Temper Developments Using Secondary Ageing

Abstract Secondary ageing may occur if age hardenable aluminium alloys are first underaged at an elevated temperature (eg. 150°C), quenched and then exposed to a lower temperature (eg. 25-65°C). At these lower temperatures, nucleation of fine precipitates occurs that further depletes the microstructure of solute elements and gives rise to additional strengthening. This treatment has been designated the T6I4 temper (I= interrupted) by the authors, and alloys normally develop tensile properties close to those for the respective T6 tempers. If these alloys are then aged again at an elevated temperature (T6I6 temper) further increases in tensile properties are possible (e.g.10-15%), usually with simultaneous increases in fracture toughness. Microstructural changes associated with these improved properties are discussed and examples are given of other tempers that have been developed to meet specific service requirements. 1. Introduction Multi-stage heat treatments provide a means for modifying the size, composition, species, morphology and distribution of precipitate particles in aged aluminium alloys. These changes can result in improvements to mechanical and other properties [eg. 1-6]. An example of a multi-stage heat treatment is the T73 temper in which artificial ageing at one temperature (eg. 100°C) is followed by a second treatment at a higher temperature (eg. 160°C). The T73 temper increases the stress corrosion resistance of 7000 (Al-Zn-Mg-Cu) series alloys by modifying the microstructure, although some sacrifice in tensile properties occurs compared with the single stage, T6 temper [1,2]. Another treatment is to naturally age at room temperature after quenching and before artificial ageing. Such a delay period, which may be unavoidable during normal industrial processing, can increase strengthening of some 7000 series alloys [1] and casting alloys such as 356 and 357, whereas tensile properties are reduced in some 6000 series alloys [3]. In other alloys, a duplex treatment of this kind causes retrogression in which fine clusters or precipitates formed at low temperatures re-dissolve on elevated temperature ageing. For many years, it was accepted that once an aluminium alloy was artificially aged, the microstructures and mechanical properties remained stable when the alloy was subsequently held for extended periods at a significantly lower temperature.