High Temperature Vacuum Brazing Research Articles

High temperature vacuum brazing of tungsten to tungsten alloy with structural material

The present paper offers an overview of experimental research on the vacuum brazing procedure used to join tungsten-based refractory materials and tungsten alloy with SS316L (a structural material) at high temperatures. The objective of joining tungsten (W) to tungsten alloy material (WL10) and SS316L materials is to develop a joining technique for fabricating the helium-cooled divertor target relevant to a DEMO-like fusion reactor. These divertor fingers are expected to withstand an incident heat flux of 10 MW/m2 and to be cooled by helium jets in an environment of high pressure (10 MPa) and high-temperature (600 °C) helium gas. High-temperature vacuum brazing has been performed at temperatures exceeding 1000 °C using selective brazing fillers in the Gleeble Machine-3800 thermo-mechanical simulator to join W (tungsten) to WL10 alloy (tungsten + 1 % Lanthanum oxide). Furthermore, vacuum brazing of multilayer materials, such as W-WL10-SS316L, has been conducted at 1050 °C for a 3 min hold time using AMS 4777 filler material in a single run on the Gleeble machine. The brazed specimens have undergone testing for 500 thermal cycles from 950 °C to 815 °C on the Gleeble-3800 machine to pre-qualify the brazed joints. Non-destructive testing (NDT), including ultrasonic testing (UT), microstructural analysis, and mechanical characterization, is employed to assess the quality of the brazed joints. The study presents the experimental techniques and the results of the characterization.

Vacuum High-Temperature Brazing of 3003 Aluminum Alloy

Brazing filler metals based on the Al-Si system are widely used for brazing aluminum alloys. Their melting point is 577 °С (eutectic). It is necessary to conduct comprehensive studies of the technological properties of experimental filler metals and brazed joints to create a brazing filler metal with a reduced melting temperature for vacuum brazing of thin-walled aluminum products made of alloy 3003. The paper presents the research results on high-temperature vacuum brazing of aluminum alloy 3003 with Al-Cu-(Si, Mg) filler metal. It was determined that the amount of magnesium in the filler should be limited due to the risk of porosity formation associated with magnesium vaporization. It was identified that reducing the magnesium content increases the liquidus temperature above 530–550 °C. Therefore, experimental alloys require additional alloying with depressant elements, particularly silicon, to achieve the required melting temperature level. The chemical inhomogeneity of the filler in the initial state (after rapid solidification from the liquid state) and the structure of the brazed joints were investigated using micro-X-ray spectral analysis. Through empirical means, it was determined that a magnesium content of 1.5 % by weight in the filler allows for producing high-quality brazed joints without visible defects. In this case, shear strength is in the range of 0.6–0.7 of the strength of the base material. Tests of brazed joints for three-point bending resulted in an angle close to 180°, which indicates the promising use of experimental brazing filler metal in vacuum brazing of aluminum alloy 3003.

Influence of brazing temperature on interfacial reaction layer characteristics of Cu-Sn-Ti/diamond composites

Cu-Sn-Ti/diamond composites have been manufactured by cold-press forming followed by high-temperature vacuum brazing. The effects of brazing temperature on Cu-Sn-Ti/diamond wetting behaviors and associated morphologies, interfacial reaction layer products and thickness variations, elemental distribution features, as well as corresponding orientation relationships, have been systematically investigated. It has been documented that as the brazing temperature increases, diamond particles are gradually coated by the active Cu-Sn-Ti filler alloy. The interfacial reaction layer products are determined to be primarily composed of TiC, the formation of which is driven by the diffusion of dissolved Ti and reaction with C. It has been revealed that the thickness of the TiC layer shows a definitive increasing trend till reaching 1.75 μm up to 1213 K but stops afterwards. Concurrently, TiC has been confirmed to possess a semi-coherent orientation relationship ((1 1 1)TiC//(1 1 1)diamond and [1 1¯ 0]TiC//[1 1¯ 0]diamond) with the diamond matrix, boding well for a robust chemical bonding. It is suggested that the brazing temperature shall be limited under 1213 K for optimized bonding strength and grinding capability.

