Investigation On High Strength-high Electrical Conductivity Mechanisms Of Al And Al Alloy Wires

With the rapid development of economy,electricity consumption is also increasing year by year.Sine the main power plants are located in remote areas,while the main power consumption cities are situated at the eastern coast,the distance between the power generation end and the power receiving end is relatively long.Such an unbalanced spatial distribution of production and consumption sites leads to the inevitability of long-distance transmission of electricity.Among the structural metallic materials,the conductivity of Ag,Cu,Au and Al decreases gradually.However,considering the performance and economic factors,Al is the most widely used conductor material in overhead transmission lines.Overhead transmission lines suffer various loads caused by wind,ice and the weight of the conductor when they are in service.Besides,power loss will occur during the transmission process of electric energy.Thus,for the overhead transmission lines,the tensile strength and the electrical conductivity are the most important properties.High strength can ensure the safety and reliability of overhead transmission lines in service,and high electrical conductivity can reduce the power loss in the process of power transmission.However,strength and electrical conductivity are usually mutually exclusive in metallic conductive materials.As a result,how to break such trade-off relation and prepare high-strength and high-electrical conductivity overhead transmission lines is a very important scientific problem and an urgent industrial problem to be solved.Commercially pure Al,Al-Mg-Si alloy and Al-Fe alloy are the most commonly used conductive materials for overhead conductors.In the present work,the strengthening mechanisms,the high conductive mechanisms,the strength-conductivity restriction relationship of the commercially pure Al wire,the Al-Mg-Si wire and the Al-Fe wire were investigated.Additionally,we proposed a process for preparing an Al clad Al alloy wire,and their microstructures and properties were revealed.The relationship between strength and electrical conductivity of the commercially pure Al wires with different drawing deformation,which were fabricated using the actual production line,was established.The results show that the electrical conductivity of commercially pure Al wires decreases firstly and then increases with the increase of strength.Therefore,the"abnormal strength-electrical conductivity relation" was found,that is,a simultaneous improvement of strength and electrical conductivity was achieved.Further microstructural observations show that with the increase of deformation,the grain is gradually elongated along the axial direction,and the grain thickness always decreases.However,the grain length firstly remains unchanged and then increases.Reducing grain thickness can increase the strength of Al wire with little loss of electrical conductivity.In contrast,increasing grain length can greatly increase the electrical conductivity without losing the strength.In addition,with the increase of deformation,the<001>soft texture in commercially pure Al wire gradually transforms into the<111>hard texture.Therefore,the texture transformation will play a strengthening role without affecting the electrical conductivity.In summary,the formation of elongated grains and hard texture can optimize the trade-off relation between strength and electrical conductivity.On the basis of this principle,the production process of traditional commercially pure Al wire was improved,and a new type of high conductivity commercially pure Al wire with desirable strength and extra-high electrical conductivity(exceeding 63.0%IACS)was fabricated.Pre-aged Al-Mg-Si alloy wire with ultimate tensile strength and electrical conductivity up to 352.3 MPa and 55.97%IACS was prepared by aging treatment of Al-Mg-Si alloy rod and subsequent cold drawing.The performances in strength and electrical conductivity of the pre-aged Al-Mg-Si wire in this study are superior to those reported in the literature.Besides,the strength and the electrical conductivity of pre-aged Al-Mg-Si wires were simultaneously improved compared with those of the as-received Al-Mg-Si wires processed by traditional routes,and a large number of nano-precipitates have been observed in the pre-aged Al-Mg-Si wires.When solid-solution atoms precipitate in the form of nano-precipitated phase,they can not only purify the matrix to improve the conductivity,but also play a strengthening role in the Al-Mg-Si wire.Besides,according to the theoretical deduction,the correlations between the mean radius of the precipitate and the precipitation strengthening,as well as the electrical conductivity were established.When the radius of precipitate is less than the critical size,with the increase of the radius of precipitates,the precipitation strengthening and the electrical conductivity increase simultaneously,which breaks the mutually exclusive relationship between the strength and the electrical conductivity.Based on the fact that Fe has an extremely low solid solubility in Al,an Al-Fe alloy wire with ultimate tensile strength and electrical conductivity up to 306.8 MPa and 58.94%IACS was prepared by cold drawing process.Compared with the properties of Al-Mg-Si wire reported in the literature,the electrical conductivity of the Al-Fe wire is always higher than that of the Al-Mg-Si wire on the condition that they own the same ultimate tensile strength.The mechanism for the combination of high strength and high electrical conductivity in the Al-Fe wire might be mainly attributed to precipitation strengthening of the nano-scale Al6Fe phases formed by the reaction of Fe with Al,and the extremely low solid solubility of Fe in Al,which purify the matrix in the Al-Fe wire.Moreover,the relationship between the microstructure evolution and the strength as well as the electrical conductivity of Al-Fe wires with different area reductions was established.According to the skin effect during the alternating current transmission process,an Al clad Al alloy ingot was successfully prepared by an interference fit method.The outer layer and the inner layer of the Al clad Al alloy wire are commercially pure Al and Al alloy,respectively.Then,the Al clad Al alloy wire with a ultimate tensile strength of 226.5 MPa and an electrical conductivity of 59.35%IACS was obtained by subsequent plastic deformation process.Fine grains were observed in the Al clad Al alloy wire across the pure Al/Al alloy interface,and no defects and intermetallic compounds were found at the interface.This indicates that the present method can be used to prepare bimetallic composite wires with perfect interface bonding.It is found that Al clad Al wire combines the high conductivity of pure Al and the high strength of Al alloy as compared with the performances of the pure Al wire and the Al alloy wire,which provides a new idea for optimizing the trade-off relation between the strength and the electrical conductivity and preparing Al wire with high strength and high electrical conductivity.Based on the insights into the mechanisms for the trade-off relation between strength and electrical conductivity in the commercially pure Al wire,the Al-Mg-Si wire and the Al-Fe wire,four key mechanisms for breaking the restrictive relation of strength and electrical conductivity were presented,i.e.elongated grain,hard texture,nano-scale precipitates and low solubility alloying element.High-strength and high-electrical conductivity Al and Al alloy conductive materials are the main development issues of Al conductors used for overhead conductors.This dissertation reveals the "strength-electrical conductivity restricted mechanism" of conductive Al materials by studying the strength-electrical conductivity trade-off behavior of the typical Al and Al alloys.The above work may not only lay a solid foundation for breaking the"strength-electrical conductivity restricted relationship",but also prepared the high-strength and high-electrical conductivity Al conductive materials,which has provided valuable references for the development of the preparation technology of Al conductive materials in the future.