In-Situ synthesized TiC nano-flakes reinforced C/C composite-Nb brazed joint

Brazing C/C composite to Nb is often associated with the problem of high residual stress, resulting in low-strength joints. To overcome these problems, here we carried out a simple polymer carbonization process to acquire uniform carbon-covered Cu foam composite interlayer, which was subsequently used for soundly brazing C/C composite and Nb with the assembly of C/C composite/Ag-Cu-Ti foil/C-Cu foam/Ag-Cu-Ti foil/Nb. Microstructure and mechanical properties of the joints were well investigated. The carbonization reacted with Ti elements, forming uniformly distributed in-situ TiC nano-flakes in the joint seam by virtue of the porous Cu foam skeleton. Results present that the in-situ TiC nano-flakes not only greatly reduced the thermal expansion coefficient but also effectively impeded the Cu solid solutions agglomeration. The average shear strength of the joint brazed with 3% C-Cu (wt.%) foam interlayer reached ～52.8 MPa with the brazing temperature of 880 °C for 10 min.     

Microstructure and mechanical properties of the C/C composite and Nb joints brazed with an active Ag-based filler metal

An active Ag-based filler metal, containing trace alloy elements of Al, Cu and Ti, was successfully applied to braze C/C composite and Nb. The microstructure and formation mechanism of the C/C composite and Nb brazed joint were investigated in this study. Moreover, the influence of brazing parameters on microstructural evolution and mechanical properties of brazed joints was evaluated. The typical interfacial microstructure of the joint obtained at 950 °C for 600 s was C/C/TiC + AlCu₂Ti/Ag(s, s) + AlCu₂Ti + particle Cu/AlCu₂Ti + AlCuTi + (Ti, Nb)₃Al + Nb(Ti)/Nb. The dispersive AlCu₂Ti phase was uniformly distributed in the Ag matrix, which was a beneficial structure for the brazed joint. The shear strength of the brazed joint was sensitive to the brazing temperature and holding time, which was closely related to the TiC layer bordering the C/C composite. The thickness of the TiC layer first increased as temperature increased to 950 °C, and then decreased when temperature reached 970 °C. The carbon fiber eroded by the filler at 970 °C entered to the brazing seam and reacted with Ti, resulting the reduction of the thickness of TiC, thus damaging the strength of the joint. With extension of holding time from 300 s to 1200 s, the interface reaction became more sufficient. Therefore, the thickness of the TiC layer increased. However, at 1200 s, the over-thick TiC broke the C/C substrate because of the mismatch of coefficient of thermal expansions. The maximum shear strength of the joint (950 °C/600 s) reached 55 MPa at room temperature and 35 MPa tested at 550 °C.     

Microstructures and mechanical properties of Cf/LAS composite and Ti60 alloy joints brazed with TiZrNiCu + Cf mixed powders

In this paper, carbon fiber reinforced lithium aluminosilicate (LAS) glass-ceramics matrix composites (C_f/LAS composites) are joined to Ti60 alloy using TiZrNiCu + C_f mixed powders by proper process parameters. The carbon fibers distribute uniformly in the brazing interlayer and react with Ti, Zr elements in the brazing alloy to form (Ti, Zr)C thin reactive layers, which are between the carbon fibers and the Ti, Zr elements. The effect of C_f content on the mechanical properties and microstructure of brazed joints are investigated. The microstructure of brazed joints varied obviously with the increasing of C_f content. The thickness of reactive layer between interlayer and C_f/LAS composites and Ti solid solution (Ti (s.s)) decrease gradually, and the volume of eutectic structure (Ti(s,s) + (Ti,Zr)₂(Ni,Cu)) decrease gradually. The obtained brazed joints exhibit a maximum shear strength of 73.5 MPa at room temperature using TiZrNiCu + 0.3 wt% C_f mixed powders. The enhanced shear strength can be attributed to the reduction in thermal stress and the reinforcing effect originated from the carbon fiber addition.     

