Influence of Extrusion Temperature on the Aging Behavior and Mechanical Properties of an AA6060 Aluminum Alloy

Nadja Berndt,Martin Wagner,Philipp Frint

doi:10.3390/met8010051

Abstract

Processing of AA6060 aluminum alloys for semi-products usually includes hot extrusion with subsequent artificial aging for several hours. Processing below the recrystallization temperature allows for an increased strength at a significantly reduced annealing time by combining strain hardening and precipitation hardening. In this study, we investigate the potential of cold and warm extrusion as alternative processing routes for high strength aluminum semi-products. Cast billets of the age hardening aluminum alloy AA6060 were solution annealed and then extruded at room temperature, 120 or 170 °C, followed by an aging treatment. Electron microscopy and mechanical testing were performed on the as-extruded as well as the annealed materials to characterize the resulting microstructural features and mechanical properties. All of the extruded profiles exhibit similar, strongly graded microstructures. The strain gradients and the varying extrusion temperatures lead to different stages of dynamic precipitation in the as-extruded materials, which significantly alter the subsequent aging behavior and mechanical properties. The experimental results demonstrate that extrusion below recrystallization temperature allows for high strength at a massively reduced aging time due to dynamic precipitation and/or accelerated precipitation kinetics. The highest strength and ductility were achieved by extrusion at 120 °C and subsequent short-time aging.

Highlights

Aluminum alloys of the 6xxx series, which are prominent examples for many structural applications in automotive and aviation industry, are usually processed by hot extrusion
Extrusion was performed at room temperature (RT), which results in maximum strain hardening of the aluminum alloy
We have studied the effect of extrusion temperature on the aging behavior and the resulting mechanical properties of cylindrical rods that were extruded from cast billets of an AA6060 aluminum alloy at room temperature (RT), at 120 ◦ C (IT), and at 170 ◦ C (AT), respectively

Summary

Introduction

Aluminum alloys of the 6xxx series, which are prominent examples for many structural applications in automotive and aviation industry, are usually processed by hot extrusion. Their mechanical properties are adjusted by a subsequent heat treatment since the alloying elements Mg and Si enable an effective precipitation hardening the low flow stress at elevated temperatures allows for high extrusion ratios (ratio of cross sectional area of the billet versus the extrudate) as well as pressing speeds. Enhancing the strength of the extruded semi-products by artificial aging at 160 to 180 ◦ C takes up several hours. Alternative processing strategy that considerably reduces aging times and increases strength is cold deformation of the as-solutionized material. The high densities of dislocations and vacancies produced by large plastic strains ensure effective strain hardening; they increase the diffusion rates of solute atoms, and can significantly accelerate precipitation kinetics [2,3]

Methods

Results

Conclusion