Effect of Single and Multi-Stage Aging Cycle on the Microstructure of Age Harden Aluminium Alloys - A Critical Review

Precipitation hardening involves a systematic heating and cooling approach for solid-state strengthening. Most of the partially soluble type pure metal alloys (aluminium, copper, titanium, and nickel) and a few categories of special steels can be easily targeted. The approach comprises three stages like: nonequilibrium high-temperature heating action to reduce the microsegregation of solute clusters in the alloy, normal water processing to attain nonequilibrium supersaturated solid solution, and longtime isothermal processing to allow the formation of coherent precipitates that help to increase the strength of alloy by solid-state strengthening, followed by further quenching to the arrest diffusion process. The expected strength and hardness level hike depend on the fineness, degree of dispersion, type, and coherency level of the precipitates forming by the treatment. It is possible to achieve state-of-the-art microstructure by suitably amending the conventional precipitation hardening approaches, especially adapting multiple-step aging treatments on the alloys. Modified precipitation heat treatment has now been the area of interest of investigation to achieve many folds higher strength and frictional coefficients than the recently developed precipitation hardening route. Also, it is more advantageous in reducing the long duration of conventional aging heat treatment and energy consumption. With this motive, an attempt is made in the present paper to review the published work to present the modifications carried out in precipitation hardening and their effects on the properties of alloys. The authors believe that the reported summary of literature in this work would be a document for exploring the new direction of research in the area of precipitation hardening of alloys by tailoring the aging cycles.