Effect of Ni foam on the microstructure and properties of AlN ceramic/Cu brazed joint

Aluminum nitride (AlN) ceramics and oxygen-free Cu were brazed with multilayer filler consisted of Ag-Cu-Ti +Ni foam. The microstructure and forming principle of AlN/Cu joints were studied and the influence of Ni foam on the joints was focused. The result shows that the composition of AlN/Cu joint was AlN/TiN/Ni₃Ti+Cu(s,s)+CuTi+Ni foam+Ag(s,s)/Cu. The joint strength was only 66.7 ± 3.7MPa with pure Ag-Cu-Ti solder and the fracture occurred inside AlN ceramics due to the residual stress. The foam nickel reacted with Ag-Cu-Ti filler metal to form Ni₃Ti during brazing process. Ni foam still retained the basic skeleton structure during brazing, and the mechanical capacity of AlN/Cu joint was enhanced significantly. The maximum shear strength of the brazed joint can reach 89.6 ± 4.5 MPa with .1 mm Ni foam, and the fracture position changed to the brazing filler. The result shows that nickel foam can reduce the residual thermal stress, and the mechanical properties of AlN/Cu joints were improved.     

Effects of Cu interlayers on the microstructure and mechanical properties of Al2O3/AgCuTi/Kovar brazed joints

Al₂O₃ ceramic and Kovar alloy brazed joints were achieved using three types of Ag-based interlayers: a AgCuTi foil, a AgCuTi/Cu foil/AgCuTi multi-interlayer and a AgCuTi/Cu foam/AgCuTi multi-interlayer. The effects of the addition of Cu interlayers on the interfacial microstructure and mechanical properties of Al₂O₃/AgCuTi/Kovar brazed joints were investigated. When Kovar alloy and Al₂O₃ ceramic were brazed with 50 μm Cu foil at 900°C for 10 minutes, the Cu foil was completely dissolved in the liquid filler. A nearly continuous Cu layer remained in the joint when the thickness of the Cu foil reached 100 μm under the same brazing conditions. With the increase in Cu foil thickness, the thickness of Ti–O compounds + Ti₃Cu₃O reaction layer formed nearby the Al₂O₃ ceramic first increased and then remained the same. The Al₂O₃/Kovar joints brazed with 100 μm Cu foil at 900°C for 10 minutes showed a maximum shear strength of 138 MPa. A low brazing temperature was beneficial to maintain the original structure of the Cu foam. Furthermore, when the joints were brazed at 880°C for 10 minutes, the average shear strength of the Al₂O₃/AgCuTi/Cu foam/AgCuTi/Kovar joints was 140 MPa, which was 30 MPa higher than that of a single AgCuTi interlayer.     

Control interfacial microstructure and improve mechanical properties of TC4-SiO2f/SiO2 joint by AgCuTi with Cu foam as interlayer

Brazing SiO_2f/SiO₂ ceramics to TC4 is often associated with the problems of excessive Ti from the dissolution of TC4 and high residual stress, which results in low-strength joints. To overcome these problems, here we put forward an effective method by introducing Cu foam as interlayer to obtain high-strength joints of SiO_2f/SiO₂-TC4. The effect of Cu foam on the microstructure and mechanical properties of brazed joints was investigated. Cu foam can consume Ti from TC4 and inhibit forming too many brittle compounds at the SiO_2f/SiO₂ side. Furthermore, Cu foam can react with Ti, forming the dispersed homogeneous distribution of fine-grained Ti-Cu compounds in the brazing seam, due to its unique 3D porous structure. The formation and distribution of fine-grained Ti-Cu compounds at the brazing seam could significantly help to reduce the residual stress and reinforce the mechanical properties of the joint. Maximum shear strength of 59.6 MPa is approached.     

