AAAC conductor fatigue resistance in overhead power lines is critical for electrical safety and stability of power transmission system. The article discusses the influence of the artificial aging of 6101 series aluminium alloy wires on their operational properties. Moreover, the influence of the proposed heat treatment on the fatigue strength and the nature of breakthroughs of the tested wires was compared

Kowal, R.

Nowak, A.

Sieja-Smaga, E.

Strzepek, P.

English

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801METALURGIJA 61 (2022) 3-4, 801-803A. NOWAK, R.KOWAL, E. SIEJA-SMAGA, P. STRZĘPEKINFLUENCE OF HEAT TREATMENT ON FATIGUE PROPERTIES  OF 6101 SERIES ALUMINIUM ALLOY WIRESReceived – Primljeno: 2021-10-29Accepted – Prihvaćeno: 2022-03-10Preliminary Note – Prethodno priopćenjeAAAC conductor fatigue resistance in overhead power lines is critical for electrical safety and stability of power transmission system. The article discusses the influence of the artificial aging of 6101 series aluminium alloy wires on their operational properties. Moreover, the influence of the proposed heat treatment on the fatigue strength and the nature of breakthroughs of the tested wires was compared.Keywords: 6101series aluminium alloy, wire, conductor, fatigue resistance, heat treatment,ISSN 0543-5846METABK 61(3-4) 801-803 (2022)UDC – UDK 669.15:621.778:537.312.62:66.04:539.43=111INTRODUCTIONThe most important element of the electricity trans-mission infrastructure are overhead power lines and each of the them is constantly subjected to the influence of weather conditions, which cause the occurrence of varying throughout time mechanical stresses [1]. Mate-rial fatigue being the result of varying stress caused by wind vibrations is on of the most dangerous phenomena as in extreme conditions it may be the reason for dam-aging or even destroying the power line [2,3]. The oc-curring problem is known and been studied for years among all types of overhear power lines currently in use [4]. The discussed issued is particularly significant in the case of AAAC power lines as they do not have a steel core which additionally transfers mechanical stresses. However, such power lines have a number of advantages including but not limited to increased cur-rent carrying capacity, lower weight and lack of corro-sion at the place of contact between core and wires [5-11]. The 6101 alloys which are usually chosen for these types of power lines allow for the shaping of mechani-cal and electrical properties on a wide range using low temperature heat treatment [12]. What is more the use of artificial aging makes it possible to obtain a material with increased fatigue resistance.EXPERIMENTAL PROCEDUREThe research on the heat treatment and its effect on the operational properties and fatigue strength were conducted on commercial Φ3,12 mm EN AW-6101A wires dedicated for electrical purposes with a chemical composition specified in Table 1.Heat treatment6101 aluminium alloy belongs to the series of pre-cipitation hardening alloys due to the presence of sili-con and that is why it is susceptible to heat treatment. The conducted research present results of artificial ag-ing carried out in a Memmert UFE500 laboratory con-vection oven in temperatures 140 °C, 160 °C and 180 °C with the times ranging from 15 minutes to 9 hours. Therefore, three different sets of samples with various properties were obtained and used for further research.Tensile strengthUsing ZwickRoell Z020 testing machine the me-chanical properties of the heat treated materials were determined in the uniaxial tensile test conducted in the ambient temperature with gauge length of samples of 250 mm and the results were used to determine the ag-ing curves.Electrical propertiesThe intended use of the tested wire requires a certain level of electrical properties, thus the electrical aging curves were determined using Kelvin-Thomson bridge for resistance measurements. The results were obtained using Resistomat 2304 device with measurement accu-racy of ≤ 0,01 % and the test was conducted in con-trolled conditions in ambient temperature with a gauge length of 1000 mm.Fatigue resistanceThe fatigue resistance was determined using espe-cially designed test stand presented at Figure 1 operat-A. Nowak (e-mail: annowak@agh.edu.pl), R. Kowal, E. Sieja-Smaga, P. Strzępek AGH University of Science and Technology, Faculty of Non-Ferrous Metals, Cracow, PolandTable 1  Chemical composition of EN AW-6101A wires / wt. %Al Mg Si Fe Cu other98,3 0,40-0,90 0,30-0,70 0,40 0,05 0,10802  METALURGIJA 61 (2022) 3-4, 801-803A. NOWAK et al.: INFLUENCE OF HEAT TREATMENT ON FATIGUE PROPERTIES…ing in rotational bending mode. The stress was applied by an appropriate deflection arrow and the stress varia-bility was caused by the rotation speed of 3 000 rpm. The test was carried out until the breakage of samples.Electrical properties after heat treatmentThe electrical properties of the 6101 aluminium alloy are also directly correlated with the parameters of the ap-plied artificial aging parameters. The data on electrical resistivity in the function of time are collectively present-ed at Figure 3 with temperature distinction.Figure 1 Fatigue resistance test stand (rotational bending)Microstructural analysisThe obtained fatigue fractures were analysed using Hitachi S-3500N scanning electron microscope (SEM).RESULTS AND ANALYSISMechanical properties after heat treatmentThe obtained mechanical properties of the studied materials in the function of artificial aging time with temperature distinction were presented at Figure 2.Figure 2  Tensile strength of Φ3,12 mm wire in the function of aging timeIn all of the studied cases, an increase in strength properties to a certain maximum value is visible. How-ever, at 140 °C the material reaches the maximum at 356 MPa after 5 hours and remains at the constant level regardless of the further applied time of heat treatment. For 160 °C the maximum of 350 MPa is reached after 1,5 hour, with further