Advanced aluminum alloys for aircraft brazed structures

Review of domestic and foreign scientific and technical literature in the fi eld of application of aluminum alloys for brazed structures of aircraft heat exchangers manufactured by high-temperature vacuum brazing is provided. Prospective developments of domestic aluminum alloys for application as base material for clad sheets for brazed structures of heat exchangers are presented. The possibilities for increasing of the corrosion resistance of heat transfer surfaces are considered. The possibilities for application of additive technologies for the manufacture of aluminum heat exchangers alloys are considered.

Infiltration brazed core-shell WC@NiP/NiCrBSi composite cladding

In this study, a novel Ni-P-coated WC/NiCrBSi composite cladding was prepared on a carbon steel substrate through high-temperature vacuum brazing technique with the aim of improving the wear properties of the surface. In order to improve the wettability of WC particles by a liquid phase (NiCrBSi alloy), Ni-P layer was deposited on carbide particles by electroless plating. This layer assisted to fabricate a thick coating with very low porosity, improving adhesion strength between ceramic particles and the matrix. The morphology, crystal structure and microstructure of coated carbides and the resultant composite cladding were characterized by field emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analysis. Thin electroless plated Ni-P shell with a thickness about 50–150 nm, were coated on WC particles. In the claddings, a uniform and homogenous distribution of WC particles in Ni-based matrix was observed. The pin on disk wear test was carried out to investigate the wear behavior of the fabricated cladding reinforced with and without Ni-P coated WC particles. The results indicated that the resistance against crack initiation and propagation significantly improved by using Ni-P layer in the sintered cladding. This cladding exhibited very low wear weight loss than conventional brazed cladding.

Influence of Cr addition on microstructure of vacuum brazed NiCr–Cr3C2 composite coatings

Metallurgically bonded NiCr–Cr3C2 composite coatings modified by the additions of Cr were produced on a Ni-based superalloy substrate by high temperature vacuum brazing process. Effects of Cr addition on the microstructure, phases presented and the interfacial diffusion of the coatings have been investigated. Results show that the Cr additive consumed Si element to form Cr3Si and reduced the amount of brittle phase Ni3Si at the interface. The higher was the Cr addition in the coating, the thinner was the thickness of the secondary reaction zone, and the lower was the amount of the Ni3Si at the interface.

Vacuum brazing of alumina to stainless steel using femtosecond laser patterned periodic surface structure

Ceramic surface patterning is a promising method to reduce the residual stress of ceramic-metal joints, since it needs no complex multiple interlayer and is cost effective. However, literature shows that surface patterns previously examined for ceramic-metal brazing were at a millimeter scale, which limits the application and the degree of enhancement. In this study, femtosecond laser ablation was used to fabricate microscale periodic surface patterns (grooves) on ceramic surfaces. No defects or damage were induced during the ablation. The groove dimension was tunable by controlling ablation parameters. High temperature vacuum brazing of 304 stainless steel to the Al2O3 ceramic with surface grooves was performed using a commercial AgCuTi brazing filler metal. With properly designed surface grooves, the average joint strength was 66MPa, 2.75 times of that of the flat-interface joints. Finite Element Analysis models combined with fracture path examination indicated that the redistribution of residual stress induced by periodic surface pattern accounted for the joint strength enhancement.

Experimental Investigation on the Kinetic Parameters of Diffusion Component for Vacuum Brazing SS316L/BNi-2/SS316L Joint

High temperature vacuum brazing has become the only appropriate process for thin-walled structures such as compact heat exchangers used in high-temperature gas-cooled reactor (HTGR) systems. During the typical brazing process, major attention should be paid to the isothermal solidification stage which has the most important influence on joint quality. However, the isothermal solidification process is controlled by the diffusion behavior of melting point depressant (MPD) elements, and only thorough diffusion of MPD elements into the base metal can result in the formation of the full solid-solution brazed joints. Therefore, it is significant for brazed joints to accurately obtain the kinetic parameters of diffusion component. In this study, a new type of wedge-shaped brazing specimen was proposed to study the joints properties. The experiments were carried out on the combination of SS316L with BNi-2 filler alloy at 1050, 1075 and 1100°C with different holding time ranged from 15 to 45min, respectively. Two classical models coupled with experimental data have been used to determine the kinetic parameters, including diffusion coefficient and diffusion activity energy. Then the effects of the brazing temperature and holding time on the maximum brazing clearance (MBC) value are also investigated. The results show that the diffusion coefficient increases with the increase of temperature and decreases with the increase of holding time at constant temperature. The diffusion activity energy of boron in SS316L is 134kJ/mol. Meanwhile, significant augment of the MBC value has also been observed with increase in brazing temperature and holding time.

