Novel casting processes for single-crystal turbine blades of superalloys

Abstract

This paper presents a brief review of the current casting techniques for single-crystal (SC) blades, as well as an analysis of the solidification process in complex turbine blades. A series of novel casting methods based on the Bridgman process were presented to illustrate the development in the production of SC blades from superalloys. The grain continuator and the heat conductor techniques were developed to remove geometry-related grain defects. In these techniques, the heat barrier that hinders lateral SC growth from the blade airfoil into the extremities of the platform is minimized. The parallel heating and cooling system was developed to achieve symmetric thermal conditions for SC solidification in blade clusters, thus considerably decreasing the negative shadow effect and its related defects in the current Bridgman process. The dipping and heaving technique, in which thinshell molds are utilized, was developed to enable the establishment of a high temperature gradient for SC growth and the freckle-free solidification of superalloy castings. Moreover, by applying the targeted cooling and heating technique, a novel concept for the three-dimensional and precise control of SC growth, a proper thermal arrangement may be dynamically established for the microscopic control of SC growth in the critical areas of large industrial gas turbine blades.

On the Fabrication of Metallic Single Crystal Turbine Blades with a Commentary on Repair via Additive Manufacturing

“…Therefore, before casting can begin, a mold, also known as an investment cast shell, must be fabricated. The first step in this process is to create a wax mold ( Figure 5a) [21]. To do this, the molten wax is poured into a master mold that contains several ceramic cores.…”

“…Crystal seeds cause the superalloy to assume the desired grain orientation more quickly than the blocker selectors [23]. [24]); (c) the shell mold corresponding to a wax assembly in (a) for casting SX blades (Reproduced from [21] under the terms of the Creative Commons Attribution 4.0 License from http://creativecommons.org/licenses/by/4.0/); and (d) Cross-section of a ready-to-use mold demonstrating the pin placement used to stabilize the ceramic cores and the ceramic mold (Reproduced from [26] under the terms of the Creative Commons Attribution 4.0 License from http://creativecommons.org/licenses/by/4.0/).…”

“…Once the grain selector is added, the resulting solidified wax structure is dipped into ceramic slurries containing alumina, silica, zirconia, and other binding agents to create the investment cast shell. By using a wax tree ( Figure 5c) [21], many wax models can be dipped at once. Foundries with automated wax trees see greater consistency in their resulting investment shells.…”

Optimizing Grain Selection Design in the Single‐Crystal Solidification of Ni‐Based Superalloys

“…This leads to the elimination of transverse grain boundaries, anisotropic mechanical properties, and excellent creep strength. Recently, dipping and heaving or formerly called downward directional solidification technique, has been successfully used to produce single crystals with premium properties, higher thermal gradients, lower possibility of stray grain formation, and more efficiency …”

“…Recently, dipping and heaving or formerly called downward directional solidification technique, has been successfully used to produce single crystals with premium properties, higher thermal gradients, lower possibility of stray grain formation, and more efficiency. [7] The introduction of a spiral grain selector mechanism for the first time by Pratt & Whitney industry brought about thorough removal of grain boundary which acts as the weak part of structure for initiating and propagating crack. [8] Later, the need for achieving higher thermal gradients resulted in the development of liquid metal cooling technique which substitutes radiation cooling with thermal convection cooling inside a metallic mold bath.…”