Microstructure and Characteristics of the Welded Joint between Ni-Cr Alloys and Copper

Abstract

In the field of petroleum extraction, the welding technology of the core wire (the hybrid structure of copper and the Ni-Cr alloy) in high-power oilfield heaters is a key process that determines the efficiency of the heater. Using the tungsten inert gas (TIG) welding method of filling pure copper wire, this work effectively joins the dissimilar metals of red copper and the Cr₂₀Ni₈₀ nickel–chromium alloy. The microstructure, mechanical properties, and conductivity of the joint were analyzed. The results showed that the surface of the welded dissimilar metal joint was smooth and uniform; radiographic nondestructive testing did not reveal any macroscopic forming defects such as pores or cracks. The microstructure of the joint fusion zone exhibits an equiaxed grain morphology. The interface between the copper and the fusion zone displays a columnar grain structure, growing perpendicular to the fusion line. An interdiffusion layer of elements was formed at the interface between the Ni-Cr alloy and the fusion zone. The microhardness of the joint shows a stepwise decreasing trend, with the highest hardness on the nickel–chromium alloy side, followed by the fusion zone, and the lowest on the copper side. The joint fractures at the copper base material, with a tensile strength greater than 220 MPa, indicating a ductile fracture mode. During the electrical heating process, the joint temperature does not significantly increase compared to the copper side, demonstrating good thermal stability.

1. Introduction

The core wire, as the key component of oil-layer-extraction electric heating technology, is mainly composed of a copper alloy and a nickel–chromium alloy [1]. The hybrid structure of the Cu-NiCr core wire uses copper as the conductive end material and the Ni-Cr alloy as the heating end material. The different parts (the copper and nickel–chromium alloys) of this core wire need to be welded. However, due to significant differences between dissimilar metals in terms of melting point, thermal conductivity, and other properties, the welding process faces great challenges [2,3,4,5]. Therefore, the reliability of dissimilar metal welding becomes extremely important in the connection of long-distance core wires.

To achieve high-quality welding between copper and other metals, researchers have conducted many studies on welding methods, welding metallurgy, and interface control. Using filler-wire arc brazing, Zhou et al. [6] achieved effective joining of aluminum and brass. The results indicated that the Al-12% Si flux-cored wire exhibited poor wettability on the brass base material surface, making it difficult to obtain high-quality brazed joints. The transition zone near the interface layer on the brass side contained a high aluminum content, and large strip-shaped Al-Cu intermetallic compounds (IMCs) formed at the interface, severely affecting the mechanical properties of the joint. Singla et al. [7] studied the effect of heat treatment on the mechanical properties of copper–stainless steel welded joints and found that the tensile strength and microhardness of the samples increased after heat treatment.

The brittle and hard intermetallic compounds (IMCs) might form at the interface of dissimilar metals, which significantly reduces the mechanical properties of the joint. It is essential to control the interfacial phases to obtain a high-quality dissimilar welded joint [8,9,10]. Zuo et al. reported that increasing the welding speed resulted in a reduction in both the thickness of the IMCs’ phase and the shear tensile strength of the joint. They also found that the presence of eutectic and hypoeutectic structures could lead to cracking in the joint [11]. In another study, Zhu et al. used a nanosecond-pulsed laser to weld aluminum to copper in an overlapping configuration. They examined how different laser scanning paths affected weld formation, microstructure, and bonding strength [12]. One challenge with laser welding is the concentration of the laser beam. In comparison, arc welding offers advantages over laser welding. The arc has a heat diameter that provides benefits such as lower welding heat input, higher connection reliability, and better on-site operability [13,14].

Cheng et al. [15] performed a new arc welding method, combined metal inert gas welding (MIG) and tungsten inert gas welding (TIG), and successfully obtained crack-free and pore-free joints. However, this MIG-TIG method might increase the welding cost and energy consumption. In addition, the use of ultrasound-assisted plasma arc brazing technology has also achieved the connection of dissimilar metals [16], obtaining well-formed lap joints. The addition of ultrasound significantly reduces the thickness of intermetallic compounds and increases the tensile strength of the joints [17]. On this basis, the technology of AC-assisted plasma arc fusion brazing [18], assisted by adding SiO₂ nanopowder, was proposed from the perspective of process method and interface control, and dissimilar metal joints with good shape and mechanical properties were realized. Due to the high thermal conductivity and fast heat dissipation of copper, higher heat input was required in the welding process to reach the melting point of the base metal, which led to difficult welding operation and reduced the mechanical properties of the joints to varying degrees, and it was difficult to obtain high-quality welding joints [19]. In summary, researchers have continually explored the welding of copper with dissimilar metals (such as stainless steel, aluminum, and Ni-Cr alloys), conducting many studies on the connection methods, microstructure, and joint performance. The connection of copper with other metals has always been the focus of research and the difficulty therein [20,21].

