Research on vacuum brazing of W-Cu composite to stainless steel with Cu-Mn-Co brazing metal

1 Introduction

The W-Cu composite is a kind of homogeneous microstructure composed of W and Cu element with neither any solid solution nor intermetallic compounds formed. Therefore, the excellent thermal and electrical conductivity of Cu, combined with the high temperature strength and strong arc ablation resistance of W, could expand the application fields of W-Cu composite greatly. For instance, it is usually used as heavy-duty electrical contacts, material for electronic packaging, as well as a device for cooling and heat sinking. Nowadays, an increasing number of reliable components made with W-Cu composite are applied in fields of military or aerospace industries [1–3].

The advantages of joining dissimilar materials can take effective use of the component structure obtained, respectively, by means of adopting its merits and avoiding the shortcomings [4]. Joints between W-Cu composite and stainless steel include not only high thermal and good electrical conductivity but also strong oxidation resistance of high temperature and corrosion resistance. At present, several recommended techniques for joining W-Cu composite cover brazing, transient liquid phase bonding, diffusion welding, friction welding, laser welding, and so on [5–10]. However, several problems need to be considered when connecting dissimilar materials. On one hand, the difficulty in joining W-Cu and stainless steel stems from the huge differences in their physical chemistry in aspects of melting point, thermal conductivity, and linear expansion coefficient. It may lead to poor bonding of metallurgy through destructive defects occurring, such as residual stress concentration, brittleness phase, and microcracks formed on the brazed interface. On the other hand, the plasticity and toughness of the component structure would be decreased in some ways, thereby increasing the tendency of fracture occurring on interface.

In particular, brazing possesses with small deformation for the substrates and excellent formation on bonding interface were superior to general fusion welding methods in joining dissimilar metals. Thus, it is natural to decide to choose brazing to join W-Cu composite and 18-8 stainless steel (1Cr18Ni9 steel). Due to its low melting point and good wettability, as well as the excellent properties of the joint achieved, Ag-Cu alloy is usually applied in brazing W-Cu composite [11]. Luo and Li [12] joined W-Cu alloy by using different brazing filler metals. The microstructure and property of the joints with silver-based and gold-based filler metal were superior to others. However, both of them belong to precious metals, and this restrains the application fields of W-Cu composite. In this paper, Cu-Mn-Co alloy was employed as the brazing filler alloy to braze W-Cu composite and 18-8 stainless steel, and the microstructure and performance of the obtained joint were studied by a series of standard methods.

2 Experimental procedures

The W-Cu composite adopted in this research was W55-Cu45 alloy (wt%), and the 18-8 stainless steel was austenite steel. The microstructure of the base metals is exhibited in Figure 1. The W-Cu composite and 18-8 stainless steel were machined by a linear cutting machine into blocks with sizes of 20 mm×20 mm×5 mm. The Cu-Mn-Co brazing filler metal was prepared in the form of foil with a thickness of 0.1 mm, whose solid-liquid phase transforming temperature ranged from 940°C to 1030°C. The chemical compositions of 18-8 stainless steel and Cu-Mn-Co brazing metal are shown in Table 1.

Figure 1

Microstructures of the base metals.

(A) W-Cu alloy. (B) 18-8 stainless steel.

Before being brazed, the surfaces of the base metals were polished by 400# emery papers and then ultrasonic cleaned in alcohol for 15 min. The samples were put into a special clamp, together with the blocks of W-Cu composite and 18-8 steel located in the top and bottom as well as a Cu-Mn-Co foil placed in the middle. Moreover, a suitable pressure was applied on the W-Cu alloy, with the purpose of promoting the brazing filler metal to spread out and wet adequately on the base metals. Such a set of piled-up samples was put into KJL-2 vacuum furnace, heated to a brazing temperature of 1060°C with a holding time for 1 h under a vacuum of 6×10^-3 Pa, and then cooled at a rate of 10°C/min in the furnace.

