**3.1 Microstructure of brazed joints of molybdenum with stainless steel at the application of brazing filler metal of Cu-Mn-Ni-Fe-1Si system**

In vacuum brazing of dissimilar materials such as molybdenum-stainless steel by brazing filler metal of Cu-Mn-Ni-Fe-1Si [16] system, good wetting of both the materials is observed, namely, molybdenum and stainless steel. This ensures the formation of smooth and tight fillets (**Figure 2(a)** and **(b)**).

In the central zone (matrix) of the brazed seam, a copper-based solid solution (92.58% Cu) solidifies, which contains a small amount of iron—2.87%, in addition to brazing filler metal component elements (**Figure 3(a)** and **(b)**; **Table 3**).

The more detailed study of chemical inhomogeneity of brazed seam matrix by mapping showed that dispersed particles of 0.5–1 μm size, enriched in iron and silicon, precipitate in the copper-based solid solution (**Figure 4**).

Alloys of the copper-manganese-silicon system contain two phases: a solid solution based on copper and manganese silicides [15]. In the presence of iron in the alloy, silicides are compounds having a hexagonal lattice isomorphic to the lattices Mn5Si3 and Fe5Si3 [17].

In the peripheral zone of the brazed seam, which borders on molybdenum, two reaction layers are observed, which precipitate in the form of narrow continuous bands along the brazed seam. One of them, based on molybdenum (51.21%), is enriched in iron (31.71%) and silicon (5.88%) and is located closer to molybdenum (**Figure 3(b)**). The second one—based on iron (68.02%)—is also enriched in silicon but contains no molybdenum. It borders on the copper-based solid solution. The width of these reaction layers is variable but does not exceed 5 μm (each). Their common feature is an increased concentration of silicon from 4.83 to 5.88% (**Table 3**). In some areas brazing filler metal penetrates along the grain boundaries of the stainless steel to a maximum depth down to 20 μm (**Figure 3(a)**).

It is evident that during brazing, the liquid brazing filler metal is saturated by steel component elements. Diffusion processes take place at the cooling of

#### **Figure 2.**

*The appearance of the brazed sample of molybdenum-stainless steel, produced using Cu-Mn-Ni-Fe-1Si brazing filler metal: direct (a); reverse (b) fillet.*

#### **Figure 3.**

*Microstructure (a, b) and studied regions (c) of the brazed joint at the application of Cu-Mn-Ni-Fe-1Si brazing filler metal.*


#### **Table 3.**

*Chemical composition of individual phases of brazed joint.*

brazed joints under nonequilibrium conditions and in the presence of a gradient of concentrations of chemical elements of base metal and brazing filler metal. Silicon and iron from the stainless steel diffuse into the brazing filler metal (**Figure 5(a)** and **(b)**).

In connection with limited solubility of the latter, two reaction layers form along the molybdenum-brazing filler metal interface (**Figure 5(b)**).

Longitudinal cracks (**Figure 4(a)**) are observed in the molybdenum-based reaction layer (51.21–52.59%), enriched in iron 31.71–32.07% (**Table 3**—Spectrum 1, 3), and on molybdenum-brazing filler metal interface (along the seam).

**109**

**Figure 4.**

**Figure 5.**

on copper and dispersed inclusions.

*Vacuum Brazing of Dissimilar Joints Mo-SS with Cu-Mn-Ni Brazing Filler Metal*

**3.2 Microstructure of brazed joints of molybdenum with stainless steel at the** 

*The electronic image of the microstructure of the brazed seam matrix (a) and the qualitative distribution of* 

Lowering of silicon concentration in brazing filler metal from 1 to 0.2 wt. % does not eliminate the formation of reaction layers on the interface of brazing filler metal molybdenum (**Figure 6(a)** and **(b)**). At brazing temperature of 1100°C (τ = 5 min), reaction layers also form along the brazed seam at the interface with molybdenum. The results of micro X-ray spectral analysis showed that silicon concentration in the reaction layer based on molybdenum (63.41%) does not exceed 0.92% (**Figure 6**), which is significantly lower than in the previous sample (in brazing with Cu-Mn-Ni-Fe-1Si brazing filler metal). The quantity of the other component elements, namely, iron, chromium, nickel, and manganese, is on the same level as in the previous sample (**Table 3**—Spectrum 1). The obtained investigation results show that microcracks form in some regions of the reactive layer (**Figure 6 (a)**). They are located normal to the reaction layer and base metal plates. Cracks are absent in the central zone of the brazed seam, consisting of a solid solution based

According to binary phase diagrams, consisting of metallic systems, the molybdenum-iron system has considerable regions of solubility at high temperatures.

**application of brazing filler metal of Cu-Mn-Ni-0.2Si**

*Schematic image of the formation of the brazed joint: before (a) and after brazing (b).*

*iron (b), copper (c), silicon (d), manganese (e), and nickel (f).*

*DOI: http://dx.doi.org/10.5772/intechopen.92983*

*Vacuum Brazing of Dissimilar Joints Mo-SS with Cu-Mn-Ni Brazing Filler Metal DOI: http://dx.doi.org/10.5772/intechopen.92983*

**Figure 4.**

*Welding - Modern Topics*

**108**

**Table 3.**

**Figure 3.**

*brazing filler metal.*

(**Figure 5(a)** and **(b)**).

brazed joints under nonequilibrium conditions and in the presence of a gradient of concentrations of chemical elements of base metal and brazing filler metal. Silicon and iron from the stainless steel diffuse into the brazing filler metal

*Microstructure (a, b) and studied regions (c) of the brazed joint at the application of Cu-Mn-Ni-Fe-1Si* 

 — 5.88 7.08 0.77 31.71 2.54 0.81 51.21 — 4.83 16.79 2.23 68.02 4.46 3.67 — — 5.65 6.56 0.86 32.07 2.27 0.00 52.59 — 4.92 16.50 2.48 66.79 4.63 4.68 — 1.74 — — — — — — 98.26 1.85 — — — — — — 98.15 — — 0.30 3.04 2.87 1.21 92.58 —

**O Si Cr Mn Fe Ni Cu Mo**

**Spectrum No. Chemical elements, wt. %**

In connection with limited solubility of the latter, two reaction layers form along

Longitudinal cracks (**Figure 4(a)**) are observed in the molybdenum-based reaction layer (51.21–52.59%), enriched in iron 31.71–32.07% (**Table 3**—Spectrum 1, 3),

the molybdenum-brazing filler metal interface (**Figure 5(b)**).

*Chemical composition of individual phases of brazed joint.*

and on molybdenum-brazing filler metal interface (along the seam).

*The electronic image of the microstructure of the brazed seam matrix (a) and the qualitative distribution of iron (b), copper (c), silicon (d), manganese (e), and nickel (f).*

#### **Figure 5.**

*Schematic image of the formation of the brazed joint: before (a) and after brazing (b).*
