Isothermal Solidification Research Articles

Effect of microstructure aspects on mechanical properties of nanoparticle-assisted transient liquid phase (NP-TLP) joints for AZ31 alloy

To study the effect of nanoparticles on strengthening the transient liquid phase (TLP) joints of AZ31 alloy, different interlayers including pure copper, pure copper+TiO 2 nanoparticles, pure copper+Al 2 O 3 nanoparticles and pure copper+SiC nanoparticles were used. The pure copper was coated on the surfaces to be joined via electroplating. The samples were TLP bonded at 525 °C for 1 and 2 h under the pressure of 1 MPa, in an atmosphere-controlled furnace. After that, the microstructure of the joints were studied using optical and scanning electron microscopy and phase analysis as conducted using Energy-dispersive X-ray spectroscopy. The joints' shear strength was evaluated via a tensile test machine equipped with a special designed fixture and X-Ray Diffraction was utilized to analyze the related fracture surfaces. Microstructural studies showed that athermal solidification induced eutectic compounds at the joints were reduced with increasing bonding time and therefore, the isothermal solidification became more completed. This also increased the homogenity and uniformity of the microstructure and the joint's shear strength. The highest and lowest shear strength was observed for the joints made using pure copper+TiO 2 nanoparticles and pure copper+SiC nanoparticles-containing interlayers, respectively, about 90% and 15% of the base metal's shear strength. Also, the shear strength of the joints made using pure copper+Al 2 O 3 nanoparticles was higher for the 20 nm particles, compared to the 50 nm ones. • Exploring TLP bonding process for AZ31 alloy as an Mg-based one • Using nanoparticles-including interlayers with different materials and sizes • A reverse relationship between the nanoparticle size and joint's shear strength • Obtaining the highest joint's shear strength about 98% of that of the base metal • ASZ size and nanoparticles surface area as the factors of the joint's strength

Interfacial microstructure characterization and strength evaluation of Si3N4/Si3N4 joints with Si–Mg composite filler for high-temperature applications

Silicon-nitride (Si3N4) components were joined under vacuum at 1100 °C for 10 min using Si–Mg composite fillers with Mg contents (XMg) that ranged from 0 at.% to 59 at.%. The Si3N4/Si3N4 joints were fabricated via Si layer formation at the joint interface; the molten Si–Mg liquid was transformed into a solid Si layer after Mg-evaporation-induced isothermal solidification. The joint tensile strength at room temperature increased considerably when XMg exceeded the liquidus composition of 37 at.% because of the enhanced densification/thinning of the Si layer. In these cases, some Mg atoms reacted with Si3N4 to form a fine-grained MgSiN2-based layer, whereas relatively large (>0.1 μm) and smaller MgO precipitates (<10 nm) were observed in the Si layer. At a high XMg, the MgO precipitates were arranged in a network-like structure, which improved the fracture strength of the Si layer. The joints with a high strength at room temperature were examined using a three-point bending test at 1200 °C in air and endured a maximum fracture stress of ~200 MPa, which confirmed their potential for use in oxidizing atmospheres at least 100 °C above the bonding temperature.

Experimental and Numerical Study of Transient Liquid Phase Diffusion Bonded DZ40M Superalloys

Transient liquid phase (TLP) diffusion bonding of DZ40M cobalt-based superalloy was carried out using a self-made NiCrCoWB intermediate layer. The typical microstructure of the joint was investigated. The effect of holding time on the microstructural evolution and the tensile strength of the brazed joints was studied. The tensile strength of the joints TLP bonded at 1160 °C for 60 min reached the maximum value of 487 MPa, which was 88.6% of the base metal strength. The diffusion of boron and the evolution of the eutectic zone were numerically studied. The time needed for isothermal solidification completion was calculated and predicted, which was well in accordance with the experimental results.

