ByDr.Deepak Lawrence.KMechanical Engineering DepartmentNIT Calict,Kerala,India

Laser beam machining

ME6324:Modern Machining Processes

manufacturing applicationsLaser can be used in wide range of manufacturingapplicationsMaterial removal drilling, cutting, engravingCladdingSurface hardeningAlloyingFormingRapid prototypingMeasurement

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Laser-based manufacturingLaser-based manufacturing has severaladvantages over conventional methodsAs non-contact process, it is well suited forprocessing advanced engineering materialssuch as brittle materials, electric and non-electric conductors, and soft and thin materials.It is a thermal process and materials withfavorable thermal properties can besuccessfully processed regardless of theirmechanical properties.It is a flexible process.

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Laser production

LASER is the acronym of light amplification bystimulated emission of radiation

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Schematic of (a) absorption, (b) spontaneous emission, and (c) stimulated emission.

Laser production

In an ordinary source, atoms or molecules are excited bythermal excitation, for example, electrical dischargewhich emits photons spontaneously in a random manner.Photons emitted in all directions with no correlation inwavelength, phase, and polarization between themIn lasers, photons are emitted by stimulated emission acharacteristic process that generates the photons withall properties (namely wavelength, phase, direction, andpolarization) as those of stimulated photons. Thus, thephotons get amplified in an orderly manner

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Population inversion

Consider a medium with a large number of atoms (molecules), some ofwhich are in the excited state and rest unexcited.Since the photons get absorbed by the unexcited atoms, the numberof excited atoms should be more than that of unexcited atoms for netamplification. This situation is called population inversion

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Lasing action

An active medium with a suitable set of energy levels to support laseraction.A source of pumping energy in order to establish a populationinversion.An optical cavity or resonator to introduce optical feedback and somaintain the gain of the system overcoming all losses. 7

pumping

Solid-state laser with its optical pumping unit

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Lasing MediumMany materials can be used as the heart of the laser. Depending onthe lasing medium lasers are classified as solid state and gas laser.

Solid-state lasers are commonly of the following typealumina alloy having a wavelength of

0.7Nd-glass lasers having a wavelength of 1.64Nd-YAG laser having a wavelength of 1.06

These solid-state lasers are generally used in material processing.

The generally used gas lasers are

Neon

2 etc.

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Working of a solid-state laser

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Construction of a CO2 laser

high voltage is applied at the two ends leading todischarge and formation of gas plasma. Energy ofthis discharge leads to population inversion andlasing action.

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Characteristics of common industrial lasers for materialprocessing

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Lasers are normally classified based on the physical states of the laser-active(pumping) medium as gas lasers (CO2 and excimer), solid state lasers (Nd:YAG),semiconductor lasers (diode), liquid lasers, and fiber lasers (Yb:YAG).

Different types of lasers and their applications

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capability and characteristics of common lasers.

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Process characteristics of different lasers

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Overview of various laser-based manufacturing processes

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Laser beam machining

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Laser beam machining

In laser cutting and drilling, the focused laser beam isdirected onto the surface of theworkpiece to rapidly heatit up, resulting in melting and/or vaporization, dependingon the beam intensity and workpiece material .The molten metal and/or vapor is then blown away usingan assist gas. 18

Schematic of a typical laser processing station

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The functions of assist gas during laser cutting1.protect the workpiece from undesirablereactions with the ambient gas and cool the hotcut zone by the forced convection;2. remove the molten material from cuttingfront through the narrow kerf to keep thecutting process going. The flow need to bedirected with a major component of its velocityin the direction of the expulsion;3. provide exothermic heating by chemicalreaction if reactive gas used.

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Chemical reactivity of common used gases with the common metals

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Laser cuttingLaser cutting is one of the largest applications of lasersin metal-working industry and is a well-establisheduniversal cutting tool enables cutting of almost allknown materials including diamondWhen compared with other cutting processes (such asoxy-fuel cutting, plasma cutting, sawing and punching),its advantages are numerous, namelya narrow cut, minimal area subjected to heat,a proper cut profile,smooth and flat edges,minimal deformation of a workpiece,the possibility of applying high cutting speed,intricate profile manufacture and fast adaptation tochanges in manufacturing programs

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laser cuttingThe laser cutting uses different cuttingmechanisms to cut different materials.Some of the mechanisms arevaporization,scribing,melt and blow,melt blow and burn, andthermal stress cracking

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Laser cutting: Vaporization:Vaporization: In vaporization cutting, the focusedbeam heats the surface of the material to boilingpoint and generates a keyhole.The keyhole leads to a sudden increase inabsorptivity and it results in quickly deepening thehole.As the hole deepens and the material boils,generated vapors erode the molten walls thatejects out the wall material, further enlarging thehole.Organic materials such as wood, Perspex,thermoset plastics, fiber-reinforced plastics areusually cut by this method.

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Melt and blow: laser cuttingMelt and blow: Melt and blow or fusion cuttingmode uses high-pressure gas to blow moltenmaterial from the cutting area, greatly decreasingthe power requirement.First the material is heated to melting point then agas jet blows the molten material out of the kerfavoiding the need to raise the temperature of thematerial any further.Materials cut with this process are usually metals.The distinct feature of this cutting mode is that thecut-edge material is same as the base metal andcan be put to weld without cleaning the edge.

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Scribing:laser cuttingScribing: This mode of cutting involves thedrilling of small overlapping hole along thedesired cut line by vaporization of the material.The sheet is then crack along the cut lineGenerally, hard and brittle materials likeceramics and glass are cut by this mode ofcutting.

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Melt blow and burn:laser cuttingblow and burn is a reactive cutting.This mechanism is successfully demonstrated inmetals especially mild and stainless steel duringoxygen-assisted cutting.This mechanism allows the cutting of very thicksteel plates with relatively little laser power.The distinct feature of this cutting mode is thatthere is edge hardening due to existence of oxideand thermal cycle on the cut edge.

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Thermal stress crackingBrittle materials are particularly sensitive tothermal fracture, a feature exploited in thermalstress cracking.A beam is focused on the surface causinglocalized heating and thermal expansion. Thisresults in a crack that can then be guided bymoving the beam.The crack can be moved as fast as in order offew m/s.It is generally used in cutting of glasses.

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laser cuttingThe cutting capacity for particular set of processingparameters can be estimated by severance energy (SE).SE= P/vtwhere P is laser power (W), V is cutting speed (mm/sec),and t is thickness of the materialThe quality of laser cut, that is, width of laser cut or kerfand the quality of cut edges depend upon laser, laserpower (average for continuous wave (cw) and peak forpulsed), the motion of laser beam and workpiece

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Laser Beam Machining Advantages

In laser machining there is no physical tool.Thus no machining force or wear of the tooltakes place.Large aspect ratio in laser drilling can beachieved along with acceptable accuracy ordimension, form or locationMicro-holes can be drilled in difficult tomachine materials

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Laser Beam Machining Limitations

High initial capital costHigh maintenance costNot very efficient processPresence of Heat Affected Zone specially ingas assist CO2 laser cutting

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Thank You.

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