Evaluation of the microstructure and mechanical properties of diffusion bonded joints of titanium to stainless steel with a pure silver interlayer

Solid-state diffusion bonding of commercially pure titanium to 304 stainless steel using an Ag interlayer was carried out at 825–875 °C under a uniaxial pressure of 8 MPa for 20 min in vacuum. The microstructural observations revealed that the resultant joints were composed of the remnant Ag interlayer, TiAg intermetallic phase and Ti–Ag solid solution. An optimized bonding strength of up to 414 MPa was achieved. Fracture took place through the remnant Ag interlayer during tensile tests and the interfacial TiAg phase exhibited no detrimental effect on the bonding strength. Extensive dimples were observed on the fracture surfaces, indicating that the joints were ductile in nature.     

Reliability studies of Cu/Al joints brazed with Zn–Al–Ce filler metals

This paper examines the effect of different Ce content on the properties and microstructures of Zn–22Al filler metals and Cu/Al brazing joints. The results indicate that, the spreading area on Cu substrates of Zn–22Al filler metal could be improved by 29.7% with the addition of 0.03 wt% Ce, whereas the oxidation resistance of the alloy increased significantly. The thermal behaviors of Zn–22Al filler metals were minimally influenced by the addition of Ce. The Zn–22Al–xCe filler metals show finer and more uniform microstructures when the added Ce content is in the range 0.03–0.05 wt%. Particularly, the addition of trace Ce into the Zn–22Al filler metal can refine the microstructures and decrease the thickness of the layer of intermetallic compounds produced in the Cu/Al brazing joints. Some bright (Zn,Al)–Ce intermetallic compounds particles were observed in the alloy when the Ce content exceeds 0.08 wt%. The results also indicate that the shear strength of Cu/Al joint brazed with Zn–22Al–0.05Ce is 30.3% higher than that of the Zn–22Al filler metal. Some hard and brittle Ce-bearing intermetallic compounds particles appear in the fracture surface when the Ce content is 0.25 wt%, which resulted in the weakening of the mechanical properties of Cu/Al brazing joints.     

Effect of calcium on the microstructure and mechanical properties of brazed joint using Ag–Cu–Zn brazing filler metal

The induction brazing of 316LN stainless steel using Ag–Cu–Zn filler metal containing various content of Ca was carried out to investigate the influence of impurity element Ca on the microstructure and mechanical properties of the brazed joint. The results showed that Ca additions caused the coarser of the grains and their irregular distribution. Increase of the Ca content resulted in the formations of brittle intermetallic compounds (IMCs) CaCu which perhaps lead to the formations of voids. All of the calcium-containing brazed joints performed better in microhardness than calcium-free ones and brazed joints containing 0.003 wt.% Ca showed the highest microhardness of 203HV. While the tensile strength decreased with the increment of Ca, from 460 MPa to 400 MPa. The combination effects of coarser grains, brittle IMCs and voids conduced to the reduction of tensile strength and microhardness of the brazed joints.     

Characteristics of dissimilar laser-brazed joints of isotropic graphite to WC–Co alloy

The effect of Ti serving as an activator in a eutectic Ag–Cu alloy filler metal in dissimilar laser-brazed joints of isotropic graphite and a WC–Co alloy on the joint strength and the interface structure of the joint is investigated in this study. To evaluate the joint characteristics, the Ti content in the filler metal was increased from 0 to 2.8 mass%. The laser brazing was carried out by irradiating a laser beam selectively on the WC–Co alloy plate in Ar atmosphere. The threshold content of Ti required to join isotropic graphite to WC–Co alloy was 0.4 mass%. The shear strength at the brazed joint increased rapidly with increasing Ti content up to 1.7 mass%, and a higher Ti content was found to be likely to saturate the shear strength to a constant value of about 14 MPa. The isotropic graphite blocks also fractured at this content. The concentration of Ti observed at the interface between isotropic graphite and the filler metal indicates the formation of an intermetallic layer of TiC.     