Development of a Novel Ni-Based Multi-principal Element Alloy Filler Metal, Using an Alternative Melting Point Depressant

Brazing is a crucial joining technology in industries where nickel-superalloy components must be joined. Nickel-based brazing filler metals are extensively employed, possessing excellent mechanical properties, corrosion resistance, and retained strength at elevated temperatures. To function as a filler metal, the alloy melting point must be reduced to below that of the materials being joined, but the addition of melting point depressants (MPDs) such as boron, silicon, and phosphorus can, however, lead to the formation of brittle intermetallics, potentially compromising the joint performance. In the present work, a novel multi-principal element brazing alloy (in the style of a high entropy alloy), utilizing Ge as an alternative MPD along with a reduced B addition, is investigated. The design process considered binary phase diagrams and predictions based on Thermo-Calc software and empirical thermodynamic parameters. The alloy was used to vacuum braze nickel-superalloy Inconel-718, and microstructural and mechanical investigations are reported. The maximum shear strength achieved was 297 MPa with a brazing temperature of 1100 °C and 60-minute hold time, with isothermal solidification completed. Shear strength was only slightly reduced with increased joint width. Assessments are made of the ability to accurately predict properties of multi-principle element alloys using Thermo-Calc software and empirical thermodynamic parameters.

Correlation Between Potentiodynamic Polarization Behaviors of Transient Liquid Phase Bonded 2205 Duplex Stainless Steel and the Bonding Constituents

In order to precisely evaluate the contribution of each bonding constituent to the pitting corrosion resistance of transient liquid phase (TLP) bonded 2205 duplex stainless steel (DSS), we have undertaken potentiodynamic polarization (PDP) and microstructural analytic measurements all across the TLP bonded area. The PDP results show that the pitting corrosion resistance of TLP bonded specimens is significantly affected by the presence of certain bonding constituents across the TLP bonded area. Electron microscopy analysis indicates that the formation of complex (Fe,Ni,Cr,Mo)3P phosphide in the bonding zone (BZ) before the completion of isothermal solidification (IS) as well as the formation of P-rich sigma phase in the diffusion-affected zone (DAZ) following the completion of the IS provides the most preferential sites for the occurrence of pitting corrosion. The PDP results also confirm that the pitting potentials (Epit) of the TLP bonded specimen before and after IS completion are, respectively, closer to the Epit of the BZ and the Epit of the DAZ rather than to those of other TLP BZs.

Investigation on the microstructure and mechanical properties of CoCrFeNi high-entropy alloy joint bonded with BNi2 interlayer

• The athermal solidification zone and isothermal solidification zone of CoCrFeNi joint are determined to be γ phase and high entropy Ni-rich FCC phase, respectively. No eutectic products containing borides precipitated in the athermal solidification zone. • With increasing the bonding temperature or prolonging the holding time, the growths of ISZ and DAZ present a synchronous trend. The precipitating location of CrxBy is changed from grain interior to grain boundaries. • The maximum bonding strength of CoCrFeNi joint is ∼ 429 MPa, which is about 75 % of base metal strength. • The impact of Cr depletion on the performance of CoCrFeNi joint is less severe than that on conventional alloys joints. The strategy of bonding high-entropy alloys (HEAs) is of vital significance for producing complex engineering components available to extreme conditions. In this work, transient liquid phase (TLP) bonding was applied to join CoCrFeNi HEAs using an amorphous Ni-Cr-Si-B interlayer (BNi2). The interfacial microstructure evolution, mechanical properties and bonding mechanism of the joints were evaluated. Results indicated three reaction products in the different zones of joints, including the γ phase in the athermal solidification zone (ASZ), high-entropy Ni-rich face-centered cubic (FCC) phase in the isothermal solidification zone (ISZ) and a high density of CrxBy precipitates with different morphologies (blocky, punctate and acicular) in the diffusion affected zone (DAZ 7 ). With increasing the bonding temperature or holding time, the width of ISZ and DAZ almost presented a synchronous growing trend, and the location of CrxBy precipitates was changed from grain interior to grain boundaries. Tension shear test showed excellent bonding strength of all the bonded joints due to the formation of the ASZ free of eutectic products containing borides, and the maximum bonding strength could reach 429 MPa, which is about 75 % of the base metal (BM) strength. The joint ruptured at the CoCrFeNi BM adjacent to the lapping area in ductile fracture mode. This work is expected to facilitate the fabrication and repair of complex CoCrFeNi-related components in future.

