Microstructural Modification by Redesigning the Chemical Composition of Ni 620 Filler Metal

Ni 620 is a widely used brazing alloy, especially in cases where high‐temperature strength and corrosion resistance are required. The formation of undesirable intermetallic phases in the brazing joint due to the addition of boron and silicon as melting point depressants affects the mechanical properties of the joint. Reducing the formation of intermetallic phases during brazing is an important issue in the application of Ni 620. In this study, two approaches, inoculation with Nb and variation of the B and Si content in Ni 620, are pursued to selectively influence the microstructure of the brazed joint. Therefore, the solidus and liquidus temperatures of the new brazing alloys are investigated by means of differential scanning calorimetry measurements. Also, the microstructure of brazed joints is analyzed and evaluated by scanning electron microscope/electron‐dispersive spectroscopy as well as the hardness properties using nanoindentaion. It is observed that the addition of Nb, as well as the variation of the B and Si content, leads to a change in the brittle phase band. Especially brittle borides can be reduced in this way. The results contribute to produce brazing joints with more adapted properties, despite low brazing temperatures and short holding times.


Introduction
Ni-based brazing alloys have been used to manufacture a wide range of products since their first appearance in the 1940s. [1]n the tooling industry, the manufacturing of injection molds is one of the greatest challenges.This requires, among other things, the use of high-strength materials and joints that can withstand high cyclic and thermal loads.These tools are made from hot work steel like the X37CrMoV5-1 which can withstand the conditions of use.Based on its minimal distortion, the X37CrMoV5-1 hot work steel is usually hardened, tempered, and annealed to improve the tool's performance and durability. [2]or joining this type of steel, brazing is typically done using Ni-based filler metals like Ni 620.Both brazing and hardening can be performed in one process due to the fact that the austenitizing temperature for X37CrMoV5-1 steel and the processing temperature of Ni 620 are both in the temperature range of 1000 °C < T < 1050 °C. [3]owever, due to the presence of the melting point lowering elements B and Si in the brazing alloy, brittle intermetallic phases such as borides and silicides are formed. [4]his occurs through isothermal solidification of the melt at the interface to the base material.In this process, B diffuses primarily into the base material, so that the local proportion of B in the melt decreases and its melting point is increased as a result.Solidification of the Ni solid solution takes place at a constant temperature.As a result, the residual melt is also enriched in the elements Si and B, which leads to the increased formation of brittle phases in the athermal solidification zone in the middle of the brazing seam.In the case of unsuitable process parameters, in particular if temperatures are too low, holding times are too short and, above all, brazing widths are too large, these brittle phases tend to manifest themselves during solidification in the form of brittle phase bands.This brittle phase band mainly affects the mechanical properties, and represents a metallurgical notch.Heat-induced stresses as well as residual stresses in the component, but also mechanical stresses during operation, can therefore result in the unexpected failure of these brittle connections.
For these reasons, efforts were made to avoid brittle phases in the brazed joint, which are based on the approach that the elements B and Si diffuse out of the brazing gap.This can be achieved by technically limiting the critical brazing gap width w crit , which for B-containing brazing alloys is about w crit ≤ 35 μm. [5]However, this requires a costly mechanical pretreatment of the components to be joined, since the tolerances with regard to surface quality, degree of precision, and the parallax of the components are very narrow.In addition, the deformation of the components due to thermal expansion and the release of residual stresses in the brazing process must also be considered.This ensures that the economic efficiency of the process is reduced.Another way to avoid brittle intermetallic phases is to control the process parameters temperature and time during brazing. [6]For example, the brazing temperature may be well above the austenitizing temperature of the steel, leading to greater diffusion of the elements B and Si and thus to a dissolution of the brittle phases.However, this leads to increased porosity at the interface between the brazed seam and the base metal, which weakens the joint. [7]By extending the holding time during brazing, the brittle intermetallic phases can also be dissolved as more time is available for the diffusion processes.However, this leads to an unwanted grain growth in the base metal, which is known to influence the mechanical properties. [8]To produce high-strength brazed joints using Ni-based filler metals, the previous approaches require increased costs, for example, due to high energy costs caused by long holding times and high brazing temperatures.The technical efforts to maintain the critical gap width also push up the costs for high-strength brazing joints.Therefore, there is a clear need for a methodology to modify the microstructure selectively without the long process times, high temperatures, or high level of engineering to limit the brazing gap width.
