L 11. LASER Machining 2.ppt - Nathi | Loves Jesus · PDF file1 LASER Machining Production...

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LASER Machining

Production Engineering I

(MENG 3221)

Content

• Laser and Light HistoryG ti f L Li ht• Generation of Laser Light

• Laser Source• Laser Delivery and Optic System• Laser Material Interaction• Laser Application CuttingLaser Application Cutting• Laser Application Welding• Laser Application Others

Introduction

THE FIRST PUBLIC DEMONSTRATION OF LASER MATERIAL PROCESSING

Four years earlier (1960),newspapers had described anew invention –the laser. Theheadlines had mainly beenvariation on a theme; a“death ray”. This combinationof mystique and terror wasperfect for the film’s producerperfect for the film’s producer

A scene from the 1964 film Goldfinger

LASER AND LIGHT HISTORY

It seems that the first theory of ray light (developed several year(developed several year B.C.) was that:

light rays come out to the eyes and irradiate the object.

there was also an empirical prove: if you p p yclose your eyes you can’t see.

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300 B.C. The Greek define different laws to describe how the ray light is reflected on the surface (reflection phenomena)


• 1000 –The physic, Al–Hazendiscovered the refractionphenomena. His work includes aphenomena. His work includes astudy on the reflection andrefraction with realizedexperiments with the help ofdifferent mirrors (spherical,parabolic, cylindrical, concaveand convex). A study on themagnifying glass, research onshade, colors, rainbow and adiscussion on light, that is thefirst philosophical scientific treatyon vision.

After the Middle Age the question that most of all scientists were discussing was: What is light?


• Sir Isaac Newton said that the light was based on

In the end of 1600 two different theory were developed about this topic:

gcorpuscles and these particles propagate follow a linear trajectory (particle theory).

• Christian Huygens said that• Christian Huygens said that light was based by waves that propagates through air


Both theories explain the reflection and refraction phenomena so itwas impossible to understand which one was the true one till 1801.

• 1801 ‐Thomas Young discovered the interference phenomena. This discovery give the aim to discover the diffraction phenomena. But both theory could be

l d l kexplained only taking into account the waves theory. So the particles theory was leaved.

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• 1865 –Maxwell published a theory of electromagnetism, y gwhich concluded that light comprised electrical and magnetically vectors oscillating in orthogonal planes: an electromagnetic waves.


• 1900 –Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory, and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction, and interference

By the way few years later Hertz discovered some photoelectric phenomena thatcould be explained only taking into account the particle theory. That creates panicinto the science community, maybe Newton theory was right?!


• 1900 ‐Max Planck developed a new theory. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) onlyelectromagnetic radiation) only as discrete bundles or packets of energy. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton.

So finally both Newton than Huygens theory was right. Light and, more in general,electromagnetic radiation are based on a dual nature: energy is transported asphotons (close to the Newton’s corpuscle) moving in a waves field (close to theHuygens’s waves)


Now that light nature was known how laser was discovered???

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• 1913

Bohr gives a fundamental contributions to understanding atomic structure and quantum mechanics.

• 1917

Einstein, based on Born studies,developed a theory about thede e oped a t eo y about t einteraction betweenelectromagnetic radiation andmaterial: the stimulated emissionof a photon.


1950 –Both Charles Townes in USA thenAlexander Prokhorov and Nicolai Basovin Soviet Union realized a system ablein Soviet Union realized a system ableto generate and amplify electromagnetwaves thanks to the stimulatedemission phenomena.

This system was called MASER(Microwave Amplification byStimulated Emission of Radiation)because the wavelength of the emittedradiation was into the microwaveregion.


16 May 1960 –The first working laser( the ruby laser) was demonstratedby Theodore Mainman. A pink rubycylinder 1 cm in diameter and 2 cmlong was mounted on the axis of ahelical xenon flash lamp, which wasplaced inside a polished aluminumcylinder. The end of the crystal wereground and polished flat and parallel,and coated with silver A hole 1 mmand coated with silver. A hole 1 mmin diameter was made in one of thefaces to allow light to escape.

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Different uses need lasers with different output powers.

