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Diffusion Bonding Process
... Atoms diffuse across the interface and form the joint between plates. As shown in (Bartle, 1979). ...
... 6, the stages of diffusion bonding are: (a) the initial "point" of contact that shows a residual oxide contaminant layer, (b) yielding and creep, leading to reduced voids and a thinner contaminant layer, (c) final yielding and creep, where some voids remain with a very thin contaminant layer, (d) continued vacancy diffusion, which eliminates the oxide layer, leaving few small voids, and (e) bonding is complete(Bartle, 1979). ...
The primary mission of very-high-temperature reactors (VHTRs) is to generate electricity and provide high-temperature process heat for industrial applications with high efficiency, which relies on an effective intermediate heat exchanger (IHX) that transfers heat from the primary fluid (i.e., helium) to a secondary fluid. A printed circuit heat exchanger (PCHE) is one of the leading IHX candidates to be employed in VHTRs due to its compactness and capability for high-temperature, high-pressure applications with high effectiveness.
In this study, the thermal-hydraulic performance of a fabricated zigzag-channel PCHE was investigated. New pressure drop and heat transfer correlations were developed based on the experimental data. Local thermal-hydraulic performance of the PCHE indicated that fully-developed flow conditions were never achieved, which was attributed to the wavy nature of zigzag flow channels as well as the large temperature variations along the flow direction. This study concluded that heat transfer discrepancies between the hot side and cold side were caused by the differences in both thermal boundary conditions and thermophysical properties. Several effects on the PCHE’s thermal-hydraulic performance were also studied, including the fluid and solid thermophysical properties, radiuses of curvature at zigzag bends, channel configurations, channel pitch lengths in the fluid flow direction, and zigzag pitch angles. It was found that the mean pressure loss factors and mean Nusselt numbers in zigzag channels with sharp bends were 4–6% larger than those in zigzag channels with bends with a curvature radius of 4 mm. A multi-objective optimization for the PCHE’s geometry was conducted based on the numerically obtained correlations. A total number of 142 points on the Pareto front were obtained. Users can select the optimal geometrical parameters for the zigzag-channel PCHE designs from the obtained Pareto front based on their needs. In addition, the stress field of a simplified three-dimensional geometry was obtained. It was observed that the highest stress occurred at diffusion-bonded interfaces and that it was less than the maximum allowable stress of the structural material.
Furthermore, a computer code was developed to predict both the steady-state and transient behaviors of both straight-channel and zigzag-channel PCHEs. Comparisons of the numerical results with the experimental data indicated that the dynamic model was successful in predicting the experimental transient scenarios. The numerical results could also provide useful insight on control strategy development for an integrated high-temperature reactor system with process heat applications, for instance, using the helium mass flow rates or helium inlet temperatures variations to adjust the heat exchanger effectiveness and heat transfer rate.
Finally, the heat exchanger model was implemented into a system dynamic code to simulate the transient behavior of a 20-MWth Fluoride-salt-cooled High-temperature Reactor (FHR). Results were obtained for three initiating events: a positive reactivity insertion, a step increase of the helium flow rate, and a step increase of the helium inlet temperature to the secondary heat exchanger (SHX). The results demonstrated that the FHR reactor, for the three transient scenarios analyzed, had inherent safety features. The results also showed that the intermediate loop consisting of the IHX and SHX played a significant role in the transient progression of the integral system.
This study provides critical insights into the thermal-hydraulic performance of PCHEs that can be applied to nuclear power, offshore industry, solar power, dual cycles for process heat applications, and cooling of electronics and fuel cells.
... Atoms diffuse across the interface to form the joint between plates. As shown in Figure 2.10, the sequences of bonding is complete (Bartle, 1979). (1) Surface preparation: Surface preparation is of significance prior to diffusion bonding. ...
... 10. Schematic representation of diffusion bonding sequences(Bartle, 1979) Many diffusion bonding techniques are available for the joining of a plenty of configurations and materials. Diffusion bonding is a reliable joint process with minimum of distortion. ...
