Athermal Solidification Zone Research Articles

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.

New insights into microstructural changes during transient liquid phase bonding of GTD-111 superalloy

The effects of joining temperature (TJ) and time (tJ) on microstructure of the transient liquid phase (TLP) bonding of GTD-111 superalloy were investigated. The bonding process was applied using BNi-3 filler at temperatures of 1080, 1120, and 1160 °C for isothermal solidification time of 195, 135, and 90 min, respectively. Homogenization heat treatment was also applied to all of the joints. The results show that intermetallic and eutectic compounds such as Ni-rich borides, Ni−B−Si ternary compound and eutectic-γ continuously are formed in the joint region during cooling. By increasing tJ, intermetallic phases are firstly reduced and eventually eliminated and isothermal solidification is completed as well. With the increase of the holding time at all of the three bonding temperatures, the thickness of the athermally solidified zone (ASZ) and the volume fraction of precipitates in the bonding area decrease and the width of the diffusion affected zone (DAZ) increases. Similar results are also obtained by increasing TJ from 1080 to 1160 °C at tJ=90 min. Furthermore, increasing the TJ from 1080 to 1160 °C leads to the faster elimination of intermetallic phases from the ASZ. However, these phases are again observed in the joint region at 1180 °C. It is observed that by increasing the bonding temperature, the bonding width and the rate of dissolution of the base metal increase. Based on these results, increasing the homogenization time from 180 to 300 min leads to the elimination of boride precipitates in the DAZ and a high uniformity of the concentration of alloying elements in the joint region and the base metal.

Microstructure and mechanical properties in TLP joint of FeCoNiTiAl alloy and IC10 superalloy using Mn-Ni-Cr filler

A filler metal of Mn-Ni-Cr was used to transient liquid phase (TLP) bonding high-entropy alloy (HEA) of FeCoNiTiAl and Ni3Al base IC10 superalloy for 10 min ~ 120 min at 1150 °C and a pressure of 2 MPa. There was no diffusion affected zone (DAZ) existed in the bonded zone as the filler material Mn-Ni-Cr contained no conventional melting-point depressants (MDPs) of Si/B. Hence, the TLP joint was composed of two isothermally solidified zones (ISZs) and an athermally solidified zone (ASZ). The ISZ beside IC10 consisted of strip Ni(Al, Mn) phases, resulted from the diffusion of Al atoms, and only γ-(Ni, Mn) phases formed in another ISZ adjacent to HEA base metal (BM). Moreover, sluggish diffusion of HEA can prevent the BM from dissolving and formed a straight bond line consisting of “(Hf, Ta)C + Al0.89Mn1.1” compound in the interface between HEA and bonded zone. Furthermore, increase of bonding time can lead to enrichment of Al in the bonded zone and induce composition segregation during solidification. A compound of “Ni(Al, Mn) + Ni3Ta” formed in ASZ of joint bonded for 30 min ~ 120 min. Hardness results revealed that formations of Ni(Al, Mn) phases and the compounds in ASZ can cause the alternant hardness distribution of valley and peak across the joint and lead to the generation of stress concentration. Therefore, the maximum shear strength, 269.9 MPa, was achieved in the joint dwelled for a period of 10 min.

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.

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.

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.

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.

Interfacial microstructure evolution and mechanical properties of Ni3Al-based alloy TLP joints with BNi-2 interlayer

Ni3Al-based alloys were successfully bonded via transient liquid phase (TLP) bonding technique with a BNi-2 interlayer. The typical joint consisted of three characteristic zones including: athermal solidification zone (ASZ) composed of Ni3B phase, isothermal solidification zone (ISZ) with dual-phase γ+γ′ structure and diffusion affected zone (DAZ) containing borides precipitation. With the increase of bonding temperature or holding time, the ASZ was gradually replaced by the ISZ, and Ni3B with high hardness disappeared. The shear strength of the joint improved from 391 MPa at 1050 °C for 30 min to 712 MPa at 1100 °C for 120 min, and the fracture occurred in the ISZ. To eliminate the borides precipitation in the DAZ of the as-bonded joints, a post-bond heat treatment (PBHT) process was implemented. The microstructure of the ISZ was homogenized and the hardness distribution across the whole joint was uniform. Therefore, the shear strength of the joint bonded at 1100 °C for 120 min increased to 838 MPa after PBHT at 1150 °C for 540 min.

