Composite Brazing Research Articles

Evaluation of the efficiency of high-temperature steel heat exchangers brazed with a multilayer composite brazing alloy of the Cr–Ni–Mn system

The process of brazing heat exchangers made of stainless, creep-resisting and cryogenic steels 12Cr18Ni10Ti, VNS 16 and VNS 25 with a multilayer composite brazing alloy is developed. It is shown that the distribution of the elements of the brazing alloys Cr, Ni and Mn in the form of coatings eliminates the problems associated with wetting, spreading of the brazing alloy and clogging up of the cooling channels with a small cross section. The following parameters are determined: the chemical composition of the brazing alloy, the optimum thickness of the layers and the sequence of deposition of the layers of the smooth and finned surfaces. The experimental results show that the optimum brazing conditions (1150°C, 30 min+homogenizing annealing at 1000°C, 60 min) do not lead to any large decrease in the mechanical properties of the steels. The corrosion and mechanical properties of the brazed joints satisfy the service requirements. The strength of the brazed joints is equal to that of 12Cr18Ni10Ti steel. The process has been introduced in a number of important, large assembling systems. The mass of the components has been reduced by 30%.

Main technological special features of brazing steels with a composite brazing alloy of the chromium–nickel–manganese system

The efficiency of application of multilayer composite brazing alloys of the Cr–Ni–Mn system is shown on the example of brazing heat exchangers of 12Cr18Ni10Ti, 06Kh15N6MVFB-Sh and 03Kh12NYuMTR-VD steels. The strength of brazed joints produced in the optimum brazing conditions, the optimum ratio of the thickness of the coatings, and the sequence of deposition of the coatings is equal to that of 12Cr18Ni10Ti steel, and the corrosion resistance of the joints is also high. In the deposition of elements of the composite brazing alloy in the form of coatings with a total thickness of 60 μm, the spreading area is more than doubled in comparison with the strips of the brazing alloys.

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%.

Effects of Composite Nickel-Base Boracic Brazing Filler on the Boride near the Brazing Seam of 316L Stainless Steel

Composite filler（1-x%）BNi-2+ x% BNi-5 with 3 different B content was used to braze 316L stainless steel. The boride near brazing seam was determined by energy spectrum, XRD Micro- area scanning and thermodynamics, dynamics calculation software THERMO –CALC to study the impacts of B content on boride type and quantity. The results showed Cr5B3 and Cr2B were found at and around brazing seam of 1# specimen and Cr2B were found at and around both 6# and 8# specimens. In addition, the Cr2B content of 6 # specimen was greater than 8 # specimen. With the increase of BNi-5 content and decrease of B in the brazing filler, the type and content of the chromium boride reduced.

Joining of Cf/SiC composite to TC4 using Ag–Cu–Ti–SiC composite filler material

Cf/SiC was successfully joined to TC4 with Ag–Cu–Ti–SiC mixed powders. Microstructures of the brazed joint were investigated by SEM, EDS and XRD. The mechanical properties of the brazed joints were measured by mechanical testing machine. The results showed that the performed joints have dense bonding layers reinforced by residual SiC and reaction products from reaction of SiC and Ti in the bonding layers. These composite brazing layers relaxed the thermal stress of the joint effectively. These characteristics were beneficial to the joint, of which the shear strengths were remarkably higher than the optimal shear strengths of the joint brazed with Ag–Cu–Ti.

Reactive composite brazing of Cf/SiC composites to Ti alloy with (Ag–6Al)+Ti+C composite filler materials

Cf/SiC composite and Ti alloy (TC4) were successfully joined by the mixed powders of Ag, Al, Ti and short carbon fibres under vacuum condition. The Microstructure of the joints was investigated by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. The results show that the good chemical bonding and reactive composite brazing joints are obtained by reactive composite brazing with both (Ag–6Al)+Ti and (Ag–6Al)+Ti+C filler materials. The addition of short carbon fibres contributes a lot to the shear strength by alleviating the thermal stress of the joints and to the in situ synthesised TiC which surround the uniformly distributed short carbon fibres. Ti element can improve filler metal’s wettability to Cf/SiC composite and restrain the oxidation of Al element. The max shear strength obtained is 90·8 MPa and the optimal parameter is brazing at 910°C within 10 min.

Brazing of carbon fibre reinforced SiC composite and Ti alloy using Cu–Ti–C filler materials

Carbon fibre reinforced SiC (Cf/SiC) was successfully joined to Ti alloys with Cu–Ti–C mixed powder by brazing. Microstructures of the brazed joints were investigated by SEM, EDS and XRD. The mechanical properties of the brazed joints were measured by a mechanical testing machine. The results showed that the performed joints have dense bonding layers reinforced by residual graphite and in situ synthesised TiC from the reaction of C and Ti in the bonding layers. The composite brazing layers relaxed the thermal stress of the joints effectively. These characteristics were beneficial to the joints, of which the shear strength was remarkably higher than the optimal shear strengths of the joints brazed with pure Cu–Ti.

