Wire and Sinker EDM of Molybdenum Materials

The hardness and brittleness of molybdenum make conventional milling awkward, so wire EDM and electrical discharge machining have become common choices. The same machine with different parameters produces very different surface conditions, and the recast layer in particular often only shows up in later use.

How to Set the Discharge Parameters

Cutting efficiency is determined jointly by discharge energy and wire travel speed. With higher energy, cutting is faster but the surface is rougher and the affected layer deeper; with lower energy, the surface is finer but efficiency falls and wire consumption rises. In roughing, energy can be opened up; in finishing it is reduced step by step, producing a multi-pass cut. Molybdenum has a high melting point and good thermal conductivity, so molten material at the edge of the discharge crater cools quickly; the recast layer is relatively dense but markedly brittle.

Wire Electrode and Dielectric

Molybdenum wire and zinc-coated wire each have their place: the former's tensile strength and durability suit thick sections and long travels, while the latter is more common for fine wire and high-precision work. Wire tension must be stable; vibration is copied directly onto the workpiece surface. The dielectric removes chips and provides cooling, and its concentration, conductivity and filtration state need to be checked each shift, since dirty fluid increases short circuits and wire breakage. When machining thin-walled parts, wire tension and feed strategy must be adjusted accordingly to avoid deformation and wire trapping.

Wire wear changes the kerf width, so in batch production compensation has to be applied according to the machined area. If the size difference between the first and last part exceeds tolerance, check wire diameter wear and guide roller condition before suspecting the program.

Before machining, confirm the insulation and grounding of the machine; changes in contact resistance in the discharge circuit also affect parameter stability. Before batch delivery, re-check key dimensions and geometric tolerances against the drawing.

Effects of the Recast Layer and Its Removal

An EDM surface consists of a recast layer, a heat-affected layer and the base metal. The recast layer may contain microcracks and pores, and the grain structure of the heat-affected layer differs from the base metal. For molybdenum parts used in vacuum furnaces, glass furnaces or high-purity environments, the affected surface layer brings outgassing, contamination and arcing risks, and usually has to be removed.

1. Mechanical grinding or polishing, stepping down through grit sizes, with removal depth controlled to cover the affected layer.
2. Electrolytic polishing, suitable for complex shapes and areas that mechanical tools cannot reach.
3. Vacuum annealing, which relieves stress at a suitable temperature but cannot remove the recast layer itself.
4. Subsequent chemical cleaning to remove grinding residue, with attention to waste liquor disposal.

Removal depth is usually set at somewhat more than the depth of the affected layer, and the actual depth can be confirmed by metallographic sectioning or stepwise polishing. Molybdenum powder and dielectric mist generated during machining must be collected; where molybdenum dust is suspended, ventilation and fire precautions should be observed, and waste fluid handled according to local requirements.

Precautions for Thin and Precision Parts

When cutting thin sheet, the workpiece tends to warp under heat and discharge reaction forces, so clamping method and cutting path must be planned in advance, with process tabs left where necessary. The final pass of a multi-pass cut only trims the contour and does not aim at stock removal. After cutting the first part, measure dimensions and roughness and confirm the parameters are stable before batch production. Kerf width and compensation should be established on the first part and then fine-tuned according to wire diameter wear. Inspection items and acceptance criteria after machining, and process parameter recommendations if required, should be settled before the job is set up on the machine.