Evolution of Work Hardening and Hemming in 6xxx Aluminum Alloys in the Course of Natural Aging Following Continuous Annealing

Abstract

Hemming is one of the formability-related properties which is difficult to describe with the forming limit curve (FLC) and unlike other forming features, its relationship with work hardening behaviour is not known. It is generally known that the formability of Al sheet material is determined by its work hardening behavior up to the onset of strain localization. Under industrial production conditions of a continuous annealing line, the material undergoes various thermomechanical processing steps which can have an adverse effect on work hardening, i.e. natural aging, pre-straining, and pre-aging. Therefore, it is essential to understand the evolution of work hardening during these treatments after solution heat treatment and quenching. In this work, the impact of the main post-quench processing parameters of a continuous annealing line on the work hardening behaviour of AA6016 and AA6005 were investigated experimentally by laboratory simulations. Based on the true stress σ–true strain ε curve, the work hardening θ = dσ/dε was characterized by two parameters, namely the initial work hardening rate θ0 and the dynamic recovery rate β = dθ/dσ. The investigation revealed that both parameters θ0 and β decrease most in the course of natural aging after solution heat treatment and quenching, but subsequent processing steps also influence the work hardening behavior.KeywordsAluminumMechanical propertiesPhase transformationsWork hardeningNatural aging