Rate Of Isothermal Solidification Research Articles

Kinetics of isothermal solidification during transient liquid phase bonding of Inconel 718 superalloy by differential scanning calorimetry

In the transient liquid phase bonding of Inconel 718 alloy, the time for isothermal solidification is measured by using the cyclic differential scanning calorimetry method. The enthalpy of solidification of the remnant liquid is used in the current investigation to determine the isothermal solidification rate and time. The differential scanning calorimetry results reveal a reduction in interlayer width during the heating and melting stage of transient liquid phase bonding due to the rapid interdiffusion of elements. The result also reveals that the isothermal solidification time for a eutectic-free joint depends upon the actual width of the interlayer material. The actual width of the interlayer material prior to isothermal solidification is found to be less than the initial width of the interlayer material due to rapid diffusional solidification. The measured isothermal solidification time and rate for transient liquid phase bonding of the Inconel 718 joint are in good agreement with the experimental and theoretical results.

Effect of bonding temperature and bonding time on microstructure of dissimilar transient liquid phase bonding of GTD111/BNi-2/IN718 system

The effect of bonding temperature and bonding time on the microstructure of transient liquid phase (TLP) bonding named GTD111 and IN718 superalloys, using a commercial Ni–B–Cr filler alloy (BNi-2) interlayer were evaluated. The sandwich assembly was kept in a vacuum furnace at temperatures of 1050, 1100, and 1150 °C for 1, 15, 30, 45, 60, and 80 min until the TLP process occurred. Microstructural characterization was carried out via optical microscopy, scanning electron microscopy (SEM) equipped with field emission energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Microstructural assessments displayed those in little bonding times, the joint microstructure includes continuous eutectic intermetallic phases and longer times cause eutectic free microstructure. The bonding temperature affects the isothermal solidification rate, while, at low bonding temperatures microstructure of the joint centerline is controlled by diffusion of melting point depressant (MPD) elements. Despite, at high bonding temperature effect of base metal alloying elements on the joint microstructure development was more marked. The results showed that athermally solidified zone (ASZ) size reduces with increasing bonding temperature and time due to diffusion of boron into the base metal.

A new numerical model for simulating transient liquid phase (TLP) bonding of dissimilar materials involving 1D, 2D and 3D solid–liquid interface migration

ABSTRACT A new numerical model which incorporates the occurrence of liquid-state diffusion (LSD) during isothermal solidification is developed to simulate transient liquid phase (TLP) bonding of dissimilar materials in contrast to previous dissimilar TLP bonding models where LSD during isothermal solidification is ignored. The new model which conserves solute and avoids the assumption of concentration-independent solid-state diffusivity applies Murray–Landis–Space-Transformation-based self-adaptive spatial discretisation with a unique adaptation of up/down-wind estimation scheme in a hybrid explicit-implicit finite difference approach. It is found that although LSD can increase isothermal solidification rate during dissimilar-bonding, yet, it is possible for the isothermal solidification completion time, to be longer during dissimilar bonding compared to similar bonding, notwithstanding the higher solidification rate of the dissimilar bonding. Moreover, contrary to general notion, numerical analyses show that solid-state diffusion exerts significant influence on during TLP bonding that involves LSD and so should be adequately considered.

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.

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.

Assessing the Influence of Cr and Fe in the Filler Metal on Dissolution and Isothermal Solidification Kinetics During TLPB of Ni-Based Superalloys

The influence of chromium and iron’s absence (BNi-3) and presence (BNi-2) in boron-containing filler metals (FM) on the TLPB behavior of IN718 base metal (BM) was examined. Results indicate that the absence of Cr and Fe in the FM significantly aggravated BM dissolution and gap widening, particularly when similar degrees of brazing beyond the FM liquidus temperature occurred. This led to a higher complete isothermal solidification time, despite a higher rate of isothermal solidification (IS) and initial boron uptake displayed by the IN718/BNi-3 joint. EDS and quantitative image analysis performed within the DAZ concluded that differences observed in the rate of IS were related to the amount and type of borides forming in the DAZ. The lack of Cr in the BNi-3 favored the formation of Cr-poor:B-rich Cr(Mo,Nb)B borides, leading to a higher rate of B uptake by the base metal. Joints made from the Cr containing BNi-2 FM displayed a combination of Cr-rich:B-poor CrB and Cr(Mo,Nb)B borides leading to lower rates of B uptake.