Dissolution and isothermal solidification behaviour of commercially pure titanium brazed using a pure nickel filler under TLPB conditions

Transient liquid phase bonding of Ti using elemental Ni foils represents a potential low-cost manufacturing route to join aerospace components. To fully develop this technique, the dissolution behaviour and kinetics of isothermal solidification in Ti/Ni joints were determined in this study. During heating, Ti-Ni intermetallic compounds form at the solid/liquid interface, which slows down the dissolution of the Ni foil, while promoting diffusion of Ni into the Ti base metal. These two features limit the amount of liquid gap widening that occurs during initial heating such that it only reaches 55% of the theoretical maximum. Once at the braze temperature of 1050 °C, the rate of isothermal solidification becomes constant with the square root of braze time. The IS rate in the Ti-Ni joints was twice as high as that reported for Ti-Cu joints. Comparison of experimental IS rates with model predictions indicated excellent agreement and demonstrated that the difference in IS rate in Ti-Ni compared to Ti-Cu was largely due to a higher rate of diffusion of Ni in Ti compared to Cu in Ti. The actual time required to achieve isothermal solidification depended both on the amount of dissolution/gap widening and on the rate of IS.

Research on evolution of tilted eutectic structure based on phase field simulation

The phase field model is established for the eutectic growth system in this paper, and the finite difference method is used to solve the model. The evolution of tilted eutectic interface morphology under isothermal solidification and directional solidification conditions, respectively, was investigated. The effects of solid-solid interface anisotropy, solid-liquid interface anisotropy, eutectic spacing and pulling speed on the evolution of the tilted eutectic structure were simulated to reveal the growth mechanism of the tilted eutectic structure. It is found that under isotropic and directional solidification conditions, eutectic growth is influenced by both of the direction of heat flow and the solid-liquid interface anisotropy. When the solid-solid interface anisotropy is small, the direction of heat flow dominates the growth direction of eutectic structure, and as the solid-solid interface anisotropy increases, the growth direction of eutectic structure starts to tilt. Two kinds of instability phenomena, bifurcation or merger and fault line, are also found in the eutectic growth.

Investigation of solidification modes of high Manganese steels

ABSTRACT The solidification modes of high manganese steels were measured by experiments and comparatively calculated by different predicted methods. The DSC experiments indicate the same phase transformation sequence as the isothermal solidification experiments. It is demonstrated that the solidification modes of the Fe–0.028C–21.1Mn–5.72Al, Fe–0.28C–21.0Mn–5.80Al and Fe–0.64C–21.0Mn–5.80Al alloys are F mode, FA1 mode and FA2 mode. The change of heating/cooling rate influences the position and shape of the peaks in the DSC curves but has no effect on the solidification mode. By comparison with different calculation methods, the equilibrium model is shown to be a valuable tool to predict the solidification sequence of the high-Mn steel, from which the critical carbon range for the Fe–C–20Mn–5Al alloy is calculated to be 0.062∼0.427 wt-%.

The effect of bonding time on the microstructure and mechanical properties of dissimilar transient liquid phase bonding between UNS N08825 alloy and UNS S32750 super duplex stainless steel using the BNi-2 interlayer

• The dissimilar transient liquid phase (TLP) bonding of the UNS N08825 alloy to UNS S32750super duplex stainless steel was successfully accomplished using the BNi-2 interlayer. • The complete isothermal solidification occurred at 1050 °C and the bonding time of 300 s. • Shear strength of the TLP joints improved by increasing the holding time. In this study, the microstructure and mechanical properties of transient liquid phase, dissimilar joints between UNS N08825 alloy and UNS S32750super duplex stainless steel was investigated. The joining process was carried out at 1050 °C for 0, 60, 120, and 300 s using an BNi-2 interlayer. The transient liquid phase bonding area was divided into three zones: (i) the isothermal solidification zone which contained γ-Ni solid solution, (ii) the a-thermal solidification zone which consisted of Ni- and Cr-rich borides and Ni 3 Si precipitates, (iii) and the diffusion affected zone in which Cr-Ni-Fe-Mo-Cu-rich borides constituents existed. The hard and brittle eutectic compounds in the bonding zone posed deleterious effects on the mechanical properties of the joints. By increasing the holding time, the amount of these detrimental phases in the bonding zone decreased before finally a eutectic-free joint was achieved at the bonding time of 300 s, which was notably low as compared to the previous studies. The joint held for 0 s had the widest a-thermal solidification zone and the highest hardness was measured at the center of it (546 Hv). The average hardness of the diffusion affected zone on the UNS S3750 super duplex stainless steel side was approximately 450 Hv, which was markedly higher than that of the diffusion affected zone on the UNS N08825 side (302 Hv). The joint held for 0 s showed the lowest shear strength (239 MPa), while the maximum shear strength was obtained for the joint with a holding time of 300 s, approximately 487 MPa. Moreover, it was demonstrated that eutectic compounds were the principal cause of the brittle fracture in the joints containing the a-thermal solidification zone, whereas the joint with a complete isothermal solidification experienced a brittle fracture that occurred at the diffusion-affected zone of UNS N08825 alloy.