Carbon nanotubes-reinforced Ni foam interlayer for brazing SiO2-BN with Ti6Al4V alloy using TiZrNiCu brazing alloy

Brazing SiO₂-BN with Ti6Al4V is often associated with the problems of high residual stress and excessive Ti-based compounds formed. To overcome these problems, we report a new type interlayer of carbon nanotube (CNT) reinforced Ni foam fabricated by plasma enhanced chemical vapor deposition. The in-situ grown CNT are homogenously dispersed on 3D structure Ni foam, which could effectively avoid damage and agglomeration in the brazing seam. Result shows that Ni foam could consume excessive dissolved Ti, and CNT was beneficial for restricting the growth of phase, improving joining strength and releasing residual stress rapidly. The average shear strength of the joint brazed with CNT-Ni foam is about 50 MPa, and this value is about 5 times higher than that of joints brazed with pure TiZrNiCu. Further, a new simulated experiment was carried out innovatively to solving the difficulty of investigating interfacial behavior of CNT in brazing seam. Then the results clarify that integral structure of CNT can prevent the reaction with Ti. In situ growth of CNT on Ni foam could provide a way for introducing CNT into brazing seam without damaging structure of CNT.     

Vacuum brazing Nb and BN-SiO2 ceramic using a composite interlayer with network reinforcement architecture

A novel composite interlayer with a reinforced network was designed using a SiC ceramic with a network structure and Ti-Ni-Nb composite filler foils, to which the Nb and BN-SiO₂ ceramic were successfully brazed under vacuum. For a brazing temperature of 1160 °C and holding time of 10 min, the interfacial microstructure of the Nb/BN-SiO₂ ceramic joint was Nb/(βTi,Nb)-TiNi eutectic structure+(βTi,Nb)₂Ni+SiC+TiC/TiN+Ti₂N+TiB+Ti₅Si₃+TiO/BN-SiO₂ ceramic. In addition, the shear strength and nano-hardness were analyzed to evaluate the effect of the composite interlayer with a network reinforcement architecture on the mechanical properties of the joint. During brazing, the Ti-Ni-Nb filler metal infiltrated and reacted with the SiC to form the network reinforcement architecture, resulting in the residual stress being relieved and the mechanical performance of the joint being significantly improved. A maximum shear strength of 102 MPa was achieved, which was 60 MPa (142%) higher than that of the joint brazed without the network reinforcement architecture. A reduction in the residual stress on the BN-SiO₂ ceramic side from 328 MPa to 210 MPa was observed with the network reinforcement architecture, and the fracture path of the joint changed from the surface of the BN-SiO₂ ceramic to the interfacial reaction zone.     

Low residual stress C/C composite-titanium alloy joints brazed by foam interlayer

Metallic foam was introduced as an interlayer to improve the performance of the brazed C/C composite-titanium alloy joint, and the interfacial microstructure and residual stress of the brazed joint were investigated. Compared with the brazed joint without foam, introducing foam interlayer could achieve the uniform bonding interface, and Ag-based solid solution (Ag(s,s)) became more dispersed and smaller in the center of the brazing seam. The thickness of reaction layer close to C/C composite side was less than 1 μm. Some Cu-based solid solution (Cu(s,s)) was detected, indicating that Cu foam still existed after brazing. The residual stress and its distribution calculated by finite element method (FEM), and the residual stress of the brazed joint decreased from 293 MPa to 228 MPa. The introduction of the foam interlayer could obtain homogeneous microstructure, change stress distribution, and improve mechanical properties of the brazed joints.     

Strength optimization of two-step-bonded Ti-6Al-4V/Si3N4 joint with Nb interlayer via transient-liquid-phase bonding and active-metal brazing

A novel two-step bonding of Ti-6Al-4V/Si₃N₄ joint was developed with Nb interlayer as residual-stress reliever via low-pressure transient-liquid-phase bonding (TLPB) of Ti-6Al-4V/Nb side prior to active-metal brazing of Nb/Si₃N₄ side. While 1.75 mass% of Ti in a 50-µm-thick CUSIL-ABA® filler was sufficient for sound bonding at Nb/Si₃N₄ side when brazed at 1103 K for 10 min, one-step-brazed joints with bonding area of 10 × 10 mm² were prone to failure at the Ti-6Al-4V/Nb side due to brittle Cu-Ti intermetallic compounds (IMCs). Replacing brazing of Ti-6Al-4V/Nb side with TLPB using pure Cu and Ni foils as filler at 1213 K for 180 min eliminated the formation of brittle IMCs via homogenization of (α + β)-Ti; bending strength increased to 193 MPa with residual-stress-induced failure from Si₃N₄ ceramics. Finally, effectiveness of stress-accommodation via Nb interlayer and filler’s plastic flow was quantitatively verified with reasonable fidelity by finite-element analysis incorporating temperature-dependent elasto-plastic properties.     

