Issue 42

A. Brotzu et alii, Frattura ed Integrità Strutturale, 42 (2017) 272-279; DOI: 10.3221/IGF-ESIS.42.29 272 Investigation on some factors affecting crack formation in high resistance aluminum alloys A. Brotzu, F. Felli, D.Pilone Dip. ICMA, Sapienza Università di Roma andrea.brotzu@uniroma1.it , ferdinando.felli@uniroma1.it , daniela.pilone@uniroma1.it A BSTRACT . Aluminum alloys having good mechanical properties are Al-Zn- Mg alloys (7xxx) and Al-Cu-Li alloys (Weldalite ®) . These alloys may be subjected to stress corrosion cracking. In order to overcome this problem the Al 7050 alloy has been developed and it is widely used for aerospace applications. Despite that, some components made of this alloy cracked during the manufacturing process including machining and chemical anodization. In a previous work cracked Al 7050 components have been analyzed in order to identify possible causes of crack formation. In this work the susceptibility of this alloy to intergranular corrosion has been analysed and compared with that of other high resistance aluminum alloys. K EYWORDS . Aluminum alloys; Al 7050; Fracture; Intergranular corrosion. Citation: Brotzu, A., Felli, F., Pilone, D., Investigation on some factors affecting crack formation in high resistance aluminum alloys, Frattura ed Integrità Strutturale, 42 (2017) 272-279. Received: 17.07.2017 Accepted: 13.08.2017 Published: 01.10.2017 Copyright: © 2017 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION l 7XXX alloys find useful applications in the field of aerospace engineering due to their attractive properties, such as low density, high strength, toughness and resistance to fatigue [1, 2]. Despite that Al–Zn–Mg–Cu alloys can be subjected to local corrosion, like inter-granular corrosion (IGC), and stress corrosion cracking (SCC) [3]. Many studies have been carried out in order to study the effect of the alloy microstructure on the material strength [4]. Mg and Cu form precipitates after solution heat treatment and aging. The strengthening effects of these precipitates depend on their size, spacing and distribution. In these alloys strength can be further increased by means of solid solution strengthening and work hardening. In Al 7050 and 7055 alloys Cr, Zr and Mn that have low solubility in the aluminum matrix, form intermetallic compounds. They are dispersoids that pin grain boundaries and avoid grain growth during heat treatments. Considering that the low corrosion resistance of many alloys limit their application, many efforts have been done to improve it. In the studies reported in the literature, two mechanisms have been proposed: anodic dissolution and hydrogen embrittlement [5]. Several studies highlighted that over-aging treatments could significantly reduce the corrosion susceptibility, but this determines a reduction of about 10-15% of the alloy strength [7]. Because of that, a retrogression and re-ageing (RRA) treatment was studied with the aim of obtaining both high strength and good corrosion resistance [8, A