Issue 42

A. Brotzu et alii, Frattura ed Integrità Strutturale, 42 (2017) 272-279; DOI: 10.3221/IGF-ESIS.42.29 273 9]. RRA consists of performing on the alloy in the T6 temper a two-stage thermal treatment: re-ageing follows a retrogression carried out at high temperature. More recently a non-isothermal ageing (NIA) has been developed with the aim of improving mechanical properties. The variations of mechanical properties and corrosion resistance during NIA were associated with the size, quantity and distribution of intra-grain and grain boundary precipitates [10, 11]. In a previous work [12] a cracked component made of an Al 7050 alloy was analyzed in order to identify possible causes of crack formation. The results highlighted that both the intermetallic phases and the associated microvoids, formed in the alloy during solidification and hot rolling process, probably determine crack formation during the material-removal processes. Considering that the studied Al 7050 component was subjected to anodization during the production process, it is interesting to highlight the susceptibility of this alloy to intergranular corrosion and to compare it with that of other high resistance alloys such as Al7075 T6, Al 7075 T7351 and Al 2195 T8 (Weldalite ® ). E XPERIMENTAL n this work four different high resistance aluminum alloys have been analyzed. These alloys differ by composition, heat treatment and manufacturing technique. The analysed specimens are described in Tab. 1. The heat treatments parameters are reported in Tab. 2. Alloy designation Heat treatment Components form Alloy nominal composition 1 7050 T7451 Hot rolled plate mechanically machined 2.3 Cu - 2.3 Mg - 6.2 Zn - 0.12 Zr and Fe,Si<0.1% 2 2195 T8 Rolled plate 0.94Li-0.39Mg-4.1Cu-0.5Fe-0.02Ti- 0.4Ag-0.13 Zr 3 7075 T6 Rolled Plate 1.6 Cu -2.5 Mg - 0.23 Cr - 5.6 Zn 4 7075 T7351 Hot ring rolled 1.6 Cu -2.5 Mg - 0.23 Cr - 5.6 Zn Table 1 : Compositions and heat treatments of the alloys studied in this work. Alloy designation Temper Heat treatment description Temperature 1 7050 T7451 -solution treatment 476 °C -Quenching (max. quenching bath T 43 °C) -double aging treatment: first step 121 °C for 3-60 hours second step 161 °C for 24-30hours 2 2195 T8 -solution treatment 460-490 °C -Quenching -artificial aging treatment: 160 °C for 14 hours 3 7075 T6 -solution treatment 460-498 °C -Quenching ) -artificial aging treatment: 121 °C for 24 hours 4 7075 T7351 -solution treatment 460-498 °C -Quenching (max. quenching bath T 43 °C) -double aging treatment: first step 107 °C for 3-60 hours second step 163 °C for 24-30hours Table 2 : Heat treatment description of the alloys studied in this work ( ASTM B918 Standard Practice for Heat Treatment of Wrought Aluminum Alloys). From each studied aluminum component, several specimens have been taken in order to fully characterize the metallographic structures of the material, to check the possible presence of microdefects, and to obtain specimens for the intergranular corrosion tests. Each aluminum component has been cut in order to get surfaces in the main metallurgical directions (longitudinal, short and long transverse). Specimens for both metallographic inspection and for intergranular corrosion tests have been ground to a mirror like surface using SiC papers up to 2400 mesh followed by 1 µm alumina. The specimens have been inspected by means of optical and electronic microscopes both before and after chemical I