In this study, the influence of the speed of downward semi-continuous casting on the crystal size and mechanical properties of recycled copper after casting and cold deformation was investigated. It was shown that the increase in the casting speed from 0,09 m / min to 0,22 m / min resulted in refinement of the macrostructure and reduction of the crystal cross-sectional area from 3,221 mm2 to 1,2 mm2, which resulted in an increase in microhardness after casting by 8 %, while in the state after cold working, an increase in microhardness by 3 % and ultimate tensile strength by 2,5 %

Domagała-Dubiel, J.

Głuchowski, W.

Maleta, M.

Rdzawski, Z.

Łagoda, M.

English

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387METALURGIJA 62 (2023) 3-4, 387-390M. MALETA, W. GŁUCHOWSKI, Z. RDZAWSKI, M. ŁAGODA,  J. DOMAGAŁA-DUBIELINFLUENCE OF THE SPEED OF DOWNWARD  SEMI-CONTINUOUS CASTING ON THE CRYSTAL SIZE  AND MECHANICAL PROPERTIES OF RECYCLED COPPERReceived – Primljeno: 2023-03-03Accepted – Prihvaćeno: 2023-03-24Original Scientific Paper – Izvorni znanstveni radIn this study, the influence of the speed of downward semi-continuous casting on the crystal size and mechanical properties of recycled copper after casting and cold deformation was investigated. It was shown that the increase in the casting speed from 0,09 m / min to 0,22 m / min resulted in refinement of the macrostructure and reduction of the crystal cross-sectional area from 3,221 mm2 to 1,2 mm2, which resulted in an increase in microhardness after casting by 8 %, while in the state after cold working, an increase in microhardness by 3 % and ultimate tensile strength by 2,5 %.Keywords: copper, semi-continuous casting, macrostructure, crystal size, mechanical propertiesISSN 0543-5846METABK 62(3-4) 387-390 (2023)UDC – UDK 669.3:621.74.04:620.18:620.17=111INTRODUCTIONCopper and its alloys, due to their properties, are used in many branches of the industry, such as construc-tion, energy, electronics, maritime and automotive in-dustry. High mechanical properties of copper and its alloys can be achieved through the use of many strength-ening mechanisms of metallic materials, such as solu-tion hardening, precipitation hardening, strain harden-ing, grain and subgrain boundary strengthening. One of the most effective mechanisms of material strengthen-ing without loss of ductility is the refinement of the mi-crostructure. In addition to increasing the strength prop-erties of the material, homogeneity of properties in the entire volume, better susceptibility to cold working and increased fatigue resistance and cracking resistance are obtained [1-4]. In the case of cast materials, there are many possibilities of grain refinement. One of the way of refining the microstructure of castings is the intro-duction of alloying elements to the chemical composi-tion, which form oxides or intermetallic phases and constitute nuclei of crystallization during solidification. Effective grain refinement by alloy additions is difficult because, apart from ensuring a sufficient amount and size of highly dispersed particles, special melting and casting conditions must be met, including an appropri-ate casting temperature and a high degree of undercool-ing [5-7]. Refinement of the microstructure in the con-tinuous casting process is possible primarily by control-ling the casting parameters such as feed, stops and the flow of the cooling medium in the primary and second-M. Maleta (marcin.maleta@imn.lukasiewicz.gov.pl), W. Głuchowski, Z. Rdzawski, M. Łagoda, J. Domagała – Dubiel, Łukasiewicz Research Network – Institute of Non-Ferrous Metals, Gliwice, Polandary cooling system. In addition to the parameters men-tioned above, the crystallization conditions are greatly influenced by the roughness and quality of the outer surface of the crystallizers in contact with the surface of the cooler. Higher roughness of the crystallizer results in poorer heat transfer. Ensuring an appropriate heat balance, both by accelerating the casting process and by fast heat dissipation, contributes to microstructure re-finement [8-9]. In the case of continuous casting pro-cesses, good results in terms of microstructure refine-ment are obtained by using a mixing electromagnetic field in the crystallization system. This interaction causes homogenization of the temperature at the front of the solid-liquid interface, disturbing the direction of heat flow, which affects the remelting of dendrites and the formation of equiaxed crystals [10-11].This paper presents the results of the study of the influence of variable speed of downward semi-continu-ous casting of recycled copper on the macrostructure and mechanical properties. The research focused