Through a series of carefully designed experiments, characterization and some modeling tools, this work is aimed at studying the role of thermal profiles on different microstructural zones and associated properties like strength and corrosion through a variation of weld parameters, thermal boundary conditions and material temper. Two different alloys belonging to the Al-Cu and Al-Cu-Li system in different temper conditions- peak aged (T8) and annealed (O) were used. A 3D-thermal pseudo mechanical (TPM) model is developed for the FSW process using heat transfer module in COMSOL Multiphysics and is based on a heat source wherein the temperature dependent yield shear stress is used for the heat generation. The precipitation and coarsening model is based on the Kampmann and Wagner theoretical framework and accounts for the competition between the various nucleation sites for both metastable and equilibrium precipitates. The model predicts different precipitate mean radius and volume fraction for the various zones in the friction stir welded material. A model for the yield strength is developed which considers contributions from different strengthening mechanisms. The predictions of the each models have been verified against experimental data and literature. At constant advance per rotation, the peak temperature decreases with a decrease in traverse speed and increases with an increase in tool rotation. Weld properties were significantly affected by choice of thermal boundary conditions in terms of backing plate diffusivity. Weld conditions with a higher peak temperature and high strain rate results in more dissolution of precipitates and fragmentation of constituent particles resulting in a better corrosion behavior for the weld nugget. For a peak aged temper of 2XXX alloys, the weld nugget experiences dissolution of strengthening precipitates resulting in a lower strength and the Heat affected zone (HAZ) experiences coarsening of precipitates. For an annealed material, both the weld nugget and HAZ experiences dissolution of precipitates with an increase in strength in the weld nugget.

In this work, the relationship between microstructure and mechanical properties of dissimilar friction stir welded AA6061-to-AA7050 aluminum alloys were evaluated. Experimental results from this study revealed that static strength increased with the tool rotational speed and was correlated with the material intermixing. Fully-reversed low cycle fatigue experimental results showed an increase in the strain hardening properties as well as the number of cycles-to-failure as the tool rotational speed was increased. Furthermore, under both static and cyclic loading, fracture of the joint was dominated by the AA6061 alloy side of the weld. In addition, inspection of the fatigue surfaces revealed that cracks initiated from intermetallic particles located near the surface. In order to determine the corrosion resistance of the dissimilar joint, corrosion defects were produced on the crown surface of the weld by static immersion in 3.5% NaCl for various exposure times. Results revealed localized corrosion damage in the thermo-mechanically affected and heat affected zones. Results demonstrated a decrease in the fatigue life, with evidence of crack initiation at the corrosion defects; however, the fatigue life was nearly independent of the exposure time. This can be attributed to total fatigue life dominated by incubation time. Furthermore, two types of failure were observed: fatigue crack initiation in the AA6061 side at high strain amplitudes (>0.3%); and fatigue crack initiation in the AA7050 side at low strain amplitudes (

Friction Stir Processing of 2XXX Aluminum Alloys including Al-Li Alloys is the latest edition in the Friction Stir Welding and Processing series and examines the application of friction stir welding to high strength 2XXX series alloys, exploring the past and current developments in the field. The book features recent research showing significant benefit in terms of joint efficiency and fatigue performance as a result of friction stir welding. Friction stir welding has demonstrated significant benefits in terms of its potential to reduce cost and increase manufacturing efficiency of industrial products including transportation, particularly the aerospace sector. The 2XXX series aluminum alloys are the premium aluminum alloys used in aerospace. The book includes discussion of the potential future directions for further optimization, and is designed for both practicing engineers and materials scientists, as well as researchers in the field. Provides comprehensive coverage of friction stir welding of 2XXX series alloys Discusses the physical metallurgy of the alloys Includes physical metallurgy-based guidelines for obtaining high joint efficiency Features illustrated examples of the application of FSW in the aerospace industry

Friction Stir Welding of High Strength 7XXX Aluminum Alloys is the latest edition in the Friction Stir series and summarizes the research and application of friction stir welding to high strength 7XXX series alloys, exploring the past and current developments in the field. Friction stir welding has demonstrated significant benefits in terms of its potential to reduce cost and increase manufacturing efficiency of industrial products in transportation, particularly the aerospace sector. The 7XXX series aluminum alloys are the premium aluminum alloys used in aerospace. These alloys are typically not weldable by fusion techniques and considerable effort has been expended to develop friction stir welding parameters. Research in this area has shown significant benefit in terms of joint efficiency and fatigue performance as a result of friction stir welding. The book summarizes those results and includes discussion of the potential future directions for further optimization. Offers comprehensive coverage of friction stir welding of 7XXX series alloys Discusses the physical metallurgy of the alloys Includes physical metallurgy based guidelines for obtaining high joint efficiency Summarizes the research and application of friction stir welding to high strength 7XXX series alloys, exploring the past and current developments in the field

Metallurgy and Design of Alloys with Hierarchical Microstructures covers the fundamentals of processing-microstructure-property relationships and how multiple properties are balanced and optimized in materials with hierarchical microstructures widely used in critical applications. The discussion is based principally on metallic materials used in aircraft structures; however, because they have sufficiently diverse microstructures, the underlying principles can easily be extended to other materials systems. With the increasing microstructural complexity of structural materials, it is important for students, academic researchers and practicing engineers to possess the knowledge of how materials are optimized and how they will behave in service. The book integrates aspects of computational materials science, physical metallurgy, alloy design, process design, and structure-properties relationships, in a manner not done before. It fills a knowledge gap in the interrelationships of multiple microstructural and deformation mechanisms by applying the concepts and tools of designing microstructures for achieving combinations of engineering properties—such as strength, corrosion resistance, durability and damage tolerance in multi-component materials—used for critical structural applications. Discusses the science behind the properties and performance of advanced metallic materials Provides for the efficient design of materials and processes to satisfy targeted performance in materials and structures Enables the selection and development of new alloys for specific applications based upon evaluation of their microstructure as illustrated in this work

Rising demand for improved fuel economy and structural efficiency are the key factors for use of aluminum alloys for light weighting in aerospace industries. Precipitation strengthened 2XXX and 7XXX aluminum alloys are the key aluminum alloys used extensively in aerospace industry. Welding and joining is the critical step in manufacturing of integrated structures. Joining of precipitation strengthened aluminum alloys using conventional fusion welding techniques is difficult and rather undesirable in as it produces dendritic microstructure and porosities which can undermine the structural integrity of weldments. Friction stir welding, invented in 1991, is a solid state joining technique inherently benefitted to reduces the possibility of common defects associated with fusion based welding techniques. Weldability of various 2XXX and 7XXX aluminum alloys via friction stir welding was investigated. Microstructural and mechanical property evolution during welding and after post weld heat treatment was studied using experimental techniques such as transmission electron microscopy, differential scanning calorimetry, hardness testing, and tensile testing. Various factors such as peak welding temperature, cooling rate, external cooling methods (thermal management) which affects the strength of the weldment were studied. Post weld heat treatment of AL-Mg-Li alloy produced joint as strong as the parent material. Modified post weld heat treatment in case of welding of Al-Zn-Mg alloy also resulted in near 100% joint efficiency whereas the maximum weld strength achieved in case of welds of Al-Cu-Li alloys was around 80-85% of parent material strength. Low dislocation density and high nucleation barrier for the precipitates was observed to be responsible for relatively low strength recovery in Al-Cu-Li alloys as compared to Al-Mg-Li and Al-Zn-Mg alloys.