Lecture 06: Non-traditional Machining using Thermal Energy

Thermal Removing Techniques

  • Thermal energy melts/vaporizes material.
  • Heat sources:
    • Electric discharge (Arc)
    • Electron beam
    • Laser beam
    • Plasma
  • Technologies:
    • Electrical Discharge Machining (EDM)
    • Electron Beam Machining (EBM)
    • Laser Beam Machining (LBM)
    • Plasma Beam Machining (PBM)
  • EDM is the oldest and most widely used.
  • EBM and LBM are newer and widely accepted.
  • Plasma-arc cutting used for thicker materials (3-75 mm).
  • Heat-affected zone (HAZ) always present. Material deposition is possible in EBM, LBM and arc machining.

Electron and Laser Beam Machining

  • EBM uses high-energy electrons; LBM uses high-energy photons.
  • EBM requires a vacuum.
  • Both EBM and LBM are electro-optical-thermal processes.
  • EBM and LBM provide accurate cutting/boring with better surface finish and narrower kerf width.

Electron Beam Machining (EBM)

  • High-velocity electrons concentrated into a narrow beam.
  • Causes rapid melting and vaporization.
  • Used for drilling, cutting, annealing, and welding.
  • Electron beam accelerated to ~75% of light speed, focused by electromagnetic lens.
  • Kinetic energy converts to thermal energy, vaporizing material.
  • Vacuum minimizes contamination and electron scattering.
  • Best for small parts in compact vacuum chambers.
  • CNC table allows machining of any shape.

Main Components of EBM

  • Working chamber with vacuum.
  • Movement system (CNC).
  • Vacuum generation system.
  • Electron gun.
  • Magnetic lens.
  • High voltage power supply.
  • Cooling system.
  • Compressed air system (sometimes).

Electron Gun

  • Generates, accelerates, and focuses electron beam.
  • Cathode (tungsten or tantalum) heated to ~2500°C for thermo-ionic emission.
  • Anode attracts and accelerates the electron beam.

Magnetic Lenses

  • Focus electron beam using Lorentz forces.
  • Computer-controlled electromagnetic deflection for positioning.

EBM Process Parameters:

  • Accelerating voltage.

  • Beam current.

  • Pulse duration.

  • Energy per pulse.

  • Power per pulse.

  • Lens current.

  • Spot size.

  • Power density.

  • High power density needed for fast evaporation (up to 10^7 W/mm^2).

  • Electrons transfer kinetic energy to heat, evaporating material.

  • Molten material expelled by high vapor pressure.

  • Uses voltages from 50 to 200 kV.

Drilling by EBM

  • Backing material used to expel molten material.
  • Precision drilling of small holes in metals.
  • Holes as small as 0.025 mm, slots as narrow as 0.025 mm in materials up to 6.25 mm thick.
  • Spot size: 10-100 microns.
  • Hole diameters: 100 microns to 2 mm, depth up to 15 mm.
  • High depth-to-diameter ratios (>100:1).
  • Pulsed mode operation.
  • Pulse durations: 50 microseconds to 15 milliseconds.
  • Requires well-trained personnel due to X-ray hazard.

EBM Applications

  • Machines steel, stainless steel, titanium, nickel alloys, aluminum, plastics, ceramics.
  • Aluminum and titanium alloys are more readily machined compared to steel.
  • Thin heat-affected zone (20-30 microns).
  • No cutting forces.
  • Unconventional geometries possible via electromagnetic coil deflection.
    • Tapered or barrel-shaped holes, reverse tapers.

Advantages of EBM

  • High drilling rates for small, high aspect ratio holes.
  • Machines almost any material.
  • Low clamping/fixturing costs.
  • Burr-free products.
  • Accurate complex hole shapes.
  • Small heat-affected zones.

Disadvantages of EBM

  • Expensive equipment.
  • Requires specialized technicians for vacuum systems.
  • Long pump-down times.
  • Recast layer can be an issue.
  • X-ray health hazard.

Laser Beam Machining (LBM)

What is a Laser?

  • LASER: Light Amplification by Stimulated Emission of Radiation.
  • Produces highly directional light through stimulated emission.
  • Differs from conventional light in coherence, directionality, mono-chromacity, and high intensity.
  • Laser light is coherent, monochromatic, directional, and of high intensity.

Lasing Principle:

  • Stimulated emission produces in-phase photons.
  • Overlapping wave trains result in stable, directional radiation.

Laser Component

  • Main components: Laser active medium, light amplifying medium and Optical resonator (Mirrors)
  • Energy pumped into active medium converts to radiation.
  • Active media can be solid, liquid, or gas.

Types of Laser

  • Solid-state laser
  • Gas laser
  • Liquid laser
  • Semiconductor laser

Solid-state laser

  • Uses solid as laser medium (glass or crystalline).
  • Ions (e.g., cerium, erbium, terbium) doped into host material.
  • Nd:YAG is most commonly used.
  • Ruby laser was the first solid-state laser.

