CNC Machining Aluminum 6063-T6 vs. 6063-T651 in Rapid Prototyping: Differences and Mitigating Flatness Issues

Introduction

In rapid prototyping, achieving dimensional stability and precise tolerances is paramount. When machining aluminum alloys like 6063, understanding the subtle differences between tempers is crucial to preventing issues like stress-induced warping, especially on large, thin-walled, or flat components. The flatness of a part is highly susceptible to these internal stresses.

Part 1: The Key Difference Between 6063-T6 and 6063-T651

Both 6063-T6 and 6063-T651 start with the same base alloy (6063) and undergo the same fundamental heat treatment: solution heat-treated and artificially aged (the "T" designation). This process gives them their high strength.

The critical difference lies in the additional mechanical processing after heat treatment, indicated by the extra digit in the "T651" temper.

• Aluminum 6063-T6:

• Process: After the T6 heat treatment, the extruded bars or plates are cooled. This cooling can introduce significant internal (residual) stresses within the material. These stresses are locked in because the outer surfaces and core of the material cool and contract at different rates.

• Characteristics: T6 material has high strength but can have an unpredictable and non-uniform stress state.

• Aluminum 6063-T651:

• Process: After the T6 heat treatment, the material undergoes an additional step: ****Stress-Relieving by Stretching. The product is stretched a precise amount (typically 1-3% of its length) permanently. This stretching process plastically elongates the material, bringing the internal stresses to a near-zero and uniform level.

• The "1" in T651 specifically indicates that the material has been stress-relieved and is suitable for machining.

• Characteristics: T651 material has virtually the same mechanical properties (yield strength, tensile strength) as T6 but with a dramatically improved and predictable internal stress profile.

Part 2: Why This Difference Matters for Machining and Flatness

When you remove material during CNC machining, you alter the balance of these internal stresses. Think of the material as a compressed spring held in equilibrium.

• Machining 6063-T6: Removing material distorts the part as the unbalanced internal stresses are relieved unevenly. This is the primary cause of warping, twisting, and poor flatness after machining, especially once the part is unclamped from the machine bed. The distortion is often unpredictable.

• Machining 6063-T651: Because the internal stresses are minimal and uniform, the material removal has a much smaller effect on the part's stability. The part is far more likely to remain flat and dimensionally stable after machining and unclamping.

Part 3: How to Avoid Stress-Induced Flatness Issues

Here is a combined strategy, starting with material selection and followed by machining best practices.

1. Material Selection: The First and Most Important Step

• For Critical Flatness: Always choose 6063-T651 (or its equivalent, like 6061-T651) for rapid prototyping parts where flatness is a key requirement. The higher initial cost is negligible compared to the cost of scrapped parts and lost time. It is the most effective way to mitigate the root cause of the problem.

2. Machining Strategies (Essential for both materials, but critical for T6)
If you must use 6063-T6, these strategies are non-negotiable:

• A. Symmetrical and Balanced Material Removal:

• Principle: Avoid removing material from only one side, which creates a new stress imbalance.

• Practice: Machine both sides of the part in sequential operations. For example, rough out one side, flip the part, rough out the second side, then return to the first side for finishing, and finally finish the second side. This "semi-finish, flip, semi-finish, finish, flip, finish" approach allows stresses to redistribute evenly.

• B. Avoid Over-Clamping:

• Principle: Excessive clamping force can "pre-stress" the blank, forcing it flat against the machine table. When unclamped, the internal stresses of the material combine with the released clamping stress, causing the part to spring back to a warped state.

• Practice: Use just enough clamping force to secure the part. Use strategic clamping points and consider using soft jaws machined to match the part contour for better force distribution.

• C. Use a Multi-Step Roughing and Finishing Process:

• Roughing: Remove the bulk of the material, leaving a small, consistent stock allowance (e.g., 0.5-1.0 mm). This allows the majority of stress redistribution to occur before the final dimensions are cut.

• Finishing: Take light final passes (e.g., 0.1-0.2 mm depth of cut) to cleanly remove the remaining stock. Light finishing passes generate less heat and cutting force, minimizing the introduction of new stresses.

• D. Optimal Tooling and Feeds/Speeds:

• Sharp Tools: Always use sharp, uncoated or ZrN-coated carbide tools. Dull tools generate excessive heat, which can thermally distort the part and induce new thermal stresses.

• Climb Milling: Use climb milling (down milling) whenever possible. This technique shears the material cleanly and pulls the part down against the table, promoting stability and a better surface finish.

• Coolant: Use a flood coolant or, at minimum, a compressed air blast to keep the part cool and evacuate chips. Managing heat is critical.

• E. Stress Relieving Before Final Machining (For T6 only):

• If you have a block of 6063-T6, you can send it out for professional stress relief before you start machining. This is an extra step that effectively turns T6 into a condition similar to T651.

Summary Table

Conclusion:

For rapid prototyping where time, cost, and first-time success are critical, specifying 6063-T651 is the single most effective decision you can make to prevent stress-related flatness issues. This should be combined with sound machining practices like symmetrical machining and light finishing passes to ensure the highest quality and dimensional stability of your aluminum prototypes.