High Content of Boride-Forming Elements in IN617 Nickel-Based Superalloy Enables Short Isothermal Solidification Time During Transient Liquid Phase Bonding

Isothermal solidification stage during transient liquid phase bonding is the key to preclude intermetallic phase formation during solidification of the liquid phase. The rate of isothermal solidification is controlled by solid-state diffusion. Therefore, the bonding time required to complete isothermal solidification is generally long. This paper reports a very short isothermal solidification time for transient liquid phase bonding of IN617 solid solution nickel-based superalloy using a boron-containing filler metal. A eutectic-free bond with limited grain growth in the base material with high shear strength is achievable by utilizing a short (i.e., 5 minutes) thermal bonding strategy. The fast isothermal solidification of IN617 is unveiled by considering the role of in-situ boride precipitation during TLP bonding. The presence of a high amount of boride-forming elements (Cr and Mo) in the base material composition promotes the formation of a high volume fraction of the boride precipitates in the diffusion affected zone. The boron-free regions in the vicinity of boride precipitates cause a steep concentration gradient for boron diffusion during isothermal solidification, which enables fast isothermal solidification.

The Role of Base Metal Chromium in Determining the TLPB Behavior of Ni-Based Alloys Using a Boron-Containing Braze

The TLPB behavior of Ni/Ni-Si-B and Ni-Cr-Fe/Ni-Si-B base metal/braze joints was examined to determine the role chromium in the base metal has during joining of Ni-based alloys using a boron-bearing braze metal. The isothermal solidification (IS) rate in these two braze joints at 1075 °C was measured using DSC. Microstructural evolution within the diffusionally affected zone (DAZ) was examined both in terms of the type of borides formed and their volume fraction, using image analysis and phase equilibria predictions. Overall, the results show that when the base metal (BM) contains an appreciable concentration of Cr the IS rate decreases significantly. This was due to the formation of a low volume fraction of CrB within the DAZ, dictated by phase equilibria. Conversely, in the pure Ni base metal (BM) the thermodynamically favored boride was Ni3B, present at high volume fractions, resulting in rapid IS. A direct link between the boride microstructure of the DAZ and the rate of IS was established.

Investigating the transient liquid phase bonding (TLPB) behaviour of a Palladium containing Ni-B based braze filler metal

ABSTRACT A cyclic DSC method has been adapted to study the TLPB behaviour and isothermal solidification rate (IS) in an IN625/CPW475 base metal/filler metal joint at 1000°C. Analysis of the DSC data resulted in a prediction of the maximum brazing clearance (MBC) as a function of braze time. The rate of IS was determined to be linear with the square root of time with a rate constant k = 4.59 µm/min1/2, once the braze temperature and isothermal conditions were reached. During transient heating from the solidus temperature up to the braze temperature there is a region of rapid boron uptake by the base metal and an accelerated growth rate of the IS zone (ISZ). This initial ISZ layer, 2wISZi was predicted to be 29.5 µm. In the current case of an initial gap width of 80 µm, this initial ISZ represents 36% of the gap width.

Influence of variable diffusion coefficient on solid-liquid interface migration kinetics during transient liquid phase bonding

ABSTRACTA new finite difference numerical model with variable diffusion coefficient is developed by using an explicit-fully-implicit hybrid method and Landau transformation with adaptable spatial discretization to study the solid–liquid interface migration kinetics during transient liquid phase bonding. It is found that notwithstanding the fact that diffusion coefficient varies with concentration, the kinetics of the solid–liquid interface displacement can still be represented by a constant parameter, known as isothermal solidification rate constant. Nevertheless, the results of the numerical model, which are verified with experimental data, reveal a key reason why the extent of diffusion-controlled isothermal solidification during TLP bonding deviates from a linear relationship with, i.e. deviation from parabolic law behaviour. The deviation occurs when the concentration dependency of diffusivity changes with time. This key finding constitutes a new explanation for the occurrence of deviation from the parabolic law behaviour and it can explicate the deviation occurrence in systems where prior suggested reasons in the literature do not apply.