Effect of gradient thermal distribution on butt joining of magnesium alloy to steel with Cu–Zn alloy interlayer by hybrid laser–tungsten inert gas welding

Experimental investigations on butt welding of magnesium alloy to steel by hybrid laser–tungsten inert gas (TIG) welding with Cu–Zn alloy interlayer are carried out. The results show that the gradient thermal distribution of hybrid laser–TIG welding, controlled by offset adjustment, has a noticeable effect on mechanical properties and microstructure of the joints. Particularly, at the offset of 0.2 mm, defect-free joints are obtained, and the tensile strength could attain a maximum value of 203 MPa. Moreover, the fracture of the joint with the 0.2 mm offset happens in the weld seam of Mg alloy instead of the Mg/Fe interface. Owning to the addition of the Cu–Zn alloy interlayer, a metallurgical bonding between Mg alloy and steel is achieved based on the formation of intermetallic compounds of CuMgZn and solid solutions of Cu and Al in Fe. Meanwhile, the same element distribution tendency of Fe and Al indicates the intimate interaction between Fe and Al in current experimental conditions.     

Improvements of the Si3N4 brazed joints with intermetallics

The bonding of Si₃N₄ ceramics with Ag–Cu–Ti, Ni and Ti was performed. The influencing factors on joint strength were investigated. Cu–Ni–Ti intermetallic particles formed in situ were observed in the joints. Scanning electron microscopy photographs show that the interfacial reaction layer is constituted of two layers. The intermetallic particles are homogeneously distributed in the matrix so that they could contribute to the decrease in the residual stresses and the improvement of the joint strength. When bonded with proper parameters, the joint shear strength can reach more than 200 MPa, with a peak experimental value of 215.33 MPa.     

TIG–MIG hybrid welding of ferritic stainless steels and magnesium alloys with Cu interlayer of different thickness

The joining of ferritic stainless steels and magnesium alloys is light and economic for weight reduction of automobiles. Unlike previous conventional welding method, a novel TIG–MIG hybrid welding is applied for the joint successfully in this study. The melted Mg weld metal wets the ferritic stainless steels surface to form a brazed Mg–Cu to steel connection when the interlayer thickness is 0.02 mm. When the interlayer thickness is 0.1 mm, the intermetallic compounds transition layer determined the tensile-shear strength of joints. Intermetallic compounds transition layer has been found in the 0.1 mm thick interlayer joints and no particle has been found in the 0.02 mm thick interlayer joints. Based on the analysis of microstructure and properties, joining and strengthen mechanisms of the joint were discovered. As the thickness of the Cu interlayer increases, the joining mechanism changed. The joining and strengthen mechanisms are mainly determined by the thickness of the interlayer. The tensile-shear strength of 0.1 mm thickness Cu interlayer joints is improved by 47% compared to 0.02 mm Cu.     

A new filler metal with low contents of Cu for high strength aluminum alloy brazed joints

The effect of Cu with low contents of 10, 12, 15 wt.% on the microstructure and melting point of Al–Si–Cu–Ni alloy has been investigated. Results showed that low-melting-point properties of Al–Si–Cu–Ni alloys with low contents of Cu were attributed to disappearance of Al–Si binary eutectic reaction and introduction of Al–Si–Cu–Ni quaternary reaction. With raising Cu content from 10 to 15 wt.%, the amount of complex eutectic phases formed during low temperature reactions (Al–Cu, Al–Si–Cu and Al–Si–Cu–Ni alloy reactions) increased and the melting temperature of Al–Si–Cu–Ni filler metals declined. Brazing of 6061 aluminum alloy with Al–10Si–15Cu–4Ni (all in wt.%) filler metal of a melting temperature range from 519.3 to 540.2 °C has been carried out successfully at 570 °C. Sound joints can be obtained with Al–10Si–15Cu–4Ni filler metal when brazed at 570 °C for holding time of 60 min or more, and achieved high shear strength up to 144.4 MPa.     

