How to Prevent Deformation and Residual Stress in CNC Milling Aluminium?

The Impact of Residual Stress in CNC Milling

Aluminium alloys offer excellent ductility and thermal conductivity, which makes them ideal for high-speed machining. However, their crystalline structure is prone to work hardening and thermal deformation.

During machining, the removal of material can cause internal stresses to release unevenly. This often results in part deformation, twisting, or even cracking, especially in thin-walled or large-surface-area components.

4-Stage Strategy to Minimise Residual Stress

We break the process into four key stages:

• Stage 1: Rough machining with a 0.5 mm allowance on one side, adjusting the allowance according to the part size and structure.
• Stage 2: Annealing to remove stress, eliminating up to 90% of historical stress.
• Stage 3: Finishing using a stress-reducing machining strategy.
• Stage 4: Deep cryogenic aging after finishing to lock dimensional stability.

Stage 1: Rough Machining

1.1 Optimize Cutting Parameters

Use dynamic roughing (e.g., φ12 flat-bottomed milling cutter, radial cutting width 1.5 mm, axial cutting depth 25 mm, feed rate 3500 mm/min) to reduce heat buildup.

Ensure tools are sharp to lower cutting forces and reduce material tensile stress.

Apply dynamic machining from the centre outward to help reduce stress generation more effectively.

Stage 2: Stress-Relief Annealing

Purpose: This method offers the best balance between mechanical strength and stress relief. It can reduce measured residual stress from 350 MPa to below 50 MPa.

2.1 Heating Control

Maintain a heating rate of ≤ 100°C/h to avoid thermal stress, especially for thin-walled parts.

Keep a spacing of ≥ 50 mm between parts to ensure uniform furnace gas flow.

2.2 Holding Phase

Holding time = thickest part dimension (mm) × 1.5 min/mm. (For example, a 30 mm thick part requires 65 minutes.)

Use nitrogen protection to prevent oxidation and discoloration. Oxygen content should be < 100 ppm.

2.3 Cooling Specifications

Air cooling is strictly prohibited. Cooling must be done inside the furnace at ≤ 30°C/h until the temperature drops below 150°C. Faster cooling can trigger thermal stress and cause springback.

For thick parts (>50 mm), use segmented cooling. The cooling rate between 250°C and 150°C must not exceed 15°C/h.

• Application Scope: Suitable for 6061, 7075, and other high-strength forged alloys.
• Professional Tip: If you’re using cold-rolled or forged plates or bars, it’s recommended to anneal the raw material before rough machining.

Stage 3: Finishing with a Stress-Reducing Machining Strategy

3.1 Optimize Finish Machining Parameters

• Reduce cutting depth and feed rate during the finishing stage.
• Use high-speed machining (HSM) to minimise heat generation.
• Keep tools sharp to lower cutting force and avoid material pull.

3.2 Select Appropriate Tool Geometry

• Use tools with a smaller tip radius to reduce lateral cutting pressure.
• Variable helix end mills help dissipate vibration and lower local stress.
• Avoid worn tools because they generate more heat and force, particularly when cutting soft aluminium.

3.3 Re-Evaluate Clamping Methods

Improper fixturing can introduce additional stress. Instead:

• Use the minimum clamping force needed for part stability.
• Apply soft jaws or vacuum fixtures to reduce pressure on finished surfaces.
• Re-clamp between operations to release accumulated clamping stress.

Stage 4: Deep Cryogenic Aging for Stress Conversion and Locking

Three-step cryogenic cycle (should be performed within 4 hours after finishing):

Repeat the entire cycle 3 times. Total process time is about 12 hours.

Final Results

• Surface residual tensile stress is transformed into compressive stress (exceeding -150 MPa).
• Dimensional stability: < 8 μm per 100 mm.

Case Study: Preventing Deformation of Thin-Walled 7075 Aluminum Brackets

• Industry: Aerospace Instruments
• Part: 7075-T6 aluminium honeycomb-shaped mounting bracket
• Dimensions: 160 mm × 130 mm × 23.85 mm
• Wall Thickness: 2.5 mm

Problem:

Post-processing deformation measured at 0.2 mm. Positional accuracy was off by 0.12 mm, failing to meet flatness and tolerance requirements.

Initial Process:

• Material: 7075-T6 aluminium sheet
• Precision machining is performed directly after full-depth roughing.

Improvement Plan (Stress Control Measures)

• 1. Leave a 0.5 mm allowance on one side after roughing. Perform annealing within 48 hours. Stress is in a ‘sub-stable’ state, so the efficiency of stress elimination is twice as high as if treated later. Reduced stress effectively.
• 2. Add semi-finishing to allow intermediate stress release. Increased dimensional stability.
• 3. Use variable helix tools with a low radial cutting angle to reduce cutting force. Improved surface quality.
• 4. Re-clamp before final finishing. Reduced clamping-induced stress.
• 5. Apply deep cryogenic treatment to lock residual stress. Converted tensile stress to compressive.

Results:

• Final flatness deviation: < 0.02 mm;
• Positional accuracy: < 0.03 mm;
• Rework and scrap rates reduced by 95%.

Summary and Implementation Recommendations

Preventing deformation in aluminium CNC machining is fundamentally about managing stress, from raw material to final product. Key takeaways:

• Source Control: Use mechanical stretching or high/low-temperature cycling to pre-condition critical components.
• Process Optimisation: Combine sharp tools with high-speed, shallow cutting strategies to avoid thermal stress.
• Tooling Innovation: Develop adaptive fixtures to minimise clamping-induced stress.
• Stress Prediction: Build simulation models to estimate residual stress in batch parts and adjust cutting parameters in advance.

There’s no one-size-fits-all approach to stress control in aluminium machining. But by understanding stress behavior and applying targeted strategies, you can keep deformation within acceptable limits.

Precision CNC Machining for Stress-Sensitive Aluminium Parts

WayKen excels in CNC machining aluminium components and parts, including thin-walled and complex geometries prone to deformation. With expertise in stress control, advanced fixturing, and precision finishing, we ensure exceptional dimensional stability and surface quality. Contact us today for expert DFM advice and a free quote.