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

Perspectives on Titanium Science and Technology

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

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

A review of dynamic recrystallization phenomena in metallic materials

The crystallography of bainitic ferrite nucleated at austenite grain boundaries was studied in an Fe-9Ni-0.15C (mass pct) alloy. The relationship between bainitic ferrite orientations (variants) and grain boundary characters, i.e., misorientation and boundary orientation, was examined by electron backscatter diffraction analysis in scanning electron microscopy and serial sectioning observation. Bainitic ferrite holds nearly the Kurdjumov–Sachs (K-S) orientation relationship with respect to the austenite grain into which it grows. At the beginning of transformation, the variants of bainitic ferrite are severely restricted by the following two rules, both advantageous in terms of interfacial energy: (1) smaller misorientation from the K-S relationship with respect to the opposite austenite grain and (2) elimination of the larger grain boundary area by the nucleation of bainitic ferrite. As the transformation proceeds, variant selection establishing plastic accommodation of transformation strain to a larger extent becomes important. Those kinds of variant selection result in formation of coarse blocks for small undercooling.

Variant Selection in Grain Boundary Nucleation of Upper Bainite

The influence of prior deformation of austenite (γ) on the formation of hcp (∈) martensite in Fe-Mn binary alloys during subsequent cooling was investigated. The formation of bcc (α') lath martensite in deformed austenite was also examined for comparison. In both of the martensitic transformations to α'in the Fe-9%Mn alloy and to ∈ in the Fe-16%Mn and Fe-24%Mn alloys, the transformation start temperature (M s ) monotonously decreases with increasing the reduction of austenite at 773 K

Effect of prior deformation of austenite on the γ → ε martensitic transformation in Fe-Mn alloys

Mechanism of dynamic continuous recrystallization during superplastic deformation in a microduplex stainless steel

Local orientation change inside lenticular martensite plate in Fe–33Ni alloy

Crystallographic orientation relationships between intragranular ferrites, the MnS+V(C, N) complex precipitates acting as ferrite nucleation sites, and austenite matrix were studied in Fe-Mn-C alloys by scanning electron and transmission electron microscopy. VC holds a cube-cube orientation relationship (‹001›γ// ‹001›VC) when it is formed directly within austenite grains in an Fe-12Mn-0.8C-0.3V alloy. When VC precipitates nucleate on incoherent MnS particles dispersed in austenite, there is no specific orientation relationship between the three phases. Intragranular ferrite idiomorphs nucleating on the MnS+V(C, N) complex precipitate in austenite in Fe-1.5Mn-0.2C and Fe-2Mn-0.2C alloys often hold the Baker-Nutting orientation relationship ((001)α//(001)V(C, N), [110]α//[100]V(C, N)). Although several irrational ferrite/V(C, N) orientation relationships were observed, misorientation for either low-index planes or directions are relatively small between ferrite and V(C, N) for those relationships. The orientation relationships between intragranular ferrite and austenite were estimated by examining the misorientations between the ferrite and the neighboring martensite lathes from the Kurdjumov-Sachs inter-variant relationships. There is no specific orientation relationship between the intragranular ferrite idiomorph and the austenite matrix because of the low-energy orientation relationships between ferrite and V(C, N).

https://www.jstage.jst.go.jp/article/isijinternational1989/43/12/43_12_2028/_pdf

Tadashi Maki

Papers

Variant Selection in Grain Boundary Nucleation of Upper Bainite

Effect of prior deformation of austenite on the γ → ε martensitic transformation in Fe-Mn alloys

Mechanism of dynamic continuous recrystallization during superplastic deformation in a microduplex stainless steel

Local orientation change inside lenticular martensite plate in Fe–33Ni alloy

Multiphase Crystallography in the Nucleation of Intragranular Ferrite on MnS+V(C, N) Complex Precipitate in Austenite