According to the binary phase diagram, Ni and Cr have large solubility and can form a solid solution in a wide concentration range. Therefore, Ni and Cr can fully diffuse during the welding process to create an alloy phase with good mechanical properties [22]. Since the atomic number of Cu is close to that of Ni and the atomic radius and lattice constant are similar, both of them have a face-centered cubic (FCC) structure, which makes Ni and Cu completely miscible, and Ni atoms can easily diffuse into copper during welding [23]. However, Cr has a body-centered cubic (BCC) structure, and according to the Cu-Cr phase diagram, stable solid solutions or compounds rarely form between Cu and Cr [24]. During the welding process of copper and the Ni-Cr alloy, phase formation in the Ni-Cr-Cu system is limited by the solubility of the binary systems from the perspective of the ternary phase equilibrium. Due to the high miscibility of Ni and Cr, a Ni-Cr solid solution phase will form in the welded joint. At the interface between the Ni-Cr alloy and the fusion zone, a continuous Ni-Cu solid solution may form as Ni atoms diffuse. Meanwhile, due to the very low solubility of Cu and Cr, Cr-rich regions may appear at the interface between the copper and the Ni-Cr alloy joints.

In this study, the tungsten inert gas (TIG) welding method, involving filling pure copper wire, was used to effectively achieve the connection of dissimilar metals between the conductive end (the red copper) and the heating end (the Cr₂₀Ni₈₀ alloy) of the cable core wire in electric heating technology for oil field development. An interface microstructure analysis is carried out through an optical microscope, a scanning electron microscope, and other testing techniques to reveal the evolution of the microstructure at the dissimilar metal interface. X-ray diffraction technology is used to confirm the formation and transformation of new phases and mechanical performance testing is used to evaluate the strength, toughness, and elongation of the joint. The conductivity of the joint is also evaluated. The test results will provide an important reference and key technical support for the application of TIG welding in core wire welding construction sites and for the safe service of cables.

2. Materials and Experiment Details

In electric heating technology for oil reservoir exploitation, the core wire plays a key role in conducting electricity and generating heat. The heat produced by the core wire improves crude oil flow while reducing viscosity and resistance during transportation, as shown in Figure 1. This technology places the cable core into hollow oil pipes and uses them to enter the well, with a maximum heating depth of over 3000 m.

As shown in Figure 2, the tungsten inert gas (TIG) welding method is used to connect the conductive section and the heating section of the cable core. The materials used were red copper and nickel–chromium alloy round rods, both with a diameter of 10 mm and a length of 100 mm. During welding, a pure copper wire with a diameter of 2 mm, matching the chemical composition of red copper, is chosen as the filler material. The main chemical compositions of the copper and nickel–chromium alloys, as provided by the supplier, are listed in

Table 1. Chemical composition of red copper and filler wire (wt.%).

Material	Cu	Bi	Sb	As	Fe	Pb	S
Red copper	≥99.9	≤0.001	≤0.002	≤0.002	≤0.005	≤0.005	≤0.005

and

Table 2. Chemical composition of Cr₂₀Ni₈₀ alloy (wt.%).

Material	C	Mn	P	S	Si	Fe	Al	Cr	Ni
Cr20Ni80	≤0.08	≤0.6	≤0.02	≤0.0015	0.75–1.60	≤1	≤0.5	20–23	Balance

.

To avoid defects such as lack of fusion, an X-shape groove with a blunt edge was used, with a blunt edge of 1~2 mm and a reserved gap of 1~2 mm. Such an X-shape groove can reduce the amount of filler in the weld and minimize the deformation of the welded joint. The joint was filled using a multi-layer, multi-pass technique, as shown in Figure 3. The welding current was 130A and the shielding gas flow rate was 15 L/min. Through experiments to explore the welding process parameters of copper and the Ni-Cr alloy, the welding current was finally determined to be 130A and the shielding gas flow rate was 15 L/min.

