Investigation of the microstructure and properties of W75-Cu/W55-Cu brazed joint with Cu-Mn-Co filler metal

1 Introduction

The W-Cu alloy has high electrical and thermal conductivities, low coefficient of thermal expansion, and low contact resistance. Owing to these properties, the W-Cu alloy is often used in heat sink materials for high-power integrated circuits [1–3]. However, with the miniaturization, integration, and high-power development of microwave semiconductor devices, which led to a high rate of heat generation, it is difficult for existing homogeneous W-Cu alloys to meet the requirements of an electronic substrate in heat dispersion [4]. If two different ingredients of the W-Cu alloy can be joined, it might very well solve this problem. The brazing method with small deformation of substrates and excellent formation on the bonding interface is superior to general fusion welding methods and pressure welding methods in joining dissimilar metals. Thus, in this paper, we chose the vacuum brazing method to join the W75-Cu and W55-Cu alloys. The brazing process to join two different ingredients of the W-Cu alloy can take full advantage of the W-Cu alloy by adjusting the content of tungsten and copper in the W-Cu alloy, expanding the application fields of the W-Cu alloy greatly. Therefore, research on the brazing process of two different ingredients of the W-Cu alloy not only has a positive practical significance but also has good application prospects.

In recent years, considerable interest has been generated toward the joining of W-Cu alloys. Researchers adopted many methods to realize the joining of W-Cu alloys, including brazing [5–8], friction welding [9, 10], liquid-phase sintering [11, 12], hot isostatic pressing, and diffusion bonding [13, 14]. In this paper, the Cu-Mn-Co alloy was employed as the filler metal to braze the W75-Cu and W55-Cu alloys in a vacuum furnace with a vacuum level >6×10^-3 Pa, by controlling the brazing temperature at 1060°C and the holding time at 30 min. The microstructure, element distribution, phase constituents, four-point bending strength, and fracture morphology of the brazed joint were analyzed by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results provided a favorable basis for further studies on the joining of two different ingredients of the W-Cu alloy and for application of the compound structure.

2 Materials and methods

The W75-Cu and W55-Cu alloys were used as the base metals in this research, and their microstructure are shown in Figure 1. The dark region is the Cu-phase and the bright region is the W-phase. The Cu-Mn-Co alloy was used as the filler metal, and its chemical composition is shown in Table 1. The W75-Cu and W55-Cu alloys were cut by using a linear cutting machine into blocks with sizes of 20 mm×20 mm×6 mm; the Cu-Mn-Co filler metal was prepared in the form of foil with a thickness of 0.1 mm after smelting, and was cut into rectangular shape with sizes of 20 mm×6 mm by using scissors.

Figure 1:

Microstructure of the base metals: (A) W75-Cu alloy; (B) W55-Cu alloy.

Before being brazed, the oxidation film and greasy dirt on the surface of the base metals and filler metal were eliminated by using 28#-3.5# emery papers, then cleaned by ultrasonication in alcohol for 15 min. After the cleaning process, the test samples were assembled, in the sequence W55-Cu alloy, Cu-Mn-Co filler metal, and W75-Cu alloy, in a special clamp. Moreover, a 50-kPa pressure was imposed on the surface of the W55-Cu alloy, with the purpose of promoting the filler metal to spread out and wet adequately on the base metals, as shown in Figure 2. Then, the clamp and the samples in the clamp were placed together into a KJL-2 vacuum furnace for brazing under the process parameters of a brazing temperature of 1060°C and a holding time of 30 min.

Figure 2:

Sketch of the clamp used in the brazing process.

After the brazing process, samples were cut by using a linear cutting machine into blocks with sizes of 10 mm×5 mm×6 mm from the brazed joint. Then, they were ground with a series of different types of emery papers, polished, and finally etched with a mixed solution of HCl, HNO₃, and CH₃COOH (1:3:4) for 8–10 s. The microstructural feature and fracture morphology of the brazed joint were observed by using a JSM-6480 scanning electron microscope; the four-point bending strength of the brazed joint was tested using an electronic mechanical testing machine (CMT5205); and the element distribution and phase constituents on the interface were examined by means of energy dispersive spectrum (EDS) and XRD (XRD-6000) analysis, respectively.

3 Results and discussion

3.1 Microstructure

Figure 3 shows the microstructure of the brazed joint of the W75-Cu and W55-Cu alloys. It could be seen from Figure 3A that the Cu-based filler metal and the base metals formed a good metallurgical combination with a width of about 0.02 mm, and a bright diffusion layer existed between the brazing seam region and the substrates. A smooth and clear bonding interface without obvious defects such as pores and microcracks was formed, which means that the Cu-Mn-Co filler alloy exhibited good wettability and spreadability on the interface of the W75-Cu and W55-Cu alloys. Moreover, the porosity of the W-Cu alloy helped in forming a good metallurgical bonding of the liquid filler metal and the base metals under a stationary pressure of 50 kPa.

