1. Characteristics and Objectives of Cast Aluminum Alloy Heat Treatment
As mentioned earlier, the metallurgical structure of cast aluminum alloys is coarser than that of wrought aluminum alloys, leading to differences in their heat treatment. The former requires longer holding times, generally exceeding 2 hours, while the latter requires shorter holding times, sometimes only tens of minutes. This is because castings produced by metal mold casting, low-pressure casting, and differential pressure casting solidify under relatively high cooling rates and pressure, resulting in a much finer grain structure compared to castings produced by plaster mold casting or sand casting. Consequently, their heat treatment holding times are also significantly shorter. Another difference between cast and wrought aluminum alloys is the uneven wall thickness and complex structural shapes of castings, such as irregular cross-sections or internal channels. To prevent deformation or cracking during heat treatment, specialized fixtures are sometimes designed for protection. Additionally, the temperature of the quenching medium is generally higher than that used for wrought aluminum alloys. Therefore, artificial aging is often employed to shorten the heat treatment cycle and enhance the properties of the castings.
The objectives of heat treating cast aluminum alloys are to improve mechanical properties and corrosion resistance, stabilize dimensions, and enhance processability such as machinability and weldability. Since the mechanical properties of many as-cast aluminum alloys cannot meet service requirements, except for alloys like ZL102 in the Al-Si system, ZL302 in the Al-Mg system, and ZL401 in the Al-Zn system, most cast aluminum alloys require heat treatment to further improve their mechanical and other service properties. Its specific functions include the following aspects:
1) Eliminate internal stresses caused by uneven cooling rates during solidification due to the casting structure (e.g., uneven wall thickness, thick sections at transitions).
2) Increase the strength and hardness of the alloy, improve the metallurgical structure, and ensure the alloy has adequate plasticity, machinability, and weldability.
3) Stabilize the structure and dimensions of the casting, preventing or eliminating volume changes caused by high-temperature phase transformations.
4) Eliminate interdendritic and compositional segregation, achieving structural homogenization.
2. Heat Treatment Methods and Key Operational Techniques for Cast Aluminum Alloys
(1) Heat Treatment Methods
The current heat treatment processes for cast aluminum alloys include annealing, quenching (solution treatment), aging, and thermal cycling. These are described below:
1) Annealing. The role of annealing is to eliminate casting stresses and internal stresses induced by machining, stabilize the shape and dimensions of machined parts, and spheroidize some of the silicon crystals in Al-Si system alloys to improve their plasticity. The process involves heating the aluminum alloy casting to 280~300°C, holding for 2~3 hours, and then furnace cooling to room temperature. This allows the solid solution to decompose slowly, causing precipitated secondary phases to agglomerate, thereby eliminating internal stresses in the casting and achieving the goals of stabilizing dimensions, improving plasticity, and reducing distortion. The heat treatment designation for this state is T2.
2) Quenching. Quenching is also called solution treatment or rapid cooling. The process involves heating the aluminum alloy casting to a higher temperature (generally close to the eutectic melting point, mostly above 500°C), holding for more than 2 hours to fully dissolve soluble phases within the alloy. Then, the casting is rapidly quenched into water at 60~100°C. Due to the rapid cooling, the strengthening phases dissolved to the maximum extent in the alloy are fixed and retained to room temperature.
3) Aging. The process involves heating the quenched aluminum alloy casting to a specific temperature, holding for a certain time, then removing it from the furnace and air cooling to room temperature. This causes the supersaturated solid solution to decompose, stabilizing the base alloy structure.
During the aging process, the alloy typically goes through several stages: As temperature rises and time extends, atoms within the supersaturated solid solution lattice rearrange, forming solute atom-rich zones (called G-P I zones). As G-P I zones disappear, second-phase atoms segregate in a specific pattern to form G-P II zones, followed by the formation of metastable second phases (transition phases). The final stages involve the combination of large numbers of G-P II zones with small amounts of metastable phases, the transformation of metastable phases into stable phases, and the agglomeration of second-phase particles.