stabilization of properties up until 3,25 hour of aging. At 180 °C the material reaches a maximum value of 346 MPa the fastest, however, it is lower than in other cases and the increase of aging time results in a rapid decrease of the material strength.Figure 3  Electrical resistivity of Φ3,12 mm wire in the function of aging timeAmong all of the studied cases the decrease of elec-trical resistivity of the tested samples was noted as the time and temperature of the artificial aging were in-creased.Fatigue resistanceThe application of tested wires requires a certain compromise between mechanical properties and de-crease of electrical resistivity obtained as a results of heat treatment. Bearing that in mind for the comparative fatigue tests materials subjected to 160 °C were selected with a maximum strength value obtained after 2 hours and the overaged material after 9 hours with a raw ma-terial as reference. S-N Wohler curves obtained by rota-tional bending were presented at Figure 4. In all of the analysed cases with high stress values the number of Figure 4 Wohler S-N curve for selected variants of aging803A. NOWAK et al.: INFLUENCE OF HEAT TREATMENT ON FATIGUE PROPERTIES…METALURGIJA 61 (2022) 3-4, 801-803cycles to failure was similar, however, the characteris-tics diverge as the stress amplitude decreases. The rea-son for that may be that with high stresses the most sig-nificant in the fatigue failure process are e.g. material defects and as the stress decreases other mechanism such as dislocation and boundary migrations begin to matter. The obtained S-N curves clearly indicate that the material after 2 hours at 160 °C had the highest fa-tigue strength whereas the lowest was observed with the overaged material (9 hours at 160 °C).Fatigue fracturesThe fracture analysis of the wires subjected to rota-tional with the stress amplitude of 100 MPa was pre-sented at Figure 5 and suggests that the applied heat treatment has a significant influence on the surface of fatigue fracture.CONCLUSIONSThe 6101 aluminium alloys allows for a wide range of properties modification by heat treatment as applied artificial aging improves mechanical and electrical properties of the materials. In the wires used for AAAC overhead power lines it is crucial to obtain high values of strength and electrical properties at the same time. What is more it is possible to select the heat treatment parameters in such a way that it will additionally in-crease the fatigue strength of the wires used for power lines. Increasing the wires and therefore the power lines resistance to high frequency vibrations of low ampli-tude, which is the most common cause of fatigue dam-age of overhead power lines is especially important.REFERENCES[1]  CIGRE WG 22-01, Report on Aeolian Vibration, Electra 124 (1989)[2]  EPRI: Transmission Line Reference Book: Wind-Induced Conductor Motion, Electric Power Research Institute, Palo Alto, CA., (1979)[3] T. Knych, Overhead power lines - Theory - Materials – Ap-plications (in polish language), Krakow (2010)[4] A. Mamala, Model of multiwre monomaterial overhead line conductors (in polish language) monograph no 6/2012, Akapit, Krakow (2012)[5] B.Jurkiewicz, A.Nowak, T.Knych, A.Mamala, M.Jabłoński, A.,Kawecki, P.Kwaśniewski, G.Kiesiewicz Research of fatigue resistance of aluminium and copper wires used for OHL and railway traction (in polish langua-ge) ISSN 1230-3534 (2017)[6] B. Jurkiewicz, B. Smyrak, M. Zasadzińska, K. Franczak, P. Strzępek, The researches of influence of strengthening on fatigue strength of aluminium wires for OHL conductors, Archives of Civil and Mechanical Engineering 19 (2019), 862 – 870[7] K.Korzeń, A.Nowak, T.Knych, A.Mamala, B.Smyrak, Analysis of rheological and fatigue properties of AlMgSi alloy wire (in polish language) ISSN 1230-3534 (2015)[8] L.P. Borrego, L.M. Abreu, J.M. Costa, J.M. Ferreira, Analysis of low cycle fatigue in AlMgSi aluminium alloys, Engineering Failure Analysis 11 (2004) 5, 715 – 725[9] L.P. Borrego, J.D. Costa, J.A.M. Ferreira, Crack Growth Behavior of AA6082 and AA6061 Aluminum Alloys, Pro-cedia Engineering 74 (2014), 175 – 178[10] P. Kumar, A. Singh, Experimental and numerical investi-gations of low cycle fatigue behavior of cryorolled AA 5754, Procedia Structural Integrity 14 (2019), 96 - 103[11] J.A.M. Pinho-da-Cruz J.A.M Ferreira, J.D.M. Costa, L.F.P Borrego, Fatigue analysis of thin AlMgSi welded joints under constant and variable amplitude block loadings, Thin-Walled Structures (2003) 5, 389 – 402[12] B. Smyrak, B. Jurkiewicz, M. Zasadzińska, M. Gniełczyk, P. Jałowy, The Effect of Al-Mg-Si Wire Rod Heat Tre-atment on Its Electrical Conductivity and Strength, Metals 10 (2020) 1027Figure 5  Influence of the heat treatment on the surface  of AlMgSi wires after rotational bending fatigue resistance testWires with no heat treatment seem to have similar fracture to overaged samples after 9 hours of aging. In both cases the fracture surface was developed with clearly visible primary cracks, both surfaces are cov-ered with brittle stripes, and what is more both materials had many residual fractures of similar size. However, in the case of the sample after 9 hours of aging the wire had bigger and more numerous secondary cracks which indicates unfavourable effect of overaging on the fa-tigue strength. The wire with a highest strength value i.e. after 2 hours of heat treatment shows significantly greater residual fractures and less developer surface with lower amount of internal defects and structural damage. Morphology comparison of the fractures after various heat treatment shows an increase in the residual fracture as the aging time increases up to the strength maximum of the material after which the phenomenon disappears.Note:  The translator responsible for English language: Grzegorz  Kiesiewicz, AGH University of Science and Technology,  Kraków, Poland

Influence of heat treatment on fatigue properties of 6101 series aluminium alloy wires

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