Transient liquid-phase bonding in the Ni-Al-B system

Transient liquid-phase (TLP) bonding experiments were performed using a Ni-10.3 at. pct Al alloy and a Ni-10 at. pct B filler material, and the results were compared to simulations performed using the finite-difference diffusion code, DICTRA. For the simulations, a thermodynamic assessment of the Ni-Al-B system was used to define the phase diagram and the thermodynamic factors of the diffusion coefficients. Composition-dependent diffusion mobilities were assessed for the ternary system. Predicted liquid widths as functions of time were in good agreement with the experiments. The calculated and experimental Al composition profiles agree in the matrix but not in the liquid. The simulations qualitatively predicted the observed precipitation and later dissolution of the intermetallic τ phase (Ni₂₀Al₃B₆) in the base material. This research demonstrated the potential for modeling the formation of spurious phases during TLP bonding of practical superalloy systems.     

An Experimental Study of Transient Liquid Phase Bonding of the Ternary Ag-Au-Cu System Using Differential Scanning Calorimetry

An experimental approach using differential scanning calorimetry (DSC) has been applied to quantify the solid/liquid interface kinetics during the isothermal solidification stage of transient liquid phase (TLP) bonding in an Ag-Au-Cu ternary alloy solid/liquid diffusion couple. Eutectic Ag-Au-Cu foil interlayers were coupled with pure Ag base metal to study the effects of two solutes on interface motion. Experimental effects involving baseline shift and primary solidification contribute to a systematic underestimation of the fraction of liquid remaining. A temperature program has been used to quantify and correct these effects. The experimental results show a linear relationship between the interface position and the square root of the isothermal hold time. The shifting tie line composition at the interface has been shown to affect the DSC results; however, the impact on the calculated interface kinetics has been shown to be minimal in this case. This work has increased the knowledge of isothermal solidification in ternary alloy systems and developed accurate experimental methods to characterize these processes, which is valuable for designing TLP bonding schedules.     

A study of the transient liquid phase bonding process applied to a Ag/Cu/Ag sandwich joint

Transient liquid phase (TLP) bonding is a process currently used for joining heat resistant alloys, for example nickel- and cobalt-based superalloys. It involves the formation of a liquid layer between two adjoining pieces and the formation of a solid bond as the liquid disappears during annealing at a suitable constant temperature. In the present study, a model Ag/Cu/Ag sandwich joint associated with a simple eutectic phase diagram was used to study the different stages of this process. The results confirm that the TLP bonding is a diffusional process occurring in clearly distinctive stages. The two most important stages are the widening and homogenization of the previously dissolved liquid interlayer, and the subsequent solidification and shrinking of the interlayer. Whereas the former stage involves diffusional processes both in the liquid phase and in the adjoining solids, the latter is controlled mainly by the diffusion in the solid phase. A modeling approach has been explored which shows that in most eutectic systems there exists an optimal bonding temperature corresponding to the shortest time needed for complete solidification. The results of a study on a Ag/Ag-20 wt pct Cu/Ag sandwich joint provide evidence that the use of an alloy close to the eutectic composition as an interlayer material shortens the TLP process substantially.     

Interface Development in Cu-Based Structures Transient Liquid Phase (TLP) Bonded with Thin Al Foil Intermediate Layers

Proper bonding and assembly techniques are essential for fabrication of functional metal-based microdevices. Transient liquid phase (TLP) bonding is a promising technique for making enclosed metallic microchannel devices. In this paper, we report results of TLP bonding of Cu-based structures at temperatures between 823 K and 883 K (550 °C and 610 °C) with thin elemental Al foils as intermediate boding layers. In situ X-ray diffraction was utilized to examine the structure of Cu/Al interface in real time, resulting in a proposed sequence of structural evolution of the Cu/Al/Cu TLP bonding interface region. Three different types of bonding interface structures, the “γ ₁ structure,” the “eutectoid structure” (“E structure”), and the “E/γ ₁/E structure,” were observed through electron microscopy, and related to the proposed sequence of interfacial structural evolution. Tensile fracture tests were conducted on TLP-bonded Cu/Al/Cu coupon assemblies. Hardness of the various phases within the bonding interface region was probed with instrumented nanoindentation. Results of mechanical testing were correlated to the structure of the bonding interface region. The present results provide an understanding of the structural evolution within the Cu/Al/Cu TLP bonding interface region, and offer guidance to future bonding of Cu-based microsystems.     