Aging treatment is further divided into natural aging and artificial aging. Natural aging refers to the aging strengthening process occurring at room temperature. Artificial aging is subdivided into under-aging (incomplete artificial aging), peak aging (complete artificial aging), and over-aging.
* **Under-Aging (Incomplete Artificial Aging):** Heat the casting to 150~170°C (lower temperature), hold for 3~5 hours. This yields better tensile strength, good plasticity, and toughness, but reduces corrosion resistance.
* **Peak Aging (Complete Artificial Aging):** Heat the casting to 175~185°C (higher temperature), hold for 5~24 hours. This achieves sufficient tensile strength (i.e., highest hardness) but reduces elongation.
* **Over-Aging:** Also called stabilization tempering. The process involves heating the casting to 190~230°C, holding for 4~9 hours. This slightly reduces strength but increases plasticity, resulting in better resistance to stress corrosion cracking.4) Thermal Cycling. Cool the aluminum alloy casting to a sub-zero temperature (e.g., -50°C, -70°C, or -195°C) and hold for a certain time. Then, heat the casting to below 350°C. This repeated heating and cooling causes the solid solution lattice to repeatedly contract and expand, inducing slight displacements in the grains of the various phases. This places the atom clusters within the solid solution lattice and the particles of intermetallic compounds into a more stable state, thereby achieving more stable dimensions and volume for the product components. This repeated heating and cooling heat treatment process, known as thermal cycling, is only suitable for treating extremely precise parts requiring high dimensional stability during use (e.g., some parts on measuring instruments); general castings do not undergo this treatment.
(2) Key Points of Heat Treatment Operation Techniques
1) Heat treatment should be carried out according to the heat treatment process specified based on the casting’s structural shape, dimensions, and alloy characteristics.
2) Before heat treatment, check whether the heat treatment equipment, auxiliary equipment, and instruments are qualified and functioning normally, and ensure the temperature difference within the furnace chamber is within the specified range (±5°C).
3) Before charging into the furnace, castings should be sandblasted or rinsed to be free of oil stains, dirt, and soil. Alloy grades should not be mixed.
4) Castings prone to deformation should be placed on dedicated bases or supports; cantilevered sections should not be left unsupported.
5) Test bars (separately cast or attached) used to check casting properties should be heat-treated in the same furnace as the workpieces to accurately reflect the casting’s properties.
6) During the holding period, constantly check and calibrate the temperature throughout the furnace chamber to prevent local overheating or melting.
7) If power cannot be restored shortly after an outage, castings undergoing holding should be quickly removed from the furnace and quenched. Resume normal heat treatment (charging, holding) after power is restored.
8) Castings quenched in a salt bath should be rinsed immediately with hot water after quenching to remove residual salt and prevent corrosion.
9) If deformation is detected after quenching, correct it immediately.
10) Castings requiring aging treatment should be aged within 0.5 hours after quenching.
11) If properties are found to be non-conforming after heat treatment, the heat treatment can be repeated, but the number of repetitions should not exceed two.
3. Designations and Process Parameters for Cast Aluminum Alloy Heat Treatment States
(1) Designations, State Names, Objectives, and Applications of Cast Aluminum Alloy Heat Treatment States
See Table 2-29.
(2) Heat Treatment Process Parameters for Selected Cast Aluminum Alloys
The heat treatment process parameters for commonly used cast aluminum alloys (27 types) will be introduced in Chapter 6 and are omitted here. The parameters for the additional alloys listed below are also summarized from decades of production practice, as shown in Table 2-30.
