Abstract
The addition of Cr and Zr into the Cu base is applied to overhead contact wires and lead frames due to its high tensile stress and electrical conductivity. Results show that the length of 45 m for Cu–Cr–Zr alloy billets can be successfully prepared by horizontal electromagnetic continuous stirring. The optimised parameters on Cu–Cr–Zr billets with 87 mm diameter are as follows: the current is 50 A, the frequent is 30 Hz and the casting speed is 108 mm min−1. The rotating magnetic field (RMF) can decrease the casting temperature by 22°C. The average grain size with RMF is 1·35 mm, reduced by 55 compared to that without RMF. The tensile stress is 212 MPa, and elongation is 33. The microstructure with RMF on non-vacuum casting is similar to that of the vacuum. At the same time, RMF makes Cr, Zr and Cu atoms uniformly distributed.
Keywords
Introduction
Cu based alloys are widely used because of their excellent properties, such as high corrosion resistance, high tensile strength, electrical conductivity, etc. The addition of Cr and Zr into Cu is applied to contact wires used in high speed railway and the lead frames used in IT due to the high mechanical and operating properties.1–3 Recently, Cu–Cr–Zr alloy has attracted considerable interest because of its superior combination of high electrical conductivity and high strength,4–8 which leads to the application of Cu–Cr–Zr alloy in the overhead contact wire of high speed railway.
Nowadays, Cu–Cr–Zr alloy is prepared by traditional technology on vacuum casting and extruded methods. However, the special properties of the contact wire used in over 350 km h−1 high speed railway is for single continuous lengths of wire up to 1·59 km long and 2500 kg in weight. Therefore, traditional preparation technologies of vacuum casting and extroded methods on Cu–Cr–Zr alloy cannot be completed. Cast roll process (CRP) as a short process technology includes the following processes: horizontal continuous casting billet→surface milling→planetary rolling→drawing→annealing. The contact wires for over 350 km h−1 high speed railway can only be prepared by CRP. The horizontal continuous casting billets changed by rotating magnetic field (RMF) are key process of CRP.
Magnetohydrodynamics is a subject that studies the law of electric liquid's movement in electromagnetic field, and it has been widely used in the metallurgy industry in recent years.9–14 Electromagnetic field is also used in the horizontal continuous casting of tubes and billets. Rodriguez 15 used an alternating magnetic field of commercial frequency on horizontal continuous casting (with diameter of 30 mm and wall thickness of 6 mm) copper tubes, whose solidifying structure was refined. Li et al. applied commercial frequency to the manufacturing process of BFe10-1-1 tubes and obtained prominent progress. 16 Nevertheless, few studies have been studied for Cu–Cr–Zr alloy billets by RMF.
This paper explores the parameters of horizontal electromagnetic continuous stirring on Cu–Cr–Zr alloy billet with diameter of 89 mm and length of 48 m by non-vacuum, studies the RMF effect on solidifying macrostructure, element distribution and mechanical properties, and discusses its mechanism of action.
Experimental
A preproduction system of horizontal electromagnetic continuous stirring has been built up. The experimental equipment contain the horizontal continuous furnace, mould, cooling system, electromagnetic stirrer system and drawing system, as seen in Fig. 1. The mould is composed of a graphite inner mould and a copper outer jacket. The RMF is generated by three poles induced by coils outside the graphite inner mould.
Diagram of experimental equipment
The chemical composition of Cu–Cr–Zr billets is as follows: Cr is 0·40–0·60 wt-, Zr is 0·05–0·09 wt- and Cu is balance. The casting temperature of the alloy was studied by the simultaneous differential thermal analyser (TGA/SDTA 851e) with the rate 5°C min−1 on heating and cooling, and the shape of samples is 3×3×0·9 mm protected by argon. The microdistribution of the alloy elements was analysed by electron probe microanalyser (EPMA-1600). Samples were etched by a mixed solution with 3 g FeCl3, 2 mL HCl and 96 mL C2H5OH for 1–2 min to study the microstructure.
