An investigation on microstructure and mechanical properties of Al7075 to Ti–6Al–4V Transient Liquid Phase (TLP) bonded joint

Transient Liquid Phase (TLP) bonding of two dissimilar alloys Al7075 and Ti–6Al–4V has been done at 500 °C under 5 × 10⁻⁴ torr. Cu was electrodeposited on Al7075 and Ti–6Al–4V surfaces, 50 μm thick Sn–4Ag–3.5Bi film was used as interlayer and bonding process was carried out at several bonding times. The microstructure of the diffusion bonded joints was evaluated by Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The eutectic and intermetallic compounds formation along Al7075 grain boundaries and Ti/Al interface such as θ(Al₂Cu), TiAl and Ti₃Al were responsible for joint formation at the aluminum and titanium interfaces. Microhardness and shear strength tests were used to investigate the mechanical properties of the bonds. Hardness of the joints increased with increasing bonding time which can be attributed to the intermetallics formation at the interface. The study showed that the highest bond strength was 36 MPa which was obtained for the samples joined for 60 min.     

Effect of holding time on microstructure and mechanical properties of ZrO2/TiAl joints brazed by Ag–Cu filler metal

Reliable joining of ZrO₂ ceramic to TiAl alloy is crucial for the success of the engine industrial application. However, there have been few systematic investigations on joining of ZrO₂ ceramic and TiAl alloy. In this study, reliable brazing of ZrO₂ ceramic and TiAl alloy was achieved using inactive AgCu filler metal. The interfacial microstructure of the joints was characterized by SEM, XRD and TEM. Effects of holding time on the microstructure and mechanical properties of the joints were investigated in details. The results revealed that Cu₃Ti₃O + TiO layers were formed adjacent to ZrO₂ ceramic while AlCu₂Ti layer was formed at TiAl substrate. The thickness of Cu₃Ti₃O + TiO layers increased and the granular AlCu₂Ti coarsened gradually with the prolongation of holding time. The hardness and Young's modulus of reaction phases were characterized by nano-indentation to reveal the plastic deformability. The highest shear strength of 48.4 MPa was achieved when brazed at 880 °C for 10 min.     

Titanium aluminide (Ti-48Al) powder synthesis,size refinement and sintering

Titanium aluminide based alloys have shown significant potential in high temperature applications, but the high production cost of TiAl considerably limits its utilisation. Although the use of powder metallurgy processes can reduce the cost by minimising post-machining, an economical powder production route is still required. Therefore, in the present study a pre-alloyed Ti-48Al powder is developed using an elemental Ti and Al powder blend prepared using a simple vacuum heat treatment. A formation model of the intermetallic phases (i.e. TiAl, Ti₃Al, TiAl₂, TiAl₃) during powder synthesis is proposed. In order to improve the sinterability, various milling methods (i.e. ball, attrition and shatterbox milling) are examined to reduce the particle size. The sintered microstructures, particularly the two-phased (α₂-Ti₃Al γ-TiAl) lamellar structures are also investigated. Improved densification is achieved at 1300 °C, held for 2 h, using the manufactured powder, compared to the elemental powder blend (～55%). With higher sintering temperatures or longer hold periods, increased density TiAl components are possible.     

Formation of a single-phase Ti3AlC2 from a mixture of Ti,Al and TiC powders with Sn as an additive

A high purity of Ti₃AlC₂ powder has been synthesized by pressureless sintering a mixture of Ti/Al/TiC/Sn (Sn as a sintering additive) powders with a mole ratio of 1:1:1:0.1 in the temperature range of 1350–1500 °C for 10 min in an Ar atmosphere. Sn is an effective additive and its effect on the formation of Ti₃AlC₂ has been discussed. The formation mechanism of Ti₃AlC₂ has been proposed. X-ray diffraction analysis and scanning electron microscopy were used to characterize the samples.     

