CNC Machining Aluminum 6061-T6 vs. 6061-T651 in Rapid Prototyping: Differences and Mitigating Flatness Issues

In the world of rapid prototyping, speed, precision, and cost-effectiveness are paramount. Aluminum 6061 stands as the undisputed champion of CNC machining materials, prized for its excellent strength-to-weight ratio, good machinability, and widespread availability. However, when specifying this alloy, engineers and machinists encounter two common tempers: 6061-T6 and 6061-T651. While often used interchangeably, understanding their subtle yet critical differences is essential for preventing costly issues, particularly when flatness is a non-negotiable requirement.

The Common Ground: Aluminum 6061

First, it's important to note that both T6 and T651 tempers start from the same base alloy. The "T" designation indicates that the material has been solution heat-treated and artificially aged to achieve higher strength. Both tempers share identical mechanical properties:

• Tensile Strength: ~45 ksi (310 MPa)

• Yield Strength: ~40 ksi (275 MPa)

• Hardness: 95 Brinell

For many prototypes—brackets, enclosures, mechanical components—where ultimate strength is the primary concern, the two tempers are functionally equivalent. The critical distinction lies not in their composition or final strength, but in the additional processing step that defines the T651 temper.

The Crucial Difference: Stress Relieving

• 6061-T6: After the solution heat treatment and aging process, T6 material is typically stretched just enough for handling (around 1-2%). This leaves a significant amount of internal (residual) stress locked within the material. These stresses are a natural byproduct of the quenching process, where the outer surface cools faster than the core.

• 6061-T651: This temper undergoes an additional, critical step. After heat treatment, the material is stress-relieved by stretching. It is pulled to a permanent set of 1-3%,
  which plastically deforms the material and effectively neutralizes a majority of the internal stresses. The "1" in T651 specifically denotes that it has been stress-relieved.

Why This Matters for Flatness in Rapid Prototyping

In rapid prototyping, parts are often machined from plate or bar stock. The inherent residual stress in 6061-T6 is the primary culprit behind flatness issues. When a stressed block of material is fixtured on a CNC machine and material is removed from one side, the internal stress equilibrium is disrupted. The material will warp or twist to find a new state of balance, much like a bi-metallic strip bends with temperature changes.

This manifests in several ways:

1. Warping After Machining: A part that measures perfectly flat on the machine table may warp noticeably once it is unclamped.

2. Difficulty Achieving Tight Tolerances: Attempting to machine a large, thin-walled or flat plate from T6 material can be a frustrating exercise, as the part will move during the machining process itself.

3. Increased Scrap Rates: For critical applications, this unpredictable movement can lead to parts falling outside flatness tolerances, resulting in wasted time and material.

6061-T651, with its significantly lower internal stress, is far more dimensionally stable. It is the superior choice for:

• Large, flat plates or mounting surfaces.

• Thin-walled components.

• Any prototype where holding a tight flatness, straightness, or parallelism tolerance is critical.

Mitigating Flatness Issues: A Practical Guide

While specifying T651 is the most straightforward solution, it may not always be readily available or cost-effective for every rapid prototyping project. Here are strategies to mitigate flatness issues when using 6061-T6.

1. Material Selection: The First Line of Defense

• For Critical Flatness: Always choose 6061-T651. The premium paid for the material is often less than the cost of a scrapped part and lost time.

• For Non-Critical Parts: 6061-T6 is perfectly acceptable and more economical for smaller, bulkier parts where warping is less likely.

2. Strategic Machining Practices:

• Roughing and Finishing: Employ a two-step process. Rough the part to within a few millimeters of its final dimensions, then unclamp it and allow it to "relax." Re-fixture it lightly and then perform the finishing passes. This allows the stress to redistribute after the roughing cut before the final, precise dimensions are machined.

• Symmetrical Machining: Whenever possible, remove material symmetrically from both sides of a part to balance the stress relief. For example, machine both sides of a plate in alternating steps instead of completely finishing one side before starting the other.

• Avoid Excessive Clamping Force: Over-tightening clamps can "pre-stress" the part, which will then spring back to its original, stressed state once released. Use just enough force to secure the part.

3. Design for Manufacturability (DFM):

• Avoid Unnecessary Thin Sections: If flatness is critical, design ribs or reinforcing features to add stiffness and resist warping.

• Communicate with Your Machinist: A good prototyping partner will recognize flatness risks and can suggest design modifications or machining strategies upfront.

Conclusion

The choice between 6061-T6 and 6061-T651 in rapid prototyping hinges on the criticality of dimensional stability. For the vast majority of projects, 6061-T6 offers a perfect balance of performance and cost. However, when your design calls for large, flat surfaces or thin geometries, the investment in 6061-T651 is not just prudent—it is essential. By understanding the science of internal stresses and employing intelligent machining strategies, you can ensure your prototypes are not only strong and functional but also precisely flat, saving time, cost, and frustration in the race to innovation.