Sharif Digital Repository

A high quality joint of zinc based pressure die cast alloys can be hardly achieved by conventional fusion welding. In this study, a novel technique to achieve the globular weld structure using stirring the localized semisolid zone during butt joining of Zamak-3 pressure die cast alloy is proposed and effect of welding parameters on mechanical and microstructural properties are investigated. A gas heating system was designed to heat up the nitrogen gas to desired temperature accurately to create a localized semisolid pool. At this stage a fine stirrer was introduced into the weld seam in order to mix the two sides into a single uniform joint. By presented process, globular microstructure of... 

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... 

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... 

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... 

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  

The paper aims at addressing the effect of the bonding time on the metallurgical and mechanical properties of a transient-liquid-phase (TLP) bonded IN718 nickel-based superalloy using an Ni-15.2Cr-4B (%) amorphous filler alloy. The results showed that after a partial isothermal solidification, the residual liquid in the joint centerline transformed, during the cooling, to non-equilibrium eutectic-type microconstituents composed of Ni-rich boride, Cr-rich boride and eutectic γ-solid solution phase. The complete isothermal solidification which resulted in a formation of an intermetallic-free joint centerline occurred after a holding period 40 min at the bonding temperature of 1100 °C. An... 

The production of robust joints after diffusion brazing necessitates the advanced understanding of phase transformations during the bonding process. This paper aims to investigate the solidification and the solid state precipitation during diffusion brazing of wrought IN718 nickel base superalloy using Ni-15Cr-4B (wt-%) filler alloy. It was found that intermetallics containing eutectic type microconstituents were formed in the joint centreline by solidification which is controlled by segregation behaviour of B and its low solubility in Ni rich solid solution. In addition, extensive Cr-Mo-Nb rich precipitates were formed in the substrate region by solid state precipitation induced by B... 

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... 

This paper addresses solidification and solid state precipitation phenomena during transient liquid phase (TLP) bonding of wrought IN718 nickel base superalloy using Ni-4.5Si-3.2B (wt.%) ternary filler alloy. The solidification sequence of the residual liquid in the joint centerline was found to be (1) formation of proeutectic γ, followed by (2) γ/Ni3B eutectic reaction, followed by (3) ternary eutectic of γ/Ni 3B/Ni6Si2B. Extensive fine Ni3Si formed within the eutectic-γ via solid state precipitation during cooling. Extensive Cr-Mo-Nb rich boride precipitates were formed in the substrate region due to boron diffusion into the base metal during bonding process. The implications of the phase... 

Atomistic simulation was used to study the deformation and fracture mechanisms of Ni-Si interfaces under tensile and shear loads dependent on the crystal structure of interface zone. Modified embedded atom method (MEAM) potential was utilized for molecular dynamics (MD) modeling. The simulation includes analysis of common neighbors, coordination number, least-square atomic local strain, and radial distribution function. The profound effect of interface crystallography on the tensile and shear deformation is shown. The highest tensile strength is obtained for interfaces with high plane density due to lowest atomic disorder while under shear loading planes with low density exhibit a high local... 

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... 

In this research, production of ferrite - martensite dual phase Steels with different martensite morphology was considered during transient liquid phase bonding of a low carbon steel. The steel was bonded using an iron base interlayer with melting point of 1443 K and 40 μm thickness. Bonding process carried out at 1473 K, under pressure of 0.5 MPa, at different holding time of 10, 20, 30 and 40 minutes. Microstructural studies of joint region showed that isothermal solidification completed at the bonding time of 40 minutes. Microstructure of joints made at the bonding time of 10, 20, and 30 minutes consists of two distinct region, athermal and isothermal solidified zones. Microstructure of... 

The effect of bonding temperature on microstructure of a transient liquid phase (TLP)-bonded GTD-111 nickel base superalloy, using a Ni-Si-B interlayer, was investigated. At low bonding temperatures microstructure of the joint centerline is controlled by B diffusion. However, at high bonding temperature effect of base metal alloying elements on the joint microstructure development was more pronounced. Effects of bonding temperature on the formation of secondary precipitates in diffusion-affected zone and isothermal solidification time were also analyzed. It was found that above a critical temperature, formation of borides in diffusion-affected zone is suppressed and the time required for... 

The microstructure of a transient liquid phase (TLP) bonded nickel base superalloy using B-containing filler metal after completion of isothermal solidification can usually be described by a eutectic-free joint centerline with extensive in-situ boride precipitation in the diffusion affected zone which in turn can affect the joint properties. Therefore, designing a proper post-bond heat treatment is needed to produce a robust joint. This paper addresses the microstructure evolution mechanism during post-bond heat treatment (PBHT) of TLP bonded wrought IN718 nickel base superalloy. PBHT at 1150 °C, which is lower than the solvus temperature of the borides, for 12 h resulted in boride-free... 

Reactive friction stir processing (RFSP) has been employed to modify the surface properties of AA5052 Al-Mg alloy through grain refinement and the distribution of ultrafine hard nanoparticles (2 to 6% volume fractions of 50 nm titanium dioxide). The heat generated induced solid state reactions between the metal matrix and the reinforcement to form Al3Ti/MgO inclusions. The role of grain refinement and distributed hard nanoparticles on the tribological behavior of the alloy under dry sliding wear condition was evaluated. The wear rates and friction coefficient as well as macro- and micro-features of the worn surfaces indicate that the wear mechanism (at 3–7 kgf and 0.5 m/s) is abrasive. The... 

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... 

Relationship between microstructure and mechanical properties of transient liquid phase-bonded joints of GTD-111 nickel-base superalloy using a Ni-Si-B interlayer was investigated. Shear strength and hardness profile of the joints were discussed with respect to the bond microstructure. In the bonding condition, in which isothermal solidification has not been accomplished completely, eutectic constituent which has the highest hardness in the bond region is the preferential failure source. Additionally, it was found that when isothermal solidification is completed, the extent of γ′ formation in the bond region is the controlling factor for joint strength. Shear strength of bonds after... 

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... 

The densification and microstructural evolution during co-sintering of M2 tool steel/316L stainless steel composite layers with and without boron addition was studied. A pressureless sintering method in conjunction with a powder layering technique was used to fabricate the stepwise graded composite layers. Isothermal and non-isothermal sintering response of the individual and composite layers was examined and the microstructural features of the bonding zone were studied. Shear strength and microhardness of the bonding zone were also measured. It was shown that an enhanced densification is obtained in the composite layers due to (i) sintering shrinkage incompatibility between two steels, (ii)... 

Joining of yttria-stabilized zirconia (YSZ) to stainless steels is considered for various applications such as solid oxide fuel cells, thermal barrier coatings, and medical implants. In the present work, a series of brazing pastes utilizing silver and nickel nanoparticles (∼80 nm) mixed with carboxymethyl cellulose as the organic phase were developed to join 5 mol% YSZ to AISI 420 steel. Effects of processing parameters including time and temperature of brazing and the composition of the nanopastes on the microstructure of joints and their shear rupture strength were studied. Elaboration of the joining mechanism by scanning electron microscopy and electron probe micro-analyzer showed...