Determining the Influence of Braze Temperature on the Dissolution Behaviour and Kinetics of Isothermal Solidification During Transient Liquid Phase Bonding (TLPB) Ni-Based Superalloys

Determining the Influence of Braze Temperature on the Dissolution Behaviour and Kinetics of Isothermal Solidification During Transient Liquid Phase Bonding (TLPB) Ni-Based Superalloys

Flame spray assisted TLP bonding of Ti alloy to Al alloy

Thermal spray is a critical and widely used surface engineering technique that forms a protective and relatively rough coating on a substrate by spraying small, high-velocity, molten particles on the substrate. The present study experimentally investigates an innovative approach of transient liquid phase (TLP) bonding of Al2024 alloy to Ti–6Al–4V using Cu–Zn flame wire spray coat interlayer as an alternative to conventional foil interlayer. The optimized variables used in this work are bonding temperature and time of 580 °C and 60 min, respectively. Aggregation and growth of brittle Al2Cu and Al2CuMg eutectic phases in the vicinity of the interface of the specimen made with Cu–Zn foil as an interlayer was observed. The investigation confirms flame spraying reduces the possibility of aggregation and growth of brittle intermetallic phases and eliminates the harmful effects of Ti and Al oxide films. Moreover, flame spraying creates a rough surface which increases the shear strength of the joint up to 44 MPa. Defects in the flame spray interlayer in this experiment act as diffusion channels. As a result, the base metals dissolution in the vicinity of the joint interface and the isothermal solidification process occur more and faster in comparison to the conventional foil interlayer.

Transient liquid phase bonding of Ni3Al based superalloy using Mn–Ni–Cr filler

A new 70Mn–25Ni–5Cr (wt. %) filler with no traditional melting-point depressants (MPDs) of Si/B was used for transient liquid phase (TLP) bonding a directionally solidified (DS) superalloy based on Ni3Al intermetallic compound (IMC) at 1150 °C, a pressure of 2 MPa for 10–120 min. Experimental results show that the joint can be divided into 3 zones, isothermally solidified zone (ISZ), athermally solidified zone (ASZ) and diffusion-affected zone (DAZ), which was similar to microstructure of typical TLP joint bonded by Ni based filler. The joint microstructures were mainly composed of γ-Ni, γ′-Ni3(Al, Ta), MC and M23C6 carbides. Besides, in the ISZ, due to the enrichment of Al, which diffused from the base metal (BM), strip-shaped Ni(Mn, Al) phases formed in such region. The ASZ included γ-Ni and compounds of “equiaxed Ni(Mn, Al) + acicular Ni3Ta” owing to the composition segregation during the later stage of solidification. Moreover, as the bonding time increased, the volume fraction of Ni(Mn, Al) and MC carbides decreased owing to homogenization of alloying elements. After bonding for 120 min, sufficient isothermal solidification (IS) of filler can also decrease segregation of Mn/Al in front of the S/L interface and the Ni(Mn, Al)+Ni3Ta compounds were eliminated almost. Thus, the highest tensile strength was 816.9 MPa of the TLP joint of 120 min, which has been equivalent to that of BM (812 MPa). However, due to the existence of a few residual large MC carbides in the joint, which can induce the stress concentration and decreased the ductility of the joints, the highest joint elongation of 4.4% was only about 40% of the BM.

