Have you ever faced challenges in sourcing aluminum casting parts, such as “dimensions not meeting drawings,” “distortion occurring after machining,” or “strength being lower than expected”? Many of these issues are not actually due to the casting itself, but rather stem from selection errors or lack of management in the subsequent process: “Heat Treatment.” In aluminum alloys, particularly representative materials like AC4C and AC4CH, it is no exaggeration to say that heat treatment determines over 50% of the final mechanical properties.
In particular, accurately understanding the differences between “T6 Treatment,” which seeks high strength, and “T7 Treatment,” which emphasizes dimensional stability, and specifying them appropriately according to the application, leads directly to not only improved product quality but also total cost reduction through reduced defect rates.
In this article, based on the technical expertise cultivated by Daiwa Aluminum Vietnam over many years, we will thoroughly explain the differences in mechanisms between T6 and T7, compare their performance based on numerical data, and discuss the merits of high-quality heat treatment procurement at our Vietnam base. We provide practical judgment criteria for procurement and purchasing managers aiming for optimization from the design stage and supply chain resilience.
Basics of Aluminum Heat Treatment: Why is “Heat Treatment” Necessary?
Strengthening Mechanism of Aluminum Alloys
Aluminum alloys in their as-cast state (F temper) have a non-uniform metal structure and cannot achieve their full potential. The main purpose of heat treatment is to dramatically increase the strength and hardness of the alloy by utilizing a phenomenon called “Precipitation Hardening.”
Specifically, alloying elements (such as magnesium and silicon) are dissolved into the aluminum matrix at high temperatures, then rapidly cooled to create a supersaturated state. Subsequently, by reheating at an appropriate temperature, fine compounds are uniformly precipitated. This inhibits “dislocation movement,” which prevents metal deformation, thereby improving strength.
In the case of general AC4C alloy, compared to F temper material without heat treatment, applying appropriate heat treatment (T6) improves tensile strength by about 1.5 to 2 times and yield strength by about 2 times or more.
Explanation of Major Heat Treatment Symbols (Tempers)
In JIS standards (JIS H 0001), “Temper designations” are defined to represent the state of heat treatment. The following three are frequently used in manufacturing sites:
- F (As Fabricated): A state where no special heat treatment is performed after casting. Strength is low, but the cost is the lowest.
- T6 (Solution Heat Treatment + Artificial Aging): A treatment to obtain maximum strength and hardness.
- T7 (Solution Heat Treatment + Stabilization): A treatment that enhances dimensional stability and heat resistance at the expense of some strength.
In the next chapter, we will delve deeper into the differences between T6 and T7, which are particularly easy to confuse and critical to select correctly.
Decisive Difference Between T6 and T7 Treatments: Strength vs. Dimensional Stability
T6 Treatment: Seeking Maximum Strength
T6 treatment is the most common and standard heat treatment for aluminum castings. Its process consists of the following two stages:
- Solution Heat Treatment: The product is held at a high temperature, such as 535℃ ±5℃, for 4 to 8 hours to dissolve the alloying elements. It is then rapidly cooled (quenched) in warm water at 60℃-80℃.
- Artificial Aging: After quenching, it is reheated at around 150℃-170℃ for 6 to 10 hours.
Through this treatment, microscopic clusters of atoms called GP zones are formed, and hardness and tensile strength reach their peak. It is ideal for parts requiring high mechanical strength, such as automotive suspension parts and brackets. However, residual stress associated with rapid cooling tends to remain internally, posing a risk of manifesting as “distortion” during subsequent machining processes.
T7 Treatment: Pursuit of Dimensional Stability and Heat Resistance
On the other hand, T7 treatment is also called “Over-aging treatment.” The process up to solution heat treatment and quenching is the same as T6, but the temperature setting for the subsequent aging treatment differs.
- Stabilization Treatment (Over-aging): Heated at a higher temperature than T6, specifically 200℃-240℃, for 4 to 6 hours.
This high-temperature treatment causes the precipitates to coarsen and stabilize. As a result, while the maximum hardness decreases by about 10% to 15% compared to T6, internal residual stress is significantly released. This results in extremely minimal dimensional changes during machining, and dimensional secular changes are less likely to occur even when used in high-temperature environments.