Microstructure evolution of Si 3N 4/Si 3N 4 joint brazed with Ag–Cu–Ti + SiCp composite filler

The Si 3N 4 ceramic was brazed by Ag–Cu–Ti + SiCp composite filler (p = particle) prepared by mechanical mixing. Effects of the content of Ti and SiC particles on microstructure of the joint were investigated. A reliable Si 3N 4/Si 3N 4 joint was achieved by using Ag–Cu–Ti + SiCp composite filler at 1173 K for 10 min. A continuous and compact reaction layer, with a suitable thickness, forms at the Si 3N 4/braze interface. The SiC particles react with Ti in the brazing layers, forming Ti 3SiC 2 thin layers around the SiC particles themselves and Ti 5Si 3 small particles in the Ag[Cu] and Cu[Ag] based solid solution. The higher content of SiC particles in the filler (≥10 vol%) depresses interfacial bonding strength between the Si 3N 4 substrate and composite brazing layer due to the thinner reaction layer and the bad fluidity of the filler. The Ti 3SiC 2 → TiC + Ti 5Si 3 reaction occurs when Ti concentration around SiC particles in the filler increases.

Joining of Cf/SiC composite to Ti alloy using composite filler materials

Cf/SiC was successfully joined to Ti alloy with Ag–Cu–Ti–W, Ag–Cu–Ti–SiC and Ag–Cu–Ti–TiC mixed powders by some suitable brazing parameters. Microstructure and shear strengths of the preformed joint were investigated. The results showed that the W particulate and reaction products can uniformly distribute in the brazing layer of the performed joint. These composite brazing layers relaxed the thermal stress of the joint effectively. These characteristics were beneficial to the joint, which had shear strengths that were significantly higher than the optimal shear strengths of the joint brazed with pure Ag–Cu–Ti at room temperature and 500°C.

Selection and Design of Brazing Fillers for Metal-Ceramic Joints

Three different approaches for metal to ceramic brazing are compared using the example of Si3N4/TiN-steel joints: the use of an active filler metal (single layer brazing system), the metallisation of the ceramic and brazing with a non active filler (two-layer brazing system), the use of a composite brazing filler system (three layer brazing system). Different aspects are analysed: the development of the joint’s microstructure in the as-brazed state, the thermally induced residual stresses and the resulting bend strength of the joint’s strength. With two layers and three layers brazing system, bend strength of about 400 MPa, about 15% higher then single filler metal, were achieved. The three layer brazing system has two advantages: firstly one step processing secondly lower scattering of joint bend strength compared to two layers brazing system. The key factors in all cases are the selection of the brazing fillers and the braze design. In all cases a careful selection of the brazing fillers and the braze design are the key factors. The first criteria for the selection of the brazing fillers for joints of dissimilar materials can be done by considering only the main filler characteristics like titanium content, processing temperature and yield stress. It’s necessary to simulate the joint behaviour by finite element simulation for verifying the final selection of filler metals. It was clearly seen that the plasticity of the filler metal is the main factor affecting residual stresses for the joint geometry in this current work.

Diffusion-controlled melting and re-solidification of metal micro layers on a reactive substrate

The paper offers a theoretical approach to a prediction of residue formation inherent to melting and subsequent solidification of micro layers of molten aluminum alloys. The residue formation follows a reactive flow of a portion of the melt that is removed by a surface tension action. The residue portion solidifies in situ. The phenomenon studied is associated with materials’ processing during controlled atmosphere brazing of aluminum. The model assumes that diffusion of Silicon, present in an Al+Si clad of a brazing sheet, has a twofold role. First, a solid state Si diffusion prior to melting and across the clad–core interface of a composite brazing sheet takes place and modifies alloys’ composition on both sides of the interface. Subsequently, Si diffusion within clad controls the melting process. Both processes are essential for clad residue formation. The approach advocated in this paper leads to a prediction of the residue formation through a modeling of the non-equilibrium diffusion-controlled melting. A heuristic interpretation of physical mechanisms was discussed and a related mathematical model devised. The model was solved numerically in terms of Si concentration distributions for a moving boundary problem and corroborated with empirical data. Empirical data were gathered using an experimental controlled atmosphere brazing facility. The results of the modeling and their corroboration with the experimental data indicate a strong dependence of residue formations on the pre-melting state of the clad, in particular on the grain size within Al-clad matrix. A good agreement between numerically predicted residue mass and experimental findings is documented in detail.

Dendritic growth in Al–Si alloys during brazing. Part 1: Experimental evidence and kinetics

This work provides empirical evidence needed for an in depth phenomenological study of dendrite growth phenomena during brazing of aluminum alloys in form of composite brazing sheets. The main objective of this study was (1) a collection of experimental evidence associated with heat and mass transfer modeling of the Al + Si solid solution dendrite macro morphology evolution inherent to joint formation during brazing, and (2) the dendrite growth kinetics analysis. The isothermal dwell and the quench that follow the clad molten aluminum binary alloy surface-tension-driven flow into the joint at the peak brazing temperature upon melting lead to the solidification of the metal micro layer and joint formation. Before, during and after the isothermal dwell a significant reduction of Si content in the melt is found. So, a subsequent dissolution may affect the interface zone between the molten clad and substrate. α-phase dendrite assemblies imbedded in an irregular eutectic in the joint zone are the main morphological features of the solidification microstructures. The major characteristic of the phenomenon is a sensitivity of the dendrite pattern selection and dendrite population on brazing process parameters, in particular on the temperature during the dwell.

Deformation of POS 61 composite brazing alloy under constant loading conditions

Deformation of POS 61 composite brazing alloy under constant loading conditions

Kinetics of formation of brazed joints in steel sections using composite brazing alloys

Kinetics of formation of brazed joints in steel sections using composite brazing alloys

A composite brazing alloy for brazing stainless and creep‐resisting steels

(1996). A composite brazing alloy for brazing stainless and creep‐resisting steels. Welding International: Vol. 10, No. 9, pp. 735-738.