One of the very-high-temperature reactor (VHTR) missions is to produce electricity and/or to provide process heat for applications with high efficiency. The electricity generation or process heat applications of these advanced reactors greatly rely on an effective intermediate heat exchanger (IHX) that transfers heat from the primary fluid (i.e., helium) to the secondary fluid, which can be helium, molten salt, water/steam, or supercritical carbon dioxide. The IHX performance is directly related to the efficiency and safety of the overall nuclear system. A printed circuit heat exchanger (PCHE) is one of the leading IHX candidates due to its high effectiveness and compactness, as well as its robustness.
In the current study, a scaled-down PCHE was fabricated using Alloy 617 plates and Alloy 800H headers. The PCHE fabrication processes, i.e., photochemical etching, diffusion bonding and brazing, are described. This PCHE has eight hot and eight cold plates with 11 semicircular wavy (zigzag) channels in each plate with the following channel dimensions: 1.2 mm hydraulic diameter, 24.6 mm pitch in the flow (stream-wise) direction, 2.5 mm pitch in the span-wise direction, and 15º wavy pitch angle. The thermal-hydraulic performance of the PCHE is investigated experimentally in the high-temperature helium test facility (HTHF) at The Ohio State University. The PCHE inlet temperatures and pressures are varied up to 350 ºC/2 MPa for the cold side and 700 ºC/2 MPa for the hot side, respectively, while the maximum mass flow rate of helium reaches 30 kg/h. The corresponding maximum channel Reynolds number for both the hot and cold sides is about 3,000, including the laminar flow and laminar-to-turbulent transitional flow regimes. Comparisons between the obtained experimental data and available empirical correlations in the literature have been carried out. Both hot-side and cold-side friction characteristics of the PCHE with the wavy channels follow the trend established in the empirical model well, while large deviation is presented in the low Reynolds number region. Heat transfer characteristics obtained from model available in the literature show a discrepancy from the experimental results. Large deviation appears in the low Reynolds number region as well. A new heat transfer correlation based on experimental data has been subsequently proposed for the current wavy-channel PCHE.
Finally, transients that involve variations of the mass flow rate and temperature on the hot and cold sides of the scaled-down PCHE are investigated by numerical method. A dynamic model has been verified using a commercial software DYNSIM and validated using the experimental data. The model predicts the dynamic trends well and is available for use in the future.
Diffusion bonding Al2O3-TÍC ceramic matrix composites with Cr18-Ni8 austenitic stainless steel has been performed with a Ti/Cu/Ti multi-interlayer. The Al2O3-TiC/Cr18-Ni8 joints were characterized by means of scanning electron microscope (SEM) with energy-disperse spectrometry (EDS), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The results indicate that the Ala2O 3-TiC/Cr18-Ni8 diffusion-bonded joint with shear strength of 106 MPa was obtained by using the Ti/Cu/Ti multi-interlayer. An interfacial transition zone was formed by diffusion reaction between Ti, Cu and the elements from the substrates, Al2O3-TiC and Crl8-Ni8. TiO, TiC, α-Cu, CuTi, FeTi and Cr2Ti are produced near the interface of the Al 2O3-TiC/Crl8-Ni8 joint.
In the present investigation, the evolution of interface microstructure and mechanical properties of diffusion bonded joints of titanium to Type 304 stainless steel with a niobium interlayer were studied. The joints were processed in the temperature range of 800 to 1 000°C for 0.5 h in vacuum. The stainless steel/niobium interface was free from intermetallic phase up to 900°C; however, Fe 2Nb+Fe 7Nb 6 phase mixture was observed at and above a processing temperature of 950°C. The niobium/titanium interface was free from intermetallic compounds at all processing temperatures. A maximum tensile strength of 297MPa (̃93% of Ti) and shear strength of 217MPa (̃75% of Ti) along with a 6.9 % ductility were achieved, when processed at 900°C processing temperature. The failure of bonded samples took place through the stainless steel-niobium interface at all processing temperatures during loading.
