Development of Promising Technology for Manufacturing Parts of Gas Turbine Engines

INTRODUCTION

In the context of import phase-out and proactive import substitution, the rate of the technological process becomes crucial. The competitiveness of the finished product will depend on how quickly and efficiently the technological process is implemented.

The use of computer modeling systems for casting in existing production helps, through iterative computer analysis, to predict the occurrence of casting defects at various stages of the technological process, as well as to propose measures and recommendations aimed at minimizing and eliminating defects. Therefore, the domestically developed PoligonSoft computer modeling system was chosen as a tool for casting analysis. This system helps to analyze the hydrodynamic nature of the melt flow, investigate the solidification of the casting block, predict the appearance of porosity in casting products, and simulate the stress-strain state of the casting and elements of the gating and feeding system. [1-9]

Another element of this promising technology being developed is the use of modern manufacturing trends such as 3D printing of products from various materials, which helps to eliminate financial and time costs associated with the setup of equipment. The use of 3D printing for master models of blanks will make it possible to obtain products of the required quality in a short time, which is especially important in conditions of generic manufacturing and manufacturing in small batches. [10-16]

Thus, the use of computer modeling and rapid prototyping constitutes a promising technological process for manufacturing parts of competitive gas turbine engines.

Relevance and scientific significance

Currently, the most important task of competitive engine building is to improve the reliability of gas turbine engines. Therefore, the main efforts should be aimed at improving the manufacturing of the main engine parts, such as turbine blades, since they bear the biggest load and determine the life of an engine. Turbine blades are characterized by complex geometry, as well as strict requirements for the accuracy of geometric parameters and the quality of the surface layer.

A review of the existing manufacturing processes of rotor blades of turbines [17-22] showed that the manufacturing of non-allowance blanks of turbine blades is extremely unstable and is characterized by a significant percentage of waste (approximately 50%). This is primarily due to the instability of the manufacturing process's conditions. A significant part of the total volume of defects is macrostructure discrepancies.

A qualitative assessment of the manufacturing process of turbine rotor blades, determined by the accuracy factor which establishes the relationship between the scattering band and the dimension tolerance [23-24], showed that the manufacturing process of blade castings is unstable and with a small margin of accuracy.

Thus, the relevance of this paper lies in developing measures aimed at reducing waste castings associated with macrostructure discrepancies. The scientific significance of the problem consists of developing reliable mathematical models of the blade casting process, designing complex digital models of copies, which allow virtual modeling and optimization of technological modes, as well as introducing rapid prototyping into the existing manufacturing process of blade blanks.

Theoretical problem statement

The following initial parameters were used for computer modeling of casting turbine rotor blades:

• Melting and pouring in the ПМП-2 (PMP-2) batch pusher-type furnace using the ЖС30-ВИ (ZS30-VI) heat-resistant alloy;
• Dusting material (for ceramic molds) is alumina Al2O3;
• Pouring time is 4 to 6 seconds;
• Pouring temperature is 1510°C;
• Temperature conditions in the corresponding zones of the ПМП-2 (PMP-2) furnace:
  

• Zone I: 1290 ± 10°C;
• Zone II: 1370 ± 10°C;
• Zone III: 1400 ± 10°C;
• Zone IV: 1510 ± 10°C;
• Zone VI: 1510 ± 10°C;
  
• Zone VII: 1450 ± 10°C;
  
• Zone VIII: 1390 ± 10°C;
  
• Zone IX: 1370 ± 10°C.
  

The temperature patterns obtained from solving the problem of the initial heating of the graphite flask were used as the initial thermal conditions for all elements of the computational region.

For layered growth of master models of turbine rotor blades, the authors have chosen the PolyJet technology. It helps to obtain products with an accuracy of ± 0.02 mm by means of ultraviolet curing of the photopolymer material. It is important to note that the thickness of the grown layer is 0.016 mm and the average roughness of the surface layer after growing is Ra = 1.45 ... 3.7 µm, which helps to avoid additional polishing to obtain the required surface finish.

