Brazing of refractory metals and AISI 304 stainless steel to Copper – The use of

ABSTRACT

Brazing of several refractory metals, namely, Tungsten, Molybdenum and Niobium as well
as austenitic stainless steel AISI 304 to pure Copper was performed in vacuum using CuGe10 filler
alloy. Interface between refractory metal and brazing seam in the W/Cu and Mo/Cu joints fabricated
at 1060 ºC using CuGe10 alloy is found to be sound and no intermetallic compounds are formed in
the transition zone. On the contrary, interaction between molten CuGe10 filler and Nb-substrate
during brazing process leads to the formation of continuous layer of product Nb 5 Ge 3 intermetallic. In
all cases, diffusion of Ge from the liquid filler into the Cu-substrate takes place during the brazing
cycle, but this phenomenon is expected to have no significant influence on structural integrity of the
product joints.
Austenitic stainless steel AISI 304 can also be brazed to Cu in vacuum at 1040 ºC using
CuGe10 filler. However, interaction of the AISI 304 stainless steel with liquid CuGe10 alloy results
in intergranular penetration of the melt into the steel substrate. Upon to the exposure to the brazing
conditions significant enrichment of Ge at the AISI 304/brazing seam interface was found, which
attributed to the formation of α 1 -(Fe-based) solid solution containing some Cu, Ni and Cr.

I. Introduction

In many advanced applications, refractory metals and austenitic steels are to be joined to
other materials, including Copper and its alloys. Although Tungsten and Molybdenum can, in
principle, be brazed in vacuum using Au-based filler alloys [1, 2], the high price of Gold resulted in
the current drive for cheaper alternatives. In this respect, the use of Co-Ge solid solution alloys as
brazing fillers attract special attention due primarily to the following reasons:

  • Suitable melting range of the alloys [3]
  • Relatively low cost
  • Good mechanical properties (ductility) [4]
  • Availability in various wrought forms (foil, strip, wire)
  • These alloys are non-magnetic

The Cu-Ge fillers are also worthy of serious consideration for brazing austenitic stainless
steels to Copper.

Obviously, for further progress in this field, it is necessary to have a clear understanding of
reaction phenomena occurring in the assembly during brazing. To the best knowledge of the present
authors, no information about interfacial reactions occurring upon vacuum brazing of refractory
metals and austenitic steels to Cu using Cu-Ge alloys is available. Therefore, here we will look at
some outstanding issues related to the high-temperature interaction of CuGe10 filler with AISI 304
stainless steel, high-purity Copper and some refractory metals, namely, Tungsten, Molybdenum and
Niobium.

The present work addresses three main issues:

  • Wetting behavior of the commercial CuGe10 brazing alloy in vacuum on the selected
    refractory metals (W, Mo and Nb), on oxygen-free Copper and AISI 304 stainless steel
    mating surfaces
  • Examination of the product brazing seam microstructure and rationalization of interfacial
    reactions occurring during brazing procedures
  • Evaluation of merits of the CuGe10 filler alloy for brazing of the selected refractory metals
    and/or AISI 304 stainless steel to Copper in high-temperature applications

II. Experimental

Plane-parallel slices of Tungsten (99.95%), Molybdenum (99.95) and Niobium (99.95), high-
purity Copper (CW009A) and AISI 304 stainless steel (0.08 C; 2.0 Mn; 1.00 Si; 18.0-20.0 Cr; 8.0-
10.5 Ni; 0.045P; 0.03 S; Bulk Fe) were used as initial materials. Prior to assembling, the mating
surfaces were cleaned in isopropanol and air dried. The brazing alloy CuGe10 was used in the form
of 50 μm foil. In order to ensure proper positioning of the components within the product
brazement, a small load was applied at the top of the assembly. Brazing was carried out in vacuum
(~5 x 10 -2 mbar).
After visual inspection, product joints were cross-sectioned and after standard metallographic
preparation, the brazement cross-sections were first examined by optical microscopy. Further
investigation was conducted with Scanning Electron Microscopy (SEM) and Electron-probe
Microanalysis (EPMA) using Energy Dispersive X-ray Spectroscopy (EDS).

III. Results and Discussion

3.1 Interaction of Copper and the selected refractory metals with CuGe10 filler alloy during
brazing

In Fig. 1 a series of micrographs of the brazed joint (cross-sectional views) based on Copper
and Tungsten fabricated with CuGe10 filler is presented.

a.
b.
c.
d.

Fig. 1: Microstructure of the cross-section of the W/Cu joint brazed in vacuum with CuGe10 filler: a) general view of the joint; b) brazement microstructure in the vicinity of the “triple” point and c) in the central part (bright-field optical images) together with d) Secondary Electron Image (SEI) of transition zone in the central area of the test sample. Note. The Vickers microhardness indentations are visible in the cross-sectional view of the transition zone (c).

