Seal Welding Research Articles

Microstructure analysis of pressure resistance seal welding joint of zirconium alloy tube-plug structure

Pressure resistance welding is usually used to seal the connection between the cladding tube and the end plug made of zirconium alloy. The seal welded joint has a direct effect on the service performance of the fuel rod cladding structure. In this paper, the pressure resistance welded joints of zirconium alloy tube-plug structure were obtained by thermal-mechanical simulation experiments. The microstructure and microhardness of the joints were both analyzed. The effect of processing parameters on the microstructure was studied in detail. The results showed that there was no β-Zr phase observed in the joint, and no obvious element segregation. There were different types of Widmanstätten structure in the thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ) of the cladding tube and the end plug joint because of the low cooling rate. Some part of the grains in the joint grew up due to overheating. Its size was about 2.8 times that of the base metal grains. Due to the high dislocation density and texture evolution, the microhardnesses of TMAZ and HAZ were both significantly higher than that of the base metal, and the microhardness of the TMAZ was the highest. With the increasing of welding temperature, the proportion of recrystallization in TMAZ decreased, which was caused by the increasing of strain rate and dislocation annihilation.

Surface defects detection in metal materials repaired by laser surfacing of seal welds

Laser surfacing repair technology for sealing welds is widely used in metal repair. Due to welding technology and usage scenarios, process defects on the metal surface are inevitable. Therefore, ultrasonic surface wave technology is used to analyze the surface defects of metal materials. Principal Component Analysis (PCA) is used to extract the main defect signals on the metal surface, and synthetic aperture focusing technology is used to reduce imaging errors. Considering the lack of PCA in imaging defects, wavelet domain hidden Markov models (WHMM) are combined to optimize the signal, thereby improving the inspection effect of metal defects. In the test results of the relationship between the propagation distance of 316 L steel and the defect echo signal, the echo signal gradually fitted as the propagation distance increased. When the propagation distance was greater than 10 mm, the image acquisition defect signal had significant noise points. Various techniques were used to process the original echo signals of metal surface defects. The improved PCA-WHMM algorithm had significant advantages with the SNR value of the defect image increased by 13.65 % compared to PCA-WHMM. At the same time, the surface repair effects of laser surfacing 316 L metal before and after optimization were compared. The hardness, toughness, and corrosion resistance of the optimized metal were significantly improved. The proposed technological innovation combines traditional laser surfacing repair with deep learning fault diagnosis, which not only greatly improves the efficiency of fault diagnosis, but also proves that this research can effectively avoid common focus issues of laser surfacing repair technology, providing important technical reference for the application of ultrasonic technology in metal defect detection.

Rapid Picosecond Laser Seal Welding of Glass Using a Two‑Step Scanning Strategy

Rapid Picosecond Laser Seal Welding of Glass Using a Two‑Step Scanning Strategy

Laser welded lip seal for UHV class fusion vessels–A methodical study

Lip seals are considered, the most promising options for the vacuum boundary joints in large size vacuum vessels (like rectangular neutral beam vessel with an opening size of ∼30 m) of fusion machines. Lip seals are designed with the corner welding of 2 mm thickness sheets of SS304L with the welding penetration requirement of 3 mm. With the first phase of development, laser welding parameter optimization and the sensitivity analysis was carried out to understand the effect of manufacturing tolerances (like air gap between the plates, laser beam misalignment and plate misalignment) on the weld quality. To ascertain the defined tolerances, a specialized clamping tool was configured, designed, manufactured and tested by integrating that with the assembly of laser welding head and robotic arm. For the prototype welding demonstration, the Lip seal welding of a rectangular configuration and an elliptical configuration of size 3 m × 1.5 m (∼1/3rd size of Diagnostic Neutral Beam (DNB) vessel lip seal); and 1 m length with 1.2 m diameter (exact size of DNB high voltage bushing lip seal) are considered. Welding qualification was done with the Non-Destructive and Destructive examinations. Further, the camera based feedback system for the seam tracking was incorporated to the system, which enables the remote detection and correction of the weld path. This development also extended to enable the repair of welding operation.

Neutron irradiation of tungsten in hydrogen environment at HFIR

Neutron irradiation of tungsten with and without the presence of hydrogen is needed to understand the influence of hydrogen on microstructure development under fusion reactor conditions. However, there is a risk of ignition if air ingress occurs during seal welding of irradiation capsules in a pressurized hydrogen environment. Therefore, an irradiation capsule was designed that contains several disks of vanadium hydrides at a 30% hydrogen-to-metal atomic ratio. During irradiation, the hydrogen is released from the hydrides as the capsule temperature increases, so the irradiation capsule environment is mostly hydrogen when the capsule reaches its 400 °C design temperature. This paper describes the design and operating characteristics of this first-of-a-kind irradiation capsule.

