Influence of Retrogression and Re-Aging Treatment on Microstructure, Mechanical and Fatigue Crack Growth Behavior of Aluminum Alloy 7010

Abstract

The high strength aluminum alloys are widely used in construction of aircrafts structural components. From the safety and durability point of view these structures need to be highly damage tolerant under service loads. The damage tolerance behavior of these alloys when treated in retrogression and re-aging condition (RRA) is rarely studied. In the present research work the heat treatment of aluminum alloy 7010 is carried out at different aging conditions to study its influence on microstructure and mechanical properties. The mechanical properties and fatigue crack growth behavior is correlated to the modified microstructure of the alloy. The alloy is heat treated in standard peak-aging (T6), retrogression and re-aging (RRA) and over-aging (T7451) conditions. The microstructure is characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). The mechanical characterization of the alloys is performed by conducting hardness tests, standard tensile tests and fracture toughness tests. The fatigue crack growth performance of the alloy in different aging conditions is determined by conducting constant amplitude (CA) fatigue crack growth rate (FCGR) tests and spectrum fatigue crack growth tests under fighter aircraft loading standard for fatigue evaluation (FALSTAFF). The fatigue crack growth behavior is also predicted adopting a two parameter crack growth law using already determined CA FCGR data. The results are compared with the experimentally determined spectrum fatigue crack growth behavior under FALSTAFF loading. The alloys are characterized for electrochemical corrosion behavior and exfoliation corrosion (EXCO) behavior. The alloy microstructure in RRA condition consists of two different kinds of matrix precipitates, namely large sized η' (MgZn2) of size 15-20 nm along with the presence of metastable precipitates η' (MgZn2) of size 5-8 nm. Unlike RRA treated alloy, T6 alloy microstructure consists of only η' (MgZn2) of size 2-4 nm. The grain boundary precipitates in RRA treated condition is more similar to that of the over-aged condition i.e., discontinuous in nature unlike the continuous one present in the T6 condition of alloy. The retrogression performed at 200 ˚C for about 20 minutes of duration is optimized to obtain equivalent tensile strength as that of the T6 alloy. The FCGR behavior of RRAtreated alloy exhibited a considerable reduction in crack growth rate in the nearthreshold regime compared to the T6 treated alloy. The crack growth reduction by about 2-3 time in RRA alloy along with increase in threshold SIF (∆Kth) of about 0.6- 1 MPa√m is evident. The fatigue crack surface roughness is measured to be higher in the near-threshold regime of the RRA alloy. The alloy microstructure consisting of increased inter-precipitate distance is correlated to the increased resistance to fatigue crack growth rate. The observed behavior of improved crack growth resistance has lead to higher damage tolerance of the RRA alloy under service loads. An improvement of fatigue crack growth life by about 22 % is evident. The corrosion and EXCO performance of the RRA alloy is better compared to the standard T6 treated alloy. The chemistry of grain boundary precipitates characterized by using TEM-EDS determined a higher content of copper on grain boundary precipitate (η) which leads to exfoliation corrosion resistance of the alloy.