Manufacturers widely use aluminum alloy cylinder castings, renowned for their lightweight and high-strength properties, to produce critical structural components such as underwater equipment.

The dimensional accuracy of their internal cavities, internal quality, and mechanical properties directly impact product reliability.

In underwater environments, these cylinder components endure prolonged exposure to complex conditions including water pressure, corrosion, and vibration.

Any dimensional deviation within the cavity can compromise the overall sealing performance of the equipment.

Defects such as internal porosity or inclusions can have the same effect.

In addition, these dimensional deviations and internal defects may induce stress concentration.

This can lead to premature structural failure and, in severe cases, threaten operational safety.

Limitations of Conventional Forming Technologies for Aluminum Alloy Cylinders

However, existing forming technologies have significant shortcomings:

Traditional sand casting relies on manual or mechanical core making.

This makes it difficult to precisely control complex internal cavity shapes.

As a result, dimensional deviations often occur in the internal cavities of castings.

Even when mold forming is used, uneven sand core shrinkage or unstable metal flow during filling frequently causes localized dimensional deviations exceeding tolerances.

Manufacturers correct dimensional deviations by performing post-processing through machining or manual grinding to remove excess material.

This approach increases material waste and extends production cycles.

Repeated processing may also compromise the surface integrity of castings.

As a result, manufacturing costs rise indirectly. Consequently, this method fails to meet the demands for efficient and low-cost production.

Advantages of 3D-Printed Sand Cores 

3D printing technology provides significant advantages through sand cores. It builds these cores using the principle of layered deposition.

This technology enables rapid fabrication of complex internal cavity cores without relying on traditional molds.

This process drastically shortens core preparation cycles.

The digitally controlled printing process achieves an overall dimensional accuracy of ±0.1 mm. It also provides uniform shrinkage characteristics.

This effectively eliminates dimensional fluctuations caused by manual operations in traditional core-making processes.

Integrating this technology with differential pressure casting utilizes the pressure differential between upper and lower tanks to drive smooth metal filling.

This approach reduces air entrapment and oxidation during metal flow while promoting the filling of internal shrinkage cavities and porosity defects through pressure action.

The result is dense castings with a density exceeding 99.5%, significantly enhancing their mechanical properties.

Deformation Mechanisms and Research Gaps

However, cylindrical castings feature a “two-end caps + central barrel” structure with pronounced wall thickness variations: barrel walls typically measure 8–15 mm, while the transition zone between caps and barrel reaches 20–30 mm.

This thickness disparity creates temperature gradients during solidification:

Thinner sections cool faster and solidify earlier, while thicker sections cool slower and solidify later.

As these regions contract asynchronously, thermal stresses develop.

• Heat Treatment–Induced Stress Accumulation and Dimensional Distortion

During subsequent heat treatment, uneven thermal expansion occurs across different sections during solution annealing.

The disparity in cooling rates during quenching further exacerbates stress accumulation.

This ultimately leads to deformation issues such as casting warping and excessive ovality.

For some large-diameter cylindrical castings, deformation can reach 3–5 mm, far exceeding design tolerance limits.

Therefore, addressing deformation issues is crucial for enhancing product quality, holds significant research value, and serves as the core motivation for this study.

• Limitations of Current Research on Integrated Casting and Core Technologies

Although progress has been made in 3D-printed sand cores, differential pressure casting, and forming control of cylindrical castings, the following shortcomings remain:

First, collaborative process research between 3D-printed sand cores and differential pressure casting is relatively weak.

Existing studies predominantly focus on sand core printing accuracy or on differential pressure casting parameter optimization.

Researchers often study these two aspects in isolation. Consequently, they do not fully integrate their respective technical characteristics.

For instance, engineers do not adjust parameters such as filling pressure and holding time in differential pressure casting.

They do not take into account the permeability of 3D-printed sand cores.

Engineers also overlook the high-temperature strength properties of these cores.

This oversight prevents them from fully leveraging the combined advantages of both technologies in complex core production and casting densification.

As a result, casting defects often occur.

These defects are typically characterized by “incomplete core collapse” or “insufficient internal shrinkage compensation.”

 Second, for large-diameter cylindrical castings (diameter ≥500 mm), the stress evolution mechanism and precise control technology across the entire casting-heat treatment process remain unclear.

• Gaps in Full-Process Stress Control and Multi-Dimensional Optimization

Existing research primarily focuses on stress analysis of individual processes.

