High-temperature Brazing Research Articles

Changes in the structure of materials in brazing niobium with sapphire using a glass–ceramic solder

The changes in the structure of joints between sapphire and niobium produced by high-temperature brazing using a CaO–Al2O3 brazing alloy are investigated. The effect of temperature on the recrystallization processes in niobium and its alloys with zirconium on the formation of the second phase in the CaO–Al2O3 system and the diffusion of niobium in the glass–ceramic solder is investigated. The optimum conditions of brazing the niobium alloy with sapphire are determined.

Basic principles of creating a new generation of high- temperature brazing filler alloys

The development of new materials is based on the formation of a structural-phase state providing the desired properties by selecting the base and the complex of alloying elements. The development of amorphous filler alloys for a high-temperature brazing has its own features that are due to the limited life cycle and the production method of brazing filler alloys. The work presents a cycle of analytical and experimental materials science investigations including justification of the composition of a new amorphous filler alloy for brazing the products from zirconium alloys at the temperature of no more than 800 °C and at the unbrazing temperature of permanent joints of more than 1200 °C. The experimental alloys have been used for manufacture of amorphous ribbons by rapid quenching, of which the certification has been made by X-ray investigations and a differential-thermal analysis. These ribbons were used to obtain permanent joints from the spacer grid cells (made from the alloy Zr-1% Nb) of fuel assemblies of the thermal nuclear reactor VVER-440. The brazed samples in the form of a pair of cells have been exposed to corrosion tests in autoclaves in superheated water at a temperature of 350 °C, a pressure of 160 MPa and duration of up to 6,000 h. They have been also exposed to destructive tests using a tensile machine. The experimental results obtained have made it possible to propose and patent a brazing filler alloy of the following composition: Zr-5.5Fe-(2.5-3.5)Be-1Nb-(5-8)Cu-2Sn-0.4Cr-(0.5-1.0)Ge. Its melting point is 780 °C and the recommended brazing temperature is 800°C.

Peculiarities of the structural-phase state of a brazing zone of dissimilar metal joints

The present work is devoted to the study of the structural-phase state of the brazed joints of dissimilar materials obtained by rapidly quenched filler metals before and after tests in the working conditions. The constructive elements of the International Thermonuclear Experimental Reactor (ITER) working chamber were selected: mock-ups of the first wall (a beryllium cladding joined with the heat-removing bronze by high-temperature brazing) and the divertor (a tungsten cladding with built-up copper, brazing of copper with bronze). An analysis of the investigated samples and the data on the structural-phase state of the brazed joints and its evolution as a result of a cyclic thermal impact was carried out by metallography and electron microscopy.

Development of rapidly quenched nickel-based non-boron filler metals for brazing corrosion resistant steels

Corrosion-resistant steels are stably applied in modern rocket and nuclear technology. Creating of permanent joints of these steels is a difficult task that can be solved by means of welding or brazing. Recently, the use rapidly quenched boron-containing filler metals is perspective. However, the use of such alloys leads to the formation of brittle borides in brazing zone, which degrades the corrosion resistance and mechanical properties of the compounds. Therefore, the development of non-boron alloys for brazing stainless steels is important task. The study of binary systems Ni-Be and Ni-Si revealed the perspective of replacing boron in Ni-based filler metals by beryllium, so there was the objective of studying of phase equilibrium in the system Ni-Be-Si. The alloys of the Ni-Si-Be with different contents of Si and Be are considered in this paper. The presence of two low-melting components is revealed during of their studying by methods of metallography analysis and DTA. Microhardness is measured and X-ray diffraction analysis is conducted for a number of alloys of Ni-Si-Be. The compositions are developed on the basis of these data. Rapidly quenched brazing alloys can be prepared from these compositions, and they are suitable for high temperature brazing of steels.