After the above brazing process, the obtained specimens were conducted to measure the flexural strength through the four-point bend test with normal dimensions of 40 mm×5 mm×5 mm. The specimens were grinded and polished by emery papers, cloths, and diamond grind paste and then etched to reveal the brazed seam by the mixture solution (FeCl₃:HCl:H₂O=5 g:10 ml:100 ml). Finally, microstructure and element distribution, along with fracture morphology, across the W-Cu/18-8 steel joint were measured via SEM/EDS, and microhardness was carried out by MH-5 micro sclerometer. Moreover, X-ray diffraction (XRD) was adopted to detect the existence of phase composition in the brazed interface.

3 Results and discussion

3.1 Microstructure and microhardness

3.1.1 Microstructure

The microstructure of the W-Cu/Cu-Mn-Co/18-8 steel joint is shown in Figure 2. A smooth and clear bonding interface formed, attributed to the capability of Cu-Mn-Co brazing metal and well wettability on the base metals. A virtually flat interface without solidification defects such as microcracks and pores was formed on the bonded interface, which promoted a good metallurgical combination for the base metals. Moreover, a reaction layer with a width of about 1–2 μm formed on the interface close to the 18-8 stainless steel side. It was not difficult to find that the brazed seam was basically composed of two phases in appearance (as marked in Figure 2): the gray punctate phase existed dispersively adjacent to the interface near W-Cu composite, together with part of the continuous white particle phase located in the brazed seam on the 18-8 steel side.

Figure 2

Microstructure of the brazing joint of W-Cu and 18-8 steel.

(A) Low amplification. (B) Higher amplification.

3.1.2 Microhardness

Microhardness across the brazed seam region and the boundaries was measured by MH-5 micro sclerometer with the following parameters: 25 gf loading force, 10 s load time, and interval 0.25 mm indentation. The specified area underwent testing five times, and the average value of the microhardness was calculated, as shown in Figure 3.

Figure 3

Microhardness distribution near the interface of the brazed joint.

It could be clearly seen that the microhardness of W-Cu composite ranged from 170 to 320 HM, which was higher than that of 18-8 steel (110–270 HM). Meanwhile, the microhardness of the brazed seam region (70–120 HM) was obviously lower than that of both W-Cu composite and 18-8 steel. However, the microhardness values of the joint obtained increased near the interface and decreased close to the center of the brazed seam, which was basically influenced by element distribution and whether the brittle phase or solid solution occurred or not. According to the low microhardness obtained in the brazed seam, some certain solid solution was formed without obvious brittle phase. Hence, the W-Cu/18-8 steel joint had good plasticity and toughness to obtain a high bend strength of 780 MPa.

3.2 Element distribution

For further analysis of microstructure behavior involved in the brazed seam and its vicinity, element distributions of tungsten, copper, manganese, cobalt, and iron that cross the interface of the W-Cu/18-8 steel joint are given in Figure 4. Elements from the filler metal diffused into the base metals in some ways, and elements of the base metals dissolved in the brazing filler metals. Thus, a small diffused layer formed at the interface to provide a good combination of metallurgical reaction for the substrates.

Figure 4

Element distribution near the interface of the W-Cu/18-8 brazed joint.

(A) Element distribution of W-Cu/18-8 steel joint. (B) Element distribution of W, Cu, Mn, Fe, and Co.

In accordance with the binary phase diagram of the Cu-Mn system, Cu participated in the solid solution reaction together with Mn. Moreover, due to the porosity that existed in W-Cu composite, partial Cu from the filler metal transited on the interface, surrounding W particles to form liquid phase bonding under a high brazing temperature. No obvious W element appeared in the brazed seam. The peak of Co element concentration on the 18-8 steel side might have depended on a lower chemical potential in the Fe element than in Cu, leading to a strong diffusion phenomenon between Fe and Co. As supported by mutual solubility, Fe-Co solid solution formed where Mn was dissolved. The brazed seam region consisted of Cu(Mn) and Fe(Co) solid solution, which were successively identified as the gray punctate phase and white particle phase, respectively. Doubtlessly, for joining W-Cu composite and 18-8 stainless steel, an evident connection of diffusion, metallurgy, and liquid phase bonding was carried out on the interface, as clearly observed in Figure 4B.