Less than 1 mW–laser pointers

5 mW–CD-ROM drive

5 10 mW DVD player or DVD ROM drive5–10 mW–DVD player or DVD-ROM drive

100 mW–High-speed CD-RW burner250 mW–Consumer DVD-R burner

1 W –green laser in current Holographic Versatile Disc prototype development

1–20 W –output of the majority of commercially available solid-state lasers used for micro machining

30–100 W –typical sealed CO2surgical lasers

100–3000 W (peak output 1.5 kW) –typical sealed CO2lasers used in industrial laser cutting

1 kW –Output power expected to be achieved by a prototype 1 cm diode laser bar

Content

• Laser and Light HistoryG ti f LASER Li ht• Generation of LASER Light


Introduction

Lasers are devices that produce intense beams of light which aremonochromatic, coherent, and highly collimated.

The wavelength (color) of laser light is extremely pure (monochromatic) whenThe wavelength (color) of laser light is extremely pure (monochromatic) whencompared to other sources of light, and all of the photons (energy) thatmake up the laser beam have a fixed phase relationship (coherence) withrespect to one another.

Light from a laser typically has very low divergence. It can travel over greatdistances or can be focused to a very small spot with a brightness whichexceeds that of the sunexceeds that of the sun.

Because of these properties, lasers are used in a wide variety of applicationsin all walks of life.

(L)ight

Electromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magneticfield components which oscillate in phase perpendicular to each other andperpendicular to the direction of energy propagation.

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(L)ightElectromagnetic radiation is classified into several types according to the

frequency of its wave.

The wavelength of a laser source has a range between the UV and IR.

Most of industrial lasers have an IR wavelength. So laser beam is invisible forthe human eye.

(L)ight

• Electromagnetic radiation has particle properties as discretepackets of energy, or quanta, called photons. The frequency ofthe wave is proportional to the particle's energy. Becausephotons are emitted and absorbed by charged particles, they actas transporters of energy. The energy per photon can becalculated from the Planck–Einstein equation

eph=hf

where eph is the energy, h is Planck's constant, and f is frequency

(L)ight

• Property of a laser beam:– Monochromatic

– One phase

– Low divergenceLow divergence

The result of these properties is that:100 W of a lamp are able to light a room

100 W of a laser are able to cut metal, paper, wood etc with high speed

(S)timulated(E)mission

To understand stimulatedemission, we start with the Bohratom.

In 1915, Neils Bohr proposed amodel of the atom. This simplemodel became the basis for thefield of quantum mechanics and,although not fully accurate bytoday’s understanding, still isuseful for demonstrating laser In Bohr’s model electrons orbit theuseful for demonstrating laserprinciples.

In Bohr s model electrons orbit the nucleus of an atom. Unlike earlier “planetary” models, the Bohr atom has a limited number of fixed orbits that are available to the electrons.

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Under the right circumstances an

As example if e0 is the atom ground level a photon is absorbed g

electron can go from its groundstate (lowest‐energy orbit) to ahigher (excited) state, or it candecay from a higher state to alower state, but it cannot remainbetween these states. Theallowed energy states are called“ ” d

by this atom if:

e0 = e1 + h. f10

This phenomena is called b i“quantum” states and are

referred to by the principal“quantum numbers”1,2, 3, etc.

absorption


• After a Δt, the atom comes back tothe ground level and release ah t Thi h i ll dphoton. This phenomena is called

spontaneous emission.

• But if a photon interact with an atomthat just absorbed a photon it willrelease two photons and comes backto the ground level. This phenomenais called stimulated emission

• The advantage of the stimulatedi i i h h hemission is that these two photons

will have same phase, samefrequency and same direction (that isthe base of a laser beam).

(A)mplification• Imagine to take into account a

photon’s flow that pass inside amedium (gas for example). The gascould act as a dumper (an example

• It considers a section of a medium with a length equal to dx and with thickness and width infinite. Imagine that this is the

is the atmosphere for the sunbeams) or as an amplifier.

• Based on the concept of absorptionand stimulated emission in order toamplify the photons the gas mustbe in an un‐steady energy level

atoms configuration in side this medium

called “population inversion”

What happen if a flux of photons (Phi) pass inside this medium?

(A)mplification

The outgoing photons flux p(Pho) will be equal to:

Is less than [DUMPER] orIs less than [DUMPER] or greater than [AMPLIFIER] zero.