High power tests of an electroforming cavity operating at 11.424 GHz

The achievement of ultra high accelerating gradients is mandatory in order to fabricate compact accelerators at 11.424 GHz for scientific and industrial applications. An extensive experimental and theoretical program to determine a reliable ultra high gradient operation of the future linear accelerators is under way in many laboratories. In particular, systematic studies on the 11.424 GHz frequency accelerator structures, R&D on new materials and the associated microwave technology are in progress to achieve accelerating gradients well above 120 MeV/m. Among the many, the electroforming procedure is a promising approach to manufacture high performance RF devices in order to avoid the high temperature brazing and to produce precise RF structures. We report here the characterization of a hard high gradient RF accelerating structure at 11.424 GHz fabricated using the electroforming technique. Low-level RF measurements and high power RF tests carried out at the SLAC National Accelerator Laboratory on this prototype are presented and discussed. In addition, we present also a possible layout where the water-cooling of irises based on the electroforming process has been considered for the first time.

Development of brazing process for W–EUROFER joints using Cu-based fillers

A successful joint between W and EUROFER using high temperature brazing technique has been achieved for structural application in future fusion power plants. Cu-based powder alloy mixed with a polymeric binder has been used as filler. Microstructural analysis of the joints revealed that the joint consisted mainly of primary phases and acicular structures in a Cu matrix. Interaction between EUROFER and filler took place at the interface giving rise to several Cu–Ti–Fe rich layers. A loss of hardness at the EUROFER substrate close to the joint due to a diffusion phenomenon during brazing cycle was measured; however, the joints had an adequate shear strength value.

V-containing-active high-temperature brazes for ceramic joining

V-containing-active high-temperature brazing alloys were designed for ceramic joining, including PdCo–V, AuPdCoNi–V, AuCuPd–V, and CuPd–V system alloys. The joined ceramics or ceramic composites include Si3N4 ceramic, AlN ceramic, carbon fiber-reinforced silicon carbide composite (Cf/SiC), and C/C composite. The dynamic wettability of the V-containing brazes on ceramic or composite was studied with the sessile drop method. Based on the wetting experiment results, several alloys were chosen as the brazing filler metal. It was verified that during the brazing process, the reactions between the filler metal and the Si3N4 or AlN ceramic caused the formation of V–N layer at the interface close to the ceramic surface. The V–C compound was detected at the reaction interface between the V-containing-active filler metal and C/C composite or Cf/SiC composite. The formation of V–N or V–C layer played an important role in the joining of the ceramic or the ceramic composite. Both of the Si3N4–Si3N4 and Cf/SiC–Cf/SiC joints brazed with V-containing-active braze exhibited stable high-temperature property, and this should be attributed to the presence of refractory Pd2Si compounds within the joints. The newly developed V-containing-active brazes should have a potential to meet service temperatures of 973~1073 K, about 300 K higher than the AgCu–Ti brazing filler metals.

Development of High-Temperature Nicrobrazed Joints Using Induction Heating in an Argon Gas Environment

For the fabrication of instrumented capsules for fast reactor applications, there is need to produce brazed joints for service at high temperatures. Fabrication of instrumented capsules requires development of thin-walled joints capable of withstanding high temperatures (>673 K). Out of the various joining methods available for thin-walled joints, development of high-temperature brazed joint method is found to be advantageous. An induction heating technique has been used for this development. Protective atmosphere is required for high-temperature brazing to prevent oxidation of the base metal and the filler metal during the brazing operation (~1303 K). Commercial argon gas (99.996 % pure) has been used and brazed joints have been developed. Trials were carried out during the development of high-temperature brazed joints using induction heating system in an argon gas environment, and the results obtained are discussed in this paper.

High-temperature brazing alloys produced by mechanical alloying

Mechanical alloying (MA) is proposed and tested as an alternative method of producing high-temperature brazing alloys. The oxygen content and distribution in the volume of the particles were determined in the powders of the high-temperature brazing alloy, produced by MA and dispersion of the melt. To reduce the oxygen content, the mechanically alloyed powders of the brazing alloy were refined in nitrogen. The data are presented for the experimental brazed joints produced with high-temperature brazing alloys prepared by MA with or without subsequent refining.

