Materials Processing
Semisolid-Liquid Fabrication of Cu–Cr Pseudobinary Alloy by Laser-Directed Energy Deposition
2023 Volume 64 Issue 5 Pages 1025-1028
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2023 Volume 64 Issue 5 Pages 1025-1028
Copper–Chromium (Cu–Cr) pseudobinary alloys are promising materials for electrical application. Nevertheless, their fabrication remains limited by traditional techniques, which are not suitable for manufacturing homogeneous Cu–Cr alloys with uniformly distributed elements. Recently, semisolid-liquid manufacturing method has emerged as a promising technique route to fabricate Cu–Cr alloys. In this work, the laser-directed energy deposition (L-DED) as one of the laser additive manufacturing (LAM) technology, was successfully introduced for semisolid-liquid fabricating the homogeneous Cu–30 mass%Cr alloys. The obtained Cu–Cr alloys are constituted by fully melted Cu metallurgically combined with semisolid-liquid Cr. The close combination of Cu and Cr is benefit to improve the electrical conductivity of the alloy.
Schematic showing formation process of the Cu-Cr alloys fabricated under different laser energies. By adjusting the laser energy input, the Cr in the molten pool can be un-melted, partially melted and fully melted.
Cu–Cr pseudobinary alloys, combining the outstanding electrical/thermal conductivities and excellent fracture toughness of Cu with the high melting point and strength of Cr, are widely used as contact material in medium-voltage and high-current vacuum interrupters.1–3) At present, there are three main techniques for fabricating the Cu–Cr alloys. The first commonly used traditional technique is the casting. Due to the immiscible and density contrasts of the two elements, the elemental distribution of Cu–Cr alloys prepared by this method is always nonuniform, which not only deteriorates the electrical conductivity, but also reduces the hardness and withstand voltage ability of the Cu–Cr alloy.4) The second commercial method used for fabricating the Cu–Cr alloys is infiltration technology. In this method, infiltration of molten Cu into the Cr skeleton can obtain a relatively uniform element distribution.5,6) Nevertheless, the electrical conductivity of the alloys fabricated by this technology is limited since the high Cr content. The third technology that commonly used to prepare the Cu–Cr alloys is the mechanical alloying method.7,8) The Cu–Cr alloys prepared by this method have a lower Cr content and a more uniform element distribution. But the mechanical bonding between Cu and Cr particles in the Cu–Cr alloys obtained by this method are not beneficial for improving the electrical conductivity. Therefore, the traditional preparation techniques for the Cu–Cr alloys have their own disadvantages.9–11) New preparation techniques are urgently needed to solve the above problems to obtain Cu–Cr alloys with desirable properties.
Recently, the semisolid-liquid manufacturing method has emerged as a promising technique route to fabricate the Cu–Cr alloys. The so-called the “semisolid-liquid” manufacturing method is to make Cu complete melted and Cr partial melted during the production process. This technique makes the distribution of Cu and Cr elements more uniform, and can obtain an excellent metallurgical bonding between Cu and Cr. Laser-directed energy deposition (L-DED), as a kind of laser additive manufacturing (LAM) is a valid approach to realize the semisolid-liquid manufacturing process.12–14) The temperature of molten pool can be tailored precisely through controlling the input laser energy, then obtain the partially melted Cr and completely melted Cu.15,16) Moreover, the extremely high cooling rate of the molten pool of L-DED (∼105 Ks−1) is expected to further refine the crystalline grains of the solidified alloys, which enhances the hardness, improve the withstand voltage ability and decrease the chopping current of Cu–Cr alloys.17–21)
In the present study, a Cu–30 mass%Cr alloy, chosen as the model material, was successfully fabricated by the L-DED technology. The L-DED technology is resoundingly presented for semisolid-liquid fabricating the uniform Cu–30 mass%Cr alloys, which are constituted by fully melted Cu metallurgically combined with semisolid-liquid Cr. Uniform distribution of elements and the close combination of Cu and Cr are benefit to improve the electrical conductivity of the alloys. The mechanism of uniform of the Cu–Cr alloys prepared by L-DED is discussed. The relationship between different microstructures and electrical properties are compared according to regulate the laser energy inputs. The purpose of the present paper is to provide a reference for the manufacture of Cu–Cr alloys and other immiscible alloys.