Determining the kinetics of transient liquid phase bonding (TLPB) of inconel 625/BNi-2 couples using differential scanning calorimetry

A cyclic DSC method was developed which can measure the rate of isothermal solidification (IS) of the centerline eutectic in a Ni-based superalloy TLPB couple using a single sample. This was done by measuring the enthalpy of solidification of the remaining eutectic liquid, in a half braze joint when cooled from the brazing temperature. The analysis revealed that a rapid diffusional solidification (DS) process occurs during initial melting such that approximately 50% of the eutectic liquid is already diffusionally solidified by the time the peak brazing temperature is reached. This initial DS process greatly reduces the total time for solidification at the brazing temperature. Experimental DSC determinations of the complete isothermal solidification time agree well with theoretical predications.

Analysis of Diffusional Solidification in a Wide-Gap Brazing Powder Mixture Using Differential Scanning Calorimetry

The diffusional solidification (DS) of a mixed powder system, consisting of pure Ni base metal powder and BNi-2 braze powder, suitable for wide-gap brazing was investigated using differential scanning calorimetry (DSC) and parallel microstructural examination. It was determined that very little interdiffusion between the powders developed in the solid state prior to braze powder melting. Once liquid formed, rapid DS occurred such that, when the powders were loosely mixed together, only 20 to 50 wt pct of the potential liquid fraction actually developed, leading to poor densification. Separating the braze and Ni into a layered structure leads to less DS, increased liquid formation (i.e., 35 to 80 wt pct of the potential liquid fraction) and improved densification. The rate of isothermal solidification in layered structures consisting of 30 and 40 wt pct BNi-2 braze material was determined using DSC. After 30 minutes of braze time at 1323 K (1050 °C), complete IS occurred, thus avoiding the formation of a continuous network of large borides. The final microstructure of the IS mixtures consisted of a continuous matrix of solid solution Ni, with isolated and dispersed borides.

Effect of Temperature and Sheet Temper on Isothermal Solidification Kinetics in Clad Aluminum Brazing Sheet

Isothermal solidification (IS) is a phenomenon observed in clad aluminum brazing sheets, wherein the amount of liquid clad metal is reduced by penetration of the liquid clad into the core. The objective of the current investigation is to quantify the rate of IS through the use of a previously derived parameter, the Interface Rate Constant (IRC). The effect of peak temperature and initial sheet temper on IS kinetics were investigated. The results demonstrated that IS is due to the diffusion of silicon (Si) from the liquid clad layer into the solid core. Reduced amounts of liquid clad at long liquid duration times, a roughened sheet surface, and differences in resolidified clad layer morphology between sheet tempers were observed. Increased IS kinetics were predicted at higher temperatures by an IRC model as well as by experimentally determined IRC values; however, the magnitudes of these values are not in good agreement due to deficiencies in the model when applied to alloys. IS kinetics were found to be higher for sheets in the fully annealed condition when compared with work-hardened sheets, due to the influence of core grain boundaries providing high diffusivity pathways for Si diffusion, resulting in more rapid liquid clad penetration.

TLP bonding of IN738/MBF20/IN718 system

The effects of temperature (1050, 1100 and 1150 °C) and time (1–20 min) on the microstructure of the transient liquid phase (TLP) joints of IN-738/MBF-20/IN-718 system were investigated using OM, SEM and SEM/EDS analysis. During bonding, no pressure was applied and just the weight of the upper specimen was effective. A continuous eutectic structure was observed at the joint centerline of the samples bonded at 1050 °C for 30 min. The same was seen at the joints made at 1100 °C for 1 min which were detected as Cr- and Ni-borides. The morphology and composition of phases at diffusion affected zone (DAZ) were somewhat different in IN-718 and IN-738 halves. Increasing the bonding time at 1050 and 1100 °C bonding temperatures increased the degree of isothermal solidification. In case of the samples, TLP-bonded at 1150 °C, the diffusion of the liquid phase into the grain boundaries of the base metals occurred and a wavy bonding interface and no DAZ phases were observed. A critical bonding temperature (between 1100 and 1150 °C) was seen above which the rate of isothermal solidification was reduced. Larson-Miller equation (P = TB [C + ln(tB)]) was used to estimate the required time for completion of the isothermal solidification. The calculated value of 33 for system-dependent constant C, resulted in the best estimate at different bonding temperatures.

