Microstructure and strength of TiC cermet/cast iron brazed with Ag – Cu – Zn filler metal

Abstract

The brazing of TiC cermet to cast iron was carried out at 1223 K for 5 – 30 min using Ag – Cu – Zn filler metal. The formation phases, interface structures and shear strengths of the joints were investigated. The experiment result and analysis identify that three new phases, namely Cu base solid solution, Ag base solid solution and (Fe, Ni) have formed during the brazing of TiC cermet to iron. The interface structure of the joints can be expressed as TiC cermet/Cu base solid solution/Ag base solid solution + a little Cu base solid solution/Cu base solid solution + (Fe, Ni)/cast iron. The highest shear strength of the joints is 292.0 MPa, obtained with a brazing time of 20 min.     

Effect of Reaction Layers on the Residual Stress of the Brazed TiC Cermets/Steel Joints

For the first time, considering the effect of reaction layers, numerical simulation calculation of residual stress on brazed TiC cermets/steel joint was studied by finite element method (FEM). The calculation results show that, when the joint is brazed at 1123 K for 300 s (low brazing parameters), the maximum shear stress value occurs on (Cu, Ni) layer near TiC cermets, which is 92.16 MPa as the temperature is 300 K. When the joint is brazed at 1273 K for 900 s (high brazing parameters), the maximum shear stress value occurs on (Cu,Ni)+(Fe, Ni) layer, which is 39.18 MPa as the temperature is 300 K. The fracture sites of the joints obtained from numerical simulation calculation accord with experimental results.     

Ti/Nb作中间层脉冲加压扩散连接TiC金属陶瓷与不锈钢

用Ti/Nb作中间层,在温度890℃、时间4~12min、脉冲压力2~10MPa、频率f=0.5Hz、恒压10MPa下,对TiC金属陶瓷和304不锈钢(304SS)进行脉冲加压与恒压扩散焊,获得了牢固的固相扩散焊接头。通过扫描电镜SEM、能谱EDS、X射线衍射XRD与剪切性能测试,对接头的显微组织、界面产物与强度进行分析。结果显示:两种接头的界面物相相似,主要有σ相,(β-Ti,Nb)与α+β-Ti固溶体。连接时间10min时,恒压下的TiC/304SS接头抗剪强度为55.6MPa,而脉冲加压下的接头抗剪强度达110MPa。恒压下接头断裂方式为TiC陶瓷断裂,而脉冲压力下接头断裂方式为TiC陶瓷与界面产物间交替进行的混合断裂。     

Simulation of the shear-tensile mode transition on dynamic crack propagations

We propose an approach to the simulation of the shear-tensile transition in dynamic crack growth based on two points: a new crack propagation criterion which is suitable for shear, and an algorithm which is capable of handling the transition from shear mode to tensile mode and back in the same simulation. The new crack propagation criterion for brittle crack growth is based on the maximum shear stress rather than the maximum hoop stress. The shear stress direction becomes the new crack??s direction in which propagation is initiated for shear-type failure. The stress state at the crack??s tip is obtained through a local approach which can be used even in the case of extensive plasticity. Additionally, we propose to control the transition from shear mode to tensile mode during the simulation of crack propagation using an equivalent strain estimated at the crack??s tip. Depending on a threshold strain, the propagation direction is predicted using the maximum shear stress (in the shear case) or the maximum hoop stress (in the tensile case).     

TiC陶瓷/NiCrSiB/铸铁钎焊连接的界面组织和强度分析

采用NiCrSiB钎料对TiC陶瓷与铸铁进行钎焊连接,分析了接头的界面组织和剪切强度.结果表明:当连接规范一定时,在钎料内部、钎料与母材的界面处有TiC从TiC陶瓷侧扩散过来,同时在钎料内部和界面处有[Ni,Fe]和Ni基固溶体生成.当连接温度为1373K,连接时间为20 min时,接头的剪切强度最高可达78.6 MPa.     

Development and partial relaxation of internal stresses in thin TiC layers chemically vapour deposited on Fe-C substrates

TiC layers were chemically vapour deposited at 1273 K mn Fe-C substrates with carbon contents between 0.06 and 1.20w%C. X-ray diffraction stress analyses showed that large compressive stresses are present in the IC coatings and that small tensile stresses any present in the substrates. The stresses developed during cooling from the deposition temperature to room temperature, owing to the difference in thermal shrink between coating and substrate. However, stress relaxation was also evident. This was provoked by the phase transformation processes occuring in the substrate on cooling. Stress relaxation was hindered when grain-boundary cementite formed in the substrates. The stresses present in the TiC coatings on substrates without grain-boundary cementite can be predicted quantitatively.     