The effect of fabrication processes on the mechanical and interfacial properties of SiCf/Cu–matrix composites

Three groups of SiC_f/Ti/Cu composites were prepared under conditions of 650 °C + 105 min (sample 1#), 750 °C + 85 min (sample 2#) and 840 °C + 50 min (sample 3#), respectively, by foil-fiber-foil method (FFF), and their room temperature tensile strengths were established. The aim is to model the reactive bonding states between Ti and SiC fiber and between Ti and Cu when Ti is used as interfacial adhesion promoters in SiC_f/Cu–matrix composites. The fracture surfaces, SiC_f/Ti interfaces and Ti/Cu interfaces were investigated by scanning electron microscopy (SEM), optical microscopy and energy dispersive spectroscopy (EDS). The tensile tests show that the tensile strengths of samples 1# and 2# are not obviously enhanced due to the weak bonding strength between SiC fiber and Ti, while those of sample 3# are achieved above 90% of ROM (the rule of mixtures) strength because of excellent bonding between SiC fiber and Ti. However, there are distinct Ti/Cu interfacial reaction zones after the three processes, which are approximately 5.4, 9.0 and 13.3 μm thick, respectively. The Ti/Cu interfacial reaction products are mainly distributed in four layers. In samples 1# and 2#, the products are predicted to be Cu₄Ti, Cu₃Ti₂, CuTi and CuTi₂ according to their chemical compositions determined by EDS, while in sample 3#, the products are Cu₄Ti, Cu₄Ti₃, CuTi and CuTi₂. Additionally, the relationships between the thickness of Ti interlayer and its reaction with C and Cu are also discussed, and an optimal thickness of Ti is introduced.     

Dissimilar joining of Ti3Al-based alloy to Ni-based superalloy by arc welding technology using gradient filler alloys

In this study, Ti–Al–Nb, Ti–Ni–Nb and Ni–Cr–Nb system alloys were designed and incorporated in order to construct a gradient structure at the surface of the joined Ti₃Al base material. And the Ti₃Al-based alloy and Ni-based superalloy were successfully joined together using gas tungsten arc (GTA) welding technology. The microstructure evolution, mechanical properties and fractured behaviors of the joints were investigated. The gradient structure remarkably decreased the formation tendency of brittle phases within the joints compared with a single filler alloy and thus improved the joint strength effectively. The average room-temperature tensile strength of the Ti₃Al/In718 dissimilar joint reached 353 MPa, and the strength value at 873 K was 245 MPa. At the Ti–Ni–Nb/Ni–Cr–Nb interface, some Ni₃(Nb, Ti) + (Nb, Ti)Cr₂ and TiNi₃ phases were detected in the Ti–Ni–Nb matrix. It was believed that their presence decreased the room-temperature strength of the Ti–Ni–Nb alloy but improved its high-temperature strength.     

Experimental study of diffusion welding/bonding of titanium to copper

In the present study, Ti–6Al–4V alloy was bonded to electrolytic copper at various temperatures of 875, 890 and 900 °C and times of 15, 30 and 60 min through diffusion bonding. 3 MPa uniaxial load was applied during the diffusion bonding. Interface quality of the joints was assessed by microhardness and shear testing. Also, the bonding interfaces were analysed by means of optical microscopy, scanning electron microscopy and energy dispersive spectrometer. The bonding of Ti–6Al–4V to Cu was successfully achieved by diffusion bonding method. The maximum shear strength was found to be 2171 N for the specimen bonded at 890 °C for 60 min. The maximum hardness values were obtained from the area next to the interface in titanium side of the joint. The hardness values were found to decrease with increasing distance from the interface in titanium side while it remained constant in copper side. It was seen that the diffusion transition zone near the interface consists of various phases of βCu₄Ti, Cu₂Ti, Cu₃Ti₂, Cu₄Ti₃ and CuTi.     

Phase transformation in Ti–48Al–6Nb porous alloys and its influence on pore properties

Ti–48Al–6Nb porous alloys were synthesized by the powder metallurgy (PM) method, and the associated phase transformation and pore parameter were investigated in order to reveal the pore-formation mechanism. The present results indicate that the Nb–Al and Ti–Al phase transformations contribute to the pore-formation. It was found that the five-step phase transformations for the Ti–48Al–6Nb porous alloys occur as follows: (1) Ti + Al → TiAl₃ at 600–700 °C; (2) Nb + Al → NbAl₃ at 700–900 °C; (3) TiAl₃ + Ti → TiAl at 900–1100 °C; (4) TiAl + Ti → Ti₃Al/TiAl at 1100–1350 °C; (5) NbAl₃ + Nb → Nb₂Al and the Ti₃Al turns to the major phase at 1350 °C. These phase transformations made the pore-diameter increasing continuously from 1.71 μm to 12.10 μm and also made the pore volume distributing widely. At the second step of 700–900 °C, the Nb–Al phase transformation leads to 5% more volume expansion compared to the Ti–Al based porous alloys. Meanwhile, the porosity and total pore area initially increase and then decrease at this step, but they increase intensely at the final step, which is needed as a catalytic carrier.     

