Stress Corrosion Cracking of Friction Stir-Welded AA-2024 T3 Alloy.

Marina Cabrini,Sergio Lorenzi,Claudio Giardini,Cristian Testa,Tommaso Pastore,Sara Bocchi,Gianluca D'Urso

doi:10.3390/ma13112610

Abstract

The paper is devoted to the study of stress corrosion cracking phenomena in friction stir welding AA-2024 T3 joints. Constant load (CL) cell and slow strain rate (SSR) tests were carried out in aerated NaCl 35 g/L solution. During the tests, open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) were measured in the different zones of the welding. The results evidenced initial practical nobilty of the nugget lower compared to both heat-affected zone and the base metal. This effect can be mainly ascribed to the aluminum matrix depletion in copper, which precipitates in form of copper-rich second phases. In this zones, no stress corrosion cracking was noticed, but well-evident stress-enhanced intergranular corrosion occurred. This is due to the uneven distribution of platic deformation during the slow strain rate tests. Higher strain values are localized at the heat affected zone, where softening occurs. On the contrary, stress values at the nugget are not sufficient to favor both the initiation and propagation of stress corrosion cracks. In the range of processing parameter studied in this experimental work, the stress corrosion cracking susceptibility of the friction stir welding (FSW)-ed alloy is then similar to that of the base metal.

Highlights

Friction stir welding (FSW) is a solid-state joining process that is gaining a lot of attention especially for high-strength age-hardening aluminum alloys, in which mechanical properties are strictly dependent upon heat treatments at relatively lower temperatures [1,2,3,4]
Increase of OPC followed by a decrease of about 150 mV
The applied load enhances the intergranular corrosion at the nugget of the AA 2024-T3 alloy owing to the presence of micrometric copper-rich precipitates at the border of the recrystallized grains

Summary

Introduction

Friction stir welding (FSW) is a solid-state joining process that is gaining a lot of attention especially for high-strength age-hardening aluminum alloys, in which mechanical properties are strictly dependent upon heat treatments at relatively lower temperatures [1,2,3,4]. The joint is produced by the friction generated between a rotating pin put directly in contact with the base metal. The thermo-mechanical action causes the material recrystallization that is localized in the central area of the weld, called nugget characterized by outstanding tensile properties [7,8]. The zones close to the nugget, which has the microstructure altered both by the action of the mechanical straining and the thermal effect, is called thermo-mechanically affected zone (TMAZ); after this zone, moving away from the nugget toward the base metal, there is an area that has not Materials 2020, 13, 2610; doi:10.3390/ma13112610 www.mdpi.com/journal/materials

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