The effectiveness of heat treatment for aluminum die castings is directly influenced by their microstructure. T5 heat treatment, at low temperatures of 150-200°C, effectively improves dimensional stability and mechanical properties by precipitating strengthening phases without causing porosity defects. In contrast, T6 heat treatment requires high temperatures exceeding 500°C, which can cause internal porosity to expand in conventional die castings, resulting in surface blistering and rejection.
This article will analyze the underlying mechanisms of T5 heat treatment for conventional die castings from a materials science perspective, clarifying the fundamental reasons why they struggle to withstand T6 heat treatment, and exploring the unique advantages of squeeze casting in overcoming this technical bottleneck.
Microstructure Determines the Limits of Heat Treatment
Aluminum die castings are formed under high pressure and high speed. This process directly shapes their unique microstructure, including α-Al dendrites, eutectic silicon, and subcutaneous porosity, which together determine the selection and effectiveness of subsequent heat treatment processes.
Eutectic Silicon Limits Heat Treatment Strengthening Effectiveness
The typical microstructure is primarily composed of α-Al dendrites and eutectic silicon networks.
This continuous eutectic silicon network not only restricts the material’s plasticity and toughness but also hinders the effectiveness of heat treatment.
During heat treatment, this brittle phase is difficult to effectively spheroidize and refine using conventional processes, thus limiting further improvements in material properties.
The figure below illustrates the α-Al dendrite and eutectic silicon network structure within an aluminum die casting:
α-Al Dendritic and Eutectic Silicon Structure
Porosity Limits Heat Treatment Process Selection
The inherent microscopic pores in traditional aluminum die castings severely restrict the choice of heat treatment process.
During heat treatment, these pores can cause blistering on the part surface due to internal gas expansion.
This makes the T6 process, which requires a high-temperature solution treatment, difficult to implement for traditional die castings.
Therefore, traditional die castings are typically restricted to the lower-temperature T5 heat treatment to avoid quality issues caused by pore expansion.
T5 Heat Treatment for Aluminum Die Castings
For conventional die castings with microscopic pores, T5 heat treatment is the preferred option for improving mechanical properties without introducing defects.
This process eliminates the risk of pore expansion through low-temperature treatment, achieving reliable performance improvements at a low cost.
Principle of T5 Heat Treatment
T5 is an artificial aging treatment that does not involve high-temperature solution treatment.
This process directly utilizes the supersaturated solid solution formed by rapid cooling during die casting. By holding the solution at 150-200°C, strengthening phases (such as Mg₂Si) precipitate from the matrix.
This entire process is based on the diffusion and segregation of alloying elements, without involving phase dissolution and reconstruction.
Improvements in Mechanical Properties Through T5 Heat Treatment
Improved Strength and Hardness
The precipitated nanoscale strengthening phases effectively hinder dislocation motion, significantly increasing the material’s hardness, yield strength, and tensile strength.
For example, after T5 heat treatment, the yield strength of A380 aluminum alloy can be increased from approximately 130 MPa in the as-cast state to 150-180 MPa, while the tensile strength increases by approximately 15-20%.
The following table shows typical mechanical properties of major die-cast aluminum alloy grades in the T5 temper used in actual production:
|
Alloy Grade |
Standard System | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Brinell Hardness (HB) |
| A380 | AA | 240–290 | 150–180 | 0.5–1.2 |
85–95 |
|
ADC12 |
JIS | 230–280 | 140–170 | 0.4–1.0 | 85–95 |
| A360 | AA | 250–300 | 150–180 | 0.8–1.5 |
80–90 |
|
A383 |
AA | 230–280 | 140–165 | 0.6–1.2 | 80–90 |
| A413 | AA | 230–270 | 130–160 | 0.8–1.5 |
85–95 |
|
B390 |
AA | 280–330 | 230–260 | <0.5 | 120–130 |
|
518 |
AA | 170–210 | 120–140 | 2.0–4.0 |
65–75 |
| AlSi9Cu3 | EN | 230–280 | 140–170 | 0.5–1.2 |
90–100 |
|
AlSi10Mg |
EN | 230–290 | 140–170 | 0.8–1.8 | 90–100 |
| AlSi12(Cu) | EN | 240–300 | 150–180 | <1.0 |
95–105 |
Note: The above data is based on the typical performance range of various die-cast aluminum alloys in actual production after a standard T5 heat treatment process (150-200°C, 2-8 hours). Actual values may vary due to specific process parameters, casting wall thickness, and chemical composition fluctuations.
