Content LASER Machining Production Engineering I (MENG 3221) Laser and Light History Generation of Laser Light Laser Source Laser Delivery and Optic System Laser Material Interaction Laser Application Cutting Laser Application Welding Laser Application Others Introduction THE FIRST PUBLIC DEMONSTRATION OF LASER MATERIAL PROCESSING Four years earlier (1960), newspapers had described a new invention the laser. The headlines had mainly been variation on a theme; a death ray. This combination of mystique and terror was perfect for the film s producer It seems that the first theory of ray light (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 close your eyes you can t see. A scene from the 1964 film Goldfinger 1
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 Hazen discovered the refraction phenomena. His work includes a study on the reflection and refraction with realized experiments with the help of different mirrors (spherical, parabolic, cylindrical, concave and convex). A study on the magnifying glass, research on shade, colors, rainbow and a discussion on light, that is the first philosophical scientific treaty on vision. After the Middle Age the question that most of all scientists were discussing was: What is light? In the end of 1600 two different theory were developed about this topic: Sir Isaac Newton said that the light was based on corpuscles and these particles propagate follow a linear trajectory (particle theory). Christian Huygens said that light was based by waves that propagates through air Both theories explain the reflection and refraction phenomena so it was 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 explained only taking into account the waves theory. So the particles theory was leaved. 2
1865 Maxwell published a theory of electromagnetism, which 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 that could be explained only taking into account the particle theory. That creates panic into 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) 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 as photons (close to the Newton s corpuscle) moving in a waves field (close to the Huygens s waves) Now that light nature was known how laser was discovered??? 3
1913 Bohr gives a fundamental contributions to understanding atomic structure and quantum mechanics. 1917 Einstein, based on Born studies, developed e a theory about the interaction between electromagnetic radiation and material: the stimulated emission of a photon. 1950 Both Charles Townes in USA then Alexander Prokhorov and Nicolai Basov in Soviet Union realized a system able to generate and amplify electromagnet waves thanks to the stimulated emission phenomena. This system was called MASER (Microwave Amplification by Stimulated Emission of Radiation) because the wavelength of the emitted radiation was into the microwave region. 16 May 1960 The first working laser ( the ruby laser) was demonstrated by Theodore Mainman. A pink ruby cylinder 1 cm in diameter and 2 cm long was mounted on the axis of a helical xenon flash lamp, which was placed inside a polished aluminum cylinder.theendofthecrystalwere ground and polished flat and parallel, and coated with silver. A hole 1 mm in diameter was made in one of the faces to allow light to escape. 4
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 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 History Generation of LASER Light Laser Source Laser Delivery and Optic System Laser Material Interaction Laser Application Cutting Laser Application Welding Laser Application Others Introduction (L)ight Lasers are devices that produce intense beams of light which are monochromatic, coherent, and highly collimated. The wavelength (color) of laser light is extremely pure (monochromatic) when compared to other sources of light, and all of the photons (energy) that make up the laser beam have a fixed phase relationship (coherence) with respect to one another. Light from a laser typically has very low divergence. It can travel over great distances or can be focused to a very small spot with a brightness which exceeds that of the sun. Because of these properties, lasers are used in a wide variety of applications in all walks of life. Electromagnetic radiation is a ubiquitous phenomenon that takes the form of selfpropagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. 5
(L)ight Electromagnetic radiation is classified into several types according to the frequency of its wave. (L)ight Electromagnetic radiation has particle properties as discrete packets of energy, or quanta, called photons. The frequency of the wave is proportional to the particle's energy. Because photons are emitted and absorbed by charged particles, they act as transporters of energy. The energy per photon can be calculated from the Planck Einstein equation e ph =hf where e ph is the energy, h is Planck's constant, and f is frequency 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 for the human eye. (L)ight Property of a laser beam: Monochromatic One phase Low 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 stimulated emission, we start with the Bohr atom. In 1915, Neils Bohr proposed a model of the atom. This simple model became the basis for the field of quantum mechanics and, although not fully accurate by today s understanding, still is useful for demonstrating laser In Bohr s model electrons orbit the principles. nucleus of an atom. Unlike earlier planetary models, the Bohr atom has a limited number of fixed orbits that are available to the electrons. 6
