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Due to Typhoon Hagibis (Typhoon No. 19) in October 2019, also known as the Reiwa 1 East Japan Typhoon, many parts of the Chikuma River’s levee in Nagano Prefecture were damaged. There were instances of levee collapse due to overtopping, and an electric railway bridge also collapsed when an eroded revetment broke. In the present study, these disasters were investigated. The investigation considered both the geology - geomechanics and the hydraulics of the levee collapse due to overtopping. The results suggest that the most significant factor in the collapse of the levee was structural weakness. There were cases in which the tenacity of a levee was improved by taking simple measures against seepage. It was found that the bridge abutments washed away mainly because the river levee became a water-colliding front due to changes in the water route following the erosion of a sandbar in the process of river level decline after the peak.

Welding and relevant studies are indispensable to employ high-entropy alloys for practical applications. In this study, Al0.3CoCrCu0.3FeNi high-entropy alloy with single FCC phase was used to make “bead-on-plate” friction stir welds at different rotational speeds, and the effects on microstructure and mechanical properties were studied. Several banded structures containing oxide or nitride particles were observed in the stir zone (SZ), and the chemical wear of the polycrystalline cubic boron nitride tool was confirmed. The microhardness distribution of the welds showed higher hardness in the SZ because of grain refinement and the presence of deformed grains. The electron backscattered diffraction results suggested that the high-entropy alloy with low stacking-fault energy experienced recrystallization during friction stir welding, which was similar to other conventional materials with low stacking-fault energy.

This work aims to develop a reliable method to predict mechanical properties of friction-stir-welded 6xxx-series alloys with experimentally measured welding heat input. A calorimetrical method was utilized to experimentally measure the welding heat input in the friction stir welded of aluminum alloy 6063-T5. Good correlations between the input variables, i.e., welding parameters and physical properties of the materials, and the welding heat inputs obtained with experimental measurements were discovered. The welding heat input can be predicted using the empirical equation derived based on these correlations. Moreover, the results suggested that the thermal conductivities of the welded alloys affected the welding heat input significantly. Mechanical properties, including hardness and tensile properties, of friction-stir-welded aluminum alloy 6063 were in good correlation to the heat input obtained with experimental measurement. These correlations were explained by the evolution of the strengthening precipitates during welding. This work proposed a reliable new route to predict these mechanical responses through the estimation of heat input.

In this work, the current state of understanding of grain structure evolution during friction-stir welding is briefly reviewed. The broad aspects of this process and experimental techniques for its examination are critically addressed. The specific character of the microstructural evolutions in body-centered cubic, face-centered cubic and hexagonal close-packed metals are considered in details. In all cases, the grain structure evolution is shown to be a relatively complex process, which usually involves geometric effect of strain, continuous recrystallization and discontinuous recrystallization. Moreover, mechanical twinning, annealing twinning and grain convergence may also occur in particular cases. It is also demonstrated that activation of a specific microstructural mechanism is primarily governed by crystal structure and stacking fault energy but may also be influenced by welding temperature. Specifically, microstructure evolution in cubic metals with high stacking-fault energy is primarily governed by the continuous recrystallization whereas grain structure development in materials with low stacking-fault energy is mainly driven by the discontinuous recrystallization. In the case of transient stacking-fault energy, the materials may experience a transition from the continuous to the discontinuous mechanism. In hexagonal metals, microstructural changes are shown to be directly linked with crystallographic texture. Specifically, a formation of very sharp texture may promote the grain convergence.

Easy and simple fabrication of a lap joint of high-temperature superconducting (HTS) tapes is desirable for various HTS applications such as power cables and current feeders. We have developed mechanical joints with a low-temperature heat treatment at around 370 K where indium foils are inserted between joint surfaces. Though the joint showed a good performance, it took over 10 min to fabricate one joint and ideally the fabrication time should be further shortened. In this study, we proposed to use ultrasonic welding (UW) to assist a heat generation at the joint for lap joints with indium foil, which can be completed within 1 s. We prepared various lap joint samples of HTS tapes: REBCO (REBa 2 Cu 3 O 7 , RE: rare-earth) or BSCCO (Bi 2 Sr 2 Ca 2 Cu 3 O 10 ) tapes were pressed together with an indium foil using a UW machine or pressing machine with cartridge heaters. Then, the current-voltage curve was evaluated at 77 K and the joint resistance characteristics were discussed depending on the joint methods and HTS tapes. The samples with copper-stabilized REBCO tapes all showed joint resistivities of 31.6-35.6 nΩcm 2 without critical current (I C ) degradation for both joint methods. The samples with silver-sheathed BSCCO tapes showed joint resistivities of 18.6-46.5 nΩcm 2 for both joint methods; however, some of those fabricated by the UW showed I C degradation. The degradation was able to be prevented by optimizing UW conditions.