Brazing of 48-2-2 intermetallic alloys with a brazing alloy of the Ti–Zr–Mn system

The results of investigation of the alloys of the Ti–Zr–Mn system are presented. It is shown that the alloys of the system can be used efficiently as brazing alloys for high-temperature vacuum brazing of 48-2-2 intermetallic titanium alloys. Morphological special features of the formation of the structure of the brazed joints are investigated.

Development of the armoring technique for ITER Divertor Dome

This paper describes the current status of the technique for armoring of Plasma Facing Units (PFUs) of the ITER Divertor Dome with flat tungsten tiles planned for application at the procurement stage. Application of high-temperature vacuum brazing for armoring of High Heat Flux (HHF) plasma facing components was traditionally developed at the Efremov Institute and successfully tried out at the ITER R&D stage by manufacturing and HHF testing of a number of W- and Be-armored mock-ups [1,2]. Nevertheless, the so-called “fast brazing” technique successfully applied in the past was abandoned at the stage of manufacturing of the Dome Qualification Prototypes (Dome QPs), as it failed to retain the mechanical properties of CuCrZr heat sink of the substrate. Another problem was a substantially increased number of armoring tiles brazed onto one substrate. Severe ITER requirements for the joints quality have forced us to refuse from production of W/Cu joints by brazing in favor of casting. These modifications have allowed us to produce ITER Divertor Dome QPs with high-quality tungsten armor, which then passed successfully the HHF testing. Further preparation to the procurement stage is in progress.

Development of multi-channel high power rectangular RF window for LHCD system employing high temperature vacuum brazing technique

A 3.7 GHz., 120 kW (pulsed), lower hybrid current drive (LHCD) system is employed to drive non-inductive plasma current in ADITYA tokamak. The rf power is coupled to the plasma through grill antenna and is placed in vacuum environment. A vacuum break between the pressurized transmission line and the grill antenna is achieved with the help of a multi (eight) channel rectangular RF vacuum window.The phasing between adjacent channels of 8-channel window (arranged in two rows) is important for launching lower hybrid waves and each channel should have independent vacuum window so that phase information is retained. The geometrical parameter of the grill antenna, like periodicity (9mm), channel dimensions (cross sectional dimension of 76mm × 7mm), etc. is to be maintained. These design constraint demanded a development of a multi channel rectangular RF vacuum window. To handle rf losses and thermal effects, high temperature vacuum brazing techniques is desired. Based on the above requirements we have successfully developed a multi channel rectangular rf vacuum window employing high temperature vacuum brazing technique. During the development process we could optimize the chemical processing parameters, brazing process parameters, jigs and fixtures for high temperature brazing and leak testing, etc. Finally the window is tested for low power rf performance using VNA. In this paper we would present the development of the said window in detail along with its mechanical, vacuum and rf performances.

Brazing Technology for Plasma Facing Components in Nuclear Fusion Applications Using Low and Graded CTE Interlayers

In Plasma Facing Components (PFCs) for nuclear fusion reactors, the protective material, carbon based or tungsten, has to be joined to the copper alloy heat sink for optimum heat transfer. High temperature vacuum brazing is a possible joining process as long as a proper interlayer is introduced to mitigate the residual stresses due to the mismatch of thermal expansion coefficient (CTE). Pure copper can act as plastic compliant layer, however for carbon based materials a proper structuring of the joining surface is necessary to meet the thermal fatigue lifetime requirements. In this work pure molybdenum and tungsten/copper Metal Matrix Composites (W-wires in Cu-matrix) interlayers have been studied as alternative to pure copper for carbon based protective materials in flat tile configuration. Finite element simulations of the brazing process have been performed to evaluate the expected residual stress reduction near the metal-carbon interface. In fact it has been demonstrated that stiff low CTE interlayers can shift the peak stresses from the weak carbon-metal interface to the strongest metal-metal one. Relevant samples have been manufactured and subjected to preliminary metallographic and thermal shock tests. Results obtained so far are encouraging and active cooled mock-ups are being prepared for high heat flux testing. Research work is in progress as regards monoblock configuration with both Wf/Cu MMC and graded Cu/W plasma sprayed and HIPped layers.