Effect of bonding temperature on the microstructure and mechanical properties of Ti-6Al-4V to AISI 304 transient liquid phase bonded joint

Transient liquid phase (TLP) bonding process was performed to join Ti–6Al–4V and AISI 304 austenitic stainless steel using a copper interlayer. The effect of the bonding temperature on the microstructure and mechanical properties was studied in the range of 870–960°C. Microstructural characterization was carried out through optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that the rate of isothermal solidification was increased by increasing the bonding temperature. In addition, when the bonding temperature was increased to 960°C, the width of the eutectic and intermetallic zones was completely removed, leading to complete isothermal solidification. According to the results, a minimum hardness value of 216VHN was measured in the joint zones and the highest shear strength was 374MPa, as obtained for the sample joined at 960°C. The fracture analysis showed different fracture morphologies for different bonding temperatures.

Influence of bonding variables on transient liquid phase bonding behavior of nickel based superalloy IN-738LC

The effect of process variables on the microstructure and properties of transient liquid phase (TLP) bonded IN-738LC superalloy was investigated using AMS 4776 filler metal. Continuous centerline eutectic phases, characterized as nickel-rich and chromium-rich borides, were observed at the joints with incomplete isothermal solidification. The eutectic width decreased with the increase of holding time and the increase in initial gap size resulted in thicker eutectic width in the samples bonded at the same temperature and for equivalent holding times. In contrast to the conventional expectation of the increase in the rate of isothermal solidification with the increase of temperature, rate decrease was observed with the increase of temperature to 1150°C. The investigations demonstrated that low isothermal solidification rate was not only due to the enrichment of liquid phase with the base alloying elements such as Ti but also because of the reduction of solid solubility limit of B in the base metal contributed to the reduction of isothermal solidification rate. Microhardness and shear strength tests were carried out in order to investigate mechanical properties of the bonded samples. In the bonding condition in which isothermal solidification did not completely occur, eutectic constituent with the highest hardness in the bond region was the preferential failure source. The results showed that homogenized joints had the highest shear strength.

Microstructure evolution and mechanical properties of transient liquid phase (TLP) bonded joints of TiAl intermetallics

TiAl intermetallics were joined by transient liquid phase (TLP) bonding technique using Ti/Ni filler metal. The effects of bonding parameters and composition of filler metal on isothermal solidification and interfacial microstructure of the joints were studied. It was found that a continuous α2 layer was formed at the joint interface when the bonding temperature was below 1125 °C. This α2 layer hindered the atom interdiffusion between the TiAl substrate and liquid filler metal, which resulted in a long holding time required for complete isothermal solidification. When the bonding temperature was 1150 °C, no continuous α2 layer was formed in the joint and the isothermal solidification rate was enhanced. Furthermore, with decreasing of Ni content in the filler metal, the isothermal solidification rate was reduced due to the decrease in dissolution of TiAl substrate into the liquid filler metal. The shear testing results showed that the highest shear strength at ambient temperature (281 MPa) and at high temperature (800 °C) (243 MPa) were achieved, when the joint was bonded at 1150 °C for 5 min.

Elements Diffusion Asymmetry in Transient Liquid Phase Diffusion Bonding of Dissimilar Metals

12Cr2MoV steel pipe and T91 steel pipe were joined by transient liquid phase diffusion bonding process (TLP) in argon atmosphere using FeNiCrSiB amorphous filler. TP304H steel pipe and 12Cr2MoV steel pipe were joined by the same method using Fe78Si9B13 and BNi2 amorphous filler. The qualified welding joint can be achieved by suitable adjustment of process parameters. The diffusion asymmetry in TLP of different metals has been analyzed. The research shows that there exists a diffusion asymmetry in TLP of different metals. The main reason is the difference of isothermal solidification rate in different materials, which results in a deviation of the bonding interface from the original centerline to T91 and TP304H side.

Effects of boron and silicon on microstructure and isothermal solidification during TLP bonding of a duplex stainless steel using two Ni–Si–B insert alloys

The influence of B and Si on microstructure and isothermal solidification during transient liquid phase (TLP) bonding of a duplex stainless steel using MBF-30 (Ni–4·5Si–3·2B, wt-%) and MBF-35 (Ni–7·3Si–2·2B, wt-%), was investigated. Based on experimental studies, the formation of Ni3B within the joint centreline is dependent on B content; the morphology of Ni3Si is dominated by Si concentration. There was a deviation between the times for complete isothermal solidification obtained by the experiment and the conventional TLP bonding model. Isothermal solidification of the liquid dependent on different solidification regimes is suggested to be a controlling factor contributing to the change in the rate of isothermal solidification observed using the two filler metals.

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.