Dimensional Stability Solutions for Complex Aviation Aluminum Alloy Structures

This paper addresses the deformation issues that arise during the machining of thin-walled aluminum alloy parts when not subjected to heat treatment for stress relief. It analyzes and enhances the structural characteristics and process control, implementing an effective clamping method to minimize the risk of deformation in these thin-walled components.

 

01 Introduction

Currently, when addressing deformation control in thin-walled aluminum alloy parts, the common approach is to manage deformation by releasing thermal stress. However, in the aerospace industry, certain thin-walled aluminum alloy components cannot undergo thermal stress release or constraint measurement during processing. If traditional methods are applied in these cases, it can lead to deformation and vibration during manufacturing, resulting in a low acceptance rate and poor surface quality of the aviation parts.

This paper presents a specific case study on processing such parts, analyzing their structure, processing challenges, and manufacturability. It outlines anti-deformation measures and explains how to effectively control the deformation of easily deformable thin-walled aluminum alloy components without relying on thermal stress release.

 

02 Local deformation structure control

2.1 Part structure analysis

As illustrated in Figure 1, the upper right corner of the part features numerous reinforcing ribs, which help prevent deformation during processing. In contrast, the lower left corner lacks ribs, and the wall thickness in this area measures only 1.6 mm. This results in an overall material removal rate exceeding 90%.

If a two-step forward and reverse machining method were employed, the upper right corner would exhibit a stronger structure, while the lower left corner would have a significant amount of material removed. This discrepancy would lead to uneven stress distribution between the lower left and upper right corners, ultimately causing deformation in the thin-walled area of the lower left section, which lacks reinforcing ribs.

2.2 Processing technology analysis

The part is made from Al6061 aviation aluminum alloy, which is known for its light weight, high strength, good machinability, and corrosion resistance. However, this material is prone to significant deformation during processing. To minimize the final deformation of the part, it is essential to limit stress release before processing. Since thermal stress release is not an option, controlling deformation throughout the entire process is crucial.

During the machining phase, improper process design or clamping methods can lead to vibrations, elastic deformation, and internal stress due to uneven force distribution. To ensure that the parts remain stable during processing, we focus on three main aspects: reasonable process design, effective anti-deformation clamping design, and high-speed cutting technology.

The process is organized as follows: rough machining of the front and back sides of the part is done prior to aging, followed by natural aging, and then finishing the front and back sides. Given that the part contains local reinforcing ribs, the finishing process primarily utilizes side top clamping methods to enhance stability.

 

2.3 During the roughing process

It is essential to consider several factors, including the cutting depth for each operation, the degree of interference of the parts caused by various external forces, and the different stresses that may develop in the parts during processing. A careful assessment of these elements helps minimize uncontrollable factors.

(1) Fixture processing

Before clamping, it is important to calibrate clamping tools such as vises, jaws, and fixtures to ensure horizontal stability. This calibration helps prevent raw materials from being clamped unevenly, which can lead to forced clamping and deformation of the materials during the process.

 

(2) Clamping position

To minimize the impact of external forces on the raw materials, aluminum alloy raw materials should be securely clamped in their natural state using vise jaws. In the first step, the clamping area is processed to ensure that the front and back of the rough clamping surface are parallel. This careful setup prevents forced clamping and deformation of the raw materials during installation.

 

(3) Front roughing allowance control

Due to the removal of a significant amount of aluminum alloy material, the parts may deform because of internal stress. Therefore, it is necessary to reserve a deformation allowance of 1.5 to 3.0 mm. The front roughing process is shown in Figure 2.

 

(4) The depth of each cut during roughing

Aluminum alloys have relatively low rigidity, which means that if the cutting depth is too great or the processing speed is too high, the internal stress within the part can vary significantly. To minimize internal stress deformation, it is advisable to choose a cutting depth of 1.0 to 2.0 mm. Smaller cutting depths result in a reduced cutting area, facilitate faster heat dissipation, and are less influenced by the thermal stresses generated during the cutting process compared to larger cuts.

 

(5) Control of the roughing allowance on the back side

When roughing the backside (see Figure 3), ensure that the allowance on both sides is uniform and kept within a range of 1.5 to 3.0 mm. Additionally, when clamping the workpiece, make sure it is held in a natural position. The clamping force should not be excessive and should be maintained within a range of 15 to 20 N.

 

(6) Roughing process design

The component is a concave, thin-walled part with a low center and elevated front and back sections. When designing the roughing process, it is important to incorporate auxiliary clamping reinforcement ribs for subsequent fine processing (refer to Figure 4). This will help prevent deformation of the part due to insufficient clamping force in the center during the front and back clamping.

 

2.4 Natural Aging

The ambient temperatures in winter and summer vary, which affects the deformation of parts after natural aging. Depending on the material, roughened parts can be left to settle for 7 to 14 days based on the specific situation. During this time, some internal stress in the aluminum alloy will be partially released. To prevent the overly cold winter environment from hindering deformation, it may be necessary to adjust the placement environment to maintain an ambient temperature above 25°C.

 

2.5 Finishing

In the finishing process, it is also necessary to consider from different angles and minimize uncontrollable factors.

(1) Tool selection

During the finishing process, it’s important to remove the remaining material from the part through careful processing. Using different tools in various areas can lead to varying degrees of deformation in the product. To minimize this risk, try to use the same tool throughout the process. This ensures that the contact surface between the tool and the product remains consistent, which also helps maintain uniform heat distribution. Avoid using different tools on the same continuous surface area whenever possible.