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(A)mplification

For a normal population of atoms,there will always be more atoms inthe lower energy levels than in theupper ones. Since the probability

• So finally a medium must be energetically pumped in order to act as an amplifier for the incoming photons A mediumpp p y

for an individual atom to absorb aphoton is the same as theprobability for an excited atom toemit a photon via stimulatedemission, the collection of realatoms will be a net absorber, not anet emitter, and amplification will

t b ibl C tl t

incoming photons. A medium where the population of e1is larger than e0is called “activemedia”. But this is not enough to generates a laser beam we need more energy.

not be possible. Consequently, tomake a laser, we have to create a“population inversion.”

(A)mplification

This resonator is a system of mirrors that reflects undesirable (off‐axis)photons out of the system and reflects the desirable (on‐axis) photons backinto the excited population where they can continue to be amplified.

Efficiency As mentioned before different energy transition must be done to obtain a laser

beam. Each transition is associated to a specific efficiency.

Basically most of industrial laser source has an efficiency less than 5%. Therest of energy is loss in heat. For this reason a laser source must be cooledby a specific device that is called chiller.

Traverse Electromagnetic Mode (TEM)The TEM index is usually reported as:

TEMxyWhere x and y are respectively the number of minimum on the x and y axis.

A parameter that it describes the trend of thepower density inside the laser beam is theTransverse Electromagnetic Mode (TEM).

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Temporal ModeThe temporal mode is the trend of the laser power as a function of time.Laser sources can be emitted in a continuous state (Continuous Wave CW)or in a pulsed state (Pulsed Wave PW)

Pulsed lasers are useful in many applications in which continuous wave (cw) lasersPulsed lasers are useful in many applications in which continuous-wave (cw) laserswon't work because the energy from a pulsed laser is compressed into littleconcentrated packages. This concentrated energy in a laser pulse is more powerfulthan the natural-strength energy that comes from a continuous-wave laser.

Beam Shape and Divergence

• Θ. do = constant = k. λ → kG = 4/π = 1.27

• K = θG/θ = kG/k = 4/(πk) …[0,1] beam propagation factor

• M2 = 1/K, …….M2 >=1 beam propagation ratio.

Beam Shape and Divergence• Sometimes, for the industrial environment, the quality of the beam is not

qualified by the beam propagation ratio, rather by the beam productparameter BPP that is the product between the focus radius and halfdivergence angle:

In case of Nd:YAG lasers: BPP = 0.34 M2

In case of CO2lasers: BPP = 3.4 M2



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Laser Sources

Industrial laser are normally classified by active medium

• Gas:molecule: CO2

atoms He/Neatoms: He/Ne

ions: Kr and Ar

excimers

• Liquid: scarce industrial relevance

• Solid: Nd:YAG

diode

Active fiber

GAS (CO2) laser

GAS LASER

CO2 Slab Source

GAS LASER

Fast axial flow GAS laser

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Nd:YAG sources • The active medium of a solid‐state laser consists of a passive

host crystal and the active ion, and it is these componentsthat give the laser its name. An Nd‐YAG laser, for example,consists of a crystal of YAG with a small amount of Nd addedconsists of a crystal of YAG with a small amount of Nd addedas an impurity. The population inversion is created in the Ndion (Nd3+), and this ion generates the photon of laser light.The Nd:YAG laser is the most prevalent of today's solid‐statelasers.

Nd:YAG sources

Nd:YAG sources

Diode pumping are characterized by a monochromatic irradiation. The pumpingdirection is longitudinal. The wavelength of the photons is particularly suited forNd:YAGlasers and so the efficiency is better than lamps (about 10%).

Fiber active laser source

The laser consists of a coil of appropriate double‐clad doped fiber, two reflectors and a pump source.

The laser beam generation mechanisms are vary close to the Nd:YAGlaser sources. The active medium is passive host crystal and theactive ion, and it is these components that give the laser its name

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Fiber active laser sourceThe emission wavelength is a function of choices in the doped fiber andby any type of reflector (a typical example would be Bragg gratings).Fiber laser configurations include single-mode continuous, which can berapidly modulated to beyond 100 kHz. Output covers the UV, visible and

i f d tnear infrared spectrum.

