Microstructures and Mechanical Properties of Vacuum Brazed Ti_3Al/TiAl Joints Using Two Ti-based Filler Metals

Vacuum brazing of Ti3Al-based alloy to Ti Al was firstly carried out by Ti—15Cu—15Ni(wt%) filler metal.A continuous Ti3 Al band, Ti2 Ni and Ti2Cu/Cu3 Ti phases formed and the joint showed a shear strength of53.8–112.4 MPa at room temperature. For the improvement of the joint strength, a new Ti—Zr—Cu—Ni—Fe filler alloy was designed, and its wettability on Ti3 Al and Ti Al substrate was studied with the sessile drop method. After holding for 20 min at 1010 °C the Ti—Zr—Cu—Ni—Fe filler showed a low contact angle of20° and 21° on (Ti)_3Al and Ti Al substrate, respectively. The joint brazed with this novel filler mainly consisted of Ti-rich area, (Ti)_3Al reaction layer and residual filler metal. With the increase of the brazing temperature, the amount of residual filler metal decreased and the (Ti)_3 Al reaction layer thickened. The (Ti)_3Al/Ti Al joint brazed with Ti—Zr—Cu—Ni—Fe filler exhibited a lower hardness than that brazed with Ti—Cu—Ni filler. The corresponding joints brazed at 950 °C for 5 min presented the shear strength of257.6 ± 33.6 MPa at room temperature and 304.8 ± 9.9 MPa at 600 °C.     

Effects of Nb content in Ti–Ni–Nb brazing alloys on the microstructure and mechanical properties of Ti–22Al–25Nb alloy brazed joints

Vacuum brazing was successfully used to join Ti–22Al–25Nb alloy using Ti–Ni–Nb brazing alloys prepared by arc-melting. The influence of Nb content in the Ti–Ni–Nb brazing alloys on the interfacial microstructure and mechanical properties of the brazed joints was investigated. The results showed that the interfacial microstructure of brazed joint consisted of B2, O, ?3, and Ti2 Ni phase, while the width of brazing seams varied at different Nb contents. The room temperature shear strength reached359 MPa when the joints were brazed with eutectic Ti40Ni40Nb20 alloy at 1180?C for 20 min, and it was321, 308 and 256 MPa at 500, 650 and 800?C, respectively. Cracks primarily initiated and propagated in ?3compounds, and partially traversed B2+O region. Moreover, the fracture surface displayed typical ductile dimples when cracks propagated through B2+O region, which was favorable for the mechanical properties of the brazed joint.     

Microstructure and brazing mechanism of porous Si3N4/Invar joint brazed with Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler

Porous Si_3N_4 was brazed to Invar alloy in this study, and Ag-Cu-Ti/Cu/Ag-Cu multi-layered filler was designed to inhibit the formation of Fe_2Ti and Ni_3Ti intermetallic compounds. The effects of the brazing temperature and the thickness of Cu interlayer on the microstructure and mechanical properties of brazed joints were investigated. The typical microstructure of the joint brazed with multi-layered filler was porous Si_3N_4/TiN + Ti_5Si_3/Ag-Cu eutectic/Cu/Ag-Cu eutectic/Cu-rich layer + diffusion layer/Invar. When the brazing temperature increased, the reaction layer at the ceramic/filler interface grew thicker and the Cu interlayer turned thinner. As the thickness of Cu interlayer increased from 50 to 150μm, the joint strength first increased and then decreased. In this research, the maximum shear strength(73 MPa) was obtained when being brazed at 1173 K with a 100μm Cu interlayer applied in the filler, which was 55% higher than that brazed with single Ag-Cu-Ti brazing alloy and had reached 86% of the ceramic. The release of residual stress and the barrier effect of Cu interlayer to inhibit the formation of Fe_2Ti and Ni_3Ti intermetallics played the major role in the improvement of joint strength.     