Results and discussion
Decrease in casting temperature by RMF
Figure 2 shows the curves of heating and cooling with TGA/SDTA 851e. From the figure, we can see that through 900°C→1220°C→900°C, the downward curve expresses endothermic process, and the upward curve does exothermic. In the heating curve, the temperature of the first endothermic peak is 1074·79°C, which indicates that the solid begins to melt, and the temperature of the second endothermic peak is 1217·75°C, which indicates that the solid is completely melted. In the cooling curve, the temperature of the first exothermic peak is 1209·75°C, which indicates that the liquid begins to solidify. The second peak is 1056·29°C, which indicates that the liquid is completely solidified. In fact, the average melting point of the alloy is 1065·54°C, averaged with 1074·79 and 1056·29°C due to thermal hysteresis. During the casting process, we find that without RMF, the melting temperature is 22°C higher than that of with RMF.
Differential scanning calorimetry curves of alloy at heating and cooling stages
The schematic diagram of solidification shown in Fig. 3 can help explain the above results. We can see that without RMF, there is higher cooling intensity on the bottom area and lower cooling intensity on the top, both leading to longer depth of sump to decrease the strength of billets, as seen in Fig. 3a. At the same time, the metals solidified will wrap the leading edge (arrow no. 4) of the graphite shell due to the higher cooling intensity, while most top metals are in liquid, so billets will be easily pulled off or pulled out the mould to breakout because of sticking in the mould. To prevent the accident, the casting temperature is raised, and drawing speed is decreased without RMF.
Schematic diagram of solidification: 1, graphite shell; 2, molten metal; 3, solidification metal; 4, wrapping area; 5 stirrer
However, with RMF, from Fig. 3b, we can see that the temperature field of molten metal is made uniform by RMF owing to reducing the temperature gradient between the top and bottom areas. Therefore, the depth of sump is shorter than that without RMF to increase the strength of billets and will not wrap the bottom area of the graphite shell. Finally, billets can be produced in the lower temperature. From above, we can draw a conclusion that RMF can decrease the casting temperature.
Study of electromagnetic parameters
It is important to study the parameters of current and frequency as the following parameters: the casting speed is 108 mm min−1, and the casting temperature is 1255–1260°C on the bottom of furnace, and the inlet water temperature is 26°C, and the outlet water temperature is 40°C, and the cooling water flowrate is 1·9 m3 h−1.
Current
Figure 4a shows the macrostructure without RMF, from which we can see that there are many developed columnar crystals on the bottom and edge area, and macrostructures are uneven and grains are coarse. However, with RMF, the parameters are the following: the current is 30 A, and the frequency is 30 Hz, and we obtain the magnetic field B of 23·31 mT. The macrostructure can be seen in Fig. 4b, from which we can see that the structure of the billet is in countercurrent direction to the centre, the grain size becomes refined and the structure becomes more uniform. When the frequency is unchanged, the current is improved to 50 A, and then B is increased to 30·00 mT. The macrostructure is seen in Fig. 4c, from which we can see that the solidification structure is more refined, and the coarse columnar crystals are completely eliminated, leading to a uniform macrostructure.
Effect of current
The current increase make the B increase, so the electromagnetic stirring force Fφ is increased. The corresponding equation can be seen as
is the average magnetic field, r is radius of molten pool and ω0 is the angular velocity of the eddy current.
From equation (1), we can see that electromagnetic stirring force that apply to molten metal is gradually increased from the inner radius to outer radius, so the liquid is formed to the convective flow to make columnar crystals broken, and more new crystalline core is generated to make grains refined. Simulation results show that when B is increased to 39·8 mT, the joule heating generated by eddy current will become very large to make nucleated nuclei remelt again, so the total number of nuclei will be relatively reduced to decrease the refined effect. Therefore, B of 39·8 mT cannot refine the grain size.
Frequency
When the current is maintained to 50 A, frequencies of 10, 30 and 50 Hz are investigated to study the macrostructure, as seen in Fig. 5. When the frequency is 10 Hz, there is a distinct coarse structure as the decreased stirring force cannot stir the solid–liquid metal, as seen in Fig. 5a. When raising the frequency to 30 Hz, the effect of grain refinement is obvious, as seen in Fig. 5b. However, with frequency raised to 50 Hz, the centre of the billet finds cracks, which is of length of 20 mm and width of 0·5 mm, as seen in Fig. 5c. Moreover, these faults can also be found without RMF. The reason can be indicated by equation (2)
Effect of frequency
From equation (2), we can see that the higher frequency, the more decay of the magnetic field will generate. When the frequency changes from 30 to 50 Hz, the electromagnetic stirring force becomes smaller in the centre of billet, so liquid metal cannot flow to faults timely to form cracks. This action also appears without RMF in the alloy.