The macroscopic forming of the Cr₂₀Ni₈₀ nickel–chromium alloy and the T2 copper filler wire TIG welding joint is shown in Figure 4. In Figure 4a, the welding joint is formed uniformly and smoothly, without forming defects such as undercutting and cracks. The partially melted nickel–chromium alloy base material, the copper base material, and the melted pure copper filler wire form the welding joint, and the overall cross-sectional morphology shows an “X” shape, as shown in Figure 4b.

The forming quality of welded joints was tested using the XXQ2005 X-ray flaw detector purchased from Red Star Instrument Factory in Dandong, China. The tube voltage was 130 kV, the tube current was 3 mA, and the exposure time was 1.5 min. The test results are shown in Figure 5. The internal structure of the joint area is continuous and uniform and no obvious defects such as pores, slag inclusions, cracks, or other discontinuities were found.

After welding, wire-cutting technology was used to prepare metallographic and tensile specimens from the center of the welded joint, as shown in Figure 6. The specimens were polished and etched using a solution composed of 10 g FeCl₃, 6 mL HCl, 20 mL C₂H₅OH, and 80 mL H₂O. Subsequently, the fusion zone and interface of the welded joint were analyzed for microstructure using a Zeiss AXIO Scope A1 optical microscope (OM) from the German company Carl Zeiss (Jena, Germany), an FEG-450 field emission scanning electron microscope (SEM), and an energy-dispersive spectrometer (EDS). Phase analysis was conducted using a D8 DISCOVER high-resolution X-ray diffractometer produced by Bruker, Berlin, Germany, combined with Jade 6.0 software, with a diffraction angle range of 20° to 110° and a scanning speed of 15°/min. To evaluate the mechanical properties of the welded joint, a VH1102 microhardness tester produced by Wilson Company (Fort Worth, TX, USA) of the United States was employed with a load of 200 g and a loading time of 15 s. Tensile strength tests were performed at room temperature using a WDW-100KN universal testing machine produced by Shandong Wanchen Testing Machine Co., Ltd. (Jinan, China) with a loading speed of 0.5 mm/min, and three tensile specimens were tested to ensure data accuracy and reliability. Nanoindentation tests were carried out using an Anton Paar nanoindenter produced by Graz, Austria, with a load of 1000 mN, and both loading and unloading rates set to 2000 mN/min.

The working condition of the core wire depends on its ability to withstand heat, so we designed a core wire heating test to simulate the temperature changes in the welded joint. The conductivity performance of the joints was tested using an electrified test system, as shown in Figure 7. A non-contact infrared thermometer was used to measure and collect the joint temperature. Two points were selected at the joint for temperature measurement, and the average value was calculated. Finally, the conductivity at different positions of the joint was measured using an RTS-II metal four-probe tester.

3. Experimental Results and Discussion

During the welding process of dissimilar metals, due to the rapid heating and cooling of the heat source, as well as the different melting points of the base metal, severe and complex metallurgical reactions could occur at the joint. In this section, the micromorphology, phase composition, element distribution, and mechanical properties of the samples were studied by OM, SEM, and XRD. The electrical and mechanical properties of the joint are also studied.

3.1. Microstructure and Phase Composition

The microstructure of the copper side, the nickel–chromium alloy side, and the fusion zone are shown in Figure 8. The welded joint can be divided into four regions: copper, the interface between Cu and the fusion zone, the fusion zone, and the interface between the fusion zone and the Ni-Cr alloy. The microstructure at different regions was observed by the OM. The microstructure of the nickel–chromium alloy side is mainly composed of austenite, with columnar crystals as the main morphology, accompanied by a small amount of equiaxed crystals. The microstructure of the fusion zone is all alpha-phase, as shown in Figure 8b, with columnar crystals perpendicular to the fusion line and equiaxed crystals at the center. The average size of the isaemic crystals measured by Image-J 2.14 software is 11.8 μm. During the welding process, the heat of the molten pool diffuses outwards from the fusion line and the direction of heat flow is perpendicular to the fusion line, resulting in the formation of columnar crystals perpendicular to the fusion line in the region. In the center of the fusion zone, the heat loss is more uniform, the region is relatively far from the fusion line, and the influence of heat diffusion from the fusion line is less, so the grains grow in multiple directions at the same time and finally form equiaxed crystals. The base material on the copper side is mainly composed of small equiaxed crystals, which is consistent with the results in references [25,26].

To test the diffusion behavior of alloy elements at the interface, EDS line scanning and map analysis were performed across the interface. The element distribution at the interface between the copper side and the fusion zone is shown in Figure 9. Small amounts of Cr and Ni diffuse to the copper side in the fusion zone and the Cu element distribution in the fusion zone is uniform without segregation or enrichment.