After exposure to the vacuum brazing conditions, surface of the end-members of the
assembly remains shiny, and a grained-type microstructure was revealed as a result of thermal
etching. Interface between the W end-member and brazing seam is sound and formation of well-
developed fillets was observed in the vicinity of the “triple” points of the product assembly (Fig. 1a).
Interaction of liquid CuGe10 filler with the Cu end-member under these conditions leads to some re-
distribution of the alloy constituents, and therefore, Ge can still be detected far away (up to
approximately 150 μm) from the product W/brazing seam interface. However, microhardness
measurements show no significant difference in the Vickers microhardness values between the annealed Cu (end-member) and the product Cu-Ge (solid-solution) formed in the brazing seam (Fig.
2c).

It was repeatedly demonstrated [5] that morphology of the transition zone developed during
interfacial reactions can be (to a large extent) rationalized just using information about pertinent
phase diagrams. Unfortunately, literature search revealed no information about phase relations in the
ternary W-Cu-Ge system. As to the constituent binary systems, it is to be noticed that binary phase
diagram Cu-W is rather simple (Fig. 2a): no intermediate phases are present, and terminal solid-state
solubility of the components is negligible [3], whereas several intermetallic phases are present on
the Cu-Ge phase diagram (Fig. 2b). The equilibrium phases of the Cu-Ge system are: (1) Liquid; (2)
two terminal solid-solutions, with negligible solubility of Cu in Ge and with a maximum solubility
of ~13 wt. % Ge in Cu at the peritectic temperature 824 ºC, (3) the cubic intermediate phase  2 ,
which is stable at 614 to 698 ºC; (4) the orthorhombic intermediate phase, , which is stable above
549.5 ºC and melts congruently at 747 ºC; (5) the intermediate phase,  1 , which is stable below 636
ºC, with a trigonally distorted cubic structure; and (6) the hexagonal intermediate phase, , which
decomposes peritectically at 824 ºC.

a.
b.

Fig. 2: The binary phase diagrams: a) Cu-W and b) Cu-Ge

On the other hand, no experimental information was found about binary W-Ge system. The
only available variant of the W-Ge phase diagram is (tentatively) given in Ref. [6]. According to
these authors, an intermediate phase with a stoichiometry W 2 Ge 3 may exist below 1000 ºC, although
its thermodynamic stability should be very low.
Interaction of another refractory metal of the Sixth Group, namely, Molybdenum, with
molten CuGe10 filler alloy during joining to Cu-substrate leads to the morphology of the brazing
seam similar to that observed in the case the W/Cu joints. Again, no intermetallic compounds are
formed in the transition zone, and reaction of the liquid CuGe10 filler with Cu end-member results
in re-distribution of the alloy constituents: Ge was detected by EPMA at about 140 μm from the
product Mo/brazing seam interface.

a.
b.

Fig. 3: Bright-field optical image (a) and Secondary Electron Image (b) of the transition zone in the

Mo/Cu joint brazed in vacuum with CuGe10 filler.

No information about ternary phase diagram Mo-Cu-Ge is available. Nevertheless, it is again possible to understand the observed brazing seam microstructure with the help of corresponding binary phase diagrams given in Fig. 4 [3].

a.
b.

Fig. 4: The binary phase diagrams: a) Cu-Mo and b) Mo-Ge.

Like in the Cu-W system, no intermetallic phases exist also in the binary Cu-Mo system, and terminal solid-state solubility of the components at brazing temperature is negligible. In contrast, four intermediate phases are present on the Mo-Ge phase diagram (Fig. 4b), but thermodynamic stability of these intermetallic compounds is, obviously, still not sufficient to form germanides in reaction with molten CuGe10 alloy.

However, situation is completely different when CuGe10 filler is used for brazing of Cu to Nb. This can be appreciated by looking at micrographs taken from the cross-section of the Nb/Cu joint brazed with CuGe10 alloy (Fig. 5).

a.
b.

Fig. 5: Bright-field optical image (a) and Secondary Electron Image (b) of the transition zone in the

Nb/Cu joint brazed in vacuum with CuGe10 filler.

A continuous layer of tetragonal (tI32) Nb5Ge3 intermetallic phase containing up to 4 at. % of Cu is formed in the reaction between molten CuGe10 filler and Nb-substrate. Such reaction behaviour can be understood from the topology of the relevant phase diagrams given in Fig. 6. (Note. Hereafter various intermetallic phases will be denoted in their binary formulae.)

a.
b.

Fig. 6: The binary phase diagrams: a) Cu-Nb and b) Nb-Ge.