Commercially pure Ti has been diffusion bonded using silver and copper interlayers and without any interlayer. The microstructure of the bonded zone is affected by the bonding temperature, bonding time and interlayer type. With a silver (Ag) interlayer at 980 °C for 10 h, the diffusion zone consisted of a solid solution of Ag at the center and a zone of AgTi intermetallics on both sides of it. As the temperature and time increased (1030 °C, 30 h), AgTi with small amount of dispersed AgTi2 formed at the center of the diffusion zone and next to it a eutectoid mixture of Ti solid solution and AgTi appeared. When Cu was used as an interlayer at 900 °C for 10 h, Cu–Ti solid solution, a zone of different intermetallics, and Ti–Cu solid solution formed in the bonded zone. However, at 1000 °C or higher temperatures, no continuous zone of intermetallics was found in the bonded region, only eutectic mixtures and Ti–Cu solid solutions appeared. The maximum tensile strengths achieved were 160 MPa, 502 MPa, and 382 MPa when Ag, Cu and no interlayers were used, respectively.
Nanostructured surface layer was synthesized on the end face of Tí-4Al-2V alloy and OCr18Ni9Ti austenite stainless steel rods by means of Surface self-nanocrystallization(SSNC). Making treated end surfaces as bonding interfaces, transition joint of Ti-4Al-2V alloy and OCr18Ni9Ti stainless steel bars was prepared by pluse pressuring diffusion bonding (PPDB) on Gleeble-1500D tester at 850°C for 80s, the maximum and minimum pluse pressuring were 8MPa and 50MPa respectively, and cycle (N) and frequency (f) of pulse load were 40 times and 0.5 Hz respectively. Bonded joints were tensed on CMT5105 style instron. Microstructure of transition joint was investigated by scanning electron microscope (SEM) and X-ray energy dispersive spectroscope (EDS). The reaction products on the fracture were detected using X-ray diffraction (XRD). Research results showed that the maximum tensile strength reached 384.0MPa, cleavage fracture took place while tension test of joints. Brittle intermetallic compounds such as Fe2Ti, FeTi and σ phase presented on the fracture, and on the titanium alloy side, α-Ti transformed into Β-Ti in the vicinity of interface while diffusion bonding.
Nanostructured surface layers were synthesized on the end face of Ti-4AI-2V titanium alloy and 0Cr18Ni9Ti austenite stainless steel rods by means of high energy shot peening (HESP). Making treated end surfaces as bonding interfaces, Ti-4AI-2V and 0Cr18Ni9Ti rods were bonded by pulse pressuring diffusion bonding (PPDB) on Gleeble-1500D tester at 650-750°C. Joints were tested on tensile testing machine, the fractures and microstructures of joints were researched. Results showed that the maximum tension strength of 262.0 MPa was achieved, cleavage fracture took place while tension test of joints, and the grains in the vicinity of the diffusion layer are fined. But, brittle intermetallic compounds were absence on the bonding interface.
In the present study, diffusion bonding of titanium and micro-duplex stainless steel was investigated in vacuum. Diffusion interfaces were characterized using light microscopy, scanning electron microscopy and X-ray diffraction technique. The inter-diffusion of the chemical species across the diffusion interfaces were evaluated by electron probe microanalysis (EPMA). The maximum tensile strength of ̃97% and shear strength of ̃80% of those of Ti along with 6.9% elongation were obtained for the diffusion couple processed at 900°C joining temperature. Fracture surface observation in SEM using EDS demonstrates that, failure takes place through ß-Ti phase when bonding was processed up to 850°C, however, failure takes place through α+FeTi+ß-Ti for the diffusion couples processed at 900°C and above bonding temperature.
