beam Drip Melting (EBDM) technology to fabricate GH4151 superalloy casting with uniform microstructures. The results indicate a significant reduction in the content of gaseous impurity elements in the casting, with an O content of only 2.0 ± 0.1 ppmw. The microstructure of the casting is dense, with grains growing axially, and the secondary dendrite arm spacing (λ₂) at the center of the casting is approximately 50 μm. The sizes of low-melting point phases between dendrites are small, with a total area fraction of <0.4 %, and the number and size of these phases significantly decrease as the height of the casting decreases. The microsegregation coefficients (k) for three typical easily segregated elements are k_W < 1.45, k_Ti > 0.70, and k_Nb > 0.45, respectively. The γ′ phases within the casting are all square-shaped, with sizes of approximately 300 nm for dendrite cores and 400 nm for the inter dendritic γ′ phases. After homogenization treatment, the primary γ′ phase size reaches 550 nm. Calculations show that during the EBDM preparation of the casting, the temperature gradients (G) at the center and at R/2 are 7.2 K/mm and 15.1 K/mm, respectively. Compared to other melting methods, the ability to replenish the mushy zone is enhanced, and the sensitivity to cracking is reduced. The EBDM-GH4151 casting exhibit higher levels of both yield strength and elongation compared to those produced by other melting methods. This study demonstrates that the EBDM process can refine the as-cast microstructure of high-alloyed superalloys, providing castings with a higher uniformity for subsequent heat treatment and thermal processing.