•Super-Ni/NiCr laminated composites were successfully diffusion bonded to Ti-6Al-4V by designing two configurations.•The width of each interfacial layer was directly proportional to the square root of holding time at a certain temperature.•The property of joint is influenced by the formation of intermetallic compounds and interfacial composition homogenization.Transient liquid phase (TLP) diffusion bonding super-Ni/NiCr laminated composite to Ti-6Al-4V alloy was performed at 950 °C with the pressure of 5 MPa. Two configurations (super-Ni/Cu/Ti-6Al-4V and Ni80Cr20/Cu/Ti-6Al-4V) were designed to explore interfacial microstructure evolution and mechanical properties. Results showed that the joint of laminated composite and Ti-6Al-4V alloy was mainly divided into two diffusion layers and a reaction layer. The interface was composed of Ni(Cu), Cu(Ni), Ti2Cu, TiCu, Ti3Cu4, Ti2Cu3 and Ti-Cu eutectic microstructure. The width of each layer increased with prolonging the bonding time and the growth rate of interfacial layers was dominated by the diffusion rate of alloying elements i.e. Cu, Ni and Ti. The maximum value of shear strength for both two configurations was about 58 MPa.Download high-res image (395KB)Download full-size image

•Super-Ni/NiCr laminated composite joined with Ti-6Al-4V by diffusion bonding without interlayer at 950 °C.•The diffusion of Ni and Ti contributed to the formation of interfacial Ni3Ti, NiTi and NiTi2 layers.•The morphology of NiTi2 layer strongly depended on the bonding time.•The maximum shear strength of joints is 69.2 MPa obtained at 60 min.Super-Ni/NiCr laminated composite was successfully joined with Ti-6Al-4V by vacuum diffusion bonding at 950 °C temperature. Results show that the bonding interface was composed of sequentially Ni3Ti, NiTi and NiTi2 layers. The interfacial phase constituents were similar although the joints were obtained with different bonding time. But the bonding time had a significant effect on the interfacial microstructure when the bonding time was prolonged from 30 min to 90 min. The morphology of NiTi2 layer transformed from serrate to straight and eutectoid products (Ni3Ti and NiTi2) were formed in NiTi layer whose width increased significantly. The maximum shear strength of joint is 69.2 MPa obtained at 60 min. Most joints fractured at the super-Ni/Ni3Ti interface. The shear strength of joint is mainly attributed to the plastic deformation and shifting of super-Ni crystals.

Butt welding of titanium alloy TA15 to aluminum alloy Al2024 dissimilar lightweight metals was conducted using gas tungsten arc welding. Pulsed current was adopted in the welding process. Influence of pulsed current on morphologies and microstructure of Ti-Al intermetallics near the Ti/Al interface was investigated. Microstructure characteristics and phase constitution of weld zone near the Ti/Al interface were analyzed. In top surface and upper region of the joint, Ti base metal was partially melted, and continuous intermetallic layers with Ti3Al, TiAl, and TiAl3 were formed in the fusion zone. In middle and bottom regions of the joint, Ti base metal was not melted and a thin TiAl3 layer was formed near the Ti/Al-brazed interface. Most of the Ti-Al intermetallics formed into discrete TiAl3 precipitations in the weld metal in upper and middle regions of the joint. No precipitation was observed in bottom region of the joint. Thickness of continuous Ti-Al intermetallic layers in the fusion zone was controlled at a low degree by adopting pulsed current in the welding. Crack sensitivity of weld zone near the Ti/Al interface was decreased.

Vacuum brazing of super-Ni/NiCr laminated composite and Cr18-Ni8 stainless steel was carried out using Ni-Cr-Si-B amorphous filler metal at 1060, 1080, and 1100 °C, respectively. Microstructure and phase constitution were investigated by means of optical and scanning electron microscopy, energy-dispersive spectroscopy, x-ray diffraction, and micro-hardness tester. When brazed at 1060-1080 °C, the brazed region can be divided into two distinct zones: isothermally solidified zone (ISZ) consisting of γ-Ni solid solution and athermally solidified zone (ASZ) consisting of Cr-rich borides. Micro-hardness of the Cr-rich borides formed in the ASZ was as high as 809 HV50 g. ASZ decreased with increase of the brazing temperature. Isothermal solidification occurred sufficiently at 1100 °C and an excellent joint composed of γ-Ni solid solution formed. The segregation of boron from ISZ to residual liquid phase is the reason of Cr-rich borides formed in ASZ. The formation of secondary precipitates in diffusion-affected zone is mainly controlled by diffusion of B.

Vacuum brazing of super-Ni/NiCr laminated composite and Cr18–Ni8 steel was performed using NiCrP filler metal. Microstructure, elemental distribution, microhardness and shear strength were investigated by means of scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and electromechanical universal testing machine. An excellent joint was obtained at 1040 °C for 20 min and the shear strength was as high as 137 MPa. The brazed region was divided into solid solution zone and eutectic zone. Elemental distribution indicated that P mainly concentrated in the eutectic zone in the form of Ni3P. Ni2Cr particles precipitated in NiCr base layer under the influence of thermal cycling.Highlights► Vacuum brazing of super-Ni/NiCr laminated composite and Cr18–Ni8 steel with NiCrP filler was performed at 940, 980 and 1040 °C for 20 min. ► Excellent joint was obtained at 1040 °C for 20 min and the shear strength was 137 MPa. ► Brazed region consisted of γ-Ni solid solution and eutectic of γ-Ni(P) and Ni3P. ► There was Ni2Cr particles precipitated in NiCr base layer with the influence of heating cycle. ► Due to the same matrix of filler metal and super-Ni/NiCr laminated composite, interfacial region between them is not clearly observed.

