There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Welding Metallurgy of

The comprehensive body of knowledge that has built up with respect to the friction stir welding (FSW) of aluminium alloys since the technique was invented in 1991 is reviewed The basic principles of FSW are described, including thermal history and metal flow, before discussing how process parameters affect the weld microstructure and the likelihood of entraining defects After introducing the characteristic macroscopic features, the microstructural development and related distribution of hardness are reviewed in some detail for the two classes of wrought aluminium alloy (non-heat-treatable and heat-treatable) Finally, the range of mechanical properties that can be achieved is discussed, including consideration of residual stress, fracture, fatigue and corrosion It is demonstrated that FSW of aluminium is becoming an increasingly mature technology with numerous commercial applications In spite of this, much remains to be learned about the process and opportunities for further research a

Friction stir welding of aluminium alloys

Friction stir welding (FSW) is a widely used solid state joining process for soft materials such as aluminium alloys because it avoids many of the common problems of fusion welding. Commercial feasibility of the FSW process for harder alloys such as steels and titanium alloys awaits the development of cost effective and durable tools which lead to structurally sound welds consistently. Material selection and design profoundly affect the performance of tools, weld quality and cost. Here we review and critically examine several important aspects of FSW tools such as tool material selection, geometry and load bearing ability, mechanisms of tool degradation and process economics.

Review: friction stir welding tools

Friction stir welding (FSW) is a relatively new welding process that may have significant advantages compared to the fusion processes as follow: joining of conventionally non-fusion weldable alloys, reduced distortion and improved mechanical properties of weldable alloys joints due to the pure solid-state joining of metals. In this paper, a three-dimensional model based on finite element analysis is used to study the thermal history and thermomechanical process in the butt-welding of aluminum alloy 6061-T6. The model incorporates the mechanical reaction of the tool and thermomechanical process of the welded material. The heat source incorporated in the model involves the friction between the material and the probe and the shoulder. In order to provide a quantitative framework for understanding the dynamics of the FSW thermomechanical process, the thermal history and the evolution of longitudinal, lateral, and through-thickness stress in the friction stirred weld are simulated numerically. The X-ray diffraction (XRD) technique is used to measure the residual stress of the welded plate, and the measured results are used to validate the efficiency of the proposed model. The relationship between the calculated residual stresses of the weld and the process parameters such as tool traverse speed is presented. It is anticipated that the model can be extended to optimize the FSW process in order to minimize the residual stress of the weld.

https://www.smu.edu/~/media/Site/Lyle/RCAM/Publications/Finite%20element%20modeling%20of%20friction%20stir%20weldingthermal%20and%20thermomechanical%20analysis.ashx

Finite element modeling of friction stir welding—thermal and thermomechanical analysis

Interfacial reaction in steel–aluminum joints made by friction stir welding

Titanium alloy TiAl6V4 and aluminium alloy 2024-T3 were successfully joined by friction stir welding. Microstructure, hardness and tensile strength of the butt joint were investigated. The weld nugget exhibits a mixture of fine recrystallized grains of aluminium alloy and titanium particles. Hardness profile reveals a sharp decrease at titanium/aluminium interface and evidence of microstructural changes due to welding on the aluminium side. The ultimate tensile strength of the joint reached 73% of AA2024-T3 base material strength.

Friction stir welding of titanium alloy TiAl6V4 to aluminium alloy AA2024-T3

https://www.cavs.msstate.edu/publications/docs/2004/01/752In%20situ%20observations.pdf

In-situ observations of high cycle fatigue mechanisms in cast AM60B magnesium in vacuum and water vapor environments

Effects of Weld Imperfections and Residual Stresses on the Fatigue Crack Propagation in Friction Stir Welded Joints

We examine the growth of microstructurally small fatigue cracks in cast AM60B magnesium (Mg) cycled in a water vapor environment. The behavior and growth rates of the small cracks were measured in situ during cycling using a fatigue loading stage contained within an environmental scanning electron microscope (ESEM). We provide quantitative data describing the interaction of representative small fatigue cracks with microstructural features, along with the average growth rate data for approximately 20 different cracks. Small surface and corner cracks, with sizes ranging from 20 to 200 μm, are observed to interact strongly with the surface microstructure during growth. The small cracks preferentially propagate through the dendrite cells, and the particle laden interdendritic regions typically act as barriers to fatigue crack propagation. As the small cracks approach the interdendritic boundaries, measured growth rates decrease and the cracks sometimes becomes temporarily pinned at the boundary. Cracks smaller than 100 μm experience more significant disruptions in crack growth rates at interdendritic boundaries compared to the larger cracks that interact with the boundaries, but with less change in crack growth rates. Under nominally identical loading conditions, isolated microstructurally small cracks grow, on average, two orders of magnitude faster in a sample containing a higher fraction of porosity. The significantly higher crack growth rates in the more porous sample were attributed to local amplification of the nominal stress field in the vicinity of the microstructurally small cracks rather than explicit interactions between growing cracks and pores. Analogous to the wrought materials, the growth rate of microstructurally small cracks is observed to be significantly higher compared to long fatigue cracks at equivalent maximum cyclic stress intensity values.

/pdf/environmentally-influenced-microstructurally-small-fatigue-mhobaaa78y.pdf

Gerhard Biallas

Papers

Interfacial reaction in steel–aluminum joints made by friction stir welding

Friction stir welding of titanium alloy TiAl6V4 to aluminium alloy AA2024-T3

In-situ observations of high cycle fatigue mechanisms in cast AM60B magnesium in vacuum and water vapor environments

Effects of Weld Imperfections and Residual Stresses on the Fatigue Crack Propagation in Friction Stir Welded Joints

Environmentally influenced microstructurally small fatigue crack growth in cast magnesium