Recent Progress in the Development of Helium-Cooled Divertor for DEMO

A helium-cooled divertor concept for DEMO has been continuously developed over the past decade at the Karlsruhe Institute of Technology within the framework of the former European Fusion Power Plant Conceptual Study. Over the years, research results and progress of the divertor development with numerous earnings representations have been continually reported. This paper first gives a retrospect of the past results achieved so far and then reports on recent progress of the divertor development. In the course of developing the conceptual design with the goal of reaching a divertor heat flux performance of 10 MW/m2, the He-cooled modular divertor with jet cooling (HEMJ) was selected in the early 2000s as the reference concept out of a series of conceptual design studies. For verification of the design principle, a combined high-heat-flux (HHF) test facility with helium loop was built in 2004 at the Efremov Institute for the divertor experiments under specified DEMO conditions. There, the cooling performance of the divertor finger with helium under the heat load of 10 MW/m2 was confirmed already at an early stage. In parallel, the HEMJ divertor design was successively improved in terms of its robustness and quality of production in order to achieve a long service life against thermocyclic loading. A breakthrough was achieved in 2010 when an optimized HEMJ cooling finger survived more than 1000 HHF cycles at 10 MW/m2 without damage. In the context of long-term planning for DEMO divertor development, research and development work on the development of larger divertor components has been started, particularly focusing on certain fabrication techniques covering, e.g., high-temperature brazing and mass production of the divertor components. Recent progress—a part of this paper—was achieved in the HHF experiment of the tungsten nine-finger module in Efremov, development of nondestructive testing methods for testing multifinger modules in collaboration with CEA, and a study on the integration of multifinger modules on the target plate.

A nickel-based brazing alloy for brazing creep-resisting alloys and steels

The main types of brazing alloys used for brazing creep-resisting alloys and steels are investigated and their characteristics and main special features described. The aspects of the selection of brazing alloys for brazing different materials depending on the brazing conditions and the operating conditions of the components are discussed. Examples of the application of the technology of high-temperature brazing in the manufacture of aviation technology components are presented.

He-Cooled Divertor for DEMO: Technological Study on Joining Tungsten Components with Titanium Interlayer

A modular He-cooled divertor concept for DEMO has been pursued at KIT with the goal of reaching 10 MW/m2. The reference design uses small tungsten-based cooling fingers of about 20 mm in size. They consist of a tungsten tile as a thermal shield that is to be connected to a thimble heat sink structure from W–1 wt% La2O3 (WL10) tungsten alloy. The lower boundary of the divertor operating temperature window is predicted by the ductile-to-brittle temperature and the upper boundary by the recrystallization temperature of WL10 material, currently assumed at 600°C and 1300°C, respectively. The important requirements for the joint between the W tile and WL10 thimble are (a) functioning as a crack stopper, (b) resisting a high operating temperature of about 1200°C, and (c) using low-activation material as an interlayer. Previously used PdNi brazing material has been successfully tested at a brazing temperature of about 1270°C. The mock-ups produced in this way are sufficient for the HHF tests without neutrons. In a further step to approach the DEMO requirements with higher demands, the use of low-activating titanium with a melting point of 1668°C as bonding material was examined both for brazing and for diffusion welding of tungsten parts. This paper reports on the first successful test results of both high-temperature brazing and diffusion bonding techniques.

C/C複合材／Ni基耐熱超合金異材間ろう付用高温ろう材の研究

C/C複合材／Ni基耐熱超合金異材間ろう付用高温ろう材の研究

Interface Reaction Thermodynamics of AgCuTi Brazing Filler Metal and Alumina Ceramic

In this work, the interface reaction between Al2O3 ceramic and Ag70.5Cu27.5Ti2 brazing filler metal at 845-860°C was investigated. Based on the data of thermodynamics and kinetics, the Gibbs free energies of the main interface reactions in the real brazing system condition were calculated. But the values of normal equilibrium reaction condition and the real interface reaction brazing system were different; and the main influential factor was the brazing temperature, and the system vacuum of brazing condition can lead the change of equilibrium constant (Kα). The results revealed that the high temperature and vacuum active brazing is a non-equilibrium interface reaction especially to titanium alloy, the vacuum and alloy liquid solution are beneficial to the brazing process, and the by-product formation of titanium-oxygen are affected by the diffusion process.

