Jian Cao

Welding and Joining of Titanium Aluminides

Welding and joining of titanium aluminides is the key to making them more attractive in industrial fields. The purpose of this review is to provide a comprehensive overview of recent progress in welding and joining of titanium aluminides, as well as to introduce current research and application. The possible methods available for titanium aluminides involve brazing, diffusion bonding, fusion welding, friction welding and reactive joining. Of the numerous methods, solid-state diffusion bonding and vacuum brazing have been most heavily investigated for producing reliable joints. The current state of understanding and development of every welding and joining method for titanium aluminides is addressed respectively. The focus is on the fundamental understanding of microstructure characteristics and processing–microstructure–property relationships in the welding and joining of titanium aluminides to themselves and to other materials.

Effect of Silver Content on Microstructure and Properties of Brass/steel Induction Brazing Joint Using Ag-Cu-Zn-Sn Filler Metal

The induction brazing of brass to steel using Ag-Cu-Zn-Sn filler metal was investigated in this study. The influence of Ag content on the microstructure and properties were analyzed by means of optical microscopy, scanning electron microscopy and electron probe microanalysis. Defect free joint was achieved using Ag-Cu-Zn-Sn filler metal. The microstructure of the joint was mainly composed of Ag-based solid solution and Cu-based solid solution. The increase of Ag content and the cooling rate both led to the increase of the needle like eutectic structure. The tensile strength decreased with the increase of Ag content. The tensile strength at room temperature using Ag25CuZnSn filler metal reached 445 MPa. All fractures using Ag-Cu-Zn-Sn filler metal presented ductile characteristic.

Joining of SiO2–BN ceramic to Nb using a CNT-reinforced brazing alloy

Brazing SiO2–BN ceramics with Nb are often associated with the problems of high residual stress caused by the difference in the thermal expansion coefficients and poor mechanical properties under high temperature. To overcome these problems, here we report a new type of carbon nanotube (CNT)-reinforced TiNi brazing alloy via an “in situ growth” method by PECVD. The CNT/TiNi brazing alloy has very homogeneously dispersed CNTs within the TiNi brazing alloy, produced by in situ growth of CNTs on TiH2 and Ni powders mixed evenly into the CNT/TiH2 powders. It can be applied in the brazing of SiO2–BN and Nb. Results show that the addition of CNTs into the TiNi brazing alloy is very beneficial for the dissolution and diffusion of Nb in the brazed joint, because it can promote more TiNi–(Nb,Ti) eutectics which emerge in most of the brazing seam. Furthermore, the average shear strength of the brazed joint at room temperature is raised from 49 to 85 MPa with 1.5 vol% CNTs added into the TiNi brazing alloy. In particular, at 800 °C the brazed joint still has a very high shear strength of 51 MPa, about 1.7 times stronger than that of the TiNi brazing alloy. These results suggest that the CNTs played a key role in reducing the residual stress and reinforcing the mechanical properties (at both room and high temperature) of the brazed joint. It provides a way for the development of CNT-reinforced brazing alloys.

Anisotropic growth of Ni3(BO3)2 nanowhiskers on nickel substrates and its application in the fabrication of superhydrophilic surfaces

One-dimensional (1D) single-crystalline nickel borate [Ni3(BO3)2] nanowhiskers were successfully grown on Ni substrates using the facile molten-salt method in air with MnO2 as the agent. The products were characterized via X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The Ni3(BO3)2 nanowhiskers that were synthesized at 950 °C, with diameters ranging from 120 to 250 nm and lengths of up to 50 μm, possessed an ultra-high aspect ratio (more than 100u2006:u20061) and were found to grow along the [021] crystallographic direction. The effects of the growth temperature, holding time and amount of agent on the product morphology were investigated in detail. The products retained a nanowhisker morphology at growth temperatures below 1000 °C but transformed into microtubes when synthesized at 1050 °C. The oxygen liberated by the manganese oxides facilitated the nucleation of Ni3(BO3)2 nanowhiskers. In addition, the surfaces coated with the Ni3(BO3)2 nanowhiskers exhibited good superhydrophilic properties, and the wetting dynamics of such surfaces was investigated.

One-dimensional nickel borate nanowhiskers: characterization, properties, and a novel application in materials bonding

The growth of nickel borate [Ni3(BO3)2] nanowhiskers was successfully achieved by simply heating a mixture of nickel and boron oxide (B2O3) powders at 950 °C in air. The products were characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The synthesized nanowhiskers, with diameters of 150–500 nm and lengths of 10–30 μm, possessed high aspect ratio, and were found to grow along the [012] crystallographic direction. Stacking faults, hollow interiors, and high-energy facets were found occasionally. The high-temperature stability of Ni3(BO3)2 nanowhiskers was investigated. The nanowhiskers partially decomposed after being treated at 1200 °C. Based on the solid–liquid–solid mechanism, the possible growth process of Ni3(BO3)2 nanowhiskers was discussed. Ni3(BO3)2 nanowhiskers were successfully used for bonding nickel in air. The nickel/nanowhisker/nickel joint was characterized in detail by scanning electron microscopy and laser scanning confocal microscopy.

Effect of catalyst film thickness on the structures of vertically-oriented few-layer graphene grown by PECVD

The growth behavior of vertically-oriented few-layer graphene (V-FLG), produced by radio-frequency plasma enhanced chemical vapor deposition (PECVD), is studied here as a function of catalyst and catalyst film thickness. The Co catalyst and catalyst film thickness were observed as being crucial for the growth states and morphology of V-FLG. V-FLG showed a slower growth rate and lower density on substrates without catalyst than with catalyst due to the V-FLG nucleation on the former being prevented. It demonstrates that the growth mechanism of V-FLG on the Co catalyst film is not similar to the catalytic growth mechanism of CNTs, which is a defect nucleation mechanism. During PECVD process, the highly reactive C radicals can preferentially form CNTs or graphite-shell by Co catalysis, then trigger nucleation on the surface of CNTs or graphite-shell for V-FLG growth. The excellent field emission properties of the V-FLG and FLG-CNTs make them promising candidates for high-performance field emission emitters.