Short electric arc machining (SEAM) is a non-conventional machining process that utilizes
an arc to melt and vaporize workpiece material. A low voltage, high current supply is employed to
produce a continuous arc in either a water-air mixed medium or in air. This paper describes an
investigation into the optimization of the SEAM machining efficiency based on experimental design
which considers both electrical parameters and machine parameters. The work uses the orthogonal
experiment processes to research the influence of the process indicators on machining efficiency.
Austenitic stainless steel (SUS304) specimens were sputter etched by using argon ions, and the formation process of protrusions on the surface was investigated by changing the sputter etching time. In addition, tensile and bending tests of the specimens were carried out to examine whether the delamination between the protrusion and matrix occurs or not. When the sputter etching time is 0.9 ks, fine conical protrusions with diameter smaller than 2 mm are formed relatively homogeneously on the surface. When the sputter etching time is 1.8 ks, they grow to more than 5 mm. In some cases, the holes and cracks are formed on the wall of protrusions. When the sputter etching time is 3.6 ks, the protrusions grow further and occupy whole surface, and finally they collapse to form ring shaped protrusions. The tensile test of the specimens with conical protrusions shows that, even at a fracture strain of 0.53, the shape of the cones is almost maintained and the plastic deformation occurs mainly in the region around them without delamination of the cones. Also in the bending test, the similar behavior to the tensile test is observed. The tensile test of the specimen with ring shaped protrusions also shows that the deformation does not largely occur in the protrusion but occurs in the region outside of them. The reasons for the above characteristics seem to be that the outer shell of the protrusion with higher Cr content is strong and the protrusion grows from interior of specimen keeping high coherency to matrix.
The sputter etching of SUS304 stainless steel specimens was carried out, and the formation and growth processes of conical protrusions was investigated. In addition, the polarization behavior of the specimens in 3.5NaCl aqueous solution and the surface morphology after the polarization test were examined. At an early stage of the sputter etching, the conical protrusions form mainly along grain boundaries. However, with increase in the sputter etching time the protrusions form also within the grains. With further increase in the sputter etching time, some protrusions grow largely and the entire surface is covered by the huge conical protrusions and the irregular small ones around them. The outer layer of the huge protrusion contains larger chromium but smaller nickel amount than matrix, and the interior has almost the same composition as the matrix. This means that the protrusion has a shell structure. For the specimen with short time sputter etching, the corrosion potential becomes nobler and the pitting corrosion potential becomes smaller, while the passive current density is smaller than those of non sputter etched specimen. In these specimens, the corrosion occurs preferentially along grain boundaries and around protrusions in grains. For the specimen with long time sputter etching, where the entire surface is covered by protrusions, the pitting corrosion potential increases and the corrosion progresses mainly from the interior of the specimen leaving the surface layer with many protrusions. In some cases, only the outer layer of the shell remains separately as a result of the corrosion of the interior and root of the huge protrusion.
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