Analysis of temperature effect on isothermal solidification completion time during transient liquid phase bonding

ABSTRACT The occurrence of the anomalous behaviour of increased isothermal solidification completion time, with increasing temperature in transient liquid phase (TLP) bonding, is investigated for cases of one-dimensional and multi-dimensional migration of solid–liquid boundary, using a numerical model. The analysis shows that the anomalous behaviour is not restricted to the generally reported one-dimensional case but also occurs during multi-dimensional displacements of the solid–liquid interface. It is found that it is possible for isothermal solidification rate constant to increase with increasing temperature and yet increase. Instead of a mere increase in with temperature, it is the extent of the increase that determines if will reduce with an increase in bonding temperature. Increase in with temperature occurs when the increase in is inadequate to overcome the concomitant increase in the volume of liquid produced at a higher bonding temperature. Additionally, the study shows that in contrast to general expectation, an increase in temperature can cause the diffusion-controlled to decrease, notwithstanding a concomitant increase in the solute diffusion coefficient. This unique condition also causes to increase with an increase in bonding temperature. The theoretical findings in this work are corroborated by experimental observations reported in the literature.

Unravelling the metal borides evolution in the transient liquid phase bonding of Ni-based alloys via high-throughput transmission electron microscopy and first-principles thermo-kinetic calculations

The understanding of temperature and time-dependent metal borides precipitation/dissolution is crucial for the design of the transient liquid phase (TLP) bonding process of Ni-based alloys. It however remains elusive despite substantial research efforts for many years mainly owing to the uncertainty on the precipitated metal borides and the complex thermo-kinetics in the process. In this paper, we have unambiguously constructed the micro/nanoscale map of the precipitated metal borides in the TLP bonded Ni-based Inconel 718 superalloy via a high-throughput transmission electron microscopy (TEM) analysis. Five types of metal borides were found to precipitate in the diffusion affected zone (DAZ) when the isothermal solidification is completed. They are the M5B3 with stacking faults, Ti-rich M3B4, and Nb-rich M3B4 at the grain boundaries as well as single-crystalline M5B3 and M3B2 inside the grains. Notably, the crystal structure of the faulted M5B3 was rationalized by a hybrid modelling approach integrating first-principles calculation and TEM experiments. The sublattice model, with the optimized thermodynamic model parameters, was used to reproduce the metal borides precipitation map in the TLP process using DICTRA software. Coupling with multiscale simulation and experimental data, the present work built a modified thermo-kinetic model, which enables the design of the TLP bonding process of Ni-based alloys that often involves complicated time-temperature schedules and the precipitation/dissolution of a variety of different phases. The strategy can be applied to the design of the brazing process of other alloys or hybrid materials such as ceramics and metal.

Tailoring microstructure and mechanical performance of Hastelloy N-Hastelloy N superalloy joint through modifying brazing processing parameters and post thermal exposure

In this work, the Hastelloy N superalloy was joined using a newly-designed BNiCrWBSi brazing filler alloy. Effects of brazing processing parameters and post thermal exposure on microstructure and mechanical performance of the joints were evaluated. The microstructure in the joints was revealed with the aid of scanning electron microscope, electron back-scattered diffraction and transmission electron microscope analyses. The nano-indentation technique and shear test were employed to appraise the mechanical properties (such as nano-scale elastic modulus/hardness of specific phases and joint bond strength) of the joints. The results showed that the typical joint constitution could be depicted as Hastelloy N alloy/diffusion-affected zone (DAZ) or fine-grained zone (FGZ)/isothermal solidification zone (ISZ)/diffusion-affected zone (DAZ) or fine-grained zone (FGZ)/Hastelloy N alloy. The appearance of the FGZ was dependent on the brazing temperature, irrespective of the soaking time. The optimum joint bond strength of 588.9 MPa was produced at 1240 °C for 10 min. The typical fracture procedure and mechanism were discussed based on the detailed microstructural examinations and theoretical calculations. Apart from the brazing processing parameters, the post thermal exposure could also modify the microstructure through importing tiny boride phases into the ISZ, posing a reduction in shear strength. The microstructural evolution of Hastelloy N alloy joints unveiled and relationship built between the microstructure and mechanical properties will give a significant guidance for the design and application of the superalloy brazed joint.

