The aim of this study is to investigate the machining characteristics of gas media used in electrical discharge machining (EDM). The process parameters were designed based on Taguchi method to manage the experimental work. The main process parameters such as machining polarity, gas pressure, peak current, pulse duration, type of gas media, and servo reference voltage were adopted to explore their effects on machining performance in terms of material removal rate (MRR) and surface roughness (SR) for SKD 61 tool steel. The experimental observed values were transferred to signal-to-noise (S/N) ratios, and then the significant machining parameters affecting obviously the machining performance were determined by analysis of variance (ANOVA). The optimal combination level of machining parameter for each machining characteristic was obtained by the S/N ratio analysis according to Taguchi methodology.

In this study, nine as-cast Ni-Al-Cr-Os alloys were prepared, and their constituent phases and microstructure were examined using X-ray diffraction and electron probe microanalysis techniques. The solidification paths of all the alloys in a Ni-rich corner were revealed based on a detailed analysis of the as-cast microstructure. The liquidus cube of the quaternary Ni-Al-Cr-Os system in a Ni-rich corner was established accordingly. A eutectic-type invariant reaction on the liquidus surface was explicitly identified, and its reaction can be expressed as L → α + β + γ. No quaternary invariant reaction was found in the alloys following the addition of Os. The Ni-Al-Cr-Os alloy points were then vertically mapped onto the Ni-Al-Cr liquid phase projection to better observe the effect of Os addition on the solidification path of the Ni-Al-Cr system. It was found that the addition of a small amount of Os has no significant effect on the solidification path of the Ni-Al-Cr system. Furthermore, the microhardness of each alloy, which was determined to be in the range of 207 HV to 565 HV, was found to be closely related to the phase constitution and phase fraction of the alloy.