Aluminum-alloy properties and dimensional stability can be adjusted through annealing, quenching or solution heat treatment, natural and artificial aging, overaging, and cyclic treatment. This guide explains the purpose, temperature range, holding time, cooling method, and expected effect of each process. It also summarizes the basic F, O, H, W, and T temper designations defined for wrought aluminum products, helping readers connect processing history with strength, ductility, corrosion resistance, internal stress, and dimensional stability. The article preserves the source’s reported process conditions, units, material grades, and technical relationships for practical reference.
Annealing relieves casting stress and internal stress caused by machining, stabilizes part shape and dimensions, and spheroidizes some silicon crystals in Al-Si alloys to improve ductility. Aluminum-alloy castings are heated to 280 to 300°C, held for 2 to 3 hours, and cooled in the furnace to room temperature. The solid solution decomposes slowly and precipitated second-phase particles coalesce, relieving internal stress, stabilizing dimensions, improving ductility, and reducing deformation and warpage.
Quenching heats an aluminum-alloy casting to a high temperature, generally near the eutectic melting point and usually above 500°C, and holds it for more than 2 hours so that soluble phases dissolve fully. The casting is then quenched rapidly in water at 60 to 100°C. Rapid cooling retains the maximum amount of strengthening constituents in solution at room temperature. This treatment is also called solution heat treatment or cold treatment.
Aging, also called low-temperature tempering, heats a quenched aluminum-alloy casting to a specified temperature, holds it, and then air-cools it to room temperature. The process decomposes the supersaturated solid solution and stabilizes the alloy matrix. As temperature and time increase, atoms first rearrange in the supersaturated-solid-solution lattice to form solute-rich G-P I zones. The G-P I zones disappear, atoms of the second phase segregate according to a defined pattern, and G-P II zones form. Subsequent stages include formation of a metastable second phase, coexistence of many G-P II zones with a small amount of metastable phase, transformation of the metastable phase into the stable phase, and coalescence of second-phase particles. Aging is divided into natural and artificial aging. Natural aging occurs at room temperature; artificial aging includes partial artificial aging, full artificial aging, and overaging.
The casting is heated to 150 to 170°C and held for 3 to 5 hours. This produces good tensile strength, ductility, and toughness, but relatively low corrosion resistance.
The casting is heated to 175 to 185°C and held for 5 to 24 hours. This produces sufficient tensile strength and the highest hardness, but lower elongation.
The casting is heated to 190 to 230°C and held for 4 to 9 hours. Strength decreases and ductility increases, producing improved resistance to stress and corrosion. The process is also called stabilization tempering.
The casting is cooled to a subzero temperature such as −50, −70, or −195°C and held, then heated to below 350°C. Repeated contraction and expansion of the solid-solution lattice causes small grain displacements in each phase, placing atomic segregation zones and intermetallic particles into a more stable condition. This improves dimensional and volumetric stability. Cyclic treatment is used for highly precise, dimensionally stable parts, including some inspection-instrument components, but is not normally applied to general castings.
ISO 2107:2023, Aluminium and aluminium alloys – Wrought products – Temper designations, defines basic temper designations using one uppercase letter.
| Code | Name | Description and Application |
|---|---|---|
| F | As fabricated | Products for which no special control over work hardening or heat-treatment conditions is applied during forming. Specific mechanical-property limits are not normally specified. |
| O | Annealed | Wrought aluminum and aluminum-alloy products annealed to obtain lower strength and higher ductility. |
| H | Strain hardened | Products strengthened by strain hardening. Additional heat treatment may be applied after strain hardening to reduce strength partially. |
| W | Solution heat-treated | Products that naturally age at room temperature after solution heat treatment. This unstable temper is normally used only during natural aging. |
| T | Thermally treated to a stable temper other than F, O, or H | Products brought to a stable temper by heat treatment, with or without additional strain hardening. |
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