Abstract
Railway freight car coupler is the key component to connect vehicles, transfer traction, and ensure the safe operation of trains. This paper focuses on the numerical simulation and process optimization design of investment casting for large complex heavy haul freight coupler, aiming to improve the quality and yield of the coupler castings.
1. Introduction
1.1 Background and Research Significance
Railway transportation plays a vital role in the socio-economic development of China. With the rapid growth of the national socio-economy and the surge in railway freight volume, China’s high-speed and heavy-haul railway transportation has achieved remarkable results. However, the failure rate of couplers, especially the crack faults, has increased year by year due to the increase in traction tons of combined locomotives on the Datong-Qinhuangdao Railway and Shenmu-Huanghua Railway. Therefore, it is of high application value and practical significance to conduct in-depth research on the casting process optimization of freight car couplers to improve their overall quality and ensure train operation safety.
Table 1-1 Main Development Stages of Freight Car Couplers in China
| Time | Model | Material | Characteristics |
|---|---|---|---|
| 1970s-1980s | 13# C-grade steel | – | Suitable for 60t freight cars; adapted to 5000t train marshalling and expanded transportation capacity |
| 1990s | 13A C-grade steel | – | Upgraded version of the 13# coupler; improved safety and reliability |
| 1990 | 16#, 17# C-grade steel | – | 16# is an interlocking rotary coupler; 17# is an interlocking fixed coupler |
| 2004 | 13E, F E+ grade steel | – | Lightweight; good curve negotiation performance; meets export requirements for freight car couplers in different countries |
| 2007 | 13B E-grade cast steel | – | Improved structure of the hook body and hook tongue; uses integral core technology to improve manufacturing accuracy |
| 2007 | Improved 16#, 17# E-grade cast steel | – | Improved hook tail pin hole traction surface structure; increased traction contact area; improved fatigue strength |
1.2 Overview of Coupler Research at Home and Abroad
With the rapid development of railway freight transportation worldwide, various industrially developed countries are actively expanding their railway transportation systems and conducting research and manufacturing of freight cars and key components suitable for their national conditions. The coupler is the core component of railway freight cars, playing a key role in connecting vehicles and transmitting longitudinal forces of trains.
The United States is the first country to independently develop the field of heavy-haul railway transportation. The Association of American Railroads (AAR) standardized freight car couplers are the core products of global heavy-haul transportation. Their coupler technology has a history of more than 100 years, leading the development of freight car couplers for heavy-haul railway transportation worldwide.
2. Numerical Simulation Theory and Experimental Scheme for Casting Process
2.1 Main Functions of ProCAST Software
ProCAST is a professional simulation software for the casting process, which can simulate the mold filling, solidification, stress field, and microstructure of the casting process.
Table 2-1 Mathematical Models for Casting Process Simulation
| Model | Description |
|---|---|
| Mold Filling Process Model | Simulates the flow of molten metal in the mold |
| Solidification Process Model | Simulates the solidification process of molten metal |
| Shrinkage Cavity and Porosity Criterion Model | Predicts the formation of shrinkage cavities and porosity in the casting |
| Stress Field Model and Thermal Crack Criterion | Simulates the stress field in the casting and predicts the occurrence of thermal cracks |
| Microstructure Simulation Model | Simulates the microstructure of the casting |
2.2 Quality Detection Methods for Investment Casting Coupler
Table 2-2 Quality Detection Methods
| Method | Description |
|---|---|
| X-ray Non-destructive Flaw Detection | Detects defects such as cracks and shrinkage cavities in the casting |
| Metallographic Organization Observation | Observes the microstructure of the casting |
| Scanning Electron Microscope (SEM) Detection | Observes the microstructure and defects of the casting at a higher magnification |
| Mechanical Tensile Test | Measures the tensile strength of the casting |
| Hardness Test | Measures the hardness of the casting |
3. Numerical Simulation Analysis and Optimization Design of Investment Casting for Coupler
3.1 Quality Requirements and Defects of Coupler
The coupler, as a critical load-bearing component of high-speed trains, undergoes various complex loads during operation. Defects such as pores, inclusions, and thermal cracks formed during the casting process have a severe impact on the continuity of the material, significantly reducing its strength and plasticity.
3.2 Simulation Analysis of Initial Gating System Scheme for Coupler
The simulation results of the initial gating system scheme showed that the coupler casting did not follow the principle of sequential solidification and simultaneous solidification during the solidification process, resulting in a large area of isolated liquid phase at the end of solidification. Shrinkage cavities, porosity, and crack defects concentrated in key areas such as the intersection of the coupler head and coupler tongue.
Table 3-1 Simulation Results of Initial Gating System Scheme
| Defect Type | Location | Impact |
|---|---|---|
| Shrinkage Cavity | Intersection of coupler head and coupler tongue | Severely affects casting quality and yield |
| Porosity | Intersection of coupler head and coupler tongue | Reduces material continuity and strength |
| Crack | Intersection of coupler head and coupler tongue | May lead to casting failure |
3.3 Optimization Design of Gating System Scheme for Coupler
The horizontal single-side gate gating system was modified to a vertical symmetrical gating system to increase the number of gates and the volume of the gating system. The simulation results of the optimized scheme showed that the shrinkage cavity and porosity defects in the key parts of the casting disappeared.
3.4 Optimization of Pouring Process Parameters
An orthogonal experiment was designed with the volume of shrinkage cavity and porosity and the average value of effective stress at the selected point in key parts as reference indicators, and pouring temperature, mold shell temperature, and pouring time as influencing factors.
Table 3-2 Optimal Pouring Parameters from Orthogonal Experiment
| Pouring Temperature (°C) | Mold Preheating Temperature (°C) | Pouring Time (s) |
|---|---|---|
| 1570 | 425 | 29 |
The range analysis showed that the pouring temperature had the greatest impact on the casting quality, followed by the pouring time and mold shell temperature.
4. Mold Shell Preparation and Pouring Test for Investment Casting of Coupler
4.1 Investment Casting of Coupler
The wax mold and shell of the optimized pouring system were prepared, and the optimal pouring process parameters were used for pouring forming.
4.2 Test Verification and Performance Testing
Random samples were taken for fluorescent magnetic particle flaw detection and X-ray flaw detection, and no cracks, shrinkage cavities, and other defects were found.
Table 4-1 Results of Non-destructive Testing
| Testing Method | Result |
|---|---|
| Fluorescent Magnetic Particle Flaw Detection | No defects found |
| X-ray Flaw Detection | No defects found |
Microstructural analysis and mechanical property tests were conducted on the samples. The results showed that the room temperature tensile strength of the coupler casting samples after heat treatment was between 8501500 MPa, up to 1450 MPa, and the elongation was up to 4.5%. The hardness value of the casting remained in the range of 352375 HBW.
5. Conclusion and Outlook
This paper conducted in-depth research on the numerical simulation and process optimization design of investment casting for large complex heavy haul freight couplers. The optimized gating system and optimal pouring process parameters were obtained through simulation analysis and experimental verification. The research results have high application value and practical significance for improving the quality and yield of coupler castings and ensuring train operation safety.