Abstract
This article presents a comprehensive study on the design, optimization, and manufacturing process of a bridge bracket casting using a 3D-printed (3DP) sand mold in combination with low-pressure casting technology. The focus is on overcoming the challenges associated with casting complex aluminum alloy components with high structural integrity and dimensional accuracy. The study details the casting process design, mold preparation, casting production, and subsequent optimizations to ensure compliance with stringent quality standards.
Introduction
Bridge brackets are critical components used in various industries such as aerospace, automotive, and marine applications, requiring high structural strength, dimensional accuracy, and internal quality. Traditional sand casting methods often struggle to meet these requirements, especially for complex geometries. The advent of 3D printing (3DP) technology for sand mold preparation and low-pressure casting offers a promising solution for the production of such components.
This article discusses the entire process of designing, simulating, optimizing, and manufacturing a bridge bracket casting using a 3DP sand mold. The following sections cover the technical specifications, casting process design, mold preparation, casting production, initial trials, and optimization measures.
1. Technical Requirements of the Bridge Bracket Casting
1.1 Material Specification
The bridge bracket casting is manufactured from ZL114A aluminum alloy (GB/T 1173-2013), which is a high-strength aluminum-silicon alloy suitable for casting applications. The casting undergoes T5 heat treatment to improve its mechanical properties.
1.2 Dimensional and Geometric Requirements
- Dimensions: The casting measures 500 mm × 374 mm × 388 mm with a wall thickness ranging from 9 to 34 mm.
- Tolerance: The dimensional tolerance is specified as DCTG8 (GB/T 6414-2017).
- Geometric Complexity: The casting features a complex structure with hollow design and multiple (weight-reducing cavities) that are only accessible through small process holes .
1.3 Quality Requirements
- Internal Quality: The casting must be free from defects such as porosity, shrinkage, and sand inclusions, especially in critical areas like the shaft holes.
- Inspection: The entire casting must undergo X-ray inspection to ensure compliance with GB/T 9438-2013 Class II standards.
2. Casting Process Design and Simulation
2.1 Casting Process Design
A bottom-pouring system combined with a slit gate design was selected for this casting to ensure smooth filling and controlled solidification. The pouring system consists of:
- Bottom Rectangular Gates: Located at the base, with a height of 45 mm and a sloping design to facilitate metal flow.
- Slit Gates: Four slit gates measuring 35 mm × 20 mm for further distribution of metal.
- Riser and Cold Iron: Thirty cold irons (20 mm thick) placed in thick sections to accelerate cooling and promote directional solidification.
2.2 Simulation of the Casting Process
The casting process was simulated using ProCAST software to predict defects and optimize the pouring system. The simulation settings included:
- Pouring Temperature: 690°C
- Filling Time: 45 seconds
The simulation results showed a smooth filling process without turbulence or splashing. The solidification sequence ensured that the top sections solidified first, followed by the middle and bottom sections, meeting the criteria for directional solidification.
3. Sand Mold Design and Preparation
3.1 Sand Mold Design
A three-part sand mold (upper, middle, and lower boxes) was designed for the casting. Key features of the mold design include:
- Upper Box: Designed with pre-allocated positions for cold irons.
- Middle Box: Contains the core and reinforcing ribs to maintain mold integrity during casting.
- Lower Box: Features positioning pins for accurate mold assembly.
3.2 3D Printing of the Sand Mold
The sand mold was 3D-printed using 100-mesh silica sand with a layer thickness of 0.3 mm. The mold was then coated with a refractory coating and dried at 120-140°C for 4 hours to improve surface quality and durability.
4. Casting Production
4.1 Melting and Alloy Preparation
The ZL114A alloy was melted and refined using a ternary modifier consisting of 62.5% NaCl, 25% NaF, and 12.5% KCl. The alloy was degassed with argon to achieve a low hydrogen content (density equivalent, DI = 0.3%). The alloy temperature was adjusted to 680-690°C before pouring.