Table 2-29: Designations and Characteristics of Heat Treatment States for Cast Aluminum Alloys
| State Designation | Heat Treatment State Name | Purpose and Application Examples | Explanation |
|---|---|---|---|
| T1 | Unquenched Artificial Aging | For alloys cast in metal molds or green sand molds, the faster cooling rate results in a certain degree of supersaturated solid solution, providing partial quenching effect. Subsequent artificial aging and precipitation strengthening can improve hardness and strength, and enhance machinability. Used for treating die castings bearing low loads. | More effective for improving the strength of alloys like ZL104, ZL105, etc. Aging temperature approx. 150~180°C, holding time 1~24h. |
| T2 | Annealing | Eliminate internal stresses in castings (casting stresses and machining-induced stresses), stabilize casting dimensions, and spheroidize Si crystals in Al-Si system alloys to improve their plasticity. Used for castings requiring high dimensional stability during service. | More effective for Al-Si system alloys. Often performed after casting or rough machining. Annealing temperature 280~300°C, holding time 2~4h. |
| T4 | Quenching (Solution Treatment) + Natural Aging | Through heating and holding, soluble phases are dissolved, followed by rapid quenching. This fixes a large amount of strengthening phase dissolved in the α-solid solution, obtaining a supersaturated solid solution to improve the alloy’s hardness, strength, and corrosion resistance. Used for castings subjected to dynamic loads and impact. | Final heat treatment for Al-Mg system alloys; preparatory heat treatment for other alloys requiring artificial aging. Quenching temperature approx. 500~535°C, for Al-Mg alloys approx. 435°C. |
| T5 | Quenching + Under-Aging (Incomplete Art. Aging) | Achieve higher strength and yield strength, improve plasticity, but corrosion resistance may decrease, especially intergranular corrosion may increase. Used for castings bearing high static loads and operating at not very high temperatures. | Low aging temperature, short holding time. Aging temp. approx. 150~170°C, holding time 3~5h. |
| T6 | Quenching + Peak Aging (Complete Art. Aging) | Achieve maximum strength with a slight reduction in plasticity, corrosion resistance also decreases. Used for parts bearing high static loads without impact. | Performed at higher temperature and longer time. Aging temp. approx. 175~185°C, holding time >5h. |
| T7 | Quenching + Stabilization Tempering (Over-Aging) | Used to stabilize casting dimensions and structure, improve corrosion resistance (especially stress corrosion resistance), while maintaining relatively high mechanical properties. Used for castings operating at high temperatures (below 300°C). | Best performed at a tempering temperature close to the casting’s operating temperature. Tempering temp. 190~230°C, holding time 4~9h. |
| T8 | Quenching + Softening Tempering | Cause the solid solution to fully decompose, and the precipitated strengthening phases to agglomerate and spheroidize. This stabilizes casting dimensions, improves alloy plasticity, but reduces tensile strength. Used for castings requiring high plasticity. | Tempering temperature higher than T7, approx. 230~270°C, holding time 3~6h. |
| T9 | Thermal Cycling | Used to further stabilize casting dimensions and shape. The heating/cooling temperatures and number of cycles depend on the part’s operating conditions and the alloy’s properties. Used for castings requiring extremely precise dimensions and high shape stability. | Post-machining cryogenic treatment temp. -50°C, -70°C, or -195°C, hold 3~6h. Thermal cycling determined by specific conditions. |
Table 2-30: Heat Treatment Process Parameters for Selected Cast Aluminum Alloys
| Alloy Code | State① | Solution Treatment (Quenching) | Aging | Cooling | Remarks |
|---|---|---|---|---|---|
| Heating Temp. /°C | Holding Time /h | Cooling Medium & Temp /°C | Heating Temp. /°C | ||
| T1 | — | — | — | 180±5 | |
| T2 | — | — | — | 290±10 | |
| ZL103 | T5 | 515±5 | 3～6 | Water 60~100 | 175±5 |
| T7/T8 | 515±5 / 510±5 | 3～6 / 5～6 | Water 60~100 / Water 60~100 | 230±5 / 330±5 | |
| ZL106 | T1 | — | — | — | 230±5 |
| T5 | 515±5 | 5～12 | Water 80~100 | 150±5 | |
| T7 | 515±5 | 5～12 | Water 80~100 | 230±5 | |
| ZL108 | T1 | 515±5 | 6～8 | Water 40~80 | 200±10 |
| T6 | 515±5 | 3～8 | Water 40~80 | 175±5 | |
| T7 | 230—250 | ||||
| ZL110 | T1 | 480～495 | 3～8 | Water 40~100 | 210±10 |
| T6 | 210±10 | ||||
| T2 | 290±10 | ||||
| ZL202 | T6 | 510±5 | 12 | Water 80~100 | 155±5 (S) / 175±5 (J) |
| T7 | 510±5 | 3～5 | Water 80~100 | 200～250 | |
| ZL209 | T6 | 530±5 / 538±5 | 0.5 / 10～18 | — / Water 20~100 | — / 170±5 |
| ZL305 | T4 | 435±5 / 490±5 | 8 / 6 | — / Water 80~100 | — / — |
① Meanings of T1-T8 see Table 2-29.