Figure 3:

Microstructure and feature regions of the W75-Cu/W55-Cu brazed joint: (A) microstructure; (B) feature regions.

Figure 4 shows the EDS analysis results of the a, b, and c (shown in Figure 3B) areas in the brazing seam region. Area c at the center of the brazing seam region was composed of Cu 95.34%, Mn 0.46%, Co 0.26%, and W 3.94% (wt.%, shown in Figure 4C). On both sides of the brazing seam region, areas a and b were composed of Cu 89.24%, Mn 0.26%, Co 0.48%, and W 10.02% (wt.%, shown in Figure 4A) and Cu 91.17%, Mn 0.31%, Co 0.58%, and W 7.94% (wt.%, shown in Figure 4B), respectively. According to the binary phase diagram of Cu-Mn and Co-Cu, the main microstructure of the brazing seam region of Cu(Mn,Co) solid solution of Mn, Co dissolved in the Cu phase, and a high content of Cu in the brazing seam region could be seen from Figure 4, which greatly improved the solid solution reaction. The Cu-based solid solution had good plasticity and toughness, ensuring a high bending strength of 950 MPa by effectively releasing the residual stress in the brazed joint. Moreover, it could be obviously found that a little element W from both sides of the substrates transited into the brazing seam region from the EDS analysis results of areas a, b, and c.

Figure 4:

EDS results of different areas in the brazing seam region: (A) area a; (B) area b; (C) area c.

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 brazed joint of the W75-Cu and W55-Cu alloys was examined by using the INCA Energy Diffraction Spectrum, as shown in Figure 5. Figure 5A shows the line scan location, and Figure 5B demonstrates that Cu and a little W transited from both sides of the base metals into the brazing seam region, which indicated that W was not involved in a strong metallurgical reaction in the brazing seam region. Neither solid solution nor intermetallic compounds formed according to the binary phase diagram of W-Cu; therefore, tungsten that transited into the brazing seam region existed in the form of a simple substance. 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 Cu contained in the brazing filler metal diffused into the interface of the sides of the base metals, surrounding the W particle to form liquid-phase bonding under the brazing temperature because of the porosity of the W-Cu alloy. Mn and Co were evenly distributed throughout the interface of the brazed joint, which was helpful in improving the wettability of the Cu-based brazing filler metal for the substrates. A high concentration level of Cu was observed in the brazing seam region and the interface of two sides of substrates, which participated in the solid-solution reaction together with Mn and Co to form a Cu(Mn,Co) solid solution, showing that the element Cu in the brazing filler metal had good wettability on these two substrates.

Figure 5:

Element distribution near the interface of the W75-Cu/W55-Cu brazed joint: (A) line scan location; (B) element distribution of Cu, W, Mn, and Co.

3.3 Phase analysis

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

Figure 6 shows the phase constitution of the W75-Cu/W55-Cu brazed joint. The main phases of the brazed joint consisted of Cu-rich phase Cu(Mn,Co), simple substance of W, simple substance of Cu, together with trace amounts of Cu_0.4W_0.6 by XRD analysis. The Cu-rich phase promoted the ductility of the brazed joint by overcoming free energy and constraining dislocation stress during brazing. These formed phases helped enhance the mechanical properties of the W75-Cu/W55-Cu brazed joint.

Figure 6:

XRD patterns of the W75-Cu/W55-Cu brazed joint.

3.4 Bend testing and fracture morphology

3.4.1 Four-point bending strength

The bending strength of the brazed joint was evaluated by using the four-point bending test method at room temperature. The bending test samples were cut by using a linear cutting machine into blocks with sizes of 40 mm×5 mm×6 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 bending test samples were prepared for the four-point bending test, and a sketch of the four-point bending test is shown in Figure 7. From a load-displacement curve, the maximum strength (σ) was calculated by using the following formula:

σ=3FL1/bh2.

Figure 7:

Sketch of the four-point bending test.

In this formula, F is the average maximum load, L₁ is the distance between two effect points, and b (mm) and h (mm) are the specimen width and thickness, respectively. The four-point bending strength of the W75-Cu/W55-Cu brazed joint was about 950 MPa, with an average maximum load of 4.0 kN after the calculation. Furthermore, bending fracture occurred at the interface near the W75-Cu alloy side, which indicated that a high bonding strength was obtained for the W75-Cu and W55-Cu alloy joint, as shown in Figure 8.