Microstructure and Strength of Brazed Joints of Ti3Al Base Alloy with Cu-P Filler Metal

Brazing of Ti3AI alloys with the filler metal Cu-P was carried out at 1173-1273 K for 60-1800 s. When products are brazed, the optimum brazing parameters are as follows: brazing temperature is 1215-1225 K; brazing time is 250-300 s. Four kinds of reaction products were observed during the brazing of Ti3AI alloys with the filler metal Cu-P, i.e., Ti3AI phase with a small quantity of Cu (Ti3AI(Cu)) formed close to the Ti3AI alloy; the TiCu intermetallic compounds layer and the Cu3P intermetallic compounds layer formed between Ti3AI(Cu) and the filler metal, and a Cu-base solid solution formed with the dispersed Cu3P in the middle of the joint. The interfacial structure of brazed Ti3AI alloys joints with the filler metal Cu-P is Ti3AI/Ti3AI(Cu)/TiCu/Cu3P/Cu solid solution (Cu3P)/Cu3P/TiCu/Ti3AI(Cu)/Ti3AI, and this structure will not change with brazing time once it forms. The thickness of TiCu+Cu3P intermetallic compounds increases with brazing time according to a parabolic law. The activation energy Q and the growth velocity K0 of reaction layer TiCu+Cu3P in the brazed joints of Ti3AI alloys with the filler metal Cu-P are 286 kJ/mol and 0.0821 m2/s, respectively, and growth formula was y2=0.0821exp(-34421.59/T)t. Careful control of the growth for the reaction layer TiCu+Cu3P can influence the final joint strength. The formation of the intermetallic compounds TiCu+Cu3P results in embrittlement of the joint and poor joint properties. The Cu-P filler metal is not fit for obtaining a high-quality joint of Ti3AI brazed.     

CuTiNiZrV Amorphous Alloy Foils for Vacuum Brazing of TiAl Alloy to 40Cr Steel

Vacuum brazing of TiAl alloy to 40Cr steel sheets was conducted with newly developed CuTiNiZrV amorphous foils. It was found that a diffusion layer,filler metal and reaction layer existed in the brazed seam. The diffusion layer in the joint brazed with Cu43.75Ti37.5Ni6.25Zr6.25V6.25(at.%) foil was flat and thin,containing Ti19Al6 and Ti2Cu intermetallic compounds; however,the diffusion layer brazed with Cu37.5Ti25Ni12.5Zr12.5V12.5 foil was uneven with bulges,consisting of essentially Ti-based solute solution. The foil with 12.5 at.% V showed inferior spreadability compared to that with 6.25 at.% V at brazing temperature. However,fracture happened along the diffusion layer with 6.25 at.% V foil due to the formation of brittle intermetallic phases,but the joints brazed with 12.5 at.% V foil failed through the TiAl substrate. These results show that designing amorphous alloy with less Ti and more V for brazing TiAl alloy to steel is appropriate.     

High-efficiency Joining of C_f/Al Composites and TiAl Alloys under the Heat Effect of Laser-ignited Self-propagating High-temperature Synthesis

The aim of this study was to develop a high-efficiency joining method of Cf/Al composites and TiA l alloys under the heat effect of laser-ignited self-propagating high-temperature synthesis(SHS). The SHS reaction of Ni–Al–Zr interlayer was induced by laser beam and acted as local high-temperature heat source during the joining. Sound joint was obtained and verified the feasibility of this joining method. Effect of filler metals on the joint microstructure and shear strength was evaluated. When the joining pressure was 2 MPa with additive filler metals, joint shear strength reached the maximum of 41.01 MPa.     

Active metal brazing of SiO_2–BN ceramic and Ti plate with Ag–Cu–Ti+BN composite filler

SiO_2–BN ceramic and Ti plate were joined by active brazing in vacuum with Ag–Cu–Ti+BN composite filler.The effect of BN content,brazing temperature and time on the microstructure and mechanical properties of the brazed joints was investigated.The results showed that a continuous Ti N–Ti B_2reaction layer formed adjacent to the SiO_2–BN ceramic,whose thickness played a key role in the bonding properties.Four Ti–Cu compound layers,Ti_2Cu,Ti_3Cu_4,Ti Cu_2and Ti Cu_4,were observed to border Ti substrate due to the strong affinity of Ti and Cu compared with Ag.The central part of the joint was composed of Ag matrix,over which some fine-grains distributed.The added BN particles reacted with Ti in the liquid filler to form fine Ti B whiskers and Ti N particles with low coefficients of thermal expansion(CTE),leading to the reduction of detrimental residual stress in the joint,and thus improving the joint strength.The maximum shear strength of 31 MPa was obtained when 3 wt%BN was added in the composite filler,which was 158%higher than that brazed with single Ag–Cu–Ti filler metal.The morphology and thickness of the reaction layer adjacent to the parent materials changed correspondingly with the increase of BN content,brazing temperature and holding time.Based on the correlation between the microstructural evolution and brazing parameters,the bonding mechanism of SiO_2–BN and Ti was discussed.     

