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Traditionally, hot water is often selected as an alternative to cold-water quenching of heat-treatable aluminum alloys for distortion reduction. The levels of reduction are often modest, however, and more effective means are required.
Over the years, Type-I polymer quenchants - as defined by SAE AMS 3025 - have been used increasingly because the levels of distortion reduction typically are dramatically lower than that achievable with hot water while still meeting Mil Handbook 5 design minimums. Distortion and residual-stress reduction achievable with Type-I quenchants are discussed here, and reasons for this behavioral advantage are discussed.
Introduction
Aluminum is solution treated at temperatures generally in the range of 400-540°C (750-1000°F). During solution treatment, some alloying elements are re-dissolved to produce a solute-rich solid solution. The objective of this process is to maximize the concentration of hardening elements, including copper, zinc, magnesium and/ or silicon in the solid solution. The concentration and rate of dissolution of these elements increases with temperature. Therefore, solutionizing temperatures are usually near the liquidus temperature of the alloy.[1,2]
If an aluminum alloy is slowly cooled from an elevated temperature, alloying elements precipitate and diffuse from solid solution to concentrate at the grain boundaries, at small voids, on undissolved particles, at dislocations and at other imperfections in the aluminum lattice.[2] For optimal properties, it is desirable to retard this diffusion process and maintain the alloying elements in solid solution. This is done by quenching from the solution temperature.
For quench-hardenable wrought alloys 2xxx, 6xxx and 7xxx and casting alloys such as 356, this is accomplished by the quenching process. The objective is to quench sufficiently fast to avoid an undesirable concentration of the alloying elements in the defect and grain-boundary structure. After quenching, aluminum alloys are aged, and a fine dispersion of elements and compounds are precipitated that significantly increase material strength. The diffusion process and precipitation kinetics varies with the alloy chemistry.
The cooling process of age-hardenable aluminum alloys not only affects properties such as strength and ductility, but it also affects thermal stresses. Thermal stresses are typically minimized by reducing the cooling rate from the solutionizing temperature. If the cooling rate is too slow, however, undesirable grain-boundary precipitation will result. If the cooling rate is too fast, there is an increased...