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

The current understanding of the fundamentals of recrystallization is summarized. This includes understanding the as-deformed state. Several aspects of recrystallization are described: nucleation and growth, the development of misorientation during deformation, continuous, dynamic, and geometric dynamic recrystallization, particle effects, and texture. This article is authored by the leading experts in these areas. The subjects are discussed individually and recommendations for further study are listed in the final section.

/pdf/current-issues-in-recrystallization-a-review-12181rj4pn.pdf

Current issues in recrystallization: a review

Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions

A review of dynamic recrystallization phenomena in metallic materials

Constitutive equations including an Arrhenius term have been commonly applied to steels with the objective of calculating hot rolling and forging forces. The function relating stress and strain rate is generally the hyperbolic-sine since the power and exponential laws lose linearity at high and low stresses, respectively. In austenitic steels, the equations have been used primarily for the peak stress (strain) associated with dynamic recrystallization (DRX) but also for the critical and steady state stresses (strains) for nucleation and first wave completion of DRX. Since the peak strain is raised by the presence of solutes and fine particles, the stress is raised more than by simple strain hardening increase, thus causing a marked rise in activation energy in alloy steels. In contrast, large carbides, inclusions or segregates, if hard, may lower the peak strain as a result of particle stimulated nucleation. Due to the linear relation between stress and strain at the peak, flow curves can be calculated from the constitutive data with only one additional constant. Maximum pass stresses can also be calculated from a sinh constitutive equation determined in multistage torsion simulations of rolling schedules. Comparison is made between carbon, micro-alloyed, tool and stainless steels and to ferritic steels which usually do not exhibit DRX. Parallels to the effects of impurities and dispersoids on the constitutive equations for Al alloys are briefly discussed.

Constitutive analysis in hot working

Current issues in recrystallization: A review

Although hot working had been defined as deformation above the recrystallization temperature (determined after cold working), it was only about 1965 that dynamic recrystallization (DRX) was confirmed to be occurring during the deformation; two decades were required to clarify the similarities to, and the differences from, static recrystallization. In classical discontinuous DRX in Cu, Ni, and γ-Fe, successive necklaces of new grains cause work softening; however in steady-state, the nuclei are uniformly distributed as reestablished dislocation structure limits growth. In high recovery metals at high strains, the grain boundary (GB) serrations meet across the elongated thinned grains thus pinching them off into almost equiaxed grains containing a substructure, thus geometric DRX.

H.J. McQueen

Papers

Current issues in recrystallization: a review

Constitutive analysis in hot working

Current issues in recrystallization: A review

Development of dynamic recrystallization theory

Recovery and Recrystallization during High Temperature Deformation