Obtaining ultrathin copper alloys wires using modified dieless drawing technique

Opracowanie metody otrzymania ultra cienkich drutów ze stopów miedzi

The project aim is to obtain the ultra-thin wires (with the diameter about 10 μm) from copper with increased mechanical characteristics. The novel research in the area of the micromechanics allows summarizing that a decrease in the wire diameter made from monocrystal to several μm results in a substantial increase in the mechanical properties without loss of the plasticity. The observed increase in material strength is in the range from 10-30% up to several hundred percents depend on the diameter of the wire and its material. This phenomenon is explaining by the geometrical limitation in dislocation motion free paths by decreasing the wire diameter. However, the known methods of obtaining such thin wires consist of cutting the wire from monocrystals by ion beam or are based on nanotechnology. These methods do not allow for the economical production of the sufficiently long wire and thus it cannot be taken advantage of the described above phenomenon of increasing in the wire strength in techniques.

The research hypothesis of this proposal is based on presumption, that phenomenon of increasing in the wire strength will be observed also for the wires made from polycrystalline material if the diameter of the wire will be comparable with the diameter of the material grain and the crystal lattice will be oriented (i.e., shows a string axial crystallographic texture). In the preliminary studies, the Authors of the Proposal proved that the drawing process of the Mg alloy from the initial wire diameter 1 mm to final diameter 50 μm causes an increase in the wire strength from 230 MPa to 340 MP- the recrystallized state. In the classical drawing process of copper, it is possible to obtain a wire with a final diameter of 20 μm. This limitation is connected with the need of using the diamond dies and results from the minimal available die diameter. This limitation can be overcome by applying the Dieless Drawing process (DD). DD process consists of the controlled tensile of local heated wire, which allows for production ultra-thin wire without using dies. The main goal of the proposal is to develop the modified dieless drawing technology (MDD) for the production of the ultra-thin wires with increased strength resulting from geometrical limitation of the dislocations motion. The main problem in the DD process is the value of the elongation limit. Two reasons for this were found during our preliminary experiments. The first one is related to the fluctuations in the wire diameter along the obtained wire length. The second reason is the limit in the technological plasticity and breaking of the wire. The solution to these problems proposed in the project based on the material rheological properties utilization and control of the microstructure defects appearing during the process. It is known, that the stress-strain curve for different conditions has a different shape. If the intensity of the strain is high enough, the mechanism of compensation of the localization of strain activates, which leads to strong hardening of the material in the place of the maximum strain. The appropriate selection of the process parameters allows for control of the material properties in such a way to take the advantages of this mechanism of the material strengthening and as result, it can be possible to achieve a lower diameter of the wire and higher values of the elongation in the MDD process. Moreover, analysis of the process parameters resulting in the material micro-fractures allows determining the set of parameters for the critical state of the deformed material. The restoration of the plasticity in the MDD process is impossible after reaching the critical state of the material. The model of the material critical state will be based on the in situ test mechanical tests or on the tensile test performed in the Gleeble simulator in vacuum for the temperature range 20-800 ⁰C. The model should help us to define the MDD process conditions in such a way to not allow for appearing the critical defects state of in the deformed material and to make possible the plasticity restoration of the material. The experimental research will be supported by FEM modeling of the MDD process and analysis of the impact of yield stress model on the behavior of the artificial fluctuations in initial wire diameter. The input material for the MDD i.e., wire with a diameter of 20μm is planned to be obtained by the drawing process developed by the Authors of the proposal. Then, the parameters of the MDD process will be developed

and the ultra-thin wire with the diameter about 10μm will be performed. The next step will be the experimental verification of the obtained wire properties carrying out by using its machine constructed especially for testing the ultra-thin wires. The advanced metallographic analysis of the obtained wires and modeling of the yield stress of the wires with different diameters by using the molecular dynamic will be also done. For the basic experiment, the polycrystalline copper will be used. This metal has a different crystallographic structure and the plastic deformation mechanisms than the magnesium alloy used in the preliminary study. Such a combination allows us to generalize the knowledge that will be obtained during the realization of the proposed project. The results of the project will have an impact on the progress in the field of plastic forming of materials, materials engineering, micromechanics and the new fast-growing branches of engineering such as microelectronics. In particular, it will be opportunities to apply the fundamental phenomenon of improving the strength characteristics of copper in new technologies demanding practical application of the electrical microwires.