(S)timulated(E)mission (S)timulated(E)mission Under the right circumstances an electron can go from its ground state (lowest energy orbit) to a higher (excited) state, or it can decay from a higher state to a lower state, but it cannot remain between these states. The allowed energy states are called quantum states and are referred to by the principal quantum numbers 1,2, 3, etc. As example if e 0 is the atom ground level a photon is absorbed by this atom if: e 0 = e 1 + h. f 10 This phenomena is called absorption After a Δt, the atom comes back to the ground level and release a photon. This phenomena is called spontaneous emission. But if a photon interact with an atom that just absorbed a photon it will release two photons and comes back to the ground level. This phenomena is called stimulated emission The advantage of the stimulated emission i is that these two photons will have same phase, same frequency and same direction (that is the base of a laser beam). Imagine to take into account a photon s flow that pass inside a medium (gas for example). The gas could act as a dumper (an example is the atmosphere for the sun beams) or as an amplifier. (A)mplification It considers a section of a medium with a length equal to dx and with thickness and width infinite. Imagine that this is the atoms configuration in side this medium The outgoing photons flux (Ph o ) will be equal to: (A)mplification Based on the concept of absorption andstimulatedemissioninorderto amplify the photons the gas must be in an un steady energy level called population inversion What happen if a flux of photons (Phi) pass inside this medium? Is less than [DUMPER] or Is less than [DUMPER] or greater than [AMPLIFIER] zero. 7
(A)mplification (A)mplification For a normal population of atoms, there will always be more atoms in the lower energy levels than in the upper ones. Sincetheprobability for an individual atom to absorb a photon is the same as the probability for an excited atom to emit a photon via stimulated emission, the collection of real atoms will be a net absorber, not a net emitter, and amplification will not be possible. Consequently, to makealaser,wehavetocreatea population inversion. So finally a medium must be energetically pumped in order to act as an amplifier for the incoming photons. A medium where the population of e 1 is larger than e 0 is called active media. But this is not enough to generates a laser beam we need more energy. This resonator is a system of mirrors that reflects undesirable (off axis) photons out of the system and reflects the desirable (on axis) photons back into 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. Traverse Electromagnetic Mode (TEM) The TEM index is usually reported as: TEM xy Where x and y are respectively the number of minimum on the x and y axis. A parameter that it describes the trend of the power density inside the laser beam is the Transverse Electromagnetic Mode (TEM). Basically most of industrial laser source has an efficiency less than 5%. The rest of energy is loss in heat. For this reason a laser source must be cooled by a specific device that is called chiller. 8
Temporal Mode The 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) Beam Shape and Divergence Pulsed lasers are useful in many applications in which continuous-wave (cw) lasers won't work because the energy from a pulsed laser is compressed into little concentrated packages. This concentrated energy in a laser pulse is more powerful than the natural-strength energy that comes from a continuous-wave laser. Θ. do = constant = k. λ k G = 4/π = 1.27 K = θ G /θ = k G /k = 4/(πk) [0,1] beam propagation factor M 2 = 1/K,.M 2 >=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 product parameter BPP that is the product between the focus radius and half divergence angle: In case of Nd:YAG lasers: BPP = 0.34 M 2 In case of CO2lasers: BPP = 3.4 M 2 Laser and Light History Generation of LASER Light Laser Source Laser Delivery and Optic System Laser Material Interaction Laser Application Cutting Laser Application Welding Laser Application Others 9
Laser Sources GAS (CO2) laser Industrial laser are normally classified by active medium Gas:molecule: CO2 atoms: He/Ne ions: Kr and Ar excimers Liquid: scarce industrial relevance Solid: Nd:YAG diode Active fiber CO 2 Slab Source GAS LASER Fast axial flow GAS laser GAS LASER 10