A new strategy for precision control over the microstructure of brazing joints created with Ni-based alloys is to add a minimal percentage of a refractory metal like Ti, Nb, or Mo to the brazing alloy, a process called inoculation.This study defines inoculation as the incorporation of refractory metals in small percentages with the goal of selectively manipulating the microstructure of the filler metal.The addition of the refractory metal is intended to reduce the formation of the very brittle and hard Ni-Sicontaining and Ni-B-containing phases by primarily binding to the B and Si by the refractory metal.This can be done because refractory metals usually have a higher affinity to the elements B and Si in comparison to Ni.The objective is to induce branching or breaking down of the brittle phase band through the influence of precipitates containing refractory metals, which act as nucleating elements for B and Si in the brazing material.
The influence of nucleation has already been investigated by Böttger et al. using phase field studies for Ni-based filler materials. [9,10]Previous research on of the austenitic stainless steel X5CrNi18-10 and the high-chromium brazing paste Ni 650 joints have already demonstrated a substantial effect of adding small amount of refractory metals on the microstructure of brazed joints. [11]In this study, a mass fraction of ω Ti = 3 wt% Ti was found to have a significant impact on the tensile strength of the joints.This effect was linked to the formation of Ti-containing precipitates, and their effect on the formation of brittle phases, which precipitate in a more homogeneous distribution.Further investigations were carried out with Ti-inoculated Ni 620 brazing pastes. [12]X38CrMoV5-1 hot work tool steel was used as the base materials to be joined.The brazing filler metal was prepared by mixing Ni 620 powder and Ti powder in amounts of 1 ≤ ω Ti ≤ 5 wt%.It appears likely that the presence of Ti in the precipitates causes heterogeneous nucleation, which accounts for the observed impact of Ti on the microstructure.This may occur when small, high-melting Ti precipitates or dispersoids such as TiC or TiB are present in the molten filler metal, promoting solidification at multiple points at higher temperatures.However, the flow properties of the brazing alloy were reduced by Ti inoculation, which was due to the formation of TiC with C diffused from the base material.A disadvantage that can be eliminated by using amorphous brazing foils, as the brazing material is placed directly in the brazing gap.
In this study, the inoculation effect is further investigated to improve the precision in brazing of Ni-based alloys.Therefore, the addition of Nb to Ni 620 brazing alloy and the adjustment of B and Si content are carried out to modify the microstructure in vacuum brazing of X37CrMoV5-1 hot work steel.Instead of using a brazing paste, the brazing alloys will be produced by melt spinning to create amorphous foils.Nb is used here as it is less reactive than Ti and thus the formation of carbides in the brazing seam can be avoided.At the same time, Nb is suitable as a nucleating element to break up the brittle phases.The addition of Nb in Ni-based superalloy is well known as the strengthener of the γ-phases by a dispersion of fine Ni 3 Nb in the matrix. [13]In addition to the precipitation-strengthening effect, the addition of Nb in Ni-based alloy system has some benefits, for example, improvement of high temperature strength and corrosion resistance. [14]The properties of Nb in Ni brazing alloys are already used in the Ni 810 brazing alloy in accordance with the ISO 17 672 standard, whereby the alloy also contains Mo and Cu. [15]It is important to note that the maximum amount of Nb that can be added to promote the inoculation effect is limited by its solubility in Ni, which is ω Nb % 5 wt% in the temperature range of brazing processes. [16]Therefore, this study will test the inoculation effect using ω Nb = 1.5 wt% and ω Nb = 3 wt% Nb additions in the system.Another way to modify the microstructure is by adjusting the amount of B and Si in the brazing alloy.The effect of a low B content in Ni base brazing alloys has already been investigated by Ghasemi et al. [17] It was found that a lower B content leads to an increased proportion of Ni solid solution in the filler metal, which increases the ductile content of the joint and can thus have a positive effect on the hardness properties.The difficulty with this method is to decrease the B content while keeping the brazing temperature range similar to that of Ni 620.The lowest amount of B needed to maintain the amorphous structure for the brazing foils is between 1.2 ≤ ω B ≤ 1.6 wt%, which allows for the liquid atomic structure to be preserved during quick solidification. [18]Reducing the amount of element B in a brazing alloy results in a rise of the melting point.To maintain a similar temperature range for brazing, the amount of the element Si must be increased.The contents of B and Si were chosen here based on conventionally available Ni-based brazing alloys.The purpose of this study is to examine the effects of Nb, as well as the variation of the B and Si amount in Ni 620 filler metal on its melting behavior and microstructure when brazing X37CrMoV5-1 hot work steel.

