Binggang Zhang

Microstructure and strength of diffusion-bonded joints of TiAl base alloy to steel

Abstract Diffusion bonding of TiAl-based alloy to steel was carried out at 850–1100 °C for 1–60 min under a pressure of 5–40 MPa in this paper. The relationship of the bond parameters and tensile strength of the joints was discussed, and the optimum bond parameters were obtained. When products are diffusion-bonded, the optimum bond parameters are as follows: bonding temperature is 930–960 °C, bonding pressure is 20–25 MPa, bonding time is 5–6 min. The maximum tensile strength of the joint is 170–185 MPa. The reaction products and the interface structures of the joints were investigated by scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA) and X-ray diffraction (XRD). Three kinds of reaction products were observed to have formed during the diffusion bonding of TiAl-based alloy to steel, namely Ti3Al+FeAl+FeAl2 intermetallic compounds formed close to the TiAl-based alloy. A decarbonised layer formed close to the steel and a face-centered cubic TiC formed in the middle. The interface structure of diffusion-bonded TiAl/steel joints is TiAl/Ti3Al+FeAl+FeAl2/TiC/decarbonised layer/steel, and this structure will not change with bond time once it forms. The formation of the intermetallic compounds results in the embrittlement of the joint and poor joint properties. The thickness of each reaction layer increases with bonding time according to a parabolic law. The activation energy Q and the growth velocity K0 of the reacting layer Ti3Al+FeAl+FeAl2+TiC in the diffusion-bonded joints of TiAl base alloy to steel are 203 kJ/mol and 6.07 mm2/s, respectively. Careful control of the growth of the reacting layer Ti3Al+FeAl+FeAl2+TiC can influence the final joint strength.

Electron beam welding of Ti-15-3 titanium alloy to 304 stainless steel with copper interlayer sheet

Electron beam welding of Ti-15-3 titanium alloy to 304 stainless steel with a copper sheet as interlayer was carried out. Microstructures of the joint were studied by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffractometry (XRD). In addition, the mechanical properties of the joint were evaluated by tensile test and the microhardness was measured. These two alloys were successfully welded by adding copper transition layer into the weld. Solid solution with a certain thickness was located at the interfaces between weld and base metal in both sides. Regions inside the weld and near the stainless steel were characterized by solid solution of copper with TiFe2 intermetallics dispersedly distributed in it. While weld near titanium alloy contained Ti-Cu and Ti-Fe-Cu intermetallics layer, in which the hardness of weld came to the highest value. Brittle fracture occurred in the intermetallics layer when the joint was stretched.

Temperature and stress fields in electron beam welded Ti-15-3 alloy to 304 stainless steel joint with copper interlayer sheet

Abstract Electron beam welding of Ti-15-3 alloy to 304 stainless steel (STS) using a copper filler metal was carried out. The temperature fields and stress distributions in the Ti/Fe and Ti/Cu/Fe joint during the welding process were numerically simulated and experimentally measured. The results show that the rotated parabola body heat source is fit for the simulation of the electron beam welding. The temperature distribution is asymmetric along the weld center and the temperature in the titanium alloy plate is higher than that in the 304 STS plate. The thermal stress also appears to be in asymmetric distribution. The residual tensile stress mainly exists in the weld at the 304 STS side. The copper filler metal decreases the peak temperature and temperature grade in the joint as well as the residual stress. The longitudinal and lateral residual tensile strengths reduce by 66 MPa and 31 MPa, respectively. From the temperature and residual stress, it is concluded that copper is a good filler metal candidate for the electron beam welding of Ti-15-3 titanium alloy to 304 stainless steel.

Fourier transformation infrared spectrum studies on the role of fluorine in SnO2:F films

In this letter we employed Fourier transform infrared spectroscopy (FTIR) to study fluorine-doped SnO2 films deposited by spray pyrolysis. The role of oxygen vacancy and substitution of fluorine for oxygen are demonstrated by FTIR. It is found that at low doping levels, fluorine ions prefer to occupy oxygen positions in SnO2 lattice. While beyond a certain doping level, fluorine ions start to occupy interstitial positions, which has a negative effect on carrier concentration that, in turn, affects the infrared reflectivity of SnO2 films. FTIR also shows the increase of the disorder of SnO2 films with increasing fluorine doping.