Microstructure and Property of AlN Joint Brazed with Au–Pd–Co–Ni–V Brazing Filler

An Au–Pd–Co–Ni–V brazing alloy was designed for Al N ceramic joining. Its wettability on Al N was studied with the sessile drop method. The results showed that the contact angle was decreased gradually with increasing temperature and the prolonging of holding time. Sound Al N/Al N joints were achieved with the brazing alloy at 1170 °C for 10 min. The microstructure of the Al N/Al N joints was examined by scanning electron microscopy(SEM). It was found that element V played the active role in the interfacial reaction between the ceramic and the brazing alloy. V reacted with N decomposed from Al N, resulted in the formation of V–N compound. Based on the energy-dispersive spectroscopy(EDS)and X-ray diffraction(XRD) analysis results, the V–N reaction product was verified as V2 N. The overall reaction during the brazing process can be described by the following equation: 2V + AlN + 2Pd = V2 N + Pd2 Al.The Al N/Al N joints brazed with the Au–Pd–Co–Ni–V brazing alloy exhibited three-point bend strength of 162.7 MPa at room temperature, and under the bend test the fracture of the joint occurred at the Al N ceramic substrate.     

Joining of C_f/SiBCN composite with two Ni-based brazing fillers and interfacial reactions

C_f/SiBCN ceramic composite was joined using Ni-19Cr-10Si(BNi5) and Ni-33Cr-24Pd-3.5Si-0.5B filler alloys at 1170 °C for 10 min. Two kinds of Ni-based filler alloys exhibited good wettability on the C_f/SiBCN composite, with a contact angle of 13° and 4°, respectively. The microstructures of the brazed joints were investigated by electron-probe microanalysis(EPMA), and three-point bend test was conducted for the joints at room temperature. When being brazed with BNi5 filler alloy, no evident reaction layer was observed at the surface of the joined composite, and the joint microstructure was characterized by Ni_2Si matrix with scatteringly distributing mixture compounds of Cr_(23)C_6, Ni_2Si and CrB. While Ni-Cr-Pd(Si,B)brazing alloy was used, a Cr_(23)C_6 reaction layer with a thickness of 11 μm was formed at the surface of the base composite. In the central part of the brazed joint, the phases were composed of Ni(Cr,Si) solid solution and complex compounds including Pd_2Si,(Ni,Pd)_2Si and Ni-B. The strength of C_f/SiBCN joint brazed with BNi5 filler alloy was 62.9 MPa at room temperature, whereas that with Ni-Cr-Pd(Si,B) filler alloy was at the same level.     

Interfacial Structure and Formation Mechanism of Ultrasonic-assisted Brazed Joint of SiC Ceramics with Al—12Si Filler Metals in Air

Ultrasonic-assisted brazing of SiC ceramics was performed by filling with an Al—12Si alloy at a low temperature of 620 °C in air. The interfacial characteristics and formation mechanism were investigated. The joint shear strength reached 84–94 MPa using the ultrasonic time of 2–16 s. The fracture morphology showed that the fracture path initiated and propagated in the joint alloy. The thin film of amorphous SiO 2 that formed on the SiC surface was non-uniformly decomposed and diffused into the liquid Al—12Si alloy under the cavitation erosion effect of ultrasound. Abnormal isolated blocks of Al_2 SiO_5 compounds formed at the interface between Al—12Si and a thicker SiO_2 layer formed during the thermal oxidation treatment of the Si C ceramic. The SiO_2 layer on the Si C ceramic did not hinder or impair the wetting and bonding process, and a stronger bond could form between Al—12Si and SiO_2 or Si C in ultrasonicassisted brazing.     

Transmission Electron Microscopy Study of the Infrared Brazed High-strength Titanium Alloy

The transmission electron microscopy was employed to investigate the microstructure of infrared brazed highstrength Ti alloy using the Ti-15Cu-15Ni filler metal. Coarse primary Ti2Ni and transformed β-Ti are observed in the 300 s brazed specimen. Blocky Ti2Ni and eutectoid Ti2Cu intermetallics are disappeared from the joint with increasing the brazing time to 1800 s. Both acicular α-Ti and retained β-Ti dominate the entire brazed joint.     

