Physical metallurgy of recycling wrought aluminum alloys

Abstract

It is quite characteristic of aluminum alloys that if the amount of any element addition is gradually increased, then an upper limit (a tolerance limit) is eventually reached where some important aspect of the alloy’s overall behavior is affected harmfully. Although alloy compositional registers and specifications commonly list upper limits for only the main alloying additions and usual impurities, in principle a practical “compositional tolerance limit” exists for essentially every element in the Periodic Table. Fairly complete knowledge of these tolerance limits for all elements is especially needed in recycling operations where unexpected and unusual impurities can creep in inadvertently, and even normal impurities may tend to accumulate and build up to a disastrous degree. This paper reviews and summarizes what is presently known about the various compositional tolerance limits—and the consequences of exceeding them—in wrought aluminum alloys. Because processes are not available either to remove or to reduce significantly the amounts of various elements that occur in unsegregated scrap, the nearly universal alternative for controlling such elements in recycled aluminum alloys is to dilute them with purer alloy grades or virgin pig. To reduce the dilution requirements, scrap recycling programs must be carefully planned and executed to assure a high degree of scrap segregation into selected grades during collection.