Transient liquid phase diffusion bonding of Ti–22Al–25Nb (at.%) alloy with Ti–15Cu–15Ni (wt.%) was performed. The factors influencing the microstructure and strength of the joints were studied. A suitably long holding time and high bonding temperature would benefit the formation of joints with homogeneous compositions and high strength. Nb element is the mainly controlling one to the formation of composition-homogenous bonding zone. The bonding zone of joint with a rapid cooling technology after dwelling at bonding temperature is composed of B2 phase. Slow cooling technology is beneficial to improving the joint strength. The tensile strength at room temperature of the joint under the joining conditions of bonding temperature 970°C for 90min with a slow cooling technology reaches up to 1018MPa, which is equal to 93% of base material tensile strength, obviously higher than the joint strength of 931MPa with a rapid cooling technology.
Surface preparation is essential for the Hot Isostatic Pressing (HIP) diffusion bonding of RAFM steels. Hot Isostatic Pressing (HIP) diffusion bonding experiments on China Low Activation Martensitic (CLAM) steel was performed to study the effect of surface preparation. A few approaches such as hand lapping, dry-milling and grinding etc., were used to prepare the faying surfaces of the HIP joints. Different sealing techniques were used as well. The HIP parameters were 150MPa/3h/1150°C. After post HIP heat treatment (PHHT), the tensile and Charpy impact tests were carried out. The results showed that hand lapping was not suitable to prepare the faying surfaces of HIP diffusion bonding specimens although the surface roughness by hand lapping was very low.
An investigation was conducted to study: (1) the mechanism of liquid-phase pulse-impact diffusion welding (LPPIDW); and (2)
the influence of pulse-impact on the microstructure and tensile strength of LPPIDW-welded joints of the aluminum matrix composite
(AMC) SiCp/A356. The results showed that, during LPPIDW: (1) the interface state between the SiC particles and matrix was prominently
affected by the pulse-impact; (2) the initial pernicious contact-state of reinforcement particles was changed from reinforcement
(SiC)/reinforcement (SiC) to reinforcement (SiC)/matrix/reinforcement (SiC); (3) the harmful microstructure/brittle phase
of Al4C3 was restrained from the welded joint; (4) the density of dislocation in the matrix neighboring to and away from the interface
in the matrix was higher than its parent composite; and (5) the intensively mutual entwisting of dislocation was taking place.
Studies illustrated that: (1) deformation mainly occurred in the matrix grain; and (2) in the deformation of rapid thermal
pressing, the matrices around SiC particles engendered intensive aberration and offered a high-density nucleus area for matrix
crystals, which was in favor of forming nano-grains and improved the properties of the successfully welded composite joints.
Such distinctly welded composite joints gave: (1) a tensile strength of up to 179 MPa, which was about 74.6% of the stir-cast
SiCp/A356; and (2) a corresponding radial deformation of below 3%, which conformed well to the deformation specification of the
welded specimens.
A diffusion-bonding procedure at a low temperature, i.e. 500°C, based on the high mobility of silver atoms was developed with a newly designed plate-and-frame type hydrogen purification membrane module consisting of a unit cell and a housing. Two membranes made of palladium and copper sputtered on polished porous nickel supports (PNS) followed by Cu-reflow at 750°C, respectively, were assembled in a unit cell to verify that the low temperature diffusion-bonding method could be applied to gas-tight membranes. Ring-shaped silver foils with a thickness of 50μm were placed between the membranes and the unit cell body made of nickel plate. A pair of membranes, a pair of silver foils and the unit cell body were compressed with a pair of covers and eight screws by a 17cm long torque wrench at 12Nm. The diffusion-bonded unit cell was welded in a module housing comprised of a feed port and a retentate port by a laser-operated welder. After the module was constructed, gas-tightness tests were carried out using helium and the measured helium leakage was 8×10−5molm−2s−1 at 0.7MPa, which is the same as the value detected before diffusion bonding with a Viton O-ring. The hydrogen permeation test and durability test consisting of three cycles of alternately changing the temperature and transmembrane pressure difference were carried out using a single gas, hydrogen, and it was found that the hydrogen permeation flux remained constant during the durability test and that the helium leakage did not increase after the durability test.