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.

/pdf/friction-stir-welding-and-processing-3hu3ejo7ub.pdf

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

The fracture toughness of friction stir welded Invar36 alloy,obtained with travelling speed of 2 mm/s and rotational speed from 200 r/min to 1 000 r/min,were estimated at low temperature(77 K)by small punch test,and were compared with that at room temperature(298 K)as well.At 298 K,all welds have higher small punch energy than the base material.For the welds acquired with the same welding parameters,the small punch energy tested at 77 K is always higher than the ones at 298 K.The small punch energy of welds trends to increase with decreasing of the rotational speed at both 77 K and 298 K. With rotational speeds equal or less than 400 r/min,the small punch energy of welds is significantly higher than that of the base material at 77 K.The results indicate that the friction stir welds keep excellent ductility at low temperature.With the increase of travelling speed,higher heat input produces a coarser microstructure and leads to the reduction of fracture toughness in the weld.

Fracture toughness of friction stir welded Invar 36 alloy at low temperature

Grain boundary carbide precipitation and intergranular corrosion in sensitized austenite stainless steel were examined by transmission electron microscopy (TEM) to clarify the effect of grain boundary structure on precipitation and corrosion. A type 304 steel, which had been solutionized at 1350 K was heat-treated at temperatures of 800-1300 K. Oxalic acid etch and Strauss tests showed that the frequency of grain boundaries with M23C6 carbide precipitation and corroded boundaries increased with holding time at sensitizing temperatures. The grain boundary carbide precipitation was observed during heat treatment at 1000 K by TEM. Grain boundaries were characterized on the basis of the Coincidence Site Lattice (CSL) theory using electron diffraction Kikuchi patterns. The observations revealed that the propensity to intergranular precipitation depends strongly on the grain boundary structure. Carbide precipitates tend to be detected at grain boundaries with higher Σ -values or larger deviation angles (Δθ) from low- Σ CSL misorientations. The border lines between precipitation and no precipitation can be drawn by a deviation parameter of Δθ/ΔθC, where Δθc is the maximum deviation angle by Brandon’s criterion. The border line of Δθ/Δθc decreased with the increase in the holding time at 1000 K. This means that the more ordered boundary needs the longer time for intergranular carbide precipitation and corrosion than less ordered or random boundaries.

Dependence of Carbide Precipitation on Grain Boundary Structure in Sensitized Austenitic Stainless Steel

Effects of nitrogen on microstructure, shape recovery behavior and tensile property of Ti-Ni alloy weldments were investigated. A near-equiatomic Ti-Ni shape memory alloy was welded by a gas tungsten arc (GTA) welding technique under Ar-N2 welding atmosphere. The nitrogen absorbed into the weld metal increased with rising nitrogen partial pressure and welding current. Second phase particles were observed in the weld metal containing nitrogen. The second phase was identified as TiN by transmission electron microscopy. The shape recovery stress of the weldment was equivalent to that of the base material. As temperature of the weld metal decreased with increasing nitrogen content. Tensile strength of the alloy decreased by welding process, and the further reduction was observed for the weld metal absorbed nitrogen. Elongation also decreased by welding and rising nitrogen content.

https://www.jstage.jst.go.jp/article/qjjws1983/18/2/18_2_318/_pdf

Effect of nitrogen on shape recovery and tensile properties of Ti-Ni alloy weldments - GTA welding of Ti-Ni shape memory alloy (Report 2)

A 304 austenitic stainless steel was friction stir welded using polycrystalline cubic boron nitride (PCBN) tool. Relationship between corrosion properties and microstructure was examined in the weld using several corrosion tests and microstructural observation techniques. The stir zone (SZ) had better corrosion properties than the base material (BM). The corrosion tests showed that the intergranular corrosion was developed in the heat-affected zone (HAZ) compared to the BM and SZ although the grain boundaries were not severely corroded. TEM/EDS analysis revealed that Cr depletion zone near grain boundary in the HAZ was shallow and narrow. Friction stir welding (FSW) suppressed sensitisation in the HAZ, which could be explained by short duration at sensitisation temperatures during welding. On the other hand, many grain boundaries were deeply corroded in the AS, where the corrosion resistance was significantly degraded. The microstructural observation revealed that sigma phase was formed in the AS during FSW. Sigma formation produced the wide and deep Cr depletion zone with the minimum Cr content less than 12 wt % in the vicinity of the grain boundary in the AS, which severely deteriorated the corrosion resistance in the AS.