Depressing effect of 0.1 wt.% Cr addition into Sn–9Zn solder alloy on the intermetallic growth with Cu substrate during isothermal aging

In this paper, the effect of 0.1 wt.% Cr addition into Sn–9Zn lead-free solder alloys on the growth of intermetallic compound (IMC) with Cu substrate during soldering and subsequent isothermal aging was investigated. During soldering, it was found that 0.1 wt.% Cr addition did not contribute to forming the IMC, which was verified as the same phase structure as the IMC for Sn–9Zn/Cu. However, during solid-state isothermal aging, the IMC growth was remarkably depressed by 0.1 wt.% Cr addition in the Sn–9Zn solder, and this effect tended to be more prominent at higher aging temperature. The activation energy for IMC growth was determined as 21.2 kJ mol^? 1 and 42.9 kJ mol^? 1 for Sn–9Zn/Cu and Sn–9Zn–Cr/Cu, respectively. The reduced diffusion coefficient was confirmed for the 0.1Cr-containing solder/Cu. Energy-dispersive X-ray mapping and point analysis also showed ZnCr phase existing in solder matrix, which can reduce diffusion rate of Zn atoms.     

An investigation on microstructure and mechanical properties of Al7075 to Ti–6Al–4V Transient Liquid Phase (TLP) bonded joint

Transient Liquid Phase (TLP) bonding of two dissimilar alloys Al7075 and Ti–6Al–4V has been done at 500 °C under 5 × 10⁻⁴ torr. Cu was electrodeposited on Al7075 and Ti–6Al–4V surfaces, 50 μm thick Sn–4Ag–3.5Bi film was used as interlayer and bonding process was carried out at several bonding times. The microstructure of the diffusion bonded joints was evaluated by Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The eutectic and intermetallic compounds formation along Al7075 grain boundaries and Ti/Al interface such as θ(Al₂Cu), TiAl and Ti₃Al were responsible for joint formation at the aluminum and titanium interfaces. Microhardness and shear strength tests were used to investigate the mechanical properties of the bonds. Hardness of the joints increased with increasing bonding time which can be attributed to the intermetallics formation at the interface. The study showed that the highest bond strength was 36 MPa which was obtained for the samples joined for 60 min.     

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.     

Development of novel CsF–RbF–AlF3 flux for brazing aluminum to stainless steel with Zn–Al filler metal

Brazing 6061 Al alloy to 304 stainless steel by flame brazing has been carried out with an improved CsF–RbF–AlF₃ flux which matched Zn–xAl filler metals. The results showed that, the spreading area on stainless steel of Zn–xAl filler metals has been improved with the addition of RbF to CsF–AlF₃ flux. It is found that a Zn-rich phase appeared between the brazing seam and the intermetallic compound (IMC) layer in the joints brazed with Zn–2Al and Zn–5Al filler metals, and the thickness of the IMC layer was approximately 1.76–6.45 μm which increased with the increase of Al added to the filler metals. Moreover, a Fe₄Al₁₃ phase formed in the IMC layer, while a Fe₂Al₅ phase appeared as the second layer in Zn–25Al brazed joint. Neither the Zn-rich phase nor Fe₂Al₅ phase was found in the joint brazed with Zn–15Al filler metal, so that the joint was exhibited the maximum shear strength which was up to 131 MPa. All the lap joints were fractured at the interfacial layer of the brazing seam and stainless steel.     

Ultrasound-assisted brazing of Cu/Al dissimilar metals using a Zn–3Al filler metal

Ultrasound-assisted brazing of Cu/Al dissimilar metals was performed using a Zn–3Al filler metal. The effects of brazing temperature on the microstructure and mechanical properties of Cu/Al joints were investigated. Results showed that excellent metallurgic bonding could be obtained in the fluxless brazed Cu/Al joints with the assistance of ultrasonic vibration. In the joint brazed at 400 °C, the filler metal layer showed a non-uniform microstructure and a thick CuZn₅ IMC layer was found on the Cu interface. Increasing the brazing temperature to 440 °C, however, leaded to a refined and dispersed microstructure of the filler metal layer and to a thin Al_4.2Cu_3.2Zn_0.7 serrate structure in the Cu interfacial IMC layer. Further increasing the brazing temperature to 480 °C resulted in the coarsening of the filler metal and the significantly growth of the Al_4.2Cu_3.2Zn_0.7 IMC layer into a dendrite structure. Nanoindentation tests showed that the hardness of the Al_4.2Cu_3.2Zn_0.7 and CuZn₅ phase was 11.4 and 4.65 GPa, respectively. Tensile strength tests showed that all the Cu/Al joints were failed in the Cu interfacial regions. The joint brazed at 440 °C exhibited the highest tensile strength of 78.93 MPa.     