Friction stir spot welding (FSSW) was applied to 2.0 mm thick steel plate cold-rolled commercial (SPCC) low carbon steel plates at a very low rotation speed that ranged from 5 to 50 rpm, which was much lower than that generally used for the conventional FSSW technique. Due to the very low heat input, the welding processes could therefore be completed at a peak welding temperature below 160 °C. As a result, a significantly refined microstructure with an average grain size of about 0.41 μm was formed in the stir zone of the joints and the J1{0–11} and J2{1–10} shear textures were the dominant components, which are different from the D1{11–2} and D2{−1–12} shear textures formed in the conventional FSSW joints. In addition, no heat affected zone could be detected along the cross-sectional plane of the joints. Although a few void-like non-bonded areas were still observed along the interface between the upper and lower steel plates, the shear tensile loads of the joints increased to about 10.0 kN when welded at a condition of 8 t, 20 rpm and 30 s, and the joints fractured through the plug failure mode.

In this work, the effect of welding temperature on material flow during friction stir welding (FSW) of AZ31 magnesium alloy was examined. To this end, FSW was conducted in the temperature range of 0.65-0.85 Tm (Tm is the melting point) and sample-scale EBSD mapping was employed to characterize texture distribution. Despite that the low-temperature welds contained macro-scale defects (which presumably affected material flow), several important observations were made. In the entire temperature range, the global material motion was shown to consist of two principal components, viz. shoulder-induced flow and probe-induced flow. In some cases, however, a synergetic effect of these two constituents resulted in transitional orientation of macro-scale shear plane thus giving rise to a transitional material flow. During low-temperature FSW, the global material transportation was dominated by the probe. However, the contribution of the shoulder and transitional components was found to increase with welding temperature. This effect was attributed to the dominant role of the shoulder in generation of FSW heat. Above ~ 0.8 Tm, however, the transitional material flow was found to abruptly disappear. This result was associated with reduction of temperature sensitivity of flow stresses and the resulting equilibration of their distribution within the stir zone.

Dissimilar metal joining between Al and Cu is effective for reducing the weight and cost of electrical components. In this study, dissimilar lap joining of pure Al to pure Cu with an Al-Ni filler material was conducted using tungsten inert gas (TIG) arc brazing, and the effect of Ni on the joint strength associated with the microstructure of the intermetallic compound (IMC) layer at the dissimilar interface was examined. The addition of Ni effectively increased the interfacial strength of the joints. Regardless of the addition of Ni, the joints fractured in the thick Al2Cu layer formed at the Al/Cu interface. However, the Ni addition reduced the thickness of the IMC layer and led to the formation of Al7Cu4Ni particles in the weakest Al2Cu layer. Both the thickness reduction and reinforcing Al7Cu4Ni particle formation are thought to contribute to the increase in joint strength of the Al/Cu dissimilar interface.

Many previous studies proposed equations and models to estimate the heat-input of FSW, but their verifications would be still insufficient. In this study, the heat-input was calorimetrically measured during FSW of aluminum alloys with various welding tools having different shapes, and the effect of tool geometries on heat-input was examined. The measured heat-input increased with increasing shoulder diameter, which was in good agreement with many previous works. Interestingly, the heat-input strongly depended on the probe diameter and length. An attempt to correlate the heat-input with the tool surface area was made, virtually showing that the heat-input linearly increased with increasing “effective surface area” defined as (half of shoulder surface area + probe column surface area + probe tip surface area). This result experimentally suggested that the effect of probe dimension on heat-input was unexpectedly high. Moreover, an empirical equation estimating the heat-input during FSW of aluminum alloys from the welding parameters and the tool geometry was successfully established.

A new solid-state bonding technique, Joule heating diffusion bonding, was used for the dissimilar bonding of commercial-purity Ti to 304 stainless steel within a short time without macroscopic deformation of the workpieces. The tensile strengths of the joints produced at various bonding parameters were examined at room temperature, and the microstructures of the joints and the fracture surfaces were analyzed to clarify the effect of the microstructural factors on the tensile strength of the joints. The tensile strength of the joints increased with the increase in the fraction of the sufficiently bonded interface. In the joints with the well-bonded interface, the tensile strength decreased with the increase in the thickness of the brittle Fe-Ti-type intermetallic compound layers at the joint interface. This study suggested that the high tensile strength could be achieved in the Joule heating diffusion bonded joints with the well-bonded interface where the thickness of the Fe-Ti-type intermetallic compound layers was thinner than 0.5 &micro, m.