The elements at the interface between the nickel–chromium side and the fusion zone are shown in Figure 10. The Cr and Ni elements diffuse to the fusion zone and are uniformly distributed on the nickel–chromium alloy side, with a small amount of Cu element also present at the interface.

The line scan results at the interface between the copper side and the fusion zone are shown in Figure 11. Pure copper is selected as the filling material, and the main element in the fusion zone is Cu, accompanied by a small amount of Cr and Ni. The content of Cr and Ni decreases sharply as they approach the interface, while the content of the Cu element gradually stabilizes. The thickness of the copper-side interface is about 25 μm. At 3, 10, 12, and 23 µm from the starting point, the copper content decreased significantly, while the nickel and chromium content increased correspondingly. This is mainly due to the diffusion of elements such as nickel and chromium from the nickel–chromium alloy side to the copper side during welding. According to the Ni-Cr-Cu ternary phase diagram, nickel, chromium, and copper may form nickel- or chromium-rich solid solutions during welding, leading to higher nickel and chromium content in certain areas and a corresponding reduction in local copper content.

The line scan results at the interface between the nickel–chromium side and the fusion zone are shown in Figure 12. As the interface approaches, the Cr and Ni contents gradually decrease while the Cu content sharply increases. A diffusion zone was formed by the Cr, Ni, and Cu elements with a thickness of 20 μm.

The X-ray diffraction pattern of the Cr₂₀Ni₈₀ nickel–chromium side and fusion zone interface is shown in Figure 13. The main phases at the interface are Cu_0.81Ni_0.19 and NiCr and the main phase in the fusion zone is Cu_0.81Ni_0.19.

As shown in Figure 14, the phase of the fusion zone is mainly Cu_0.81Ni_0.19, with Cu as the main component on the copper side. Overall, NiCr is the main component on the nickel–chromium alloy side and the fusion zone is mainly composed of Cu_0.81Ni_0.19.

3.2. Mechanical Properties

To verify the strength and welding reliability of the joint, microhardness tests were performed on the dissimilar metal welding joints. The microhardness distribution of the joint is shown in Figure 15. The hardness of the nickel–chromium side was about 200–220 HV, the fusion zone was about 190–260 HV, and the copper side was about 60–90 HV. During the welding process, due to rapid cooling, especially on the nickel–chromium alloy side, the heat conduction is fast, which may lead to grain refinement, thereby improving the hardness of the fusion zone. At the same time, XRD results show two solid solutions in the fusion zone near the nickel–chromium alloy side. These solid solutions cause lattice distortion and hinder the movement of dislocations, thereby further improving the hardness of the material, ultimately leading to a slight increase in microhardness [23,27].

To obtain a deeper understanding of what occurred near the interface, the nanoindentation methods were applied at different areas on both sides. Figure 16 shows the typical loading–unloading curves of different regions of the interface on both sides. Figure 16a shows the loading–penetration depth curve at the interface between the Ni-Cr alloy and the fusion zone. The penetration depth of the diffusion layer is significantly smaller than that of the Ni-Cr side but greater than that of the copper. The lower penetration depth at the diffusion layer indicates that its deformation resistance is higher than copper and a strong connection is achieved at the interface. In other words, the interface between the Ni-Cr and the copper is well metallurgically bonded during welding. Similarly, solid solution strengthening might occur, leading to a higher hardness than that of copper [28].

The hardness and elastic modulus values of different regions of the joint were obtained using the Oliver Pharr method [29], as shown in

Table 3. Hardness and elastic modulus at different regions.

Region	Hardness (MPa)	Elastic Modulus (GPa)
Ni-Cr side	2232.5	266.39
Ni-Cr/Fusion zone interface	1847.3	163.62
Fusion zone	1473.3	157.37
Fusion zone/Cu interface	1031.6	164.86
Cu side	666.76	137.06

. The hardness and elastic modulus measured of the nickel–chromium alloy were the highest, at 2232.5 MPa and 266.39 GPa, respectively. It is worth noting that the decreasing trend of the elastic model in different regions of the joint is as follows: nickel–chromium > fusion zone > copper. The hardness change trend in different regions is as follows: H_copper < H_{fusion zone} < H_{nickel–chromium}.

