Microstructure and properties of W-Cu/1Cr18Ni9 steel brazed joint with different Ni-based filler metals

3.1 Microstructure

Figure 1 shows the microstructure of the W-Cu/1Cr18Ni9 steel brazed joint with NiCrSiBFeTi brazing filler metal. It could be seen from Figure 1 that the NiCrSiBFeTi brazing filler metal and the base metals formed a good metallurgical combination with a width of approximately 0.12 mm, and an obvious diffusion layer existed in the brazing seam region near the W-Cu alloy side. A smooth and clear bonding interface without obvious defects such as pores and microcracks was formed, which means that NiCrSiBFeTi brazing filler metal exhibited good wettability and spreadability on the interface of W-Cu alloy and 1Cr18Ni9 steel. Moreover, some dark areas could be found in the brazing seam region.

Figure 1:

Microstructure and feature regions of the NiCrSiBFeTi brazed joint.

The EDS analysis results of A, B, C, and D areas (shown in Figure 1) in the brazing seam region are shown in Table 3. According to the binary phase diagram of Ni-W, Ni-Cu, and Ni-Cr, the main microstructure of the brazing seam region near the W-Cu alloy side (area A) consisted of γ-Ni(W) solid solution and γ-Ni(Cu, Cr) solid solution. The solid solution reaction of between Ni from brazing filler metal and W or Cu from W-Cu alloy was the reason for the emergence of the diffusion layer, which improved the bonding strength of the joint. According to the binary phase diagram of Ni-Fe, the main microstructure of the brazing seam region near 1Cr18Ni9 steel side (area B) was γ-Ni(Fe, Cr), showing that the brazing filler metal and 1Cr18Ni9 steel formed also a good metallurgical combination. It could be seen from Table 3 that the dark area (area C) mainly contained elements Cr, B, and Ti. The elements Cr and B reacted to form the intermetallic compound CrB, and the intermetallic compound TiCr₂ was formed by the reaction between Ti and Cr. They were dispersed in the brazing seam region and became the crack originals as brittle compounds in the process of fracture, showing that the content of B and Ti in the brazing filler metal needed to be strictly controlled. The main microstructure of the center of brazing seam region (area D) was a Ni-based solid solution of Cr, Fe, and Cu dissolved in the Ni phase, and a high content of Ni in the brazing seam region could be seen from Table 3, which greatly improved the solid solution reaction. The Ni-based solid solution with good plasticity and toughness ensured a high shear strength of 285 MPa by effectively releasing the residual stress in the brazed joint.

Figure 2 shows the microstructure of the W-Cu/1Cr18Ni9 steel brazed joint with NiMnSiCuZr brazing filler metal. It could be seen from Figure 2 that the width of brazing seam region was approximately 0.02 mm, and an obvious diffusion layer existed in the brazing seam region near the W-Cu alloy side. No obvious defects such as microcracks or pores were observed, which indicates that NiMnSiCuZr brazing filler metal had good wettability on these two substrates.

Figure 2:

Microstructure and feature regions of the NiMnSiCuZr brazed joint.

The EDS analysis results of A, B, and C areas (shown in Figure 2) in the brazing seam region are shown in Table 4. According to the binary phase diagram of Ni-Cu, Ni-Si, and Mn-Si, the main microstructure of the brazing seam region near the W-Cu alloy side (area A) consisted of γ-Ni(Cu) solid solution and intermetallic compounds Mn₅Si₃, Ni₃Si, and Ni₇Zr₂. A proper amount of element Si and Zr could reduce the melting point of the brazing filler metal, and excess Si and Zr might cause a large number of silicides and zirconium compounds in the brazing seam region, which could reduce the mechanical properties of the joint as brittle compounds in the course of fracture. The main microstructures of the brazing seam region near 1Cr18Ni9 steel side (area B) were Fe(Ni,Cr) solid solution and Fe(Ni,Cu) solid solution. No obvious silicides and zirconium compounds were formed, showing that the mechanical properties of area B were higher than that of area A. The main microstructure of the center of brazing seam region (area C) was γ-Ni(Cu, Fe) solid solution, and a large number of Ni-based solid solutions helped enhance the mechanical properties of the brazed joint.

