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

Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships.

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Friction Stir Welding and Processing

[Lu, K.; Lu, L.] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China. [Lu, L.; Suresh, S.] MIT, Sch Engn, Cambridge, MA 02139 USA.;Lu, K (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China;lu@imr.ac.cn ssuresh@mit.edu

/pdf/strengthening-materials-by-engineering-coherent-internal-4fkjmv3btq.pdf

Strengthening Materials by Engineering Coherent Internal Boundaries at the Nanoscale

Recent advances in friction-stir welding : Process, weldment structure and properties

This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.

Recent developments in stainless steels

Dislocation structures of grain boundaries whose sliding behaviour had already been studied, have been observed by transmission electron microscopy of the slid grain boundaries. The ordered (coincidence) and random boundaries exhibited quite different sliding behaviour and dislocation structures. In the case of ordered boundaries, sliding was difficult and remarkable slide-hardening was shown. Transmission electron microscopy evidenced the presence of a number of EGBDs in these grain boundaries. In contrast, sliding of random boundaries easily took place showing slight slide-hardening. EGBDs were hardly observed in random boundaries. A systematic change in grain boundary misorientation during sliding was observed. An off-coincidence boundary changed into an almost exact coincidence boundary by the absorption of lattice dislocations. These results can be qualitatively explained by a dislocation model of sliding in which the absorption of lattice dislocations into the grain boundary is considered t...

Sliding behaviour and dislocation structures in aluminium grain boundaries

The precipitation sequence in friction stir weld of 6063 aluminum during postweld aging, associated with Vickers hardness profiles, has been examined by transmission electron microscopy. Friction stir welding produces a softened region in the weld, which is characterized by dissolution and growth of the precipitates. The precipitate-dissolved region contains a minimum hardness region in the aswelded condition. In the precipitate-dissolved region, postweld aging markedly increases the density of strengthening precipitates and leads to a large increase in hardness. On the other hand, aging forms few new precipitates in the precipitate-coarsened region, which shows a slight increase in hardness. The postweld aging at 443 K for 43.2 ks (12 hours) gives greater hardness in the overall weld than in the as-received base material and shifts the minimum hardness from the as-welded minimum hardness region to the precipitate-coarsened region. These hardness changes are consistent with the subsequent precipitation behavior during postweld aging. The postweld solution heat treatment (SHT) and aging achieve a high density of strengthening precipitates and bring a high hardness homogeneously in the overall weld.

Precipitation sequence in friction stir weld of 6063 aluminum during aging

Microstructural evolution during friction stir-processing of pure iron

Microstructural evolution of ultrahigh carbon steel during friction stir welding

A “stop-action” technique was employed to study the grain structure and texture development during friction stir welding of thin 6016 aluminum sheets. The microstructural evolution ahead of the tool pin was found to be a complex process involving geometrical effects of strain as well as simultaneous development of continuous and discontinuous recrystallizations. At the circumference of the pin, the material flow was shown to be governed by the simple shear deformation induced by the rotating pin, which led to the formation of a pronounced {1 1 2} simple shear texture. Beyond the pin, the material experienced additional hot deformation, possibly caused by the influence of the tool shoulder, and limited static annealing during the weld cooling cycle. As a result, a well-recrystallized grain structure having a characteristic {1 0 0} cube texture finally developed in the stir zone.

Hiroyuki Kokawa

Papers

Sliding behaviour and dislocation structures in aluminium grain boundaries

Precipitation sequence in friction stir weld of 6063 aluminum during aging

Microstructural evolution during friction stir-processing of pure iron

Microstructural evolution of ultrahigh carbon steel during friction stir welding

Grain structure and texture evolution during friction stir welding of thin 6016 aluminum alloy sheets