Complete Isothermal Solidification Research Articles

Effect of Bonding Time, Composition of Interlayer, and Postbond Heat Treatment on Transient Liquid Phase Joints of Inconel 718 Superalloy

The investigation aims to explore the relationship between the microstructure and mechanical properties of transient liquid phase‐bonded joints of Inconel 718 (IN718) nickel‐based superalloy, utilizing BNi‐1 and BNi‐2 interlayers with a thickness of 50 μm, produced at 1150 °C for varying durations. Microstructural analysis, facilitated by scanning electron microscopy, and phase analysis, utilizing energy‐dispersive spectrometer, reveal a diverse range of microstructures, including continuous eutectic intermetallic phases and eutectic‐free configurations under different bonding conditions. Additionally, the interfacial diffusion of IN718/BNi‐2 is simulated by molecular dynamics. Notably, higher mass fractions of Cr in the interlayer are found to prolong the time of isothermal solidification. The use of BNi‐2 interlayers, 50 μm in thickness, results in joints achieving isothermal solidification within a remarkably brief 10 min, a significantly shorter duration compared to previous studies. Mechanical tests, encompassing microhardness and uniaxial tensile experiments, are carried out on all samples. The microhardness of the isothermally solidified zone decreases with increasing bonding time. The tensile strength of bonded joints increases with extended bonding time before isothermal solidification completion, followed by a subsequent decline. The highest tensile strength (893.3 MPa) is observed in joints using BNi‐2 interlayer after applying postbond heat treatment, representing approximately 88.5% of the base metal tensile strength.

Comprehensive microstructural and mechanical characterization of transient liquid phase bonded austenitic stainless steel by Ni–Cr–Si–B amorphous interlayer

In this research, along with the joining of the gas turbine fuel nozzle part from austenitic stainless steel 316L alloy with BNi-2 interlayer by the transient liquid phase bonding process, microstructure test, shear strength and tensile strength tests have been placed. Bonding at a temperature of 1080 °C and a holding time of 45 min has led to the completion of isothermal solidification in the bonding zone. Microstructural and phase investigations showed that austenitic stainless steel 316L undergoes phase transformation after exposure to temperature conditions and the δ ferrite phase is formed as separate areas due to the diffusion of the solid state of ferrite-forming elements, and the grains of the base metal are 32 μm have reached 43 μm, which has caused a decrease in the shear strength of the base metal from 362 MPa to 347 MPa. Despite the completion of isothermal solidification in the joint centerline due to the boron diffusion of the solid state into the base metal and achieving the maximum shear strength of the joint (402 MPa), two generations of boride deposits have been formed. The first-generation borides based on nickel-iron-chromium-molybdenum are formed during heating up to the bonding temperature, and the second-generation boride deposits are based on iron-chromium-molybdenum, which were deposited in situ during the isothermal solidification stage. The microhardness test confirms the uniform distribution of hardness in each of the bonding zones, which feedback was determined in the tensile strength test. The tensile strength of the bonded sample was 485 MPa, which is approximately 82 % of stainless steel 316L. In both shear and tensile tests, the presence of small dimples indicates the ductile failure of the joint samples in base metal.

Brazing manufacturing technology of plate-fin heat exchanger for solid oxide fuel cells

The plate-fin heat exchanger (PFHE) is the core equipment used to achieve the high efficiency of solid oxide fuel cell (SOFC), however, the issues of high-performance brazed joints in the manufacturing of the PFHE have been a challenge due to the poor mechanical properties. This study proposes a brazing manufacturing technology of isothermal solidification with the optimized post bonding heat treatment strategy, to synergistically improve the strength-ductility property and homogenize microstructure of brazed joints, aiming at guaranteeing the high energy efficiency of SOFC. The results show that brazing at 1065 °C for 25 min achieves the complete isothermal solidification and an intermetallic-free joint centerline with numerous borides generating in the diffusion-affected zone. Solution treatment then dissolves large quantities of acicular and blocky borides. The uniformity of grain size and kernel average misorientation distribution is also improved due to recrystallization, which becomes more pronounced after solution aging treatment. In addition, solution aging treatment results in an improvement in the ultimate tensile strength of the brazed joint, which is more prominent than that after solution treatment. However, the increase in elongation after solution aging treatment is smaller than after solution treatment, while still much higher than the as-brazed joint due to the dissolution of boride precipitates and growth of twin boundaries. The results demonstrate that the proposed brazing manufacturing technology not only homogenizes microstructure, but also significantly improves strength and ductility, further promoting the long-life operation of SOFC.

