Abstract
This paper presents the casting process design and manufacturing of a bridge bracket using 3D printed (3DP) sand molds and anti-gravity low-pressure casting technology. A bottom-pouring and slit-gating system was designed, and the casting process was simulated using simulation software. Based on the simulation results, the casting process was optimized, and the 3DP sand mold was designed. The production of qualified castings was successfully completed using the anti-gravity low-pressure casting method. This study demonstrates the feasibility and advantages of using 3DP sand molds and simulation techniques in the production of complex aluminum alloy castings with high structural and dimensional accuracy.
1. Introduction
In modern industries such as weaponry, aerospace, and shipbuilding, aluminum alloy castings are required to have complex structures, high dimensional accuracy, and excellent internal quality. Traditional sand gravity casting methods often struggle to meet these high-quality and high-precision requirements. Low-pressure casting, with its advantages of smooth filling, dense microstructure, and low impurity content, has emerged as a modern specialty casting technique in the current casting field.
3D printed (3DP) sand molds enable moldless casting and rapid sand mold production, overcoming the challenges of complex shape castings, such as difficulty in demolding, molding complexity, low dimensional accuracy, and high mold costs. Casting simulation technology can simulate the temperature field during the forming process of castings, predict the locations of shrinkage porosity and shrinkage cavities in advance, and shorten the trial production cycle.
In this study, the casting process design and simulation of a bridge bracket casting were carried out, followed by optimization. Qualified castings were successfully produced using the 3DP sand mold low-pressure casting process. The following sections detail the process design, simulation, optimization, and production process.
2. Process Design and Simulation of Bridge Bracket Casting
2.1 Technical Requirements of the Casting
The bridge bracket casting has a complex structure. The material used is ZL114A according to GB/T 1173-2013, with T5 heat treatment applied. The external dimensions are 500 mm × 374 mm × 388 mm, and the wall thickness ranges from 9 to 34 mm. The dimensional tolerance grade is DCTG8 according to GB/T 6414-2017. The mass of the casting is 27 kg.
The internal quality requirements at the shaft hole are high, with no shrinkage porosity, shrinkage cavities, or sand inclusion defects allowed. The casting has a complex structure with a hollow design underneath the base. There are two weight-reducing cavities at the bottom, which are only connected to the outside world through four φ20 mm and four φ10 mm process holes. X-ray inspection is required for the entire casting, and the internal quality must meet Class II of GB/T 9438-2013.
2.2 Casting Process Design and Simulation
A low-pressure casting process with counter-gravity was adopted. This process ensures smooth filling and solidification under pressure, reducing defects caused by uneven flow and improving the internal quality of the casting. The gating system structure.
To prevent deformation of the casting, two process tie bars were designed at the top. The pouring system uses a bottom-pouring and slit-gating method. The dimensions of the four slit gates are 35 mm × 20 mm, and the dimensions of the four slag traps are φ80 mm × 400 mm. There are 12 rectangular gates at the bottom, located on the internal ribbing, with a gate height of 45 mm, a small end width consistent with the wall thickness of the bottom internal ribbing, and a slope of 8°. The bottom cross-gates have widths of 90 mm and 45 mm, a height of 50 mm, and 30 chill blocks with a thickness of 20 mm are placed in thick areas to provide a chilling effect on the casting.
The casting process was simulated using ProCAST simulation software, with a pouring temperature of 690 °C and a filling time of 45 s. The temperature field during the filling process. It can be observed that the alloy liquid enters the mold from the bottom rectangular gates, and the filling process is smooth with no splashing or turbulence.
The simulated temperature field distribution during the solidification process, positions A and B, which are at the top of the casting and equipped with chill blocks, have a high degree of undercooling and solidify first. The next to solidify are the shaft hole positions at both ends, which also have chill blocks. that the upper end has the lowest temperature, followed by the middle, and the bottom has the highest temperature. The bottom gates solidify last, achieving the designed sequential solidification.
The locations of predicted shrinkage porosity and shrinkage cavity defects in the casting, as simulated. These defects are located at the top process tie bars and the bottom surface. There is slight shrinkage porosity near the bottom gates, but the process is generally feasible, and a trial production was conducted based on this gating system.
3. Sand Mold Design and Printing
3.1 Sand Mold Structure
The sand mold adopts a three-box structure, comprising an upper box, a middle box, and a lower box,. Positions for the chill blocks are pre-reserved in the upper box, and the formed chill blocks are later fixed to the sand mold using a casting binder. The sand core is a single unit located in the middle box, ensuring both the dimensional accuracy and strength of the casting. The middle box is designed with two reinforcing ribs to ensure sand mold strength and prevent deformation during sand core drying. The lower box is designed with four positioning cores to ensure sand mold positioning. The casting shrinkage rate is set at 1%, with a sand mold positioning undercut slope of 8° and a fit clearance of 0.5 mm, and a sand consumption of 35 mm.
3.2 3D Printing of the Sand Mold
The sand mold was printed using 3D printing technology, with 100-mesh silica sand as the printing material and a layer thickness of 0.3 mm. the printed sand mold.
4. Casting Production
The chill blocks were sandblasted and dried, then fixed to the upper box using a casting binder. The printed sand mold surface was brushed with paint twice and dried at a temperature of 120-140 °C for 4 hours. Finally, the sand mold was assembled for pouring. During the melting process, the alloy liquid was modified using a ternary modifier of 62.5% NaCl + 25% NaF + 12.5% KCl at 730 °C and refined with argon gas at 720 °C. The hydrogen content of the aluminum liquid was evaluated using the density equivalent DI, where DI = (1 – ρ2/ρ1) × 100%. The denser the sample, the more compact it is, with lower porosity and a lower density equivalent. Conversely, a higher density equivalent indicates higher porosity. After refining, the density equivalent was 0.3%. The alloy liquid temperature was adjusted to 680-690 °C for counter-gravity low-pressure pouring, with a filling speed of 45 mm/s and a filling pressure of 50 kPa, and a crystallization time of 600 s. The casting underwent T5 heat treatment, with solution treatment at 535 °C for 12 hours followed by water cooling at 80 °C, and aging at 160 °C for 6 hours.