Experimental Section
The base material for the brazing experiments in this study were square bars made of X37CrMoV5-1 steel that were electro-slag remelted with a cross-sectional area of A = 25 mm 2 and were in an annealed condition.The chemical composition of the base material is listed in Table 1.
For the production of the brazing alloys, powders with high purity's Ni (99,99%), Cr (99,00%), Fe (99,99%), B (98,00%), Si (99,99%), and Nb (99,85%) from Goodfellow Cambridge Ltd. were used.The exact compositions of the brazing alloys investigated can be seen in Table 2. Pure Ni 620 brazing powder was used as a reference.For the production, the brazing powder mixture was placed in the amount of m = 10 g in an Al 2 O 3 crucible and homogenized in the laboratory tube furnace R50-250-13 from Nabertherm GmbH.This process took place at T = 1250 °C for t = 2 h in an argon atmosphere.
The brazing alloys created in this way were then processed into amorphous foils in the Melt-Spinner SC from Edmund Bühler GmbH.A schematic representation of the melt-spinning process is shown in Figure 1.
The previously melted brazing alloys were placed in a boron nitride (BN) crucible and remelted by means of inductive heating.To avoid oxidation, the manufacturing process took place under a vacuum atmosphere at p ≤ 10 À5 mbar.After complete melting of the brazing alloy, the liquid brazing alloy was pressed through a nozzle onto the rotating copper wheel by means of an overpressure Δp in the BN crucible.Cooling rates of up to Ṫ ≤ 10 6 K s À1 could be achieved.The foil produced in this way was then removed from the collection tube.The process parameters set for the melt spinning process can be found in Table 3.
The minor deviations of the process parameters were based on the adjustment of the process to produce foils with the same quality as far as possible.
After the foils were manufactured, they were analyzed using differential scanning calorimetry (DSC) to investigate the melting points and detect any solid phase transitions from amorphous to crystalline.The DSC/thermogravimetry system SETARAM SETSYS Evolution from Setaram Instruments was used for this purpose.A material of approximately m = 0.01 g was placed in an Al 2 O 3 crucible for the DSC analysis.The sample was heated from room temperature to T = 1300 °C at a rate of Ṫ = 0.17 K s À1 , followed by cooling to T = 200 °C at a rate of Ṫ = 0.25 K s À1 .Two heating cycles were run to validate the data.
For brazing, two rod-shaped base materials with a crosssectional area of A = 5 Â 5 mm 2 were brazed with a butt joint.For this purpose, the brazing foils were placed between the two rods and a force of F = 10 N was applied.The schematic structure can be seen in Figure 2.
A PVA MOV 553 high-vacuum furnace from PVA TePla AG was used for brazing.The entire brazing process was carried out under high-vacuum conditions with a pressure of less than p ≤ 10 À5 mbar.A brazing temperature of T = 1000 °C was used.The heating rate was set at Ṫ = 10 K min À1 , and the holding time was t = 10 min.After brazing, the furnace was cooled down to room temperature.
After brazing, samples were cut and embedded in epoxy resin.They were then ground and polished using a 1 μm diamond suspension.Microstructural observation and analysis were performed using a Phenom XL scanning electron microscope (SEM) from Thermo Fisher Scientific Inc.The SEM was equipped with electron-dispersive spectroscopy (EDS) and operated at an acceleration voltage of U = 15 kV in backscattered electron mode.
To visualize the distribution of hardness across the brazed joints, microhardness indentation was performed using the Nanoindenter FISCHERSCOPE HM 2000.And, 21 indents were made in the x-direction and 31 indents in the y-direction, resulting in a total of 651 indents, with a spacing of s = 5 μm between each indent on all samples.A test load of F = 20 mN was applied for each measurement using a Berkovich indenter.The converted hardness in HV Calc was measured and the location coordinates associated with the measurement were recorded.From these data, a map of the hardness distribution could then be created, which allowed statements to be made about the properties in the joint.

Manufacturing of Brazing Foils
Figure 3 shows the foils produced by melt spinning with the obtained thicknesses d 2 .With all the alloys investigated here, it was possible to produce foils that are suitable for further use in brazing.The foils have all the maximum possible width b s , which corresponds to the width of the slot nozzle of the BN crucible of about b s = 10 mm.The foils also exhibit extremely ductile behavior, which is a first indication of an amorphous or partially amorphous structure of the microstructure.Furthermore, it is noticeable that the edges of the foils inoculated with Nb show  significantly more shape deviations and thus these foils exhibit the lowest quality of the alloys investigated here.The highest quality was achieved in the production of the reference foil Ni 620.

