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

Friction stir welding (FSW) is a solid-state joining process. During FSW, microstructure drastically changes in local region of the workpiece by introduction of frictional heat and severe plastic deformation arising from rotation of the welding tool, which results in inhomogeneous microstructural distribution in the welds. To maintain high reliability of the structure produced by FSW, precise understanding of microstructural factors governing weld properties is required. In the present paper, microstructural factors governing mechanical properties, especially hardness profile and tensile properties, of friction stir welded Al and Mg alloys are reviewed.

Microstructural Factors Governing Mechanical Properties in Friction Stir Welds

Pb(ZrxTi1-x)O3 (PZT) thin films were coated on Pt/Ti/SiO2/Si substrates by a sol-gel method and then crystallized by 28 GHz microwave irradiation. The crystalline phases and microstructures as well as the electrical properties of the microwave-irradiated PZT films were investigated as a function of the elevated temperature generated by microwave irradiation. X-ray diffraction analysis indicated that the PZT films crystallized well into the perovskite phase at an elevated temperature of 480°C by microwave irradiation. Scanning electron microscopy images showed that the films had a granular grain structure and most of the grains were approximately 1.5 µm in size. With increasing elevated temperature from 480°C to 600°C by microwave irradiation, the breadth of grain boundaries of the films became narrow and the remanent polarization of the films increased slightly. It is clear that microwave irradiation is effective for obtaining well-crystallized PZT films with good properties at low temperatures in a short time.

Electrical Properties and Microstructures of Sol-Gel-Deposited Lead Zirconate Titanate Thin Films Crystallized by 28 GHz Microwave Irradiation

Initial oxide layer on the butt surface fragments during friction stir welding (FSW) and then often remains as a faint zigzag-line pattern on the cross section. When remnant of the oxide layer often adversely affects the mechanical properties in the weld, it is called as "kissing-bond". The present study systematically examines effect of oxide array on bend property in the root of friction stir (FS) welded A1 alloy 1050 by transmission electron microscopy (TEM), and then clarifies identity of the kissing-bond.

Kissing-Bond Characteristics in a Friction Stir Welded Aluminum Alloy by Transmission Electron Microscopy

The effect of the nitrogen absorption on the transmission electron microscopic structure of SUS304L austenitic stainless steel weld metal was investigated by varying the nitrogen partial pressure of the Ar-N2 gas mixture welding atmosphere in an atmosphere-controlling chamber. An electron diffraction study suggested that the primary solidification mode of the weld metal alters from ferritic mode to austenitic one with the nitrogen partial pressure and the nitrogen content of weld metal. The austenite frequently adopts a near Kurdjumov-Sachs orientation relationship with the adjacent primary ferrite, while the eutectic ferrite does not adopt the K-S one with the adjacent austenite. The amount of the eutectic ferrite phase increases with the nitrogen partial pressure and the nitrogen content. In the weld metal with no ferrite, precipitation on dislocations and grain boundaries was observed.

The effect of nitrogen absorption on the TEM structure of SUS304L austenitic stainless steel weld metal

Summary Type 329J1 duplex stainless steel was welded by gas tungsten arc welding in argon‐nitrogen mixed gas atmospheres. The tensile properties and microstructures of the weld metals were examined. The nitrogen content increases and the ferrite content decreases with increasing nitrogen partial pressure of the atmosphere. The ferrite content linearly decreases with an increasing nitrogen content. The tensile strength and elongation of the weld metal produced in the argon atmosphere are much lower than those of the base metal, but they increase with an increasing nitrogen content and approximate those of the base metal at around 0.4 mass% nitrogen content. The fractographs suggest that only the base metal and high‐nitrogen weld metal clearly show dimple patterns. The tensile‐tested base metal and high‐nitrogen weld metal have complex crack paths, whereas the other weld metals have relatively straight paths. The tensile properties of the weld metal are affected by the ferrite content and chromium nitride.

Hiroyuki Kokawa

Papers

Microstructural Factors Governing Mechanical Properties in Friction Stir Welds

Electrical Properties and Microstructures of Sol-Gel-Deposited Lead Zirconate Titanate Thin Films Crystallized by 28 GHz Microwave Irradiation

Kissing-Bond Characteristics in a Friction Stir Welded Aluminum Alloy by Transmission Electron Microscopy

The effect of nitrogen absorption on the TEM structure of SUS304L austenitic stainless steel weld metal

Effects of nitrogen on tensile properties of duplex stainless steel weld metal