Difference between aluminum quality symbols "T6" and "T651

Here, aluminum processingDifference between T6 and T651."It is a note about.

When selecting an aluminum alloy, I have had the experience of being perplexed by the fact that a search using the keywords "aluminum T6 T651" shows almost identical values for the strength properties (tensile and yield strength) of the material.

Many information websites were all about comparing these strength characteristics, and tended to select inexpensive T6 material based only on the slight difference between the two.　　However, when we actually checked high-precision CNC-machined parts at a manufacturing site, we found that parts made of T6 material had "machining distortion," which causes significant warping after machining.

This experience has convinced me that the real difference between T6 and T651, which appear to have nearly identical strength properties, lies in their behavior during processing and dimensional stability, but in this memo, I will discuss the issue of residual stresses that inevitably arise in the manufacturing process of T6 treated materials, which are not well explained on other sites, and the specific stress relief implemented in the T651 treatment. This memo will delve deeply into the technical mechanism of the T651 treatment.

Aluminum T6 T651 has residual stress problems

In this section, we will follow the technical process, from the basic definitions of T6 and T651, to why the problem of residual stress occurs and how T651 solves that problem.

Basis of Aluminum Quality Code T6 and T651

Aluminum alloysquality mark　is an important code defined by JIS, ASTM, and other standards that indicates the heat treatment and processing history applied to a material.　　It defines the final mechanical properties and physical condition of the material.

The "T" refers to the state of being stabilized by heat treatment, i.e., having undergone age-hardening treatment.　　Both T6 and T651 have undergone an artificial age-hardening treatment and have high strength and hardness in common.　　Therefore,On the spec table showing the fracture strength of the material, the two values are almost equalThe first is the

However, the critical difference between T6 and T651 lies in the number following the "T".　　T6 indicates that age-hardening treatment was performed after solution annealing.　　T651, on the other hand, indicates that the solution annealing treatment was followed by tensile processing (drawing) and further age hardening.This tensile process is the special step to intentionally remove residual stresses that are unavoidable in T6 materialand is the most important technical factor that distinguishes T651 material from T6 material.

Definition of T6 treatment and sources of residual stress

T6 treatment is the standard heat treatment method for maximizing the strength of precipitation hardened aluminum alloys such as Series 6000 and Series 7000.　　This process consists of three main steps

1. Solution heat treatment: The alloy is heated to high temperatures to ensure uniform solid solution of the strengthening elements into the aluminum matrix.
2. Rapid cooling (quenching): In order to suppress precipitation of strengthening elements and obtain high strength, the material is rapidly cooled with water to obtain a state of supersaturated solid solution of the elements (supersaturated solid solution).
3. Artificial age-hardening treatment: Reheating at the appropriate temperature and over time allows fine precipitates of strengthening elements to form, maximizing the strength and hardness of the material.

In this process.residual stressThe rapid cooling process in Step 2 is a major source of　　Rapid cooling is necessary to maximize the strength of the material, but this creates a large difference in cooling rate between the surface and the interior of the material.　　Naturally, the outside of the material cools first and tries to shrink, while the inside cools later.　　This non-uniformity in cooling rate leads to non-uniformity in thermal shrinkage and inevitable inside of the material.thermal residual stressIt traps the

Therefore,Although T6 material has high strength, it is a material that is always at risk of "work strain," which is residual stress in a non-uniform state inside the materialThe first is the

Mechanism of stress relaxation by T651 treatment

T651 is a quality designation specifically designed to eliminate residual stresses introduced by the T6 treatment while maintaining the high strength obtained by the T6 treatment.

T651 suffix "51" after solution treatment and quenching,Before age hardening treatmentby tensile processing (stretching/stretching) that imposes a specified amount of permanent strain on theStress Relief Treatmentwas implemented.

The critical difference between the T6 and T651 heat treatment processes is described in the table below.