Joining of Cf/SiC composite to Ti-6Al-4V with (Ti-Zr-Cu-Ni)+Ti filler based on in-situ alloying concept

In this paper, a novel brazing process based on in-situ alloying concept was carried out to join C_f/SiC composite to TC4 alloy at 940 °C for 20 min. Mixed powders of Ti-Zr-Cu-Ni alloy and pure Ti metal were used as interlayer. In the process, Ti-Zr-Cu-Ni alloy melted and then dissolved pure Ti metal via liquid-solid reactions, achieving in-situ alloying of the interlayer. The interfacial microstructure and formation mechanism of the brazed joints were investigated. The effect of Ti powder content on the microstructure and the mechanical properties of joints were analyzed. The results showed that: the maximum lap-shear strength of the in-situ alloying brazed joints was 283±11 MPa when using (Ti-Zr-Cu-Ni)+40 vol% Ti composite filler, and this value was 79% higher than the mechanical strength when using Ti-Zr-Cu-Ni alone. A reaction layer of (Ti,Zr)C+Ti₅Si₃ formed near C_f/SiC composite side, while a diffusion layer of Ti₂Cu+Ti(s,s) formed near Ti-6Al-4V side. In the interlayer, lots of Ti(s,s) were distributed uniformly and few of Ti-Cu compounds were found, contributing to the plasticity of joints. Adding moderate Ti powder was beneficial for improving the interfacial reaction between C_f/SiC composite and filler material, which affected the lap-shear strength of joints.     

Microstructure and mechanical properties of ZrB2-SiC/Nb joints brazed with CoFeNiCrCuTix high-entropy alloy filler

ZrB₂-SiC ceramics and Nb alloy were brazed at 1160°C for 60 min with CoFeNiCrCuTi_x high-entropy alloy filler. The influence of Ti content on the interface structure and mechanical properties of ZrB₂-SiC/Nb joint was systematically studied. It is found that the rich-Ti Laves phase was formed due to the addition of large atomic size Ti fill into the filler alloy or brazing joint, and its content increases with Ti content. The joint brazed by high-entropy alloys filler without Ti can be divided into a tooth-shaped Cr₂B reaction layer and a central area composed of a eutectic mixed structure of FCC phase and rich-Nb lamellar Laves phase. Ti and Nb are mutual solid solution elements. The increase of Ti content in the joint makes the FCC phase and the rich-Nb lamellar Laves phase to transform into a big bulk Ti-rich Laves phase and the quadrilateral (Ti, Nb)B phase. The tooth-shaped Cr₂B was disappeared. The residual stress generated in the joint during the brazing process tends to cause defects such as holes and microcracks in the bulk Ti-rich brittle Laves phase. Therefore, with the addition of Ti, the normal temperature performance of the joint decreases from 216 MPa to 52 MPa. However, with the increase of Ti, the high-temperature mechanical properties of the joint first decrease, and then increase. It was mainly due to the formation of rich-Ti Laves phase and quadrilateral (Ti, Nb)B with excellent high-temperature mechanical properties. When brazing with CoFeNiCrCuTi_1.5 filler, the high-temperature performance of the joint reached 92% of its room temperature performance.     

Microstructure and mechanical property of SiC whisker coating modified C/C composite/Ti2AlNb alloy dissimilar joints prepared by transient liquid phase diffusion bonding

