Feasibility of FS as a deposition layer technique to apply as fatigue crack propagation stopper

Abstract

The search for innovative technologies with the purpose of increasing service life, thus reducing maintenance and frequency of inspections, is the main objective for many aircraft industries, such as EMBRAER S.A. Techniques concerning the damage tolerance improvement of fuselage structures are becoming an interesting solution when it comes to aluminum structures exposed to fatigue. There is a rising interest in the use of friction surfacing (FS) as a technique to retard fatigue crack propagation (FCP). FS is a deposition process, used to deposit layers of material through plastic deformation and high temperature. Once the process temperature achieved during FS is below the melting point, the deposition occurs entirely in a solid state This characteristic is advantageous regarding the joining of “non-weldable” alloys, such as the high alloyed aluminum alloys from 2XXX and 7XXX series. The first part of this study is to identify the best parameter set, regarding satisfactory superficial morphology, effective bonding width, and bonding strength. Thus, light microscopy (LM), scanning electron microscopy (SEM) equipped with a field emission gun (FEG), Vickers microhardness, and three-point bending tests are used to evaluate the best set, in terms of microstructural features and mechanical behavior. Then, the second part consists of understanding the mechanical and FCP behavior of friction surfaced AA2024 on AA7475 substrate, by evaluating the yield strength (YS), ultimate tensile strength (UTS), and elongation with uniaxial tensile tests. For the FCP tests, three specimen configurations are used to compare the influence of the advancing side (AS) and retreating side (RS) in the fatigue crack growth rate (FCGR) and the influence of a post surface machining process. To identify features on the fractured surfaces after tensile and fatigue failure, LM and SEM-FEG fractography were performed. Furthermore, this work has resulted from the industrial research entitled “Friction Surfacing for Damage Tolerance Improvement in Aircraft Structure” from the joining effort among partners from Department of Solid-State Materials Processing from the Institute of Materials Mechanics (Helmholtz-Zentrum Hereon) in Germany, Embraer, State University of Ponta Grossa (UEPG).