Picosecond laser seal welding of perovskite films

Perovskite solar cells (PSCs) are a promising application industry, but its stability is not very ideal at present. The bottleneck of the PSCs industrialization is that perovskite material is very sensitive to humidity, and it is easy to produce hydrolysis and failure. Therefore, it is necessary to develop an effective encapsulation method to achieve a better long-term stability. Ultrashort pulse laser glass welding technology is one of the effective encapsulation methods, but it is still difficult to be used in engineering due to the harsh conditions of optical contact. To solve these problems, a sealing welding method of perovskite films based on ultrashort pulse laser under the natural stacking conditions was proposed, the effectiveness of the sealing weld was verified by experiments, which provides support for the engineering application of perovskite solar cells, and also provides a possible direction for the application of ultrashort pulse laser glass welding technology.

Follow-up ultrasonic vibration assisted laser welding dissimilar metals for nuclear reactor pump can end sealing

A novel follow-up ultrasonic vibration assisted welding process was proposed to reduce the common defects in laser welding of dissimilar metals for nuclear reactor pump can end sealing, such as unmixed zone, precipitated phase, and uneven element distribution. The influences of ultrasonic vibration on morphology, microstructure and property of C-276/304 laser-welded joint was analyzed. The results indicated that, the most significant ultrasonic vibration at welding position was achieved at parameters of 25 N pressing force and 13 mm distance from ultrasonic action point to welding position. By the application of follow-up ultrasonic vibration, the width of unmixed zone was reduced and the macro element distribution was homogenized due to the enhancement of molten convection. The average grain radius was refined from 1.03 μm to 0.55 μm because acoustic pressure in the molten pool reduced critical nucleus size rk and caused grain fragmentation. The higher diffusion coefficient and cooling rate caused by ultrasonic vibration suppressed element segregation, and thus the amount of precipitated phase decreased from 2.15% to 0.62%, which finally improved the intergranular corrosion resistance of weld metal. The load capacity of weld joint was improved by 12.6%, and the micro-hardness of weld metal was also increased, attributing to the grain refinement strengthening. The follow-up ultrasonic vibration process proposed in this paper provided an efficient, convenient and widely applicable process for laser seal welding of dissimilar metals for nuclear reactor pump can end sealing with higher qualities.

Single‑Stroke Attachment of Sheets to Tube Ends Made from Dissimilar Materials.

This paper presents a new joining method by a forming process for attaching sheets to tube ends. The process consists of two different forming stages carried out sequentially in a single stroke. Firstly, the free tube end is flared by compression with a contoured die, then is squeezed (indented) against the sheet surface to create a mechanical interlocking. The new process is carried out at an ambient temperature and, in contrast to existing joining by forming operations based on tube expansion, it avoids seal welds, tube protrusions above the sheet surfaces, and machining of grooves on the sheet holes to obtain the form-fit joints. The paper starts by analyzing the process deformation mechanics and its main operating variables and finishes by presenting examples that demonstrate its effectiveness for attaching sheets to tube ends made from polyvinylchloride and aluminum. Experimental and numerical simulation work provides support to the presentation.

An experimental study of effects on tube to tube sheet expansion joint grip strength due to seal welding or blanking of adjacent bank tubes of bi-drum boiler

Industrial boilers used in processing industry and auxiliary units for start up steam supply of super critical boilers generate steam with the help of bi-drum arrangement. The tube connecting upper and lower drums are called bank tubes and situated in the flue gas path. Bank tubes are fitted in both the drum with the help of tube to tube sheet expansion joint. During prolonged operation this tubes got punctured due to flue gas erosion or becomes loose at its expansion joints due to improper operation, during drum repair and started leaking. Leaking tubes were repaired either by blanking the tubes or seal welding depends on the nature of failure. As drum is of higher thickness and tubes are of lower thickness, as a procedural drum surface required to be pre-heated before seal welding. This pre-heating and welding affects the joint strength of surrounding tubes. This study focuses on lose of grip strength of surrounding tubes due to pre-heating and welding of central tube during repair, by comparing its micro structure, hardness and pull out load strength. During micro structure analysis polygonal grains of ferrite and pearlite were observed. No significant change in microstructure dimensions between each expansion trail was observed visually. Hardness value falls around 20 Hv near 0° angle of HAZ and falls around 10 Hv at 180° angle of HAZ for 12% expansion. For wall thinning above 10% and ligament efficiency less than 0.4 grip strength falls drastically, and requires complete re expansion of surrounding tubes.

