## 3 Results and Discussion
### Effect of Different Frequencies  
#### 3.1.1 Z-Direction Specimens  
The stress-strain relationship between specimens \(Z^{0}_{0}\_i\), \(Z^{0.1}_{100\_i}\), \(Z^{0.1}_{200\_i}\), \(Z^{0.1}_{300\_i}\), \(Z^{0.1}_{400\_i}\), \(Z^{0.1}_{500\_i}\), \(Z^{0.1}_{600\_i}\), \(Z^{0.1}_{700\_i}\), \(Z^{0.1}_{800\_i}\), and \(Z^{0.1}_{900\_i}\) (\(i\) =1-9) was compared and analyzed in Figure 4. The influence of applied vibrations of the same acceleration amplitude (0.1 g) but different frequencies (0-900 Hz) on the tensile properties of the specimens can be determined. As can be seen, the tensile properties (tensile strength and plasticity) of the specimens built with applied vibration have been greatly improved, and they are further enhanced with the increase in frequency. Table 2 shows the details. Figure 3: The servo-hydraulic machine for tensile test and the specimen. It can be seen that the average tensile strength of the ordinary specimen (without applied vibration) \(Z^{0}_{0}\_i\) is 23.68 MPa and the average plasticity (the ability to resist deformation under a certain external force, expressed by elongation here) is 5.7%; when vibration is introduced, both specimens' tensile strength and plasticity are obviously increased. For example, the average tensile strength and plasticity of specimen \(Z^{0.1}_{100}\_i\) are 26.16 MPa and 5.95%, respectively, and they are increased by 10.5% and 4.4%, respectively, compared with those of \(Z^{0}_{0}\_i\). In addition, the average values for specimen \(Z^{0.1}_{900}\_i\) are, respectively, 38.92 MPa and 7.65%, with the growth being 64.3% and 34.2%, respectively. This is because the layer in the Z direction is deposited above the other and it is perpendicular to the applied load during the tensile test. The Z-direction specimens' tensile properties mainly rely on the bonding strength between material fibers instead of the fibers themselves. Applied vibration oscillates the molten pool inside the extrusion liquefier, making the gas escape quickly and reducing the occurrence of pores and thermal cracks in the extrudate. Meanwhile, the melt apparent viscosity is decreased and its fluidity is enhanced, making the extrudate finer and more uniform [20]. In addition, the applied vibration generates a reciprocating excitation pressure in the ME process, assisting to make the connection between the layers closer. With an increase in the applied vibration frequency, these improvements will be further enhanced, leading to a better forming quality. Therefore, the tensile performance of Z-direction specimens built with applied vibration is better than that of the normal ones, and the property will be further improved with increasing vibration frequency. Figure 5 shows the SEM micrographs of the specimens' cross sections. As can be seen, there are fewer layer separations in specimens to which vibrations were applied, and the bonding gap is much smaller. The layer height and extrusion width are more uniform, with less distortion and deformation. With increasing vibration frequency, the defects mentioned above are further reduced. Therefore, applied vibration can significantly improve the forming quality of ME parts and it will be further improved with increasing vibration frequency.