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

Microstructural evolution during friction stir welding of Ti-6Al-4V alloy

OFHC copper plates were welded with Cu and Cu-Ti electrode wires using Ar-N2 gas shielding. The effects of welding conditions, N2% in the shielding gas and titanium content of the electrode wire on the porosity and the microstructure of the weld metal were systematically examined. The results are as follows:(1) Using copper electrode wire, the weld metal shows better surface appearance, less porosity and deeper penetration with increase of the welding current.(2) As N2% in the shielding gas increases, the porosity of the weld metal increases. However, the tendency becomes very slight in the case of using Cu-Ti electrode wire.(3) The nitrogen content of the weld metal increases with N2% in the shielding gas. This tendency becomes more remarkable with increase of the titanium content of the electrode wire.(4) Using Cu-Ti electrode wire, a secondary phase is observed in the weld metal. The fractional area of the secondary phase in the weld metal increases with N2% in the shielding gas and titanium content of the electrode wire.(5) The electron diffraction patterns of the secondary phase are in good agreement with those of TiN.(6) The temperature of molten copper pool was measured and the formation of TiN was discussed by using thermodynamic data.

Effects of Nitrogen and Titanium on Blow Hole Formation and Microstructure in Copper Weld Metal by Ar-N2 Gas Metal Arc Welding

Transmission Electron Microscope Observations of SUS304L Stainless Steel Weld Metal

Effect of Nitrogen on Sigma Transformation in Duplex Stainless Steel Weld Metal at High Temperatures

To improve intergranular corrosion resistance of 304 stainless steel, a novel method, laser surface remelting combined with annealing treatment was adopted, which resulted in a high population of low  CSL boundaries, especially, twin boundaries (3) on the surface of the processed specimens. The grain boundary character distribution and effect of laser processing parameters on it were investigated. The experimental results showed that the maximum frequency of the low  CSL boundaries could attain 88.6% under the optimal processing conditions. The high fraction of the low  CSL boundaries led to a high corrosion resistance to intergranular corrosion.

Hiroyuki Kokawa

Papers

Microstructural evolution during friction stir welding of Ti-6Al-4V alloy

Effects of Nitrogen and Titanium on Blow Hole Formation and Microstructure in Copper Weld Metal by Ar-N2 Gas Metal Arc Welding

Transmission Electron Microscope Observations of SUS304L Stainless Steel Weld Metal

Effect of Nitrogen on Sigma Transformation in Duplex Stainless Steel Weld Metal at High Temperatures

Application of Laser Surface Remelting in Grain Boundary Engineering