Exothermic of electrolytic cell cathode bar

Guodong Zhang1 , Fei Zheng1 , Longjian Xue1 , Jingdang Fei2 , Hongwei Yang3

( 1. School of Power and Mechanic of Wuhan University, Wuhan 430072, China; 2. Wuhan Billion Science & Technology Development Co., Ltd, Wuhan, 430071, China; 3. China Power Investment Corporation Ningxia Energy Aluminum Engineering and Overhaul Co., Ltd, Wuzhong 750002, China )

Abstract: The use of exothermic welding technology to achieve the molten cathode steel bar welding of the electrolytic cell under the strong magnetic field was introduced. The microstructures, microhardness and several other properties Prof. Guodong Zhang Email: [email protected] of welding joints were analyzed from OM, SEM, EDS, etc. The results showed that the exothermic welding was not impacted by the strong magnetic field of the Guodong Zhang , an associate professor electrolytic cell, the welding interface was well fused, and there were no obvious in School of Power and Mechanic of Wuhan welding defects. The intermetallic metallurgical bonding was realized between the University, Ph.D. One of the members of base and the weld metal. The mechanical properties of steel bar after the the Committee China Brazing and Special exothermic welding could meet the safety requirements of the repair or overhaul Joining, the executive director of Hubei of the electrolytic cell. Welding Commitee. The main researches Key words: electrolytic cell; cathode steel bar; exothermic welding; mechanical are advanced material joining technology, properties simulation of materials' , compu- DOI: 10.7512/ j.issn.1001-2303.2017.13.11 tational materials science, surface engin- eering and nanomaterials' fabrication.

0 Introduction or operated seriously restrict the further extend of electrolytic cell life. So In order to improve the current efficiency and aluminum production the cathode steel bar welding under strong magnetic field is an urgent amount in the produce of electrolytic aluminum industry, the current and important technical problem to be solved [2--5]. intensity of the aluminum electrolytic cell is increasing, the maximum Exothermic welding technology, which by means of reducing agent current in the world has reached 600 kA [1]. Such a large DC current in the reaction to restore the metal in the oxide, which produces result in a strong magnetic field around the cell and its diameter reaches high-temperature liquid metal in a short time and releases large amount several meters which can be calculated up to hundreds of Gaussian, of heat meanwhile, heating of the workpiece to form a certain shape magnetic field distributes along the three-dimensional disorderly. An directly or indirectly, and the size of welding joint is also certain. Because important part of the electrolytic cell overhaul is the welding of the there is no arc, the interference of strong magnetic field could be cathode steel bar. In the implementation process, the cathode steel bar avoided, which ensures the quality of welding, reduces labor intensity and the cathode busbar are separated at first with the old steel bar, and and makes operation simple and quick. This paper attempted to use then connected with the new steel bar. The connection between the the exothermic welding technology to melt the welding of the cathode new cathode steel bar and the small busbar is a plurality of parallel steel steel bar and researched on the contains and components of welding plates, and the welding process is carried out by manual welding. Under materials. The design of the welding mold and the optimization of the the effect of electromagnetic field, the arc shape and metal movement welding process were also researched in the paper. The microstructures, change, resulting in some welding defects subsequent to magnetic bias microhardness, chemical composition and mechanical properties of and welding pool asymmetry, in this situation, the welding quality will welding joints were measured to see whether it meets the practicality in be decreased. The uneven of the cathode steel bar current when started the engineering field [6--7].

·142 · 1 Experimental details blowtorch to preheat the mold so as to remove the water vapor. Place 1.1 Welding Materials and Molds the self-fusing plug at the bottom of the and cover the For the cathode steel bar, several major indicators of the exothermic trough. The configured flux is loaded into the crucible and compacted. flux are: (1) the heat of the self-propagating reaction is sufficient to melt The ignition powder should be spread evenly on the sfigureuppurface the metal. (2) the surface of the base material is wetted in advance of the flux, insert the bar. With the ignition the magnesium, when it exists the oxide layers and is sufficiently spread on the surface magnesium ignited the ignition powder, then the flux will also be of the base material, which means the infiltration is good. (3) the ignited, which release a lot of heat, get high temperature overheat mechanical properties ensure the safety of welding joints. metal flowing from the liquid hole into the groove, melting the steel