The stress–strain curve of the specimen after being pulled apart is shown in Figure 17. In tensile tests, the applied stress is usually concentrated at the weakest point. As shown in Figure 17a, all joints broke on one side of the base material, and a distinct “necking” pattern was observed. The results show that when the welded joint is subjected to the same stress as the base material, no fracture occurs at the joint, so the connection strength of the joint is greater than that of the copper base material and a strong metallurgical connection is formed between the alloy and copper. The maximum tensile strength of all joints exceeds 200 MPa and the elongation is about 25%, showing ductile fracture, as shown in Figure 17b.

3.3. Electrical and Thermal Conductivity Characteristics of Joints

To ensure long-term electrical heating of the core wire, we conducted an electrical testing experiment. This experiment aimed to investigate the thermal stability of the welded joint under continuous current load conditions and assess how the joint’s performance changes during prolonged operation. The applied current was 100 A and the electrical durations were 10 min, 20 min, and 30 min, respectively. The infrared thermography of the welded joint is shown in Figure 18. Unlike connections of the same metal, the electrical resistance at different positions of the core wire is different (Ni-Cr > fusion zone > copper) [30], resulting in varying amounts of Joule heat generated at different locations of the joint. The temperature of the entire core wire gradually increases from the copper side to the nickel–chromium alloy side.

The temperature at different positions of the joint varies with the duration of the current flow, as shown in Figure 19. After 10 min of current flow, the joint temperature is approximately 94 °C, while the temperature on the copper side is about 78.3 °C. After 20 min, the joint temperature stabilizes at approximately 111.4 °C and the copper-side temperature is about 92 °C. After 30 min, there is no significant increase in the joint temperature, remaining around 114.8 °C, with the copper-side temperature at about 94 °C. Neither the joint temperature nor the copper-side temperature increased significantly.

The electrical conductivity of dissimilar metal joints at different positions is shown in Figure 20. The copper side exhibits the highest electrical conductivity, followed by the fusion zone, while the nickel–chromium alloy side has the lowest conductivity. According to Joule’s law, when the Joule heat generated during the electrification process is minimal, the temperature change of the joint remains insignificant. In contrast, nickel–chromium alloys have relatively low conductivity, which leads to the generation of more Joule heat during electrification. As a result, the joint experiences a more noticeable temperature change during the heating process. This pattern of temperature change aligns with the results obtained from the experiment.

4. Discussion

This work joined red copper to a Ni-Cr alloy by the TIG filler wire method. The microstructure, electric conduction, and mechanical properties were studied. It is worth noting that in past dissimilar metal welding procedures, the fracture location of the joint was mostly at the interface between the two different metals [31]. However, in this work, the fracture of the welded joint occurred in the copper base material, which indicates that the strength of the joint structure is significantly greater than that of the copper base material (as shown in Figure 16). Previous studies have shown that the microstructure of the interface determines the mechanical strength of the joint [32]. Additionally, if intermetallic compounds were formed at the interface, they could severely reduce the strength of the joint. In this work, through observation and testing, no intermetallic compounds were found. Instead, only a diffusion layer was observed at the interface. This section will discuss the formation process of the interfacial layer and the strengthening of the related joint microstructure.

Firstly, the molten copper and the Ni-Cr alloy mix with each other in a liquid state at the fusion zone and the interface, as shown in Figure 21a,b. Next, as the temperature decreases, Cu-Ni can dissolve and form a strengthening phase, Cu_0.81Ni_0.19. During the melting process, the concentration gradient of atoms on both sides of the interface causes Cr, Ni, and Cu to diffuse and mix. In the cooling process, the solid solution phase precipitates and forms a diffusion layer on one side of the interface, as shown in Figure 21c,d. The presence of this microstructure enhances the joint, resulting in the hardness and tensile strength at the connection being greater than that of the copper base material.

5. Conclusions

Copper was joined to a Ni-Cr alloy by the TIG filler wire welding method. The microstructure, phase composition, mechanical properties, and electrical conductivity of the joints were studied. The main conclusions are as follows:

1.

Using filler wire TIG welding allows for the effective joining of nickel–chromium alloys and copper as dissimilar metals. The joint forms well, with no defects such as pores or cracks on the surface, and the joint strength exceeds 220 MPa, meeting the performance requirements.

2.

There is a mutual transition of alloy elements at the interface on both sides of the fusion zone, and the tensile fracture mode is a plastic fracture with an obvious necking phenomenon during the tensile process.

3.

The results of the joint power-on test showed that the temperature change of the joint reached a stable state after 20 min of power-on, about 114 °C, and the temperature on the low-temperature side was about 92 °C. Compared with the low-temperature side, the temperature of the joint did not increase significantly.