Diffusion bonding between Ti–6Al–4V alloy and ferritic stainless steel

In the present study, Ti–6Al–4V alloy was diffusion bonded to a ferritic stainless steel. The effect of bonding temperature on the microstructural development across the joint region was investigated. After diffusion bonding, microstructural analysis including metallographic examination, energy dispersive spectrograph (EDS) and shear strength was conduced. From the results, it was seen that bonding on the temperature was affecting the Fe and Ti mutual diffusion which controls the interface microstructure. The microstructure of the interface region was formed, consisting of Fe and Ti intermetallics.     

Challenges of Diffusion Bonding of Different Classes of Stainless Steels

Solid state diffusion bonding is used to produce monolithic parts exhibiting mechanical properties comparable to those of the bulk material. This requires diffusion of atoms across mating surfaces at high temperatures, accompanied by grain growth. In case of steel, polymorphy helps to limit the grain size, since the microstructure is transformed twice. The diffusion coefficient differs extremely for ferritic and austenitic phases. Alloying elements may shift or suppress phase transformation until the melting range. In this paper, diffusion bonding experiments are reported for austenitic, ferritic, and martensitic stainless steels possessing varying alloying elements and contents. Passivation layers of different compositions are formed, thus affecting the local diffusion coefficient and impeding diffusion across faying surfaces. As a consequence, different bonding temperatures are needed to obtain good bonding results, making it difficult to control the deformation of parts, since strong nonlinearities exist between temperature, bonding time, and bearing pressure. For martensitic stainless steel, it is shown that it is very easy to obtain good bonding results at low deformation, whereas ferritic and austenitic stainless steels require much more extreme bonding parameters.

     

Diffusion bonding of ferritic oxide dispersion strengthened alloys to austenitic superalloys

The superior high temperature mechanical strength and oxidation resistance of ferritic oxide dispersion strengthened (ODS) tubular alloys are compromised by the difficulties encountered in joining. Conventional fusion welding techniques generate a weld fusion zone which is devoid of the mechanical strength exhibited by the base material. Therefore, more sophisticated solid state joining techniques, such as diffusion bonding, must be employed when joining ODS materials. This paper describes a series of solid state diffusion bonding experiments carried out between two tubular ferritic ODS alloys and two high temperature austenitic alloys. Careful control of bonding conditions produced pressure retaining joints between one of the tubular ODS alloys and both austenitic alloys. The successful joint design was incorporated into the manufacture of a tubular creep component, which enabled a series of internally pressurized creep tests to be carried out. The microstructure developed at the bond interface of each of the four separate material couples is described and the high temperature performance of the pressure retaining joints is discussed.     

Effectiveness of Ni-based diffusion barriers in preventing hard zone formation in ferritic steel joints

A numerical procedure based on finite difference method was used to simulate the formation of ‘hard’ and ‘soft’ zones, in dissimilar weldments of 9Cr–1Mo and 2¼Cr–1Mo steels during high temperature exposure. Kinetic analysis of the calculated diffusion profiles showed that the activation energy for carbon diffusion in Cr–Mo steels is marginally higher than that in Fe–C system. Calculations were extended to incorporate the effect of Ni-based interlayers between 2¼Cr–1Mo and 9Cr–1Mo ferritic steels. The presence of a diffusion barrier was found to reduce the propensity for formation of hard and soft zones, which is related to the interaction parameter \( \varepsilon_{\rm C}^{\rm M}. \) Thickness of the interlayer required to suppress the formation of hard zone was optimized by the calculations. Transition joints of ferritic steels with Inconel 182 as the interlayer of thickness close to that predicted by the computations were fabricated and exposed to elevated temperature. Microstructural studies and hardness measurements further confirmed the effectiveness of Ni-based interlayers in preventing hard zone formation.     

Tensile strength of five metals and alloys in the nanosecond load duration range at normal and elevated temperatures

The paper presents results of measurements of the resistance to tensile fracture at spallation in nickel, cobalt, stainless steel, AlMg6% alloy, and Inconel IN 738 LC alloy. In the experiments carried out with a high-power ion beam as a shock-wave generator the load pulse duration was in the range of 50 ns. The measurements were performed at peak stresses varying by a factor up to 2 which had no influence on the dynamic tensile strength of the materials tested. For cobalt and Inconel measurements were also done at elevated temperatures. Whereas the response of cobalt was practically insensitive to temperature, IN 738 LC demonstrated a transition from viscous to relatively brittle fracture accompanied by a significant increase of the spall strength at higher temperatures.     

