Brazing Cycle Research Articles

Radio Frequency Properties of a 3D Printed Klystron Circuit

The manufacturing of active RF devices like klystrons is dominated by expensive and time-consuming cycles of machining and brazing. In this article, we characterize the RF properties of X-band klystron cavities and an integrated circuit manufactured with a novel additive manufacturing process. Parts are 3D printed in 316 L stainless steel with direct metal laser sintering, electroplated in copper, and brazed in one simple braze cycle. Stand-alone test cavities and integrated circuit cavities were measured throughout the manufacturing process. The un-tuned cavity frequency varies by less than 5% of the intended frequency, and Q factors reach above 1200. A tuning study was performed, and unoptimized tuning pins achieved a tuning range of 138 MHz without compromising Q. Klystron system performance was simulated with as-built cavity parameters and realistic tuning. Together, these results show promise that this process can be used to cheaply and quickly manufacture a new generation of highly integrated high power vacuum devices.

Ultrasonic-Assisted Brazing of Titanium Joints Using Modified Al-Si-Cu Based Fillers: Brazing at Liquid—Semisolid States under Load

Brazing joints of Ti/Ti under ultrasonic vibration (USV) and compression load were investigated using optimized and modified filler alloys of Al-Si-Cu-(Ni)-(Sr) group prepared in the lab. Preliminary trails at semisolid to liquid states were conducted using the ready Al-Si-Cu-(Mg) alloy as a filler, then the brazing cycle was redesigned and enhanced according to the microstructural observations of the produced joints. USV assisted brazing at the semisolid state of low solid fraction was able to produce joints with round silicon morphology and granular α−Al, while at a high solid fraction, USV was only able to affect the silicon and intermetallic particles. Applying a compression load after ultrasonic vibration, at a designed solid fraction, was proved to be a successful technique for improving the quality of the joints by reducing the porosity, enhancing the soundness of the joint, and the diffusion at the interface. Based on alloy composition and the improved brazing cycle, joints of thin intermetallic layer and high shear strength (of 93 MPa average value) were achieved. The microstructures and the mechanical behavior were discussed based on the filler compositions and brazing parameters.

Microstructural and mechanical characterization of self-passivating W-Eurofer joints processed by brazing technique

Self-passivating tungsten alloys have arisen as a potential candidate for the blanket first wall due to their high oxidation resistance and good thermal shock resistance. However, joining technologies have to provide a suitable process to conform the component. In this investigation, brazing technique was applied to join a self-passivating tungsten alloy to Eurofer using 50 μm thick Cu interlayer. In addition, the use of post-brazing thermal treatment (PBTT) was also investigated to recover the as-received hardness of steel after the brazing cycle. The results showed the achievement of high brazeability joints in terms of metallic continuity and strength. With the PBTT the initial hardness of Eurofer was recovered but it gave rise to lower shear strength of the joint due to the softening of copper. The high strength and the fracture mechanism observed in some joints, propagating the fracture through the base material instead of the joint interfaces, showed the high adhesion properties achieved in both studied conditions.

Diffusion braze homogenisation and contraction during re-repair heat treatments of a single crystal nickel-based superalloy

In this work, nickel-based braze reservoirs embedded in single crystal nickel-based superalloy substrates were cycled between one and four braze diffusion heat treatments to analyse the effect on microstructural transformation of the braze and substrate materials. Nickel-based superalloy turbine components can undergo several braze repair cycles during service life, and little has been reported on how previously repaired braze joints are affected in additional heat treatments. 1 mm wide braze joints with an initial composition of 6.8 at.% boron embedded in a single crystal substrate do not homogenise when brazed for 2.5 h above 1150 °C, resulting in the formation of eutectic boride phases. Subsequent heat treatments caused braze remelting and contraction leading to surface depressions with the potential to act as a stress raiser for crack nucleation, possibly requiring reworking which impacts on both profits and turnaround times. After four braze diffusion heat treatments, the braze had still not fully homogenised, even though four discrete bands of boride precipitates, separated by boride-free zones, could be observed in γ channels, indicating boron removal from the braze reservoir. Analysis of the metallurgy at the braze-substrate interface and the diffusion-affected zones showed that the most probable reason for controlling boron removal from the braze were: 1) decreasing boron concentration gradient across a thickening braze-substrate interface; 2) restricted boron transport across boride precipitates in the diffusion-affected zones; 3) slower transport of boron in γ’, the area fraction of which changes over the braze cycle temperature range.