Figure 8:

Sketch of the fracture location of the brazed joint.

3.4.2 Fracture morphology

The bending strength is closely related to the fracture morphology. The fracture morphology of the W75-Cu and W55-Cu alloy brazed joint was analyzed in order to further explain the high bending strength. Figure 9 shows the SEM fracture morphology of the brazed joint with different amplifications. Figure 9A shows the macro fracture morphology of the brazed joint. It could be observed from Figure 9A that the fracture surface was rough and the color was gray. Figure 9B shows the micro fracture morphology of the brazed joint. It could be clearly seen from Figure 9B that there were two different types of characteristics on the fracture surface. The lower part of Figure 9B shows many different sizes and depths of the dimples, with diameters ranging from 10 to 50 μm. Dimples are a typical characteristic of ductile fractures; therefore, the preliminary judgment on the fracture was that it was a part of ductile fracture.

Figure 9:

Fracture morphology of the W75-Cu/W55-Cu brazed joint: (A) macro image; (B) micro image.

EDS analysis of point a, point b, and area c with different morphologies on the fracture surface was realized, and the results were as follows. The chemical composition of point a at the center of the dimple was Cu 97.90%, Mn 1.25%, and Co 0.85% (wt.%, shown in Figure 10A); the chemical composition of point b on the wall of the dimple was Cu 96.56%, Mn 2.02%, and Co 1.42% (wt.%, shown in Figure 10B); and the chemical composition of area c was W 53.32% and Cu 46.68% (wt.%, shown in Figure 10C). The results indicated that the phase constituents of points a and b were confirmed to be the solid solution phase Cu(Mn,Co) obtained in the brazing seam region. It could release the interface stress and prompt the plastic deformation occurring on the bonding interface prior to the fracture, so the fracture mode of the brazing seam region was a ductile fracture. It was concluded that area c was the fracture morphology of the W75-Cu alloy from EDS analysis. Because the W-Cu alloy was produced by infiltrating molten copper into the open pores of the sintered W-skeleton, the W particles started to resist the increasing dislocations caused by the external loading force. While it added up to a certain value, the W/W bonding interface was constrained to be separated due to the insufficient plasticity to release the interface stress. Finally, the W particles were stripped away from each other, resulting in a typical brittle fracture. Consequently, the fracture mode of the W75-Cu/W55-Cu brazed joint was identified as mixed ductile-brittle.

Figure 10:

EDS results of the different morphologies on the fracture surface: (A) point a; (B) point b; (C) area c.

4 Conclusions

1. The W75-Cu and W55-Cu alloys were successfully brazed with Cu-Mn-Co brazing filler metal in a vacuum furnace with a vacuum level >6×10^-3 Pa, by controlling the brazing temperature to 1060°C and the holding time to 30 min. High-magnification SEM showed a smooth and clear bonding interface without obvious gas holes, cracks, and other microdefects, which means that the Cu-Mn-Co filler alloy exhibited good wettability and spreadability on the interface of the W75-Cu and W55-Cu alloys. The main microstructure of the brazing seam region was a Cu-based solid solution of Mn and Co dissolved in the Cu phase. The Cu-based solid solution had good plasticity and toughness, ensuring a high bending strength of 950 MPa by effectively releasing the residual stress in the brazed joint and prompting the plastic deformation occurring on the bonding interface prior to fracture.

2. EDS analysis indicated that Cu and a little W from the two sides of the base metals transited into the brazing seam region, which indicated that W was not involved in a strong metallurgical reaction in the brazing seam region. A part of Cu contained in the brazing filler metal diffused into the sides of the base metals, and Mn and Co evenly distributed throughout the interface of the brazed joint, which was helpful in improving the wettability of the Cu-based brazing filler metal for the substrates. A high concentration level of Cu was found in the brazing seam region and the interface of the two sides of the substrates, improving the solid-solution reaction between Cu, Mn, and Co and ensuring the performance of the W75-Cu/W55-Cu brazed joint.

3. XRD analysis revealed that the main phases that existed in the brazed joint were Cu-rich phase Cu(Mn,Co), simple substance of W, simple substance of Cu, together with trace amounts of Cu_0.4W_0.6. The Cu-rich phase promoted the ductility of the brazed joint by overcoming free energy and constraining dislocation stress during brazing. These formed phases helped enhance the mechanical properties of the W75-Cu/W55-Cu brazed joint.

4. The four-point bending test suggested that the four-point bending strength of the brazed joint was about 950 MPa, and fracture occurred at the interface near the W75-Cu alloy side. The fracture morphology was identified as having the mixed characteristics of a brittle fracture of the W-W interface and a plastic fracture of the brazing seam region.