Single, double, and triple-layer WC-40Co coatings were prepared by electron beam (EB) cladding to improve the wear resistance of Ti-6Al-4V. The microstructure, microhardness, wear resistance, and corrosion resistance of the coatings were investigated. The results demonstrated that the three types of coatings consisted of β (Ti, W), TiCo, (Ti, W) C1-X, (W, Ti) C1-X, and a small amount of Ti2Co. The core/shell structure of (W, Ti) C1-X/ (Ti, W) C1-X was observed by TEM, and the phase interfaces were studied. The hardness and wear resistance of triple-layer coating was the highest, and its wear resistance was improved by 11.5 times that of the substrate. The wear mechanisms of the substrate, single- and double-layer coating was oxidative and abrasive wear, while the triple-layer coating exhibited abrasive wear. The corrosion resistance of the single- and double-layer coatings was better than that of the substrate; however, the triple-layer coating surface was significantly damaged owing to the cracks.

Dissimilar metal joining between Al and Cu is effective to reduce the cost and weight of electrical components. In this study, dissimilar laser lap brazing of Al containing Ni to pure Cu was conducted, and the effect of the addition of Ni on the joint strength and microstructure at the dissimilar interface was examined. The addition of Ni higher than 2.8 at. pct improved the joint strength effectively, even though the thickness of the IMC layer at the dissimilar interface increased. The addition of Ni additionally produced not only (Ni,Cu)Al particles in θ-Al2Cu but also a (Ni,Cu)Al layer at the θ-Al2Cu/γ1-Al4Cu9 interface. This study implied that the addition of Ni to Al resulted in the formation of a (Ni,Cu)Al layer at the weakest interface between θ-Al2Cu and γ1-Al4Cu9, drastically increasing the strength of the Al/Cu dissimilar joint.

Recently, weight reduction and improvement of crashworthiness of auto bodies have become important issues. At the same time, stiffness of auto bodies is also needed to ensure a smooth ride. Using h...

Dissimilar welds of Al alloy 1050 to Cu were prepared via ultrasonic spot welding in order to understand the formation of welds and interfacial microstructures. To observe oxide layer behavior at the weld interface, the anodized Al alloy 1050 was also welded to Cu. Mechanical mixing and material flow during ultrasonic welding broke and dispersed the oxide layers into the Al matrix. This material flow is attributed to compressive deformation that occurred due to ultrasonic vibrations. Direct bonding in the Al/Cu region increased with welding time. Once micro-bonds were generated between Al and Cu, the Al matrix in the vicinity of the weld interface was severely deformed in the direction of the ultrasonic vibrations. Consequently, a recrystallized microstructure with shear texture was formed in the Al matrix. Deformation heating and severe shear deformation formed Al2Cu intermetallic compound layers at the weld interface. The growth rate of the Al2Cu layer was much faster than had previously been estimated based upon the peak temperature and ultrasonic welding heating time. This is likely due to the increased rate of Cu diffusion into Al in the severely deformed region.

In this work, electron backscatter diffraction and digital image correlation techniques were employed to study the microstructure and microstructure-property relationship in friction-stir welded TRIP steel. It was found that the thermal effect of the welding process led to material softening in the heat-affected zone and promoted martensite transformation in the stir zone. These microstructural changes provided rapid strain localization during subsequent transverse tensile tests and thus resulted in premature failure of the welds. Material softening in the heat-affected zone was deduced to be a combined result of dissolution and spheroidisation of retained austenite as well as recovery in bainitic ferrite. The stir zone martensite was characterized by significant orientation spread and therefore its orientation relationship with austenite essentially deviated from the ideal Kurdjumov-Sachs relation. Moreover, the martensite transformation was found to be influenced by variant selection.

In this work, the current understanding and development of friction-stir welding and processing of Ti-6Al-4V alloy are briefly reviewed. The critical issues of these processes are addressed, including welding tool materials and design, tool wear, processing temperature, material flow, processing window and residual stresses. A particular emphasis is given to microstructural aspects and microstructure-properties relationship. Potential engineering applications are highlighted.