Nanocomposite protective coatings based on Ti–N–Cr/Ni–Cr–B–Si–Fe,their structure and properties

The first results of manufacturing and investigations of a new type of nanocomposite protective coatings are presented. They were manufactured using a combination of two technologies: plasma-detonation coating deposition with the help of plasma jets and thin coating vacuum-arc deposition. We investigated structure, morphology, physical and mechanical properties of the coatings of 80–90 μm thickness, as well as defined the hardness, elastic Young modulus and their corrosion resistance in different media. Grain dimensions of the nanocomposite coatings on Ti–N–Cr base varied from 2.8 to 4 nm. The following phases and compounds formed as a result of plasma interaction with the thick coating surface were found in the coatings: Ti–N–Cr (200), (220), γ-Ni₃–Fe, a hexagonal Cr₂–Ti, Fe₃–Ni, (Fe, Ni)N and the following Ti–Ni compounds: Ti₂Ni, Ni₃Ti, Ni₄Ti, etc. We also found that the nanocomposite coating microhardness increased to H = 31.6 ± 1.1 GPa. The Young elastic modulus was determined to be E = 319 ± 27 GPa – it was derived from the loading–unloading curves. The protective coating demonstrated the increased corrosion resistance in acidic and alkaline media in comparison with that of the stainless steel substrate.     

Microstructural characteristics and evolution of Ti2AlN/TiAl composites with a network reinforcement architecture during reaction hot pressing process

The microstructural evolution of TiAl matrix composites with a novel network distribution of Ti₂AlN particle reinforcement was studied. The composites were synthesized by reaction hot pressing method using pure Al and nitrided Ti powders as initial materials. Pure Ti powders nitrided at 600 °C for a certain time in an atmosphere of flowing nitrogen turned into new compound Ti(N) powders, which have a shell of titanium nitrides (such as TiN, Ti₂N and TiN_0.3) and a core of Ti–N solid solution. Within the composites synthesized, Ti₂AlN particles, produced by in situ reaction, exhibit a network distribution. The special shell/core structure of the compound Ti(N) powders contributes to this architecture. Nitriding time of the Ti powders greatly affects the microstructure of the composites. Increasing the nitriding time is beneficial to the distribution of Ti₂AlN particles in a continuous network form. However, too long nitriding time can result in the aggregation of Ti₂AlN particles and thus destroy the uniformity of the network structure. The in-situ synthesized Ti₂AlN/TiAl composites with uniform network structure have a superior mechanical property, and their compressive strengths at 800 °C and 1000 °C are 1112 MPa and 687 MPa, respectively.     

Development and characterization of laser clad high temperature self-lubricating wear resistant composite coatings on Ti–6Al–4V alloy

To enhance the wear resistance and friction-reducing capability of titanium alloy, a process of laser cladding γ-NiCrAlTi/TiC + TiWC₂/CrS + Ti₂CS coatings on Ti–6Al–4V alloy substrate with preplaced NiCr/Cr₃C₂–WS₂ mixed powders was studied. A novel coating without cracks and few pores was obtained in a proper laser processing. The composition and microstructure of the fabricated coating were examined by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) techniques, and tribological properties were evaluated using a ball-on-disc tribometer under dry sliding wear test conditions at 20 °C (room-temperature), 300 °C, 600 °C, respectively. The results show that the coating has unique microstructure consisting of α-Ti, TiC, TiWC₂, γ-NiCrAlTi, Ti₂CS and CrS phases. Average microhardness of the composite coating is 1005 HV_0.2, which is about 3-factor higher than that of Ti–6Al–4V substrate (360 HV_0.2). The friction coefficient and wear rate of the coating are greatly decreased due to the combined effects of the dominating anti-wear capabilities of reinforced TiC and TiWC₂ carbides and the CrS and Ti₂CS sulfides which have excellent self-lubricating property.     

Transient liquid phase (TLP) bonding of Al7075 to Ti–6Al–4V alloy

TLP diffusion bonding of two dissimilar aerospace alloys, Ti–6Al–4V and Al7075, was carried out at 500 °C using 22 μm thick Cu interlayers for various bonding times. Joint formation was attributed to the solid-state diffusion of Cu into the Ti alloy and Al7075 alloy followed by eutectic formation and isothermal solidification along the Cu/Al7075 interface. Examination of the joint region using SEM, EDS and XPS showed the formation of eutectic phases such as, ?(Al₂Cu), T(Al₂Mg₃Zn₃) and Al₁₃Fe along grain boundaries within the Al7075 matrix. At the Cu/Ti alloy bond interface a solid-state bond formed resulting in a Cu₃Ti₂ phase formation along this interface. The joint region homogenized with increasing bonding time and gave the highest bond strength of 19.5 MPa after a bonding time of 30 min.     