In the Cu-Nb system, terminal solubility of the components in the solid state is very low, while in the binary Nb-Ge system there exist three intermetallic compounds, with two of which, namely, Nb5Ge3 and NbGe2 being congruently melting phases. The pronounce difference in reactivity of the selected refractory metals in contact with liquid CuGe10 alloy can qualitatively be explained by comparing thermodynamic stability of the corresponding germanides. The enthalpy values of germanide formation of the selected refractory metals are listed in Table 1 [6].

Table 1

Enthalpy of formation, ΔH, at 298 K ( k J ) for germanides of the selected mole of atoms

refractory metals

GermanideW2Ge3Mo3GeMo5Ge3Mo13Ge23MoGe2βNb3GeNb5Ge3NbGe2
ΔH (298 K), kJ
mole of atoms
+4*)-15-19-15-14-15-40-29
*) calculated value

It follows from the Table that Niobium germanides are (generally) thermodynamically more stable than binary Mo-Ge compounds. It is, therefore, not surprizing that only the most stable (congruently melting) intermetallic phase, namely, Nb5Ge3 (containing some Cu) is formed during brazing at the reaction interface between Nb-substrate and molten CuGe10 alloy.

3.2 Reactivity of the CuGe10 filler in contact with AISI 304 stainless steel and Copper during brazing

In Fig. 7 a montage of bright-field optical images obtained from the cross-section of the brazed joint based on AISI 304 and Cu fabricated with CuGe10 filler is presented.

a.
b.
c.

Fig. 7: Optical (bright-field) images of the AISI 304/Cu joint brazed with CuGe10 filler alloy in vacuum: a) and b) general views of the brazement cross-section taken in the central part of the assembly and in the vicinity of the joint edge, respectively and c) magnified domain in the center part of the cross-section. Note the penetration of the molten filler into the steel substrate indicated by black arrow in (c) and the presence of some brazing alloy on the “vertical” surface of the steel end-member as indicated by white arrow in (b)).

It was observed that interface between AISI 304-end member and brazing seam is, generally, sound in the central part of the assembly as well as in the vicinity of the “edges”. During brazing at

1040 oC for 30 min interaction of liquid CuGe10 filler with the base Cu-component leads to some re-distribution of the alloy constituents, and therefore, Ge can still be detected far away (up to ~120 μm) from the AISI 304/brazing seam interface. It was also found that upon exposure to the brazing conditions, a significant enrichment of Ge at the AISI 304/brazing seam interface (up to ~14 wt. %) occurs and a notable penetration of the molten CuGe10 alloy into the stainless steel substrate (most likely, along grain boundaries) takes place. Because of fast diffusion in the liquid state, some Ni and

Fe are also detected inside the final Cu-based brazing seam.
To rationalize the observed reaction behaviour and the brazing seam microstructures

developed in the AISI 304/Cu joints, let us first turn attention to the phase relations in the alloy systems based on Cu or Ge with the major constituents of the AISI 304 stainless steel. The pertinent binary phase diagrams are collected in Fig. 8 [3] and the Cu-Ni-Fe ternary system is shown in Fig. 9 [7].

a.
b.
c.
d.

Fig. 8: The binary phase diagrams: a) Cu-Cr; b) Fe-Ge; c) Ni-Ge and d) Cr-Ge.

Fig. 9: The ternary Cu-Ni-Fe phase diagrams. The position of the immiscibility gap in the solid-state at various temperatures is indicated. The location of the critical point as a function of temperature is indicated by the dotted line and the extend of the solid miscibility gap between 1094 oC and 1220 oC, corresponding to the three-phase equilibrium L+ γ-Fe = (Cu) by the heavy line.

One can see that amongst the major components of the AISI 304 stainless steel, only Ni is soluble in Cu in both liquid as well as solid state. On the contrary, Fe and Cr are miscible in the liquid state, but exhibit a terminal solid-state solubility. If we now look at the ternary Cu-Ni-Fe system (Fig. 9), we can see the presence of a miscibility gap in the solid-state, with its size increasing with decreasing temperature. In ensuing discussion, we will keep in mind this information.

As to the phase relations involving Ge and major components of the AISI 304 stainless steel (viz. Fe, Ni, Cr), several features pertain to the present discussion are to be mentions:

MEMO-2023002-WEB (The use of CuGe10 for brazing refractory metals)-S

  • Solid-state solubility of the Fe, Ni and Cr in Ge is negligible.
  • On the contrary, solid-state solubility of Ge in the major constituents of the AISI 304 stainless steel is substantial.
  • In all binary systems discussed, there exists a non-stoichiometric phase with an average composition corresponding to the formula, “Me3Ge” (where Me = Fe, Ni or Cr), which is in equilibrium with the terminal solid solution. However, crystal structure of these intermediate phases are different: the ε-Fe3Ge has hexagonal (D019; hP8) structure, whereas the non- stoichiometric phases, “Ni3Ge” and “Cr3Ge” are cubic with L12 (cP4) and A15 (cP8) structure, respectively.
  • At 1040 oC and at Ge-concentration over ~13 at. % a superstructure of BCC (B2; cP2) ordered distribution of atoms is formed in the Fe-Ge system. It is denoted α1 and stems from a second-order reaction (transition) [8].
  • The solubility of Ge in α1 decreases with decreasing temperature (1130 – 900 °C) but increases below 900 °C.