Brazing continuous carbon fiber reinforced Li2O–Al2O3–SiO2 ceramic matrix composites to Ti–6Al–4V alloy using Ag–Cu–Ti active filler metal

C_f/LAS composites and TC4 alloy were brazed successfully by vacuum brazing using Ag–Cu–Ti active filler metal. The interfacial microstructure was characterized by a scanning electron microscope, energy dispersive spectrometer and X-ray diffraction. The effects of brazing temperature on the interfacial microstructure and joint properties were investigated in details. Various phases including TiC, TiSi₂, Ti₃Cu₄, Cu (s,s), Ag (s,s), TiCu and Ti₂Cu were formed in the brazed joints. Interfacial microstructure varies greatly with the increase of brazing temperature, while the amount of Ti₂Cu reduced, but no new phase is generated. The optimal shear strength of the joint is 26.4 MPa when brazed at 890 °C for 10 min. Shear test indicated that the fracture of the brazed joints went through the TiSi₂ + TiC layer close to the C_f/LAS composites interface.     

Microstructure characteristics and mechanical properties of cold metal transfer welding Mg/Al dissimilar metals

AZ31B Mg alloy and 6061 Al alloy were joined by using cold metal transfer (CMT) welding with pure copper (HS201) as the filler metal. The microstructure of Mg/Al CMT weld joint was studied by means of Optical Microscopy, Scanning Electron Microscope (SEM), Energy Dispersive X-ray (EDX), X-ray Diffraction (XRD). Results showed that dissimilar metals of Mg/Al could be successfully joined by CMT under proper processing parameters. The bonding strength of the joint was 34.7 MPa. A variety of Al–Cu intermetallic compounds, i.e. AlCu, CuAl₂, Cu₉Al₄, presented in the fusion zone of Al side, and Cu based solid solution was generated in weld zone, while Cu₂Mg and Al–Cu–Mg ternary eutectic structure was formed in the fusion zone of Mg side. The micro-hardness in the both sides of fusion zones increased sharply, which were 362 HV in Mg side and 260 HV in Al side. The joint was brittle fractured in the intermetallic compound layer of the fusion zone of Mg side, where plenty of Cu₂Mg intermetallic compounds were distributed continuously.     

Vacuum brazing of sapphire with Inconel 600 using Cu/Ni porous composite interlayer for gas pressure sensor application

In this research, sapphire as a ceramic was brazed to Inconel 600 as a metal with a commercially available Cusil ABA (63Ag–1.75Ti–35.25Cu) filler foil as braze alloy where Cu/Ni porous composite introduced as an interlayer so it could be used in a particular gas pressure sensor application. Several brazing processes were carried out in a high vacuum furnace in order to investigate the effects of brazing parameters on the joint interface and mechanical properties. The common brazing temperature and time were in the ranges of 830–900 °C and 15–30 min respectively, while vacuum pressure was remained constant at 1 × 10⁻⁴ Pa. SEM-EDS and XRD analyses of the joint microstructure and interface composition revealed five distinct phases; Ni₃Ti, AlNi, Cr_1.97Ti_1.07, Fe_0.2Ni_4.8Ti_5, (TiO_1.06)_3.32. The brazing area formed two “ocean” structures near to Inconel and sapphire interfaces whereas a reaction layer was developed at the surface of Inconel 600. Under the mechanical property analyses the brazed joint at 900 °C for 30 min obtained the maximum shear strength of 58.5 MPa which is adequate for particular gas pressure sensor application.     

