1. Introduction
1.1 Background
Digital Array Module (DAM) is a crucial component in phased array radar systems. With the rapid development of radar equipment technology in various fields such as vehicle – mounted, airborne, and satellite – borne, DAM components are evolving towards high integration, multi – functionality, and lightweight. As a result, the box bodies of DAM components often feature thin – walled, compact, and precisely fitted structures.
1.2 Traditional Manufacturing Challenges
Traditionally, mechanical machining methods are commonly used to manufacture DAM box bodies. However, this approach presents several challenges. Firstly, machining blind spots may occur, especially for internal features like wire grooves and through – holes in the cavity, which are difficult to access and machine precisely. Secondly, the material removal rate is extremely high, with over 90% of the base material being cut away, leading to significant waste of materials. Moreover, the machining process is time – consuming and labor – intensive, resulting in low production efficiency. Additionally, for features that cannot be machined mechanically, such as some complex internal cavities, electrical discharge machining (EDM) is often required as an auxiliary process. This not only further increases the manufacturing cost but also extends the production cycle.
1.3 Investment Casting as an Alternative
Investment casting, also known as the lost – wax process, is a near – net – shape manufacturing technique with distinct advantages. It offers high material utilization rates, as it minimizes material waste by closely approximating the final shape of the part. The process is capable of producing complex geometries with excellent dimensional accuracy, typically within the range of ±0.1 mm to ±0.05 mm or even better, depending on the specific requirements. Moreover, investment casting can achieve good surface finishes, with surface roughness values (Ra) as low as 0.8 μm to 3.2 μm, reducing the need for extensive post – machining operations. This makes it a promising alternative for manufacturing DAM box bodies, especially when considering the requirements for high precision and complex shapes.
2. Process Analysis of DAM Box Body
2.1 Structural Characteristics
The DAM box body under study exhibits a complex structure with numerous features. It is an aluminum alloy flat plate structure with overall dimensions of 271 mm × 236 mm × 23.1 mm and a mass of 0.75 kg. Internally, it contains eight large and small cavities, which are distributed with various features such as long and short rib plates, convex and concave assembly steps, wire grooves, through – holes, lugs, and screw mounting holes. On the outside, there are two protruding thin – walled mounting plates.
One of the notable characteristics is the presence of multiple thin – walled structures. While the frame outer wall of the box body, which serves as the main load – bearing structure, has a thickness ranging from 5 mm to 10 mm, other components such as the rib plates inside the box, the bottom surface of the cavities, and the two protruding thin – walled mounting plates on the outside are relatively thin, with a thickness of only 1 mm to 1.5 mm. This thin – walled nature results in poor rigidity, making the parts susceptible to stress deformation during machining or casting processes.
In addition to the structural complexity and thin – walled characteristics, the DAM box body also demands high dimensional accuracy. The important dimensions of the box body have a tolerance requirement within ±0.05 mm, while the remaining dimensions are required to be within ±0.1 mm. The back surface of the box body, which serves as a mating surface, has a flatness requirement of 0.1. For the casting process, the dimensional tolerance of the casting is required to meet the CT6 level, and the surface quality (Ra) is controlled within 3.2.
2.2 Process Difficulties
The complex structure and thin – walled nature of the DAM box body pose significant challenges in the casting process. One of the primary difficulties is ensuring complete filling of the mold cavity during casting. The presence of multiple thin – walled sections and complex internal features can impede the smooth flow of molten metal, leading to potential defects such as incomplete filling or misruns. This is especially critical considering the high dimensional accuracy and surface quality requirements of the part.
Another major challenge is controlling the deformation of the casting during cooling. Due to the different wall thicknesses within the box body, the solidification and contraction rates of different parts will vary, resulting in internal stresses that can cause significant deformation. Minimizing this deformation to meet the strict dimensional and flatness requirements is a key concern.
Furthermore, achieving the required surface finish and dimensional accuracy necessitates careful selection and control of the shell material and the casting process parameters. The shell material must possess appropriate properties such as high strength, good thermal stability, and low reactivity with the molten metal to ensure the reproduction of fine details and prevent surface defects. Additionally, the casting process parameters, including temperature, pressure, and casting speed, need to be optimized to achieve the desired metallurgical quality and mechanical properties of the casting.