Picosecond laser seal welding of glasses with a large gap.

Ultra-fast lasers can realize selective welding of glasses through nonlinear effect and have the advantages of having high welding accuracy, having small heat-affected zone, having high joint strength and not requiring intermediate absorption layer, which provides a new idea for chip packaging. However, this method is limited to the condition of optical contact, which is difficult to use in engineering applications. To solve this problem, an innovative seal welding method by picosecond laser is presented in this paper. In this method, a picosecond laser performs firstly rapid oscillating scan local welding of the two natural overlap glasses with a large contact gap to form a closed area consisting of spot welds. The glass contact gap in the closed area can be reduced to about 1.5 μm through the solidification shrinkage effect of spot welds. Then a good seal weld without plasma ablation, micro-hole, and micro-crack defects can be achieved by picosecond laser in this closed area to realize seal welding of glasses with a large contact gap. The sealing test results show that the seal welding samples can keep good sealing performance for more than one week.

Ultrasonic high frequency guided wave for detection and identification of defect location in seal welds of sodium cooled fast reactor fuel subassemblies

Hexcan seal welds in fuel subassemblies of prototype fast breeder reactor (PFBR) pose serious problem during ultrasonic inspection for defects due to lower thickness involved and its complex structure. This paper presents detection of defects and identification of its localization across the thickness in fuel subassembly hexcan seal welds using high frequency guided (plate) waves (HFGW). The generation and propagation characteristic of the optimized guided wave mode is extensively studied by using 2D finite element method (FEM). The energy transfer between the hexcan sheath surfaces results in a better sensitivity for detection of small defects on both surfaces. The study demonstrates that the through-thickness location of the defects can be accurately determined by using the frequency content rather than time-of-flight information.

Effects of Power Modulation, Multipass Remelting and Zr Addition Upon Porosity Defects in Laser Seal Welding of End Plug to Thin-Walled Molybdenum Alloy

Aiming to solve the serious porosity defects in laser welded girth joints of thin-walled tube and end plug made of nano-sized Ce2O3 doped Mo alloy (NC-Mo), the influences of laser power modulation, multipass remelting and zirconium (Zr) addition on the number, size and distribution of porosity defects were experimentally studied. By utilizing X-ray computed tomography (XCT), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) analysis, transmission electron microscope (TEM) and Raman spectrum (RS), the porosity feature of welded joints achieved under various conditions was analyzed. The results showed that welding cycles had a significant influence on the porosity ratio of fusion zone (FZ) while the amplitude and frequency of laser power waveform slightly influenced the porosity. When the welding cycles increased from 2 to 8, the porosity ratio of FZ decreased from about 1.00% to about 0.48% and the maximum and average pore diameters reduced by about 53% and 27%, respectively. Adding minor Zr in molten pool can further reduce the RP of FZ to about 0.35%. Through analysis, it can be seen that the pores in FZ can be divided into irregularly-shaped keyhole-induced pores and spherical metallurgy-induced pores. The latter was generated possibly because some impurity elements (including O and H) pre-existing in base metal (BM). Increasing welding cycles can promote the gas in molten pool to float and outflow, thus significantly decreasing the porosity. Moreover, Zr added in molten pool can be preferentially reacted with O to generate ZrO2, which can inhibit the precipitation of volatile MoO2 to thus suppress the generation of metallurgy-induced pores.

Cracking of AISI T22 Reheater Pendant Assembly Tubing of Steam Boiler

The Power Station Unit steam boiler was supplied in 1995. In the period of 2008–2013, a number of forced outages were documented at the reheat pendant assemblies. Based on laboratory analysis, it has been concluded that all investigated tubes failed due to propagation of fatigue cracks along the high crown seal welds, caused by sway movement fluctuations of assemblies while combustion gas flows. The main factor in the formation of fatigue cracks was the poor quality of the crown seal welds, and the main reason for the development of fatigue cracks in the tube wall was the rigid and non-flexible fixing of the tubes in the crown seal boxes. It has been assumed that fatigue cracks currently exist in many reheater tubes. In an attempt to locate leaks in the reheater tubes, leak tests were conducted with acoustic equipment, and validation tests were conducted using soap bubbles and a borescope. Using this method, a high number of leaks were revealed during the long planned outage in April–December 2016. The most surprising fact was that in every one of approximately ten tests there were revealed a few new additional leaks. It was necessary to carry out brief failure analysis due to the outage, to understand the causes of failure, to avoid similar failures and to return the unit to operation.