High Temperature Tensile Behavior of a Nickel-Based Superalloy 55Ni-17Cr-12Fe-9Mo-2Nb-1.5Al Used in Launch Vehicle Applications

55Ni-17Cr-12Fe-9Mo-2Nb-1.5Al alloy is a nickel-based superalloy (Russian designation is XH55MбЮ or KhN55MBYu, XH55) without any equivalent in American/European alloy designation. It is used in cryogenic engine of satellite launch vehicles application in two different heat-treated conditions: (1) standard aged (STA) at 730 °C/15 h + 650 °C/10 h and (2) STA + BC (brazing cycle) treatment carried out in vacuum at 1030 °C with holding time of 30 min. Due to the braze cycle adopted for manufacturing, it is essential to study the deterioration in mechanical properties, if any. Hence, the present work is carried out to understand the material behavior in tensile mode (25, 425, 575, 700 and 900 °C) for XH55 alloy in STA condition and STA + BC conditions, compared with corresponding microstructural analysis, morphology and composition using microscopy at various length scales. The tensile stress–strain curve shows characteristic sudden drops in stress with respect to strain, attributed to dynamic strain aging at different temperatures for both STA and STA + BC conditions. In STA condition, the yield strength of the material decreased with increase in temperature. In STA + BC condition, the yield strength decreased up to 425 °C, increased up to 700 °C as the material was subjected to artificial aging during testing and finally decreased at 900 °C. Marginal deterioration in mechanical properties have been observed due to the braze cycle adopted against STA condition.

Vacuum Brazing Ti–15–3 with a TiNiNb Braze Alloy

Among all types of brazing fillers, Ti-based fillers show satisfactory joint strengths in brazing titanium alloys. However, the major concern in using such fillers is the formation of Cu/Ni/Ti intermetallic compound(s) in the joint. In this study, a Ti–15–3 alloy was vacuum brazed with a clad Ti–35Ni–25Nb foil. The brazed zone consisted of a Ti2Ni intermetallic compound in a (β-Ti,Nb)-rich matrix for specimen brazing at 1000 °C/600 s. Raising brazing temperature and time resulted in the Ti2Ni dissolving into the (β-Ti,Nb)-rich matrix. For the specimen brazing at 1100 °C/600s, Ti2Ni could only be observed at the grain boundaries of the (β-Ti,Nb)-rich matrix. After further raising it to 1200 °C/600 s, the Ti2Ni intermetallic compound was all dissolved into the (β-Ti,Nb)-rich phase. The average shear strength was significantly raised from 140 (1000 °C/600 s) to 620 MPa (1100 °C/3600 s). Crack initiation/propagation in the brittle Ti2Ni compound with the cleavage fractograph were changed into the Ti–15–3 base metal with a ductile dimple fractograph. The advantage of using Nb in the TiNiNb filler foil was its ability to stabilize β-Ti, and most of the Ni in the braze alloy was dissolved into the β-Ti matrix. The brazed joint could be free of any intermetallic phases with a proper brazing cycle applied, and the joint was suitable for a few harsh applications, e.g., repeated stresses and impact loadings.

The development of technology of Be/CuCrZr joining using induction brazing

The article describes the method of vacuum induction brazing of beryllium tiles covering the surface of the first wall of ITER. To prevent the formation of brittle intermetallic and preservation of the mechanical properties of the CuCrZr alloy, it leads to the need to minimize the brazing temperature and time of the brazing cycle. Designed according to the results of calculations and optimized in experiments, the inductor allows heating locally the beryllium tile/CuCrZr alloy region with required heating rate. With the aim to minimize brazing temperature this paper presents the result of selection of the optimal eutectic of STEMET 1101 brazing alloy thereby reducing the temperature of the brazing to 680 °C. To fix each of the beryllium tiles during the brazing cycle, a unique clamping tool was developed and applied. As a confirmation of the reliability of this method, along with the results of ultrasonic testing of brazed joints, the paper presents the successful results of high heat flux testing of the mock-ups throughout the full life-time of the ITER first wall panels (15,000 heat cycles).

Characterization of SiC Ceramic Joints Brazed Using Au⁻Ni⁻Pd⁻Ti High-Temperature Filler Alloy.

