Sharif Digital Repository

This paper investigates the effect of microstructure development on mechanical properties of diffusion brazed IN718 nickel-based superalloy using a Ni-Cr-Si-B-Fe filler metal. The phase transformations during diffusion brazing of IN718/Ni-Cr-Si-B-Fe/IN718, which dictate the microstructure of the bonds are governed by diffusion-induced isothermal solidification, cooling-induced athermal solidification, and diffusion-induced solid-state precipitation. It was found that when partial isothermal solidification occurs at the bonding temperature, the residual liquid is transformed into eutectic-type microconstituents. Considering solute redistribution and segregation behavior of the melting point... 

The time required to ensure completion of the isothermal solidification process is the key in obtaining an intermetallic free and robust joint after diffusion brazing. In view of economic considerations, it is interesting to perform bonding process trials using standard heat treatment cycles. This paper addresses the effect of applying the standard heat treatment cycle including homogenisation, solution and double aging treatments on the microstructural and mechanical response of diffusion brazed IN718. A commercial Ni-7Cr-4·5Si- 3?2B-3Fe (wt-%) brazing alloy was used as the interlayer. Results showed that bonds made with the standard homogenisation treatment, at 1100°C, exhibited a eutectic... 

Transient liquid phase (TLP) bonding was used to join a nickel-based superalloy GTD-111 using a Ni-Si-B amorphous interlayer, MBF30. Bonding was carried out at 1100 °C with different holding time under vacuum. Joint region microstructure was studied by optical and scanning electron microscopy. Microstructural studies showed that before completion of isothermal solidification, bond region consists of four distinct zones: centerline eutectic structure due to athermal solidification, solid solution phase due to isothermal solidification, diffusion-induced boride precipitates and base metal. Complete isothermal solidification, which prevents formation of centerline eutectic constituent, occurred... 

In transient liquid phase (TLP) bonding for commercial applications, one of the important key parameters is the holding time required for complete isothermal solidification tIS, which is a prerequisite for obtaining a proper bond microstructure. The objective of the study is to analyse the isothermal solidification kinetics during TLP bonding of cast IN718 nickel based superalloy. Experiments for TLP bonding were carried out using a Ni-7Cr-4.5Si-3Fe-3.2B (wt-%) amorphous interlayer at several bonding temperatures (1273-1373 K). The time required to obtain TLP joints free from centreline eutectic microconstituents was experimentally determined. Considering the solidification behaviour of... 

This paper aims at addressing the microstructure-strength characteristics relationship during transient liquid phase (TLP) bonding of cast IN718 nickel based superalloy using Ni-7. Cr-4.5. Si-3. Fe-3.2. B (wt%) amorphous interlayer. The progress of the isothermal solidification at different temperatures for different times is analyzed using a Larson-Miller parameter (LMP). It was found that there is direct relationship between isothermal solidification zone (ISZ) size and LMP. Results showed that in situation where isothermal solidification has not been completed, the ratio of athermal solidification zone (ASZ) size to the width of the total solidified zone is the controlling factor for... 

Redistribution of alloying elements in the transient liquid phase (TLP) bonding zone of IN738LC/ BNi-3/IN738LC was studied to investigate microstructural evolution in this area. Wavelength dispersive spectrometry and electron probe microanalysis revealed that, during non-isothermal solidification in the TLP bonding zone, enrichment of residual liquid phase with the positive segregating elements caused formation of intermetallic in the bonding zone. Scanning electron microscopy observation Indicated that the redistribution of alloying elements, between TLP bonding zone and base alloy, resulted In formation of a γ′ boundary layer, containing high density of fine γ′, around the bonding zone.... 

The as-cast samples of FSX-414 Co-based superalloy were heat treated at the standard solution and aging heat treatment cycles. Specimens for joining were cut from the as-cast ingot. Transient liquid phase (TLP) bonding was carried out at the standard solution cycle, using MBF-30 interlayer. Also, bonded specimens were homogenized at the standard aging condition. The microstructure of the as-cast, heat treated and TLP bonded specimens were studied. These studies showed that the non-uniform distributed carbides of the as-cast specimen were replaced by uniform ones during the heat treatment. Eutectic M 23C 6 carbides were observed in all specimens. Due to the complete isothermal solidification,... 

Aging response of transient liquid phase Nb bearing wrought IN718 nickel base superalloy is studied. The aging behaviour of the joint is influenced by low Nb+Al+Ti content of isothermal solidification zone (ISZ) and formation of Nb-Cr-Mo based boride precipitates in diffusion affected zone (DAZ). It was shown that applying a post-bond heat treatment which was able to eliminate the diffusion induced boride precipitates in DAZ and increase the Nb+Al+Ti content of the ISZ improved the aging response and shear strength of the joint  

Transient liquid phase (TLP) diffusion bonding is a preferred joining/repairing technique for nickel based superalloy. In this research, TLP bonding process carried out according to the standard heat treatment cycles for wrought IN718 base material in order to permit the combining of these two thermal operations. The results showed that the standard solution treatments at 1000 and 1050. °C for 60. min can result in completion of isothermal solidification in IN718/Ni-Cr-Fe-Si-B/IN718 system producing an entirely intermetallic free joint centerline. Two distinct microstructural zones were formed in the bonding affected zone: isothermal solidification zone (ISZ) which consists of single phase... 