3. Process Implementation
3.1 Overall Process Route
The overall process route for manufacturing the DAM box body using investment casting is designed to address the specific requirements and challenges of the part. It begins with the preparation of the wax pattern, which is a crucial step in accurately replicating the complex geometry of the box body. The wax pattern is then assembled and invested in a ceramic shell, which provides the necessary support and shape for the molten metal during casting.
After the shell is formed, the wax is removed through a dewaxing process, leaving a hollow cavity in the shell. The shell is then preheated to a specific temperature to ensure proper filling and solidification of the molten metal. The molten aluminum alloy is prepared by melting and refining the raw materials, followed by adding appropriate alloying elements and performing degassing and modification treatments to improve the mechanical properties and quality of the casting.
The molten metal is then poured into the preheated shell under controlled conditions, typically using vacuum suction casting or other appropriate casting methods to ensure smooth filling and minimize the formation of defects. After solidification, the casting is removed from the shell and undergoes a series of post – processing operations, including heat treatment to relieve internal stresses and improve mechanical properties, as well as machining and surface finishing operations to achieve the final dimensional accuracy and surface quality requirements.
3.2 Material Selection
The selection of the appropriate material for the DAM box body is based on several factors, including the required mechanical properties, thermal conductivity, weldability, corrosion resistance, and casting characteristics. Considering the application requirements of the box body, which involves heat dissipation from high – power components and exposure to various environmental conditions, the material needs to have good thermal conductivity and corrosion resistance.
Among the available casting aluminum alloys, Al – Si – Mg series alloys, specifically ZL101A alloy, are chosen for this application. ZL101A alloy offers a combination of desirable properties. It has excellent casting characteristics, allowing for the formation of complex shapes with good dimensional accuracy. The alloy exhibits good thermal conductivity, which is essential for efficient heat transfer from the internal components to the external environment. It is also weldable, enabling the repair of any potential casting defects through welding. Moreover, ZL101A alloy can be heat – treated to enhance its mechanical properties and relieve internal stresses, thereby improving the overall performance and reliability of the casting.
3.3 Structural Process Optimization Design
To address the issue of deformation in the thin – walled sections of the DAM box body, structural process optimization is carried out. This involves adding additional reinforcing features to enhance the rigidity of the thin – walled areas. For example, 2 mm thick vertical ribs are incorporated on the two thin – walled mounting plates on the sides of the box body. These ribs effectively increase the stiffness of the thin – walled structure, reducing the likelihood of deformation during casting and subsequent processing.
3.4 Precision Investment Casting Process Design
3.4.1 Wax Pattern Preparation
The wax pattern for the DAM box body is prepared using a combination of techniques to ensure accurate replication of the complex geometry. The overall structure of the box body, which has a continuously varying wall thickness in one direction, is molded using a compression molding process. For the side walls with multiple wire grooves and holes, a core – pulling mechanism is employed to form these features accurately.
To achieve high – quality wax patterns with good dimensional accuracy and surface finish, a low – temperature wax material is selected. The low – temperature wax offers excellent flowability and filling ability, ensuring that the intricate details of the box body are reproduced precisely. The process parameters for wax injection are carefully optimized to control the shrinkage and deformation of the wax pattern. These parameters include the room temperature, mold clamping pressure, wax cylinder temperature, injection temperature, injection pressure, injection time, holding pressure, holding time, and cooling time. By maintaining a lower injection temperature and higher injection/holding pressures and times, the shrinkage and deformation of the wax pattern can be minimized, thereby ensuring the dimensional accuracy of the final casting.
3.4.2 Shell Preparation
The shell for the investment casting of the DAM box body is made using a plaster – based shell system. The plaster shell offers several advantages, including the ability to accurately replicate the shape of the wax pattern, resulting in a good surface finish of the casting with a surface roughness ranging from 0.8 μm to 3.2 μm. The low thermal conductivity of the plaster shell is beneficial for casting thin – walled sections, as it allows for better control of the solidification process and reduces the risk of defects such as hot tearing. Additionally, the plaster shell has good collapsibility, facilitating the removal of the shell from the casting after solidification.