A thin wall of well-formed dense 2Cr13 martensitic stainless steel was successfully fabricated by wire arc additive manufacturing (WAAM). Heterogeneous distributions of the hardness and microstructure were found in the wall consisting of 20 layers along the building direction. The hardness profile of the thin wall mainly depended on the degree of tempering of the martensite. Based on the simulated and tested thermal history, the inter-pass temperature was lower than the martensitic transformation start (Ms) temperature in the first 10 layers. The thermal effect of the subsequent passes partially tempered the quenched martensite formed in the previous pass during cooling. By contrast, the inter-pass temperature was greater than the Ms temperature in the last 10 layers. The martensitic transformation mainly occurred after the final pass, resulting in a relatively homogeneous microstructure. This study suggests that the inter-pass temperature is a dominant factor in controlling the microstructural evolution during the WAAM of 2Cr13 martensitic stainless steel.

In this study, WC-10Co/Ti-6Al-4V coatings were fabricated under varying cladding voltages via electron beam cladding technology. The microstructure, microhardness, and wear performance of the composite coatings were studied. In addition, temperature field simulations were performed for the cladding process applying ABAQUS. The thickness of the coatings ranged from 350 to 850 μm. The presence of α-Ti, (Ti, W)C1-X, and small amounts of WC, TiC, (W, Ti)C1-X, W2C, and β-Ti in the coatings was confirmed. WC and TiC were partially dissolved and, in accordance with the simulated temperature field, resulted in the appearance of (W, Ti)C1-X around the WC particles. Moreover, the consumption of C atoms was beneficial for the diffusion of W atoms into the TiC lattice to form (Ti, W)C1-X. In the comparison of experimental and numerical simulation results, it was found that a deeper penetration and further settlement of WC particles was observed experimentally, when a higher cladding voltage was applied. The coating prepared under 65 kV exhibited the best mechanical performance and its friction mechanism entailed abrasive and adhesive wear.

A novel strategy with shot peening (SP) pretreatment was proposed to prevent heat-affected zone liquation cracking during laser cladding of K447A nickel-based superalloy powder over its non-weldable cast structure. High dislocation density accumulated by the SP pretreatment drove the high-temperature zone of the heat-affected zone to recrystallize during the laser cladding process, as a result, the coarse columnar grain with an average dendrite stem and secondary dendrite arm width of 102.67 ± 4.24 μm was transformed into a fine equiaxed grain with an average size of 10 μm. SP could effectively inhibit liquation cracking. With the increase of SP duration, the tendency of the liquation cracking reduced. Compared with no SP BM, the total length of the liquation cracking in the longitudinal section of the laser cladding sample reduced from 1649 μm to 248.8 μm. In-situ experimental observation and process simulation were opted to study the evolution behavior of recrystallization. Recrystallization firstly developed in the region of the intergranular zone at ~1214 °C and gradually the base metal recrystallized between 1232 and 1242 °C except for the region with γ/γ′ eutectic structure. As the temperature rose further, a fine liquid film network developed in the range of 1260–1267 °C. Finally, the stress and strain-based criteria were exercised to evaluate the crack susceptibility of the liquid film. The fine liquid film network adequately constrained the liquation cracking by reducing the driving stress applied to the liquid film as well as by lowering the pressure drop of the liquid film caused by driving strain.

Electron backscatter diffraction (EBSD) was used to examine the microstructural evolution in a one-step thermomechanically processed 304 austenitic stainless steel specimen during the thermomechanical process of grain boundary engineering. Solution-treated materials were cold-rolled to 3% reduction and subsequently annealed at 1220 K for different annealing times. The EBSD observation of the specimen showed an increase in the frequency of coincident site lattice (CSL) boundaries and a decrease in the percolation probability of random boundaries. Additionally, the specimen exhibited heterogeneous growth of clusters of grains that contained a high frequency of CSL boundaries. These clusters of grains were developed in the entire observed area by strain-induced grain growth according to the results of grain orientation spread analysis. The details of the growth of the clusters and the disconnection of random boundaries were successfully observed in situ using EBSD and a heating stage. The frequency of CSL boundaries increased with the growth of the clusters. Disconnection of random boundaries between the clusters was achieved by the formation of annealing twins through the impingement of the growing clusters during the thermomechanical process. Twin variant selection to introduce CSL boundaries into a random boundary network was observed by the in situ EBSD observation.

