Research On Processing And Properties Of Mg/Cu Clad Bimetal Wires Obtained Through Co-extrusion And Drawing

Bimetallic wires combine the properties of two different metals into a hybridmaterial to obtain improved performance such as corrosion resistance, mechanicalstrength, and electrical conductance or enhance weight to strength ratio. By carefulmaterial selection, properties of bimetallic materials can be optimized for particularservice conditions. However, difficulties in manufacture limit the number of materialcombinations in actual use. One of the main aspects in production of clad wires is toensure uniformity in sleeve thickness. Co-extrusion is a process of concurrentlyextruding two or more materials from a single composite billet.Magnesium exhibit high strength to weight ratio, good thermal conductivity,high dimensional stability, good electromagnetic shielding characteristics, highdamping properties, good machinability and it is easily recyclable. Magnesium hasalso unattractive properties like low corrosion and wear resistance, poor creepresistance and high chemical reactivity that have limited its widespread use in manyapplications. Copper on the other hand possess outstanding resistance to corrosion,exceptional formability, and good strength and fatigue resistance. Copper can beused in relatively high temperature applications. The above mentioned properties ofmagnesium and copper can be blend together to obtain a bimetal product havingsuperior properties.In this research a processing method combining extrusion and drawing wascarried out to produce bi-metal wires consisting of a magnesium core clad by acopper sleeve. The initial step was co-extrusion carried out at high die temperature intwo passes. A first pass from a diameter of52mm-18mm and a second one from18mm-7mm were successful in obtaining a bimetal rod. The extruded bimetal rodwas used as the starting material for drawing. Drawing process was started at hightemperatures and reduced gradually to room temperature with decrease in wirediameter. Wire samples were obtained at different stages of wire drawing. Eventually,the composite billet was reduced from an overall diameter of52mm to a0.4mmdiameter bimetal wire. Extrusion process was preliminary to drawing, with anobjective to improve workability of magnesium through grain refinement andenhance integrity of the interface between copper sleeve and magnesium core. Lowtemperature workability of magnesium was derived from the extrusion process. Annealing was performed to improve mechanical properties of the bimetal wire.Wire properties were studied including: geometric parameters, microstructure,mechanical properties, electrical conductivity and defects analysis. Annealingtemperature of250°C was sufficient to obtain recrystallized grains in45minutes.Increasing the annealing time led to grain growth and increase in average grain size.Average grain size decreased with decrease in wire diameter. Average grain size of6.6μm was obtained for the magnesium core of0.4mm wire diameter. Yield strengthand tensile strength increased with decrease in wire diameter, while total elongationdecreased. Electrical conductivity in the range of50-54%IACS was measured in theselected samples.DEFORM-2D software was used to simulate the co-extrusion and drawingprocesses. The material properties were obtained from literature. In the simulation ofthe co-extrusion, a comparison between cup sleeve and pipe sleeve was made toanalyze flow behavior especially in the variation of sleeve thickness. The cup sleeveresulted to a thinner sleeve diameter as compared to the pipe sleeve. At low thicknessof sleeve (ts <7mm), sleeve failure occurred. A copper sleeve of7mm thick was theoptimum thickness for pipe sleeve and the minimum thickness for cup sleeve to cladmagnesium core. In general a cup promotes concurrent flow of material but this isaccompanied with material wastage. Concurrent flow of material can be promoted ina pipe sleeve set up by making a longer sleeve in the leading side of extrusion. Thisset up reduces the amount of scrap material. In the simulation of wire drawing, aconstant ratio of the core radius to sleeve radius was obtained which is in goodagreement with the actual results.