A Study of the Effect of Nanosized Particles on Transient Liquid Phase Diffusion Bonding Al6061 Metal–Matrix Composite (MMC) Using Ni/Al2O3 Nanocomposite Interlayer

Transient liquid phase (TLP) diffusion bonding of Al-6061 containing 15 vol pct alumina particles was carried out at 873 K (600 °C) using electrodeposited nanocomposite coatings as the interlayer. Joint formation was attributed to the solid-state diffusion of Ni into the Al-6061 alloy followed by eutectic formation and isothermal solidification of the joint region. An examination of the joint region using an electron probe microanalyzer (EPMA), transmission electron microscopy (TEM), wavelength-dispersive spectroscopy (WDS), and X-ray diffraction (XRD) showed the formation of intermetallic phases such as Al₃Ni, Al₉FeNi, and Ni₃Si within the joint zone. The result indicated that the incorporation of 50 nm Al₂O₃ dispersions into the interlayer can be used to improve the joint significantly.     

Microstructure and Interfacial Reactions During Active Metal Brazing of Stainless Steel to Titanium

Microstructural evolution and interfacial reactions during active metal vacuum brazing of Ti (grade-2) and stainless steel (SS 304L) using a Ag-based alloy containing Cu, Ti, and Al was investigated. A Ni-depleted solid solution layer and a discontinuous layer of (Ni,Fe)₂TiAl intermetallic compound formed on the SS surface and adjacent to the SS-braze alloy interface, respectively. Three parallel contiguous layers of intermetallic compounds, CuTi, AgTi, and (Ag,Cu)Ti₂, formed at the Ti-braze alloy interface. The diffusion path for the reaction at this interface was established. Transmission electron microscopy revealed formation of nanocrystals of Ag-Cu alloy of size ranging between 20 and 30 nm in the unreacted braze alloy layer. The interdiffusion zone of β-Ti(Ag,Cu) solid solution, formed on the Ti side of the joint, showed eutectoid decomposition to lamellar colonies of α-Ti and internally twinned (Cu,Ag)Ti₂ intermetallic phase, with an orientation relationship between the two. Bend tests indicated that the failure in the joints occurred by formation and propagation of the crack mostly along the Ti-braze alloy interface, through the (Ag,Cu)Ti₂ phase layer.     

Improved Resistance to Laser Weld Heat-Affected Zone Microfissuring in a Newly Developed Superalloy HAYNES 282

Gleeble thermomechanical simulation and microstrucutural analyses of laser beam weldability of a newly developed precipitation-hardened nickel-base HAYNES alloy 282 were performed to better understand the fundamental cause of heat-affected zone (HAZ) cracking and how to prevent the cracking problem in the material. Submicron size intergranular M₅B₃ particles are identified for the first time in the present work by transmission electron microscopy, and were found to be the primary cause of HAZ grain boundary liquation cracking in the alloy. Complete dissolution of the liquating M₅B₃ particles by preweld heat treatment exacerbated rather than reduced susceptibility to cracking, which could be attributed to nonequilibrium intergranular segregation of boron atoms, liberated by the complete dissolution of the boride particles, during cooling from heat treatment temperature. Consequently, to reduce the HAZ cracking, a preweld heat treatment that reduces the volume fraction of the M₅B₃ particles while minimizing nonequilibrium grain boundary boron segregation is necessary, and this is possible by heat treating the alloy at 1353?K to 1373 K (1080?°C to 1100 °C). Further improvement in cracking resistance to produce crack-free welds is achieved by subjecting the alloy to thermomechanically induced grain refinement coupled with the preweld heat treatment at 1353 K (1080 °C). A Gleeble hot ductility test showed that formation of the crack-free welds is unexplainable by mere reduction in grain size without considering the effect of grain refinement on intergranular liquid produced by subsolidus liquation of the M₅B₃ borides.     

Diffusion induced isothermal solidification during transient liquid phase bonding of cast IN718 superalloy

In transient liquid phase (TLP) bonding for commercial applications, one of the important key parameters is the holding time required for complete isothermal solidification t_IS, 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 residual liquid, t_IS could be predicted by a mathematical solution of the time dependent diffusion equation based on Fick’s second law.

