QUENCHING FUNDAMENTALS--QUENCHING OF ALUMINUM ALLOYS: COOLING RATE, STRENGTH, AND INTERGRANULAR CORROSION

摘要

Aluminum is solution treated at temperatures generally in the range of 400 to 540°C (750 to 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. 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, small voids, on undissolved particles, at dislocations, and other imperfections in the aluminum lattice as shown in Fig. 1. 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 undesirable concentration of the alloying elements in the defect and grain boundary structure while at the same time not quenching faster than necessary to minimize residual stresses, which may lead to excessive distortion or cracking. After quenching, aluminum alloys are aged, and during this process, a fine dispersion of elements and compounds are precipitated that significantly increase material strength. The diffusion process and precipitation kinetics vary with the alloy chemistry.