Research and Optimization of Investment Casting Process for K403 Shell Castings

Abstract

This article focuses on the investment casting process of K403 shell castings. It analyzes the casting defects and technical difficulties, and presents detailed process optimization measures for mold making, shell making, and melting pouring. Through experimental research and improvement, the casting quality has been significantly enhanced, providing a valuable reference for similar casting processes.

1. Introduction

Investment casting is a crucial manufacturing process for producing high-quality and complex-shaped components, especially in the aerospace industry. The K403 shell casting studied in this article is a key part of an aero-engine, and its quality directly affects the performance and reliability of the engine. However, due to its complex structure and strict quality requirements, there are many challenges in the casting process, such as casting defects and dimensional inaccuracies. Therefore, it is necessary to conduct in-depth research and optimization on the investment casting process to ensure the production of high-quality castings.

2. Casting Structure and Technical Difficulties Analysis

2.1 Casting Structure

The K403 shell casting has a complex structure with significant variations in wall thickness. It consists of pillars with a height of 129 mm, a length of 111 mm, a maximum diameter of 32 mm, a minimum diameter of 14 mm, and a wall thickness of 6.5 mm. The complex shape and thickness changes result in numerous hot spots, making it a high-difficulty precision casting without machining allowance.

2.2 Technical Difficulties

  • Difficulty in Mold Removal and Dimensional Deformation: The complex structure makes it challenging to remove the mold, and the dimensions are prone to deformation.
  • Shell Cracking and Defects during Pouring: Due to structural limitations, shell cracking, running out of metal, and burrs are likely to occur during the pouring process.
  • Porosity, Cracks, and Cold Shuts: The presence of multiple hot spots leads to difficulties in filling, resulting in defects such as porosity, cracks, and cold shuts.

3. Casting Process Optimization

3.1 Mold Making

  • Influence of Process Parameters on Wax Mold Quality
    • Wax Material Temperature: If the wax material temperature is too low, the fluidity is poor, leading to cold shuts and insufficient filling in the wax mold. If it is too high, the shrinkage of the wax material increases, resulting in flow marks and depressions in the wax mold. The optimal temperature range is controlled at 55 – 63 °C.
    • Mold Temperature: The mold temperature affects the solidification of the wax mold. A suitable temperature range of 25 – 35 °C is selected.
    • Injection Pressure: Appropriate increase in injection pressure can reduce the shrinkage rate of the wax mold and improve its dimensional accuracy. However, excessive pressure can cause problems such as mold sticking, cracks, and blisters. The injection pressure is set within 15 – 25 bar.
    • Holding Time: A proper holding time can ensure the quality of the wax mold. The holding time is set at 15 – 20 s.
  • Wax Mold Dimensional Control
    • In the initial trial production, the wax part was composed of three parts pressed by the mold and then assembled using a combination fixture. Due to the limitations of the fixture’s positioning, human factors had a significant impact on the dimensional accuracy. After machining, some dimensions were found to be severely out of tolerance. To address this issue, a new integrated mold for the casting blank structure was designed and manufactured, eliminating the potential for dimensional errors caused by splicing.

3.2 Shell Making

  • Shell Structure Optimization for Heat Dissipation
    • To solve the porosity defect caused by poor heat dissipation in some parts after casting, the thick parts of the shell were thinned. After applying the fourth layer of coating, wax was pasted on the upper and lower holes of the casting wax mold, and a soft wax was placed around each group of 8 ingate ports. This operation effectively improved the heat dissipation at the hot spots and reduced the porosity defect.
  • Selection of Shell Making Process Parameters
HierarchySlurryViscositySandingAir Drying / Self – dryingAmmonia DryingVentilation
1Silica Sol – Zircon Powder40 – 50 sWhite Corundum WAF70≥12 h
2Ethyl Silicate Hydrolysate – Shangdian Powder37 – 42 sShangdian Sand 36 Mesh≥20 min10 min10 min
3 – 8Ethyl Silicate Hydrolysate – Shangdian Powder13 – 15 sShangdian Sand 24 Mesh≥20 min10 min10 min
Sealing LayerEthyl Silicate Hydrolysate – Shangdian Powder13 – 15 s12 h
  • Shell Preheating and Pouring Temperature Selection
  • Shell Preheating and Pouring Temperature Selection
    • The shell needs to be preheated before pouring to remove moisture and ash and prepare for hot pouring. The preheating temperature of the shell is generally selected as 950 – 1000 °C. This temperature can effectively reduce the temperature difference between the alloy melt and the mold, slow down the temperature reduction rate, and improve the alloy filling ability. At the same time, a suitable pouring temperature is crucial for ensuring casting quality. For this casting, considering its complex structure, the pouring temperature is selected as 1430 °C ± 10 °C.

3.3 Melting Pouring

  • Pouring System Design for Shrinkage Compensation
    • The pouring system in investment casting not only supports the entire module and shell but also functions as a riser. It should have sufficient strength and ensure the sequential solidification of the casting, allowing for adequate shrinkage compensation and keeping the shrinkage holes in the runner.
  • Selection of Pouring Speed
    • The pouring speed directly affects the heat exchange between the liquid metal and the mold. A slow pouring speed can lead to insufficient casting and cold shuts. Therefore, in production, the pouring speed is increased as much as possible. The selected pouring speed is 2 – 3 s per mold.

4. Effect Verification

After implementing the above process improvements for mold making, shell making, and melting pouring, 40 castings were trial-produced. According to the product’s specific acceptance standards, the metallurgical quality and full dimensions of the trial-produced castings were detected and evaluated. The results showed that 35 castings were qualified, with a qualification rate of 87.5%, indicating that the improved process can effectively be used for the mass production of such parts.

5. Conclusions

  • Mold Making Optimization: For shell castings with complex structures and significant wall thickness variations in investment casting, selecting appropriate mold making processes can obtain high-quality wax molds, effectively ensuring the surface finish and dimensional accuracy of the castings. Using an integrated mold for shell casting wax molds can improve production efficiency and reduce dimensional errors and deformations caused by human factors.
  • Shell Making Optimization: In the shell making process, applying wax to local areas after the fourth layer of coating and thinning the shell locally can improve the heat dissipation effect at hot spots. Selecting appropriate pouring temperatures and speeds can effectively solve problems such as porosity and cold shuts.
  • Overall Significance: The research and optimization of the investment casting process for K403 shell castings have achieved significant results, providing a valuable reference for the development and optimization of similar casting processes in the future. It also emphasizes the importance of continuous improvement and innovation in the casting industry to meet the increasingly stringent quality requirements of high-tech products.

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