Feng Jicai

Microstructure and strength of the SiC/TiAl joint brazed with Ag-Cu-Ti filler metal

SiC ceramics are considered one of the most promising structural materials for special applications. The development of bonding technology is widening the application field of SiC ceramics. There have been many reports on diffusion bonding and brazing of SiC ceramics to metals [1–3]. TiAl intermetallics have a great potential to become important candidates for advanced applications in aerospace and military industries. The researches on diffusion bonding and brazing of TiAl intermetallics to other materials have progressed in recent years [4–6]. The concept of utilizing ceramic, intermetallic and metallic materials to attain one complete armor system by bonding process is a recent approach for defeating armor projectiles [7]. Therefore, a previous study of diffusion bonding of SiC to TiAl was carried out [8]. This letter aims to demonstrate the feasibility of brazing of SiC to TiAl, and the focus is placed on the microstructures and strengths of the SiC/TiAl joints brazed with Ag-Cu-Ti filler metal. The materials used in experiments were cylindrical SiC rods (diameter 6 mm, height 4 mm), and cylindrical TiAl rods (diameter 10 mm, height 4 mm) with an average composition of Ti-43Al-1.7Cr-1.7Nb (at.%). The chemical composition of the Ag-Cu-Ti filler metal foils (thickness 20 μm) was Ag-27Cu-4.5Ti in weight percentage. The surfaces to be brazed were ground and polished through diamond paste and cleaned in ethanol and acetone prior to brazing. The coaxial SiC/TiAl assemblies were brazed at 1173 K for 5–40 min under a vacuum of 6.6 mPa in a vacuum furnace (Centorr-3520). The cross-sections of the brazed SiC/TiAl joints were prepared for metallographic analysis by standard polishing techniques. The microstructures of the SiC/TiAl joints were examined by scanning electron microscopy (SEM, S-570), electron probe X-ray microanalyzer (EPMA, JXA-8600) and X-ray diffractometer (XRD, JDX-3530M). The room-temperature shear strengths of the SiC/TiAl joints were evaluated by means of a specially-designed fixture in an electron tension testing machine (Instron-1186), and the average strength of the three joints brazed under the same conditions was used. Fig. 1 shows the back-scattered electron image of the cross-section of the SiC/TiAl joint brazed at 1173 K for 15 min. It can be found from the figure that three kinds of different microstructural zones have occurred in the brazing seam between SiC and TiAl. For the sake of convenience, these zones are marked by D, E and F, respectively. Fig. 2 shows the concentration profiles of major elements across the brazing seam of the SiC/TiAl joint brazed at 1173 K for 15 min. The AB line in the figure indicates the position analyzed by EPMA. Obviously, the distribution of each element across the brazing seam is not even. There is almost no Ag in E zone, and the concentration profile of Ag is undulating in D zone and flat in F zone. Cu exists in all zones, and the concentration of Cu in D zone is lower than in E zone and much higher than in F zone. There is almost neither Ti nor Al in D and F zones, and the concentration profiles of Ti and Al are both flat in E zone. These results reveal that D zone is an Ag-rich and Cu-rich one, and F zone is

Microstructure evolution and fracture behaviour for electron beam welding of Ti-6Al-4V

The effect of microstructural characteristics on fracture behaviour mechanism for electron beam welding of Ti-6Al-4V was investigated. The results indicated that the welded microstructure composed of coarse needle α + β phases presenting disordered and multidirectional short needle morphology to make fracture mechanism complex. The coarse grains in weld seam with microhardness 536 HV were easy to be fractured in the region where welding heat input was ≥ 68.8 kJ/m. There exists flat curves of Ti, Al and V, Fe concentration distribution fluctuation to cause microstructural amplitude-modulated decomposition to increase the joint ductility and cleavage strength. The uneven distribution of the partial micropores located at the interior of the specimen acting as crack initiation sites lead to non-linear branch propagating path. The α + β interlaced structure results in the fracture location near α/β interface. The existence of stacking fault structure caused pile-up of dislocation to produce micropores to be new fracture initiation sites.

Interface structure and formation mechanism of diffusion-bonded SiC/Ni-Cr joint

It is necessary to bond SiC ceramics to metals in order to expand the engineering applications of SiC ceramics. There have been some reports on the interface structures and reaction mechanisms of SiC ceramics to pure metals and intermetallics such as Ni [1–3], Cr [4, 5] and TiAl [6]. For SiC/Ni-Cr joints, however, only partial information is available and the understanding of the interface reaction is still missing. This letter aims to study the interface structure and to clarify the reaction process of diffusion-bonded SiC/Ni-Cr joints to lay a foundation for their practical use. The materials used in experiments were cylindrical SiC rods (diameter 6 mm, height 4 mm), and Ni-Cr alloy foils (thickness 0.2 mm) with a normal composition of Ni-25at.%Cr. The SiC/Ni-Cr/SiC couples were diffusion-bonded at 1273 K for 1.8–7.2 ks under a pressure of 7.2 MPa in a vacuum furnace equipped with a graphite heating tube. The cross-sections of diffusionbonded SiC/Ni-Cr joints were prepared for metallographic analysis by standard polishing techniques. The morphologies, crystal structures and chemical compositions of the formed phases were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe X-ray microanalysis (EPMA). Fig. 1 shows the back-scattered electron image of the cross-section of the SiC/Ni-Cr joint bonded at 1273 K for 3.6 ks. It can be seen from the figure that three kinds of reaction layers have occurred between SiC ceramic and Ni-Cr alloy. For the sake of convenience, the layer adjacent to SiC ceramic and the layer adjacent to Ni-Cr alloy are called A layer and C layer, respectively, and the layer between A and C is called B layer. Obviously, A layer and B layer are composed of duplex phases, and C layer is a single-phase one. Fig. 2 shows the XRD patterns from the revealed surfaces, parallel to the original bonding interface, of the reaction layers of the SiC/Ni-Cr joint bonded at 1273 K for 3.6 ks. Obviously, the phases identified from the revealed surface on the SiC side are SiC, Ni2Si and graphite where SiC is an original phase existing in the SiC base material. The phases identified from the revealed surface on the Ni-Cr side are Ni(ss.Cr), Ni5Cr3Si2 and Cr3Ni2SiC where Ni(ss.Cr) is the solid solution of Cr in Ni and is the original phase in the Ni-Cr base material. Therefore, four kinds of new reaction phases have formed during the diffusion bonding of SiC ceramic to Ni-Cr alloy. They are orthorhombic Ni2Si, hexagonal graphite, cubic Ni5Cr3Si2 and cubic Cr3Ni2SiC. Table I shows the chemical compositions of each phase in the reaction layers. Based on the XRD results mentioned above, the synthetical analysis to stoichiometric compositions of each phase indicates that the white and black phases in A layer are Ni2Si and graphite, respectively; the grey and blackish phases in B layer are Ni5Cr3Si2 and Cr3Ni2SiC, respectively; and the grey phase in C layer is also Ni5Cr3Si2. In other words, A layer, B layer and C layer are composed of

