Julien Boselli

Fatigue Crack Growth Behavior of 2099-T83 Extrusions in two Different Environments

Aluminum-lithium alloy 2099-T83 is an advanced material with superior mechanical properties, as compared to traditional alloys used in structural applications, and has been selected for use in the latest generation of airplanes. While this alloy exhibits improved fatigue crack growth (FCG) performance over non-Li alloys, it is of interest to simulate the impact of fluctuating loads under variable temperature during airplane service, particularly in terms of the potential effects of material processing history. In the present paper, the FCG behavior in an Integrally Stiffened Panel (ISP) has been investigated both at room temperature and at 243 K. It has been shown that the resistance to crack growth in a cold environment was higher than in ambient laboratory air. Results of this investigation are discussed from the microfractographic point of view, with regard to the variation of the local extrusion aspect ratio, a parameter which correlates with both the crystallographic texture and the grain structure.

Fatigue Crack Propagation Rates and Local Texture Relationship in 2099-T83 Al-Li Alloy

An integrally stiffened panel (ISP) made from extruded 2099-T83 Al-Li alloy was subjected to fatigue loadings to investigate the influence of both the local texture and grain structure on fatigue crack propagation (FCP) behavior. The microstructure was mainly unrecrystallized. Grains were mostly layered in the web and fibrous in the other locations. Fiber texture components were present in the stiffener locations, and a rolling-type texture in the web. Resistance to FCP decreases as the local aspect ratio increases. Changes in FCP rates in the web, stiffener base and stiffener web were consistent with the microstructural features and texture. The stiffener cap with a strong fiber texture similar to that of the stiffener base exhibited a lower resistance to FCP, suggesting that the influence of the texture is convoluted in the stiffener cap by the markedly different grain structure. Therefore, FCP behavior in this alloy appears to be governed by both texture and grain structure.

Rapid Solidification of a New Generation Aluminum‐Lithium Alloy via Electrospark Deposition

Electrospark deposition (ESD) is a rapid solidification processing technique capable of depositing a metal onto a conductive substrate. The short pulse duration and high pulse frequency, combined with the small amount of material transferred during each pulse, results in high cooling rates being realized, on the order of 105–106 C/sec. This study investigates the ability to induce solute trapping behavior, for a new generation aluminum-lithium alloy, AA2199, using ESD.

The Metallurgy of High Fracture Toughness Aluminum-Based Plate Products for Aircraft Internal Structure

A significant volume of “thick” aluminum plate products is used in the manufacture of an aircraft’s internal structure in applications such as ribs, spars, frames, bulkheads, etc. With the recent launch of more fuel efficient and primarily metallic single aisle aircraft as well as the introduction of composite-intensive twin-aisle aircraft, a number of opportunities exist for upgrading alloys developed more than 30 years ago with a new generation of thick plate products. These include 7xxx aluminum alloys that show significant improvements in both strength and toughness along with Al-Li alloys that show high strength, low density and very high corrosion resistance with significantly improved toughness over previous generation Al-Li. This paper will review these improvements and provide insights into the metallurgy behind better fracture toughness, particularly in the short transverse direction, by considering the impact of composition and processing on quench sensitivity.

Al-Li Alloy 2099-T83 Extrusions: Static Mechanical Properties, Microstructure and Texture

Utilization of aluminium-lithium alloys in aerospace applications requires an understanding of how processing and product geometry impact their microstructure, crystallographic texture and mechanical properties. In this paper, the effect of various microstructural features as well as deformation textures on the static mechanical properties of Al-Li extruded components has been investigated. These relationships are discussed with regard to two 2099-T83 extruded sections, i.e. a cylindrical extrusion and an integrally stiffened panel (ISP). The ISP typically shows an unrecrystallized microstructure with varying texture depending on the location along its cross section while the cylindrical extrusions present a strong fibre texture. The anisotropy is noticeable in tensile and compressive tests for both types of extrusions.