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 components that give the laser its name. An Nd YAG laser, for example, consists of a crystal of YAG with a small amount of Nd added as an impurity. The population inversion is created in the Nd ion (Nd3+), and this ion generates the photon of laser light. The Nd:YAG laser is the most prevalent of today's solid state lasers. Nd:YAG sources Nd:YAG sources 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:YAG laser sources. The active medium is passive host crystal and the active ion, and it is these components that give the laser its name Diode pumping are characterized by a monochromatic irradiation. The pumping direction is longitudinal. The wavelength of the photons is particularly suited for Nd:YAGlasers and so the efficiency is better than lamps (about 10%). 11
Fiber active laser source The emission wavelength is a function of choices in the doped fiber and by any type of reflector (a typical example would be Bragg gratings). Fiber laser configurations include single-mode continuous, which can be rapidly modulated to beyond 100 khz. Output covers the UV, visible and near infrared spectrum. Fiber active laser source high absorption at diode λ high efficiency: 75-80% single emitting diode coupled in fiber : 5-7 W LASER System Laser and Light History Generation of LASER Light Laser Source Laser Delivery and Optic System Laser Material Interaction Laser Application Cutting Laser Application Welding Laser Application Others 12
Beam Delivery System Water cooled reflective mirrors: CO 2, high power (>5kW) fiber: Nd:YAG, Yb:Glass, Diode Beam delivery by reflective mirror simple all lasers, CW, PW straight path natural or forced convection, depending on power Beam delivery by fiber It was a well known fact that, as light travels in straight lines, it is impossible to make it follow a curved path to shine around corners. [Boston, Mass., USA, 1870.] Beam delivery by fiber To protect the optic fiber from surface scratches, we add a layer of soft plastic to the outside of the cladding. This extra layer is called the primary buffer. An Irish physicist by the name of John Tyndall gave a public demonstration of an experiment which not only disproved this belief but gave birth to a revolution in communications technology. EXPECTED WHAT HAPPENED 13
Laser Material Interaction Laser and Light History Generation of LASER Light Laser Source Laser Delivery and Optic System Laser Material Interaction Laser Application Cutting Laser Application Welding Laser Application Others So 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, in particular it depends on the refraction index. Ii is possible to assert that the direction of the wave propagation depends on the real part of this index instead of the dumping depends on the imaginary part. Laser Material Interaction Laser Material Interaction 14
Heat flow and laser machining parameters Heat flow and laser machining parameters Interaction time It 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 Rectangular Beam Laser and Light History Generation of LASER Light Laser Source Laser Delivery and Optic System Laser Material Interaction Laser Applications. Laser Cutting The material either melts, burns, vaporizes away, or is blown away by a jet of gas, leaving an edge with a high quality surface finish. Industrial laser cutters are used to cut flat-sheet material as well as structural and piping materials. where: P = Incident power (W), w = Average kerf width (m) t = Thickness (m), V = Cutting speed (m/s) 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) 15
Melt and blow cutting As 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 only smaller 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 is maximized when the focal point is slightly below the surface of the workpiece. A continuous or pulsed laser beam may be used depending upon the application. Milliseconds long pulses are used to weld thin materials such as razor blades while continuous laser systems are employed for deep welds Laser welding Other Laser Applications Hardening Cladding Marking (Engraving) There are two modes of welding with the laser Conduction mode Deep penetration mode: Key-hole mode 16
Laser Hardening Heat treatments are transactions or series of transactions which metals or metal alloys are imposed in order to obtain a specific structure and specific final properties. These treatments are performed on a material in a solid state and in a controlled environment. Laser Cladding Laser cladding is a method of depositing material by which a powdered or wire feedstock material is melted and consolidated by use of a laser in order to coat part of a substrate or fabricate a near-net shape part. Laser cladding offers many advantages over conventional coating processes such as arc welding and plasma spraying. The laser cladding technique can produce a much better coating, with minimal dilution, minimal distortion, and better surface quality. Laser Marking Laser marking is the practice of using lasers to engrave or mark an object. The technique can be very technical and complex, and often a computer system is used to drive the movements of the laser head. Despite this complexity, very precise and clean engravings can be achieved at a high rate. The technique does not involve tool bits which contact the engraving surface and wear out. This is considered an advantage over alternative engraving technologies where bit heads have to be replaced regularly. 17