LASER BEAM MACHINING

Laser Beam Machining or more broadly laser material processing deals with machining and material processing like heat treatment, alloying, cladding, sheet metal bending etc. Such processing was carried out utilizing the energy of coherent photons or laser beam, which was mostly converted into thermal energy upon interaction with most of the materials.

WORKING PROCESS

 •         A laser machine conswasts of the laser, some mirrors or a fiber for beam guidance,  focusing optics and a positioning system. The laser beam was focused onto the work-piece and could be moved relatively to it. The laser machining process was controlled by switching the laser on and off, changing the laser pulse energy and other laser parameters, and by positioning either the work-piece or the laser focus.

•         Laser machining was localized, non-contact machining and was almost reaction-force free. Photon energy was absorbed by target material in the form of thermal energy or photochemical energy. Material was removed by melting and blown away (long pulsed and continuous-wave lasers), or by direct vaporization/ablation (ultra-short pulsed lasers). Any material that could properly absorb the laser irradiation could be laser machined. The spectrum of laser machinable materials includes hard and brittle materials as well as soft materials. The very high intensities of ultra-short pulsed lasers enable absorption even in transparent materials.

FIG. 1 a) Schematic illustration of the laser-beam machining process. (b) and (c) Examples of holes produced in non-metallic  parts by LBM.

                                                                                   

•         For a given beam, I₀ will be at a maximum in the focal plane where w = w₀, the minimum beam waist.

The most fundamental feature of laser/material interaction in the long pulse regime (e.g., pulse duration 8 ns, energy 0.5 mJ) was that the heat deposited by the laser in the material diffuses away during the pulse duration; that was, the laser pulse duration was longer than the heat diffusion time. This may be desirable for laser welding, but for most micromachining jobs, heat diffusion into the surrounding material was undesirable and detrimental to the quality of the machining

•         Here are reasons why one should avoid heat diffusion for precise micromachining:

Heat diffusion reduces the efficiency of the micromachining process as it takes energy away from the work spot—energy that would otherwise go into removing work piece material. The higher the heat conductivity of the material the more the machining efficiency was reduced

Laser Beam Machining:Heat Affected Zone – HAZ

The most fundamental feature of laser/material interaction in the long pulse regime (e.g., pulse duration 8 ns, energy 0.5 mJ) was that the heat deposited by the laser in the material diffuses away during the pulse duration; that was, the laser pulse duration was longer than the heat diffusion time. This may be desirable for laser welding, but for most micromachining jobs, heat diffusion into the surrounding material was undesirable and detrimental to the quality of the machining

Here are reasons why one should avoid heat diffusion for precise micromachining:

Heat diffusion reduces the efficiency of the micromachining process as it takes energy away from the work spot—energy that would otherwise go into removing work piece material. The higher the heat conductivity of the material the more the machining efficiency was reduced.

Heat-diffusion affects a large zone around the machining spot, a zone referred to as the heat-affected zone or HAZ. The heating (and subsequent cooling) waves propagating through the HAZ cause mechanical stress and may create micro cracks (or in some cases, macro cracks) in the surrounding material. These defects are "frozen" in the structure when the material cools, and in subsequent routine use these cracks may propagate deep into the bulk of the material and cause premature device failure. A closely associated phenomenon was the formation of a recast layer of material around the machined feature. This resolidified material often had a physical and/or chemical structure that was very different from the unmelted material. This recast layer may be mechanically weaker and must often be removed.

Heat-diffusion was sometimes associated with the formation of surface shock waves. These shock waves could damage nearby device structures or delaminate multilayer materials. While the amplitude of the shock waves varies with the material being processed, it was generally true that the more energy deposited in the micromachining process the stronger the associated shock waves. 

 

Laser Beam Machining – Advantages  

• In laser machining there was no physical tool. Thus no machining force or wear of the tool takes place.
• Large aspect ratio in laser drilling could be achieved along with acceptable accuracy or dimension, form or location
• Micro-holes could be drilled in difficult – to – machine materials
• Though laser processing was a thermal processing but heat affected zone specially in pulse laser processing was not very significant due to shorter pulse duration

  Laser Beam Machining – Limitations

• High initial capital cost
• High maintenance cost
 • Not very efficient process
• Presence of Heat Affected Zone – specially in gas assist CO2 laser cutting 
• Thermal process – not suitable for heat sensitive materials like aluminium glass fibre laminate

There are many useful applications and uses for Laser Cutting in the current manufacturing market. Many new innovations are on their way and new uses for this versatile cutting process are cropping up each year