Strength of deformation processed Cu–Fe–Ag in situ composites

Three Cu–Fe–Ag in situ composites, i.e. Cu–14 wt.%Fe–1 wt.%Ag, Cu–14 wt.%Fe–3 wt.%Ag and Cu–11 wt.%Fe–6 wt.%Ag, were prepared by cast and drawn process. Strength of the composites was investigated in detail. The results show that the presence of Ag can refine the primary Fe dendrites, leading to much higher strength of Cu–Fe–Ag composites, which reached 1578 and 1357 MPa for Cu–14Fe–3Ag and Cu–11Fe–6Ag respectively at η = 6.1, however that of the same processed Cu–12 wt.% Fe is only 978 MPa. Strength of Cu–Fe–Ag can be expressed as σ = 1319 × λ^− 1 / 2 + 57 × C_Fe–M^1 / 2 + Δσ_Ag, where λ is filament spacing, C_Fe–M is the Fe content in the matrix and Δσ_Ag is strengthening effect of Ag in the matrix. As Ag content is lower, Δσ_Ag = 100 × C_Ag ^1 / 2 and as it is near or beyond 6 wt.%, Δσ_Ag should be determined by specific experiment due to a second-stage work hardening of the matrix. The calculated strength of Cu–Fe–Ag composites fits the experimental results well.     

An investigation of residual stresses in brazed cubic boron nitride abrasive grains by finite element modelling and raman spectroscopy

Joining cubic boron nitride (CBN) abrasive grains and tool body made of steel using brazing always creates residual stress due to thermal mismatch of the components when cooling down from the brazing temperature. A large tensile stress perhaps causes grain fracture during the grinding process with single-layer brazed CBN abrasive tools. To evaluate the residual stresses occurring in brazed CBN grains, values and distribution of residual stresses are calculated using the finite element method. Effects of bonding materials, embedding depth, gap thickness and grain size on brazing-induced residual stresses are discussed. Results show that the Cu–Sn–Ti bonding alloy always results in a larger tensile stress in the CBN grains, when compared to Ag–Cu–Ti alloy during the cooling phase of the brazing process. The maximum tensile stress is obtained at the grain–bond junction region irrespective of the choice of bonding material and embedding depth. When the grain side length is 100 μm, gap thickness is 10 μm and grain embedding depth is 30%, the maximum magnitude of the tensile stresses is obtained. The maximum stress is 401 MPa with Ag–Cu–Ti alloy and 421 MPa with Cu–Sn–Ti alloy. The brazing-induced residual stresses have been finally measured experimentally by means of the Raman spectroscopy. The current simulated results are accordingly verified valid.     

Effect of aging temper on the thermal stability of Al–Cu–Mg–Ag heat-resistant alloy

The thermal stability of Al–Cu–Mg–Ag alloy was tested by thermal exposure and creep performance. The results show that the under-aged Al–Cu–Mg–Ag alloy possessed better thermal stability compared to the peak-aged one, due to the secondary precipitations of the strengthening phases. The number of the precipitations in the under-aged sample increased and then decreased with increasing the thermal exposure or creep time while that of the peak-aged sample decreased gradually. The tensile strength of the under-aged sample increased and then decreased with increasing the thermal exposure time with a peak value of 524 MPa after exposed for 20 h, which was 19 MPa higher than that of the peak-aged alloy. The steady creep rate of the under-aged sample was much lower and the creep-life was much longer to that of the peak-aged one, indicating that the under-aged sample possessed excellent creep resistance property at elevated temperatures.     

The application of equal channel angular pressing to join dissimilar metals,aluminium alloy and steel,using an Ag–Cu–Sn interlayer

Joining cylindrical and bar-shaped components manufactured from dissimilar materials is frequently required in various industrial applications. The current study focuses on developing equal channel angular pressing (ECAP) as a severe plastic deformation process for solid state joining of tubular aluminium alloy 6061 components and SAE 1018 steel rods. The influence of using a 0.1 mm thick 60Ag–30Cu–10Sn interlayer in addition to annealing at 220, 320, 420 and 520 °C for 60 min is investigated as well. Finite element analysis (FEA) is performed in order to evaluate the deformation behaviour of the workpieces during the ECAP joining process. XRD and EDX analyses as well as nanoindentation and shear tests are carried out to evaluate the joints' characteristics. The FEA outcomes show remarkable accumulation of equivalent plastic strain with relatively low strain inhomogeneity. Moreover, the experimental results indicate that with increasing annealing temperature, joint strength exhibits improvement as well. It is also revealed that the application of an interlayer at any specific annealing temperature leads to achieving higher shear strength values. According to the results, shear strength of up to 32 MPa is feasible by having an interlayer and with subsequent annealing at 520 °C.