In a word, the optimised parameters on Cu–Cr–Zr rod with Φ-87 mm are 50 A and 30 Hz under RMF.
Formation mechanism of surface cracks on RMF
Without RMF, there are some large cracks that appear one per length of 5 mm with depth of 3 mm and width of 2 mm on the surface of the billets, as seen in Fig. 6a. However, with RMF, these cracks are totally disappeared, as seen in Fig. 6b, so RMF may inhibit the formation of cracks. The reason is that the depth of sump becomes short owing to forming the thicker and uniform solidified shell, so the tensile stress of billets is increased to prevent surface cracks.
Effect of rotating electromagnetic field (REF) on crack of surface
Properties of Cu–Cr–Zr billets on RMF
Tensile stress and fracture micrograph
Without RMF, there are many cracks on the billets surface, and the macrostructure is coarse and uneven, so billets cannot be rolled by the planetary rolling mill. Nevertheless, with RMF, cracks on the billet surface disappear; at the same time, the macrostructure and segregation of elements are improved. In the end, the length of 45 m for Cu–Cr–Zr alloy billets can be cast successfully by horizontal electromagnetic continuous stirring on RMF. The average tensile stress is 212 MPa, and the elongation rate is 33. The fracture micrograph of scanning electron microscopy (SEM) on tensile fracture can be seen in Fig. 7, which further reveals that the plastic processing properties are excellent and suitable for large deformation, as shear lips are wide and dimples are big.
Fracture micrograph
Change of grain size
Figure 8 shows the change of grain size with and without RMF. Without RMF, the average grain size is 3·00 mm. However, with RMF, the average grain size becomes 1·35 mm, reduced by 55. Therefore, RMF can refine the grain size.
Change of grain size
Microstructures and distribution of elements on RMF
Microstructures of Cu–Cr–Zr alloy on RMF are shown in Fig. 9. Figure 9a shows the microstructure magnified to 50 times, from which we can see that grains grow through the reversible direction of electronic magnetic field to solidify, and structures are uniform. When the microstructure is magnified to 200 times, as can be seen in Fig. 9b, we can see that the Cr rich phase appearing spheroid or bar shape distributes as semicontinuous or single style on the based Cu, and the average diameter of the Cr rich phase is ∼10 μm. The microstructure remarked by circle is magnified to 1000 times, as seen in Fig. 9c, from which we can see that on the Cu base, there are many small Cr rich phases that show as regular spheroid with diameter less 2 μm. The microstructures show that there are not the gas porosity and slag with RMF on the non-vacuum, so it is similar to that of the vacuum casting.
Microstructures with RMF
Figure 10 shows the micrographs of EPMA, from which we can see that curve variations of Cr, Zr and Cu are small to indicate that the distribution of elements is nearly uniform. Scanning images of Cr, Zr and Cu also show that there is a certain point of segregation in a microarea, but on scanning area all elements are evenly distributed. All show that the RMF has an effect to improve the segregation of Cr, Zr and Cu.
Elements distribution with RMF
Conclusions
The length of 45 m for Cu–Cr–Zr alloy billets can be successfully prepared by horizontal electromagnetic continuous stirring. The optimised parameters on Cu–Cr–Zr billets with Φ-87 mm are the following parameters: the current is 50 A, the frequent is 30 Hz and the casting speed is 108 mm min−1. The RMF can decrease by 22°C of casting temperature.
The average grain size with RMF is 1·35 mm, reduced by 55 compared to that without RMF. The tensile stress is 212 MPa, and the elongation is 33. The microstructure with RMF on non-vacuum casting is similar to that of the vacuum. At the same time, RMF makes the elements of Cr, Zr and Cu evenly distributed. All properties reveal that the plastic processing properties are suitable for large deformation.
Footnotes
Acknowledgement
This project was supported by the Fundamental Research Fund for the Zhejiang Provincial Laboratory of Testing and Deep Processing for Measuring and Cutting Tools (grant no. ZD 201202) and by the National Natural Science Foundation of China (grant no. 51206179).