It lacks a systematic investigation of the entire sequence: molten metal filling, solidification cooling, solution heating, and quenching cooling.

This prevents precise identification of critical stress peak locations, making it difficult to simultaneously ensure dimensional accuracy and internal quality.

Thirdly, there is a lack of systematic optimization approaches spanning materials, processes, and structural design.

Engineers often limit existing optimizations to single aspects, such as adjusting the casting alloy composition or refining heat treatment processes.

They do not consider the interrelationship between material composition, process parameters, and structural design.

This results in limited optimization effectiveness, making it difficult to meet the stringent demands for high precision and reliability in castings for fields like underwater equipment.

This paper addresses these challenges by focusing on the integrated technology of 3D-printed sand cores and differential pressure casting.

It delves into deformation control and precision enhancement mechanisms during the forming process of aluminum alloy cylindrical castings.

By establishing a process-structure co-optimization system, it provides theoretical and technical support for solving the precise forming challenges of large-diameter cylindrical castings.

Experimental Subjects and Methods

• Experimental Subjects

The study focuses on an existing aluminum alloy cylinder.

Figure 1 details the cylinder structure, showing external dimensions of φ550 mm × 770 mm and a main wall thickness of 12 mm.

The inner cavity features main annular ribs measuring 10 mm × 15 mm (width × height).

The annular ribs at both ends, designed to bear higher loads, measure 20 mm × 30 mm (width × height).

The overall structure combines load-bearing capacity with lightweight characteristics.

      ♦ Material Selection and Alloy Property Requirements

The cylinder body material is selected as ZAISi7MgY aluminum alloy.

This alloy possesses excellent casting properties and mechanical stability, making it the preferred material for critical structural components in various equipment.

Its chemical composition, including elements such as Si, Mg, and Y, must strictly comply with specifications.

Its mechanical properties, such as tensile strength, yield strength, and elongation, must also meet the requirements.

All of these must conform to the GB/T 12499.4 “Aluminum and Aluminum Alloy Castings” standard.

      ♦ Dimensional Inspection Method and Tolerance Criteria

Engineers use 3D scanning technology to inspect the dimensional accuracy of the cylinder’s inner cavity.

To ensure comprehensive and accurate inspection results, the distribution of inspection data points must include no fewer than 72 points.

Additionally, the dimensional tolerance on each side must meet the following control criteria:

1. At least 80% of inspection points must fall within the -0.3 to +0.8 mm deviation range.

2. No more than 20% of inspection points may fall within the +0.8 to +1.4 mm deviation range.

      ♦ Analysis of Dimensional Deviations under the Existing Casting Process

Under the current casting process, the mold value inspection results for the cylinder’s inner cavity are shown in Figure 2, with the statistical distribution of mold value point cloud deviations presented in Table 1.

Among these, points with mold value deviations between -0.3 and +0.8 mm account for 59.79%, those between +0.8 and +1.4 mm account for 35.85%, and those between +1.4 and +2.48 mm account for 4.35%.

It is evident that the internal cavity mold dimensions fail to meet product specifications, primarily due to localized dimensional deviations caused by deformation during the heat treatment process.

• Experimental Materials and Equipment

Technicians melted the alloy using a 150 kg resistance furnace.

Raw materials included high-purity aluminum (99.99 wt%), 99.95 wt% magnesium ingots, AITi5 intermediate alloy, and 1101 metallic silicon.

After melting, technicians cast the alloy using a differential pressure casting system.

Additional raw materials used during the experiment included 3D-printed molding sand, Dalinsand, resin, and curing agents.

Technicians fabricated sand cores via 3D printing, while they prepared the outer molds using Dalinsand.

The casting process employed a differential pressure pouring system.

By precisely controlling the pressure differential between the upper and lower tanks, stable metal filling was achieved, minimizing defects such as entrapped gas and oxidation.

Materials for sand molds and cores included 3D printing-specific molding sand, Dalinsand, phenolic resin, and corresponding curing agents.

Technicians produced complex internal cavity cores via 3D printing to ensure dimensional accuracy, while they used resin sand mixed with Dalinsand for the outer molds to balance mold strength and compliance.

• Experimental Plan

This study focuses on achieving precise forming of aluminum alloy cylinder castings as its core objective.

Centering on a “process-structure” co-optimization approach, it addresses the challenge of heat treatment deformation in cylinders by designing a multi-dimensional optimization scheme.

