The effect of EDM wire cutting on the fatigue properties of 4340 steel.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Electric discharge machining (EDM) is a metal removal process that has seen increasing use in the recent years. EDM is replacing the use of conventional machining in certain applications due to its ability to machine conductive materials of any hardness into highly complex shapes. One such application is the manufacture of structural components for skis used by Antarctica-bound Hercules aircraft. As fatigue properties are important for structural components used in the aeronautical industry, the effect of EDM on fatigue properties is significant. Literature indicates that EDM causes degradation in fatigue properties; however, there is insufficient information relating EDM wire cutting (EDWC) (the process to be used) to fatigue. Results indicate that EDWC is greatly detrimental to fatigue properties. Fatigue limits of EDWC specimens are reduced from approximately 900MPa (ground specimens) to 300MPa. Numerous cracks in the surface and sub-surface of the specimens, together with a surface roughness of 3μm Ra, indicate that surface topography contributes to the reduction in fatigue limit. A phase change in the surface layer and the presence of a residual tensile stress are also possible contributing factors. The effect of shot peening after EDWC is also investigated, as there is little information on this subject. Shot peening after the EDWC process has returned the fatigue limit to its initial value, and in some cases has even increased it (1000MPa). This result is attributed to the introduction of residual compressive stresses in the surface. The use of EDWC alone to manufacture the aircraft structural components is not practical. By adding the shot peening process after EDWC, the manufacturing process can become a viable alternative to conventional machining.