on verifying the possibility of obtaining pure copper rods with higher mechanical properties after casting and plastic working by refinement the casting macrostruc-ture crystals using a higher casting speed.EXPERIMENTAL PROCEDUREMaterialThe test material consisted of rods with a diameter of 25 mm cast in a downward semi-continuous manner from copper. Pieces of pure Cu-ETP copper and recy-cled copper from power cables (51 % of the charge weight) were used to produce the copper rods. A small 388  METALURGIJA 62 (2023) 3-4, 387-390M. MALETA et al.: INFLUENCE OF THE SPEED OF DOWNWARD SEMI-CONTINUOUS CASTING…amount of magnesium addition was used to deoxidize the liquid copper. The average chemical composition of tested material measured on the spark emission spec-trometer is presented in Table 1. Three measurements of the chemical composition were made for each sample.structure into a binary image using the ImageJ software is shown in Figure 1.Tensile testThe static tensile test was carried out on the Instron 100 kN universal testing machine. Three samples of each material at a speed of 2 mm / min were tested after cold deformation. The ultimate tensile strength Rm and the yield strength Rp0,2 were determined. The A50 elon-gation was calculated on the basis of the dimensional changes of the applied datum on the samples.Microhardness testMicrohardness measurements were made for sam-ples as cast and for samples after cold working. Micro-hardness was measured by the Vickers method with the universal Future-Tech FM-700 hardness tester. A load of 1 kg of the indenter was applied for 15 seconds. After casting, 25 measurements were made on the samples, and 20 measurements after cold working. The mean value was calculated based on the measured values.Cold workingAfter casting, samples with a diameter of 25 mm were subjected to groove rolling (square cross-section) to the size of 7 x 7 mm. The samples were rolled with a unit reduction of 20 %. After the rolling process, the rods were subjected to a drawing process with a unit reduction of 20 % to a diameter of 4,5 mm. Drawing was carried out on a bench drawing machine. The total relative cold deformation level was 97 %.RESULTS AND DISCUSSIONMacrostructure analysisThe macrostructure of tested alloy as cast is shown in Figures 2 - 4. The results of the crystal size measure-ments are shown in Table 3 and Figure 5. The highest degree of macrostructure refinement in the cross-section and longitudinal section was found for sample no. 3 cast at the highest speed of 0,22 m / min. Table 1 Average chemical composition of samples / ppmZn Pb Sn P Fe Ni Si* Mg Cr Te<15 <3 4,2 <2 10 <2 78 138 <2 3,9As Sb Cd Bi Ag Co Al S Be Zr<2 <10 <1 <5 16 <4 <3 9 <1 4,4Au B Ti Se Pt Cu<5 <2 <2 <3 45 rest* increased Si content caused by grinding the sample before  testing on SiC sandpaperAn INDUTHERM VCC3000 installation was used for melting and downward semi-continuous casting. Melting in a graphite crucible in an induction furnace at a temperature of 1250 °C was carried out. The liquid copper was protected by an argon blast. Rods with a diameter of 25 mm and a length of 1,5 m were cast. Casting parameters of the tested samples are presented in Table 2.Table 2 Downward semi-continuous casting parametersSample Casting temp. / °CCrystalizer temp. / °CFeed / mmStop / sCasting velocity / m / min1 1200 1130 2 0,7 0,092 1198 1138 2,5 0,6 0,133 1193 1146 4 0,5 0,22Method of macrostructure analysisThe samples for macrostructure analysis were ground on sandpaper with a gradation of 500# to 4000#. A water solution of HNO3 was used for etching. Obser-vations of the macrostructure were performed using a Keyence-VHX 7100 digital microscope. The crystal size was analysed using the ImageJ analysis software (developed in the public domain at the National Insti-tutes of Health). In the binary image of the macrostruc-ture, the number of crystals in the cross-section and their area were counted. The mean crystal size, standard deviation, minimum and maximum values were calcu-lated. The view of the transformed image of the macro-8-bit macrostructure  imageBinary macrostructure  imageFigure 1  Converting an 8-bit image macrostructure to binary in ImageJ softwareFigure 2  Macrostructure of sample 1 – casting speed  0,09 m / min389M. MALETA et al.: INFLUENCE OF THE SPEED OF DOWNWARD SEMI-CONTINUOUS CASTING…METALURGIJA 62 (2023) 3-4, 387-390The measurements carried out in the cross-section showed 388 crystals with an average surface area of 1,2 mm2. In this sample, most crystals were in the range under 0,5 mm2 (153 pcs.) and from 0,5 to 1 mm2 (96 pcs.). For sample no. 1 cast at the lowest speed, the measurements showed 148 crystals with an average