Through adjusting heat treatment process parameters and incorporating structural stiffeners in a combined configuration, precise deformation control is achieved.

1. Optimization of Heat Treatment Process Parameters

To mitigate thermal stresses caused by temperature gradients during heat treatment, engineers optimized the solution treatment process parameters across four dimensions:

First, reducing the initial charging temperature to prevent thermal shock from rapid heating of castings;

Second, slowing the heating rate to control temperature uniformity;

Third, engineers extend the isothermal holding time to ensure complete dissolution of alloying elements and achieve a uniform microstructure.

Fourth, they lower the quenching cooling rate to reduce stress accumulation during cooling.

2. Axial Reinforcement Ribs

To enhance axial stiffness and suppress bending deformation during quenching, engineers implemented the following structural optimization on the vertical cylinder wall:

First, they removed redundant material to reduce stress concentration points and reserved a 25 mm × 20 mm (width × height) gap gate to ensure unobstructed metal flow during casting.

Then, longitudinal stiffeners are constructed along the cylinder’s axial direction.

The supporting function of these stiffeners enhances overall structural stability, disperses axial thermal stresses, and reduces axial deformation.

3. Radial Stiffeners

To address stress concentration in the annular region of the cylinder’s inner cavity, engineers radially distribute four 10 mm × 25 mm (width × height) stiffeners around the annular section of the cavity.

This design utilizes the mechanical constraint effect of the ribs to:

Distribute thermal stresses arising from wall thickness variations during casting solidification; 

Cushion structural stresses during heat treatment processes (solution treatment and quenching).

This prevents excessive local stresses that could cause ovality exceeding specifications or localized concave deformation, achieving comprehensive stress control throughout the entire process.

Experimental Results and Analysis

• Optimization of Heat Treatment Process and Results Analysis

1. Adjustment Plan for Solution Treatment Process Parameters

This experiment aims to reduce internal thermal stress in aluminum alloy cylinders.

It also seeks to ensure compliance with mechanical property standards.

To achieve this, temperature parameters were systematically adjusted during the solution treatment stage.

The specific plan is as follows:

Lowered the initial charging temperature from the conventional maximum of 300°C to below 150°C to mitigate thermal shock caused by rapid heating of castings;

During the heating phase, a stepwise temperature control approach was adopted.

First, the cylinder was heated at 80°C/h to 400°C, followed by a 2-hour soak to equalize temperatures between the interior and exterior.

Next, heating continued at 60°C/h to 535°C, which is the optimal solution treatment temperature for ZAISi7MgY aluminum alloy.

Finally, an 8-hour soak was applied to ensure complete dissolution of the alloying elements.

During quenching, the cooling medium temperature is adjusted to 70°C to slow cooling rates and reduce thermal stress accumulation.

Finally, thermal aging treatment is performed to eliminate residual stresses.

The optimized solution treatment process curve is detailed in Figure 3.

This curve enables precise matching of temperature changes with casting thermal response, thereby suppressing deformation at the process level.

2. Analysis of Thermal Stress Generation Mechanism

During solution treatment of cylinder castings, thermal stress generation is directly related to temperature gradients:

If the heating rate is too rapid, the outer surface of the cylinder experiences direct furnace temperature exposure and heats up quickly, exhibiting expansion tendencies.

Meanwhile, the core experiences delayed thermal conduction, resulting in slower temperature rise and minimal expansion.

This core expansion constrains the surface’s free expansion, leading to compressive stress on the surface and tensile stress within the core.

Similarly, during quenching cooling, the outer surface rapidly exchanges heat with the high-temperature medium, causing a sharp temperature drop and contraction tendency.

The core, cooling more slowly with delayed contraction, impedes surface shrinkage.

This creates a constraint relationship where the surface experiences tensile stress and the core experiences compressive stress.

Further analysis reveals that the cooling rate is the key factor influencing thermal stress and deformation magnitude.

Faster cooling increases the temperature difference between the cylinder’s interior and exterior.

This leads to more significant stress accumulation and, ultimately, greater deformation.

Research indicates: During the initial quenching phase, thermal stress exceeds the material’s yield strength, causing irreversible plastic deformation in the cylinder.

In the middle to late quenching stages, temperature reduction lowers thermal stress below yield strength, resulting predominantly in elastic deformation that transforms into retained residual stress.

This residual stress gradually shifts from an “external tensile-internal compressive” pattern to an “external compressive-internal tensile” pattern.