DSC Measurements
The results of the DSC analysis of Ni 620 and its modifications are shown in  c). [19]Looking at the DSC measurement of Ni 620 1.5Nb, the very pronounced peak a can also be seen.In this case, peak b is shifted to higher temperatures and is less strong.Peak c is less pronounced than in comparison with Ni 620 and also falls off much more flatly after reaching its maximum.What is also noticeable in the case of the filler metal Ni 620 1.5Nb is the appearance of a new fourth peak, which is referred here as peak x.This peak is only very weakly pronounced, but can still be seen between the two peaks a and b.This can probably be attributed to the melting of an Nb-containing phase.Looking at the filler alloy Ni 620 3.0Nb, it is immediately apparent that peak a is much less pronounced than in the case of Ni 620 and Ni 620 1.5Nb.Peak x is much more noticeable, which suggests that the proportion of the Nb-containing phase (peak x) has increased compared with the (Ni,Cr,Fe) 3 B phase (peak a).Peak b moreover has been further shifted to higher temperatures and peak c can no longer be identified in the measurement.A superposition of peak c with peak b is possible here.Looking at the two DSC measurements of Ni 620 1.5B 7.5Si and Ni 620 2.3B 7.5Si, the clearly broader and at the same time flatter peak a is evident compared to Ni 620.This is primarily due to the fact that a higher sample mass was used for the measurements of the two samples, which leads to a flattening of the peaks.The onset temperatures nevertheless correspond almost exactly to those of Ni 620.Peak b, in contrast, is shifted to higher temperatures in both samples and is also only very weakly pronounced.Peak c cannot be identified in the measurement.Overall, the DSC measurements show that the   melting interval was only slightly influenced by the manipulation of the chemical composition of the brazing alloy and is still within a range relevant for brazing.Furthermore, in the case of the Nb-containing brazing alloys, a phase probably containing Nb was identified within the melting interval between the two peaks a and b.

Microstructural Analysis
Figure 5 shows the microstructure of the brazed joint Ni 620/X37CrMoV5-1.The typical brittle phase network for this compound can be seen in the brazing material.Three different phases, as well as the solid solution, can be recognized on the basis of the cross section.On the one hand, these are light gray globular phases, which extend over the entire cross section of the joint.Based on EDS measurements and comparison with the literature, these can be identified as (NiFeCr) 3 B, which are the last to solidify in the solidification process.Around the globular phases, both dark gray Ni-silicides (Ni 3 Si) and black finely distributed Cr borides (CrB) can be seen.In correlation with the DSC measurements, the CrB phases are the first to solidify.
In contrast, the precipitation of Si-rich Ni 3 Si phases is expected to occur only in the solid state as precipitation from the Ni solid solution, as already described by Ruiz-Vargas et al. [19] The solid solution is mainly composed of Ni, Cr, Fe, and Si and can be detected in the intermetallic phase band and at the interface between the filler metal and the base metal.Cr borides can also be measured in the diffusion affected zone (DAZ).These occur due to the strong diffusion of the B along the grain boundaries of the base material.Due to the fact that Cr is present in the base material, these phases form in the DAZ.In the detection of the elements, it must be pointed out at this point that the quantitative determination of B is not possible.However, the analysis of the intermetallic phases in Ni 620 brazing alloys is well known from the literature. [19,20]igure 6a    composed of Ni-rich solid solution.The diffusion zone is characterized by a widespread distribution of dark gray phases.As already observed in Figure 4, these are Cr-rich borides.
As the amount of Nb added increases to ω Nb = 3 wt%, a slightly different microstructure can be observed in Figure 6b compared to the case of Ni 620 1.5Nb.The distribution of intermetallic phases changes from a network to a fractured phase band.This phase band consists of the same phases that are seen in Ni 620 1.5Nb except for the phase (Ni,Cr,Fe) 3 B, which could not be detected here.However, the presence of relatively large Nb-rich phases, which could not be observed in Ni 620 1.5Nb, is notable.This can most likely be attributed to the higher Nb content in this filler alloy.
Here, the phases Ni 3 Si, solid solution and CrB can be detected.The absence of the (Ni,Fe,Cr) 3 B phase can be explained by the  Possible phase.
significantly reduced content of B in the brazing alloy.The proportion of Ni 3 Si is also significantly higher, which can be expected from the higher Si content.The chemical compositions with the corresponding standard deviations of the phases measured by EDS are shown in Table 4.The phases have been estimated using the chemical compositions and literature values.It should also be noted that a quantitative determination of element B using EDS is not possible.

