Abstract
The L21 front-end case, with its complex structure and numerous internal cavities, poses significant challenges in casting production. The primary issues encountered are porosity and shrinkage defects, which severely impact the quality of the castings. To address these challenges, a comprehensive process optimization and improvement initiative was undertaken, focusing on optimizing the core venting and pouring systems. This article delves into the structural characteristics, technical requirements, problem identification, root cause analysis, and detailed improvement measures implemented. The results demonstrate a marked improvement in casting quality, with a reduction in porosity defects and an enhancement in surface finish, fulfilling the requirements for mass production.
1. Introduction
The L21 front-end case, a vital component in engine systems, is characterized by its intricate internal structure and stringent quality requirements. Due to its weight (2180 kg) and large dimensions (1800 mm × 1100 mm × 950 mm), the casting process for this component is inherently complex. This paper outlines the casting process optimization undertaken to mitigate porosity and shrinkage defects, resulting in an improvement in casting quality and production efficiency.
2. Structural Characteristics and Technical Requirements
The L21 front-end case, cast from HT300 material, features a complex internal structure consisting of 27 cores distributed across three layers, creating numerous interconnected cavities. The primary wall thickness is 12 mm, with a maximum thickness of 125 mm, leading to significant differences in cooling rates and increased susceptibility to porosity and shrinkage defects. The mechanical properties specified for this component include a minimum tensile strength of 300 N/mm² and a hardness range of 185 to 278 HB.
Table 1: Technical Specifications of the L21 Front-End Case
| Parameter | Specification |
|---|---|
| Material | HT300 |
| Tensile Strength (σb) | ≥ 300 N/mm² |
| Hardness (HB) | 185 – 278 |
| Gross Weight | 2180 kg |
| Dimensions (L x W x H) | 1800 mm x 1100 mm x 950 mm |
| Primary Wall Thickness | 12 mm |
| Maximum Wall Thickness | 125 mm |
3. Problem Identification and Root Cause Analysis
During the initial production runs, the L21 front-end cases exhibited significant porosity defects, primarily manifested as large, smooth-walled pores on the casting surface. These defects, while infrequent in number, were detrimental to the component’s integrity and resulted in a high rejection rate. The primary root causes of these defects were identified as follows:
3.1 Gas Entrapment
As the molten metal enters the sand mold, the intense heat generates gases within the sand and cores. These gases can penetrate the metal-mold interface, forming bubbles that may become entrapped if they fail to escape.
3.2 Turbulent Filling
The original pouring system design featured an upper-gate configuration, resulting in a significant height difference between the pouring basin and the casting cavity. This height differential led to high-velocity metal flow and turbulent filling, increasing the likelihood of gas entrapment.
3.3 Core Ventilation
The complex core arrangement, particularly the large 24# core, hindered effective ventilation and contributed to gas accumulation within the mold.
4. Optimization and Improvement Measures
Based on the root cause analysis, a multifaceted approach was adopted to address the porosity defects and improve casting quality.
4.1 Core Ventilation Enhancements
To mitigate gas entrapment, several modifications were made to the core ventilation system:
4.1.1 24# Core Ventilation Improvement
The 24# core, being the largest and most obstructive, was modified to enhance its ventilation capabilities. The top center of the core was hollowed out to create a gas reservoir, and recycled coke was introduced to further facilitate gas escape.
4.1.2 Additional Core Venting Techniques
For other cores, a combination of nylon vent ropes and withdrawal strings was employed. Nylon ropes were placed along the core skeleton during molding, and withdrawal strings were used to extract the ropes after sand filling, creating stable vent channels.
Table 2: Core Ventilation Enhancements
| Core Number | Ventilation Enhancements |
|---|---|
| 24# | Hollowed-out top center, recycled coke |
| Others | Nylon vent ropes + withdrawal strings |
4.2 Pouring System Optimization
To address turbulent filling and metal entrainment, the pouring system was revamped, focusing on reducing metal flow velocity and ensuring a smooth, laminar filling process.
4.2.1 Bottom Gating and Buffer Design
The original upper-gate system was replaced with a bottom-gating configuration, featuring a horizontal runner system at the casting base. This design significantly reduced the metal flow velocity, as demonstrated by filling simulations.
4.2.2 Introduction of Filtration
To further refine the pouring process, ceramic filters were integrated into the in-gates. These filters not only controlled metal flow velocity but also trapped impurities, enhancing casting cleanliness.
Table 3: Pouring System Modifications
| Modification | Description |
|---|---|
| Bottom Gating | Horizontal runner system at casting base |
| Ceramic Filtration | In-gate ceramic filters for flow control and impurity trapping |
5. Production Verification and Results
After implementing the optimized core ventilation and pouring systems, a series of test castings were produced. These castings underwent rigorous quality inspections, focusing on surface finish, porosity, and mechanical properties.
5.1 Surface Quality and Porosity Inspection
Visual and non-destructive testing (NDT) methods, including radiographic inspection, confirmed a significant reduction in porosity defects. The casting surfaces exhibited a smooth finish, with no visible porosity or shrinkage defects.
5.2 Mechanical Property Verification
Tensile tests and hardness measurements were conducted on sampled castings, confirming compliance with the specified mechanical properties.
Table 4: Mechanical Property Results
| Property | Specification | Measured Value |
|---|---|---|
| Tensile Strength | ≥ 300 N/mm² | 320 N/mm² |
| Hardness (HB) | 185 – 278 | 250 HB |
5.3 Production Stability
Continuous production runs confirmed the stability and reproducibility of the optimized casting process. The porosity defect rate was significantly reduced, and the overall casting quality met or exceeded customer specifications.
6. Conclusion
The comprehensive optimization and improvement of the casting process for the L21 front-end case resulted in a marked enhancement in casting quality. By addressing core ventilation challenges and refining the pouring system, porosity defects were effectively mitigated, leading to a significant improvement in surface finish and mechanical properties. The success of this initiative underscores the importance of a meticulous approach to casting process design and optimization, emphasizing the need for continuous improvement efforts to enhance production efficiency and product quality.