Therefore, this experiment aims to suppress plastic deformation and residual stress accumulation by reducing thermal stress through a lower temperature gradient.

This is achieved by increasing the quenching medium temperature and slowing the cooling rate.

These adjustments ensure that no secondary phases precipitate in the alloy during quenching.

As a result, the decline in mechanical properties is prevented.

3. Mold Value Inspection Results and Analysis After Process Optimization

For the cylinder castings processed using the optimized technique described above, 3D scanning technology (with ≥72 inspection data points) was employed to inspect the mold values of the inner cavity.

The results are shown in Figure 4, with the statistical distribution of mold value point cloud deviations presented in Table 2.

Data indicates: 67.35% of inspection points fall within the -0.3 to +0.8 mm range, 29.05% within +0.8 to +1.4 mm, and 3.5% within +1.4 to +2.48 mm.

Compared to the product technical requirements (‑0.3 to +0.8 mm ≥ 80%, +0.8 to +1.4 mm ≤ 20%), the optimized shape accuracy showed significant improvement over the traditional process.

However, it still failed to meet the qualification standards.

These results indicate that relying solely on single-process optimization methods has limited effectiveness in controlling cylinder heat treatment deformation.

Examples of such methods include reducing the solution treatment heating rate and slowing the quenching cooling rate.

This approach cannot fully resolve deformation challenges.

Further structural optimization measures are required to achieve coordinated deformation control through a “process-structure” synergy.

• Design and Experimental Analysis of Axial Rib Structures

1. Axial Rib Design Scheme

This study builds upon optimized heat treatment parameters, including lowering the charging temperature, controlling the heating rate, and slowing the cooling speed.

It further enhances the cylinder’s deformation resistance by designing axial ribs.

These ribs are based on structural optimization principles, as illustrated in Figure 5.

By utilizing the removal process of the gap sprue on the vertical cylinder wall, the design eliminates redundant material while retaining a 25mm x 20mm (width x height) gap sprue structure.

This allows the region to naturally form longitudinal stiffeners distributed along the cylinder axis after casting.

This design significantly enhances the overall structural rigidity of the cylinder through the mechanical support provided by the stiffeners.

Specifically addressing the axial bending deformation prone to occur during the quenching cooling stage, the stiffeners distribute axial stress and constrain the tendency for axial deformation.

This synergistic approach of “process optimization + structural reinforcement” further suppresses heat treatment deformation.

2. Mold Value Inspection Results and Analysis

For the cylinder casting with added axial stiffeners, 3D scanning technology (inspection data points ≥72) was employed to perform mold value inspection of the inner cavity.

The results are shown in Figure 6, with statistical data on the deviation distribution of the mold value point cloud presented in Table 3.

Test data indicates that adding axial stiffeners significantly improves the forming accuracy of the cylinder body compared to the optimized single heat treatment process:

(1) The proportion of inspection points with dimensional deviations within the acceptable range of -0.3 to +0.8 mm reached 76.11%, an increase of 8.76 percentage points from the 67.35% achieved by the optimized single heat treatment process.

(2) The proportion of inspection points with dimensional deviations within the +0.8 to +1.4 mm tolerance range was 20.63%.

Although this represents a decrease of 8.42% compared to the 29.05% achieved by the single heat treatment process optimization, it remains slightly above the 20% technical upper limit.

(3) The proportion of inspection points with dimensional deviations exceeding +1.4mm was 3.25%, essentially unchanged from the 3.5% achieved with the optimized single heat treatment process, showing no significant exceedance.

These results indicate that axial ribs effectively reduce axial bending deformation during the quenching stage of heat treatment.

They achieve this by enhancing the overall stiffness of the cylinder body. This demonstrates a significant improvement in deformation control.

However, due to the ineffective dispersion of radial stresses, a small number of inspection points still exceeded the required deviation range of (+0.8 to +1.4) mm.

Further optimization of the radial structure is necessary to achieve comprehensive deformation control across all dimensions.

• Radial Rib Structural Design and Experimental Results Analysis

1. Radial Rib Structural Design Scheme 

Building upon optimized heat treatment processes and the addition of axial ribs, this experiment further incorporates four radially distributed ribs at the annular rib location within the cylinder cavity to control deformation.

The structure is illustrated in Figure 7.

This design aims to mitigate deformation caused by uneven structural distribution.