Effect of material and geometric parameters on deformations near the notch-tip of a dynamically loaded prenotched plate

We analyze plane strain thermomechanical deformations of a prenotched rectangular plate impacted on one side by a prismatic body of rectangular cross-section and moving parallel to the axis of the notch. Both the plate and the projectile are made of the same material. Strain hardening, strain-rate hardening and thermal softening characteristics of the material are modeled by the Johnson–Cook relation. The effect of different material parameters, notch-tip radius, impact speed and the length of the projectile on the maximum tensile principal stress and the initiation and propagation of adiabatic shear bands at the notch-tip is analyzed. It is found that for high impact speeds or enhanced thermal softening, two shear bands, one at −10° to the notch ligament and the other at −128° to it, propagate from the notch tip. Otherwise, only one shear band nearly parallel to the notch-ligament initiates at the notch-tip. The notch-tip distortion for high strength materials is quite different from that for low strength materials. The maximum tensile principal stress occurs at a point on the upper surface of the notch-tip and for every set of values of material parameters and impact speeds studied equals about 2.3 times the yield stress of the material in a quasistatic simple tension or compression test. We assume that the brittle failure occurs when the maximum tensile principal stress equals twice the yield stress of the material in a quasistatic simple tension test and a shear band initiates when the effective plastic strain at a point equals 0.5. The effect of material and geometric parameters on the time of initiation of each failure mode is computed. It is found that for low impact speeds (<30 m/s) a material will fail due to the maximum tensile principal stress exceeding its limiting value, and at high impact speeds due to the initiation of a shear band at the notch-tip. Results are also computed for a C-300 steel with material parameters given by Zhou et al. For an impact speed of 50 m/s, the shear band speed and the maximum effective plastic strain-rate before a material point melts are found to be 350 m/s and 5×10⁷/s respectively. Key words: Failure-mode transition, shear bands, thermoviscoplasticity, numerical simulations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of interfacial compounds on mechanical properties of titanium–steel vacuum roll-cladding plates

The titanium–steel clad plates were prepared by vacuum roll cladding. Ti–Fe compounds and TiC were observed at different cooling rates after rolling. Optical microscopy, electron microprobe analyser, X-ray diffraction and shear test studies were carried out to study the effect of Ti–Fe compounds and TiC on the ultimate microstructure and mechanical properties of titanium–steel clad plates. At a cooling rate of 6.2°C/min, TiC and Ti–Fe compounds seriously impacted the mechanical properties of the clad plate. At a cooling rate of 1.8°C/min, the thickness of the TiC layer was optimal much that the maximum shear strength of 296?MPa was obtained. At a cooling rate of 0.6°C/min, the thickness of the TiC layer was relatively thick, which affected the mechanical properties of clad plates.     

Finite element analysis of the effects of comers on residual stresses in protective oxide scales

Finite element simulations are used to examine residual thermal stresses and strains in corner regions of protective Al₂O₃ scales on Fe₃Al specimens, both during cooling from oxide formation temperatures and during subsequent thermal cycling. The effects of a corner's radius of curvature and oxide thickness, as well as the impact of aluminide plasticity, are considered. Localized plasticity is found to have a major influence on net deformation and on the magnitude and location of maximum stress. As the ratio of corner curvature to oxide thickness (r_s/t) is reduced, stresses within the oxide corner shift from highly compressive to tensile and the location of the maximum principal stress moves from the substrate to the oxide scale. Based on these stress distributions prior to the development of any flaws, key implications about the tendencies for damage are addressed. The stress evolution during cooling and thermal cycling is presented; these results demonstrate the effects of temperature-dependent material properties. For the material behavior assumed in this study, thermal cycling does not cause significant stress relaxation.     