Stress Redistribution and Technical Effects of Stretching Treatment

The mechanism of stress relaxation treatment is the material'sStress above yield pointis applied uniformly to the macroscopic area.　　This causes plastic deformation of the material.　　Through this plastic deformation, the internal stresses that remained unevenly in the material during quenching are forcibly redistributed, and the stress level is significantly reduced.

JIS and other standards specify that the amount of permanent strain applied by this drawing process is in the range of 1% to 3% for plates and bars.　　This intentional plastic deformation changes the material into a "pre-stressed" state, i.e., a stable state in which the internal stresses are equalized and distortion caused by cutting is less likely to occur.

Thanks to this stress relief, T651 offers designers the benefit of reduced need for post-machining adjustments and improved machining accuracy.

Work distortion and dimensional stability prevented by stress relaxation

The main reason machine designers choose T651 is that this stress relief process providesDimensional stabilityIt is located at the height of

When high-precision cutting is performed on T6 material, residual stress trapped in the removed portion is released when a portion of the material is removed by milling or other processes.　　The stress balance in the remaining material is then disrupted and the entire part is deformed, resulting in what is known as "warpage.Machining distortionThis distortion is the result of the distortion of the material.　　When cutting deep or complex pocket shapes, the effect of this distortion is significant, and the final dimensional accuracy is compromised.

On the other hand, T651 material does not deform significantly when stress is released during cutting because internal stress is removed by stress relaxation treatment, providing stability.　　This allows T651 material to maintain high dimensional stability after cutting, enabling high precision cutting.

The stability of T651 is considered a prerequisite for achieving high-precision designs, especially in geometries that are susceptible to deformation, such as thin-walled components, large plate materials, or components with high aspect ratios.　　For example, in thin-walled cases for electronics, where the use of T6 material may further bend the material due to external cutting processes, T651 has the stability to hold the equipment in place with a perfectly shaped box.

Differences in mechanical strength properties and fatigue strength

Strength properties are the first thing many people check as "the difference between T6 and T651." Taking aluminum alloy 6061 as an example, the main mechanical properties such as tensile strength and yield strength are,approximately equaland treated as such.

The following table provides a detailed comparison of the properties of 6061 alloys.

An important point that these data show is that the choice of T6 and T651 does not compare the superiority of the strength performance of the material when it fails, but rather the material'sProcessing CharacteristicsとDimensional stabilityThe fact is that the comparison of

Also,Fatigue strengthThe values for T6 and T651 are slightly higher than those for T6 (96 MPa and 95 MPa, respectively), but this difference is negligible in practical design.　　Considering the risk of micro cracks caused by processing strain in T6 material, the stability of T651 material is considered to be more advantageous in many cases in durability design where long life is required.

Quantitative evaluation of machinability in CNC machining

The technical superiority of the T651 material is clearly evident in its machinability.　　In the 6061 alloy, the machinability of T6 is rated "Fair" while that of T651 is rated "Great.

The main reason for the excellent machinability is that T651 is in a state of stress relaxation.

Because T651 is stress-relieved, the material is very stable during cutting and the load on the tool tends to be more uniform.　　This allows deeper cuts to be made without concern for distortion and makes it easier to set more demanding cutting conditions.　　With T6 material, the release of internal stresses during cutting increases the risk of erratic vibration and chipping, making it difficult to maintain stable cutting conditions.

The T651 is not only easy to machine, but its stability and lack of distortion during machining is what makes it "excellent," resulting in both high accuracy and high speed machining.　　The T651 also has the advantage of providing a smoother, more uniform surface finish.

Manufacturing strategies and costs change with the choice of aluminum T6 T651

Now that the differences between T6 and T651 have been clarified, how this knowledge should be applied to actual design and manufacturing decisions will be explained in terms of cost efficiency and specific applications.

T651 material cost efficiency and overall decision making

T651 is generally more expensive than T6 because it requires an additional stretching step (stress relaxation treatment) to the T6 treatment.Initial material costs are highThe first choice would be T6.　　If material cost is the highest priority, T6 would be the natural choice.

However, the mechanical designer must be able to provide a comprehensivecost effectivenessWhen evaluating the cost of a product, it is important to include not only the initial material cost, but also post-processing adjustment costs and disposal costs due to defective products.