SiC whisker coating was prepared on the surface of C/C composite successfully by CVD, and transient liquid phase (TLP) diffusion bonding was employed to realize the joining of SiC whisker coating modified C/C composite and Ti₂AlNb alloy using Ti–Ni–Nb foils as interlayer. The microstructure, shear strength and fracture behavior were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD) and universal testing machine. The results show that SiC has good compatibility with C/C composite, and gradient interface formed between SiC-modified C/C composite and Ti₂AlNb alloy. When the bonding experiment was carried out under bonding temperature of 1040 °C and holding time of 30min with 5 MPa pressure in vacuum, the joints formed well and no obvious defects can be observed. The typical microstructure of joints is C/C composite/SiC + TiC/Ti–Ni compounds + Ti–Ni–Nb solid solutions/residual Nb/diffusion reaction layer/Ti₂AlNb alloy. With the increasing of bonding temperature, the thickness of joining area increased due to sufficient element diffusion. However, when bonding temperature is elevated to 1060 °C, some defects such as cracks and slag inclusions exist in the interface layer between interlayer and Ti₂AlNb. The joints with maximum average shear strength of 32.06 MPa are bonded at 1040 °C for 30min. C, SiC and TiC can be found on the fracture surface of joints bonded at 1040 °C which indicated that fracture occurred at the interface layer adjacent SiC layer.     

Diffusion bonding of Ti-coated Cf-SiCf/SiBCN composites to Nb using Ag–Pd interlayer

The Ti-coated C_f-SiC_f/SiBCN ceramic was diffusion-bonded to Nb using an Ag–Pd interlayer. At a ceramic/Ag–Pd interface, it is found that Ti first reacted with C and Pd to form a TiC_x/Ti(Pd) double-layer structure, and then Ti(Pd) completely converted into TiC_x with the increase of holding time or temperature. Meanwhile, the infiltration of Pd along TiC_x grain boundary reacted with Si to form brittle Pd₂Si compound. By contrary, only a simple NbPd₃ layer was formed at the Nb/Ag–Pd interface during the whole process. A maximum shear strength of 16 ± 3 MPa is obtained for the joint prepared at 900°C for 30 min. The plastic deformation of the Ag–Pd interlayer and pullout of fibers inside ceramic contributed to the superior performance. Nevertheless, as the holding time and temperature reached 90 min and 950°C, the high chemical affinity of the Pd–Si system and enhanced atomic diffusion led to the massive formation of Pd₂Si, which increased the joint brittleness and degraded the ceramic performance.     

Ni interlayer induced strengthening effect in alumina/alumina joint bonded with Ti/Cu/Ni/Cu/Ti composite foils

In this work, alumina ceramic was bonded to itself with composite foils (Ti/Cu/Ni/Cu/Ti). The thermodynamics analysis indicates that Ti-Cu liquid firstly formed and reacted with alumina to produce Ti₃Cu₃O reaction layer. Meanwhile, the consumption of element Ti within the Ti-Cu liquid by Ni interlayer optimized the microstructure of the joint. By thickening Ni interlayer from 0 µm to 100 µm at 1020 ℃, the joints' strength could be significantly improved by 130.6%. While the joints' strength decreased with the decomposition of reaction layer at the brazing temperature higher than 1020 ℃. The FE simulations show that the high-level stress concentration within the reaction layer could be ameliorated by Ni interlayer effectively, which was in accordance with the corresponding fracture characteristics. Although the joint's strength could be improved to 203.9 MPa by using 300 µm Ni interlayer, its improvement rate was limited with Ni interlayer constantly thickening.     

Microstructural evolution during the brazing of Al2O3 ceramic to kovar alloy by sputtering Ti/Mo films on the ceramic surface

Alumina (Al₂O₃) ceramics were metallized by magnetron sputtering Ti/Mo bilayer films on the surface with a subsequent high temperature sintering and were brazed to Kovar alloy (Fe-Ni-Co) using Ag-Cu eutectic alloy. The Ti/Mo metallization film and the brazing seam microstructures were investigated and the correlation between the Al₂O₃/Kovar joining strength and the microstructures of the brazing seam was discussed. The results show that the joining strength is related to the thickness of the Ti/Mo adhension layer which depends on the holding time during brazing. The mutual diffusion of the elements at the interface firstly increases the thickness of the adhension layer as the holding time increases and the Mo film acts as a barrier layer to block the diffusion of Ti atoms into the seam. The optimal brazing joining strength of 72.6±5.0 MPa could be achieved at a brazing temperature of 810 °C for 14 min However, if the holding time is further prolonged, Mo atoms will diffuse into the (Ni, Cu) solid solution, resulting in the diffusion of Ti atoms and the adhension layer becoming indistinguishable. Therefore, the intermetallic Ni₃Ti forms in the seam and the titanium oxide changes from TiO to Ti₂O₃ or Ti₃O₅, which leads to the joint strength decreasing.     