【Comparison Table】 Differences in Mechanical Properties and Residual Stress
The characteristic comparison between T6 and T7 for general AC4CH (Aluminum-Silicon-Magnesium alloy) is as follows:
| Property Item | T6 Treatment (Max Strength) | T7 Treatment (Dimensional Stability) | Comparison Comment |
|---|---|---|---|
| Tensile Strength | 225 MPa or higher | Around 200 MPa | T6 is approx. 10-15% higher |
| Yield Strength (0.2%) | 180 MPa or higher | Around 150 MPa | Yield point is also higher in T6 |
| Elongation | 2% or higher | 3-5% | T7 is tougher (Higher toughness) |
| Residual Stress | High (Risk of machining distortion) | Minimal | T7 is advantageous for maintaining machining precision |
| Heat Resistance | Strength drops above 150℃ | Stable even at 200℃ | T7 is advantageous around engines |
How to Choose by Application: Criteria for Failure-Free Spec Selection
Automotive Parts & Structural Components (Recommended for T6)
T6 treatment should be selected when “you want to pursue lightness and strength to the limit, even if tolerating some risk of distortion.”
- Suspension members, control arms: Need strength to withstand high loads during driving.
- Hydraulic pump bodies: Need yield strength to withstand high pressure.
- Motorcycle wheels: Balance between strength and impact resistance is important.
For these parts, it is common to enjoy the high strength of T6 while suppressing distortion by devising rib shapes at the design stage.
Precision Engine Parts & Large Casings (Recommended for T7)
T7 treatment should be selected when “micron-level machining precision is required” or “the part is used in high-temperature environments.”
- Cylinder heads, engine blocks: Since they are exposed to high temperatures (around 200℃) due to combustion heat, T6 would suffer from strength reduction and dimensional changes during use. It is necessary to stabilize the structure in advance with T7.
- Large semiconductor manufacturing equipment parts: Because the amount of machining is large and warping due to residual stress release after machining is fatal, stress relief by T7 is mandatory.
- Pistons: Thermal stability is the top priority to withstand high temperatures and intense reciprocating motion.
High-Quality Heat Treatment in Vietnam: Balancing Cost Reduction and Quality
Daiwa Aluminum Vietnam’s Heat Treatment Facilities and Management System
A concern in overseas procurement, especially in Vietnam, is “variation in heat treatment quality.” If the temperature distribution inside the furnace is uneven, variations in strength will occur even within the same lot.
Daiwa Aluminum Vietnam guarantees quality equivalent to Japan with the following system:
- Introduction of state-of-the-art continuous and batch furnaces: We select the optimal furnace according to the workpiece size and production volume. Temperature Uniformity Surveys (TUS) inside the furnace are conducted periodically, strictly adhering to the standard range of ±5℃.
- Thorough water temperature management: In quenching after solution heat treatment, water temperature is a critical factor influencing the cooling rate. We manage water temperature constantly within the range of 60℃-80℃ using chiller equipment, eliminating quality fluctuations due to seasons.
- Traceability: Heat treatment charts (temperature records) are kept for all lots, making it completely traceable when, in which furnace, and through what temperature history the product passed.
Realizing Japanese Quality × Vietnam Cost
Heat treatment uses electric furnaces, so electricity costs account for a large weight of the processing fees. Vietnam’s industrial electricity rates are cheaper compared to Japan, providing significant cost competitiveness for castings involving energy-intensive heat treatment processes.
Furthermore, Daiwa Aluminum Vietnam handles everything from “Casting -> Heat Treatment -> Shot Blasting -> Machining” through in-house integrated production.
Since there is no process like “outsourcing only heat treatment” seen in other companies, not only are horizontal transportation costs zero, but the lead time after heat treatment can be reduced by an average of 3 to 5 days. Also, in the unlikely event that distortion is found during the machining stage, we can immediately feed back to the heat treatment conditions internally and rotate the improvement cycle.
Summary
Heat treatment technology determines the performance of aluminum castings. T6 treatment provides “maximum strength,” while T7 treatment provides “dimensional stability and heat resistance.”
- T6 for suspension parts where strength is the top priority
- T7 for engine parts and large casings requiring precision machining
This distinction is the key to enhancing product reliability and optimizing total costs.
As a specialist in aluminum casting near Ho Chi Minh City, Vietnam, Daiwa Aluminum Vietnam provides solutions that fuse Japanese-quality heat treatment technology with Vietnam’s cost benefits. By getting involved in the selection of heat treatment from the casting plan design stage, we support optimal product creation without machining distortion.
If you are currently considering mold transfer from Japan or supply chain diversification, please consult us. We will present a concrete plan to solve your procurement challenges.