The CuO-Fe2O3-CaSO4-Al system is mainly used in the exothermic bar to weld. After all these steps, remove the mold, remove the fixture flux, the CaF2-SiO2-CaO system is the -forming agent, the Cu-Mn-Si and plate. Clean the weld metal surface of the welding slag to system is the alloy agent, and the Cu powder has the effect of adjusting expose the metallic luster [8]. the temperature and the speed of the reaction, as well as alloying effect; After all the welding process, make some samples to measure its other components for alloying, refining the grain and improving the role properties, use the optical microscope to observe the metallographic, of weld metal properties. According to the chemical reaction formula, use the scanning electron microscopy with EDAX to analyze the the components were weighed and mixed in a feeder, and then placed microstructure and get the EDS result, use microhardness tester to in an electrothermal oven for 120 °C / 30 min to dry [8]. measure the hardness. Make some standard tensile specimen of Graphite has high anti-heat impact performance, it’s less likely to be the cathode steel bar welding joint to measure the tensile strength broken in the welding process. It has less melt penetration, less adhesion by a tensile tester in order to check whether it’s capable to meet the to other and better slag removal, it’s used to be the mold requirement of long-term service. material. The upper mold is a crucible with a lower flow guide hole fixed to the upper part of the groove, and its shape is shown in Fig.1 2 Results and Discussion The lower mold consists of two graphite plates, which are attached to 2.1 Fusion Performance the left and right sides of the groove to be welded, together with the Fusion property directly affects the quality and of welding joints. upper plate to form a sealed cavity with a vent hole, and the outside is Fig.2 shows the manual macro samples of the cathode clamped with a steel fixture.

( a ) Manual welding joint

Fig. 1 Schematic of the upper welding mold 1.2 Welding Process and Joint Properties Test Before welding, the size of the groove of the cathode steel bar should be measured, the filling volume and quality should be calculated, and the amount of powder should also be calculated according to the rate and requirement. Clean the surface first. Use a graphite U-shaped plate with a groove to block both sides of the aluminum busbar and then clamp the graphite plates on both sides with a steel gripper. Put some high temperature glue on the graphite plate to prevent the ( b ) Exothermic welding joint molten metal outflow. Place the upper mold crucible on top of the Fig. 2 Macroscopic morphology welding joints base metal groove and align the groove with the liquid hole. Use the

·143 · steel bar and the exothermic welding one which were grinded and etched. It can be seen that under the effect of the magnetic field, various defects appeared such as pores, non- penetration, non-fusion, etc. in the manual process. Except for some parts of the slag and non- fusion defects, there was no continuous pores, cracks, non-penetration and other defects in the inside and the interface of the welding metal. Obviously, because of the high heat output of the exothermic welding, sufficient metallurgical combination was achieved. Because there was no arc and ferromagnetic material in the beam metals, the whole welding process weren’t influenced by the magnetic field, which resulted in the good quality of the welding. 2.2 Microstructure and Composition Analysis ( a ) the microstructure of base metal The joint microstructure of the cathode steel bar shown in Fig.3. It can be seen that a typical ribbon structure existed in the base material Q235 steel, which consists of pearlite and ferrite (Fig.3(a )), light- colored matrix in the weld metal is the copper-based solid solution, and the black particles is for the precipitation of the second phase (Fig.3(b )). The weld is well bonded to the substrate without the presence of various microscopic defects, and there is an apparent transition zone between the weld and the base material (Fig.3(c )) with a thickness of about 50 μm, because of the overheated weld liquid metal melted and wetted the base material of the interface. The column crystals are grown in one direction close to the fusion zone and are larger than those of the column in the transition zone. This is due to the fact that in the heat ( b ) the microstructure of the welding seam center affected zone, the heat received by the substrate near the weld is much higher and the temperature rises quickly, and grains are combined at the high temperature , grain boundary migration led to the increase of the grain size ( Fig.3(d )). Electron probe microanalyzer was used to analyze the distribution of interfacial elements in the exothermic welding joint of cathode steel bar. The results showed that the weld zone mainly consisted of Cu which varied slightly according to the different kinds of alloying elements added; the interface fusion zone mainly consisted of Fe and Cu, and a small amount of Mn, Al, etc., it indicated that the fusion zone is produced by the process in which high temperature liquid metal melted the base material and mixed together. The EPMA face distribution ( c ) the microstructure of the interface near the fusion zone is shown in Fig. 4. Fe and Cu on both sides were fully diffused, indicating that metallurgical bonding was achieved between the weld metal and the base metal, and there were a small amount of Mn, Fe, Al and other elements, which had strengthened the alloy because of the solid solution strengthening in some aspects, the mechanical properties of the weld metal was improved. 2.3 Mechanical Performance Test and Analysis The microhardness distribution of the welding joint is shown in Fig.5, and the microhardness fluctuates greatly. Because the middle weld alloy area is mainly the α-Cu phase, the hardness is lower, about HV130 or so. The highest mictohardness appeared in the fused area, because the heating process heating temperature exceeded Ac1 ( 735℃) , ( d ) the microstructure of large grain zone and a part of the tempered sorbite transformed into austenite, then Fig. 3 Microstructure of welding joint

