چكيده به لاتين
The great advantages of wire-cut or wire electrical discharge machining (WEDM) process have made it to be widely used in industrial production, molding, medical and transportation industries. However, due to defects appear on the surface after the machining, the quality of the surface integrity decreases, which could have significant effects on fatigue life of the manufactured parts. In this thesis, 4340 steel specimens were machined by WEDM with different effective parameters including pulse on-time and spark frequency. The surface roughness and the thickness of the surface layer in these specimens were considered to assess the quality of their surface integrity. Afterward, pull-push fatigue tests were performed on the specimens in high cycle regime and the stress-life curves were obtained. Moreover, scanning electron microscopy images were employed to evaluate the surface layers and observe the fracture surface of the samples. The results indicate that the formed rough surface layers are prone sites for crack initiation and propagation. The fatigue life of the specimens decreases by increasing the thickness and roughness of the surface layers. The best surface quality was obtained by applying the wire-cut parameters of 0.8 µs pulse on-time and a frequency of 75%, in which the thickness of the surface layer and the surface roughness were measured 15.4 µm and 2.3 µm, respectively. Also, the worst surface quality was resulted from the parameters of 1.0 µs pulse on-time and 100% frequency. The thickness of the surface layer and the surface roughness of this sample were respectively 29 µm and 3.3 µm. The fatigue life of the WEDM specimen with 0.8µs pulse on-time and 75% frequency under a stress amplitude of 450 MPa was approximately 30% lower than that of the ground specimens. In the case of 0.1 µs pulse on-time and 100% frequency, the samples showed a fatigue life reduction of 50%. At lower stress amplitudes (for instance 350 MPa), this difference was increased and the fatigue life was reduced by 70-95%.