3.3 Fracture morphology

The feature of plasticity and toughness on the W-Cu/18-8 steel joint was greatly affected by interfacial bonding strength. The SEM fracture morphology with different amplifications is shown in Figure 5. First, numerous dimples with different sizes and depths emerged in the fracture surface, with diameters ranging from 15 to 20 μm. This was in agreement with the plastic deformation existing on the bonding surface. Second, EDS analysis of A, B, and C with different morphology on the fracture surface of the brazed layer was conducted: A point (W 100%, wt%), B point (Cu 100%, wt%), and C point (Cu 95.18%, Mn 3.23%, Co 1.60%, wt%). This fully illustrated that the fracture position was located at the interface near the W-Cu composite side.

Figure 5

Micrograph of the fracture of the W-Cu/18-8 steel brazing layer.

(A) Low multiples. (B) Higher multiples.

Because of the lower plasticity of W than that of Cu in W-Cu composite, the reinforced phase (W particles) of A point started to resist the increasing dislocations caused by external loading force. While it added up to a certain value, the W/W bonding interface was constrained to be separated due to the lack of enough plasticity to relax the interface stress. Finally, W particles were stripped away from each other, showing a typical brittle fracture. Nevertheless, the chemical composition of point B and point C among dimples was confirmed to be the solid solution phase (Cu-Mn) obtained in the brazed seam. It could release the interface stress and prompt the plastic deformation occurring on the bonding interface prior to fracture, as shown in Figure 5B. Corresponding to the conclusions above, the bend strength of the brazed joint was measured to be 780 MPa. In short, the fracture morphology of the W-Cu/18-8 steel joint was identified as a mixed feature of fracture, containing brittle fracture of W-W interface and plasticity facture of solid solution formed in the brazed layer.

3.4 Phase analysis

With regard to joining W-Cu alloy and 18-8 steel reliably, not only the filler metal with very good wettability and spreadability required for the base metals but also a significant physicochemical interaction was expected to be carried out at the same time. Based on the standard powder diffraction of XRD analysis, the identification of different phases formed across the bending fracture surface of W-Cu composite with the brazed seam is illustrated in Figure 6.

Figure 6

XRD analysis of brazed joint on the interface with Cu-based metal.

The results corroborated that a physicochemical interaction was carried out between Cu-Mn-Co filler metal and the substrates. The main component of Cu(Mn-Fe) and W_0.4Cu_0.6 remained on the interface, together with simple substance of W, α-Fe gained. Especially, the existence of a slight intermetallic compound of Co_0.7Fe_0.3 aimed to strengthen the connection between the brazing filler metal and 1Cr18Ni9 steel. Moreover, the Cu-rich phase (Cu(Mn-Fe)) involved also promoted the ductility of the W-Cu/18-8 steel joint by overcoming free energy and constraining dislocation stress while being brazed.

4 Conclusions

Under a vacuum of 6×10^-3 Pa, a brazed joint of W-Cu/18-8 steel with excellent properties was manufactured with Cu-Mn-Co filler metal, whose bend strength was as high as 780 MPa, with a brazing temperature of 1060°C and a holding time of 1 h. The obtained joint had good metallurgical combination in the interface via the suitability of selected experimental parameters. Relatively low microhardness and high bend strength, on average, made the W-Cu/18-8 steel joint gain excellent mechanical property.

Results indicate that the brazed layer basically composed of two different phases, identified as Cu(Mn) and Fe(Co) solid solution, formed via EDS. Fracture morphology was identified as a mixed feature of brittle fracture of W-W interface and plasticity fracture of the brazed layer, with fracture occurring at the W-Cu composite side.

Analysis of XRD indicated that formation of Cu-rich phase, W_0.4Cu_0.6, α-Fe, and simple substance of W together with a small Co_0.7Fe_0.3 should have played a positive role for consolidating and enhancing the mechanical properties of the W-Cu/18-8 steel joint.