Relationship between X-ray diffraction and unidirectional solidification at interface between diamond and brazing filler metal

Brazing single crystal diamonds by using silver-copper eutectic filler containing reactive metal: titanium has been carried out. Unidirectional solidification brazing method was tried to obtain stable brazed strength. The diamond specimen was cooled down by contact with copper cooling mass of which temperature was controlled at a room temperature, 470 K and 670 K, respectively. The brazing temperature was 1080 K. The brazing filler was solidified from diamond brazing surface and we called this method as unidirectional solidification brazing. The brazed specimen was examined in shear strength by an original apparatus. In the case of diamond (100), the average shear strength shows more than 120 MPa and maximum shear strength is 240 MPa. These specimens are stronger than that made by usual brazing method. After the strength test, interface orientation between the diamond and the brazing filler was investigated by X-ray diffractometer. In the case of brazing diamond (100), diamond (100) – TiC (111) – Ag (111) orientation can be detected. In the case of brazing diamond (111), diamond (111) – Cu (111) orientation can be detected. Misfits for those orientations were calculated. The value for TiC (111) // diamond (100) is 0.05016, on the other hand the value for TiC (111) // diamond (111) is 0.2125. The brazed interface of diamond (111) is more delicate for thermal stress than diamond (100).     

Thermomechanical Analysis of Preheat Effect on Grade P91 Steel During GTA Welding

Effect of preheat on the thermal cycles, residual stress, and distortion during autogenous GTA welding of Grade P91 steel has been analyzed using FEM. Phase transformation effect on the residual stresses is also analyzed. Thermomechanical analysis has shown that preheat reduced the peak temperatures, cooling rate, and the distortion values. The phase transformation involving martensite during cooling resulted in typical “M” shaped residual stress profile with a reduction in peak tensile residual stress value and a wider distribution of residual stress. There is good agreement between the predicted and measured thermal cycles, residual stresses, and distortion of the P91 steel weld joint.     

Mo添加对Ni3Al-TiC润湿特性的影响机制研究

采用2AP-LEITZ高温显微镜对TiC-Ni₃Al的润湿接触角进行了实验测定,着重探讨了Ni₃Al对TiC的润湿特性以及Ni₃Al中添加少量Mo的影响机制.结果表明,TiC与Ni₃Al之间具有良好的润湿性能.5wt％Mo的添加使Ni₃Al向TiC基板浸渗的深度增大,并与TiC颗粒发生固溶反应置换出部分Ti,在其周围形成一个含Mo的壳层.在这个壳层里,Ti、Mo进一步与Ni₃Al固溶,这些反应降低了液-固表面张力,导致了润湿接触角的下降,从而改善了TiC-Ni₃Al的润湿性.     

Electron-beam Treatment of Tungsten-free TiC/NiCr Cermet Ⅰ:Influence of Subsurface Layer Microstructure on Resistance to Wear during Cutting of Metals

An experimental investigation were performed on the effect of the impulse electron-beam irradiation upon microstruc-ture of the surface layer and on wear resistance of a cutting tool for sintered TiC/NiCr cermet. The results showed that the surface electron-beam treatment of the TiC/NiCr cermet is an efficient method for investigating the mi-crostructure and phase composition in the surface layer of the powder composite and there are optimal regimes of electron-beam treatment, which ensure a substantial increase in the resistance of the cermet to wear during cutting of metals.     

Joining of Cf/SiC composite to Ti alloy using composite filler materials

Abstract

C_f/SiC was successfully joined to Ti alloy with Ag–Cu–Ti–W, Ag–Cu–Ti–SiC and Ag–Cu–Ti–TiC mixed powders by some suitable brazing parameters. Microstructure and shear strengths of the preformed joint were investigated. The results showed that the W particulate and reaction products can uniformly distribute in the brazing layer of the performed joint. These composite brazing layers relaxed the thermal stress of the joint effectively. These characteristics were beneficial to the joint, which had shear strengths that were significantly higher than the optimal shear strengths of the joint brazed with pure Ag–Cu–Ti at room temperature and 500°C.     

虚拟裂缝尖端拉应力大小的确定

针对单边切口的混凝土轴心受拉构件,基于虚拟裂缝模型提出一种计算极限承载力的解析模型,并在此基础上确立了虚拟裂缝尖端拉应力与混凝土轴心抗拉强度之间的关系。结果表明:二者的比值随初始缝高比的增大呈线性增加,但对混凝土强度等级的变化不敏感。其原因是由于所有的混凝土试件都存在初始缺陷,导致截面上存在明显的应力梯度,因而得到的混凝土轴心抗拉强度值是截面应力的平均值,而虚拟裂缝尖端拉应力为截面上的最大应力。很显然,轴心受拉构件的初始缺陷越长,截面的应力梯度越大,虚拟裂缝尖端拉应力与平均应力的比就越大。通常情况下,虚拟裂缝尖端的拉应力大小约为混凝土轴心抗拉强度值的1.22倍,约等于混凝土的抗折强度。     

