Application of Inner Radiation Baffles in the Bridgman Process for Flattening the Temperature Profile and Controlling the Columnar Grain Structure of Directionally Solidified Ni-Based Superalloys

Abstract

The technique of flattening the temperature profile and controlling the formation of both the dendritic microstructure and grain structure in the directional solidification of nickel-based superalloy casting, using the novel inner radiation baffles (IRBs) in the Bridgman process, is presented in this paper. These baffles matched to the shape of mold and were placed horizontally along its height at various distances from the casting base. The plate castings of CMSX-4 superalloy were fabricated without and with the use of IRBs, withdrawing the mold at the rate of 6 mm/min from the heating to the cooling area of the industrial Bridgman furnace. Thermal analysis of the directional solidification of castings was carried out using the ProCAST software for a process where the various designs of the radiation baffle were applied. The results of the solidification conditions, the shape of liquidus and solidus isotherms, and grain structure obtained for the IRBs were compared with those reached for the standard ring-shaped (AERB) or perfectly adjusted (PARB) radiation baffles. The use of IRB resulted in flattening of the temperature distribution and decrease of the curvature of liquidus and solidus isotherms, as well as an increase of temperature gradient and cooling rate, compared with the process where AERB was only used. Consequently, primary dendrite arm spacing (PDAS) reached similar values across the width of casting and equaled to approximately 370 μm, reducing its average value by 26%, compared with the standard process. The change in predicted axial temperature gradient in casting was not found when thermophysical properties of molybdenum IRBs were used. The increase in graphite IRBs number in mold from seven to 14 caused the reduction of inhomogeneity of axial temperature gradient along the casting height.

Heterogeneity of the Dendrite Array Created in the Root of Cored SX Turbine Blades during Initial Stage of Crystallization

“…Attempts are often made to determine the reasons for heterogeneity of the dendrite array and the crystal orientation by production of the model casts with a simplified shape. As an example, the part with a narrow cross-section connected with the wide platform may be presented [ 6 , 7 , 8 ]. This part allows for the verification of the theoretically calculated temperature distribution or crystallization front shape, and consequently—creation of models of the dendrite array.…”

2D single crystal Bragg-dip mapping by time-of-flight energy-resolved neutron imaging on IMAT@ISIS

“…Single crystal Ni-base superalloys are employed in gas turbine engines to improve high temperature mechanical properties 1 , 2 . They are manufactured by the Bridgman casting process 3 , which constrains the heat flow to one direction (Fig. 1 a).…”

Molding Binder Influence on the Porosity and Gas Permeability of Ceramic Casting Molds

“…Ceramic molds with firmly specified properties, made from alloys such as CMSX4 and CMSX6, allow manufacturing precision castings with the Bridgman method. This method allows to obtain a specific microstructure in the mold, in the form of mono-or poly-crystalline castings with specific grain orientation [2,3]. This type of approach is used for manufacturing heavily stressed parts for aircraft engine sections, such as high-pressure turbines.…”