Dans la brasure en phase liquide transitoire (TLP) pour applications commerciales, l’un des paramètres clés importants est le temps de maintien requis pour la solidification isotherme complète (t_IS), qui est une condition requise pour l’obtention d’une microstructure adéquate du lien. L’objectif de cette étude est d’analyser la cinétique de la solidification isotherme lors de la brasure TLP du superalliage coulé à base de nickel, IN718. On a effectué les expériences de brasage de TLP en utilisant une couche de liaison amorphe de Ni–7Cr–4·5Si–3Fe–3·2B (% en poids) à plusieurs températures de brasage (1273–1373 K). On a déterminé expérimentalement le temps requis pour l’obtention de joints TLP libres de micro constituants eutectiques à la ligne centrale. En considérant le comportement de solidification du liquide résiduel, on pouvait prédire t_IS au moyen d’une solution mathématique de l’équation de diffusion dépendante du temps basée sur la seconde loi de Fick.     

An Investigation on High-Temperature Oxidation and Hot Corrosion Resistance Behavior of Coated TLP (Transient Liquid Phase)-Bonded IN738-LC

Despite all achievements to improve nickel-based superalloy, these classes of alloys are still prone to degradation via high-temperature oxidation and hot corrosion. Repairing damaged parts could decrease the life cycle, cost of equipment, and a transient liquid phase (TLP) bonding is a favorable method that has successfully been used for this purpose. One way to increase the lifetime of the repaired parts and the main body is to utilize protective coating. In the current study, aluminized coating was applied on IN738-LC which was first bonded by TLP process. Coating performance on the joint centerline compared to the other parts of the sample was investigated using a scanning electron microscope (SEM and FESEM) and X-ray diffraction method (XRD). The oxidation test result showed that coating provided less protection on the joint centerline due to coating’s chemical composition difference in this area: particularly Fe and Cr. XRD results showed that at the initial time of oxidation, all (α, γ, δ and θ)-Al₂O₃ were formed and by prolonged exposure were transformed to α-Al₂O₃. The hot corrosion test also proved that the joint centerline and the diffusion-affected zone were less resistant to the corrosion attack of 3Na₂SO₄?+?NaCl salts and severity of damage in these zones were clearly distinguished from microscopic images.     

Metastable Phase Formation and Its Transformation in Highly Undercooled Ni75B25 Alloy

Ni₇₅B₂₅ alloy was solidified at various undercooling. The formation and subsequent transformation of metastable Ni₂₃B₆ phase were clearly identified. If undercooling prior to nucleation is less than a critical value of 240 K (240 °C), the alloy solidifies completely into Ni₃B phase. At larger undercooling, metastable Ni₂₃B₆ phase primarily forms in the melt but then is decomposed into α-Ni and Ni₃B through a eutectoid reaction. The decomposition simultaneously triggers the rapid solidification of residual liquid, due to which a second temperature recalescence occurs. The α-Ni/Ni₃B eutectoid is partially remelted if temperature exceeds the eutectic temperature during the second recalescence. Then, residual Ni₃B grows into coarse round grains while the remaining liquid re-solidifies into α-Ni/Ni₃B eutectic structure in the remelted region. In the case that the eutectic temperature is not reached, the eutectoid product with dot α-Ni distributing in Ni₃B matrix is retained in the solidification structure. A longer delay time between the two temperature recalescence events means less residual liquid, lower recalescence temperature and thus depressed remelting. The formation competition between Ni₃B and Ni₂₃B₆ phases in the alloy melt is nucleation controlled. The heterogeneous site in Ni₇₅B₂₅ alloy melt is a better nucleation substrate for Ni₂₃B₆ phase than for Ni₃B phase.

Graphical abstract

     

Diffusion Bonding of Microduplex Stainless Steel and Ti Alloy with and without Interlayer: Interface Microstructure and Strength Properties

The interface microstructure and strength properties of solid state diffusion bonding of microduplex stainless steel (MDSS) to Ti alloy (TiA) with and without a Ni alloy (NiA) intermediate material were investigated at 1173 K (900 °C) for 0.9 to 5.4 ks in steps of 0.9 ks in vacuum. The effects of bonding time on the microstructure of the bonded joint have been analyzed by light optical microscopy and scanning electron microscopy in the backscattered mode. In the direct bonded joints of MDSS and TiA, the layer-wise σ phase and the λ + FeTi phase mixture were observed at the bond interface when the joint was processed for 2.7 ks and above holding times. However, when NiA was used as an intermediate material, the results indicated that TiNi₃, TiNi, and Ti₂Ni are formed at the NiA-TiA interface, and the irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ have been observed within the TiNi₃ intermetallic layer. The stainless steel-NiA interface is free from intermetallics and the layer of austenitic phase was observed at the stainless steel side. A maximum tensile strength of ~520 MPa, shear strength of ~405 MPa, and impact toughness of ~18 J were obtained for the directly bonded joint when processed for 2.7 ks. However, when nickel base alloy was used as an intermediate material in the same materials, the bond tensile and shear strengths increase to ~640 and ~479 MPa, respectively, and the impact toughness to ~21 J when bonding was processed for 4.5 ks. Fracture surface observations in scanning electron microscopy using energy dispersive spectroscopy demonstrate that in MDSS-TiA joints, failure takes place through the FeTi + λ phase when bonding was processed for 2.7 ks; however, failure takes place through σ phase for the diffusion joints processed for 3.6 ks and above processing times. However, in MDSS-NiA-TiA joints, the fracture takes place through NiTi₂ layer at the NiA-TiA interface for all bonding times.     