In this work, (Au79Ni17Pd4)96Ti4 (wt.%) filler alloy was designed and employed to join SiC ceramics. The effects of brazing temperature and soaking time on the microstructure and fracture morphology of joints were investigated. The results show that the joint obtained can be described as SiC/reaction layer/braze/reaction layer/SiC. The reaction layer was composed of TiC and Au (Si, Ti). The wettability of the filler alloy toward the SiC ceramics was analyzed. The braze zone was mainly constituted by Pd2Si, Ni2Si, and Au (Ni, Si). A large number of nano-sized TiC particles were distributed within the Au (Ni, Si) layer. The formation mechanism of the braze containing different phases was discussed. The brazing temperature and soaking time had a significant effect on the reaction layer at the SiC/braze interface and TiC particles within the Au (Ni, Si) layer, while they showed a negligible effect on the Pd2Si and Ni2Si within the braze. The inherent reason was also clarified in detail. The joint fractography indicated that a good bonding was achieved between the filler alloy and SiC, while joint fracture was primarily induced by the thermal stresses residing after the brazing cycle.

The effect of heat treatment sequence on microstructure and mechanical properties of diffusion brazed IN-939 superalloy

Conducting the diffusion brazing cycle at 1120 °C for 45 min led to the completion of isothermal solidification and formation of a single-phase Ni-rich γ-solid solution at the joint centerline. Chromium-rich boride precipitates with blocky and acicular morphologies were detected in the diffusion affected zone (DAZ) of all the bonds. The post bond heat treatment of the brazed joint (HT-3 sample) resulted in complete elimination of DAZ boride precipitates and obtaining high levels of homogeneity across the joint region. Performing the brazing cycle between the solution treatment and the double aging cycle (HT-1 sample), as well as after the full heat treatment of the base metal (HT-2 sample), had a minor effect on the morphology of the joint region. While all the heat treatment sequences raised the microhardness values across the bonding regions, the HT-2 sequence reduced the microhardness levels in the isothermally solidified zone (ISZ). The HT-3 sequence improved the joint efficiency up to the remarkable value of 91%. In the as-brazed sample, the fracture path was observed in the ISZ and a dimple-like morphology was detected in the fracture surface. In HT-1 sample, the fracture occurred within the hard DAZ precipitates and a semi-cleavage morphology was detected in the fracture surface.

Sagging Mechanisms of Three-Layer Aluminum Clad Foils used in the Heat Exchanger in the Brazing Process

The sagging mechanisms of a three-layer aluminum clad foils (AA4343/AA3003Mod/AA4343) with final rolling reduction of 20-50% during brazing were studied. The results show that the sagging behavior of the aluminum brazed foils during the brazing cycle can be divided into four stages: Stage I is the slight sagging stage, during which sagging deformation can almost be ignored; Stage II is referred to as the accelerative sagging stage, and the sagging behavior is mainly ascribed to recovery. In stage II, the greater the reduction ratio is, the earlier the recovery ends, and the smaller the value of the sagging distance is. Stage III is the slow sagging stage, and sliding deformation along the grain boundaries is responsible for the slow sagging rate. Stage IV is the erosion stage. During stage IV, the coarsely recrystallized grains can inhibit erosion and are helpful in obtaining excellent sagging resistance, while the foils with finely recrystallized grains or non-recrystallized structure show severe erosion and limited sagging resistance.

Development of W-composites/EUROFER brazed joints for the first wall component of future fusion reactors

The present work describes a joining procedure between two different tungsten composite materials (W–2Y2O3 and W–1TiC) with reduced activation ferritic–martensitic steel (Eurofer). The results indicated the achievement, in both cases, of high quality W-composites/Eurofer joints using 80Cu–20Ti as filler material. The braze is constituted by several ternary Cu–Ti–Fe phases distributed along a Cu-matrix, which acts as ductile phase capable of relieving the residual stresses, which could be produced during the service life of the component. Some cracks growing from W–braze interface into the base material have been detected. They are originated by the stresses produced during the cooling stage of the brazing cycle. Regarding the strength of the joints, similar shear strengths of both joints were obtained (∼105 MPa). These values were slightly lower than the ones obtained when pure tungsten was used as the base metal.

