Effect Of Surface Mechanical Grinding On Microstructure And Mechanical Properties Of Pure Copper

In this paper, pure copper samples with ultrahigh strength and good ductility were produced by using the surface mechanical attrition treatment (SMAT), a gradient structure layer with the grain size decrease from matrix to top surface was obtained on the sample’s surface layer. The highest thickness of gradient grain structure layer was about 230μm and the sample’s yield strength reached to about 245MPa. This paper discussed the influence of the strain and the temperature on SMAT-ed sample’s structure and mechanical properties during SMAT processing. The yield strength of pure copper samples can be improved by increase the processing time, but the value of strengthen would reaches its saturation. Besides, the deformation temperature played a key role in SMAT processing. By using the devices such as optical microscope, scanning electron microscope, transmission electron microscope, X-ray diffractometer, microhardness tester and tensile testing machine, we have found that dynamic recovery and recrystallization can be suppressed as the deformation temperature is low in the samples treated in liquid nitrogen temperature, thus a thicker gradient structure layer with higher dislocation density and finer grain can be obtained when SMAT at lower temperature, and the sample would achieve a much higher strength compared to the sample SMAT treated at room temperature. The thickness of gradient grain structure of the sample SMAT-ed for 30 min at room temperature (RT-30) was about 160μm and the yield strength can be improved from 60MPa for annealed state to about 220MPa for RT-30 sample. In contrast, the thickness of gradient grain structure of the sample SMAT-ed for 30 min at liquid nitrogen temperature (LNT-30) is about 230μm and the yield strength can be improved to about 245MPa.In addition, the strengthening mechanism of synergetic strengthening in gradient structure pure copper has been analyzed through stress relaxation experiment and its related tests. We found that the geometrically necessary dislocations introduced by gradient strain in samples could interact with mobile dislocations during tensile test, which produced an extraordinary strain hardening to improve the samples’ strength and ductility. Thus, a more excellent synthetic properties that far beyond other traditional composite materials could reached was obtained for the pure copper samples with gradient structure layer.After the strengthening mechanism of synergetic strengthening in gradient structure was revealed, the preparation of multi-layer gradient structure pure copper sample was under took to enhance the affection of synergetic strengthening. After electroforming for 10h, the sample with thickness about 70μm recast layer was obtained which the yield strength was about 220MPa. Besides, after electroforming for 20h, a thickness about 150μm recast layer was obtained and the yield strength for that sample was about 225MPa. The double gradient structure layers pure copper samples was prepared by introduced the electroforming technology into SMAT, which could laid the foundation for further research to prepare the much higher strength SMAT-ed pure copper samples.