Study On The Evolution Law And Mechanism Of The Relationship Between Strength And Electrical Conductivity Of Oxygen-free Copper Wire

Copper and copper alloy wire is widely used in the field of electric wire and cable,which plays the role of load-bearing for power transmission.Strength and electrical conductivity are the key properties of copper wire.However,the trade-off relationship between strength and electrical conductivity limits the further development of high-performance copper wires.Therefore,it is particularly important for the preparation of high strength and high conductivity copper materials to explore the mechanism behind the trade-off relationship between strength and electrical conductivity and to break the trade-off relationship between strength and electrical conductivity.In the process of power transmission,the defects in the copper wire will interact with the electrons which results in a thermal effect.Therefore,the actual service condition of the oxygen-free copper wire is thermal service condition.The evolution of strength and electrical conductivity of oxygen-free copper wire under thermal service condition remains to be investigated.The main research contents of this paper are as follows.(1)In this study,a series of annealing parameters were designed to anneal oxygen-free copper rods,and annealed oxygen-free copper rods with different grain sizes were prepared according to Hall-Petch relation.Subsequently,the oxygen-free copper rods with different annealing states were drawn continuously to prepare oxygen-free copper wires with different area reductions.At the early stage of cold-drawing process,the strength of oxygen-free copper wires gradually increased with the increase of area reduction,while the electrical conductivity gradually decreased.The strength and electrical conductivity still maintained the traditional trade-off relationship.At the later stage of cold-drawing process,the strength and electrical conductivity of the oxygen-free copper wires increased with the increase of area reduction.That is,the strength and electrical conductivity increased simultaneously.The microstructure observation showed that dislocation,texture and grain are the main microstructure structures that affect the strength and electrical conductivity of cold-drawing oxygen-free copper wire.By quantitatively calculating the contribution of microstructure to the strength and electrical conductivity,it was found that dislocation,grain boundary and <111> texture were the main strengthening factors of continuous cold-drawing oxygen-free copper wire.The elongated grain and dislocation recovery can improve the electrical conductivity of oxygen-free copper wire.The elongated fibrous grains are the key to increase the strength and electrical conductivity of oxygen-free copper wire.It is found that adding low solid solubility alloying elements to copper,which can reduce the fault energy of copper layer,combined with mechanical processing and heat treatment,can be a suitable method for preparing copper materials with high strength and high conductivity.(2)Annealing treatment was used to simulate thermal service state of oxygen-free copper wire.Annealing temperatures of 80?,150?,210?,250? and300? are set for oxygen-free copper wires.Among them,80? is the normal service temperature,and 300? is the temperature when short circuit occurs.The annealing time gradient was set for all annealing temperatures.After annealing with different parameters,the strength and electrical conductivity of all the annealed oxygen-free copper wires were tested.The results showed that the strength of the oxygen-free copper wire decreased slightly and tended to be stable when the annealing temperature were 80? and 150?.Besides,the electrical conductivity increased slightly,indicating that the oxygen-free copper wire can serve safely at 150?.The decrease of strength is attributed to the decrease of dislocation density and the slight increase of grain size in the oxygen-free copper wire,while the increase of electrical conductivity is attributed to the decrease of dislocation density,the transformation of some non-equilibrium grain boundaries to equilibrium grain boundaries and the slight increase of grain size.When the annealing temperature reached 210? or above,with the increase of the annealing time,the strength of the oxygen-free copper wire decreased greatly but the electrical conductivity increased obviously.The recrystallization was observed in the annealed copper wire.The theoretical analysis showed that the growth of grain size,the recovery of dislocation and the weakening of texture strengthening were the main mechanisms for the strength decrease of the high-temperature annealed oxygen-free copper wire.The main reasons for the electrical conductivity increase of the high-temperature annealed oxygen-free copper wire are the growth of grain size,the recovery of dislocation and the decrease of the density of grain boundaries.