It does so by dispersing thermal stresses during casting solidification.

It also disperses structural stresses during heat treatment processes, specifically during solution heat treatment and quenching.

2. Mold Value Inspection Results and Analysis

Mold value inspection was conducted on the cylinder casting with added radial ribs, with results shown in Figure 8.

The statistical distribution of mold value point cloud deviations is presented in Table 4.

The results indicate that the specimen featuring both axial and radial ribs exhibits significantly superior forming accuracy compared to specimens without ribs or with only axial ribs.

In this specimen, 93.34% of the mold deviation points in the inner cavity fall within the range of -0.3 to +0.8 mm.

Meanwhile, 6.66% of the points fall within the range of +0.8 to +1.4 mm.

This result validates the effectiveness of adding six radial ribs at the annular ribs within the cylinder cavity, based on the optimized heat treatment plus axial ribs.

The ribs effectively disperse thermal solidification stresses during casting. They also disperse structural stresses from various heat treatment stages.

This helps mitigate deformation caused by uneven structural distribution.

• Comprehensive Analysis of Experimental Results

Comparing the core metrics of the three experimental schemes—“Single Heat Treatment Optimization,” “Heat Treatment + Axial Ribs,” and “Heat Treatment + Axial + Radial Ribs”—the following conclusions can be drawn:

(1) Limitations of Single Heat Treatment Optimization:

Controlled heating rates and slow cooling reduce thermal stress, increasing the proportion of dimensional deviations within the -0.3 to +0.8 mm range from 59.79% to 67.35%.

However, this approach fails to address stress concentration caused by insufficient structural rigidity.

The deviation of the cylinder’s internal cavity profile remains below technical requirements, confirming the limitations of single heat treatment optimization.

(2)Synergistic Effects and Shortcomings of “Heat Treatment + Axial Ribs”:

Combining stress reduction through process optimization with axial stiffness reinforcement increased the proportion of points within the -0.3 to +0.8 mm range to 76.11%.

This combination reduced axial bending deformation to 0.4 mm, essentially eliminating it.

However, uncontrolled radial stress resulted in 20.63% of points falling within the +0.8 to +1.4 mm dimensional deviation range.

This slightly exceeds the 20% upper limit requirement.

These results indicate the need for multidimensional shape control through multidirectional structural optimization.

(3) Technical Effectiveness of “Heat Treatment + Axial + Radial Reinforcement”:

This approach employs a combined strategy of “stress reduction via heat treatment + deformation resistance through dual-direction reinforcement + dimensional stability through thermal aging.”

It achieves a 93.34% proportion within the -0.3 to +0.8 mm range and a 6.66% proportion within the +0.8 to +1.4 mm range, fully meeting technical requirements.

It also further ensures dimensional stability through thermal aging.

Its core value lies in overcoming the limitations of single measures through synergistic optimization: reducing stress generation at the process source while enhancing overall cylinder stiffness through structural design.

This approach balances process feasibility with mass production demands.

It provides a reliable technical pathway for the precise forming of large-diameter aluminum alloy cylinders with uneven wall thickness.

Conclusion

(1) Fundamental Role of Heat Treatment Process Optimization:

Engineers reduced the initial charging temperature from no more than 300 °C to below 150 °C.

They implemented a stepwise heating profile and adjusted the quenching medium temperature to 70 °C.

These measures effectively minimized the temperature gradient between the casting surface and core.

Engineers eliminated residual stresses through thermal aging, which reduced deformation during cylinder heat treatment and established a process foundation for subsequent structural optimization.

(2) Shape Control Effect of Rib Structure Design: Four radial tie rods (15mm x 15mm cross-section) were uniformly distributed along the circumferential ribs in the cylinder’s inner cavity.

Engineers constructed eight longitudinal tie rods (20 mm × 15 mm cross-section) along the cylinder’s axial direction using the sprue gate reserved by the gap sprue, enhancing the overall structural rigidity of the casting.

They dispersed casting thermal stresses and controlled localized deformation within 0.5 mm, mitigating deformation issues caused by uneven structural distribution in the cylinder body.

(3) Precise forming effect through multi-technology synergy:

Engineers added radial and axial reinforcement tie-rod structures using 3D-printed sand cores and resin sand molding processes.

They combined this with thermal aging to eliminate residual stresses, controlling the final cylinder inner cavity profile deviation within specified limits and achieving precise forming of the aluminum alloy cylinder body.