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.