Effect of two-step heating transient liquid phase bonding on microstructure and mechanical behaviour of IN-738LC gas turbine components

The long isothermal solidification and homogenization time is the major concern in repairing turbine parts by the conventional transient liquid phase method. In this study, microstructure and morphology of the proposed two-step heating transient liquid phase joints were compared to results of the conventional ones conducted at the same temperature. After the isothermal solidification, deleterious precipitates such as Ni3Si, Ni3B and CrB were removed. The application of the two-step technique increased the shear strength of the joints up to 28% (516–674 MPa) due to collision of two non-planar interfaces and formation of the longer bonding line. The shear fracture surfaces of all bonds with the complete isothermal solidification stage represent a dimple pattern that is characterized by ductile fracture. The stress-rupture life of the joint at 982 °C was about 85% of the superalloy service life at this temperature.

Effect of diffusion brazing time on microstructure, isothermal solidification completion and microhardness distribution during joining of Nicrofer 5520 superalloy using a liquated Ni–Cr–B interlayer

In the present study, the effect of time on the microstructural evolution, distribution of elements, type of precipitated phases, and microhardness of diffusion brazed Nicrofer 5520 superalloy using a BNi-9 (Ni–15%Cr–3.7%B) interlayer was investigated. The diffusion brazing was carried out at 1120 °C for seven different holding times by using a 38 μm thick interlayer. The results indicated that the holding time of 30 min was sufficient for complete isothermal solidification. The eutectic gamma solid solution, Ni-rich borides, and Cr–Mo-rich borides were identified near the bond center before the completion of isothermal solidification, (Ni, Mo, Cr)-rich borides and Cr–Ni–Mo–Fe carbides with different morphologies were found in the diffusion affected zone (DAZ). In the holding time longer than 30 min and, the volume fraction of the precipitated phases in the DAZ decreased, and they transformed into rounder particles. Accordingly, the chemical composition of the interlayer became more homogeneous, resulting in a lower DAZ phases’ density that is happened over the higher brazing times. As the holding time was increased, the chemical composition in the interlayer became more similar to the base metal, and the microhardness in the bond zone and its surrounding became more uniform.

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.

Hybrid additive manufacturing of an electron beam powder bed fused Ti6Al4V by transient liquid phase bonding

Hybrid Additive Manufacturing (HAM) is a production strategy enhancing the flexibility of the already versatile Additive Manufacturing (AM) techniques. AM of Ti6Al4V, on the other hand, has been of great interest to numerous research works, thanks to the unique corrosion, biomedical and mechanical properties of the alloy. Hence, this research marks the first report on the HAM of Ti6Al4V by Transient Liquid Phase (TLP) bonding of an Electron Beam Powder Bed Fused (EB-PBF) sample to a conventional one. A copper interlayer was used for bonding, and the TLP process was performed at 890 °C and 970 °C for 60 min. Shear strength test was carried out and the results showed the highest shear strengths of 579.3 and 662.5 MPa for TLP bonding at 890 °C and 970 °C, respectively. By increasing the bonding temperature to 970 °C, no Cu-rich phases were observed in the microstructure, as opposed to the 890 °C samples, and a complete isothermal solidification without intermetallic phases was achieved. Moreover, the 970 °C TLP sample was featured with a much better microstructural integrity and homogeneity in both the base metals and the bonded zone. TLP bonding at 970 °C resulted in a more ductile fracture surface than that bonded at 890 °C. The strong differences between the two TLP bonds were primarily attributed to the faster diffusion rate of elements along the joint and base metal at higher temperatures.