In order to propose any plausible reaction mechanism, it is important to realize that here, we are dealing with several events (phenomena) which may occur simultaneously and sequentially. It can be conjectured that when liquid CuGe10 filler brought into contact with the AISI 304 surface, it will wet grain boundaries of the base austenitic stainless steel, and some Cu and Ge will also diffuse into the “bulk” of the steel grains. Penetration of the Cu-based melt into the steel-substrate can be rather deep, which can be seen in Fig. 7c: propagation of the Cu-based phase (perhaps, along grain or interphase boundaries of the stainless steel) into substrate in clear visible. At the same time, Ni as well as Fe and some Cr from the steel-substrate will leach into the CuGe-melt. As interaction proceeds, Ge from the CuGe-based melt can form with Fe an ordered α1 (Fe-based) solid solution containing some Cu, Ni, Cr.

As already explained, along the second contact surface of the assembly, i.e. at the Cu/ CuGe10-melt interphase interface, interaction of the liquid filler with the base Cu-substrate during the brazing process also results in re-distribution of the constituents, and Ge is detected quite far away from the AISI 304/brazing seam interface. As soon as the individual grains of the α1-(Fe- based) solid solution will be occluded by the liquid, they may also lift off from the substrate into the Cu-based melt. Subsequent solid-liquid interaction leads to some “spheroidization” of the α1- inclusion, which can be seen in Fig. 7c.

IV. Conclusions

Based on the findings of the present investigation the following conclusions can be drawn:

  • Tungsten, Molybdenum and Niobium can, in principle, be joined to Copper in vacuum using CuGe10 brazing filler.
  • Interface between refractory metal and brazing seam in the W/Cu and Mo/Cu joints brazed with CuGe10 alloy is found to be sound in the central part as well as at the “edges” of the assembly and no intermetallic compounds are formed in the transition zone.
  • In contrast, interaction between molten CuGe10 filler and Nb-substrate during brazing process leads to the formation of continuous layer of product Nb5Ge3 intermetallic containing up to 4 at. % of Cu.
  • During the brazing cycle, diffusion of Ge from the liquid filler into Cu-substrate takes place. However, it is expected that this phenomenon will not significantly affect structural integrity of the product brazements.
  • Austenitic stainless steel AISI 304 can also be brazed to Cu in vacuum using CuGe10 filler and interaction of the liquid CuGe10 alloy with the stainless results in intergranular penetration of the melt into the steel-substrate leading to the formation of interpenetrating- type microstructure.
  • Upon the exposure to the brazing conditions a significant enrichment of Ge at the AISI 304/ brazing seam interface was found, which attributed to the formation of the α1-(Fe-based) solid solution containing some Cu, Ni and Cr.

References

[1] M. H. Sloboda, Industrial Gold brazing alloys – Their present and future usefulness, Gold Bull., 4 (1971) 1- 8

[2] D. Easton, Yu. Zhang, J. Wood, A. Galloway, M.-O. Robbie, Ch. Hardie, Brazing Development and Interfacial Metallurgy Study of Tungsten and Copper Joints with Gold Copper Brazing Alloy, Fusion for Energy, 98 (2005) DOI: 10.1016

[3]  T. B. Massalski, Binary Alloy Phase Diagrams, ASM, Metal Park, Ohio, 1986

[4]  http://www.morganbrazealloys.co

E u t e c t i c

[5] A. A. Kodentsov, A. Paul, Diffusion Couple Technique: A Research Tool in Materials Science, in: A. Paul, S. Divinski (Eds.), Handbook of Solid State Diffusion-Diffusion Fundamentals and Techniques, Vol 2, Elsevier, 2017, pp. 207-275

[6] F. R. de Boer, R. Boom, W. C. M. Mattens, A. R. Miedema, A. K. Niessen, Cohesion in Metals-Transition Metal Alloys, North-Holland, 1989

[7]  K. P. Gupta, Phase Diagrams of Ternary Nickel Alloys, Indian Institute of Metals, 1990

[8]  O. Kubaschewski, IRON-Binary Phase Diagrams, Springer-Verlag, 1982

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