Ascending Order of Enhancement in Sliding Wear Behavior and Tensile Strength of the Compocast Aluminum Matrix Composites

The emerging demand of light weight alloys and composites for the engineering and structural applications leads to explore the possibility to develop new techniques to achieve materials of high performance. In the present study, Al–B₄C reinforced composite has been developed via semi solid technique. The influence of Boron carbide (B₄C) content on the dry sliding wear characteristics of Al6061 matrix composites has been assessed using a pin-on-disc wear test. Wear rate was found to increase in ascending order with B₄C particles content. On comparing the wear rate, it has been found that the wear resistance offered by coated B₄C reinforced Al 6061 alloy matrix composites is higher than both base Al alloy and uncoated composites with incorporation of harder phase. This shows the good interfacial bonding of coated B₄C and Al6061 alloy matrix phase.     

Predicted volume change behavior accompanying the solidification of binary alloys

For the volume changes accompanying solidification, distinctions are made between the volume changeβ _Mfor the whole freezing process, the volume changeβ _mfor liquid entrapped within the freezing zone, and the localized volume changeβ _Taccompanying the liquid-solid phase transformation at a given temperature. The first volume change is important in mold design, while the latter two are important factors in the formation of casting defects such as shrinkage pores, solidification cracks, and inverse segregation. Values ofβ _M, β_M, andβ _mare deduced for equilibrium conditions in the representative alloy systems Al-Cu, Bi-Sb, Fe-C and Pb-Sn. While the volume changeβ _Mmay vary only moderately with alloy composition,β _mis a strong function of composition and of the temperature of enclosure. The isothermal volume change,β _T, equal to the relative density difference between solid and liquid, varies during the freezing process and is strongly dependent upon composition. Isothermal volume changes and hence density differences as large as 20 pct are deduced for some Bi-Sb and Pb-Sn alloys.     

Microstructural Evolution and Bonding Behavior during Transient Liquid-Phase Bonding of a Duplex Stainless Steel using two Different Ni-B-Based Filler Materials

Microstructural evolution and bonding behavior of transient liquid-phase (TLP) bonded joint for a duplex stainless steel using MBF-30 (Ni-4.5Si-3.2B [wt pct]) and MBF-50 (Ni-7.5Si-1.4B-18.5Cr [wt pct]) were investigated. Using MBF-30, the microstructure of the athermally solidified zone was dependent on B diffusion at 1333.15 K (1060 °C). Ni₃B and a supersaturated γ-Ni phase were observed in this zone. BN appeared in the bonding-affected zone. However, using MBF-50, the influences of base metal alloying elements, particularly N and Cr as well as Si in the filler material, on the bond microstructure development were more pronounced at 1448.15 K (1175 °C). BN and (Cr, Ni)₃Si phase were present in the bond centerline. The formation of BN precipitates in the bonding-affected zone was suppressed. A significant deviation in the isothermal solidification rate from the conventional TLP bonding diffusion models was observed in the joints prepared at 1448.15 K (1175 °C) using MBF-50.     

M23C6 carbide dissolution mechanisms during heat treatment of ASTM F-75 implant alloys

The dissolution of M₂₃C₆ carbides in an ASTM F-75 alloy was experimentally followed, during a liquid-phase homogenization treatment, in as-cast and pretreated for partial carbide dissolution (PTPCD) specimens. The results revealed that before the fusion of the carbides, solid-state diffusion of the elements forming the carbides occurred. After the fusion of the carbides, a serrated interface developed. Treatment periods longer than 1000 seconds led to a liquid-carbide zone morphology showing the presence of dendrites within the liquid phase. Energy dispersion spectrometry (EDS) analysis revealed that the composition of such dendrites was very close to that of the α-phase matrix. The observed microstructure features are explained in terms of a solutal diffusion-driven mechanism leading to the growth of the matrix by consuming the liquid phase formed by the carbide fusion.     