Influence of Exposure to Multiple Brazing Cycles on the Integrity of OFE Copper Brazed Joints

A 10 MeV, 5 kW S-band travelling wave electron linear accelerator has been developed at Raja Ramanna Centre for Advanced Technology for industrial applications. The electron accelerating structure is made of oxygen-free electronic (OFE) copper and comprises of fifty-six brazed joints and is fabricated in multiple stages, as per the process and testing requirements. The accelerating structure is often required to be repaired through an additional brazing cycle with the same filler. The present study evaluates possible microstructural and mechanical property degradation of OFE copper brazed joints, as a result of its exposure to multiple brazing cycles. The study was performed on an OFE copper cavity stack brazed assembly, made through a four-stage brazing using BVAg-8 as braze filler. The resultant brazed assembly did not exhibit any leak at a sensitivity of 1 × 10−10 mbar.L/s. The results of the study demonstrate that OFE copper brazements can be safely subjected to an additional brazing cycle without causing significant degradation in its microstructure and strength. However, brazed joint’s exposure to any additional brazing cycles results in the formation of voids and cracks due to intergranular penetration of silver, with associated drop in tensile strength from about 210 MPa to about 150-180 MPa. Based on the results of study, it is recommended to qualify the accelerating structure, fabricated through multiple brazing cycles, by a suitable non-destructive technique. The results of the study are important for design, fabrication and maintenance of electron linear accelerator components.

Microstructural evolution of Si3N4/Ti6Al4V joints brazed with nano-Si3N4 reinforced AgCuTi composite filler

A novel nano-Si3N4 particle reinforced AgCuTi composite filler (AgCuTiC) was used to braze Ti6Al4V and Si3N4 ceramic in the study and brazing cycles that peak at 880 °C for 0–20 min. Effects of holding time on interfacial microstructure and mechanical property were studied by scanning electron microscopy, energy dispersive spectrometer, transmission electron microscopy, and universal testing machine. TiN and Ti5Si3 were identified as the main product of the reaction between Si3N4 and Ti. The interfacial microstructure evolved considerably with joining time, eventually leading to a high degree of inhomogeneity across the length of the joint, and the maximal shear strength of 78.3 MPa was obtained when the joint was brazed at 880 °C for 10 min. A limited number of Si3N4/Si3N4 and Ti6Al4V/Ti6Al4V joints using AgCuTiC with different content of Ti particles were also studied to clarify the influences of diffusion and dissolution behaviors of element Ti on interfacial microstructure. In terms of characterizing the interfacial phases, efforts were made to understand the microstructural evolution mechanism of Si3N4/Ti6Al4V brazed joints.

Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers

This paper studies the improvements of 80Cu-20Ti alloyed powders as filler material for W-Eurofer joints of the next generation of fusion power plants. The advantages derived from their alloyed composition allow shortening the heating stage up to 3 times with respect to pure powders. Besides, the better flowing capabilities and lower melting range make possible to work with lower brazing temperature. Operational brazeability of W-Eurofer joints using a mixture of 80Cu-20Ti alloyed powders has been achieved for brazing temperatures of 960°C and 940°C, holding the temperature during 1–5 and 10min, respectively. Hardening and softening processes of the Eurofer occurs during the brazing cycle, but the as received hardness conditions could be recovered by the application of a post-weld annealing treatment. The highest shear strength was obtained for the sample fabricated at 960°C for 5min.

Study of a novel brazing process for W-W joints in fusion applications

Next generation of fusion power plants needs to develop joining technologies for several components of the reactor. In this paper, a novel brazing process for joining two pieces of divertor component made of tungsten is proposed. A flexible tape, fabricated of a mixture of 86Fe-14Ti metallic powders and an organic binder, has been used as filler material. Different binder contents (from 5 up to 30wt.%), which provides the required flexibility to the filler tape for its fitting to the divertor curved surface, have been evaluated. The proposed flexible filler tape will improve the brazing procedure, but also it could be implemented for any curved component of the power generation industry. Two fillers contents (95/5 and 90/10 powder/binder ratios) have been used for brazing tests that were carried out in a high vacuum furnace at 1350°C for 10min. W-W brazed joints exhibited high continuity along the joint and no wetting problems were detected. The microhardness tests gave rise to similar values and revealed that the mechanical properties of tungsten base material were not modified during the brazing cycle. The shear strength of the joints (~44MPa) meets the mechanical requirements of the component.