Assessment of overall manufacturing costs

When T6 material is used to machine high-precision parts, distortion due to residual stress risks incurring significant costs in subsequent processes.

• Cost of rework/adjustment: $3,000 Time and expense required to correct and rework distorted parts.
• Increased defective product rate:Cost of rejects discarded because distortion does not meet strict tolerances.
• Delayed delivery:Delays in delivery due to the above actions and the associated credit risk.

In this light, T651 can significantly reduce these additional costs and risks, even though the initial material cost is high, because the risk of distortion after machining is extremely low.　　Especially for parts that require tight geometric tolerances and complex geometry, the introduction of T651 hedges the risk of the entire manufacturing process, resulting inMost cost-effective optionThis is often the case.

It has also been noted that some lots of 7075 alloy, a high-strength alloy, may require additional processing steps for T6 processing, making T651 more cost-effective than T6 .　　This suggests that the cost structure may vary by alloy type and supplier, and designers should always take a holistic view.

Specific applications of T6 and T651

T6 and T651 are clearly divided into fields of application according to their residual stresses and dimensional stability.　　The designer must select the appropriate material depending on the tolerances required for the part and the complexity of the geometry.

Examples of cases where T651 is mandatory

T651 is a highDimensional stabilityとreliabilityThis is an essential choice in the manufacture of components that require

• Aerospace Components: Applications where tight tolerances are required and dimensional constancy must be guaranteed after the manufacturing process.
• Precision manufactured parts: Applications where dimensional reproducibility is required and measurement accuracy must not be degraded by distortion, such as jigs and inspection equipment components.
• Thin-walled and large parts: Stress-relieved T651 is recommended for thin walls and large components, which are susceptible to bending deformation due to internal stresses.
• Parts requiring deep cutting: For efficient machining of deep pockets and complex contours without the worry of distortion due to stress release.

Examples of cases where T6 fits

T6 material is selected when its cost advantage is to be taken advantage of.

• Structural brackets and frames: General parts where stress relief is not critical and tolerances are relatively loose.
• General-purpose structural member: Applications where distortion is judged to have little effect on subsequent assembly and function.

Once tight tolerances are specified on the blueprints, the use of T651 (or other stress relief treated material) is not for the physical strength of the material, but to achieve risk hedging and tolerance management strategies throughout the manufacturing processQuality Assurance CodeIt is important to understand that it is

Final conclusion on aluminum T6 T651 that designers should know

The answer machine designers were looking for when they searched for the keywords "aluminum T6 T651" was not the strength properties of the material, but the high precision machining ofMachining distortionto prevent theDimensional stabilityIt can be concluded that the securing of

The choice of T651 is an important decision for machine designers to optimize the balance between material cost, manufacturing quality, and manufacturing stability.

The key points and conclusions explained in this article are as follows

• Tensile and yield strengths of aluminum alloys T6 and T651 are almost equal
• T6 treatment has internal thermal residual stresses due to the rapid cooling process
• Residual stress is released during the cutting process, causing "warpage" or "machining distortion" of the part.
• T651 treatment has an additional stress relaxation step by stretching process after quenching
• The quality code "51" indicates that this stress relief process has been performed.
• Stress-relieved T651 material remains stable and highly dimensionally stable after cutting
• T651 is rated "excellent" in machinability (ease of machining) over T6
• High machinability is due to the stability of the material during machining
• T651 has the disadvantage of higher initial material costs than T6
• However, for high precision parts, T651 contributes to overall manufacturing cost reduction
• T651 is essential in fields such as aerospace and precision jigs that require tight tolerances.
• T651 is strongly recommended for thin-walled and large parts, which are susceptible to internal stresses
• T6 is suitable when cost is a top priority, such as in structural components where stress relief is not critical
• Designers choose T6 and T651 based on "dimensional stability", not "strength".
• T651 selection is a risk hedging and quality assurance strategy for the entire manufacturing process

That's it.