Visual inspection of the castings revealed local wall thickness deficiencies at the bottom cavity. Analysis showed that the reduced-weight core of the middle box was only connected to the main sand mold through process holes. The A and B sand cores had low strength and broke or floated during pouring due to the impact of high-temperature alloy liquid, resulting in local wall thickness deficiencies in the castings. Subsequent consultation with the design team led to an increase in the diameter of the process holes to enhance sand core strength.
The internal cavity bottom structure of the casting. The three-dimensional model’s internal cavity bottom is a flat surface. However, the trial-produced casting’s internal cavity bottom is not flat, with a height difference of 2 mm. This could be due to the breaking of the middle box sand core during pouring, causing the sand core to float and misalign, resulting in an uneven casting bottom surface. Analysis of the assembly method between the lower and middle boxes, revealed that during pouring, the impact force from the metal liquid caused the lower box to transmit all the upward force through contact surface A to the middle box sand core, leading to sand core breaking and floating. Subsequent improvements to the sand mold structure were made to distribute the upward force on the lower box to both surfaces B and C of the middle box instead of solely through surface A.
The main dimension measurements of the castings are shown in Table 1. The theoretical dimension SR217 mm had actual measurements of SR215 mm, SR216 mm, and SR216.5 mm for the three castings, with the SR217 dimension location . The cause of the dimension SR217 mm being out of tolerance is consistent with the uneven internal cavity bottom of the casting, attributed to the breaking and floating of the middle box sand core.
Table 1: Main Dimension Measurement Results
| Seq. | Theoretical Dimension (mm) | Actual Dimension (mm) | Tolerance (DCTG8) (mm) | Out of Tolerance? |
|---|---|---|---|---|
| 1 | 465 | 465.8, 466.0, 465.6 | ±1.3 | No |
| 2 | 270 | 270.6, 270.4, 270.7 | ±1.1 | No |
| 3 | 374 | 374.5, 374.3, 374.6 | ±1.1 | No |
| 4 | 9.5 | 9.3, 9.6, 9.4 | ±0.5 | No |
| 5 | 14.5 | 14.8, 14.9, 14.8 | ±0.55 | No |
| 6 | SR217 | SR215, SR216, SR216.5 | ±1.0 | Yes |
The chemical composition of the furnace-attached specimens is shown in Table 2, meeting the requirements of GB/T 1173-2013. The structure of the furnace-attached specimens. The mechanical properties of the furnace-attached specimens are shown in Table 3, meeting the standards of GB/T 1173-2013.
Table 2: Chemical Composition of Furnace Attached Specimens
| Element | Si | Mg | Ti | Fe | Al |
|---|---|---|---|---|---|
| Standard Content (%) | 6.5~7.5 | 0.45~0.75 | 0.10~0.20 | ≤0.2 | Remainder |
| Actual Content (%) | 6.87 | 0.53 | 0.15 | 0.16 | Remainder |
Table 3: Mechanical Properties of Furnace Attached Specimens
| Test Bar | Tensile Strength (MPa) | Elongation (%) | Standard Requirement | Result |
|---|---|---|---|---|
| 1 | 316 | 3.0 | ≥290 | Pass |
| 2 | 303 | 3.0 | ≥290 | Pass |
| 3 | 312 | 3.5 | ≥290 | Pass |
The X-ray inspection results of the casting. The casting was inspected using X-rays, and no shrinkage porosity, shrinkage cavities, or cracks were found at the shaft hole and mounting boss. However, there was shrinkage porosity at the bottom surface of the casting. This area has a wall thickness of 14.5 mm with multiple gates, leading to local overheating and resulting in shrinkage porosity. This can be improved by placing chill blocks to increase undercooling.
The inspection results of the first three trial-produced castings are shown in Table 4. The castings exhibited local wall thickness deficiencies, the theoretical dimension SR217 was out of tolerance, and there was shrinkage porosity at the bottom surface, not meeting the standard requirements. Further optimization of the process and sand mold was needed.
Table 4: Detection Results of the First Trial-Produced Castings
| Inspection Item | Result |
|---|---|
| Appearance and Size | Local wall thickness deficiencies, bottom surface uneven, theoretical dimension SR217 out of tolerance |
| Chemical Composition | Conforms to GB/T 1173-2013 |
| Mechanical Properties | Conforms to GB/T 1173-2013 |
| Internal Quality | Shrinkage porosity at bottom surface |
5. Process Optimization and Practice
Based on the production practice and results analysis, process optimizations were made in terms of casting structure, addition of chill blocks, and sand mold structure. The process hole dimensions were modified, with the four φ10 mm holes changed to two φ20 mm and two φ14 mm holes, enhancing sand core strength. The other four φ20 mm process holes remained unchanged, resulting in a final configuration of six φ20 mm and two φ14 mm holes. The optimized sand core structure for the weight-reducing cavities.
Ten chill blocks were added to the bottom surface, with a thickness of 15 mm each. The optimized process was again simulated using simulation software, and the predicted locations of shrinkage porosity and shrinkage cavities. After adding chill blocks to the bottom, the defects disappeared.