Production of The Gas Turbine Exhaust Valve Shell With Complicated Shape by Investment Casting

With the continuous development of aircraft engine technology, there is an increasing demand for precision casting size accuracy and casting quality, as well as longer service life requirements.

This project focuses on the precision casting process of the exhaust valve housing of a certain type of gas turbine.

1. Structural characteristics of the shell

The following figure is a structural diagram of a certain type of gas turbine exhaust valve housing casting.

The shell has a complex shape and uneven wall thickness, with the thinnest being 3.5mm and the thickest being 24.8mm; There are also 3 ribs and protrusions inside; There are also protrusions and ears on the outside, which have different spatial angle distributions and are prone to shrinkage and porosity. Especially the holes on the two internal protrusions need to ensure their coaxiality during the casting process, as their spatial distribution structure increases the difficulty of the casting process. Two urea cores are used to form the internal space shape, and the external space shape is directly formed by the mold, so two sets of urea core molds and one set of external molds are required.

2 Shell precision casting process test

2.1 Wax mold pressing process

Use a high-pressure wax press to inject wax molds. The mold material is filled with American F28-44B wax material, which has stable performance, good fluidity, good coating and hanging properties, low ash content, and the pressed wax parts have a small shrinkage rate [1,2]. By inspecting the surface quality of the pressed wax mold and adjusting the wax mold parameters, the reasonable molding process for the shell is determined as follows: mold clamping force of 50MPa, mold material temperature of 60-70 ℃, injection pressure of 0.8-2.0MPa, and holding time of 300-600s; After batch pressing of wax molds, it was found that the molding parameters and process are stable and can meet the requirements for use.

2.2 Design of pouring system scheme

When designing the pouring system, the following factors are mainly considered [3]: ① Consider the filling problem of the shell, especially ensuring the filling of the thin-walled and various convex platforms and ear positions; ② The pouring system can meet the shrinkage needs of castings, especially ensuring the shrinkage of the two internal protrusions; ③ The pouring system facilitates the operation of processes such as shell making, cutting, and cleaning; ④ The pouring system should establish an exhaust system to avoid poor exhaust during the casting process. The combination scheme adopts two methods: shell folding (top injection type) and shell vertical (mixed top injection and bottom injection type), as shown in the following figure.

2.3 Shell making process

For the exhaust valve housing of a gas turbine, it is required that the expansion rate of the shell be low, the deformation resistance be strong, and the shell space be prevented from twisting due to expansion and deformation; At the same time, the shell should have good chemical and thermal stability to prevent the shell from catching fire during pouring. When making the shell, in addition to using ethyl silicate for the reinforcement layer

Except for the hydrolysis solution, all other layers are coated with silica sol, with a total of 10 layers, and then another layer of slurry is hung. The first layer uses corundum powder and hangs 80-100 mesh fine corundum sand; The second layer uses corundum powder with a coating viscosity of 15-20 seconds, hangs 40-60 mesh corundum sand, and air dries for more than 3 hours; The final shell making determined through experiments

2.4 Determination of Melting Process Parameters

2.4.1 Preheating of Type Shell

The preheating method of the shell adopts a higher preheating temperature and a preheating method wrapped in insulation felt with good insulation effect. After testing, the preheating temperature of the shell is determined to be 980 ℃, and the insulation time is greater than 4 hours. Moreover, the wrapping position of the shell has an impact on the cooling and solidification sequence and shrinkage effect of the module after casting. In order to achieve the sequential solidification and shrinkage effect of the casting group, a layer of insulation felt with a thickness of 10-15mm is wrapped around the sprue and runner.

2.4.2 Testing of Pouring Process

ZG1Cr17Ni3 alloy has good casting performance, low density, good plasticity, and good structural stability. A 50kg medium frequency induction furnace is used for melting. The pouring temperature is (1630 ± 10) ℃, and the pouring speed is 3-5 seconds.

2.5 Statistical results of castings

Three batches (a total of 19 pieces) of shell castings were cast using a top injection combination scheme, and no defects such as insufficient casting, cold insulation, cracks, or shrinkage were found. Only four shells had some inclusions or pores at the internal lower convex platform, which were all qualified after polishing and welding.

3 Conclusion

Reasonable precision casting process parameters were determined by precision casting the exhaust valve housing. Solved issues such as insufficient watering, cold insulation, and shrinkage of complex exhaust valve housings

Problem: Produce a gas turbine exhaust valve housing with metallurgical quality and dimensions that meet technical standards and design drawing requirements.