Microstructural evaluation of Hastelloy-X transient liquid phase bonded joints: Effects of filler metal thickness and holding time

Transient liquid phase (TLP) bonding was investigated in Hastelloy-X samples with different filler metal thicknesses (20, 35, 50, 65, and 100 μm) and holding time (5, 20, 80, 320, and 640 min) to obtain optimum bonding parameters. Microstructural evaluations using electron probe microanalysis (EPMA) and electron backscattered diffraction (EBSD) show that the central eutectic phases present in the athermally solidified zone (ASZ) are Ni3B, Ni2Si, and CrB, and the precipitates formed in the diffusion-affected zone (DAZ) are MoB, CrB2, and Mo2B5. According to the results, decreasing the filler thickness as well as increasing the holding time helps realize the completion of isothermal solidification and reduction in the density of precipitates in the DAZ, leading to a joint with more uniform properties. Diffusion of boron and silicon to longer distances with increasing holding time causes the removal of Cr-rich borides in the DAZ and the formation of Mo-rich silicide at the joint interface. Decrease in hardness of ASZ and DAZ due to the elimination of brittle phases in these zones during long holding time causes more uniform hardness distribution in the joint area. The best results are obtained for the sample joined with the 35 μm-thick filler metal for 640 min holding time.

Relationship of isothermal solidification completion and precipitate formation with mechanical properties of Inconel 939 joints vacuum TLP bonded by an amorphous Ni–Cr–Fe–Si–B filler alloy

In this research, vacuum transient liquid phase bonding of Inconel 939 superalloy was performed using an amorphous Ni–Cr–Fe–Si–B filler alloy at different temperatures from 1060 °C to 1180 °C, and the microstructure and mechanical properties of the joints were investigated. The results showed that the bonding region can be categorized into three athermal solidification, isothermal solidification, and diffusion affected zones. The ASZ region consisted of a eutectic phase while the ISZ region included a Ni-rich gamma single-phase solid solution. Also, the DAZ involved cubic-shaped, needle-like and grain boundary-shaped borides. The width of DAZ increased with increasing the bonding temperatures, and it also led to a decrease in the ASZ/joint ratio. The ASZ was disappeared at 1180 °C. The highest hardness value was obtained for the DAZ region of the sample bonded at 1180 ᵒC, and a direct relationship was detected between the bonding temperature and ISZ hardness. The addition of bonding temperature also led to the improvement of the shear strength and joint efficiency. It was also found that an inverse relationship can be detected between the shear strength and the ASZ/joint ratio. A joint efficiency of ∼79% was obtained for the bonding temperature of 1180 °C.

Role of interlayer composition in microstructure and mechanical properties during TLP bonding of GTD-111/IN-718 superalloys

An investigation was carried out to assess the applicability of transient liquid phase bonding of two dissimilar super-alloys with different interlayers. The effect of using three types of interlayer such as BNi-2, BNi-3, and BNi-9 on microstructure and mechanical properties was studied in the GTD-111/IN-718 system at 1100 °C for different bonding time. To determine the compositional changes and microstructure in the joint region, field emission scanning electron microscopy equipped with energy dispersive spectroscopy was utilized. The formation of Ni3B in the athermally solidified zone (ASZ) is controlled by the B content and, accordingly, the morphology of Ni3Si is governed by the Si content. The Cr content might impede the relocation of B from the interlayer into the base metal and the formation of CrB inside the ASZ is dominated by the Cr content. The high micro-hardness of the eutectic compounds is originated from the formation of boride matrixes such as Ni or Cr boride. The shear strength of the joint using BNi-9 after the completion of isothermal solidification is lower compared that that using BNi-3 and BNi-2, which could be related to the absence of Si in the filler metals constituent and the significant presence of Cr in BNi-9.