Gas lasers

  • Gas lasers uses mixture of gases as laser medium.
  • Examples: Helium-Neon, argon ion, carbon dioxide lasers.

Liquid laser

  • Uses liquid as laser medium.
  • Dye laser (organic dye solution).
  • Produces light from near UV to near IR.

Semiconductor laser

  • Cheap, compact, low power.
  • Also known as laser diodes.
  • Uses electrical energy as pump source.

Laser application in industry

  • Used for heat treatment, welding, rapid prototyping, measurement, scribing, cutting, and drilling.
  • Various laser types used depending on application.

Laser Cutting

  • Uses laser beam on CNC head.
  • Beam bounced by mirrors and focused onto plate.
  • Uses compressed gas (Oxygen or Nitrogen).
  • Accurate, excellent cut quality, small kerf width and HAZ.

Gas-assisted laser cutting

  • Gas removes molten material.
  • Gas can react chemically with workpiece to increase cutting speed.
  • Oxygen used for mild steel; nitrogen for stainless steel and aluminum.

Laser Beam Machining

  • Uses carbon dioxide gas lasers and solid-state lasers.
  • Localized, non-contact, reaction-force free.
  • Photon energy absorbed as thermal or photochemical energy.
  • Material removal:
    • Melting and blowing away (long pulsed and continuous-wave lasers).
    • Direct vaporization/ablation (ultra-short pulsed lasers).
  • Any material that absorbs laser irradiation can be machined.
  • Ultra-short pulsed lasers enable absorption even in transparent materials.

Micromachining Parameters

  • Wavelength
  • Spot size
  • Laser beam intensity
  • Depth of focus
  • Laser pulse length
  • Shot-to-shot repeatability

Laser Beam Machining: Heat Affected Zone - HAZ

  • Heat diffusion undesirable for micromachining.
  • Reduces efficiency; higher heat conductivity reduces efficiency more.

Material Defects due to HAZ

  • Mechanical stresses and cracks

  • Delamination due to Shock waves

  • Recast layer

  • Surface debris

  • HAZ causes mechanical stress and micro-cracks.

  • Cracks propagate, causing premature failure.

HAZ -Recast layer

  • Recast layer forms with different structure.
  • Must often be removed.
  • Continuous wave (CW) laser → remove material by mainly meting and some vaporisation
  • Short pulse laser → remove material by meting and vaporisation
  • Ultra Short pulse laser → remove material by ablation (direct vaporisation)

HAZ – Shock waves

  • Shock waves can damage structures or delaminate materials.
  • More energy = stronger shock waves.

Short Pulse Laser Beam Machining

  • High peak laser intensity with low pulse energies.
  • Reduced heating.
  • No melt zone, micro cracks, shock waves, stress, or recast layer.
  • Can machine hard, high melting point materials.
  • Pulse duration shorter than heat diffusion time.
  • High efficiency; energy does not diffuse away.

Laser Beam Machining Advantages:

  • Excellent control of laser beam.
  • Faster than conventional tool-making techniques.
  • Higher accuracy rates.
  • Quicker turnaround for parts.
  • Reduces wastage.

Disadvantages:

  • Material gets very hot; thermal expansion may be a problem.
  • Distortion caused by oxygen.
  • High energy cost.
  • Not effective on aluminum, copper alloys, crystal, glass, and transparent materials.

Plasma Beam Machining (PBM)

  • Plasma: Highly conductive ion-electron gas.

  • Electrically heated gas stream constricted through a small orifice.

  • High temperature, high velocity stream melts and blows through most metals.

  • PBM (AKA Plasma arc cutting or plasma arc machining, PAM)

  • Used for thick sections of electrically conductive materials.

  • High-temperature plasma stream blasts through workpiece.

  • Plasma confined in a narrow column.

  • Electrode negatively charged; workpiece positively charged.

  • Fast cutting speeds.

  • Uses copper nozzle to constrict gas stream.

  • Arc jumps from electrode to conductive material.

  • Only for conductive materials (mild steel, stainless steel, aluminum).

  • Other metals have difficult cutting due to melting temperatures.

Electrode Material

  • Copper with metal insert (tungsten or hafnium).
  • Tungsten burns up in oxygen; hafnium used when using oxygen or compressed air.

Oxygen in Plasma Torch

  • Reacts with mild steel to speed up cutting and improve edge quality.

  • Nitrogen or compressed air used for stainless steel or aluminum.

  • Argon gas is used when plasma marking. Mixture of Argon and Hydrogen is often used when cutting thicker Stainless Steel or Aluminum.

Specification

  • Tolerances of ±0.8 mm for thicknesses < 25 mm; ±3 mm for greater thicknesses.
  • HAZ varies between 0.7 and 5 mm.