The metallurgical behaviour during brazing of Ni-base alloy Inconel 600 to Si3N4 with Ag71Cu27Ti2 filler metal

Detailed observations were carried out on the metallurgical behaviour of joint-brazing of nickel based alloy Inconel 600 to Si₃N₄ with Ag71Cu27Ti2 filler metal, with emphasis on the interface between the filler metal and the Inconel 600 and the effects of nickel which was the predominant element in the base metal. Based on the experimental results, the mechanism of bonding Inconel 600 to the filler metal is attributed to the diffusion of silver and copper along the grain boundaries of the Inconel 600, which results in mechanical anchoring. The effects of nickel on the metallurgical behaviour of filler metal are summed up as enhancing the separation of silver- and copper-rich liquid phases from the molten filler metal; combining titanium and decreasing its activity in the reaction with Si₃N₄ at the interface with ceramics; promoting the diffusion of silver and copper into Inconel 600; and facilitating the flow of filler metal over the surface of Inconel 600.     

Microstructure and mechanical properties of diffusion bonded SiC/steel joint using W/Ni interlayer

This paper describes the design and examination of W/Ni double interlayer to produce a joint between SiC and ferritic stainless steel. Diffusion bonding was performed by a two steps solid state diffusion bonding process. Microstructural examination and mechanical properties evaluation of the joints show that bonding of SiC to steel was successful. EDS and XRD analysis revealed that W₅Si₃ and WC were formed at SiC/W interface. The diffusion products at W/Ni interface, Ni-rich solid solution Ni(W) or intermetallic compound Ni₄W, was found to be dependent on the second step joining temperature. Neither intermediate phases nor reaction products was observed at Ni/steel interface for the joints bonded at the temperature studied. The average tensile strength of 55 MPa which is insensitive to the second step process was measured for as-bonded SiC/steel joint and the failure occurred at SiC/W interface. The hardness near the various bonded interfaces was also evaluated.     

扩散连接技术在核聚变反应堆包层模块制造中的应用

国际受控热核聚变实验堆计划是全球规模最大、影响最深远的国际科研合作项目之一,有望彻底解决能源危机。核聚变反应堆关键部件——包层模块的结构复杂、体积庞大,且服役环境恶劣,焊接接头成为影响反应堆安全运行的薄弱环节。以扩散连接为代表的固相焊接技术对接头性能及组织影响较小,已逐渐取代熔化焊应用于包层模块复杂构件制造。在简要介绍扩散连接及其原理的基础上,对包层模块构件扩散连接的研究进展进行了阐述,包括低活化铁素体/马氏体钢及氧化物弥散强化钢构件的扩散连接,Be,W,Si C等其他先进高温材料的扩散连接等。     

Brazing of metallic conductors onto ceramic plates in solid oxide fuel cells Part II Attaching conducting wires

Commercialization of the solid oxide fuel cell (SOFC) will be facilitated by use of conventional materials and fabrication processes. In this paper, we discuss the results of brazing metallic wires, made of conventional heat-resisting alloys, onto metallic CrFe5Y₂O₃1 conductor plates in SOFC. Such wires would be used for the current transport between individual SOFC stacks and to the power-consuming device. Aluminum-alloyed ferritic stainless steels (iron, 20 to 25 wt% chromium, 4.5 to 6 wt% aluminum) and Inconel 617 were found to be suitable materials for the wires. They can be brazed onto CrFe5Y₂O₃1 using L-Ni 5 as filler for Inconel 617 and the ferritic steels, and Cr50Ni or L-Ni 5 for the ferritic steels. The effectiveness of the brazed conductor/CrFe5Y₂O₃1 joints was verified by monitoring their resistance at 1000°C in air for 1000 h.     

Der Einfluß der Kohlenstoff-Diffusion auf die mechanisch-technologischen Eigenschaften von ferritisch-austenitischen Schweißverbindungen im Chemie-Betrieb

The effect of carbon diffusion on the mechanical properties of ferritic-austenitic weldments . Weldments between ferritic and austenitic steels indicate a diffusion of carbon from ferritic to austenitic partner as an influence of temperature stresses and due higher solubility of carbon in γ-iron in comparison to α-iron. Investigations on two characteristic weldments (German standard H II/1.4571 and H II/Ni) have been conducted in different annealed conditions to study the effect of carbon diffusion on the various mechanical properties. There is an observable impairement of the mechanical properties of the weldments. Increased hardness in the carburised zone causes cracking and tensile strength as well as notch toughness decrease in the decarburised area.     

