Roberto J. Rioja

Fabrication methods to manufacture isotropic Al-Li alloys and products for space and aerospace applications

Abstract After a description of the evolution of Al-Li alloys, this paper first reviews the subject of anisotropy in mechanical properties of Al-Li wrought products. In plane, through thickness and axisymmetric flow anisotropies present in Al-Li wrought products are defined. The root causes for these anisotropies are discussed. Pros and cons arising from anisotropic products are then discussed in the context of designing and manufacturing for space and aerospace structures. Numerous attempts aimed to reduce anisotropy in rolled and extruded products are summarized. It is concluded that ‘isotropic’ Al-Li products can be manufactured via the selection of appropriate process, composition and processing parameters.

Processing of an experimental aluminum–lithium alloy for controlled microstructure

Abstract This paper describes the processing route that was used to obtain unrecrystallized and recrystallized microstructural versions of an experimental Al–Li alloy. The alloy, designated AF/C-489, has a nominal chemical composition of Al–2.05 Li–2.7 Cu–0.3 Mg–0.6 Zn–0.3 Mn–0.04 Zr (weight percentage). Processing schemes to obtain unrecrystallized and recrystallized conditions of the Al–Li plate were obtained by performing dynamic material modeling. Dynamic material modeling involved compression testing over strain rates ranging from 0.05 to 10 s −1 and temperatures ranging from 300 to 550°C and generating a processing map. Rolling parameters were obtained by using the constitutive equations obtained from dynamic material modeling as input to the finite-element analysis code.

Al-Li-Cu-Mg-(Ag) Products for Lower Wing Skin Applications

Al-Li-Cu-Mg alloy products, with and without Ag additions provide substantial performance advantages over conventional 2xxx products. For lower wing applications, the combination of specific ultimate tensile strength and damage tolerance is of particular importance and this is an area in which the Al-Li alloys can excel. Since Al-Li products have historically suffered with issues surrounding high property gradients through the plate thickness and high degrees of tensile in-plane anisotropy, a great deal of attention has been paid to the thermo-mechanical processing routes used in the fabrication of the current generation of alloy products. In addition, corrosion resistance is an area that has received greater attention recently since it can impact inspection intervals. In this presentation, the microstructures and properties of two new alloy products aimed for lower wing applications, 2199-T86 and 2060-T8E86, will be reviewed and compared with non-Li 2xxx products. It is concluded that the performance improvements of Al-Li alloys/products in addition to their lower density will enable significant weight savings in modern aircraft.

Development of High Toughness Sheet and Extruded Products for Airplane Fuselage Structures

High specific ultimate strength and high plane stress fracture toughness are primary requirements of aircraft fuselage skins. The performance of alloys/products used in high performance fuselage applications is first reviewed. The specific fracture toughness for products such as 2017-T3, 2024-T3, 2524-T3 and 6013-T6, is discussed as a function of their composition and microstructure. Then the performance of modern Al-Li alloys/products such as 2199 and 2060 sheet and 2099 and 2055 extrusions is examined. It is concluded that the performance of Li containing alloys/products offer significant improvements over non-Li containing conventional fuselage products because of the optimization of strengthening precipitates and grain microstructures. The role of chemical composition on resulting microstructures is discussed.

The Evolution of Plate and Extruded Products with High Strength and Fracture Toughness

From the first use of 2017-T74 on the Junkers F13, improvements have been made to plate and extruded products for applications requiring the highest attainable strength and adequate fracture toughness. One such application is the upper wing of large aircraft. The progression of these product improvements achieved through the development of alloys that include 7075-(T6 & T76), 7150-(T6 & T77) and 7055-(T77 & T79) and most recently 7255-(T77 & T79) is reviewed. The most current advancements include aluminum-copper-lithium, alloy 2055 plate and extruded products that can attain strength equivalent to that of 7055-T77 with higher modulus, similar fracture toughness and improved fatigue, fatigue crack growth and corrosion performance. The achievement of these properties is explained in terms of the several alloy design principles. The highly desired and balanced characteristics make these products ideal for upper wing applications.

Nucleation sites of the T2 phase in alloy 2090

Alloy 2090 fabricated as O-Temper sheet exhibited the presence of the T2 phase. The morphology and nucleation characteristics of the T2 phase in alloy 2090 were investigated by transmission electron microscopy. Also, the crystal structure was documented by electron and X-ray diffraction analyses. It was found that T2 precipitates formed preferentially at the interface of heterogeneities in the alloy. Several types of heterogeneities were identified. From these results it was proposed that as the impurities are eliminated, the propensity for nucleation of the T2 phase should be reduced.

Functional Gradient Products Enabled by Planar Solidification Technologies

A new class of materials with engineered compositional gradients through the thickness is introduced. Ingots with different types of composition gradients through the thickness were fabricated into plate. The composition and mechanical properties of these “Functional Gradient” plate products (metallic composites) are described, and the energy absorption performance of different architectures of functional gradient products is discussed in terms of dimensionless parameters. It is concluded that ab-initio simulations of architecture and performance are needed for the development, optimization and use of functional gradient products.

The Effect of Planar Solidification on Mechanical Properties of Al‐Zn‐Mg‐Cu‐Zr Alloy Plate

Direct-Chill (DC) casting of aluminum ingots and billets for the subsequent manufacture of wrought product forms has been the established industry standard practice for the past 80 years. The through-thickness characteristics of aluminum products are well understood and are affected by the chemical variability introduced during DC casting by the phenomenon known as macrosegregation. In the present work, an ingot of Al-Zn-Cu-Mg-Zr was cast using a novel planar solidification approach that avoids conventional DC macrosegregation and achieves a highly uniform composition through-thickness. This ingot was fabricated to 25 mm plate and its chemical, microstructural, and mechanical characteristics are compared to a DC cast and fabricated plate of similar composition, thickness, and temper.

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.