Improvement in the Characterization of the 2099 Al-Li Alloy by FE-SEM

This paper describes how state-of-the-art Field-Emission Scanning Electron Microscopy (FE-SEM) can contribute to the characterization of the 2099 aluminum-lithium alloy, and metallic alloys in general. Investigations were carried out on bulk and thinned samples. BSE imaging at 3kV and STEM imaging at 30kV along with highly efficient microanalysis permitted to correlate experimental and expected structures. Although our results confirm previous studies, this work points out possible substitutions of Mg and Zn with Li, Al and Cu in the T1 precipitates. Zinc and magnesium are also present in “rice grain” shaped precipitates at the grain boundaries. The versatility of the FE-SEM is highlighted in that it can provide information at the macro and micro scales with relevant details. Its ability to probe the distribution of precipitates from nano-to micro-sizes throughout the matrix makes Field-Emission Scanning Electron Microscopy a suitable technique for the characterization of metallic alloys.

Effects of Deformation Texture Intensities and Precipitates on the Anisotropy of Mechanical Properties of Al‐Li Alloy 2099 T83 Extrusions

The use of aluminum-lithium alloys in aerospace applications requires a thorough knowledge of how processing and product geometry impact their microstructure, texture and mechanical properties. As with other aluminum alloys, anisotropy of mechanical properties has been related to the formation of deformation textures during thermo-mechanical processes.

Friction Stir Welding of Al-Li AA2199: Parameters, Precipitates and Post Weld Heat Treatment

The aerospace industry strives to develop materials allowing an increase in payload and reducing fuel consumption. Al-Li alloys, with their low density and high strength are currently in use for such applications and have potential for additional applications. When compared to composites, utilizing Al-Li alloy products is cost effective for aerospace companies as they do not need to redesign pre-existing fabrication facilities. The joining of these alloys by conventional methods is limited by segregation of alloying elements and the formation of oxides during high temperature exposure. This study focuses on solid state joining method that has the potential to generate low heat and be defect free - Friction Stir Welding (FSW). AA2199 sheets were joined by FSW. Process variables included table force, tool rotation speed and weld travel speed. A post weld heat treatment (PWHT) was applied to improve the mechanical properties by precipitation of strengthening phases. An increase in hardness of the weld zone from 95HV to 125HV upon PWHT was recorded for selected welding conditions. The type and morphology of second phase precipitates is deemed responsible for this effect. It is suggested that the high temperature and high strain levels characteristic of welds with fast tool rotation allow for the dissolution of precipitates during welding. The re-precipitation of these second phases during PWHT allowed the welds to recover strength to the level of the base material.

Solid Freeform Fabrication of Al-Li 2199 via Controlled-Short-Circuit-MIG Welding

Aluminum-lithium (Al-Li) alloys are of interest to the aerospace and aeronautical industries as rising fuel costs and increasing environmental restrictions are promoting reductions in vehicle weight. However, Al-Li alloys suffer from several issues during fusion welding processes including solute segregation and depletion. Solid freeform fabrication (SFF) of materials is a repair or rapid prototyping process, in which the deposited feedstock is built-up via a layering process to the required geometry. Recent developments have led to the investigation of SFF processes via Gas Metal Arc Welding (GMAW) capable of producing functional metallic components. A SFF process via GMAW would be instrumental in reducing costs associated with the production and repair of Al-Li components. Furthermore the newly developed Controlled-Short-Circuit-MIG (CSC-MIG) process provides the ability to control the weld parameters with a high degree of accuracy, thus enabling the optimization of the solidification parameters required to avoid solute depletion and segregation within an Al-Li alloy. The objective of this study is to develop the welding parameters required to avoid lithium depletion and segregation. In the present study weldments were produced via CSC-MIG process, using Al-Li 2199 sheet samples as the filler material. The residual lithium concentration within the weldments was then determined via Atomic Absorption (AA) and X-ray Photoelectron Spectroscopy (XPS). The microstructure was analyzed using High Resolution Scanning Electron Microscopy (HR-SEM). Finally the mechanical properties of welded samples were determined through the application of hardness and tensile testing.