The microstructure of the two brazing seam regions contained solid solutions and intermetallic compounds, and a large number of solid solutions existed in each brazing seam region ensured the good mechanical properties of each joint. The difference between the two microstructure was the distribution of the intermetallic compounds. Intermetallic compounds CrB and TiCr₂ contained in the NiCrSiBFeTi brazing seam region were dispersed in the brazing seam region, and intermetallic compounds Mn₅Si₃, Ni₃Si, and Ni₇Zr₂ contained in the NiMnSiCuZr brazing seam region were more clustered in the brazing seam region near the W-Cu alloy side. The difference might be caused by the distribution of element Si and element B.

3.2 Element distribution

For further analysis of the microstructure behavior involved in the brazing seam region and its vicinity, the element distribution across the interface of the W-Cu/1Cr18Ni9 steel brazed joint with NiCrSiBFeTi brazing filler metal is shown in Figure 3. Figure 3A shows the line scan location, and Figure 3B demonstrates that the little elements of W and Cu transited from W-Cu alloy into the brazing seam region, which indicated that W and Cu were not involved in a strong metallurgical reaction in the brazing seam region. A part of Fe transited from 1Cr18Ni9 steel into the brazing seam region, which participated in the solid solution reaction together with Ni and Cr to form Ni(Fe,Cr) solid solution.

Figure 3:

Element distribution near the interface of NiCrSiBFeTi brazed joint: (A) line scan location; (B) element distribution.

Elements from the base metals dissolved into the brazing seam region at the same time, and the elements of the liquid brazing filler metal diffused into the substrates. A part of Ni contained in the brazing filler metal diffused into the interface of the sides of substrates, especially on the W-Cu alloy side. An obvious trough could be observed in the center of brazing seam region from the element distribution figure of Ni, whereas both elements Cr and Ti appeared to have an obvious peak, which indicates that the main microstructures here were CrB and TiCr₂. The diffusion rate of Si was relatively fast because of the small atomic radius, which caused a strong diffusion of element Si into the loose interfaces between the W particles and the interfaces between W and Cu regions under the brazing temperature because of the porosity of W-Cu alloy. The element Ti was more easily diffused into the interface of W-Cu alloy compared with that of 1Cr18Ni9 steel, showing that the element Ti in the brazing filler metal had better wettability on W-Cu alloy than on 1Cr18Ni9 steel.

Element distribution across the interface of the W-Cu/1Cr18Ni9 steel brazed joint with NiMnSiCuZr brazing filler metal is shown in Figure 4. Figure 4A shows the line scan location, and Figure 4B demonstrates that a little W transited from W-Cu alloy into the brazing seam region, and obvious diffusion phenomena of Fe and Cr from 1Cr18Ni9 steel were observed in the brazing seam region, showing that Fe, Cr, and Ni participated in the solid solution reaction to form Fe(Ni,Cr) solid solution.

Figure 4:

Element distribution near the interface of NiMnSiCuZr brazed joint: (A) line scan location; (B) element distribution.

Owing to the solid solution reactions between Ni and Cu and between Mn and Cu, the elements Ni and Mn from brazing filler metal were more easily diffused into the interface of W-Cu alloy compared with that of 1Cr18Ni9 steel. The element Si had a strong diffusion into the loose interfaces between the W particles and the interfaces between W and Cu regions because of the small radius of Si atom and the porosity of W-Cu alloy. Element Zr was evenly distributed throughout the interface of the brazed joint, which was helpful in improving the wettability of NiMnSiCuZr brazing filler metal for the substrates.

The diffusion of elements Fe and Cr from 1Cr18Ni9 steel into brazing seam region was stronger than that of W and Cu from W-Cu alloy in each brazing interface because of the different reaction abilities between elements of brazing filler metal and elements of base metals. Compared with the interface of 1Cr18Ni9 steel, the elements Ni, Si, and Ti contained in the NiCrSiBFeTi filler metal and the elements Ni, Mn, and Si contained in the NiMnSiCuZr filler metal were more easily diffused into the loose interfaces between the W particles and the interfaces between W and Cu regions because of the solid solution reactions between elements from brazing filler metal and element Cu from W-Cu alloy and the porosity of W-Cu alloy. It was worth noting that the strong diffusion of element Si into the interface of W-Cu alloy helped to reduce the silicides that reduced mechanical properties of brazed joint in each brazing seam region. The high content of element Ni could be observed in each brazing interface, which participated in the solid solution reactions to form Ni-based solid solutions, showing that these two Ni-based brazing filler metals had good wettability on these two substrates.