sur-face area of 3,221 mm2 (almost 2,5 times larger than sample no. 3). The largest number of crystals in this sample (62 pieces) was in the area above 2,5 mm2. For sample no. 2 cast at a speed of 0,13 m / min, 296 crys-tals with an average surface area of 1,585 mm2 were counted. The largest number of crystals was in the range under 0,5 mm2 (88 pcs.) and in the range above 2,5 mm2 (66 pcs.).Microhardness and tensile testThe results of the Vickers microhardness measure-ments in the cross-section of the tested samples in the as-cast condition and after cold working are shown in Figure 6, while the results of the static tensile test after cold working are shown in Figure 7 and Table 4.Figure 3  Macrostructure of sample 2 – casting speed  0,13 m / minFigure 4  Macrostructure of sample 3 – casting speed  0,22 m / minTable 3  Measurement results of the mean size of crystals in the cross-sectionCrystal Surface area / mm2Sample 1 Sample 2 Sample 3Mean 3,221 1,585 1,200Min 0,011 0,022 0,006Max 26,933 10,134 10,161SD 4,238 1,667 1,460Figure 5  Measurement results of the mean size of crystals  in the cross-sectionFigure 6  Measurement results of Vickers microhardness  in the cross-section of samples after casting process and cold workingTable 4 Results of tensile test after cold working (ε=97%)Sample Rm / MPa Rp0,2 / MPa A 50 / %1 428 410 7,22 433 413 6,83 439 413 7,2Figure 7  Stress-strain curve for selected samples after cold workingThe increase in the casting speed, in addition to the refinement of the macrostructure, increased the me-chanical properties of the copper rods in the as-cast state and after cold working. Obtained results correlate with the research in work [14], which also showed an increase in hardness after increasing the casting feed of CuMg alloys. For sample no. 2, increasing the casting speed from 0,09 m / min to 0,13 m / min resulted in an increase in microhardness from 49 HV1 to 51 HV1 (in-crease by about 4 %) in the as-cast state and from 124 390  METALURGIJA 62 (2023) 3-4, 387-390M. MALETA et al.: INFLUENCE OF THE SPEED OF DOWNWARD SEMI-CONTINUOUS CASTING…HV1 to 126 HV1 (increase by about 2 %) in the cold-deformed state. A greater increase in mechanical prop-erties was observed for the sample cast at twice the cast-ing speed. Increasing the casting speed from 0,09 m / min to 0,22 m / min for sample 3 resulted in an increase in microhardness from 49 HV1 to 53 HV1 (an increase of about 8 %) in the as-cast condition and from 124 HV1 to 128 HV1 in the as-cold worked state (increase by about 3 %). The results of the static tensile test cor-relate to the microhardness results. The increase in the casting speed increased the tensile strength while main-taining a similar level of yield stress and elongation. Compared to sample no. 1, the increase in the casting speed for sample no. 2 increased the tensile strength by 5 MPa (about 1,5 %), and for sample no. 3 by 11 MPa (about 3 %).CONCLUSIONSAfter testing the macrostructure and mechanical properties in the as-cast state and after cold working of the copper rods, the following conclusions and observa-tions were made:Increasing the casting speed contributed to the re-finement of the macrostructure of copper rod. The best effect was observed when increasing the casting speed from 0,09 m / min to 0,22 m / min. The number of crys-tals in the cross-section increased more than twofold, which resulted in a decrease in their size from 3,221 mm2 to 1,2 mm2.The refinement of the macrostructure increased the level of microhardness and tensile strength of the tested samples. For extreme casting speeds, the microhardness increased 8 % in the as-cast state and 3 % after cold working. The tensile strength increased about 2,5 % while maintaining a similar level of yield strength and elongation.AcknowledgmentsThe research was carried out as part of the CuMag+ project financed by the National Center for Research and Development under the TECHMATSTRATEG III program (TECHMATSTRATEG-III/0002/2019-00)REFERENCES[1] Copper Development Association, A Guide to Working with Copper and Copper Alloys, CDA Publication, New York, 2010[2] K. Muszka, J. Majta, Ł. Bienias, Effect of grain refinement on mechanical properties of microalloyed steels, Metallur-gy and Foundry Engineering, 32 (2006) 2, 87–97[3] K. Tamta, D. B. Karunakar, Grain refinement of cast al-loys: A review, Int. J. Mech. Eng. and Rob. Res., 1 (2014) 1, 199–212[4] M. Łagoda, W. Głuchowski, M. Maleta, J. Domagała-Dubiel, M. 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Influence of the speed of downward semi-continuous casting on the crystal size and mechanical properties of recycled copper

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