Microhardness Measurements
Figure 8a shows an SEM image of the brazing gap of the Ni 620/ X37CrMoV5-1 joint.The results of the reference sample have already been published by the authors in ref. [21].The hardness indentations can be seen both in the base material and in the brazed joint.Figure 8b shows the corresponding hardness distribution, represented by the color scaling.After heat treatment in the brazing process, the martensitic base material shows a homogeneous hardness distribution in the range of 561 ≤ HV calc ≤ 667.In the brazed seam itself, the pronounced brittle phase band can be recognized by hardness values exceeding H ≥ 1000 HV calc .The highest values measured here are in the range 1300 ≤ HV calc ≤ 1400.
The following Figure 9 shows the hardness mappings of the modified brazing alloys in the brazed state.Compared to the unmodified reference sample in Figure 8, there is a change in the hardness distribution.Figure 9a shows a significantly more homogeneous distribution of hardness in the brazing gap than the reference Ni 620, whereas Figure 9b shows a brittle phase band in a significantly more ductile brazing matrix.The measurements of the brazing material Ni 620 1.5B 7.5Si seen in Figure 9c show that the overall hardness values in the brazing gap are significantly lower than compared to the reference.A distinct hardness band is almost not visible here.Figure 9d shows the hardness distribution of the brazed joint with Ni 620 2.3B 7.5Si.Here, the brittle phase band can be seen from the increased hardness values in the brazing gap, but is somewhat lower compared to the reference with Ni 620.
For a more precise classification of the hardness values, the respective mean values of the hardness measurements within

Conclusions and Outlook
In the investigations presented here, the inoculation of Nb into Ni 620 brazing alloy to modify the brittle phases in the brazing process was investigated for the first time.Furthermore, the Si and B content of the Ni 620 brazing alloy was varied with the aim of reducing brittle B-containing phases.For this purpose, the modified alloys were investigated with regard to their producibility in the melt spinning process and their melting behavior was examined by means of DSC.Subsequently, brazing tests were carried out and the joints thus produced were characterized by means of EDS/SEM and nanoindentation measurements.The following findings can be drawn from the investigations: 1) the modifications did not negatively affect the manufacturability of the alloys into a foil by melt spinning; 2) the melting behavior of the alloys was influenced by the modification of the Ni 620 filler metal, but with regard to the melting range, the alloys are still in a relevant range for brazing; 3) inoculation with Nb leads to a change in the microstructure of the brazing gap, and in particular, the proportion of solid solution was reduced, whereas a new Nb-containing phase was identified; 4) the reduction of the B content and simultaneous increase of the Si content in Ni 620 led to a significant increase in the proportion of Ni 3 Si in the brazing gap and a significant reduction in the proportion of (Ni,Fe,Cr) 3 B; 5) the hardness measurements clearly show that the hardness values in the brazing gap were reduced as a result of the modifications made.
Further questions were raised in the course of the investigations, which will be considered in the following studies.These relate primarily to the strength properties of the modified brazing alloys, with toughness being the most important factor.In this study, the exact localization of boron was not able to be determined clearly due to limitations of the methodology used.To address this, a more comprehensive analysis of the phase composition and diffusion phenomena will be conducted using electron probe microanalysis.

Figure 4 .
Endothermic peaks can be observed in the melting interval region, which is marked in each case by the intermediate area within the dashed lines.In the case of Ni 620, three different peaks, which are named as a, b, and c, are located at T a = 984 °C, T b = 1015 °C, and T c = 1039 °C, corresponding to the melting of three different phases.These are according to Ruiz-Vargas et al. (Ni,Cr,Fe) 3 B (Peak a), Ni solid solution (Peak b), and CrB (Peak

Figure 2 .
Figure 2. a) Temperature curve over time in the brazing process and b) schematic representation of the brazed specimen.

Figure 4 .
Figure 4. Differential scanning calorimetry (DSC) analysis of Ni 620 and its modifications, measured in Al 2 O 3 crucible under flowing argon atmosphere with v = 20 mL min À1 , heating rate Ṫ = 10 K min À1 .
depicts the microstructure of Ni 620 1.5Nb/ X37CrMoV5-1 joint.The joint is observed to have a widely distributed intermetallic phase network embedded into a Ni solid solution.The intermetallic phases are primarily composed of white Ni-rich Nb phases, black Cr-rich borides, dark gray Ni 3 Si phases, and light gray (Ni,Cr,Fe) 3 B. The white Ni-rich Nb phase may have formed due to the segregation of Nb in Ni-solid solution during solidification.The interface is primarily

Figure 8 .
Figure 8. a) SEM image of a brazed joint Ni 620/X37CrMoV5-1 tested by means of nanoindentation and b) associated evaluation of the hardness measurements, according to ref. [21].

Table 1 .
Chemical composition in wt% of the hot work steel X37CrMoV5-1 according to DIN EN ISO 4957.

Table 2 .
Chemical composition of filler metal powder mixtures in wt%.

Table 4 .
Chemical composition of the phases measured by EDS in the brazing gap, B is not quantifiable using EDS. a)