The effect of single crystal and welded substrates on the development of braze microstructures

To emulate combined welding and brazing techniques, frequently used during the rejuvenation of turbine components, TIG and SWET weld overlays have been applied to the surfaces of a single crystal nickel-based alloy and subsequently overlain with braze and subjected to a typical brazing cycle heat treatment. Both welds comprised of columnar dendritic microstructures with an epitaxial [001] growth orientation near the substrate surface and equiaxed randomly oriented grains at the weld surface. While the TIG weld showed a sharp columnar to equiaxed transition, that for the SWET weld was diffuse and comprised colonies of columnar structures with their [001] misoriented to the epitaxial columnar directions as a result of shallower temperature gradients. Analysis of the microstructures of the brazed materials showed that the extents of boron penetration were significantly greater for the welds due to enhanced diffusion along grain boundaries between the equiaxed structures compared to γ channel limited transport in the single crystal. A striking feature of the diffusion zone microstructures was the development of carbo-boride needles which formed by the reaction of boron, refractory metals and TaC. It is the release of Ta from TaC which stabilises γ′ envelope formation around the needles. The post-brazed microstructures of TIG welds used in this study showed considerably fewer detrimental boride precipitates than brazed SWET welds and as a result are expected to show superior mechanical properties.

Investigation of the brazing characteristics of a new iron-based brazing filler metal

High temperature applications of new class of iron-based filler metals provide brazements with high corrosion resistance and mechanical properties. These brazements are cost-effective alternative to those made of the conventional brazing alloys. However, a wiser usage demands a deeper understanding of the wetting as well as gap filling behavior in conjunction with the resulting microstructure, which is mainly influenced by the applied brazing cycles. Therefore, this paper presents results of the investigation of specific brazing fundamentals for the new iron-based brazing alloy Fe-24Cr-20Ni-10Cu-7P-5Mn-5Si. Followed by DTA/DSC measurements, the spreading and gap filling behavior were examined by using stainless steel AISI 304 as base material. In wetting tests and wedge-gap experiments, the influence of the applied brazing temperature and the dwell time were investigated for vacuum brazing processes. The resulting microstructure was evaluated using a scanning electron microscope (SEM), equipped with an energy dispersive X-ray spectroscopy (EDS). Finally, strength tests were conducted in order to determine the influence of the brazing parameters on the mechanical properties of the joint.

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.

Thermodynamic evaluation of oxidation during brazing process of medium-voltage electrical circuit breakers

The present study is a thermodynamic analysis of the different molecular flows occurring during the vacuum brazing of electrical vacuum bottles also called Vacuum In-terrupters (mentioned later as VI). Among the available impurities either coming from the Vacuum Interrupters components materials or from vacuum technology, the analysis is fo-cused on the oxygen since this impurity leads to formation of the more stable compounds, i.e. oxides. During the brazing cycles the resistors of the vacuum furnace and the VI materials including some braze components are the main sources of vapours that partly escape or make deposits on colder parts of the furnace. Vaporization and condensation processes are evaluated, their matter flows quantified and finally their interaction with residual oxygen calculated through a balance between input oxygen from vacuum or neutral gas and the different sources of oxygen coming from the parts under treatment. The enrichment of the VI materials and deposits with oxygen is evaluated as well as the limits required for vacuum conditions. The main conclusion is that present vacuum conditions allow correct working of the brazing sequence but in any other temperature steps or ramps, oxygen is trapped as oxides at the surface of the parts or accumulated in the bulk materials of the furnace.

Effect of Brazing and Heat Treatment Cycle on the Mechanical Properties of Base Materials

Brazing is extensively used in liquid rocket engines for realizing various subsystems. In the case of cryogenic engines, brazing operation is done to realize the gas generator. Gas Generator is one of the major systems of cryogenic engine. It generates and supplies hot gases required for running turbine of main turbo pump. This uses liquid oxygen and gaseous hydrogen as propellant combination. Combustion chamber of Gas Generator is of double walled construction with the cylindrical outer shell of transition class ICSS-0716-301 austenitic-martensitic stainless steel and inner shell of ICSS-1218 321, aTi stabilized austenitic stainless steel material brazed together with Fe-Ni-Mn type braze alloy at a temperature of 1180°C. This temperature can cause the grain growth and related issues to the base material. Thus the present work focuses on the effect of the brazing/thermal cycle on mechanical properties and microstructure of the base materials in post braze condition. The results obtained on metallurgical/mechanical behavior of the material showed the different grain growth patterns in inner and outer shell materials. This helped in understanding the effect of brazing condition on the changes in mechanical properties of base materials.