Electric Discharge Machining of AZ91 Magnesium Hybrid Composites under Different Dielectric Mediums

Abstract

In this work, an attempt was made to machine the AZ91/5B4C/5Gr hybrid composites in a castor oil electric discharge medium with an objective of attaining green environment. The hybrid composites were produced using stir casting technique in a protective environment. Experiments were conducted by varying dielectric fluid (castor oil and hydrocarbon oil), tool materials (copper, graphite, and brass), polarity, current, pulse-on time, and gap distance in Al2O3 mixed dielectric medium. L36 Taguchi approach was adopted for the design of experiments, and machining performance was accessed in terms of material removal rate (MRR), tool wear rate (TWR), and surface roughness (Ra). Because of castor oil’s high thermal conductivity, high kinematic viscosity, and lower dielectric strength, the specimen exhibits 5% higher MRR than EDM oil. When the electrodes were connected to the negative polarity, the Ra was 1.72 μm and 3.66 μm at positive polarity; however, at negative polarity, the TWR was higher than the MRR. The high density and specific heat of castor oil facilitate flushing and uniform heat distribution; as a result, the composite had a Ra of 2.52 μm compared to 2.86 μm when machined with conventional EDM oil. Surface topography showed the specimen machined with hydrocarbon dielectric medium proffers black spots, which were eliminated in castor dielectric medium. Best parametric combination was selected by the Relative Index Method optimization technique.