To further enhance the quality of the shell and the casting, silica sol is added as a binder, and zircon flour and mullite sand are used as refractory materials. The silica sol binder improves the high – temperature strength and creep resistance of the shell, ensuring its integrity during the casting process. Zircon flour, with its low thermal expansion coefficient, high – temperature strength, and chemical stability, is used as the facing refractory material to withstand the thermal shock and chemical reactions during casting, preventing the formation of cracks in the casting. Mullite sand is selected as the backup refractory material to provide the shell with appropriate high – temperature strength, good collapsibility, permeability, and resistance to slag inclusion, ensuring that the shell can be easily removed after casting without leaving residues that could cause defects in the casting.
3.4.3 Aluminum Alloy Refining and Modification Treatment
The aluminum alloy used for casting the DAM box body undergoes a refining process to remove impurities and gases from the molten metal. A rotary injection method is employed for refining, where the aluminum melt is heated to a specific temperature range of (730 ± 10) °C and held at this temperature to ensure complete melting of the aluminum ingots. A rotating lance is then inserted into the bottom of the melt, and argon gas at a pressure of 0.2 MPa is injected into the molten aluminum while the lance is rotated rapidly. This creates a vortex in the melt, which helps to disperse the argon bubbles into fine bubbles and distribute them evenly throughout the melt. The hydrogen in the aluminum melt, due to the pressure difference, is adsorbed by the argon bubbles and floats to the surface, along with other impurities, thereby purifying the alloy. This refining method is efficient, fast, and cost – effective, and it results in a relatively calm molten surface without excessive turbulence.
After refining, the aluminum alloy is subjected to a modification treatment to improve its microstructure and mechanical properties. A combination of 0.15% AlSr10 and 0.2% AlTi5B1 master alloys is used as the modifier. The Al – Sr alloy modifier has the advantages of long – lasting modification effect, low dosage, and effectiveness even after remelting, making it suitable for the long – pouring – time application in investment casting of aluminum alloys. In the ZL101A aluminum alloy, which contains approximately 7% silicon, the addition of strontium (Sr) helps to modify the eutectic silicon structure. Sr adsorbs on the growth interface of silicon, inhibiting the growth of the eutectic aluminum – silicon structure and transforming it from a coarse needle – like shape to a fine fibrous shape. This refinement of the grain structure improves the mechanical properties of the casting, such as tensile strength and ductility.
3.4.4 Pouring and Solidification Process
The pouring process for the DAM box body casting is carried out using vacuum suction casting with a bottom – gated gating system. This method allows for precise control of the filling pressure, ensuring that the molten metal fills the mold cavity smoothly and rapidly, establishing a static pressure head for proper filling. To achieve good feeding and sequential solidification of the casting, several measures are taken. Cold iron is placed at the end of the casting to create a temperature gradient during solidification, promoting sequential solidification from the thinner sections to the thicker sections. Insulation is added at the gate area to delay the solidification of this region, maintaining a sufficient feeding pressure head and a supply of molten metal for feeding the casting. Additionally, internal gates are strategically located at the transitions between thick and thin sections and other potential defect – prone areas to enhance the feeding ability of the casting and prevent the formation of shrinkage cavities and porosity.
The process parameters for pouring are optimized to ensure the quality of the casting. The shell is preheated to 490 °C and held at this temperature for more than 2 hours before pouring. The pouring temperature of the molten aluminum alloy is maintained at 720 °C, and a vacuum of 0.15 MPa is maintained throughout the pouring process to prevent the formation of gas pores and ensure good filling.
3.4.5 Heat Treatment Process
After casting, the DAM box body undergoes a heat treatment process to relieve internal stresses, stabilize the dimensions and microstructure of the casting, and improve its mechanical properties. The heat treatment consists of a solution treatment followed by an aging treatment.
