Effect Of Mechanical Grinding On Surface And Mechanical Properties Of Pure Copper Under Different Experimental Parameters

Many Severe Plastic Deformation (SPD) techniques have been extensively investigated in various metals and alloys over the past decades to refine grains to the sub-micrometer or nanometer regimes. Typically, the bulk grain refinement produces very high yield strengths and ultimate strengths but very limited tensile ductility. In this paper, we obtained the gradient structure surface layer in pure copper by using surface mechanical attrition treatment (SMAT), grain size increased from sample surface to matrix, the matrix was coarse grain structure. The strength of materials were great improved and the elongation of materials keep a good value.In this paper, we selected four different experiment time: 1 min,5 min,15 min, 30 min, two different temperature:room temperature, liquid nitrogen, and five different samples thickness:1 mm,2 mm,3 mm,4 mm,5 mm, to study the effect of SMAT under different parameters on the organization and the mechanical properties of pure copper. Measuring hardness of samples by using micro-hardness tester, and analysis the tensile properties of samples by using tensile tester. The results shown that, all the samples have a hardness gradient in surface, and the surface hardness has been significantly improved. Tensile properties of samples are different under different experimental conditions, reduce the temperature, increase the experiment time, decrease the thickness of the samples can effectively improve the strength of the samples. Comparing the mechanical properties of samples to determine the optimum technical parameters that sample have both good strength and plasticity. The microstructure of the sample were observed by means of metallographic microscope and SEM. The results shown that, the grain size of SMAT samples have a gradient change, which are consistent with the hardness observations. The recrystallization may have taken place during or after SMAT processing at room temperature (R-SMAT), this may be due to the imparted kinetic energy and resultant temperature increase depends on the frequency of oscillations and the translational amplitude of the specific balls used. The samples treated by SMAT processing at liquid temperature(LN-SMAT) have a large number of twin, this is due to the dislocation motion is restrained by low temperature, make twinning easy to occur during plastic deformation. The grain size and the density of dislocation and twin were measured by X-Ray Diffraction (XRD), results shown that, LN-SMAT samples have smaller grain size and higher density of dislocation and twin than R-SMAT samples, which are consistent with the microstructure observations, indicated that SMAT at liquid temperature have more significant strengthening effect.It is important to note that, we found some interesting phenomenon in the experimental process. By carefully analyzing the tensile curve, we found a yield peak phenomenon, this phenomenon is common in the alloy but rare in the pure metal. We explained this phenomenon by using the relationship of strain rate and mobile dislocation density(s=bηv). At the same time, we found additional work hardening in material. By using the in-situ tensile observation, we found that, the plastic deformation of surface gradient structure layer and coarse grain matrix occurred in different time, which lead to materials produced stress and strain gradient. The stress and strain gradient will lead to the accumulation of geometrically necessary dislocations caused the additional work hardening.