Mechanical behaviour of TiC film coated on molybdenum by magnetron sputtering

The TiC film, which is coated on molybdenum by magnetron-sputtering, is analysed after the molybdenum substrate is tensile-tested to rupture at 300 to 1070 K. At 300 K some portion of the film exfoliated during the molybdenum substrate deformation. The degree of exfoliation is proportional to the substrate strain up to about 30% elongation, and is proportional to the square root of the film thickness. The maximum shear stress which is generated at the interface between the film and the substrate during the deformation is estimated by the measurement of the distance between cracks. From the estimated maximum shear stress, the adhesive strength of the present TiC film is evaluated to be about 400 MN m^?2.     

A unified theoretical model for tensile and compressive residual film stress

A relative simple model is elaborated for the overall behaviour of the residual stress in a coating layer, grown under continuous bombarding of energetic particles. It concentrates on the transition from tensile to compressive stress under increasing energetic particle flux, a phenomenon frequently observed in experiments. The energy density E in the film is written as a sum of an elastic deformation contribution, and a contribution due to depletion zones between the growing columns. Minimization of E leads to the value of the lateral stress in the film. It is found that the film stress shows a characteristic behaviour on the ratio J_ion/J_at (ion to atom flux ratio). Generally, for increasing J_ion/J_at the stress will first attain a maximum tensile value, and then will change from tensile to compressive values. Further we find that the maximum tensile stress and the tensile/compressive transition occur at lower J_ion/J_at ratios for higher energies of the incoming energetic particles. These findings agree with a number of experimental results.     

Long-life torsion fatigue with normal mean stresses

Relatively simple fatigue tests have been performed on two common engineering materials, cast ductile iron and low-carbon steel, using two stress states, cyclic torsion and cyclic torsion with static axial and hoop stresses. Tests were designed to discriminate between normal stress and hydrostatic stress as the most suitable mean stress correction term for high cycle fatigue analysis. Microscopy shows that cracks in low-carbon steel nucleate and grow on maximum shear planes, while for cast iron pre-existing flaws grow on maximum normal stress planes. The data illustrate that tensile normal stress acting on a shear plane significantly reduced fatigue life and is an appropriate input for fatigue analysis of ductile materials. Static normal stresses did not significantly affect the fatigue life for the cast iron because the net mean stress on the maximum normal stress plane was zero. Mean torsion significantly reduced the fatigue strength of the cast iron. A critical plane long-life parameter for nodular iron which accounts for both stress state and mean stress is proposed, and is found to accurately correlate experimental data.     

Characterization of Al/Ni multilayers and their application in diffusion bonding of TiAl to TiC cermet

The Al/Ni multilayers were characterized and diffusion bonding of TiAl intermetallics to TiC cermets was carried out using the multilayers. The microstructure of Al/Ni multilayers and TiAl/TiC cermet joint was investigated. The layered structures consisting of a Ni₃(AlTi) layer, a Ni₂AlTi layer, a (Ni,Al,Ti) layer and a Ni diffusion layer were observed from the interlayer to the TiAl substrate. Only one AlNi₃ layer formed at the multilayer/TiC cermet interface. The reaction behaviour of Al/Ni multilayers was characterized by means of differential scanning calorimeter (DSC) and X-ray diffraction. The initial exothermic peak of the DSC curve was formed due to the formation of Al₃Ni and Al₃Ni₂ phases. The reaction sequence of the Al/Ni multilayers was Al₃Ni → Al₃Ni₂ → AlNi → AlNi₃ and the final products were AlNi and AlNi₃ phases. The shear strength of the joint was tested and the experimental results suggested that the application of Al/Ni multilayers improved the joining quality.     

界面具有垂直裂纹的热障涂层的失效模拟

采用有限元分析软件ANSYS构造了一个在热生长氧化层（TGO）与陶瓷层界面具有一个垂直裂纹的纳米结构热障涂层的有限元模型。并计算了在热震过程中裂纹处的应力分布图,及裂纹尖端的应力场强度因子K1变化图。计算结果表明：裂纹处存在应力集中现象,且裂纹尖端的应力场强度因子K1在热障涂层热循环的冷却过程中随着时间的延长而减小,且在冷却最开始阶段,温度梯度变化最大,K1值也变化最大,裂纹在冷却的初始具有最大的扩展可能性。且涂层最有可能发生开裂失效。