The paper aims at understanding solidification phenomena and solid state precipitations during diffusion brazing of Hastelloy X nickel base superalloy using a ternary Ni–4.5Si–3.2 B (in wt-%) filler metal. The joint is featured by the formation of Ni-rich solid solution in an isothermal solidification zone, borides/silicide formation during eutectic-type solidification in an athermal solidification zone, on-cooling precipitation of fine nickel silicides in the joint centerline, in situ precipitation of Mo–Cr-rich borides in the diffusion affected zone and grain growth in the heat affected zone. The implication of the phase transformations on the joint properties is discussed. It is shown... 

Transient liquid phase bonding of AISI 304 austenitic stainless steel was carried out using a Co-based interlayer with 40 μm thickness. The effect of bonding time and solid-state homogenisation time on the microstructure and mechanical properties of samples was investigated. The results showed that isothermal solidification was completed within 30 min at a constant temperature of 1180°C. With increasing homogenisation time, at 1000°C, a more uniform distribution of alloying elements and hardness profile across the joint region was achieved. The average shear strength of homogenised samples was about 72% that of the base metal at the same heat treatment cycle. © 2017 Institute of Materials,... 

Ti–6Al–4V and AISI 304 stainless steel were joined by diffusion brazing with a Cu interlayer at 900 °C for different bonding times. The influence of the bonding time on the joint width, microstructure, microhardness and shear strength was studied. Interfacial characterizations and the identification of phases were done by scanning electron microscopy and electron backscatter diffraction (EBSD). The results revealed that by increasing the bonding time, the thickness of the joint was first increased and then decreased. Microstructure examinations also showed that the joints consisted of several intermetallics such as TiCu, Ti2Cu, TiCu4, TiCu2, FeTi and Fe2Ti. On the other hand, EBSD results... 

The key feature of diffusion brazing, also referred to as transient liquid phase bonding, is isothermal solidification which precludes the formation of intermetallic in the joint centreline. Analysing the available data published in the literature showed that the composition of the nickel-based superalloys plays a strong role in determining the required time for obtaining intermetallic-free joint during diffusion brazing. This effect is not predictable by the standard conventional models. It is proposed that increasing the boride-forming elements in the base superalloy which promotes in situ boride precipitation at the diffusion-affected zone can accelerate the diffusion flux of the boron... 

This investigation aims at understanding the underlying fundamentals of the isothermal solidification phenomenon during the transient liquid phase (TLP) bonding process. The isothermal solidification is governed by solid-state diffusion of the melting point depressant (MPD) into the base material, which, in turn, is controlled by both kinetic and thermodynamic parameters; however, the latter factor is generally ignored. In this work, the competition between kinetics and thermodynamics of diffusion were considered in TLP bonding of a nickel alloy, Monel 400, using two distinct filler metals including pure copper (Cu) and Ni-Si-B filler metal. The joint generated by Ni-Si-B filler metal... 

The application of the multi-component Fe-based filler metals (FMs) for transient liquid phase (TLP) bonding of AISI 304 austenitic stainless steel has been overshadowed by dissimilar interlayers merely due to their shorter isothermal solidification time. However, the latter usually suffers from low efficiency in terms of mechanical properties even after homogenization of heat treatment. This study shows that by imposing a temperature gradient across the bond line during the TLP bonding process (TG-TLP), it is possible to reduce the isothermal solidification time significantly. This renews the interest in utilizing multi-component Fe-based FMs. In this regard, the TG-TLP bonding process was... 

Understanding diffusion-induced isothermal solidification time during transient liquid phase bonding is vital in producing intermetallic-free robust joints. The isothermal solidification completion time is overestimated by the existing analytical models, even by the closest one to the real bonding conditions, known as the moving interface model. It was found that the boride formation in the diffusion affected zone of Ni-based superalloy upon using B-containing filler metals is one of the reasons behind the inability of the moving interface model to predict the isothermal solidification completion time accurately, which has received scant attention in the literature. Moreover, simplified... 

Optimization of transient liquid phase (TLP) bonding variables is essential to achieve a joint free from deleterious intermetallic constituents as well as with appropriate mechanical properties. In this research, TLP bonding of FSX-414 superalloy was performed using the MBF-80 interlayer. The effects of bonding time (1-30. min) and gap size (25-100 μm) were studied on the joint microstructure and its mechanical properties. Continuous centerline eutectic phases, characterized as nickel-rich and chromium-rich borides, were observed at the joints with incomplete isothermal solidification. The globular and acicular phases were seen at diffusion affected zone (DAZ). These phases could be...