Want to check the data of mechanical properties of die-cast aluminum alloy at the as-cast stage, click here to learn more.
Residual Stress Relief
T5 heat treatment can effectively reduce or eliminate residual stress within aluminum die casting parts.
While this does not directly improve mechanical properties, it prevents machining distortion and dimensional change, ensuring mechanical stability and reliability in actual use.
Limitations of T6 Application in Traditional Aluminum Die Casting
Although T6 heat treatment can impart peak strength to aluminum alloys, the characteristics of traditional die casting processes make it difficult to withstand this strengthening process.
Process Principle and Performance Potential
The complete T6 process consists of three stages: solution treatment + quenching + artificial aging.
High-temperature solution treatment above 500°C fully dissolves alloying elements, followed by rapid quenching and aging to form strengthening phases, thereby achieving optimal strength and hardness properties.
For heat-treatable alloys such as A356, their T6 yield strength can reach over 240 MPa, and their tensile strength exceeds 310 MPa, performance levels unattainable by traditional die castings.
Technical Barriers to Implementing T6 Heat Treatment
Effective T6 heat treatment requires that the material maintain structural stability during the high-temperature solution stage and be able to form a supersaturated solid solution through rapid quenching.
However, the internal structure of traditional die castings struggles to meet these basic requirements:
- First, internal microscopic pores expand at temperatures above 500°C, causing blistering on the part surface.
The figure below illustrates the blister defect seen in traditional aluminum die castings after T6 heat treatment:
Blister Defect of Traditional Die Casting Parts After T6 Heat Treatment
- Second, the inherent eutectic silicon network tends to coarsen at high temperatures, weakening its strengthening potential.
- Third, the inhomogeneous structure of the material makes it susceptible to deformation and internal stress during rapid quenching.
These factors collectively constitute fundamental obstacles to the application of the T6 process to traditional die castings.
The following diagram compares the T5 and T6 heat treatment process flows:
T5 & T6 Heat Treatment Process Flow
Squeeze Casting: The Ideal Foundation for T6 Heat Treatment
Successfully implementing the T6 heat treatment on aluminum castings and realizing their full performance potential requires a dense, uniform material matrix.
The squeeze casting process, with its unique technical characteristics, provides the ideal foundation for this.
Dense Matrix Ensures Heat Treatment Stability
Squeeze casting continuously applies extremely high mechanical pressure during solidification, a process characteristic that effectively eliminates shrinkage and porosity within the casting.
The resulting dense matrix can fully withstand the high-temperature solution stage exceeding 500°C during the T6 treatment, fundamentally avoiding the blistering problem caused by porosity expansion in traditional die castings and ensuring a stable heat treatment process.
The figure below shows the X-ray internal structure inspection results of an aluminum alloy squeeze casting part produced by CEX Casting. The casting’s internal structure is dense and free of pores:
X-Ray Internal Structure Inspection of Squeeze Castings Made by CEX Casting
Optimizing Microstructure and Improving Heat Treatment Response
Under continuous high pressure, squeeze castings not only achieve a dense structure but also form refined α-Al dendrites and spheroidized eutectic silicon phases.
This optimized microstructure creates favorable conditions for subsequent heat treatment: alloying elements dissolve more fully during solution treatment, and strengthening phases precipitate more uniformly during aging, resulting in a casting with a balance of higher strength and improved toughness.
CEX Casting’s Patented Squeeze Casting Technology
As an innovator in the squeeze casting field, CEX Casting has developed patented squeeze casting technology through a unique feed system design and mold process optimization.
Request the Squeeze Casting Case Study
By precisely controlling pressure parameters, the molten aluminum is consistently maintained in optimal conditions during mold filling and solidification, resulting in aluminum castings with uniform microstructure and high density, providing a reliable material foundation for subsequent T6 heat treatment.
The image below shows CEX Casting’s squeeze casting workshop. Click on the image to watch our squeeze casting introduction video:
Squeeze Casting Workshop at CEX Casting
Conclusion
The heat treatment results of aluminum die castings are fundamentally limited by their internal microstructure.
T5 treatment, due to its low-temperature properties, is a viable option for traditional die castings to avoid porosity defects while providing a slight performance improvement.
However, the high temperatures required for T6 treatment are beyond the processing capabilities of traditional die castings.
CEX Casting’s squeeze casting process eliminates internal porosity and optimizes microstructure, creating the necessary conditions for T6 heat treatment, thus overcoming the performance limitations of traditional die castings.
To evaluate the most suitable casting process and heat treatment options for your project, contact us today for customized solutions.