Surface modification of tool steel by electrical discharge machining with molybdenum powder mixed in dielectric fluid

Abstract

Research works in electrical discharge machining (EDM) with powder suspended in dielectric fluid indicate significant gains in process stability and surface roughness improvement of EDMachined components. Powder-mixed EDM (PMEDM) process is also investigated in order to obtain a resolidified layer (recast layer), which acts as a kind of coating, having superior mechanical properties to the base material. Molybdenum (Mo), commonly used as an alloying element in steels, in solid solution or in the form of various carbides, provides high resistance to wear, corrosion, and high hardness. However, as reported in literature, there are technological gaps in the use of molybdenum as an additive powder in PMEDM process. The influences of different sizes of fine Mo powder particles (˂5 and ˂15 μm) suspended in dielectric fluid in several EDM finishing regimes and machining times were investigated regarding AISI H13 tool steel surface modification. Samples were evaluated in terms of hardness, chemical composition, and microstructure using optical microscopy, scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), nanoindentation, and X-ray diffraction (XRD). Results showed an enrichment of the modified surface layer (recast layer) with Mo, in which the formation of phases of Fe-Mo and Mo x C and also the presence of Mo in solid solution were observed, resulting in a four times fold hardness increase in comparison with the metal matrix. Best results were obtained with 1-A discharge current and less than 15-μm particle size, with the formation of a Mo-enriched, 14-GPa hardness uniform modified layer, with a constant thickness of 5 μm free of pores and microcracks. Machining times did not affect the characteristics of resolidified layer (recast layer). This work provides PMEDM process referential parameters to the use of Mo as a material for providing better surface mechanical properties.