Microstructural and mechanical properties assessment of transient liquid phase bonding of CoCuFeMnNi high entropy alloy

The transient liquid phase (TLP) bonding of CoCuFeMnNi high entropy alloy (HEA) was studied. The TLP bonding was performed using AWS BNi-2 interlayer at 1050 °C with the TLP bonding time of 20, 60, 180 and 240 min. The effect of bonding time on the joint microstructure was characterized by SEM and EDS. Microstructural results confirmed that complete isothermal solidification occurred approximately at 240 min of bonding time. For samples bonded at 20, 60 and 180 min, athermal solidification zone was formed in the bonding area which included Cr-rich boride and Mn3Si intermetallic compound. For all samples, the γ solid solution was formed in the isothermal solidification zone of the bonding zone. To evaluate the effect of TLP bonding time on mechanical properties of joints, the shear strength and micro-hardness of joints were measured. The results indicated a decrement of micro-hardness in the bonding zone and an increment of micro-hardness in the adjacent zone of joints. The minimum and maximum values of shear strength were 100 and 180 MPa for joints with the bonding time of 20 and 240 min, respectively.

Analysis of microstructure, kinetics of isothermal solidification and mechanical properties of IN718/MBF-20/SS316L TLP joints

ABSTRACT In this research, transient liquid phase (TLP) bonding of IN718 and SS316L dissimilar alloys was carried out using MBF-20 amorphous interlayer. The effects of bonding temperature and time ranging from 1030°C to 1110°C and 1–30 min on the microstructure and mechanical properties of the bonded samples were studied. The mechanism of the microstructure formation and the solidification sequence at the joint area were discussed. Two diffusion models assuming stationary and moving solid/liquid interface were used to study the isothermal solidification kinetics and the time required for the complete isothermal solidification (tf) at different temperatures was predicted. The results suggest that the tf values obtained by the moving interface approach are closer to those of the experiments. The minimum difference between the experimental value of tf and that calculated by the moving interface approach was about 26%, while it was about 106% by considering the stationary interface approach. A more uniform hardness distribution across the joint area and the highest shear strength values were observed in the samples with complete isothermal solidification. The maximum shear strength of 440 MPa was obtained for the sample bonded at 1030°C for 30 min. This was attributed to the formation of a eutectic-free joint in these samples.

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.

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.

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.

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.

EBSD study of dissimilar transient liquid phase joining of duplex stainless steel SAF 2205 to nickel-based superalloy IN X-750

SAF 2205 and IN X-750 were joined using the TLP method. The joining process was carried out at 1100 °C for different times using BNi-3 interlayer. Microstructural study and identification of the present phases in the bonding regions were performed using optical microscopy and scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and electron backscatter diffraction technique. Microhardness and shear strength tests were done to measure the mechanical properties. Different phases such as BN, Ni3B, Ni2B, CrB, Cr2B, and Ni2Si were formed in the bonding area. EBSD analysis results showed that the amount of the formed phases in the bonding area is a function of the bonding time. As the holding time increased, the amount of created phases in the bonding area increased from about 8 to 11 wt.%. The microhardness profile of the joining area also behaved differently as the bonding time changed. The maximum measured microhardness was about 850 HV which appeared in the athermally solidified zone of the bonded sample for 1 min. The maximum shear strength was observed in the bonded sample with complete isothermal solidification, approximately 72% of the shear strength of SAF 2205 and 85% of IN X-750.

Evolution of microstructure and mechanical properties on dissimilar transient liquid phase (TLP) bonding of GTD-111 and IN-718 by BNi-9 (AWS A5.8/A5.8M) interlayer

Dissimilar transient liquid phase (TLP) bonding of GTD-111 and IN-718 nickel-based superalloys was investigated using BNi-9 (AWS A5.8/A5.8M) as an interlayer. The effect of the bonding time on the microstructure and mechanical properties of the TLP-bonded specimens was studied at 1100 °C using optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy-dispersive spectrometry (EDS), and micro-hardness and shear strength analyses. Joint microstructural studies revealed that at the bonding times shorter than 75 min, nickel- and chromium-rich borides were formed with the joint centerline. The volume fraction of intermetallic compounds decreased with the increase of the bonding time and the diffusion of boron element whereby the isothermal solidification eventually completed after 75 min. Micro-hardness investigations along the bonding joint showed that the hardness of an isothermal solidification zone (ISZ) decreased with the completion of isothermal solidification. The hardest zone in the joint structure belonged to the athermal solidification zone (ASZ). The shear strength test results showed that incrementing the bonding time increased the shear strength where the shear strength value at 1 min/1100 °C (320 MPa) raised to 590 MPa after the completion of isothermal solidification.

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.