In the solution treatment, the casting is heated to 535 °C and held at this temperature for 12 hours. This allows the alloying elements to dissolve into the aluminum matrix, homogenizing the microstructure. After solution treatment, the casting is quenched in water cooled to 80 °C. It is important to control the water temperature during quenching to prevent rapid cooling, which could lead to cracking of the casting.
The aging treatment is then carried out by heating the casting to 155 °C and holding it at this temperature for 8 hours, followed by air cooling. This process promotes the precipitation of fine strengthening phases in the microstructure, further enhancing the mechanical properties of the casting, such as hardness, tensile strength, and yield strength.
4. Casting Inspection and Quality Evaluation
4.1 Mechanical Properties Testing
The mechanical properties of the investment cast DAM box body are evaluated through hardness testing, tensile testing, and elongation measurement. The hardness of the casting surface is measured using a Brinell hardness tester and found to be 78.8 (HBS). Tensile testing is performed on specimens cut from the casting, and the results show that the tensile strength (Rm) is 290 MPa, and the elongation is 5.5%. These mechanical properties meet the requirements for the intended application of the DAM box body, ensuring its structural integrity and reliability under normal operating conditions.
4.2 Chemical Composition Analysis
The chemical composition of the casting is analyzed using spectroscopic techniques to ensure that it conforms to the specifications of the ZL101A aluminum alloy. The results of the chemical composition analysis are presented in the following table:
| Element | Specification Range (%) | Measured Value (%) |
|---|---|---|
| Al | Remainder | Remainder |
| Si | 6.5 – 7.5 | 6.73 |
| Mg | 0.25 – 0.45 | 0.33 |
| Cu | 0.1 | 0.08 |
| Fe | 0.20 | 0.11 |
| Zn | 0.1 | 0.06 |
| Ti | 0.20 | 0.14 |
| Ni | 0.05 | – |
| Mn | 0.1 | – |
| Sn | 0.05 | – |
| Pb | 0.05 | – |
The measured chemical composition of the casting is within the acceptable range for ZL101A alloy, indicating that the alloying elements are properly controlled during the casting process and that the casting meets the material requirements.
4.3 Microstructure Examination
Microstructural examination of the casting is carried out using optical microscopy and scanning electron microscopy (SEM) to assess the quality of the microstructure and detect any potential defects. The microstructure of the ZL101A casting after heat treatment shows a fine and uniform distribution of grains, with the eutectic silicon phase modified into a fine fibrous structure, as expected from the addition of the modifier. No visible defects such as cold shuts, cracks, shrinkage cavities, or through – porosity are observed in the microstructure, indicating good casting quality.
4.4 Dimensional Accuracy and Surface Roughness Measurement
The dimensional accuracy of the casting is measured using coordinate measuring machines (CMM) to verify that it meets the design requirements. The measurements are taken at various critical locations on the casting, and the results show that the dimensional tolerances are within the specified limits, with the important dimensions having a tolerance within ±0.05 mm and the remaining dimensions within ±0.1 mm.
The surface roughness of the casting is evaluated using surface profilometers. The measured surface roughness (Ra) values are within the range of 0.8 μm to 3.2 μm, meeting the surface quality requirements for the DAM box body. This indicates that the investment casting process is capable of producing castings with good surface finishes, reducing the need for extensive post – machining to achieve the desired surface quality.
5. Conclusion
The research and development of the precision investment casting process for the thin – walled DAM box body have been successfully carried out. Through a comprehensive analysis of the structural characteristics and process requirements of the DAM box body, an optimized process route and process parameters have been established. The selection of ZL101A aluminum alloy as the casting material, combined with appropriate refining and modification treatments, has enabled the production of castings with good mechanical properties and chemical compositions.
The investment casting process, including wax pattern preparation, shell making, casting, and heat treatment, has been carefully designed and controlled to ensure the quality of the casting. The addition of reinforcing ribs in the thin – walled areas and the optimization of the gating system and process parameters have effectively reduced the deformation of the casting and improved its dimensional accuracy and surface quality.