Effect of Nb Concentration on Thermal Stability and Glass-Forming Ability of Soft Magnetic (Fe,Co)-Gd-Nb-B Glassy Alloys

Addition of a small amount of Nb to the (Fe,Co)-Gd-B glassy alloy in (Fe_0.9Co_0.1)_71.5−x Nb _x Gd_3.5B₂₅ increased the stabilization of supercooled liquid. The largest supercooled liquid region of 104 K was obtained for the x = 2 alloy. A distinct two-stage-like glass transition was observed with further incresing Nb content. The nanoscale (Fe,Co)₂₃B₆ phase precipitated in the glassy matrix after annealing, while the two-stage-like glass transition disappeared, indicating that the anomalous glass transition behavior originates from the exothermic reaction for the formation of the (Fe,Co)₂₃B₆ phase in the supercooled liquid region. The glass-forming ability (GFA) also increased by addition of Nb, leading to formation of the bulk glass form for the Nb-doped alloys. The best GFA with a diameter of over 3 mm was achieved for the x = 4 alloy. The (Fe,Co)-Gd-Nb-B glassy alloys exhibited good magnetic properties, i.e., rather high saturation magnetization of 0.81 to 1.22 T, low coercive force of 2.5 to 5.8 A/m, and low saturated magnetostriction of 9 to 19 × 10⁻⁶. In addition, the glassy alloys also possessed very high compressive fracture strength of 3842 to 3916 MPa and high Vickers hardness of 1025 to 1076.     

Characterization and corrosion behavior of Al–Co–rare earth (Ce–La) amorphous alloy

A new Al-based amorphous alloy with excellent corrosion resistance and high thermal stability was produced in the Al–Co–RE (Ce–La) system. In this regard, two different amorphous-nanocrystalline ribbons: Al_87.6–Co_6.4–Ce_3.8–La_2.2 (M₁) and Al_82.3–Co_10.1–Ce_4.8–La_2.8 (M₂) were prepared using melt spinning. The results reveal the supercooled liquid region (ΔT_x) of 218 °C which shows that the M₂ alloy has higher thermal stability in comparison to the M₁ alloy. The M₂ ribbons present the superior corrosion resistance because of the formation of amorphous phase. The i_corr, E_corr, E_pit, and E_pit – E_corr values of fully amorphous Al–Co–Ce–La metallic ribbons have a better trend of anti-corrosion performance compared to other crystalline and amorphous aluminum alloys.     

Effects of welding and weld heat-affected zone simulation on the microstructure and mechanical behavior of a 2195 aluminum-lithium alloy

The microstructures, tensile properties, and fatigue properties of a 2195-T8 Al-Li alloy subjected to a weld heat-affected zone (HAZ) simulation and gas-tungsten-arc (GTA) welding using a 4043 filler metal, with and without a postweld heat treatment, were studied. The principal strengthening precipitate in the T8 base alloy was the T ₁ (Al₂CuLi) phase. The HAZ simulation resulted in the dissolution of T ₁ precipitates and the formation of T _B(Al₇Cu₄Li) phase, Guinier-Preston (G–P) zones, and δ′ (Al₃Li) particles. When the HAZ simulation was conducted at the highest temperature of 600 °C, microcracks and voids also formed along the grain boundaries (GBs). In the specimens welded with the 4043 alloy, T (AlLiSi) phase was found to form in the fusion zone (FZ). An elongated T _Bphase and microcracks were observed to occur along the GBs in the HAZ close to the FZ interface. The T ₁ phase was not observed in the HAZ. The postweld heat treatment resulted in the spheroidization of primary T phase and the precipitation of small secondary T particles in the FZ, the dissolution of T _B phase, and the reprecipitation of the T ₁ phase in the HAZ. Both the HAZ simulation and welding gave rise to a considerable decrease in the hardness, tensile properties, and fatigue strength. The hardness in the FZ was lower than that in the HAZ. Although the postweld heat treatment improved both the hardness and tensile properties due to the reprecipitation of T ₁ phase in the HAZ and a smaller interparticle spacing in the FZ, no increase in the fatigue strength was observed because of the presence of microcracks in the HAZ.