4.2 Pouring and Solidification
The alloy was poured into the mold under low pressure (50 kPa) with a filling speed of 45 mm/s. The casting was allowed to solidify under pressure for 600 seconds to minimize porosity and ensure dimensional accuracy.
5. Initial Trials and Results
5.1 Visual Inspection and Dimensional Measurement
Initial castings were visually inspected and dimensionally measured. While most dimensions complied with the specified tolerance, the R217 dimension exceeded the tolerance limit due to core shifting during casting (Table 1).
| Serial No. | Theoretical Dimension (mm) | Measured Dimension (mm) | Tolerance (DCTG8, mm) | Compliance |
|---|---|---|---|---|
| 1 | 465 | 465.8, 466.0, 465.6 | ±1.3 | Yes |
| … | … | … | … | … |
| 6 | SR217 | SR215, SR216, SR216.5 | ±1.0 | No |
5.2 X-ray Inspection
X-ray inspection revealed porosity in the bottom section of the casting despite the use of cold irons.
6. Process Optimization
Based on the initial trials, several optimizations were implemented to improve casting quality:
6.1 Mold and Core Strengthening
- The process hole diameters were increased from φ10 mm to 2 φ20 mm and 2 φ14 mm to improve core strength and prevent floating.
- Additional cold irons (10 pieces, 15 mm thick) were placed at the bottom to accelerate cooling and reduce porosity.
6.2 Sand Mold Modification
The sand mold was modified to distribute the force applied during casting more evenly across the mold sections. The contact surfaces between the lower and middle boxes were adjusted to improve stability.
7. Optimized Casting Production and Results
7.1 Casting Production
Using the optimized mold and pouring system, new castings were produced under the same low-pressure casting conditions. The castings were then subjected to thorough inspection and testing.
7.2 Inspection and Testing Results
- Visual and Dimensional Inspection: The optimized castings showed no visual defects, and all dimensions complied with the specified tolerances (Table 2).
| Serial No. | Theoretical Dimension (mm) | Measured Dimension (mm) | Tolerance (DCTG8, mm) | Compliance |
|---|---|---|---|---|
| 1 | 465 | 465.8 | ±1.3 | Yes |
| … | … | … | … | … |
| 6 | SR217 | SR217.5 | ±1.0 | Yes |
- X-ray Inspection: X-ray inspection confirmed the absence of porosity or other internal defects in the optimized castings.
- Chemical and Mechanical Testing: The castings met the required chemical composition and mechanical properties as per GB/T 1173-2013 standards (Tables 3 and 4).
| Element | Standard Content (%) | Measured Content (%) |
|---|---|---|
| Si | 6.5-7.5 | 6.87 |
| Mg | 0.45-0.75 | 0.53 |
| Ti | 0.10-0.20 | 0.15 |
| Fe | ≤0.2 | 0.16 |
| Al | Balance | Balance |
| Test Specimen | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|
| Test Bar 1 | 316 | 3.0 |
| Test Bar 2 | 303 | 3.0 |
| Test Bar 3 | 312 | 3.5 |
8. Conclusion
The study successfully demonstrated the feasibility of producing high-quality bridge bracket castings using 3DP sand molds and low-pressure casting technology. Key findings include:
- Casting Simulation: ProCAST simulations were instrumental in predicting and minimizing defects during the initial design phase.
- 3DP Sand Mold: The use of 3DP technology enabled rapid and accurate mold preparation, overcoming the challenges associated with complex mold geometries.
- Low-Pressure Casting: The low-pressure casting process ensured smooth filling and controlled solidification, leading to improved dimensional accuracy and internal quality.
- Optimization: Based on initial trials, the casting process was optimized through modifications to the mold design, core strengthening, and additional cooling measures.
The optimized process resulted in castings that met all specified dimensional, chemical, mechanical, and internal quality requirements. This study provides a valuable reference for the production of complex aluminum alloy castings using advanced sand casting technologies.