Fiber active laser sourcehigh absorption at diode λ

• high efficiency: 75-80%• single emitting diode coupled in fiber : 5-7 W



LASER System

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Beam Delivery System

• Water cooled reflective mirrors: CO2, high power (>5kW)

• fiber: Nd:YAG, Yb:Glass, Diode

Beam delivery by reflective mirror

simpleall lasers, CW, PWstraight pathnatural or forced

convection, depending on power

Beam delivery by fiber

It was a well known ‘fact’ that, as light travels instraight lines, it is impossible to make it follow acurved path to shine around corners. [Boston,Mass., USA, 1870.]

An Irish physicist by the name of John Tyndallgave a public demonstration of an experimentwhich not only disproved this belief but gave birthto a revolution in communications technology.

EXPECTED WHAT HAPPENED

Beam delivery by fiber

To protect the optic fiber from surface scratches, we add a layer of softplastic to the outside of the cladding. This extra layer is called the primarybuffer.

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Laser Material InteractionSo the effect of a laser radiation on a material is an increase of temperature inside the material. However on the contrary because a part of the energy is absorbed by the material, the wave will be dumped as far as it propagates inside the material.

The amount of damping depends on the property of the material, inparticular it depends on the refraction index. Ii is possible to assertthat the direction of the wave propagation depends on the real partof this index instead of the dumping depends on the imaginary part.

Laser Material Interaction Laser Material Interaction

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Heat flow and laser machining parametersInteraction timeIt is the time during the material is exposed to the laser beam. In case of PW source it is equal to the pulse time. In case of CW source with movement source, it is equal to:

Circular Beam

RectangularBeam

Heat flow and laser machining parameters


• Laser Source• Laser Delivery and Optic System• Laser Material Interaction• Laser ApplicationsLaser Applications ….

Laser CuttingThe material either melts, burns, vaporizes away,or is blown away by a jet of gas, leaving an edgewith a high quality surface finish. Industrial lasercutters are used to cut flat-sheet material as well asstructural and piping materials.

where: P = Incident power (W), w = Average kerf width (m) t = Thickness (m), V = Cutting speed (m/s) ( ) g p ( )m’= Fraction of melt vaporized Lf = Latent heat of fusion (J/kg) Lv= Latent heat of vaporization (J/kg) Τf= Fusion temperature (K), Ta=Ambient temperature (K)Α= absorption coefficient, ρ= density (kg/m3)cp=thermal capacity (J/kg K)

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Melt and blow cuttingAs show below there’s a correlation between power/thickness and the cutting speed

Laser Welding

• Laser beam welding has high power density (on the order of 1 MW/cm²)resulting in small heat‐affected zones and high heating and cooling rates.

• The spot size of the laser can vary between 0 2 mm and 13 mm though onlyThe spot size of the laser can vary between 0.2 mm and 13 mm, though onlysmaller sizes are used for welding.

• The depth of penetration is proportional to the amount of power supplied,but is also dependent on the location of the focal point: penetration ismaximized when the focal point is slightly below the surface of theworkpiece.

• A continuous or pulsed laser beam may be used depending upon theapplication. Milliseconds long pulses are used to weld thin materials such asrazor blades while continuous laser systems are employed for deep welds

Laser welding

There are two modes of welding with the laser

Conduction mode Deep penetration mode: Key-hole mode

Other Laser Applications

HardeningCladdingMarking (Engraving)

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Laser HardeningHeat treatments are transactions or series of transactions which metals or metalalloys are imposed in order to obtain a specific structure and specific finalproperties. These treatments are performed on a material in a solid state and in acontrolled environment.

Laser CladdingLaser cladding is a method of depositing material by which a powderedor wire feedstock material is melted and consolidated by use of a laser inorder to coat part of a substrate or fabricate a near-net shape part.

Laser cladding offers many advantagesover conventional coating processes suchas arc welding and plasma spraying. Thelaser cladding technique can produce amuch better coating, with minimal dilution,minimal distortion, and better surfacequalityquality.

Laser MarkingLaser marking is the practice of using lasers to engrave or mark an object.The technique can be very technical and complex, and often a computersystem is used to drive the movements of the laser head.

Despite this complexity, very precise and clean engravings can beachieved at a high rate. The technique does not involve tool bits whichcontact the engraving surface and wear out. This is considered anadvantage over alternative engraving technologies where bit heads haveto be replaced regularly.

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