In the non traditional machining process called Electrical Discharge Machining (EDM), material removal from the workpiece takes place mainly due to which combination of physical phenomena?

Introduction / Context:
Electrical Discharge Machining (EDM) is an important non traditional machining process used to shape hard and difficult to machine materials using electrical sparks rather than mechanical cutting forces. Understanding how material is actually removed in EDM is essential in manufacturing and production engineering. This question asks you to identify the correct physical mechanisms responsible for material removal in EDM.

Given Data / Assumptions:

• The machining process under discussion is Electrical Discharge Machining (EDM).
• EDM uses repetitive electrical discharges between a tool electrode and a conductive workpiece, separated by a dielectric fluid.
• The options include various combinations of erosion, cavitation, melting, evaporation and corrosion.
• We assume conventional EDM, not ultrasonic or chemical machining.

Concept / Approach:
In EDM, a series of rapid electrical discharges occur between the tool and the workpiece in a dielectric medium. Each discharge produces a tiny plasma channel and extremely high local temperatures at the point of impact, often in the range of several thousand degrees Celsius. At these temperatures, a small volume of the workpiece material melts and part of it even vaporises. The expanding vapour bubble and the collapse of the plasma channel then remove this molten and evaporated material from the crater. Thus, the primary mechanisms of material removal are thermal in nature, specifically melting and evaporation due to intense localized heating, not corrosion or cavitation in the usual fluid dynamic sense.

Step-by-Step Solution:
Step 1: Recall that EDM requires the workpiece and tool to be electrically conductive and uses a pulsed DC power supply. Step 2: Recognise that when a voltage is applied, dielectric breakdown occurs and an electrical spark jumps across the small gap between tool and workpiece. Step 3: Each spark creates a plasma channel with extremely high temperature and energy concentrated in a tiny region of the workpiece surface. Step 4: This intense local heating causes a small volume of the workpiece material to melt and some portion to evaporate. Step 5: The sudden expansion and collapse of the vapour bubble along with the dielectric flushing action remove this molten and evaporated material as tiny debris particles. Step 6: No significant chemical corrosion is involved in the primary removal mechanism, and cavitation is not the main effect the way it is in ultrasonic or hydrodynamic processes. Step 7: Therefore, material removal in EDM occurs mainly by melting and evaporation.

Verification / Alternative check:
Metallographic examination of EDM machined surfaces reveals recast layers and small craters characteristic of rapid melting and solidification. The presence of a heat affected zone and microstructural changes near the surface supports the conclusion that very high temperatures were involved. Process models and textbooks consistently describe EDM as a thermal erosion process dominated by melting and vaporisation of material due to electrical discharges. While flushing and minor mechanical effects help carry away the molten droplets, the core mechanisms are still melting and evaporation, not cavitation or corrosion.

Why Other Options Are Wrong:
Erosion and cavitation suggests a purely mechanical fluid dynamic mechanism similar to hydraulic cavitation, which is not the main process in EDM. Cavitation and evaporation again emphasises fluid effects more typical of ultrasonic machining rather than the thermal spark erosion of EDM. Melting and corrosion is incorrect because corrosion involves chemical reaction over longer times and is not the rapid, spark based removal observed in EDM.

Common Pitfalls:
Learners sometimes loosely use the term erosion for any material removal, which can cause confusion. In EDM, the word spark erosion is sometimes used, but the detailed mechanism behind that erosion is thermal, involving melting and vaporisation. It is important not to confuse EDM with processes like chemical machining or ultrasonic machining, where corrosion or cavitation may dominate. Remember that EDM is a thermal, non contact machining process relying on repeated electrical discharges to melt and evaporate material.

Final Answer:
In Electrical Discharge Machining, material is removed mainly by melting and evaporation of the workpiece at the spark sites.