Joint strength and interfacial microstructure in silicon nitride/nickel-based Inconel 718 alloy bonding

Joining of Inconel 718 alloys to silicon nitrides using Ag–27Cu–3Ti alloys was performed to investigate the microstructural features of interfacial phases and their effect on joint strength. The Si3N4/Inconel 718 alloy joints had a low shear strength in the range 70.4–46.1 MPa on average, depending on joining temperature and time. When the joining time was held for 1.26 ks at 1063 K, shear, tension, and four-point bending strength were 70.4, 129.7, and 326.5 MPa on average. The microstructures of the joints typically consisted of six types of phases. They were TiN and Ti5Si4 between silicon nitride and filler metal, a copper- and silver-rich phase, island-shaped Ti–Cu phase, a Ti–Cu–Ni alloy layer between filler and base metal, and diffusion of titanium into the Inconel 718 alloys. With increasing joining temperature, the thickness increase of the Ti–Cu–Ni alloy layer was much greater than that of the reaction layer. Thus the diffusion rate of titanium into the base metal was much greater than the reaction rate with silicon nitride. This behaviour of titanium results in the formation of a Ti–Cu–Ni alloy layer in all the joints. The formation of these layers was the cause of the strength degradation of the Si3N4/Inconel 718 alloy joints. This fact was supported by the analyses of fracture path after four-point bending strength tests.     

Estimation of homogenisation time for superalloys based on a new diffusional model

The inter-diffusion of segregating elements (Nb, Mo, and Ti) in the cast ingot of Inconel 718 superalloy during homogenisation at elevated temperature was studied by a new model based on the characteristic diffusion length. The proposed simple method was used for obtaining the homogenisation time required for back-diffusion of each element. Therefore, the developed model can be used for both studying the diffusion of alloying elements and estimation of homogenisation time, where the latter is important from the industrial standpoint.     

The effects of metal coating on the diffusion bonding in Al2O3/Inconel 600 and the modulus of rupture strength of alumina

Alumina with a sputter-deposited metal film was diffusion bonded to Inconel 600. A higher bonding strength and lower joining temperature were obtained with titanium coating compared to that for the non-coated sample. The improved joining behaviour was attributed to an enhanced interface reaction and reduction in the thermal stress. Also, the effect of various coatings of 3 m thickness on the mechanical property of alumina after heat treatment at 1000 °C for 30 min under 10^–6 torr vacuum was evaluated in terms of modulus of rupture (MOR) using a Weibull plot. While the Cu coating did not change MOR strength of alumina, the reactive Ti and Zr metal coatings caused a noticeable reduction in averaged MOR strength. The effect of co-sputtering of Ti-Cu, and bilayer coatings of Cu/Ti and Ti/Cu was also investigated.     

Evaluation of transient liquid phase bonding between nickel-based superalloys

Induction brazing of Inconel 718 to Inconel X-750 using Ni-7Cr-3Fe-3.2B-4.5Si (wt.-%) foil as brazing filler metal was investigated in this paper. Brazing was conducted at the temperature range 1373–1473 K for 0–300 s in a flow argon environment. Both interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate joint quality. The optical and scanning electron microscopic results indicate that good wetting existed between the brazing alloy and both Inconel 718 and Inconel X-750. Microstructures at joint interfaces of all samples show distinct multilayered structures that were mainly formed by isothermal solidification and following solid-state interdiffusion during joining. The diffusion of boron and silicon from brazing filler metal into base metal at the brazing temperature is the main controlling factor pertaining to the microstructural evolution of the joint interface. The element area distribution of Cr, Fe, Si, Ni and Ti was examined by energy dispersive X-ray analysis. It was found that silicon and chromium remain in the center of brazed region and form brittle eutectic phases; boron distribution is uniform across joint area as it readily diffuses from brazing filler metal into base metal. The influence of heating cycle on the microstructures of base material and holding time on the mechanical properties of brazed joint were also investigated.     

High temperature carbon corrosion in solid oxide fuel cells

Abstract

Operating conditions in a current design for a planar geometry oxide fuel cell plant are briefly reviewed and the danger of encountering “metal dusting” conditions identified. Laboratory tests were designed to produce accelerated metal dusting by exposing heat resisting alloys to a CO–26 H₂–6 H₂O (vol. pct) gas mixture at 680°C under thermal cycling conditions. The hot gas composition corresponded to a_c = 2.9 and an oxygen potential high enough to oxidise chromium and aluminium, but not iron or nickel. The alloys tested included ferritic and austenitic chromia formers and two ferritic alumina formers, all with electropolished surfaces. Thermal cycling of the chromia formers led to oxide scale damage followed by internal carburisation, metal dusting and coking. This failure occurred very rapidly on most austenitic materials (Alloy 800, Inconel 601, 690, 693, Alloy 602CA), but did not commence until after approximately 50 one-hour cycles for the ferritic steel Fe–27Cr–0.001Y (wt %). The alloy with the best performance was Inconel 625, which was still protected by its Cr₂O₃ scale after 500 cycles. The alumina forming alloys showed superior performance, with no damage apparent after 1200 cycles. Additional tests using ground metal surfaces showed that they were more resistant to dusting in the case of chromia formers, but more susceptible in the case of alumina formers, metal dusting.     