3.3 Phase analysis

To further clarify phase constitution near the interface of the brazed joint, the identification of different phases formed was conducted by XRD analysis. The obtained result was compared with data from the Joint Committee on Power Diffraction Standards to determine the existing phases.

Figure 5 shows the phase constitution of the W-Cu/1Cr18Ni9 steel brazed joint with NiCrSiBFeTi brazing filler metal. The main phases of the brazed joint consisted of Ni-rich phases γ-Ni(W), γ-Ni(Cu,Cr), and γ-Ni(Fe,Cr); intermetallic compounds CrB and TiCr₂; and a simple substance of W, together with a trace amount of Cu_0.4W_0.6 by XRD analysis.

Figure 5:

XRD patterns of the NiCrSiBFeTi brazed joint.

Figure 6 shows the phase constitution of the W-Cu/1Cr18Ni9 steel brazed joint with NiMnSiCuZr brazing filler metal. The main phases of the brazed joint consisted of Ni-rich phase γ-Ni(Cu,Fe); Fe-rich phases Fe(Ni,Cr) and Fe(Ni,Cu); intermetallic compounds Mn₅Si₃, Ni₃Si, and Ni₇Zr₂; and a simple substance of W, together with a trace amount of Cu_0.4W_0.6 by XRD analysis.

Figure 6:

XRD patterns of the NiMnSiCuZr brazed joint.

The intermetallic compounds CrB and TiCr₂ from NiCrSiBFeTi joint and the intermetallic compounds Mn₅Si₃, Ni₃Si, and Ni₇Zr₂ from NiMnSiCuZr joint could reduce the plasticity and toughness of each joint as brittle phases in the course of fracture, but a large number of solid solution phases contained in each brazing seam region could release the residual stress of each joint by overcoming free energy and constraining dislocation stress during brazing. Moreover, the other formed phases also helped enhance the mechanical properties of each brazed joint.

3.4 Shear testing and fracture morphology

3.4.1 Shear strength

The bonding strength of the brazed joint was evaluated by shear test method at room temperature. The shear test samples were cut by using a linear cutting machine into blocks with sizes of 40 mm×8 mm×5 mm and tested on an electronic mechanical testing machine(CMT5205) using a special fixture with a cross-head speed of 0.3 mm/min. Ten shear test samples were prepared for each joint to get more accurate shear strength value, and a sketch of shear test is shown in Figure 7. From a load-displacement curve, the shear strength (τ) was calculated by using the following formula:

Figure 7:

Sketch of shear test.

(1)τ=Fbh.

In Equation 1, F is the average maximum load, and b (mm) and h (mm) are the specimen width and thickness, respectively. The shear strength of the W-Cu/1Cr18Ni9 steel brazed joint with NiCrSiBFeTi brazing filler metal was approximately 285 MPa with an average maximum load 11.40 kN, and the shear strength of the W-Cu/1Cr18Ni9 steel brazed joint with NiMnSiCuZr brazing filler metal was approximately 249 MPa with an average maximum load 9.96 kN. Furthermore, the shear fracture of these two joints occurred at an interface near W-Cu alloy, which indicated that two high bonding strength values were obtained for the two W-Cu/1Cr18Ni9 steel brazed joints. These data provided a certain theoretical reference for the W-Cu/1Cr18Ni9 steel brazed joint with other Ni-based brazing filler metals and for the popularization and application of the brazed joint.

3.4.2 Fracture morphology

Shear strength was closely related to fracture morphology. Figure 8 shows the SEM fracture morphology of NiCrSiBFeTi brazed joint with different amplifications. Figure 8A shows the macrofracture morphology of brazed joint. It could be observed from Figure 8A that the fracture surface was flat and the color was gray. Some large cleavage planes and many bright edges could be found in Figure 8B. Moreover, few obvious secondary cracks could also be observed. EDS analysis of point A, point B, and area C (shown in Figure 8B) with different morphologies on the fracture surface was realized, and the results are shown in Table 5.