Vacuum brazing of Inconel 600 sleeve to 316L stainless steel sheath of mineral insulated cable

Abstract In mineral insulated (MI) triaxial cables for detectors, the 200 μm thick zirconiated copper conductors are sheathed with 200 μm thick 316L stainless steel (SS). The process conditions for high-temperature vacuum brazing of Inconel 600 sleeves to 316L SS cable sheath, with a radial gap of 125–150 μm between them, has been optimised using 40 μm thick nickel-base BNi2 brazing alloy. The brazed joints, which were subjected to thermal cycling between 473 and 923 K over 50 thermal cycles to simulate the high-temperature operating conditions, was found to have adequate tensile strength similar to those in the as-brazed condition. During tensile testing, failure occurs in the cable sheath in all cases. The leak tightness of the brazed joints, in as-brazed and thermal-cycled conditions, both before and after tensile testing is also found to be satisfactory.

Wear behavior of brazed WC/NiCrBSi(Co) composite coatings

A metallurgically bonded composite coating of a Ni–Cr–B–Si alloy reinforced with cobalt-coated tungsten carbide (WC-Co) particles was produced on a carbon steel substrate by high temperature vacuum brazing. The bond strength of the coating/substrate interface and the tensile strength of the coating itself reached 300–360 and 100–140 MPa, respectively. Effects of Co and WC-17Co contents on the wear behavior of the coating have been systematically investigated at two different wear conditions, namely the high speed rotating slurry erosive wear and the wet sand/rubber wheel abrasive wear. Results show that the higher was the Co content in the coating, the lower were the hardness and wear resistance. In the slurry erosive wear, the WC reinforcement of the composite coating plays an important role protecting the matrix from being worn-out, whereas in the abrasive wear, the wear mechanism is mainly controlled by the scratching and micro-cutting of the matrix followed by the pull out of WC particles due to the scratching action of abrasives. The wear resistance of the brazed composite coating (WC/NiCrBSi(Co)) is better than that of the flame overlaid coating.

Microstructure and bonding strength of WC reinforced Ni-base alloy brazed composite coating

A tungsten carbide (WC) reinforced NiCrBSi(Co) composite coating was produced on mild steel by high temperature vacuum brazing. The microstructure, phases present and the interface diffusion have been investigated. The cobalt coated on the surface of WC particles dissolved into the liquid nickel-base alloy but the WC was not decomposed severely during the vacuum brazing process. Microanalysis showed that the composite coating matrix was composed of γ-Ni solution and γ-Ni+(Ni,Co) 3B lamellar eutectic. Diffusion and metallurgical reaction occurred at the WC/NiCrBSi(Co) interface and composite coating/substrate interface. A 6-μm thick brazing seam or interfacial bonding zone was found between the coating and the substrate as the coated sample cooled down. Bonding strengths of the WC/NiCrBSi(Co) interface and the composite coating/substrate interface reached 100–140 MPa and 300–360 MPa, respectively, which are far higher than those of thermal spray coating. The abrasive wear resistance of brazed composite coating also appears to be better than that of the flame overlaid at the same composition.

Application of high-temperature vacuum-brazing for fabrication of compact shell-and-tube heat exchanger

Application of high-temperature vacuum-brazing for fabrication of compact shell-and-tube heat exchanger

High-temperature vacuum brazing of plate-and-rib heat exchangers of compressor units

High-temperature vacuum brazing of plate-and-rib heat exchangers of compressor units

High-temperature vacuum brazing of centrifugal-compressor impellers made of 07Kh16N6 steel

High-temperature vacuum brazing of centrifugal-compressor impellers made of 07Kh16N6 steel