Joining of Si3N4 ceramic using PdCo(NiSiB)–V system brazing filler alloy and interfacial reactions

The wettability of V-active PdCo-based alloys on Si3N4 ceramic was studied with the sessile drop method. And the alloy of Pd50.0–Co33.7–Ni4.0–Si2.0–B0.7–V9.6 (wt%), was developed for Si3N4 ceramic joining in the present investigation. The rapidly-solidified brazing foils were fabricated by the alloy Pd50.0–Co33.7–Ni4.0–Si2.0–B0.7–V9.6. The average room-temperature three-point bend strength of the Si3N4/Si3N4 joints brazed at 1453K for 10min was 205.6MPa, and the newly developed braze gives joint strengths of 210.9MPa, 206.6MPa and 80.2MPa at high temperatures of 973K, 1073K and 1173K respectively. The interfacial reaction products in the Si3N4/Si3N4 joint brazed at 1453K for 10min were identified to be VN and Pd2Si by XRD analysis. Based on the XEDS analysis result, the residual brazing alloy existing at the central part of the joint was verified as Co-rich phases, in which the concentration of element Pd was high up to 18.0–19.1at%. The mechanism of the interfacial reactions was discussed. Pd should be a good choice as useful alloying element in newer high-temperature braze candidates for the joining of Si-based ceramics.

Optimization of the thermal cycle of high-temperature brazing of steels

Results are presented on the investigation of the optimization of the temperature–time conditions of high-temperature brazing of structural steels of the austenitic (12Cr18Ni10Ti) and martensitic [06Kh15N6MVFB-Sh (VNS16) and 03Kh12NYuMTR-VD (VNS25)] grades used widely in the manufacture of heat exchangers.

Using a multilayer composite brazing alloy for high-temperature brazing of heat exchangers

A new concept of composite brazing – application of multilayer composite brazing alloys of the Cr–Ni–Mn system in the form of coatings with efficient alloying of the brazed surfaces with chromium and the production of joints with an optimum thickness smaller than 80 μm – is described. The technology can be used to produce high-quality joints at the temperature of optimum heat treatment of steels, prevent clogging of cooling channels with a small cross section, increase the chromium content of the joint to a level similar to the chromium content of steels, produce corrosion-resisting joints with a strength equal to that of 12Cr18Ni10Ti steel and reduce the weight of the component by 30%.

Dissolution of TiAl alloy during high temperature brazing

An analytical model has been established to evaluate the thickness of the dissolution layer of base metal in a liquid brazing filler metal during a high temperature brazing process. The model was validated by a study of brazing of a TiAl alloy to a 42CrMo steel. Peak brazing temperatures were within a range of 1103–1203 K. The margin of deviation between predicted and experimentally determined dissolution thicknesses is within the range of 1–14 %, thus supporting the validity of the model well.

Thermally sprayed diffusion barrier coatings on C/C light-weight charging racks for furnace applications

Charging racks are used for the positioning and transport of workpieces in heat treatment and high-temperature brazing processes. Conventionally, they are made of steel or cast iron, but in recent years, more and more companies have changed over to charging racks made of light-weight materials such as graphite or carbon fiber-reinforced carbon (C/C). However, the main problem of these carbon-based workpiece carriers is the undesired carburization of components, which are in contact with the rack, by means of diffusion processes. For that reason, within the scope of the presented work, thermally sprayed coatings were applied on C/C and extensively tested for their suitability as high-temperature diffusion barrier. The ceramic powders Al2O3, Al2O3/Cr2O3, Al2O3/TiO2 and ZrO2/Y2O3 were chosen as coating materials and processed by powder flame spraying (PFS) and atmospheric plasma spraying (APS). Molybdenum and silicon carbide served as adhesion-promoting intermediate layers. Coating characterization included materialographic analyses, tensile strength and tribometric tests as well as long-term high-temperature tests, both laboratory-scaled and in industrial practice. Especially Al2O3 and Al2O3/Cr2O3 coatings on siliconized C/C exhibited excellent results, proving the wide application potential of thermally sprayed ceramic coatings on carbon-based light-weight materials.

High-Temperature Brazing Molybdenum

High-temperature brazing molybdenum using palladium and titanium foils have been investigated in the experiment. Successful brazed joints are achieved from using the palladium filler foil. Brazed joints are fully dense and free of any intermetallic phase. Three point bending strengths of 246 and 233 MPa are obtained from joints using 100 m thick palladium filler foil brazed at 1580 and 1610 oC for 600 s, respectively. The application of palladium filler foil shows potential in brazing molybdenum for high-temperature application.