Figure 8:

Fracture morphology of NiCrSiBFeTi brazed joint: (A) macroimage; (B) microimage.

Table 5:

Chemical compositions of representative areas of NiCrSiBFeTi fracture surface.

Representative areas	Chemical compositions (wt%)
	Ni	Cr	Si	B	Fe	Ti	W	Cu
A	9.70	43.18	2.74	31.35	1.47	11.56	–	–
B	77.62	10.12	–	–	2.29	9.97	–	–
C	–	–	–	–	–	–	51.23	48.77

The results indicated that the main phase constituents of the bright edge (point A) were confirmed to be the intermetallic compounds CrB and TiCr₂, and they cracked first because of the brittle phases under shear stress, forming many bright edges in the fracture surface, which shows that the mechanical properties of point A was the worst of the whole fracture surface. The cracks extended to the brittle region along with the increase of shear stress, and the brittle fracture tendency of the joint was also increasing. The main phase constituents of cleavage plane (point B) were the solid solution phase γ-Ni(Fe, Cr) and intermetallic compound TiCr₂ contained in the brazing seam region. Although the Ni-based solid solution could release the interface stress, the intermetallic compound TiCr₂ cracked first as a brittle phase under shear stress because of the insufficient plasticity to release the interface stress. While shear stress added up to a certain value, the brittle phase TiCr₂ was separated to form a cleavage plane. It was concluded that area C was the fracture morphology of W-Cu alloy from EDS analysis, showing the fracture occurred at the interface near the W-Cu alloy side. Consequently, the fracture mode of the W-Cu/1Cr18Ni9 steel brazed joint with NiCrSiBFeTi filler metal was identified as brittle.

Figure 9 shows the SEM fracture morphology of NiMnSiCuZr brazed joint with different amplifications. Figure 9A shows the macrofracture morphology of brazed joint. It could be observed from Figure 9A that the fracture surface was rough and the color was gray. It could be clearly seen from Figure 9B that there were two different types of morphologies on the fracture surface. The left of Figure 9B shows a rock pattern, and the three-dimensional sense of this area was strong. EDS analysis of point A, point B, and area C (shown in Figure 9B) with different morphologies on the fracture surface was realized, and the results are shown in Table 6.

Figure 9:

Fracture morphology of NiMnSiCuZr brazed joint: (A) macroimage; (B) microimage.

Table 6:

Chemical compositions of representative areas of NiMnSiCuZr fracture surface.

Representative areas	Chemical compositions (wt%)
	Ni	Mn	Si	Cu	Zr	Fe	Cr	W
A	25.09	18.15	35.33	2.45	18.98	–	–	–
B	62.66	9.77	16.53	8.28	2.76	–	–	–
C	–	–	–	58.43	–	–	–	41.57

The results indicated that the main phase constituents of the bright edge (point A) were confirmed to be the intermetallic compounds Mn₅Si₃, Ni₃Si, and Ni₇Zr₂, and they greatly reduced the plasticity and toughness of point A as brittle phases under shear stress, making point A become the first location to fracture. The cracks extended to the brittle region along with the increase of shear stress. The main phase constituents of point B were solid solution phase γ-Ni(Cu) and intermetallic compounds Mn₅Si₃ and Ni₃Si. The segregation of element Si caused the bonding strength of the grain boundary, which was lower than that of the grain, and the cracks grew along the grain boundary under shear stress, so the intergranular fracture morphology could be found at point B. It was concluded that area C was the fracture morphology of W-Cu alloy from EDS analysis, showing the fracture occurred at the interface near the W-Cu alloy side. Consequently, the fracture mode of the W-Cu/1Cr18Ni9 steel brazed joint with NiMnSiCuZr filler metal was identified as brittle.

The fracture of the two joints occurred at the interface near the W-Cu alloy side. The fracture morphology of NiCrSiBFeTi joint was identified as cleavage brittle fracture, and the fracture morphology of NiMnSiCuZr joint was identified as intergranular brittle fracture. The difference might be caused by the brittle phases in each brazing seam region.