4. Quality Defects in Cast Aluminum Alloy Heat Treatment and Their Elimination/Prevention
Common quality issues in aluminum alloy castings after heat treatment include non-conforming mechanical properties, deformation, cracking, and overheating (burning). The causes and methods for elimination/prevention are described below.
(1) Non-Conforming Mechanical Properties
Manifests as low elongation (δ5) in the annealed state, or non-conforming strength and elongation after quenching or aging. Causes are varied: annealing temperature too low, holding time insufficient, or cooling too fast; quenching temperature too low, holding time insufficient, or cooling too slow (quenching medium temperature too high); under-aging/peak aging temperature too high or holding time too long; deviation in alloy chemical composition.
- Elimination: Re-anneal, increasing heating temperature or extending holding time; increase quenching temperature or extend holding time, lower quenching medium temperature; if re-quenching, adjust subsequent aging temperature and time; if composition deviates, change or adjust repeated heat treatment parameters based on the specific deviating elements and deviation amount.
(2) Deformation and Warping
Typically reflected as changes in casting dimensions or shape after heat treatment or during subsequent machining. Causes: Heating rate or quenching cooling rate too fast (too severe); quenching temperature too high; unreasonable casting design structure (e.g., large difference in thickness between connected walls, thin or fine ribs in frame structures); improper orientation during quenching immersion or incorrect loading method.
- Elimination/Prevention: Reduce heating rate, raise quenching medium temperature, or switch to a slightly slower cooling medium to prevent residual stress; apply coating or insulate thin sections with materials like asbestos fiber; select proper immersion orientation based on casting structure/shape or use special anti-deformation fixtures; immediately correct minor deformation after quenching.
(3) Cracking
Manifests as visible cracks on the casting surface after quenching, or fine microcracks detectable by fluorescent inspection but invisible to the naked eye. Cracks are often tortuous and dark gray. Causes: Heating rate too fast; quenching cooling too fast (quench temperature too high, medium temperature too low, or medium cooling rate too fast); unreasonable casting design (large wall thickness difference, thin/fine ribs in frames); improper furnace loading or immersion direction; uneven furnace temperature causing uneven part temperature.
- Elimination/Prevention: Slow down heating rate or use isothermal quenching process; raise quenching medium temperature or use a slower cooling medium; apply coating on thick/thin sections or insulate thin sections with asbestos; use special anti-cracking quenching fixtures and select correct immersion direction.
(4) Overheating (Burning)
Manifests as surface nodules on the casting and a significant drop in alloy elongation. Causes: Excessive content of low-melting-point impurity elements like Cd, Si, Sb, etc.; uneven heating or heating too fast; localized furnace temperature exceeding the alloy’s overheating temperature; malfunctioning temperature measurement/control instruments causing actual furnace temperature to exceed indicated value.