The bonding temperature effect of the diffusion brazing Inconel 625 superalloy on the microstructure changes, corrosion resistance, and mechanical properties

The aim of this survey was an evaluation of the diffusion brazing (DB) bonding properties of the Inconel 625 superalloy using a BNi-3 interlayer. During the inter-diffusion and the dissolution of the alloy elements at the bonding temperature, and also cooling-cycle process, the joining zone (JZ) divided into three section namely isothermal solidification zone (ISZ), diffusion affected zone (DAZ), and athermal solidification zone (ASZ) which included a Ni-rich solid solution phase, Mo and Cr-rich boride precipitation, and Mo-rich boride precipitation and also Ni-Si binary phase, respectively. The cyclic potential-dynamic polarization tests indicated an inverse relationship between the corrosion behavior and the width of the ASZ. Also, the bonding process leads to the loss of resistance to pitting corrosion. The completion of the ISZ, together with the highest corrosion resistance attained at the bonding temperature at 1180 ᵒC. Likewise, with increasing bonding temperature, the eutectic compounds decreased and ISZ completed. The highest hardness belonged to the JZ/DAZ boundary at 1060 ᵒC and ASZ at 1120 ᵒC measured as 422 HV and 390 HV, respectively; which decreased by increasing bonding temperature, but the ISZ hardness increased.

Microstructural Evaluation during dissimilar transient liquid phase bonding of TiAl/Ni-based superalloy

The paper aims at understanding solidification phenomena and solid state precipitations during dissimilar transient liquid phase bonding of γ-TiAl intermetallic compound to IN718 Ni-based superalloy using a quinary Ni–7Cr–3Fe–4.5Si–3.2B (wt%) filler metal. The mechanisms of intermetallic formation via solidification and solid state reactions are discussed. The joint is featured by the formation of Ni-rich solid solution in an isothermal solidification zone near to IN718, Ni-rich solid solution and isostructural binary phases solid solution designated as Al1-xNi3Six in an isothermal solidification zone near to TiAl, boride/silicide and boro-silicide formation during binary and ternary eutectic-type solidification in an athermal solidification zone, fine nickel silicide precipitation during cooling in the joint centerline, and in-situ precipitation of Cr–Mo–Nb rich borides in a diffusion affected zone. On the other hand, Ni from the filler metal reacted with γ-TiAl base metal, leading to the formation of Ti2Ni, τ2-Al2TiNi and τ4-AlNi2Ti compounds on the reaction layer adjacent to TiAl substrate. The evaluation of different zones of the bond region showed that some brittle phases with high hardness are formed in the bond region. It was identified that the presence of the eutectic gamma solid solution between boride phases within the joint centerline can decrease the detrimental effect of the intermetallic phases on the joint toughness.

Shear strength/microstructure relationship for dissimilar IN738/IN718 TLP joints

MBF20 interlayer, as a commercial foil, was used for TLP bonding of IN738/MBF20/IN718 system at different temperatures and times. Microhardness and shear strength tests were conducted for all the samples. Hardness profile across the joints made at 1050 and 1100 °C revealed the microstructure regions as isothermal solidification zone (ISZ), athermal solidification zone (ASZ), and diffusion affected zone (DAZ). With increasing bonding time at each temperature, hardness peak at the joint centerline was removed but the hardness peak at DAZ remained. Increase of joint’s shear strength, increase of ISZ hardness, and decrease of DAZ hardness with increasing bonding temperature and/or time were discussed according to the effect of Larson-Miller parameter. Partial liquation of the base metals at 1150 °C/60 min condition caused reverse trend for variation of hardness and joint’s strength with bonding temperature. Fracture path of the joints with incomplete isothermal solidification was seen through the centerline eutectics. This path for the joints with complete isothermal solidification was observed through the bonding zone and the DAZ phases.

WIDE-GAP JOINTS IN INCONEL 738 SUPER ALLOY BY VACUUM BRAZING: MICROSTRUCTURE AND MICROHARDNESS

In this investigation, the effect of the parameters during vacuum brazing of wide-gaps (0.6 to 1.8 mm) in a Nickel-based super alloy Inconel 738, were studied. The temperatures used were 1120 °C and 1150 °C with times of 120 and 240 min. The filler metal was a mix of Ni-11Cr-3.5Si-2.25B-3.5Fe paste with Inconel 738 powder with a particle size of 73µm. From observations with Scanning Electron Microscopy, three microstructural zones were identified: Athermally Solidified Zone (ASZ), Isothermally Solidified Zone (ISZ) and Diffusion-Affected Zone (DAZ). The eutectic compounds identified by X-Ray Energy Dispersive Spectroscopy in the ASZ were of the Fe-Cr-Si type. When the brazing time was increased to 240 min and the temperature to 1150 °C with a gap of 0.6 mm and the application of a mixture ratio of 80/20 (Ni paste and Inconel powder, respectively), the volumetric fraction of eutectics decreased in the ASZ favoring isothermal solidification, diffusion of B and Si in the Ni matrix; hardness reached 260 HV, while the width of the DAZ decreased by 39% on average. The latter indicates that an adequate selection of brazing parameters allows to obtain better joints, with a reduced process time and a decrease in intermetallics, which are the main reason of cracks and fractures.