In this work, microstructural changes occurring during transverse tension tests of friction-stir welded AZ31 magnesium alloy were systematically studied in order to clarify the microstructure-properties relationship. To this end, digital image correlation and electron backscatter diffraction (EBSD) were employed. As expected, plastic deformation in the stir zone was found to be preferentially concentrated at its advancing- and retreating sides. In both these locations, the microstructure evolution was shown to be a complex process consisting of two stages. The first stage was characterized by a profuse { 10 1 ― 2 } twinning giving rise to a 0001 11 2 ¯ 0 > texture component. In the second stage, enhanced slip activity led to extensive formation of low-angle boundaries and to the development of a fiber hkil 11 2 ¯ 0 > texture. The twin-induced 0001 11 2 ¯ 0 > orientation was shown to promote double { 10 1 ¯ 1 } – { 10 1 ¯ 2 } twinning which is known to lead to final failure. It was therefore suggested that the cause of the well-known premature fracture of magnesium welds at the stir zone extremity may be due to the profuse { 10 1 ¯ 2 } twinning.

High-strain-rate deformation in ultrasonic additive manufacturing was analyzed by performing microstructural characterization via electron microscopy. The micro-asperities on the top tape surface, which were formed by contact with the sonotrode surface, underwent cyclic deformation in the shear direction at high strain rates during welding with an additional tape. This caused plastic flow and crushing of the micro-asperities, and a flattened interface was formed between the upper and lower tapes. Further, surface oxide films were fractured and dispersed by ultrasonic vibration, and metallurgical welding was achieved.

In this work, electron backscatter diffraction (EBSD) and digital image correlation were employed to study tensile behavior of friction-stir welded magnesium alloy AZ31. The character of measured strain patterns indicated the prevalence of basal slip but also suggested significant activity of prism slip. A substantial strain gradient was found lengthways in the stir zone. The failure was shown to be initiated at the weld root and to originate from double { 10 1 ¯ 1 } − { 10 1 ¯ 2 } twinning. Crack propagation was influenced by the “onion-ring structure” of the stir zone.

In this chapter, the current state of understanding and development of friction-stir processing of commercial-purity titanium is briefly reviewed. The critical issues of this technique are addressed, including tool materials, working temperature, and processing window. A particular emphasis is given to the final microstructure produced in the stir zone as well as to mechanisms of microstructural evolution.

During corner AdStir fillet stationary shoulder friction stir welding (FSW), filler material with surface oxide is fed into the stir zone. In this study, the material flow of the filler material and surface oxide layer during the process was investigated by microstructure observation. To visualize the material flow, a marker insert technique was employed. Similar to conventional FSW, fine and equiaxed grain structure was observed in the stir zone. EBSD investigation revealed that the material flow was governed by the simple shear deformation induced by the rotating probe. The filler material was widely distributed in the stir zone, suggesting that some amount of the filler material moved downward due to the vertical material flow. The initial surface layer on the filler material was finely broken up by the material flow, achieving metallic bonding between the filler wire and the plates. Any harmful effects by adding the filler material were not found in the mechanical tests in this study.

Laser cladding with a paraxial powder-feeding device was used to repair as-cast K447A nickel-based superalloy, a kind of non-weldable alloy with a high Hf content and working as gas-turbine hot-section components. In the present study, the metallurgical evolution behavior of liquid film and liquation cracking mechanisms are studied. The results showed that a kind of uncommon Ni21Hf8 phase was found in the incipient melting regions (IMR) of the intergranular region. In the thermal cycle, γ/Ni21Hf8 eutectic liquefaction, M5B3 dissolved, and γ/γ’ eutectic liquefaction successively occurred in the heating process in IMR, and liquid → γ + γ/γ’ + M5B3 + γ/Ni21Hf8 → γ + γ’(3) + γ/γ’ + γ/Ni21Hf8 + M5B3 occurred in the cooling process. The inhomogeneity of the intergranular structure leads to the liquid film with the characteristics of discontinuous distribution and uneven width. The cracking criterion of the liquid film was established based on pressure drop. When the closed liquid film meets the condition of 2 γ sl 1 / K * ( ρ s / ρ l − 1 * R * dt + ∆ h e ) , the liquid film with a thickness that is greater than critical thickness will be induced into a liquation cracking.

Although laser welded Al–Li alloys show great potential for application in aerospace industry, the detailed relationship between local hardness and complex microstructure remained unclear. In the present study, relationship between precipitation behavior and local hardness in a defect-free laser pressure weld of Al–Li alloy 2198 was systematically examined by transmission electron microscopy. Hardness profile of the laser pressure weld was reduced in the vicinity of the weld center, mainly explained by size and density of the strengthening precipitates. Moreover, post-weld aging increased the density of strengthening precipitates in the precipitate-dissolved regions, increasing their hardness values markedly. On the other hand, a few precipitates were formed in the precipitate-coarsened regions during post-weld aging, which slightly increased the local hardness. The variation in hardness during post-weld aging could be well correlated with the precipitation behavior in each region of the laser pressure weld.