The Cu–30 mass%Cr alloys were produced by L-DED with a coaxial powder feeding system equipped. Gas-atomized spherical Cu powders (∼50 µm, 99.9% purity) and mechanically broken irregular Cr powders (99.1% purity) were used as the raw materials for fabrication the Cu–Cr alloys. The SEM morphologies of the Cu and Cr powders are embedded in Fig. 1. Cu powders and Cr powders were placed in two powder feeding drums, and the feed rate of Cu powders and Cr powders were 7 g/min and 3 g/min respectively by controlling the rotation speed. The YLS-6000 fiber laser produced by German IPG company is connected with the YC52 laser head through the fiber, and the output laser wavelength is 1060 nm. Compared with CO2 laser and YAG laser, YLS-6000 fiber laser produces smaller focusing spots, which is more conducive to nonferrous metal processing and precision machining. The 45 carbon steel plate was used as the substrates during the L-DED process. The substrates were polished and washed by alcohol before fixed on the water-cooled copper plate. The whole process is carried out in an argon gas environment with oxygen content less than 50 ppm.
Schematic of laser-directed energy deposition system (a) and Cu–Cr alloy prepared by L-DED (b). The SEM morphologies of the Cu and Cr powders are shown in the inset.
The schematic representation of the equipment and the L-DED process is shown in Fig. 1. Firstly, the CAD model of parts is generated by modeling software. Secondly, the CAD model is sliced into several horizontal layers of the same thickness. Finally, these layers are sequentially deposited on the substrate, producing a three-dimensional component. At the beginning of the manufacturing process, a laser beam is focused and produces a molten pool on the substrate, which is fixed to a computer numerical control (CNC) X-Y meter. When the X-Y motion table is moved according to the CAD file, a predetermined amount of Cu powders and Cr powders go directly into the molten pool to increase its volume. As the laser beam moves, the melted material rapidly solidifies and forms a thin track that binds the material firmly to the substrate or to the previously deposited layer.
The macroscopic characteristics and microstructure of L-DED samples are mainly controlled by laser powers during the L-DED process.22,23) Therefore, it is reasonable to speculate that different laser energy inputs can produce different microstructure of Cu–Cr alloys. In order to investigate the influence of laser energy inputs to the macroscopic characteristics and microstructure during the L-DED process, the Cu–Cr alloys were prepared with varying laser powers of 1500 W, 2000 W and 2500 W, respectively. All the other processing parameters were kept constant at a powder mass of 10 g/min, a beam diameter of 3 mm, a beam scanning speed of 600 mm/min, an overlapping ratio of 30% and a layer thickness of 0.5 mm.
Metallographic samples were prepared using standard grinding and polishing techniques. Subsequently, the samples cross-section surfaces were observed by SEM (Carl Zeiss, SUPRA 55, Germany). The Cu–Cr phase boundaries were investigated utilizing a transmission electron microscope (TEM) (JSM-2100F, Japan) with an accelerating voltage of 200 kV.
The typical microstructures of the Cu–Cr30 mass% alloys prepared by the LAM under different laser processing conditions are shown in Fig. 2. The light and dark contrast regions correspond to Cu and Cr, respectively. Apparently, the microstructures are strongly dependent on the laser processing parameters. For the alloy prepared under the laser power of 1500 W and scanning speed of 600 mm/min, as presented in Fig. 2(a), large irregular Cr blocks, whose size are the same scale as the original Cr powders, are observed to distribute on the Cu matrix. Some pore defects are formed at the interface between Cu matrix and Cr particles, which reduce the performances of the obtained Cu–Cr alloy.24,25) This is mainly due to the lower energy input and the faster cooling rate of the molten pool, which prevents the molten Cu from fully filling the grooves on the surface of the irregularly shaped Cr particles.
Mirostructures of the Cu–Cr alloys fabricated under varied laser energies. (a) SEM morphology of the alloy fabricated under the laser power of 1500 W and scanning speed of 600 mm/min. (b) SEM morphology of the alloy fabricated under the laser power of 2000 W and scanning speed of 600 mm/min. (c) SEM morphology of the alloy fabricated under the laser power of 2500 W and scanning speed of 600 mm/min. (d) HRTEM image in the vicinity of the Cr–Cr interface region of the alloy fabricated under the laser power of 2500 W and scanning speed of 600 mm/min.
In order to avoid the interfacial pore defects, we further increased the laser energy of L-DED to obtain a better metallurgical bonding interface between the Cr particles and Cu matrix. Keeping the composite composition and other laser fabrication parameters unchanged, we increased the laser power to 2000 W to obtain higher input laser energy. The resulting microstructure is shown in Fig. 2(b). Two morphological Cr, i.e., small Cr blocks with relatively smooth interfaces and fishbone-like Cr dendrites with the size ranging from 1 to 20 µm, are observed to uniformly distributed on the Cu matrix. When the laser power is increased to 2000 W to manufacture the Cu–Cr alloy, the laser energy can completely melt the Cu powders, but is not enough to completely melt the Cr powders. The Cr powders only partially melt at their edges, allowing the residual unmelted Cr particles to form excellent metallurgical bonds with the Cu matrix. In the subsequent solidification process, the partially melted Cr recrystallize out again, forming fishbone-like small Cr dendrites uniformly distributed on the Cu matrix and leading to microstructure with two morphological Cr. The excellent metallurgical bonding between the Cu and Cr can be verified by the HRTEM image in the vicinity of the Cr–Cr interface region (Fig. 2(d)). No obvious pore defects can be found and metallurgical bonding at atomic scale is achieved at the interface through this semisolid-liquid manufacturing process.