Manufacturing Process and Interface Properties of Vacuum Rolling Large-Area Titanium-Steel Cladding Plate

In order to obtain large-area titanium-steel cladding plate by vacuum rolling, the manufacturing process was discussed with the Hypermesh/LS-DYNA simulation software. The result showed that the reduction rates of each rolling pass were 20, 25 and 33%, respectively. And the total rate was 60%. Moreover, the vacuum rolling process mainly included the following steps: welding preparation, drilling vacuum holes, composite and assembly, vacuum extraction, accumulative seal welding, preheating and rolling. The final size of the cladding plate was 1650 × 12000 × (4 + 40) mm. It could be found that the vacuum rolling interface of the titanium-steel cladding plate was mainly divided into four parts: steel layer (I), decarburized layer (II), bonding layer (III) and titanium layer (IV). Acicular widmanstatten structure of β-titanium was formed in zone IV, which might reduce the impact toughness of joint. The microhardness test results showed that the hardness near the interface was relatively high. Macroscopically, the average shear strength (297 MPa) and the average tensile strength (590 MPa) were both much higher than the standard. However, the brittle fracture mode of shear specimens might decrease the joint property of vacuum rolling cladding plate.

Failure analysis of titanium heater tubes and stainless steel heat exchanger weld joints in nitric acid loop

A 400 l capacity nitric acid loop (NAL) test facility simulating inactive plant operating conditions was designed, constructed and evaluation of candidate materials was carried out for nuclear spent fuel reprocessing applications. Austenitic stainless steel coupons were exposed to flowing nitric acid medium at various temperatures in NAL. Assessment of critical components of NAL after 10,000 h operation revealed failure of titanium heater tubes and stainless steel heat exchanger tube sheet welds. Failure analysis and detailed characterisation of corrosion products on the failed tubes were carried out using SEM, EDX and XRD techniques. The brown colour corrosion product on titanium heater tubes comprised iron and small amounts of chromium indicating that failure of titanium tubes was due to deposition of corrosion products of stainless steel, leading to galvanic corrosion. Cracks observed on titanium heater tubes were attributed to hydrogen-induced cracking, resulting in severe degradation. Surface morphology and EDX analysis of the corrosion product on stainless steel heat exchanger tubes revealed dislodged grains from 304LSS and XRD analysis confirmed the deposit as stainless steel. Grain dropping occurred due to intergranular corrosion of 304L SS in nitric acid and deposition of the dislodged grains at the seal weld joints resulted in localised corrosion and failure. Based on the studies carried out, surface heaters instead of immersion heaters and periodic inspection and frequent removal of corrosion products from the nitric acid loop were suggested to mitigate such failures. Based on the results of the investigation, repair and refurbishment of the nitric acid loop was undertaken.

Assembly and final dimensional inspection at factory of the JT-60SA Cryostat Vessel Body Cylindrical Section

The superconducting tokamak JT-60SA is currently being assembled at the QST laboratories in Naka (Japan). Within the European contribution in the framework of the Broader Approach, Spain has been responsible for providing JT-60SA cryostat. The cryostat is a large vacuum vessel made up of 304 stainless steel which encloses the tokamak providing the vacuum environment to reduce thermal loads on the components at cryogenic temperature. It must withstand the external atmospheric pressure during normal operation and the internal overpressure in case of an accident. Due to functional purposes, the cryostat has been divided in three assemblies: the Cryostat Base (CB), the Cryostat Vessel Body Cylindrical Section (CVBCS) and the Top Lid. For transport and assembly reasons the cryostat is made up of 20 main parts: 7 making up the CB and 13 making up the CVBCS (including the top lid). The joints between them rely on bolted flanges together with light seal welds, non-structural fillet welds performed from inside and/or outside of the cryostat. The single wall is externally reinforced with ribs to support the weight of all the ports and port plugs and also to withstand the vacuum pressure. The material is SS 304 (Co < 0.05 wt%) with a permeability (μrel) below 1.1. The CVBCS made of a single wall is a stainless steel shell with a thickness of 34 mm. The CB was manufactured and assembled in-situ in 2013, while the CVBCS has been manufactured, assembled, measured by a Spanish company (ASTURFEITO S.A) and delivered to Japan in November 2017. The paper summarizes the assembly and final measurement of the CVBCS at the factory.

