A new investment casting method for manufacturing heat-resistant steel turbine blades for aircraft engines

Dear friends, today I would like to share with you a technical document《Novel approach to investment casting of heat-resistant steel turbine blades for aircraft engines》, which discusses in detail the integral turbine precision casting technology.

Abstract

Defects formed during the investment casting process of aircraft engine turbine blades pose a serious threat to flight safety and significantly increase manufacturing costs. This study used a numerical simulation based on a melt modulus-retaining model to analyze metal flow within the mold shell and determine the probability of a shrinkage defect (PSD) for aircraft engine heat-resistant steel (SCH12) turbine blades. Preliminary simulation and experimental results guided the development of a simulation based on four casting schemes. The goal of this phase was to identify the casting scheme with the lowest PSD. The optimal of the four schemes was then applied to a case where the bottom of the casting system was immersed in cold water. A virtual thermodynamic sensor was also introduced into the simulation to characterize the effect of water depth on the solidification rate and direction in the mold cavity. Finally, the optimal investment casting conditions were applied to the turbine blade production process at a well-established foundry. X-ray analysis demonstrated the effectiveness of the proposed scheme by detecting virtually no detrimental defects commonly found in such castings.

1 Introduction

Turbine blades are key components of aircraft engines. High temperature differences and harsh operating environments can generate complex stresses within the blades. Long-term operation under such conditions can easily lead to cavitation and surface crack formation. Severe defects pose a threat to safety, while the need for regular replacement significantly increases operating costs [1]. Researchers have been seeking ways to improve the resistance of turbine blades. During the investment casting process, the blades can withstand high temperatures while avoiding shrinkage and porosity formation [2, 3]. Investment casting is used in many industries that require high-precision manufacturing of complex shapes and smooth surfaces [4-8]. The process of manufacturing turbine blades by investment casting is very complex, partly because the shrinkage during the casting process is highly nonlinear [1-3]. Most previous research on casting has used a trial-and-error method, which is often imprecise, time-consuming, and costly. One of the main problems is that the flow of molten iron in the mold cavity and the direction of solidification cannot be observed, making it difficult to make reasonable predictions about the formation of casting defects. Figures 1(a)-(d) show common defects in turbine blades manufactured by investment casting, including pores formed between the inner side of the outer ring and the blade due to shrinkage, gaps in the blade due to insufficient material, and overflow of molten iron on the back of the turbine blade. Designers must be able to accurately predict shrinkage during solidification [1-3]. Advances in computer technology have made it possible to use computer-aided engineering software to predict where and how casting defects may form and in what direction during solidification, thereby eliminating many of the costs inherent in traditional trial-and-error methods.

For example, ProCAST computer software was developed to study the dynamic temperature field and shrinkage defects during the casting process [7]. Researchers used the software to observe the flow of molten iron in the mold and predict the location of casting defects [9]. The software was also used to eliminate shrinkage defects in the 200ZJA slurry pump impeller [5]. The software was even used to study the cavity contour of investment casting molds through reverse design methods [10]. Other computer software (ANSYS) was used to study the reduction of residual stress in castings during precision casting [11], and AnyCasting software was used to simulate the semi-solid thixoforming process of A1-Cu-Mn-Ti alloy [12]. Dou et al. [10] used ProCAST to establish the process parameters for investment casting of turbine blades. However, few studies have systematically optimized investment casting and verified the effectiveness of its numerical predictions through experiments. In this study, we performed numerical simulations based on the retained melt modulus (RMM) model to analyze the mold flow pattern and predict the location and manner in which defects may occur during the casting of turbine blades made of heat-resistant steel (SCH12) for aircraft engines.

 This study was divided into four stages: (1) We compared the preliminary casting scheme (Case 0) with the experimental results obtained using the lost wax investment casting process. (2) Then, using the analysis results, four casting schemes were simulated: a top gating system (Case 1), a bottom gating system (Case 2), a side gating system (Case 3), and a side gating system with different sizes and geometries (Case 4). (3) For each of the above casting systems, numerical simulations were performed under five test conditions (i.e., different ceramic shells and casting temperatures). We also simulated the solution (immersing the bottom of the casting system in cold water) applied to Case 4. The optimal casting conditions derived from the simulations were then applied to the production of investment cast turbine blades. Finally, X-ray analysis was performed to evaluate the effectiveness of the proposed solution in avoiding casting defects.