Corrosion properties in friction stir welded 304 austenitic stainless steel

摩擦攪拌接合(FSW = Friction-Stir-Welding)は難溶接材 であるアルミニウム材料の接合方法としてよく利用されて いる手法である.その接合部はブローホールや凝固割れ等 の溶接由来の欠陥を持たず,FSW特有の微細な結晶粒から なる攪拌組織を形成して高硬度特性を持つことなどから, 継手品質が優れていることでも知られている.既に FSW の適用が進んでいるアルミニウムや銅などの低融点材に比 べて,鉄鋼や Ti合金などの高融点材は塑性流動が起こりに くく,FSWによる接合には課題が多い.しかし,これらの 材料に対しても高温で優れた耐久性を有する超合金やセラ ミックを用いたツールが開発されており,高融点材への FSWの適用検討は加速している.今後は実用面での生産 性を向上させることが重要である.その方法として,接合 に要する入熱を補助するための熱源を負荷した FSWの開発 が考えられる.FSWは摩擦攪拌により被接合材を塑性流動 させて接合しているが,他の熱源を組み合わせることによ り被接合材の塑性流動を促進することができ,接合速度の 高速化や接合荷重の低減に寄与することが期待される.予 熱技術として,比較的初期に報告された誘導コイルを用い た検討において,予熱により荷重および軸トルクを低減で きることが既に報告されている.現在までに均一かつ高エ ネルギーの入熱を得られるレーザーを予熱源に用いた事例 が多数報告されており,例えば低炭素鋼を接合した事 例において,レーザー予熱を施すことで高速で無欠陥の接 合ができることや脆弱なマルテンサイトやベイナイトを形 成しにくいことが報告されている.また別の手法として通 電設備を用いて摩擦攪拌と同時にジュール熱で加熱するハ イブリッド法も報告されている.その中で本研究は GTAW (Gus-Tungsten-Arc-Welding)を予熱に用いたハイブ リッド法に着目した.GTAWはエネルギー密度が高く局所 加熱に好適であることから FSWの塑性流動を補助する熱源 として期待できる.またレーザーなどに比べると比較的安 価であり,FSWの接合ラインに沿ってツール前方にGTAW トーチを設置したシンプルな装置構成によりハイブリッド 法を構築することが可能である.具体的には,本研究に用 いた GTAW-FSWハイブリッド接合法(以下,GTAW-FSW と略す)を模した Fig.1で説明する.GTAW-FSWでは溶加 材を用いないGTAW熱源により FSWツール前方で被接合材 を一度溶融させ,再度凝固した箇所を FSWで接合するプロ セスとした.GTAWの溶接部に対して十分に大きい接合部 を FSWで作ることにより,被接合材の微細組織にGTAWの 影響が残らない組織となることを考慮する.このとき 1パ スのGTAWの溶接部と FSWの接合部は,前方から見ると両 方とも逆三角形であり,形状を重ねやすいことから,一対 のGTSW-FSWとすることで FSWの攪拌部へ効率よく予熱 することが可能と考えられる.しかし,レーザー,誘導コ Friction-Stir-Welding および Gas-TungstenArc-Welding の ハイブリット法による低炭素鋼の高速接合

Yutaka S. Sato

Papers

Fracture toughness of friction stir welded Invar 36 alloy at low temperature

Dependence of Carbide Precipitation on Grain Boundary Structure in Sensitized Austenitic Stainless Steel

Effect of nitrogen on shape recovery and tensile properties of Ti-Ni alloy weldments - GTA welding of Ti-Ni shape memory alloy (Report 2)

Corrosion properties in friction stir welded 304 austenitic stainless steel

High speed welding of friction-stir-welding for mild steel by gas-tungsten-arc-welding hybrid process