Heat treating aluminum castings: AFS Technical Dept., Schaumburg, Illinois.

Many aluminum castings meet property requirements in the as-cast condition and do not require further processing. However, depending on the alloy and casting process, aluminum castings have dimensional instability, low strength and toughness, or poor corrosion resistance that might make them inadequate for some applications in the as-cast state.

To improve these properties and enhance strength and ductility, they can be thermally processed by a series of heating and cooling cycles called heat treatment. This thermal processing involves three basic operations: solution, quenching and aging. Combinations of these processes are called tempers (Table 1).

Why Heat Treat?

The principal purpose for heat treating aluminum castings is to develop the best combination of mechanical properties that will meet the critical needs required for the specific application. The benefits of heat treatment include:

Homogenization--because alloying and impurity elements tend to segregate into the grain boundaries during cooling, the microstructure of castings in the as-cast state is not homogeneous. Concentrations of elements vary depending on alloy type and solidification rate. Homogenization is desirable to distribute elements throughout the matrix so properties in the casting will be more uniform.

Stress relief--residual stresses are created during cooling from elevated casting and solution temperatures. Heating the casting to an intermediate temperature can relieve internal stresses.

Improved stability and maehinability--changes in the microstructure can cause castings to grow over time. To maintain tight dimensional tolerances during and after machining, castings should be heat treated to form stable precipitate phases.

Mechanical property improvement--mechanical properties are improved by spheroidizing constituent phase particles and by precipitation hardening. Corrosion resistance is improved by homogenizing. Age hardening principles can be used to tailor heat treatments to applications.

Heat Treatment Cycles

The heat treatment cycle starts with solution treatment. During this stage, three events occur: the dissolution of elements that will later cause age hardening; spheroidization of undissolved constituents; and homogenization. In the as-cast condition, a large proportion of the precipitate-forming elements are contained in constituent phases. The primary purpose of solution treatment is to dissolve constituent phases and enrich the matrix phase aluminum. The second purpose of solution treatment is to spheroidize constituents that are present in quantities that cannot be fully dissolved. For example, in 3XX (AI-Si-Mg and AI-Si-Cu-Mg) alloys, excess silicon is always present. During solution treatment, silicon particles globularize and coarsen. The third purpose of solution treatment is to homogenize the microstructure when the elements in constituent phases are redistributed as they dissolve.

Rapid cooling or quenching cycles are then necessary to retain the dissolved elements in the solution. Although rapid quenching increases the response to age hardening, it also creates residual stresses and distortion. Aluminum castings are typically quenched in boiling water, but other methods, such as quenching in a glycol solution or in air using high-velocity fans, also are used.

Dissolved elements that are trapped in the solution during quenching eventually precipitate, occurring slowly at room temperature. After time at room temperature, some alloys will harden appreciably. Aging is accelerated by heating castings to intermediate temperatures, a process called artificial aging. After the initial stages of aging, the structure is said to be underaged. Increased time at age temperature or aging at a greater temperature further evolves the precipitate structure, and hardness increases to a peak hardness condition. Further aging decreases hardness, and the structure becomes overaged.

The aging process also affects ductility, which is high in the as-quenched condition and reduces during hardening. During overaging, a loss of hardening mechanisms permits extensive deformation to occur before fracture and ductility increases. Annealing, which is extreme overaging, maximizes ductility.

The recommended solution. quench and age times and temperatures are available for each alloy and temper. But these cycles are often customized to meet specific component requirements for strength and ductility.

Table 1. Heat Treatment Tempers

 Temper      Thermal Processing

   T4        Solution treat and age naturally to a substantially stable
             condition. Natural aging may continue slowly, particularly
             at elevated service temperatures, so structural stability
             may not be satisfactory.
   T6        Solution treat and age artificially. In castings, T6
             commonly describes optimum strength and ductility.
   T61       Solution treat, quench and age artificially for maximum
             hardness and strength. This variant of T6 yields additio-
             nal strength and stability but at reduced ductility.
   T7        Solution treat, quench and artificially overage or
             stabilize. This temper improves ductility, thermal
             stability and resistance to stress corrosion cracking.
   T71       Solution treat, quench and artificially overage to a
             substantially stable condition. This temper further
             increases thermal stability and resistance to stress
             corrosion cracking and reduces strength.
   T5        Age only. Stress relief or stabilization treatment. Cool
             from casting temperature and artificially age or stabilize
             (without prior solution treatment). Frequently, the as-
             cast condition provides acceptable mechanical properties
             but is accompanied by microstructural
             instability or undesirable residual stresses. Perhaps the
             possibility of in-service growth is the only constraint
             against using a casting in the as-cast state. In each
             case, the T5 temper is appropriate.

Annealing    Castings that have low strength requirements but require
             high dimensional stability are annealed. Annealing also
             substantially reduces residual stress, a need in die
             castings. Annealing is a severe stabilization treatment
             and an elevated temperature variant of the T5 temper.
             Softening occurs because annealing depletes the
             matrix of solutes, and the precipitates formed are too
             large to provide hardening.