The effects of deformation and microstructure inhomogeneities in the Accumulative Angular Drawing (AAD)

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Abstract

Angular Accumulative Drawing (AAD) is a new powerful tool that produces wires with enhanced properties and microstructure. The AAD is characterised by a complex strain path history resulting from various deformation modes. Its main purpose is to produce wires that represent increased strength and ductility due to controlled deformation inhomogeneity. This paper discusses the effect of processing routes and parameters on inhomogeneity of deformation as well as microstructure, and finally on the properties of wires. Investigated materials were: aluminium, copper, low carbon and microalloyed steels. Also, it is shown that the complex strain path history can be effectively controlled by computer simulation.

Introduction

For many years, in most metal forming applications inhomogeneity of deformation has not been desired as it leads to non-homogeneous microstructure evolution. Since it is also difficult to control, non-uniform property distributions appear in the final product. Recently, however, with the growing interest of Ultrafine-Grained (UFG) and Nanostructured (NC) materials, introduction of deformation inhomogeneity is becoming an important way to help improve weak ductility of UFG structures [1]. Although the potential of this approach has already been recognised, proper control of the properties of final products through controlled deformation inhomogeneity still remains extremely difficult [2], [3]. Computer simulation, which can be very effective and significantly minimise the cost of expensive industrial or laboratory trials, is becoming a very helpful tool in this research [4], [5].

In the present work, the recently developed Accumulative Angular Drawing process (AAD) [6] is presented and discussed with respect to its potential to produce wires with enhanced properties thanks to controlled strain accumulation that leads to local high grain refinement. In this process, a complex strain path is applied and, similarly to the Severe Plastic Deformation processes [7], [8], large strain accumulation is introduced to refine the microstructure of a drawn wire. The applicability of strain path change in the AAD process allows for the preferred distribution of effective strain in the entire cross-section of the drawn product, which, in turn, introduces microstructure inhomogeneity at the cross-section of the wire. These effects – if properly controlled – enable the achievement of a desirable combination of mechanical properties in the final product. In the present study, analysis of microstructure development and their inhomogeneity was carried out in order to understand the deformation mechanisms and to establish the correlation between deformation, microstructure and mechanical properties observed in various metals and alloys. It was proved that by proper control of the process parameters that it is possible to introduce controlled inhomogeneneity of strain and, by accumulation of deformation energy, to induce grain refinement across the cross-section of the wire. As a result enhanced properties compared to conventional wire drawing processes were obtained.

Section snippets

Experimental

Due to its design, the AAD process (Fig. 1) induces high strain accumulation in the surface layers of the wire, which allows one to achieve increased mechanical properties and ductility in wires characterised by small diameters. In the present study wires made from four different metals and alloys, with the basic chemical composition summarised in Table 1, were investigated.

Wire rods with diameters of 6.5 mm were drawn to wires of 4.0 mm diameter by applying two different schedules of the drawing

Results

As expected, high strain accumulation and inhomogeneity of the deformation resulting from application of different die positionings are reflected in the microstructure in all studied materials. Fig. 2 shows microstructure of the Al wire observed at cross sections cut in the transverse and longitudinal directions after drawing with linear and stepped die positionings. As expected, the highest grain refinement was observed near the surface of the wires. Also, concentration of the dislocation

Multiscale computer simulation of the AAD process

Major emphasis in the developed model was put on the proper representation of the evolution of strain inhomogeneity in order to control and improve the properties of produced wires. In the present study, due to the complexity of the AAD process, a multiscale approach based on two steps of submodelling was used (Fig. 7). Submodelling is an effective finite element technique and can be classified as uncoupled concurrent multiscale computing method. In the approach the macro scale material

Conclusions

Based upon results from the presented research the following major conclusions can be drawn:

  • 1.

    Application of the AAD process allows control of deformation and microstructural inhomogeneity in the drawn wires. As a result, it is possible to control the mechanical properties of the final products and obtain wires with better combination of strength and ductility compared to typical wire drawing processes.

  • 2.

    The beneficial use of the AAD process is especially visible in the case of microalloyed steels

Acknowledgements

Financial assistance of The Polish National Science Centre (UMO-2012/05/B/ST8/00215) is acknowledged. FEM Calculations were realised at the ACK CYFRONET AGH under Grant no.: MNiSW/IBM_BC_HS21/AGH/075/2010.

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