Intermetallic phase formation during brazing of a nickel alloy using a Ni–Cr–Si–Fe–B quinary filler alloy

ABSTRACTFundamental understanding of the intermetallic phase formation is the key for enhancing the robustness and reliability of the brazed joints. The paper addresses the phase transformations during brazing of the Hastelloy X nickel-base superalloy using the quinary Ni–13Cr–4.5Si–4.2Fe–2.8B (wt-%) braze alloy. The mechanisms of intermetallic formation via solidification and solid-state precipitation are discussed. The athermal solidification zone (ASZ) is featured by the formation of brittle and hard borides and boro-silicides that are formed via eutectic reactions. However, in contrast to other commercial B-bearing Ni-based filler alloys, it was identified that the presence of a high-volume fraction of eutectic gamma solid solution between boride phases within the joint centreline can alleviate the deleterious effect of the intermetallic phases on the joint toughness.

Microstructure and mechanical properties of Ni3Al-based alloy joint transient liquid phase bonded using Ni/Ti interlayer

Ni3Al-based alloy joints were fabricated by transient liquid phase (TLP) bonding with Ni/Ti composite interlayer. The typical interfacial microstructure of the bonded joint was primarily composed of three zones: isothermal solidification zone, athermal solidification zone and base metal. Various reaction products were observed in the bonded joint, including γ′-Ni3(Al, Ti), TiNi3, Cr-Mo-Fe rich precipitates and β-Ti. The TiNi3 phase in the athermal solidification zone was gradually reduced and eventually vanished by increasing the bonding parameters. The maximum room temperature shear strength of 828 MPa was obtained when the joint was bonded at 1220 °C for 60 min, while it was 625 MPa and 533 MPa at 650 °C and 800 °C, respectively. Cracks propagated primarily along the Ni3Al base metal and partially at the bonding interface when the joint was bonded at 1220 °C for 60 min, indicating the shear strength of the joint was almost equal to that of the base metal.

Influence of bonding time on the transient liquid phase bonding behavior of Hastelloy X using Ni-Cr-B-Si-Fe filler alloy

The effects of different transient liquid phase (TLP) bonding times on the microstructure and mechanical properties of Hastelloy X joints made by Ni–Cr–B–Si–Fe filler alloy were investigated. The specimens were TLP bonded at 1070 °C for holding times of 5, 20, 80, 320, and 640 min. The electron probe microanalysis (EPMA) results revealed that the main eutectic phases observed at the joints following incomplete isothermal solidification were Ni-rich borides, Ni-rich silicides, Ni–Si eutectic, and some Cr-rich borides. A high density of plate-like, blocky, and acicular (Mo and Cr)-rich borides were observed in the diffusion-affected zone (DAZ) of the samples; however, increasing the holding time decreased the contents of these phases. The solid-state diffusion was found to be a more effective transportation phenomenon than base metal dissolution at longer holding times. The increased DAZ thickness and the complete isothermal solidification as a result of the improved solid-state diffusion helped increase the uniformity of the hardness profile of the TLP bond at higher holding times (320 and 640 min). The results showed reverse relationship between the athermally solidified zone (ASZ) width and the bonding strength. The highest tensile strength (∼617 MPa) was achieved for the sample bonded at a holding time of 320 min; this strength was more than 80% of the base metal strength. A fractographic analysis of the tensile failure revealed a cellular fracture surface, exhibiting the characteristics of both brittle and ductile fractures. The sites prone to stress concentration and crack initiation were reduced with the completion of isothermal solidification.

Effect of Bonding Temperature on Microstructure and Mechanical Properties during TLP Bonding of GH4169 Superalloy

The effect of bonding temperature on the microstructure and mechanical properties of transient liquid phase (TLP) joints of GH4169 superalloy was investigated. Joining processes were carried out at 1040–1100 °C for 30 min using BNi-2 solder paste. The results showed that three distinct microstructural zones were formed in the joint region: an athermal solidification zone (ASZ), consisting of eutectic compounds; an isothermal solidification zone (ISZ), consisting of γ solid solution; and a diffusion affected zone (DAZ), consisting of Ni-Cr rich boride and Cr-Nb-Mo-rich boride compounds. With increasing bonding temperature, the amounts of eutectic compounds in ASZ first decreased and then increased. A eutectic-free joint centerline was obtained at 1080 °C. The maximum bonding shear strength reached 728.03 MPa due to the completion of isothermal solidification. Fractographic studies revealed that the boride compounds in ASZ and the intermetallic compounds in DAZ were the main causes for the failure of joints. The fracture mode of the sample bonded at 1040 °C was brittle, and the fracture path was along the ASZ. However, the fracture mode of the sample bonded at 1080 °C was ductile, and the fracture occurred along the DAZ.

