Multiphysics modeling of dendritic thermomechanical deformation during the directional solidification of nickel-based single-crystal superalloys

Abstract

Nickel-based single-crystal (SX) superalloys are the key metallic materials of aeroengines. However, thermomechanical deformation always occurs during the directional solidification of SX superalloys, negatively influencing the SX structure. Casting deformation is simulated in most of the previous studies, whereas the direct simulation of dendritic thermomechanical deformation has been largely ignored, resulting in a lack of comprehensive understanding of this process. In this study, we systematically investigate dendritic thermomechanical deformation with a model coupled with dendrite growth, fluid flow, and thermomechanical deformation behavior. Results reveal that the dendritic thermomechanical deformation-induced dendrite bending is not randomly distributed but is mainly concentrated on the casting surface. The dendritic thermal stress increases as dendrite grows and accumulates after dendrite bridging. Transverse thermal contraction mainly occurs at the edge of casting in the corner, and axial thermal contraction is larger than transverse contraction. The high-stress region of the primary dendrite trunk is mainly distributed below the dendrite bridging near the solidified part, and the stress along the transverse direction reaches its maximum value on the casting surface. Stress concentrated on the casting surface is mainly attributed to variations in transverse temperature gradients caused by heat dissipation on the lateral mold wall, and inconsistent constraints in the lateral mold walls.

1. Introduction

Nickel-based single-crystal (SX) superalloys are the main metallic materials used in the hot component of an aeroengine due to their excellent high-temperature mechanical properties, such as creep and fatigue resistance [1–5]. An SX superalloy is mainly prepared through directional solidification with grain selection or seeding method [6–7]. However, thermal stress and deformation can be induced due to the nonuniform temperature distribution during directional solidification [8–9]. Thermal stress plays an important role in microstructure formation, and a large thermal stress can lead to solidification defects [10–12].

The thermomechanical deformation of SX casting has been widely investigated in previous studies. Aveson et al. [13] studied the thermomechanical deformation of SX casting during solidification through experiments and simulations; their work shows that large thermal stress is mainly located at the starter block of blade casting. Xu et al. [8] simulated the thermal stress of SX blade casting and found that large thermal deformation and stress are located at the body and platform of blade casting. According to the as-solidified microstructure, it was proposed that the large thermal stress on the surface of the casting causes the mechanical deformation of dendrites near the surface [14–15]. The dendrite deformation is also recently observed by in-situ or ex-situ experiments [9,16–18]. In situ synchrotron radiation experiments have demonstrated that dendrite trunks are deformed during directional solidification [17–18]. Chen et al. [19] analyzed the microstructure of complex-shape castings of nickel-based SX superalloys and observed dendrite deformation, but their thermal stress model, which was based on a continuous medium, could not clearly explain dendritic deformation. As thermal stress simulations are modeled on the basis of macroscopic continuous media, they cannot provide comprehensive information on dendritic thermomechanical deformation.