High Precision Electro Discharge Machining of Molybdenum and Tungsten

Refractory metals are known to be difficult to machine. Nevertheless, their mechanical or electrical characteristics are interesting for many industrial advanced applications. The research of methods to manufacture the electrodes in Molybdenum for the future “CLIC” accelerator at CERN has provided a pretext to study these issues. Click accelerator cavities are characterized by extreme quality objectives (dimensional precision < 1 m, surface roughness Ra < 0.05 m).We have studied mainly the machining of Molybdenum and Tungsten; we have noticed that the choice of the dielectric was crucial for the Molybdenum. The formation of Molybdenum oxide prevents machining. The solubility of Mo in water also brings out problems of corrosion. This requires that these materials must be machined in oil. Frost heaving of grain from this compacted material is also a long term problem.The refractory materials behavior remains problematic due to the peculiar cracking, as “alligator cracks”, “lizard skin” or crazing that appears on the surface. The difference between cracking and C is mentioned. Furthermore, this «Alligator cracks» phenomenon also appears in the case of samples Molybdenum submitted to strong and high frequency electric fields. This badly-known phenomenon endangers the use of molybdenum in CLIC accelerator.