Performance of GH4169 brazed joint using a new designed nickel-based filler metal via cluster-plus-glue-atom model

A novel Ni-Cr-Si-B filler metal with the cluster formula of [Cr-Ni_(12)]B_2Cr + [B-Ni_8Cr]BSi Cr based on the cluster-plus-glue-atom model was designed for vacuum brazing GH4169 alloy. The effect of brazing temperature and brazing time on microstructure and shear strength of GH4169 alloy joints was investigated.The brazed seam was mainly composed of γ-Ni solid solution.(Nb, Ti)-rich phase and(Cr, Nb, Mo)-rich borides distributed in diffusion zones. The diffusion and aggregation of B, Cr, Nb, and Mo resulted in the variation of phase contrast and morphology of borides. Coarse precipitations in the joint brazed at1240℃ consisted of borides, Laves phase and δ phase. The shear strength of joints was principally dominated by the brittle precipitations in diffusion zone, and the homogenization of microstructure improved the room-temperature shear strength to 820 MPa with the high-temperature shear strength of 627 MPa for the joint brazed at 1240℃/20 min. The joint fractured in diffusion zone and brazed seam, and the existence of borides and Laves phase in diffusion zone provide the potential origin for crack growth.     

Microstructural Evolution of Infrared Brazed CP-Ti Using Ti-Cu-Ni Brazes

Microstructural evolution of infrared vacuum brazed CP-Ti using two Ti-based braze alloys,Ti-15Cu-15Ni and Ti-15Cu-25Ni,has been investigated.The infrared brazed joint consisted of eutectic Ti 2 Cu/Ti 2 Ni intermetallic compounds and Ti-rich matrix.The eutectic Ti 2 Cu/Ti 2 Ni intermetallic compounds disappeared from the joint after being annealed at 900 C for 1 h.In contrast,the depletion rate of both Cu and Ni from the braze alloy into CP-Ti substrate at 750 C annealing was greatly decreased as compared with that annealed at 900 C.Blocky Ti 2 Cu/Ti 2 Ni phases were observed even if the specimen was annealed at 750 C for 15 h.Because the Ni content of the Ti-15Cu-25Ni braze alloy is much higher than that of the Ti-15Cu-15Ni alloy,the amount of eutectic Ti 2 Cu/Ti 2 Ni phases in Ti-15Cu-25Ni brazed joint is more than that in Ti-15Ci-15Ni brazed joint.However,similar microstructural evolution can be obtained from the infrared brazed joint annealed at various temperatures and/or time for both filler metals.     

Microstructure evolution and mechanical behavior of Ni-based single crystal superalloy joint brazed with mixed powder at elevated temperature

Brazing of a Ni-based single crystal superalloy has been investigated with the additive Ni-based superalloy and filler Ni–Cr–W–B alloy at 1260℃, and attentions were paid to the microstructure evolution during brazing and the stress-rupture behavior at 980℃ of such brazed joints after homogenization. Microstructure in the brazed joint generally includes brazing alloy zone(BAZ), isothermally solidified zone(ISZ) and diffusion affected zone(DAZ). Microstructure evolution during this brazing process is discussed at the heating stage, the holding stage and the cooling stage respectively, according to the diffusion path of B atoms. Initially well-distributed γ’/γ’ microstructure in the homogenized bonded zone after heat treatment and substantial γ’ rafts enhance the post-brazed joint to obtain a stress-rupture lifetime of more than 120 h at 980℃/250 MPa. On the other hand, the decreased stress-rupture behavior of post-brazed joint, compared with parenting material, is ascribed to the presence of inside brazing porosity and stray grain boundary, which not only reduces the effective loading-carrying area but also offers preferential sites for creep vacancy aggregation to further soften stray grain boundary. And finally an early fracture of these post-brazed joints through the intergranular microholes aggregation and growth mode under this testing condition was observed.     

Effects of Al2O3 Particulates on the Thickness of Reaction Layer of Al2O3 Joints Brazed with Al2O3-Particulate-Contained Composite Filler Materials

In order to understand the rate-controlling process for the interracial layer growth of brazing joints brazed with active composite filler materials, the thickness of brazing joints brazed with conventional active filler metal and active composite filler materials with different volume fraction of AI203 particulate was studied. The experimental results indicate although there are Al2O3 particulates added into active filler metals, the time dependence of interracial layer growth is t^2 as described by Fickian law for the joints brazed with conventional active filler metal. It also shows that the key factor affecting the interracial layer growth is the volume fraction of alumina in the composite filler material compared with the titanium weight fraction in the filler material.     