Vacuum brazing of Ni–NiCr laminated composite to Cr18–Ni8 steel was carried out using Ni–Cr–Si–B amorphous foil. Microstructure characteristic of the brazed joints was investigated by scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), field-emission scanning electron microscope (FE-SEM) and microsclerometer. Ni–Cr–Si–B amorphous foil exhibited good wettability on Ni–NiCr laminated composite. The brazed region consisted of γ-Ni solid solution and Ni–B eutectic. An excellent bonding with shear of 170 MPa was obtained. An interface of Ni3B precipitated in Ni cover layer near the brazed region. Microhardness of the interface was 500 HV. There was an interface consisting of fine boride granules in Cr18–Ni8 steel close to the brazed region and microhardness of the interface was 250 HV. Bonding behavior of the brazed joint was described according to the microstructure characteristic.Highlights► Ni–NiCr laminated composite is an attractive candidate for the modern aero-engines. ► Ni–NiCr laminate was brazed to Cr18-Ni8 steel with Ni–Cr–Si–B amorphous foil. ► The brazed region consisted of γ-Ni solid solution and Ni–B divorced eutectic. ► Blocky Ni3B and (Fe, Cr)B granules precipitated near the brazed region. ► Shear strength of the brazed joint was 170 MPa.

The welding of Mo–Cu composite and 18-8 stainless steel was carried out by Tungsten Inert Gas welding process with Cr–Ni fillet wires. The microstructure, element distribution, phase constituents and microhardness of the joint were analysed. The results indicate that austenite and ferrite phases were obtained in the weld metal. Austenite and delta ferrite structures were observed in the fusion zone near 18-8 stainless steel. Cu agglomeration regions formed in Mo–Cu composite heat affected zone during welding. The microhardness near the fusion zone at Mo–Cu composite side increased from weld metal to fusion zone, and the peak value appeared near the boundary between fusion zone and Mo–Cu composite due to the generation of high hardness and brittleness Fe–Mo intermetallic compounds. The phase constituents near the fusion zone at Mo–Cu composite are Mo, Cu and γ-Fe (Ni), Cu3.8Ni and the Fe–Mo compound Fe0.54Mo0.73.

The microstructure and phase constitution near the diffusion bonding interface of Mg/Al dissimilar materials are studied using a scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The test results indicated that an obvious diffusion zone was formed near the Mg/Al interface during the vacuum diffusion bonding. The diffusion transition zone near the interface consists of various MgxAly phases. The transition region on the Mg side mainly consists of Mg crystals, and the new phase formed was the Mg3Al2 phase having a face-centred cubic lattice. This is favorable for improving the combined strength of Mg substrate and diffusion transition zone.

The microstructure near a diffusion interface was studied by means of scanning electron microscopy and electron probe microscopy, and the results indicated that the interface transition zone of Fe3Al/Q235 dissimilar materials was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the sides of the interface. Microstructures of the interface and base materials were interlaced to form the microstructure of layer characteristic. With increased heating temperature and holding time, the width of the Fe3Al/Q235 interface transition zone increased and the microstructure gradually became coarse. The microhardness in the diffusion transition zone was decreased and there was a peak value at the diffusion interface. The distribution of Al, Fe, and Cr in the interface transition zone was increased or decreased monotonically with some local concentration fluctuation. There was nearly no change in the concentration of C element near the interface.

The distribution of hydrogen in the welding zone of HQ130 high strength steel is calculated by using finite element method (FEM). The finite element program of hydrogen in the welding zone is worked out. In the program, the effects of welding heat input (q  /vv), temperature and surface-escaping coefficient of hydrogen, etc., are taken into account. Crack morphology in the fusion zone and effect of diffusion hydrogen on cracks are analyzed. Cracks originated in the partially melted zone at about 20–60 μm from the melting interface line and propagated parallel to the fusion zone or turned into the weld metal. Hydrogen accumulates seriously near the fusion zone, particularly at the root fusion zone. This is one of the importance reasons for the forming of cracks in this zone. The test results and analysis indicate that welding heat input should be controlled about q  /vv = 16.0 kJ/cm to prevent cracks being produced and propagated for HQ130 steel.

•Nickel composite coatings were modified by combining Ti addition with wide-band laser cladding.•Star-like Cr5B3 and eutectic M23C6/γ-Ni were in-situ synthesized in Ni60/WC coating.•Net structure TiC increased the nucleation rate of Cr5B3 and homogenized the particle size.•Microhardness became more uniform and wear resistance was significantly improved due to Ti addition.Ni60/WC composite coatings were fabricated by wide-band laser cladding. The effects of Ti addition on microstructure homogenization and coating properties were investigated. Coating microstructure, phase constitution, microhardness and wear resistance were studied and grading analysis of in-situ synthesized ceramic particles was carried out. Results indicated that ceramics particles of Cr5B3 and M23C6 (M represents for Cr and W) carbides were in-situ synthesized in original Ni60-20WC coatings. With Ti addition, dissolution of original WC was facilitated and lots of TiC particles were synthesized instead of M23C6 carbides. Furthermore, the block Cr5B3 particles were greatly homogenized due to the net structure formed by dispersive TiC particles. With Ti addition, D50 of particle size decreased from 8.94 μm to 4.45 μm and particle morphologies were transformed from star-like shapes to uniform square blocks. Microhardness distribution became more uniform with average value decreased from 799 ± 89 HV0.2 to 744 ± 77 HV0.2. Due to the homogenized ceramic particles, wear resistance of coatings with Ti addition was enhanced to 2.6 times that of the original coatings.