In order to check whether further increase of input laser energy can improve the microstructure more, we further increased the laser power to 2500 W while keeping other process parameters unchanged. Figure 2(c) displays the microstructure of the obtained Cu–Cr alloy. Clearly, the deposited Cu–Cr alloy completely divided into Cr-rich region (top) and Cu-rich region (bottom). Such severe elemental segregation is mainly due to the reduced cooling rate of the molten pool at higher laser energy, which allows enough time for the molten Cr elements to coalesce during the subsequent solidification.
The formation processes of the above three Cu–Cr alloys fabricated by L-DED are schematically shown in Fig. 3. The temperature of the molten pool is directly controlled by the laser power, and different molten pool temperatures will produce different structures.26) When the laser power is too low (green line), the molten pool temperature does not reach the melting point of Cr, so Cr powders will not melt and remain pores in the molten pool. When the laser power is moderate (blue line), the molten pool temperature is just to make Cr semisolid-liquid, about 0.2 seconds of liquid time will not cause Cr accumulation. When the laser power is too high (red line), the Cr stays in the liquid state for up to 0.7 seconds. The density of liquid Cu and liquid Cr is 8.9 g/cm3 and 6.9 g/cm3 respectively. Such a large density difference will cause gravity segregation of metals in the molten pool during the long solidification process, resulting in uneven distribution of elements. By comparing the above three kinds of microstructures, it can be found that the elemental distribution obtained by semisolid-liquid manufacturing method is the most uniform, and the interfacial metallurgical bonding is the best, so this method is the best choice for preparing Cu–Cr alloy with excellent properties, which are demonstrated by the electric conductivity presented in Fig. 4. Obviously, as the temperature increases, the electric conductivity of the Cu–Cr alloys decreases linearly. This is because as the temperature rising, the vibrations of atoms become more violent, which makes them more likely to collide with free electrons, thus impeding their directional movement and reducing their electrical conductivity. Due to the presence of pores in the alloy that block the free movement of electrons, the 1500 W prepared alloy has slightly lower electrical conductivity (red line). As mentioned earlier, excessive laser power (2500 W) can cause Cr to accumulate. Therefore, in this region, Cr is the main conductor with low conductivity (black line). By comparing the electrical conductivity of three-kind alloy structures, the semisolid-liquid prepared Cu–Cr alloy has a much higher electrical conductivity than that of other alloys, supporting that the microstructures obtain by semisolid-liquid method is indeed beneficial to ensure the good performance of the Cu–Cr alloy.
Schematic showing the molten pool temperature under different powers (a) and formation process of the Cu–Cr alloys fabricated under different laser energies (b). By adjusting the laser energy input, the Cr in the molten pool can be un-melted, partially melted and fully melted.
The electric conductivities of L-DED Cu–Cr30 alloys under varied laser powers and constant scanning speed 600 mm/min.
In this investigation, the Cu–30 mass%Cr pseudobinary alloys were successfully prepared by L-DED technology using optimum process parameters of laser scan speed 600 mm/min and laser power 2000 W. Because of the Cu–Cr alloys prepared by LAM have uniform elements distribution and compact microstructure, the electric conductivity is 24.784 MS/m, much higher than other microstructures of materials with the same Cr content. In the L-DED process of Cu–Cr alloys, the extremely high cooling rate refines the size of Cr grains less than 20 µm, and improves the withstand voltage ability of Cu–Cr alloy. It is found that if the laser energy inputs are too low or too high, the elements will not be uniform distribution. Optimum laser energy input makes the Cr semisolid-liquid, mix uniform with Cu. Semisolid-liquid Cr combines with Cu in metallurgy, so that the Cu–Cr alloys fabricated by L-DED are highly dense and free of apparent cracks and pores.
This work was supported by the National Natural Science Foundation of China under Grant Nos. 51971047 and 52271022, the project of Liaoning Province’s “Rejuvenating Liaoning talents plan” under Grant No. XLYC1907046, Dalian High-Level Talent Innovation Support Program under Grant No. 2020RJ07, the state Key Lab of Advanced Metals and Materials under Grant No. 2021-ZD10, the Joint Research Fund Liaoning-Shenyang National Laboratory for Materials Science under Grant No. 2019JH3/30100032.
