PSI - Issue 14

M S Nandana et al. / Procedia Structural Integrity 14 (2019) 314–321 M S Nandana/ Structural Integrity Procedia 00 (2018) 000–000

315

Keywords: Retrogression;Fatigue crack growth;Microstructure;Precipitation hardening

1. Introduction

Aluminum alloys find its wide application in aerospace structural components due to its high specific strength, good damage tolerance capability and better corrosion resistance. The 7xxx Al-Zn-Mg-Cu alloys are used for primary structural members such as lug joints, bulk heads and wing spars in aircrafts, where strength is the limiting design parameter, described by Schijve (2009). These alloys possess high strength when heat treated to peak ageing (T6) condition. The aircraft materials, since they are exposed to aggressive environments like humidity, oil and saline medium, the microstructure of alloy in T6 heat treated condition leads to failure by stress corrosion cracking (SCC) and corrosion fatigue as studied by Puiggali et al. (1998). Efforts have been put forward in the past by Chen et al. (2012) to improve the corrosion resistance properties of these alloys by various heat treatment and microstructural modifications. It was shown by Rout et al. (2015) that two step over ageing (T7) is one of the heat treatment that imparts good stress corrosion cracking resistance to the alloy, but results with reduction in peak strength by 10-15 %. The heat treatment procedure called retrogression and re ageing was patented by Cina (1974) which involves three step ageing treatment after solution treatment. This procedure subjects the solution treated alloy to peak ageing (T6) condition, subsequently followed by reversion heat treatment known as retrogression. The retrogressed alloy is re-aged to T6 condition to complete the entire heat treatment procedure. This heat treatment is known to retain the peak strength and also impart good SCC resistance compared to that of T6 treated alloy. There are not much studies related to the effect of RRA on fatigue crack growth (FCG) behavior of the 7010 alloy in the literature. The current study aims at understanding the influence of RRA treatment on the microstructure and FCGR behavior of the 7010 alloy. The FCGR behavior of the RRA treated alloy in the near-threshold regime is compared with that of the T6 treated alloy.

2. Experimental

2.1. Material

The material used for the current research work was an aluminum alloy 7010, received in T7451 condition in rolled plate form. The chemical composition of the alloy is presented in Table 1. The as received microstructure of the alloy is shown in Fig. 1.

Table 1. Chemical composition of the AA7010 (wt %)

Zn 6.3

Mg

Cu

Zr

Fe

Si

Al

2.21

1.65

0.124

0.21

0.073

Balance

Fig. 1. Optical micrograph of the AA7010 in T7451 condition