Effects of Al2O3—Particulate—Contained Composite Filler Materials on the Shear Strength of Slumina Joints

Al2O3/Al2O3 joints were brazed with a new kind of filler materials,which were formed by adding Al2O3 particulates into Ag-Cu-Ti active filler metal.The results showed that the material parameters (the Ti content,Al2O3 particulate volume fraction) of the composite filler materials affected the shear strength of brazed joints.When the Ti content was 2 wt pct in the filler metal, the shear strength of brazing joints decreased with the increasing the volume ratio of Al2O3 particulate .When the Ti content was 3 wt pct in the filler metal, the shear strength of joints increased from 93.75 MPa(Al2O3 pOvol pct) to 135.32 MPa(Al2O3 p15 vol.pct).     

Infrared Brazing Zirconium using Two Silver Based Foils

Ag-based brazing foils,BAg-8（72Ag-28Cu in wt%） and Ticusil^?（68.8Ag-26.7Cu-4.5Ti in wt%） were selected to braze Zr.Interfacial AgCu₄Zr,CuZr₂ reaction layers and Ag-rich matrix dominate BAg-8 brazed joint,and fractograph after shear test shows ductile dimple fracture with plastic sliding marks.Ticusil^? joint brazed at 910℃for 300 s is comprised of Cu₉Zr₁₁ and AgZr intermetallics,and fractograph after shear test displays brittle cleavage fracture.     

Effect of Brazing Conditions on Microstructure and Mechanical Properties of Al2O3/Ti--6Al-4V Alloy Joints Reinforced by TiB Whiskers

Al2O3 and Ti-6Al-4V alloy were brazed with Ag-Cu-Ti ＋B fillers in different brazing conditions. Effects of brazing temperature, holding time and additive Ti content on joints microstructure and shear strength were investigated by scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, transmission electron microscopy and shear testing. Results indicate that TiCu and Ti（Cu,Al） decrease, but Ti2Cu and -Ti2（Cu,Al） increase in brazing seam with increasing brazing temperature, holding time and additive Ti content. Area consisting of Ti3（Cu,Al）30 and TiO near Al2O3 becomes gradually discontinuous from continuity when brazing temperature rises or holding time extends. As Ti additive content increases, TiO is absent near Al2O3, area consisting of only Ti3（Cu,Al）30 thickens. TiB whiskers are in situ synthesized by Ti and B atoms during brazing process. The brazing temperature, holding time and additive Ti content on joints microstructure influence the joints shear strength directly. The shear strength of joints, obtained at 850 ℃ holding for 10 min, reaches the maximum of 78 MPa. According to the experimental results, phase diagram and thermodynamics calculation, the interface evolution mechanism of the Al2O3/Ti-6Al-4V alloy joint was analyzed.     

Effects of Al_2O_3-Particulate-Contained Composite Filler Materials on the Shear Strength Alumina Joints

Al2O3l2O3 joints were brazed with a new kind of filler materials, which were formed by adding AI203 particulates into Ag-Cu-Ti active filler metal. The results showed that the material parameters (the Ti content, Al2O3 particulate volume fraction) of the composite filler materials affected the shear strength of brazed joints. When the Ti content was 2 wt pct in the filler metal, the shear strength of brazing joints decreased with the increasing the volume ratio of Al2O3 particulate. When the Ti content was 3 wt pct in the filler metal, the shear strength of joints increased from 93.75 MPa(AI203p 0 vol. pct) to 135.32 MPa(AI203p 15 vol. pct).     

Improvement of Joint Strength of SiCp/Al Metal Matrix Composite in Transient Liquid Phase Bonding Using Cu/Ni/Cu Film Interlayer

The compact oxide on the surface of SiCp/Al metal matrix composite （SiCp/Al MMC） greatly depends on the property of the joint. Inlaid sputtering target was applied to etch the oxide completely on the bonding surface of SiCp/Al MMC by plasma erosion. Cu/Ni/Cu film of 5μm in thickness was prepared by magnetron sputtering method on the clean bonding surface in the same vacuum chamber, which was acted as an interlayer in transient liquid phase （TLP） bonding process. Compared with the same thickness of single Cu foil and Ni foil interlayer, the shear strength of 200 MPa was obtained using Cu/Ni/Cu film interlayer during TLP bonding, which was 89.7% that of base metal. In addition, homogenization of the bonding region and no particle segregation in interfacial region were found by analysis of the joint microstructure. Scanning electron microscopy （SEM） was used to observe the micrograph of the joint interface. The result shows that a homogenous microstructure of joint was achieved, which is similar with that of based metal.