Wetting of SiC by Al-Ti alloys and joining by in-situ formation of interfacial Ti3Si(Al)C2

Two pressureless and reliable procedures for brazing SiC-based materials have been designed. The joining was obtained by the in-situ formation of a Ti₃Si(Al)C₂ MAX phase using simple Al-Ti interlayers. Wettability studies were conducted using several Al-Ti alloys in contact with SiC at 1500?°C. The interfacial microstructures and thermodynamic analysis demonstrated that liquid Al₃Ti in contact with SiC formed a well-bonded Ti₃Si(Al)C₂ interfacial layer. These findings guided the design of two joining methods: one consisted of the simple infiltration of Al₃Ti into the brazing seam, while an Al₃Ti paste/Ti/Al₃Ti paste interlayer assembly was designed for the second process. Sound interfaces without cracks were obtained in both processes. The average shear strength was very high, 296?MPa, for the infiltration method; the drawback was the presence of residual Al. Joining through Al₃Ti/Ti/Al₃Ti interlayers avoided the presence of low-temperature melting phases, with lower shear strength: 85 or 89?MPa depending on the testing method.     

Reactive brazing of silicon nitride to Invar alloy using Ni foam and AgCuTi intermediate layers

Silicon nitride (Si₃N₄) ceramic and Invar alloy have been brazed by using AgCuTi active filler and the Ni foam was added to further improve mechanical properties of joints in this study. The microstructure of Si₃N₄/Invar brazed joint changed obviously after adding Ni foam with different thickness. Ni foam reacted with the AgCuTi active filler during brazing, but it did not completely disappear and still maintained the basic frame structure after brazing. The average shear strength of the brazed joints with 0.2 mm Ni foam could reach 180 MPa, and their thermal cycle lifetime also improved significantly. The addition of Ni foam shifted the fracture location of joints from Si₃N₄ ceramic to brazing seam. These results indicated that the Ni foam could act as a buffer layer to reduce the residual thermal stress, and improve the mechanical properties of Si₃N₄/Invar joint.     

Effect of brazing parameters on microstructure and mechanical properties of Cf/SiC and Nb-1Zr joints brazed with Ti-Co-Nb filler alloy

Reliable brazing of carbon fiber reinforced SiC (C_f/SiC) composite to Nb-1Zr alloy was achieved by adopting a novel Ti45Co45Nb10 (at.%) filler alloy. The effects of brazing temperature (1270–1320 °C) and holding time (5–30 min) on the microstructure and mechanical properties of the joints were investigated. The results show that a continuous reaction layer (Ti,Nb)C was formed at the C_f/SiC/braze interface. A TiCo and Nb(s,s) eutectic structure was observed in the brazing seam, in which some CoNb₄Si phases were distributed. By increasing the brazing temperature or extending the holding time, the reaction layer became thicker and the amount of the CoNb₄Si increased. The optimized average shear strength of 242 MPa was obtained when the joints were brazed at 1280 °C for 10 min. The high temperature shear strength of the joints reached 202 MPa and 135 MPa at 800 °C and 1000 °C, respectively.     

High shear strength and ductile ZrC–SiC/austenitic stainless steel joints bonded with Ti/Ni foam interlayer

A transient-liquid-phase (TLP) diffusion bonding method was employed to join ZrC–SiC ceramic and austenitic stainless steel by using Ti foil and various density Ni foam as interlayer. The Ti–Ni TLP contributed to a firm bonding between ceramic and foam interlayer while avoiding liquid infiltration in foam structure. The influence of holding time on the microstructure of the TLP reaction layer was investigated. The shear tests showed the joints with foam interlayer exhibited ductile failure mode and improved fracture work in comparison with the one with dense Ni interlayer. A maximum shear strength of 117.2 MPa was reached when the relative density of Ni foam was 0.57. The fracture behaviors of the joints during shear test were in-situ observed.