Study on Welding Techniques of 316L Lined Bimetallic Pipe

It is well known that welding technique was often a knotty problem for bimetallic lined steel pipes to use widely. A number of failures in secession of weld cracking and weld corrosion had been observed in oil fields in recent years, which seriously disrupted the order of oil and gas production. To solve welding problems of 316L bimetallic lined pipes, works outcome about failure analysis and welding process research were presented in this paper. Failure analysis results confirmed that Welding defects, high hardness regions was the main reasons about failure problems of weld crack while structure design defects of seal weld and bad back-protection effects of flux-cored wire resulted in weld corrosion. Welding defects in the regions of seal weld became the failure source while the high hardness both in the region of seal weld and weld joint formed the crack propagation channel, and therefore both initially contributed to weld cracking. Additionally owing to the structure design of seal weld, liner layer would be heated over and over again during the period of seal weld and then it was not enough to protect CRA layers from being damaged during the period of girth weld. As a result the corrosion resistance in the welding area was reduced to become a weak area. On the basis of failure analysis, further research work was carried out to improve welding performance. Seal weld structure and girth weld process was improved. The difference of welding wires and welding process was analyzed, and their defects were described separately. Results showed that the welding performance welding by ERNiCrMo-3 and supporting technology was more reliable than ATS-F309L and supporting technology, whether seal weld or butt welding. The distribution and value of the hardness could be effectively controlled; Moreover, corrosion resistance performance was also better. Therefore, the seal weld and girth weld conducted by ERNiCrMo-3 and supporting technology was feasible.

Laser seal welding of end plug to thin-walled nanostructured high-strength molybdenum alloy cladding with a zirconium interlayer

The effects of micro-alloying method on microstructure and properties of laser-welded molybdenum alloy joints were investigated, and zirconium was selected as the additive material. In the fusion zone of the joint without zirconium addition, the segregation of MoO2 on the grain boundary were observed, leading to the brittle fracture of the joint. When zirconium ring was added into the fusion zone, ZrO2 was preferentially formed in the grain interior which alleviated the segregation of MoO2. The grains at fusion zone were refined under the effect of heterogeneous nucleation particles of ZrO2, thus it changes the fracture mechanism of welded joint from intergranular fracture to cleavage fracture. When zirconium foil was pre-placed at the overlapped interface of cladding tube and the end plug, a parasitic brazing zone was generated due to the high melting point and high thermal conductivity of molybdenum. Tensile strength of the welded joints matched that of the base metal. These results highlight possibilities for achieving high strength joints when joining refractory materials with intrinsic brittleness in complex joint configurations.

The use of single-mode fiber laser for welding of stainless steel thin thickness

At this moment, solid-state lasers based on Nd:YAG are replaced by fiber ytterbium lasers. This is due to a number of advantages, such as: high electrical efficiency, long life of diodes for pumping, significant reduction in floor space, etc. In this regard, Nd:YAG laser was changed to YLR300 single-mode fiber laser manufacturing by IPG at the Rofin StarWeld 500 installation with scan head. Preliminary modes for welding stainless steel AISI 321 with a thickness of 0.1 - 0.4 mm were selected. In this report, the influence of the laser power and welding speed on weld geometry are presented. The welds were analyzed by optical microscopy. Lap welding of the heat exchanger-recuperator parts was carried out with the weld tightness test. Analysis of the experiments showed that the use of a single-mode fiber laser with scan head makes it possible to obtain high-quality seal welds of small thickness with a high productivity and welding speed on the order 250 mm/s.

(Invited) Safe Design of GS Yuasa Large Format Lithium Ion Cells for Industrial Applications

Since the 1990’s, GS Yuasa has developed and produced large-format Li ion cells for a variety of applications including satellites, rockets, electrified vehicles, trains, AGVs, energy storage systems, UPS, submarines, and others. Through these activities, safer Lithium-ion cell designs have evolved not only through the careful selection of electrode chemistry, but also cell shape, case materials and cell orientation within the module or battery pack. Lithium iron phosphate, for example, is stable and often considered a “safe” material for use in the positive electrodes of lithium-ion cells; however, there have been many lithium iron phosphate cell fires in the field. Fires may be caused by leakage of a flammable electrolyte in proximity to an ignition source. GS Yuasa considers the combination of using a metal cell case with laser-welded seal and a terminals-up orientation of cells in the module or battery pack to drastically decrease the risk of electrolyte leakage. Figure 1 shows the typical cell design of our large lithium ion cells for electrified vehicles and other industrial applications such as trains, AGVs, energy storage systems and UPS. The metal case with laser-welded seal offers high reliability with respect to the prevention of electrolyte leakage. This approach to cell design and orientation locates the more sensitive potential leak paths such as the rupture disk, terminal seal and case welds in the top of the cell and away from free electrolyte. In this presentation, we will detail GS Yuasa’s cell and battery systems and discuss our field experiences. Figure 1