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 morphology and crystallography of lath martensite in Fe-C alloys

The morphology and crystallography of lath martensite in alloy steels

The microstructure and the strength of the lath martensite in Fe–0.2C and Fe–0.2C–2Mn alloys were analyzed as a function of the prior austenite grain size. The size of martensite packets formed within individual austenite grains was controlled by the austenite grain size but not affected by the Mn addition. However, the further subdivision of packets into blocks differed significantly in the two alloys, and at a given austenite grain size a smaller block size was observed in the Mn containing alloy. The yield strength of the two alloys was related to the packet size and the block size, respectively, and the results suggested that the block size is the key structural parameter when analyzing the strength–structure relationship of lath martensite in low carbon steels.

Effect of block size on the strength of lath martensite in low carbon steels

Lath martensite structure has overwhelming industrial significance in most heat-treatable commercial steels. High dislocation density is one of major factors for high strength of lath martensite. It is known that the dislocation density in the lath martensite is an order of 10 or 10 m . However, the quantitative studies on the dislocation density in martensite are very few. One of the difficulties in the measurement of dislocation density by transmission electron microscopy is a determination of thin foil thickness. Kehoe and Kelly determined the foil thickness from extinction thickness fringes in an inclined boundary and measured the dislocation density within the laths in Fe–C alloys with carbon content from 0.01 to 0.1 mass%. They showed that the dislocation density increases with carbon content. However, the effect of carbon content more than 0.1% on the dislocation density is not clear yet. In the present paper, the dislocation densities within the lath martensite in Fe–C (0 to 0.8 mass% C) and Fe–Ni (0 to 23 mass% Ni) alloys were studied by TEM through precise determination of foil thickness using the convergent-beam electron diffraction (CBED) method.

Dislocation density within lath martensite in Fe-C and Fe-Ni alloys

The crystallography, microstructure and mechanical property of as-quenched martensite of Fe-0.2C-Mn(-V) alloys of which the prior austenite grain sizes are 370-2 μm were studied. The prior austenite grain, whose size is larger than 28 μm, is divided by several packets. Those packets are subdivided by blocks containing sub-blocks, each of which corresponds to the Kurdjumov-Sachs variant. When the prior austenite grain size is about 2 μm, one packet tends to grow predominantly. Each packet is divided by blocks containing sub-blocks.

Tadashi Maki

Papers

The morphology and crystallography of lath martensite in Fe-C alloys

The morphology and crystallography of lath martensite in alloy steels

Effect of block size on the strength of lath martensite in low carbon steels

Dislocation density within lath martensite in Fe-C and Fe-Ni alloys

Effect of Austenite Grain Size on the Morphology and Crystallography of Lath Martensite in Low Carbon Steels