Improving the Machining Process of Thin-Walled Aluminum Alloy Parts

Given the issues of clamping deformation, thermal deformation, and significant residual stress that aluminum alloy thin-walled frame components often encounter during processing, this analysis focuses on the main causes of deformation from the perspective of machining technology. It emphasizes the optimization of process routes and parameters to effectively address these deformation challenges. As a result, processing efficiency is enhanced, and processing costs are reduced.

 

PART. 01 Introduction

Aluminum alloy materials are lightweight, high-strength, and possess good processability, making them widely used in the fundamental components of aerospace applications. The main parachute canopy is an essential part of the simulation cabin in the return capsule of the new generation of crewed spacecraft. Its primary function is to enclose the main parachute bag within the parachute cabin.

The installation location of the main parachute canopy is illustrated in Figure 1. During operation, the main parachute bag pulls out the sling, connecting it to the main parachute canopy via an adapter. When the deceleration parachute separates, the sling of the main parachute bag is gradually extracted from under the heat protection layer, which subsequently pulls the main parachute canopy.

Once the main parachute canopy has moved a certain distance, the force straightens the towing strap of the main parachute pack, pulling it out of the parachute canopy. The towing strap passes through a limiting ring located at the bottom of the main parachute canopy and connects to the adapter seat. This connection stabilizes the canopy’s position during the pulling-out process and prevents it from flipping.

PART. 02 Structural features and processing difficulties of the main parachute canopy

2.1 Structural features

The main parachute canopy, illustrated in Figure 2, is constructed from 5A06-H112 aluminum alloy (forging). Its maximum outer dimensions are 1925 mm × 1230 mm × 200 mm, and it has a minimum wall thickness of 5 mm, with an overall mass of approximately 44 kg. The design takes the shape of a conical sector, featuring curved surfaces both inside and outside, along with bosses and lightening grooves in the reinforcing ribs. This structure qualifies as a thin-walled frame.

The blank dimensions are 2025 mm × 1330 mm × 235 mm (including re-inspection and flaw detection), and its mass is around 1735 kg. It is evident that the structural characteristics of the main parachute canopy include a large volume, low rigidity, complex geometry, a high rate of material removal, and challenges associated with clamping.

 

2.2 Processing Difficulties

The processing of the main parachute canopy presents several challenges due to its structural characteristics. As the milling volume increases during the milling process, the walls become progressively thinner, making them more susceptible to vibration and deformation caused by cutting forces. This exacerbates the difficulty of maintaining processing stability and ensuring quality control.

Additionally, both the inner and outer surfaces of the canopy feature conical arc shapes, along with steps and lightening grooves. Processing both surfaces complicates the selection of an appropriate clamping method, positioning technique, processing sequence, tool model, and cutting parameters. Furthermore, the large material removal allowance results in lengthy processing times, leading to lower overall efficiency.

 

PART. 03 Optimize the processing technology of the main parachute canopy

After analyzing the characteristics of the workpiece, it is determined to follow the principle of mutual reference in the processing steps. First, the inner hole at one end will be processed and used as the positioning reference. Next, the inner hole at the other end will be processed, and then both inner holes will serve as joint positioning references for processing the dimensions of the outer circle. This approach will ultimately result in parts that meet the required specifications.

 

3 Re-planning the processing route

Due to the challenges associated with milling processing the main parachute canopy, we take into account the residual stress, precision, and efficiency. To optimize the process, we focus on various aspects, including the clamping method, process route, tool selection, tool path, and cutting parameters.

 

3.1 Clamping method

The clamping process plays a crucial role in the machining of thin-walled frame parts. Several factors, including the clamping method, position, and direction, can lead to part deformation and inaccuracies.

Currently, the implementation of Model-Based Design (MBD) and full digital design and manufacturing technology is gaining traction in engineering applications. Innovative technologies such as collaborative design, simulation, virtual processing, and virtual verification using three-dimensional models are extensively utilized in high-end equipment manufacturing fields, including aviation and aerospace.

Within the Creo three-dimensional environment, the characteristic surfaces of the main parachute canopy design model are systematically classified and collected. To address the structural characteristics of the canopy, a process step pressure plate is added around the periphery. This design allows the clamping force to apply vertically to the process step pressure plate, effectively solving the issue of the inner and outer surfaces of the main parachute canopy being conical and lacking a positioning reference surface. As a result, this approach significantly reduces clamping deformation of the parts. The process step for the main parachute canopy is illustrated in Figure 3.

The model was analyzed using finite element analysis, where boundary conditions and loads were established accordingly. The boundary conditions included four pressure plates at the process steps and fixed constraints on the inner conical surface. A load of 125.6 N was applied to simulate the stress and deformation of the inner conical surface both with and without support.

Figures 4a and 4b display the stress distribution patterns (cloud diagrams) of the inner conical surface without and with support, respectively. The maximum stress was observed in the center of the bottom of the outer conical surface, with maximum stress values of 108.819 MPa and 150.202 MPa, respectively.

Figures 5a and 5b illustrate the deformation patterns of the inner conical surface without and with support, respectively. Again, the maximum deformation occurred in the center of the bottom of the outer conical surface, with maximum deformation measurements of 1.513 mm without support and 1.001 mm with support. As shown in Figure 5, the deformation decreased by more than 30% after introducing support, and the area affected by deformation was also significantly reduced. The maximum stress values before and after adding support were much lower than the allowable stress of the material (σ 0.2 ≥ 160 MPa).

This indicates that the strength requirements during processing are met, the measures for controlling deformation are effective, and the clamping method used is suitable.