Analysis of experimental research results

Computer modeling of the casting of turbine blades begins with loading the preprocessor (Master module) of PoligonSoft software and positioning the geometric model of the blade so that the Y axis (geometric axis of the blade airfoil) is directed vertically upward, i.e., opposite to the vector of the melt pouring rate.

After the geometric model has been imported, it is important to define its volume and borders. In the future, the volumes will be assigned the properties of materials, and the heat transfer conditions will be assigned at the borders. There are two types of volumes in PoligonSoft systems: casting and molds. Molds can consist of different materials (in our case, it is a flask, filler, or ceramic mold). Castings are marked red and molds are marked blue (see Fig. 1).

Figure 1. Purpose of the geometric model of volume types.

In order to perform correct calculations, it is necessary to set the corresponding parameters for all volumes of the 'Mold' type. In our case, the volume indicator template is redundant and is intended for investment casting. Within the framework of this problem, we are only going to use index #2 'Al2O3-based ceramics' and index #9 'Graphite, density of 1,720 kg/m³'. When simulating the investment casting process, the metal takeover point and the diameter of the poured melt stream must be specified

The target function of computer modeling of manufacturing turbine blade blanks with a monocrystalline structure in the ПМП-2 (PMP-2) discrete-continuous unit was to optimize the existing technological modes that affect the quality parameters of the casting (such as macrostructure and stability of geometric parameters)

The results of computer modeling of casting turbine blades in PoligonSoft software were analyzed in the Mirage postprocessor.

An important component of the analysis is the pattern of filling the mold with metal (see Fig. 2). It occurs sequentially, while the flow is split in the central part of the blade airfoil, as evidenced by the resulting voids. Therefore, when pouring in the presence of oxygen, gas bubbles and, as a consequence, oxide films may appear. When pouring in a vacuum, it is possible to predict the appearance of cold shuts.

Figure 2. Patterns of distribution of melted metal in ceramic molds at different points in time.

Fig. 3 shows the distribution patterns of heat fields on the surface of the casting block during solidification at different points in time, taking into account the thermal interaction of the elements of the furnace – flask – filler – mold – metal – stop system. Due to the presence of a bottom water-cooled crystallizer in the ПМП-2 (PMP-2) casting unit, a directed crystallization front along the blade airfoil and a monocrystal structure may appear.

Figure 3. Patterns of temperature distribution at different points in time.

An important parameter influencing the structure of blade castings is porosity and the possibility to predict porosity during computer modeling. Figure 4 shows the distribution of porosity in the casting. Using this figure, it can be concluded that the casting has a porosity of more than 8% distributed over the entire volume, which is unacceptable, especially in the case of thin-walled blade geometry. A shrinkage cavity forms when the pressure in the melt drops to a critical value [25, 26].

Figure 4. Patterns of porosity distribution in casting.

As already mentioned, the prospects of the technology under analysis lie not only in the use of computer modeling systems for casting processes but also in the introduction of rapid prototyping into the existing production of blade blanks. This involves growing master models using a 3D printer and producing a silicone elastic mold, followed by casting wax model masses into it. Thus, an alternative way of producing wax models of blades is found.

Figure 5 shows the developed technological process of manufacturing turbine blades using rapid prototyping. One of the most important advantages of this innovation is the reduction of financial and time costs associated with manufacturing necessary technological equipment. Among the disadvantages is the low resistance of silicone molds. Depending on the geometric complexity of the product, the resistance can amount to 60-70 models from one mold. Thus, this promising technology will be especially relevant in the context of generic manufacturing and manufacturing of parts in small batches.

Figure 5. Manufacturing of turbine blades using rapid prototyping

Conclusion

The use of the PoligonSoft casting process computer modeling system during the preparation of turbine blades production made it possible to:

• Evaluate the efficiency of the gating and feeding system during the analysis of the melt's hydrodynamics without using expensive experimental melts;
• Perfect the process of obtaining suitable castings of turbine rotor blades of gas turbine engines;
• Perfect methods of predicting casting defects (porosity and macrostructure discrepancies) and develop measures for their detection and elimination.

The use of rapid prototyping helped to reduce the cost of manufacturing 250 wax models by 15% compared to the traditional technological process, as well as to reduce the time spent on average by 5-6 months.

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