Microstructure and Properties of Electron Beam Welded Tantalum-to-Stainless Steel Joints

Abstract The microstructure, defect characteristics, and mechanical properties of electron beam welded (EBW) tantalum-to-stainless steel joints were investigated. Joint formation was influenced by the position of the EBW heat source. with a well-formed joint was obtained when the heat source was deviated by 0.2 mm towards the stainless steel. The weld is composed of fine dendrites with high elemental Fe content and homogenously-dispersed ɛ (Fe 2 Ta) and μ (FeTa or Fe 7 Ta 6 ) phases. However, ɛ and μ phases formed at the tantalum fusion line had lamellar distribution and micro-cracks are easily formed in this region under welding stress. Although the weld zone has high hardness, the poor deformation compatibility of the fusion line at the tantalum side and the existence of crack sources lead to fracturing during the tensile test. The tensile strength of the joints was found to be low, 255 MPa.

Reaction-Diffusion Bonding of High Nb Containing TiAl Alloy

Abstract In this study, high Nb containing TiAl alloy (TAN) was welded by reaction-diffusion bonding using Al foil as interlayer at 1200 °C for 2 h. The desired full lamellar microstructure ( γ +α 2 ) was obtained across the bonding seam when the joint was heat-treated according to the heat treatment process of TAN alloy. The present paper also investigated microstructure and bonding mechanism in details. The results show that liquid Al can react with TAN to form TiAl 3 intermetallic layer, and then the TiAl 3 layer will transform to γ -TiAl by solid-state diffusion at high temperature, finally a full lamellar structure is formed after heat treatment. Furthermore, sound joints with full lamellar structure can be also obtained when TAN alloy is directly bonded in accordance with the heat treatment cycle of TAN alloy.

复合活性钎料钎焊Cu与Al 2 O 3 的接头组织及性能

为改善紫铜与Al2O3陶瓷的连接强度，采用纳米-Al2O3增强的AgCuTi复合钎料(AgCuTip)对紫铜与Al2O3陶瓷进行了真空钎焊.采用扫描电镜、能谱分析以及剪切试验对钎焊接头微观组织及力学性能进行了分析.钎焊接头典型界面组织为紫铜/扩散层/铜基固溶体+银基固溶体+ Ti2Cu + Ti3(Cu, Al)3O/Al2O3.纳米-Al2O3的添加抑制了Al2O3侧反应层的生长，并促进钎缝中形成弥散分布的Ti2Cu相.随着保温时间的延长，铜侧扩散层和Ti3(Cu, Al)3O反应层的厚度逐渐增大.保温时间为20 min时，铜母材向钎料过度溶解，降低了接头性能.当钎焊温度为880 °C，保温10 min时，接头抗剪强度最高为82 MPa.纳米颗粒的加入细化了钎缝组织并降低了母材与钎缝热膨胀系数的不匹配，因此提高了接头的连接性能.保温时间可影响界面组织及反应层的厚度，进而影响接头的连接强度.

Mechanism and Process of the Intermetallic Compound Particles Reinforced TiAl/Steel Brazing Seam

Abstract In order to prevent the formation of large size intermetallic compounds during the TiAl-based alloy brazing, a specially designed heating process, which was established based on the analysis of Ti-Al-Ag ternary phase diagram, was presented and validated. The associated mechanism and process were proposed as follows: (i) a special dwell period was installed at the early stage during the melting of the filler metal for the generation of a weak primary AlCu 2 Ti intermetallic layer; (ii) with initial heating up, Ag atoms in the filler would diffuse into the TiAl substrate through the weak AlCu 2 Ti layer and a new liquid phase of Ti-Al-Ag would form due to the combined effects of thermal disturbance, convection and concentration gradient; (iii) the primary intermetallic compounds would be broken into irregular particles and pieces which were pushed into the molten filler; (iv) with heating up to the peak temperature, the intermetallic particles would disperse uniformly in the brazing seam, resulting in a brazing seam with a dispersive distribution of the intermetallic compound particles. To verify the feasibility of the proposed method, the vacuum brazing of a TiAl-based alloy and a 42CrMo steel was conducted within a temperature range of 1033∼1173 K and a brazing time range of 100∼300 s. Microstructures of the brazed joints were examined by optical microscopy, scanning electron microscopy (SEM) as well as energy dispersive spectrum (EDS). The results show that an intermetallics particles reinforced TiAl/steel brazing seam can be obtained by the proposed heating process.