Diffusion of chromium in ferritic and austenitic 9–20 wt-%chromium steels

Abstract

Radiotracer ⁵¹Cr diffusion experiments were conducted on 9–20 wt-% chromium ferritic and austenitic steels. Volume diffusion coefficients have been determined in the temperature range 881–1281 K, and triple product values of grain boundary diffusion between 795 and 1281 K. Compared with dilute solid solutions, high ratios of grain boundary and volume diffusion activation energies have been obtained. This is discussed in view of the chemical composition of the grain boundaries measured by Auger electron spectroscopy. Furthermore, in the case of ferritic steels the effects of α–γand paramagnetic–ferromagnetic phase transitions are illustrated, while for austenitic steels a classical Arrhenius relationship has been found in the investigated temperature range.     

Diffusional processes in thin films of Au/CeO2 on NiCr superalloy

Gold films deposited onto CeO₂ intermediate layers on Inconel substrates by means of thermal decomposition of organometallic solutions lose their coating identity, i.e. reflectivity, as a result of heating at temperatures between 500 and 850 °C in air. The main cause is the out-diffusion of chromium from the Inconel substrate through the CeO₂ intermediate layer to the gold film surface and the subsequent oxidation of the chromium. The predominant diffusion mechanism was found to be through grain boundaries in the gold film, with an activation energy of 1.00±0.05 eV. This relatively low value of the activation energy was attributed to in situ grain growth in the gold films that take place during the diffusion. In this temperature regime it could be assumed that the CeO₂ intermediate layer is a semidiffusion barrier, since no concentration of gold could be observed in the Inconel substrate. However, at temperatures higher than 750 °C, slight decomposition of the CeO₂ was detected.     

Potentialunterstützte Herstellung von Metall‐Keramik‐Verbundschichten

Potential assisted fabrication of metal‐ceramic composite coatings A possibility to produce uniform metal‐ceramic composite coatings with a high content of ceramic particles up to 60 vol.% will be presented in this study. This method includes a combination of electrophoretic deposition and electrolytic deposition by several steps. A yttria‐stabilized zirconia coating (Tosoh TZ‐8Y) was first electrophoretically deposited on a ferritic steel plate and then sintered by 1100 °C to an open porous layer. In the next step nickel was electrodeposited into the pores of the layer. By a final annealing step it was possible to improve the bonding of the composit coating on the substrate by diffusion of the metal components.     

Diffusion Bonding of Ti_3Al Base Alloy

: The effects of diffusion bonding temperature and holding time on the joint strength of Ti3Al base alloy has been investigated in this paper. The shear strength of Ti-14Al-21Nb-3Mo-V alloy diffusion bonding joint under pressure of 12 MPa at 990℃ for 70 min was obtained to 797.6 MPa which approaches the base material strength. In addition, a short-time diffosion bonding process was studied in order to decrease the bonding cost. With the deformation of the specimens of 2.5% and the bonding temperature of 990℃ for 15 min, the bonding strength could reach 801 MPa.     

Oxidation behaviour and phase transformations of an interconnect material in simulated anode environment of intermediate temperature solid oxide fuel cells

The oxidation behaviour and the phase transformations associated with high temperature exposure of a commercial ferritic interconnect steel, Crofer 22 H, was studied in a simulated solid oxide fuel cell (SOFC) anode atmosphere at 700 °C. Special emphasis was placed on the formation of the intermetallic sigma phase. No sigma phase was detected in the bulk alloy after 500 h of exposure of bare specimens. However, specimens which were pre-coated with a layer of nickel showed formation of an interdiffusion zone after as little as 2 h of exposure and sigma phase formation occurred after 10 h. The presence of the nickel layer, which simulates the contact between ferritic steel interconnects and a nickel mesh in a SOFC results in the formation of an austenitic zone and accelerated formation of a σ-phase rich layer at the ferrite/austenite interface. The ferritic steel is transformed into austenite due to the inward diffusion of nickel, σ-phase started to nucleate at the transformed austenite grain boundaries. The nucleation is enhanced by an increased Cr/Fe-ratio at that interface due to more pronounced diffusion of Fe, compared to Cr, in the direction of the Ni-layer. Different possible mechanisms for the nucleation and growth of σ-phase were identified. The experimental results led to the conclusion that sigma nucleates in the austenite and grows following an isothermal eutectoid-like decomposition. The kinetics of σ-phase formation and the depth of the interdiffusion zone were found to follow a traditional diffusion relationship. It was observed that as the Ni-concentration increases the sigma-phase re-dissolves and thus the zone which, contains sigma phase moves deeper into the ferritic steel with exposure time. Interdiffusion processes between the nickel layer and the ferritic steel result not only in accelerated formation of σ-phase but also in the formation of Cr-rich oxides within the nickel layer.