Influences of different filler metals on electron beam welding of titanium alloy to stainless steel

Electron beam welding experiments of titanium alloy to stainless steel were carried out with different filler metals, such as Ni, V, and Cu. Microstructures of the joints were examined by optical microscopy, scanning electron microscopy and X-ray diffraction analysis. Mechanical properties of the joints were evaluated according to tensile strength and microhardness. As a result, influences of filler metals on microstructures and mechanical properties of electron beam welded titanium-stainless steel joints were discussed. The results showed that all the filler metals were helpful to restrain the Ti-Fe intermetallics. The welds with different filler metals were all characterized by solid solution and interfacial intermetallics. For each type of the filler metal, the type of solid solution and interfacial intermetallics depended on the metallurgical reactions between the filler metals and base metals. The interfacial intermetallics were Fe2Ti+Ni3Ti+NiTi2, TiFe, and Cu2Ti+CuTi+CuTi2 in the joints welded with Ni, V, and Cu filler metals, respectively. The tensile strengths of the joints were dependent on the hardness of the interfacial intermetallics. The joint welded with Ag filler metal had the highest tensile strength, which is about 310 MPa.

Influence of electron-beam superposition welding on intermetallic layer of Cu/Ti joint

QCr0.8 was electron-beam welded to TC4 and the effect of the intermetallic layer (IMC-layer) on the mechanical properties of the joint was investigated. The IMC-layers are joint weaknesses at the Cu fusion line in centered welding and at the Ti fusion line when the beam is deviated towards Cu. A new method referred to as electron-beam superposition welding was presented, and the optimal welding sequence was considered. The IMC-layer produced by centered welding was fragmented and remelted during Cu-side non-centered welding, giving a finely structured compound layer and improved mechanical properties of the joint. The tensile strength of joint is 276.0 MPa, 76.7% that of the base metal.

Influence of Aluminum Content on the Microstructure and Properties of Electron Beam Welded Joints of TiAl/TC4 Alloy

Abstract Electron beam welding experiments of TiAl and TC4 titanium alloy under different welding parameters were carried out. The influence of aluminum content in the weld on the microstructure and mechanical properties of the joints were analyzed. The weld mainly consisted of the α 2 -Ti 3 Al phase and α-Ti phase, with a small quantity of B2 phase and YAl x phase when the beam acted on the contact face. In the joints the content of molten TiAl and TC4 was about 1/3 and 2/3, and the percentage of Al was about 28 wt%. This high content of Al led to the formation of a brittle α 2 phase, thus the ductility and toughness of the joints decreasing. In the centered beam welding, the tensile strength of the joints was low. The fracture of the joints was the classic brittle transgranular and quasi-cleavage fracture. When the beam was deflected towards the TC4 alloy, the Al content in the weld decreased and the mechanical properties of the joints improved. When the lateral deviation h s was 0.2 mm, the highest tensile strength of the joints reached 422.2 MPa.

Microstructure and defect of titanium alloy electron beam deep penetration welded joint

The microstructure, phase composition and cold shut defect of thick titanium alloy electron beam welded joint were studied. The results showed that the microstructure of weld zone was composed of α′ phase; the heat affected zone was divided into fine-grained zone and coarse-grained zone, the microstructure of fine-grained zone was primary α phase + β phase + equiaxed α phase, and the microstructure of coarse-grained zone was primary α phase + acicular α′ phase; the microstructure of base metal zone basically consisted of primary α phase, and a small amount of residual β phase sprinkled. The forming reason of cold shut was analyzed, and the precaution of cold shut was proposed.

Electron beam welding of 304 stainless steel to QCr0.8 copper alloy with copper filler wire

Abstract Electron beam welding (EBW) of 304 stainless steel to QCr0.8 copper alloy with copper filler wire was carried out. Orthogonal experiment was performed to investigate the effects of process parameters on the tensile strength of the joints, and the process parameters were optimized. The optimum process parameters are as follows: beam current of 30 mA, welding speed of 100 mm/min, wire feed rate of 1 m/min and beam offset of −0.3 mm. The microstructures of the optimum joint were studied. The results indicate that the weld is mainly composed of dendritic α phase with little globular α phase, and copper inhomogeneity only occurs at the top of the fusion zone. In addition, a melted region without mixing exists near the weld junction of copper side. This region with a coarser grain size is the weakest section of the joints. It is found that the microhardness of the weld decreases with the increase of the copper content in solid solution. The highest tensile strength of the joint is 276 MPa.

Structure and mechanical properties of aluminum alloy/Ag interlayer/steel non-centered electron beam welded joints

Electron beam welding was carried out between aluminum alloy and steel with Ag interlayer. Seam morphology, structure and mechanical properties of the joints were investigated with different action positions of the electron beam spot. The results show that with the increment of the beam offset to the silver side from the interface between silver and steel, the seam morphology was improved, and the porosity in the Ag interlayer vanished. A transition layer mainly composed of Ag2Al and Al eutectic was formed at the interface between silver and aluminum, and became thin and spiccato as the beam offset increased. When the beam offset was too large, two IMC layers composed of FeAl and FeAl3 respectively were formed at the interface between steel and Ag interlayer. The optimal beam offset was 0.2 mm, and the maximum tensile strength of the joint was 193 MPa, 88.9% that of the aluminum alloy, and the fracture occurred at the interface between steel and Ag interlayer.