Primary Crystal Orientation of the Thin-Walled Area of Single-Crystalline Turbine Blade Airfoils

Abstract

The thin-walled airfoil areas of as-cast single-crystalline turbine blades made of CMSX-4 superalloy were studied. The blades were produced by the industrial Bridgman technique at withdrawal rates of 2, 3 and 4 mm/min. The angle between the [001] crystallographic direction and blade axis, related to the primary orientation, was defined by the Ω-scan X-ray diffraction method at points on the camber line located near the tip of an airfoil and at points of a line located in parallel and near the trailing edge. Additionally, primary crystal orientation was determined by Laue diffraction at the selected points of an airfoil. The influence of mould wall inclination on the primary crystal orientation of the thin-walled areas is discussed. The effect of change in the [001] crystallographic direction, named as “force directing”, was considered with regard to the arrangement of primary dendrite arms in relation to the trailing edge and the camber line. It was stated that when the distance between the mould walls is less than the critical value of about 1.5 mm the “force directing” increases as the distance between the walls of the mould decreases. The effect may be controlled by selecting an appropriate secondary orientation using a seed crystal in the blade production process. The model of dendrite interaction with the mould walls, including bending and “deflection”, was proposed.

On Directional Dendritic Growth and Primary Spacing—A Review

“…Unfortunately, the mechanisms behind local dendritic misorientation and its influence on the upper λ 1 selection procedure are not fully understood. Multiple investigation into the origins of misorientation have identified plastic deformation through differential thermal contraction in the mushy zone as the probable cause of dendritic bending [128][129][130][131][132][133][134][135][136][137][138]. However, to the authors' knowledge, no proper analytical or numerical treatment of the driving force behind dendritic bending or its influence on the upper primary selection procedure is reported in the literature.…”

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

“…As stated in Ref. [ 11 , 24 ], each of the dendrite chains of a 1 –a 3 type, visible on the P surface, is the result of the growth of a set of tertiary dendrites in the Z direction from the laterally growing single secondary arm below the P surface which was named as the leading arm. In the SE subarea the dendrites grow directly from the selector (three double arrows, Figure 1 c), that is why the chains occur less often.…”

“…In Ref. [ 11 , 24 ], it was stated that each group of the dendrites of UGA type subarea is created by the tertiary dendrites that grow almost parallel to the Z axis from a single secondary dendrite growing laterally near the bottom surface of the root.…”

“…The methods of obtaining and interpretation of the topograms are described in Ref. [ 21 , 22 , 23 , 24 , 25 ].…”

Variation of Crystal Orientation and Dendrite Array Generated in the Root of SX Turbine Blades

“…The method of misorientation determination was described in Ref. [31]. In addition, the pairs of the topograms obtained from the ABCD and A'B'C'D' surfaces were recorded using the 1 ̅ 13 and 11 ̅ 3 reflections (Figure 4a,b).…”