·144 · Fig. 4 EPMA analysis of welding joint changed into martensite when cooling, and copper diffusion occurred combination was achieved between the base metal and weld metals. simultaneously. These changes resulted in Fe lattice elastic distortion, (4) The average tensile strength of the exothermic welding joint of which increased lattice distortion energy, so the hardness increased the cathode steel bar was about 440 MPa, which could meet the safety greatly. The microhardness of the base metal is about HV 200. requirements of the repair and overhaul.

Acknowledgment: This research was supported financially by the Suzhou nanometer special project (No.ZXG201447) and the Natural Science Foundation of Hubei Province (No.2014CFB707).

References: [1] Fist all series 600kA aluminum electrolytic cell in the world was tested in Weiqiao [J]. Energy Saving of Nonferrous , 2015 ,(05): 64. [2] Xue S. Welding Technology Manual [M]. Beijing: China Machine Press ,2006: 121-125. [3] Zhang K, Wang F. Electromagnetism [M]. Beijing: High Education

Fig. 5 Micro-hardness distribution of welding joint Press, 2011: 122-154. [4] Li Z, Xin W, Wu B, et al. Effect of thermit composition on manual The tensile test showed that the average tensile strength of the SHS welding for low steel [J]. Transactions-China Welding cathode joint is 440MPa, which can meet the safety requirements Institution, 2007, 28( 2 ): 79. of electrolytic cell repair or overhaul under strong magnetic field. The [5] Zhang B, Xin W, Li Z. Analysis on Properties and Microstructure elongation is 16.5%, slightly lower than the plastic index of manual of Welded Joints by Manual SHS Welding [J]. Hot Working arc welding process we use now, which is related to the weld structure Technology, 2007, 15: 006. which was as-cast. [6] Hu J, MA S, CHEN X, et al. Structure and performance of a new Cu-base SHS welded joint [J]. TRANSACTIONS-CHINA WELDING 3 Conclusion INSTITUTION, 2006, 27(10): 94. ( 1 ) Welding process can be carried out without cutting the power [7] Liu J, Huang X, Qiu J, et al. Study on Combustion Velocity and supply when we use exothermic welding technology. The welding Temperature of Non-electric Welding Material [J]. Journal of quality will not be influenced by the strong magnetic field, and the Academy of Armored Force Engineering, 2009, 3: 020. production will not be influenced either. [8] Hongwei YANG, Guodong ZHANG, Huiyun LIU. A welding mold ( 2) Good interface fusion was acquired, in which no obvious non- used for the SHS repairing method of the aluminum busbar [P]. penetration, non-fusion and other welding defects were found. CN200820230016.1.2010. ( 3) Weld metal consists mainly of Cu, and the good metallurgical

·145 ·