Synthesis of Ti3AlC2/Al2O3 nanopowders by mechano-chemical reaction

Ti₃AlC₂/Al₂O₃ nanopowders were synthesized by the combination of mechanically-induce self-propagating reaction (MSR) of Ti, C, Al and TiO₂ powder mixtures and subsequently heat treatment. Effects of high energy milling and heat treatment temperatures on the phase transformation were investigated in detail. X-ray diffraction (XRD) was used to characterize the powders of milled and annealed, respectively. The morphology and microstructure of as fabricated products were also studied by scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). Results show that TiC, Ti_xAl_y and Al₂O₃ transitional phases were formed when the initial powder mixtures were milled for 24 h. The desired Ti₃AlC₂/Al₂O₃ nanopowders with high purity were obtained when annealed the as-milled powders at 1100 °C. SEM image confirmed that the as fabricated Ti₃AlC₂/Al₂O₃ particles has nanocrystalline layered structural matrix of Ti₃AlC₂, and the second phase of nanosized Al₂O₃ disperses uniformly in the Ti₃AlC₂ matrix.     

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950 °C for 45 min under the compressive stress of 4 MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni₃Ti, NiTi and NiTi₂ intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ was observed in the Ni₃Ti intermetallic layer. At 950 °C processing temperature, black island of β-Ti phase has been observed in the NiTi₂ intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi₂ phase at the NiA–TiA interface when bonding was processed up to 900 °C, however, failure takes place through NiTi₂ and β-Ti phase mixture for the diffusion joints processed at 950 °C. Joint strength was evaluated and maximum tensile strength of ∼560 MPa and shear strength of ∼415 MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900 °C for 45 min.     

Evaluation on diffusion bonded joints of TiAl alloy to Ti3SiC2 ceramic with and without Ni interlayer: Interfacial microstructure and mechanical properties

Diffusion bonding of TiAl alloys and Ti₃SiC₂ ceramics were carried out in a vacuum atmosphere. The microstructures and mechanical properties of the bonded joints were investigated. Results showed that three coherent intermetallic layers formed in the TiAl/Ti₃SiC₂ joints during bonding process. The compound layer adjacent to Ti₃SiC₂ substrate was indicated to be Ti₅Si₃, in which brittle fracture of the joints took place during shear strength test. The properties of diffusion bonded joints were greatly improved attributed to the formation of a good transition in the joint as well as the relief of the residual stress when using Ni foil as interlayer. Formation mechanisms of the compound layers during bonding process were discussed. Shear test results showed that the maximum shear strength reached 52.3 MPa. Corresponding fractograph indicated that the crack mainly propagated along Ti₃SiC₂ substrate adjacent to the bonding zone, accompanied with an intergranular and transgranular fracture mode.     

Improved mechanical properties of Ti–6Al–4V alloy by electron beam welding process plus annealing treatments and its microstructural evolution

In this study, we investigated the effect of post weld annealing treatments on the mechanical properties of a Ti–6Al–4V alloy following electron beam welding (EBW). The operational parameters used in this EBW process together with suitable annealing treatments appeared to substantially enhance the tensile properties of the Ti–6Al–4V weldment, suggesting that EBW is a promising method for industrial application. Moreover, we report for the first time that γ-TiAl + α₂-Ti₃Al formed in the fusion zone of the Ti–6Al–4V EBW weldment and exhibited the same lamellar structure, orientation relationship, deformation mechanism, and slip system as common Ti–Al-based alloys do. The presence of these intermetallic compounds affected the mechanical properties of the weldment. We discuss the related phase transformation, microstructural evolution, and characteristics of the precipitates formed.     

