During the aluminum heat treatment process, a variety of methods are utilized to enhance the strength and hardness of specific aluminum alloys. These methods include annealing, homogenizing, solution heat treatment, natural aging, and artificial aging, all carried out within a temperature range of 240 to 1000°F. It is important to recognize that aluminum heat treatment differs significantly from that of steel.
Aluminum alloys are susceptible to work hardening, also known as strain hardening, when subjected to plastic deformation. To address this issue, annealing is used to reset the crystalline grain structure and relieve internal stresses. Homogenizing is another technique employed to evenly distribute precipitating elements throughout an aluminum component, especially for cast parts.
Solution heat treatment plays a key role in dissolving elements responsible for age hardening, creating a uniform structure. Quenching is a necessary step after solution heat treatment to maintain the distribution of dissolved elements. Natural aging and artificial aging further enhance the strength and hardness of aluminum alloys.
Accurate temperature control is essential for effective aluminum heat treatments, requiring top-notch furnaces and skilled technicians. In sectors like aerospace and automotive, compliance with standards such as AMS2750E is vital for certification and auditing purposes.
When it comes to critical aluminum parts heat treatment, L&L Special Furnace Co., Inc. offers a variety of solutions, including high-uniformity annealing furnaces, dual chamber heat treating ovens, bench-mounted recirculating tempering ovens, heavy-duty gas or electric recirculating tempering ovens, and smaller series furnaces typically in stock for immediate shipment.
Each of these ovens can be equipped with the Aviation Pyrometry Package to ensure AMS2750E compliance and other customized options to meet specific aluminum heat treatment requirements.
The article delves into the concept of strain hardening in aluminum alloys, elaborating on how it occurs during forming processes and enhances strength properties. It also covers standardized temper designations, microstructural changes during deformation, and essential mechanisms for optimizing aluminum alloy properties.
Strain hardening naturally boosts strength in both non-heat-treatable and heat-treatable aluminum alloys. Work hardening is a critical aspect for strain-hardened tempers, while partial annealing combines strain hardening to achieve desired properties.
Different tempers in strain-hardened aluminum alloys offer specific strength characteristics, ranging from quarter-hard to extra-hard conditions. Annealing can be used to revert products to a soft, ductile state.
| Mood | Explanation |
| F |
| As-fabricated. Not restricted by any mechanical property criteria. |
| O |
| Fully softened and reformed. Exhibits minimal strength but high ductility. |
| Heading 1 |
| Material strengthened through strain hardening process to reach quarter-hard to extra-hard states. |
| Grade H112 |
| Enhanced strength achieved through the process of strain hardening. Complies with specified mechanical requirements. |
Microstructural Changes During Deformation
Aluminum alloys experience deformation through crystallographic slip mechanisms. The process of work hardening results in an increase in dislocation densities and lattice distortions, which contributes to the strengthening of the material.
In addition to dislocation densities and lattice distortions, other microstructural changes occur during deformation. These include grain refinement, texture development, and formation of precipitates. Grain refinement refers to the decrease in average grain size as a result of plastic deformation, which can improve mechanical properties such as strength and ductility. Texture development involves the preferred orientation of grains due to deformation, leading to anisotropic properties. Precipitates form in the alloy matrix as a result of solute atoms clustering together, further strengthening the material.
Texture Development and Directionality
During the working processes, cast aluminum structures undergo a transformation into crystallographic textures, impacting the properties and characteristics of the material. These textures vary depending on the type of deformation, the shape of the workpiece, and the composition of the alloy.
The directional properties due to textures can pose challenges such as the formation of ears during the deep drawing of sheets.
“Heat treating” involves procedures aimed at enhancing the strength of heat-treatable aluminum alloys. These alloys differ from non-heat-treatable ones as they undergo a strengthening process through the manipulation of heating and cooling.