- Elimination/Prevention: Strictly control low-melting-point element content within limits; heat slowly at a rate not exceeding 3°C/min; check and control zone temperatures within ±5°C; regularly inspect and calibrate temperature instruments to ensure accurate measurement, display, and control.
(5) Surface Corrosion
Manifests as spots or patches on the casting surface, with a color distinctly different from the aluminum alloy surface. Causes: Chloride content in nitrate salt bath exceeds standard (>0.5%), corroding the surface (especially porous/porous areas); insufficient cleaning after removal from salt bath, leaving adhered nitrate salts (especially in narrow gaps, blind holes, channels) causing corrosion; presence of acid or alkali in the nitrate bath; or castings placed near concentrated acid/alkali.
- Elimination/Prevention: Minimize time between furnace removal and immersion into quench tank; check chloride content in nitrate bath and reduce if excessive; rinse castings heated in nitrate bath immediately with warm or cold water; check for acid/alkali in bath and neutralize or stop use if present; do not place aluminum castings near concentrated acid/alkali.
(6) Uneven Quenching
Manifests as low elongation and hardness in thick sections (especially internal center), and high hardness in thin sections (especially surface layer). Causes: Uneven heating and cooling of the casting; slower cooling and heat transfer in thick sections.
- Elimination/Prevention: Re-heat treat with reduced heating rate and extended holding time to achieve uniform temperature; apply insulating coating or insulate thin sections to promote simultaneous cooling; immerse thick sections first; switch to organic quenchants to reduce cooling rate.
HKX’s Heat Treatment Expertise: Adding Value
At HKX Die Casting Factory, integrating heat treatment isn’t just an add-on; it’s a fundamental part of our capability to deliver high-performance components. We focus on:
- Process Precision: Understanding that each alloy and part geometry may respond differently, we employ precise control over temperature profiles, soaking times, and quenching parameters.
- Temper Selection: Collaborating closely with customers to determine the optimal temper (T4, T5, T6, T7, etc.) based on the specific functional demands of the part – balancing strength, ductility, hardness, dimensional stability, and corrosion resistance.
- Mitigating Distortion: Heat treatment can induce stresses. Our experience and process controls help minimize potential warping or dimensional changes, ensuring parts meet tight tolerances post-treatment.
- Quality Integration: Heat treatment parameters are considered part of the overall manufacturing process, ensuring traceability and consistent results batch after batch.
The Benefits Realized
Choosing heat-treated aluminum die castings from HKX translates into tangible advantages for your products:
- Enhanced Strength & Hardness: Critical for structural components, brackets, housings, and parts under load.
- Improved Wear Resistance: Extends the lifespan of moving parts or components subject to abrasion.
- Greater Fatigue Resistance: Essential for parts experiencing cyclic stresses, preventing premature failure.
- Improved Dimensional Stability: Reduces the potential for dimensional shifts over time or under stress.
- Optimized Properties: Tailoring the material precisely to the application’s needs, potentially allowing for lightweighting or longer service life.
Applications Shining Through
Heat-treated aluminum die castings from HKX are the backbone of demanding applications across industries:
- Automotive: Engine brackets, transmission components, structural housings, steering system parts, EV battery enclosures (where applicable).
- Industrial Machinery: Pump housings, valve bodies, gearbox cases, hydraulic components, heavy-duty brackets.
- Consumer Durables: Power tool housings and gears, high-stress appliance components.
- Electronics: Heat sinks (where specific alloys/tempers are suitable), robust enclosures.
Conclusion: Beyond the Casting
Heat treatment is a powerful metallurgical tool that elevates cast aluminum from a versatile forming material into a high-performance engineering solution. HKX Die Casting Factory leverages this process with precision and expertise, ensuring our customers receive components that not only meet complex shapes and tight tolerances through die casting but also deliver the robust mechanical properties required for success in the most challenging environments. By understanding and controlling the transformation within the metal, we unlock the full potential of aluminum alloys for your critical applications.