(2) Clamping method for semi-finishing machining

Utilize special fixtures for clamping. Position the part in the fixture as it would naturally sit. Use four parallel blocks to secure the part from the front and back, clamping through the two reinforcement ribs designated for rough machining (refer to Figure 5). The locking screws for the part are designed to be positioned on the left and right sides, aligned with the two reinforcement ribs. Ensure that the locking force is not excessive, keeping it within a range of 15 to 20 N.

 

(3) Control of the positioning surface of datum A

Place the part in the fixture in its natural position and securely clamp it on both the front and back sides using the same clamping method. Choose a small cutting depth of 1.0 to 2.0 mm to machine this surface. Additionally, machine two positioning holes to prepare for the next step of positioning the part. The finishing of the front surface is illustrated in Figure 6.

 

(4) Part placement

The back surface finishing process is illustrated in Figure 7. The part is naturally positioned on the special tooling using the two positioning holes in the green area. The flatness in this natural state is controlled within 0.05 mm.

Next, tighten the eight screws located in the red area. It’s important to avoid tightening a single screw completely before moving on to the next one; the force applied at each locking position must be uniform.

Once the main area of the part is machined, proceed to machine the four pressure blocks and the two screws in the yellow area to secure the finished section until the process is complete.

 

 

(5) Description of the front and back surface finishing

To minimize the tensile stress caused by the cutting tool and the thermal stress generated between the tool and the workpiece during machining, it’s important to maintain a cutting depth of 1.0 to 2.0 mm and a finishing allowance of 0.2 mm. Before each finishing step, the parallelism of the clamping positioning surface must be naturally corrected. This ensures that when the part is secured in the tool, it will not be unnecessarily forced or deformed due to external pressures. Additionally, this practice helps prevent the clamping force from affecting other processing areas of the part during machining.

 

03 Overall deformation structure control

3.1 Part structure analysis

As illustrated in Figure 8, Part 2 does not include any reinforcing ribs. The overall wall thickness of this part is 1.6 mm, and the material removal rate exceeds 90%. Due to improper processing, any area of the part may experience deformation. Consequently, it is challenging to prevent deformation using conventional processing methods.

 

3.2 Process Analysis

The custom aluminum parts is constructed from Al6061 aviation aluminum alloy, which undergoes significant deformation during the machining process. To manage stress variations in the part, we implemented overall process optimization and design. This process was divided into the following steps:

1. Rough machining of the front and back surfaces before aging,
2. Natural aging,
3. Finishing of the front and back surfaces.

Given the risk of overall part deformation, the finishing was conducted using an inverted hanging and suction cup clamping method. The rough machining and natural aging processes were designed to address “Local Deformation Control” as previously mentioned. The following section will focus on the design of the finishing process.

 

3.3 Finishing

During the entire finishing process, careful consideration must be given to the cutting depth, finishing allowance, and the methods used for clamping and locking the component’s positioning surface. This is crucial to prevent excessive external forces from causing deformation of the part and to minimize any uncontrollable factors.

(1) Positioning Surface Treatment: Since the part lacks a suitable positioning surface, it is necessary to reserve five auxiliary bosses for reverse hanging during the roughing process. Before finishing, the surfaces of these bosses should be made parallel to ensure that the part does not deform under stress caused by the tightening force of the screws during reverse hanging. The reverse hanging position is illustrated in Figure 9.

 

(2) Reverse Lifting Clamp

As illustrated in Figure 10, the reverse lifting clamp is securely locked onto Special Tooling A (translucent blue) by five positioning surfaces of the part being processed (yellow). This connection forms Assembly B. Next, Assembly B is installed on the positioning workbench or another compatible special tooling.

Due to significant vacant areas on the lower surface of the part after the reverse lifting, we control the depth of each cut on the upper surface to a maximum of 1.0 mm. This limitation helps prevent excessive cutting depths that could lead to vibrations on the part’s surface. Additionally, the two positioning holes required for the next step are processed at this stage.

 

(3) Suction cup clamping

Suction cup clamping is illustrated in Figure 11. First, remove the part from the inverted lifting fixture and use the suction cup to hold it securely. Position the part naturally into the special fixture through the two positioning holes indicated in green. Simultaneously, gently screw the six screws located in the red position into the surface of the part.

Next, turn on the suction cup air pump switch but do not tighten it yet; instead, proceed to tighten the six screws. Once the main area of the die casting parts has been processed, place six yellow pressure plates at the front and back of the part to secure the completed area. Continue processing until the task is finished.

 

04 Conclusion

After machining the irregular thin-walled aluminum alloy part according to the specified process plan, a strict inspection was conducted to ensure compliance with the drawing requirements, and the test results met all specifications.

This article presents two case studies: one focusing on local deformation and the other on overall deformation. The aim is to enhance the process by examining aspects such as product structure, process route, processing parameters, and clamping methods. Preliminary stress relief was achieved through rough machining followed by natural aging. Natural machining of the clamping locating surfaces was performed prior to finishing to maintain the accuracy of these critical surfaces. During the machining process, various clamping methods—including side lifting, reverse lifting, and suction cups—were implemented to effectively manage product deformation. This article offers valuable technical insights regarding process innovation and control, particularly for the manufacturing of irregular thin-walled aviation parts that cannot undergo thermal stress relief.

 

 

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