The joint properties of a high-chromium Ni-based superalloy made by diffusion brazing: Microstructural evolution, corrosion resistance and mechanical behavior

The phase transformation during diffusion brazing of high chromium Ni-based Inconel 939 superalloy, using a ternary Ni-14.9 Cr-3.7 B (wt.%) interlayer was studied. The microstructural evolution during the bonding process was governed by isothermal solidification, athermal solidification and solid-state precipitation. A single-phase gamma solid solution was the only phase detected in the isothermally solidified zone (ISZ). However, the cooling-induced solidification led to the formation of proeutectic-gamma, Ni-rich boride, Cr-rich boride and eutectic-gamma at the athermally solidified zone (ASZ) which deteriorated the corrosion resistance of the joints. The ASZ width decreased by increasing the bonding time. A reverse relation between the corrosion resistance and the ASZ width was perceived. The complete isothermal solidification, as well as the highest corrosion resistance, were obtained at the bonding time of 90 min. The solid-state precipitation resulted in the formation of Cr-rich boride particles with two blocky and Widmanstatten morphologies in the diffusion affected zone (DAZ). The hardest structure compared to the other microstructural zones belonged to the ASZ in all the bonds. The shear strength of the joints increased in longer bonding times and a reverse relation between the ASZ width and the shear strength was witnessed. The highest shear strength (∼625 MPa) which was approximately 80% of that of the base metal was achieved in the 90-min sample.

Effect of bonding time on microstructure and mechanical properties of diffusion brazed IN-939

The applicability of the diffusion brazing technique for joining IN-939 superalloy using a Ni-Cr-B interlayer was assessed. The phases formed during the brazing process in the isothermally solidified zone (ISZ), athermally solidified zone (ASZ) and diffusion affected zone (DAZ) were studied. An inverse relationship between the bonding time and the width of the ASZ was perceived. The analytical model predicted the holding time of ∼36 min for completion of the isothermal solidification. This was corroborated by the experimental observations. The solid-state diffusion found to be a more effective transportation phenomenon than the base metal dissolution in increasing the concentration of solute elements in the ISZ. The formation of Cr-rich boride precipitates in DAZ was determined as the main contributing factor to the high microhardness values in this region. The shear strength of the joints raised by increasing the bonding time and reached the value of 655 MPa in the bonding time of 60 min implying a remarkable joint efficiency of ∼85%. Chromium was detected as the most influential element on the shear strength of the joints. Dimple-like and semi-cleavage morphologies were observed in the fracture surfaces of the bonds with complete and partial isothermal solidification, respectively.

Microstructural evolution mechanism during brazing of Hastelloy X superalloy using Ni–Si–B filler metal

ABSTRACTThe paper aims at understanding solidification phenomena and solid state precipitations during diffusion brazing of Hastelloy X nickel base superalloy using a ternary Ni–4.5Si–3.2 B (in wt-%) filler metal. The joint is featured by the formation of Ni-rich solid solution in an isothermal solidification zone, borides/silicide formation during eutectic-type solidification in an athermal solidification zone, on-cooling precipitation of fine nickel silicides in the joint centerline, in situ precipitation of Mo–Cr-rich borides in the diffusion affected zone and grain growth in the heat affected zone. The implication of the phase transformations on the joint properties is discussed. It is shown that Hastelloy X exhibited very fast isothermal solidification which can be attributed to its high Mo and Cr content that promotes in situ boride precipitation during brazing.

A study on the microstructure and properties of brazing joint for Cr18-Ni8 steel using a BNi7+9%Cu mixed filler metal

Vacuum brazing of Cr18-Ni8 steel was performed using the BNi7+9%Cu mixed filler metal at the different brazing temperature. Microstructure, microhardness and shear strength were investigated by means of scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), hardness tester and AG-25TA electronic universal material testing machine. The results indicated that the brazed region was divided into isothermal solidification zone (ISZ) and athermal solidification zone (ASZ). The γ-Ni (Cr, Fe, Cu) solid solution was formed in the ISZ, and its thickness would increase as brazing temperature increased. On the other hand, the generated microstructure was different by the change of chemical composition in the ASZ and its microhardness would increase as the brazing temperature increased. Elemental distribution indicated that Ni and Cu gradually gathered on both sides of the brazed joints as brazing temperature increased, while P and Cr were concentrated in the middle. In addition, the bonding behaviors of brazed joint was also illustrated based on the elemental distribution and microstructure distribution of brazed joint. Finally, an excellent brazing joint of Cr18-Ni8 steel was obtained at 980 °C for 30 min s and its shear strength was as high as 144 MPa.