Quantitative three-dimensional phase-field modeling of dendritic solidification coupled with local ensemble transform Kalman filter

Prediction of dendrite growth during alloy solidification is important for controlling the microstructures and mechanical properties of alloys. To this end, quantitative phase-field (QPF) models have been developed to accurately predict dendrite growth. However, a comprehensive method to calibrate the parameters of the QPF models has not been established. In this study, QPF modeling of dendritic solidification coupled with a data assimilation method based on the local ensemble transform Kalman filter (LETKF) is proposed. This modeling could simultaneously estimate multiple parameters and improve the prediction of dendrite growth. The modeling was applied to simulations of isothermal dendritic solidification in Fe–C–Mn ternary alloys. The results show that the solid–liquid interfacial energy and diffusion coefficients of solute atoms in a liquid phase can be estimated from the shapes of the growing dendrites. The accuracy of parameter estimation depends on the growth rate of the primary dendrite arm instead of the dendrite shape. This study demonstrates that LETKF-based data assimilation improves the prediction of solute concentration fields. The proposed QPF modeling paves a promising way to improve the prediction accuracy of conventional QPF models and derive new knowledge from the experimental data obtained by in-situ observations of growing dendrites.

Microstructural and Corrosion Properties of Ti-to-Zr Dissimilar Alloy Joints Brazed with a Zr-Ti-Cu-Ni Amorphous Filler Alloy

Titanium and Zircaloy-4 dissimilar alloys were brazed with a zirconium-titanium-copper-nickel amorphous filler alloy, and the resulting joint structures as well as their corrosion properties were examined. The microstructure of the brazed joints was investigated according to brazing holding time at 850 °C, and the corrosion property was analyzed by potentiodynamic polarization. During brazing, joints were produced by diffusion-induced isothermal solidification of the molten filler alloy. At a relatively brief brazing holding time of 5 min, a large segregation zone consisting of an active α-phase and a nobler intermetallic phase was generated in the joint center, which suffered from micro-galvanic corrosion. The presence of alloyed titanium deteriorated the nobility of the α-zirconium phase near the joint and induced galvanic coupling with cathodic base metals, resulting in massive localized corrosion. This localized corrosion caused the pitting behavior at the applied potential of −51.1~187.5 mV during anodic polarization. With a brazing holding time of 20 min, the concentration of the alloying elements was homogenized to eliminate the electrochemical potential difference and minimize the galvanic corrosion susceptibility of the joint region. This homogeneous joint resulted in a highly passive corrosion behavior comparable to that of the titanium base metal.

Formation mechanism of interfacial microstructures and mechanical properties of Ti2AlNb/Ni-based superalloy joints brazed with NiCrFeSiB filler metal

Dissimilar brazing of Ti2AlNb-based alloy and Ni-based wrought superalloy (GH536) was studied using NiCrFeSiB filler metal. The Ti2AlNb/GH536 joints were analyzed by scanning electron microscope (SEM) equipped with an electron probe micro-analyzer (EPMA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The formation mechanism of interfacial microstructure and mechanical properties of Ti2AlNb/GH536 joints were studied. The results indicated that Ti2Ni(Al,Nb), AlNi2Ti and TiB2 reaction layers were formed in the joint adjacent to Ti2AlNb base metal. These layers resulted in high micro-hardness and the weak link of the joint. γ solid solution was formed through isothermal solidification and β1-Ni3Si phase precipitated in the γ solid solution during cooling process. Ni3B, β1-Ni3Si and CrB phases appeared in the centre of the joint. Blocky and needle-like borides formed within the diffusion affected zone of GH536 base alloy. The maximum tensile strength of Ti2AlNb/GH536 joints reached 425 ​MPa ​at room temperature and the strength value of 373 ​MPa was maintained at 923 ​K.

Analysis of isothermal solidification kinetics during TLP bonding involving two-dimensional solid–liquid interface migration

ABSTRACT A new approach is used to model transient liquid phase (TLP) bonding without the simplification assumption of one-dimensional solid–liquid interface migration. In contrast to the general assumption that the single parameter, Φ, that is often used to represent the isothermal solidification kinetics during TLP bonding is constant, this work reports for the first time by coupled numerical simulation and experimental verification that a condition exists where Φ becomes a variable parameter that increases with time. This unique isothermal solidification kinetics behaviour is attributable to a combination of two-dimensional migration of the solid–liquid interface and solute diffusion in a direction along the increase in radius of curvature.