In situ joining of titanium to SiC/Al composites by low pressure infiltration

A method of in situ joining of titanium to SiC/Al composites by low pressure infiltration was proposed. The effect of infiltration temperature on microstructure and bending strength of in situ joining composites was investigated and the best infiltration temperature was confirmed to be 710 °C. The interfacial region of SiC/Al/Ti composites was consisted of Ti substrate, Al–Ti interfacial layer, Al layer and SiC/Al composite. The bending strength of SiC/Al composites kept nearly constant as the infiltration temperature changed while that of SiC/Al/Ti composites was influenced significantly by the infiltration temperature. The fracture occurred at the Al–Ti and Al–SiC/Al interfaces alternately as infiltrated at 670 °C. But as the infiltration temperature was increased to 710 °C, the fracture occurred only at the Al–SiC/Al interface which shows a great interfacial bonding at the Al–Ti interface. The formation of Al–Ti brittle intermetallics and the effect of crystallization and grain coarsening are two possible reasons which lead to the decrease of bending strength when the infiltration temperatures were increased from 710 °C to 730 °C.     

Effect of in situ synthesized TiB whisker on microstructure and mechanical properties of carbon–carbon composite and TiBw/Ti–6Al–4V composite joint

Carbon–carbon composite (C–C composite) and TiB whiskers reinforced Ti–6Al–4V composite (TiB_w/Ti–6Al–4V composite) were brazed by Cu–Ni + TiB₂ composite filler. TiB₂ powders have reacted with Ti which diffused from TiB_w/Ti–6Al–4V composite, leading to formation of TiB whiskers in the brazing layer. The effects of TiB₂ addition, brazing temperature, and holding time on microstructure and shear strength of the brazed joints were investigated. The results indicate that in situ synthesized TiB whiskers uniformly distributed in the joints, which not only provided reinforcing effects, but also lowered residual thermal stress of the joints. As for each brazing temperature or holding time, the joint shear strength brazed with Cu–Ni alloy was lower than that of the joints brazed with Cu–Ni + TiB₂ alloy powder. The maximum shear strengths of the joints brazed with Cu–Ni + TiB₂ alloy powder was 18.5 MPa with the brazing temperature of 1223 K for 10 min, which was 56% higher than that of the joints brazed with Cu–Ni alloy powder.     

The high- and low-cycle fatigue behavior of Ni-contain steels and Ni-alloys in high pressure hydrogen

The effect hydrogen on short-term strength and plasticity, high- and low-cycle durability of 15Cr12Ni2MoNMoWNb martensitic steel, 10Cr15Ni27Ti3W2BMo austenitic dispersion-hardened steel, 04Kh16Ni56Nb5Mo5TiAl and 05Kh19Ni55Nb2Mo9Al Ni-base superalloys in range of pressures 0–30 MPa and temperatures 293–1073 K was investigated. In the case of 15Cr12Ni2MoNMoWNb steel and 04Kh16Ni56Nb5Mo5TiAl alloy the dependence of low-cycle durability (N) and characteristics of plasticity (δ and ψ) on the hydrogen pressure consists of two regions. In the first region (low pressures), the N, δ and ψ abruptly drops, and in the second, the negative action of hydrogen becomes stable or decrease negligibility. This means that there exists a pressure under which the degradation of this material with hydrogen reaches its limit. The additional effect of preliminary dissolved hydrogen on the properties of 15Cr12Ni2MoNMoWNb steel and 04Kh16Ni56Nb5Mo5TiAl alloy developed at hydrogen environment pressure least of 10 MPa. In the case of 10Cr15Ni27Ti3W2BMo steel and 05Kh19Ni55Nb2Mo9Al alloy the low-cycle durability N, characteristics plasticity δ and ψ decrease in whole hydrogen pressure range. Preliminary dissolved hydrogen leads to a considerable additional decrease in the properties of this alloy.     

Effect of growth rate on microstructure and tensile properties of Ti–45Al–2Cr–2Nb prepared by electromagnetic cold crucible directional solidification

Ti–45Al–2Cr–2Nb alloy was directionally solidified at different growth rates varying from 0.2 to 1.2 mm/min by applying an electromagnetic cold crucible directional solidification technique. It was determined that well-aligned α₂ (Ti₃Al)/γ(TiAl) lamellar structures, B₂ phase and blocky γ phase were generated in columnar grains. The interlamellar spacing (λ) decreases with the increasing growth rate (V) according to the relationship λ ∝ V^− 0.48, but the volume fraction of B₂ is increased as growth rate is increasing. Results of uniaxial tensile tests show that the B₂ phase and the blocky γ phase have significant influence on tensile failure when they are presented in the matrix of α₂/γ lamellar structures, because they are usually employed to act as cracking sources during the tension process.