 

 

3.2 Process route

The process route is the key factor in determining whether parts can be processed efficiently while meeting precision requirements. Generally, it should adhere to the principle of prioritizing primary features first, followed by secondary features, and then the reference surface. Given the special shape and high technical requirements of the main parachute canopy, we adopt a process route that involves a small cutting amount, repeated milling, and multiple aging treatments to effectively control deformation.

 

(1) Traditional Processing Technology

The traditional processing route for the main parachute canopy is outlined in Table 1. Rough milling is performed on a five-axis CNC gantry milling machine. However, the significant amount of material removal and the extended equipment operation time lead to low processing efficiency and high costs.

 

(2) Optimized Processing Technology

To address the issues of low efficiency and high costs associated with traditional processing methods, we have leveraged the capabilities of CNC gantry milling machines. The rough milling and semi-finishing processes, which were previously performed on five-axis CNC gantry milling machines, along with the embedded lightening groove processing that was done on horizontal CNC milling machines, have now been adjusted to be carried out on CNC gantry milling machines. The optimized processing route is detailed in Table 2.

 

By comparing Table 1 and Table 2, it is evident that the optimized processing technology offers several advantages over traditional processing methods:

1. Rough milling and semi-finishing milling are now separated from the five-axis CNC gantry milling machine and instead performed using a CNC gantry milling machine. The CNC gantry milling machine is not only significantly more cost-effective than the five-axis machine, but it also provides high power that is suitable for rough milling with substantial material removal.

2. The frequency of using the traditional horizontal CNC milling machine has been reduced. This minimizes the impact of secondary clamping, reference conversion, tool errors, and deformation on processing accuracy.

3. Symmetrical rough openings are achieved in a single clamping process. The overall cavity openings (not fully completed, with approximately 0.1 mm remaining around the edges and about 0.15 mm at the bottom) are blocked. After processing all cavities, the bottom is removed last to minimize structural deformation.

4. Custom grinding head tooling, combined with EDM for root cleaning, is utilized to eliminate issues where the root cannot be cleaned after machining the inner groove of the lightning groove with a disc milling cutter. This approach helps to lower costs significantly.

 

(3) Advantages of the Optimized Processing Route

The optimized processing route is outlined as follows: First, the clamping and positioning reference surface is processed. Next, the inner conical surface and process step are machined. After that, the process step and outer conical surface are processed, followed by sufficient artificial aging, leading to the final finishing stage.

The optimized processing route offers several advantages:

- The large rough processing installation surface ensures reliable clamping and fixation.
- It provides high strength and precise positioning.
- It allows for the removal of more processing allowance.
- The next process can utilize the process step for clamping, with additional auxiliary support added to enhance strength at the local vibration position in the middle.
- Incorporating artificial aging between the rough processing, semi-finishing, and finishing processes helps eliminate cutting stress and effectively controls deformation.
- It enhances processing efficiency.

The actual processing situation after optimization is illustrated in Figure 6.

3.3 Tool selection

During the machining process, high-speed friction between the tool and the workpiece generates cutting heat. The combined effects of cutting force and heat can lead to material rebound, plastic deformation, and vibration. Therefore, when selecting tools, it’s important to consider the shape characteristics of the machining surface, precision requirements, and economic factors. Additionally, different tools should be used based on the specific rough and fine machining allowances to ensure minimal cutting force and high efficiency, which helps reduce part vibration and deformation.

Taking these factors into account, we use a φ63mm alloy milling cutter for rough milling, both φ63mm and φ25mm alloy milling cutters for semi-finishing milling, and the following tools for fine milling: a φ25mm alloy milling cutter, a φ14mm R7mm lollipop ball head milling cutter, a φ16mm alloy milling cutter, and a φ12mm R6mm tungsten steel ball milling cutter.

 

3.4 Tool path and cutting parameters

Based on the characteristics of the parts and our processing experience, milling in a machining center using a circular cutting path from the outside to the inside results in a more uniform distribution of surface residual stress, with relatively lower stress values. Additionally, this method produces less hardening, resulting in a smoother surface morphology and reduced surface roughness. During the finishing process, we employ CNC high-speed machining with principles that include shallow cutting depths, rapid cutting speeds, careful calculation of finishing allowances, selection of appropriate tools, minimizing the removal of large allowances, and lowering stress deformation of the CNC spare parts. The optimized cutting parameters are shown in Table 3.

PART. 04 Summary of optimization points

After comparing the results, we found that process optimization led to an increase in processing efficiency by more than 15% and a reduction in costs by 30%. The specific optimization points are as follows:

1) We utilized Model-Based Definition (MBD) technology for process analysis to thoroughly understand the structure of the main parachute canopy. By selecting CAD/CAM methods based on CNC machining for curved surface parts, we aimed to enhance production efficiency and ensure high-quality task completion.

2) We optimized the positioning and clamping scheme. For curved surfaces and thin-walled parts that are challenging to clamp, we introduced additional process steps at strategic locations to standardize the positioning reference between processes. This was aligned with the design reference to minimize errors caused by misalignment during processes and reduce deformation.

3) We focused on selecting optimal tool paths and refining cutting parameters to maintain accuracy and control deformation effectively.

 

PART. 05 Conclusion

During the processing of the main parachute canopy, we carefully considered the characteristics of the parts and materials to account for stress and deformation. We optimized the process route, tool selection, and processing parameters. This optimization has proven to be effective, enabling the efficient processing of aluminum alloy thin-walled frame parts, with the main parachute canopy serving as a typical example. This approach effectively controls deformation and meets precision requirements.

In subsequent processing phases, we recognized the low material utilization rate and decided to incorporate 3D printing technology. This change significantly improved material utilization and reduced costs while still meeting strength and precision requirements.

 

 

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