Aluminum Hardening
Heat treatment of aluminum alloys enhances their strength through a three-step process:
- Solution heat treatment: melting soluble phases
- Quenching: creating supersaturation
- Age hardening: forming solute atoms at room or high temperatures
Precipitation hardening, also known as age hardening, is a method used to increase the yield strength of various materials such as aluminum, magnesium, nickel, titanium, and specific stainless steels. This technique involves altering the solid solubility to generate fine impurity particles that impede crystal lattice defects, ultimately strengthening the material. These particles act similarly to those in particle-reinforced composites and can vary in size and properties. Age-hardening alloys are kept at elevated temperatures for a period to allow precipitation, known as aging, different from conventional tempering. Alloys may undergo solution and aging treatments, often abbreviated to “STA” in metal specifications.
Aluminum Aging Furnace
Concerning aluminum alloys, “heat-treatable” alloys focus on enhancing precipitation-hardenable wrought and cast alloys, unlike “non heat-treatable” alloys that rely on cold working for strength. The alloys can be annealed to soften them. A critical aspect of precipitation-hardening alloy systems is the temperature-dependent solid solubility, which increases with temperature. Major aluminum alloy systems utilizing precipitation hardening include aluminum-copper, aluminum-copper-magnesium, aluminum-magnesium-silicon, aluminum-zinc-magnesium, and aluminum-zinc-magnesium-copper.
Commercial heat-treatable alloys utilize ternary or quaternary solutes for precipitation strengthening, categorized in series like 2xxx, 6xxx, 7xxx for wrought alloys and 2xx.0, 3xx.0, 7xx.0 for casting alloys. The addition of magnesium along with copper, silicon, or zinc accelerates precipitation hardening, with the 6xxx series noted for its formability, weldability, machinability, and corrosion resistance.
Heat treatment procedures for aluminum alloys include solution heat treatment, quenching, and age hardening. Precipitation hardening is facilitated when the solid solubility line leans toward the center, resulting in finely dispersed precipitates during aging below the equilibrium solvus temperature. Around 10% of the alloy composition consists of elements for precipitation strengthening, with commercial alloys typically utilizing three components for strengthening purposes.
Modifying the aging temperatures can adjust the strength in many alloy systems. For instance, dry ice aging can enhance the strength of rivets used in aircraft construction, while higher aging temperatures may risk over-aging the parts. The control of the annealing process can tailor the strength levels and promote nucleation sites for disrupting dislocations.
To utilize precipitation hardening effectively, a solid solution must be achieved through solution heat treatment, aiming for a nearly homogeneous solid solution. Some alloys respond well to precipitation heat treatment following quenching, enhancing their strength without the need for a separate solution heat treatment process.
Overall, heat treatment plays a crucial role in strengthening aluminum alloys through a precise three-step process.
Quenching is a vital stage in heat-treating operations, crucial for retaining the solid solution formed during the solution heat treatment by rapid cooling to near room temperature.
To prevent detrimental types of precipitation, the solid solution formed during solution heat treatment must be rapidly quenched to attain a supersaturated solution at room temperature, ideal for precipitation hardening.
The resistance to stress-corrosion cracking in aluminum-zinc-magnesium alloys can be improved through slow quenching. Parts are typically quenched by immersion in cold water or by cold water sprays.
Age hardening
Following quenching, hardening can occur at room temperature (natural aging) or via precipitation heat treatment (artificial aging). Some alloys naturally age for a few days or undergo heat treatment to enhance their strength and hardness.
The 6xxx, 7xxx, and 2xxx alloys are subjected to solution heat treatment and quenching. While some alloys achieve useful tempers through natural aging alone for precipitation hardening, others require additional precipitation heat treatment.
The selection of optimal time-temperature cycles for precipitation heat treatment is crucial to achieve the best combination of properties. Longer times and higher temperatures lead to larger particles, impacting the properties and distribution pattern.
Precipitation heat treatment affects properties based on the initial structure of the material, resulting in the production of T6- and T7-type tempers. T7-type tempers involve sacrificing some strength to enhance other characteristics such as dimensional stability or reduce warpage.
T73 and T76 types of T7 tempers have been developed for alloys within the 7xxx series, aimed at improving corrosion and cracking resistance while enhancing fracture toughness.