Metal Additive Manufacturing (MAM) Week-3 PDF

EL PT N Dr. J. Ramkumar Professor Department of Mechanical Engineering and Design Program IIT Kanpur ▪ System Setup of AM Machines EL ▪ Laser theory ▪ Laser components ▪ Laser types PT ▪ Continuous vs pulsed laser ▪ Laser beam pr...

EL PT N Dr. J. Ramkumar Professor Department of Mechanical Engineering and Design Program IIT Kanpur ▪ System Setup of AM Machines EL ▪ Laser theory ▪ Laser components ▪ Laser types PT ▪ Continuous vs pulsed laser ▪ Laser beam properties N SYSTEM SETUP OF AM MACHINES 1. Laser Powder Bed Fusion (LPBF) EL 2. Laser Directed Energy Deposition (LDED) with Blown Powder Known as Laser Powder-Fed (LPF) 3. Binder Jetting (BJ) PT N  EL ▪ The laser beam melts the powder according to the CAD model loaded in the machine. PT ▪ The laser beam is scanned on the powder layer using appropriate positioning devices. ▪ After the first layer melts, the construction platform is lowered, N and a layer of powder is spread. ▪ Repeat until the part is printed. Unused Powder is removed during post-processing.  EL PT N https://www.engineersgarage.com/3d-printing-processes-powder-bed-fusion-part-5-8/  EL ▪ In this machine, the material is deposited through a nozzle (lateral or coaxial) mounted on a multi-axis arm or table and melted by a laser or electron beam. PT ▪ The material is added and melted layer by layer to print the part. N ▪ Powder is stored in powder feeders before flowing to the nozzle.  EL ▪ DED machines, instead of feeding powders, are equipped with metal wires, and the technique is referred to as the wire-fed deposition process. PT ▪ Polymers and ceramics can be used as feeding materials (powder and/or wires), but metals are the most used materials. N  EL PT N Laser Powder-Fed (LPF) system. Source: Image reproduced under the Creative Commons License CC BY-SA 3.0. 3. BINDER JETTING (BJ) ▪ The BJ technique prints complex parts by injecting liquid binder into a powder bed. EL ▪ Liquid binder is injected into a powder bed as an adhesive. PT N Schematic of a binder jetting system setup Additivley, “Binder Jet.” [Online]. Available: https://www.additively.com/en/learn-about/binder-jetting. 3. BINDER JETTING (BJ) 3. The binder is dispensed through an inkjet printhead on an EL x–y platform. 4. Similar to LPBF, a powder roller spreads the powder material which is then printed and stitched. 5. PT The build platform is lowered after the first layer is printed and a new layer of powder is spread and printed. The process is repeated until the part is printed. N 6. The BJ technique 3D prints stainless steel, polymers, and glass. LASER BASICS IMPORTANT PARAMETERS NEEDED TO BE EL KNOWN FOR AM 1. Laser theory Laser components 2. 3. 4. Laser types PT Continuous vs. Pulsed laser N 5. Laser beam properties ▪ "LASER" stands for "Light Amplification by Stimulated Emission of Radiation," which explains its working principle. EL ▪ Planck's law can be derived from spontaneous and stimulated emission. PT ▪ Emission is the process by which an excited atom transitions to a lower energy state by emitting "photon" electromagnetic radiation. N ▪ The ground state is the lowest energy orbital, while excited states are higher. ▪ When an electron jumps to a higher or lower orbit, the atom emits energy. An excited atom emits radiation spontaneously while returning to the ground state. EL ▪ Photon pumping (radiative) or external heating (non-radiative) raises the energy level of an electron in the ground state (orbit) E1 to an elevated level with energy E2. ▪ After a few nanoseconds, the electron decays to the ground state PT while emitting a photon (spontaneous emission) with an energy equal to the difference between the two states (hυ = E2-E1) Where h is Planck’s constant (4.1 × 10−15 eV s), N 𝑐 v= λ (c is the speed of light (299 792 458 m/s), and λ is the wavelength)  EL PT N Illustration of the absorption, spontaneous emission, and stimulated emission processes. E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177.  When incoming photons induce the emission, the process is EL called stimulated emission In stimulated emission, the emitted photons match the stimulating photon's phase, frequency, and polarisation. PT In a system with many atoms, this process can be repeated, amplifying light, the laser's fundamental process. A laser beam is coherent, mono-color, and collimated N (extremely parallel rays). RATE EQUATIONS AND EINSTEIN COEFFICIENTS ▪ Consider an atomic system with two energy levels, E1 EL (ground) and E2 (excited). ▪ N1 and N2 are atoms or electrons per unit volume in E1 and E2. PT ▪ N total = N1 + N2 since it's a two-level system. When the atomic or electron system interacts with electromagnetic radiation with frequency υ12, where hυ12 = E2 − E1, some N atoms will transit from the ground state 1 to the excited state 2. RATE EQUATIONS AND EINSTEIN COEFFICIENTS EL ▪ Einstein assumed that the stimulated absorption rate from level 1 to level 2 is proportional to the radiation energy density ρυ(υ12) and the number of atoms or electrons in the ground state N1 with a proportionality constant B12. PT N (1) 𝑑𝑁1 The absorption rate is (-) where N1(t) decreases over time. 𝑑𝑡 As mentioned, spontaneous and stimulated emissions can cause atoms to decay from level E2 to E1 ▪ RATE EQUATIONS AND EINSTEIN COEFFICIENTS ▪ The spontaneous emission is proportional only to the number EL of atoms (N2) in the excited state. Therefore, the spontaneous emission rate can be written as: PT N (2) ▪ RATE EQUATIONS AND EINSTEIN COEFFICIENTS ▪ The stimulated emission rate is proportional to radiation density ρυ(υ12) and the number of exciting ground-state atoms N2 with EL constant B21. ▪ The subscript order for proportionality constants A and B is 21, indicating that spontaneous and stimulated emission transitions PT start from excited state 2 to ground state 1 (2 ➔ 1). ▪ The stimulated emission rate is: N (3) ▪ RATE EQUATIONS AND EINSTEIN COEFFICIENTS ▪ Einstein coefficients are B12, A21, and B21. EL ▪ Absorption, spontaneous emission, and stimulated emission all occur simultaneously when an atomic system interacts with light. ▪ The ground state and excited state population rates equal the sum of absorption, spontaneous, and stimulated emission rates. PT (4) N ▪ RATE EQUATIONS AND EINSTEIN COEFFICIENTS ▪ Einstein'scoefficients are related. In thermal equilibrium, populations 1 and 2 (N1 and N2) are constant. EL (5) PT Therefore, at thermal equilibrium, Eq. (4) becomes N (6) or (7) ▪ RATE EQUATIONS AND EINSTEIN COEFFICIENTS ▪ ρυ(υ12) is the radiation energy density per unit frequency interval, and according to Planck’s law, it is given by EL (8) ▪ Where ( PT σB = 1.38 × 10−23 J/K) is Boltzmann’s constant and T is thermal equilibrium temperature. ▪ The number of atoms or molecules in a j state with energy Ej at N thermal equilibrium is given by (9) where c∗ is a proportionality constant ▪ RATE EQUATIONS AND EINSTEIN COEFFICIENTS ▪ Consider the ratio of population levels 1 and 2 at thermal equilibrium at temperature T, described by a Boltzmann distribution EL PT (10) N ▪ Therefore, (11) ▪ RATE EQUATIONS AND EINSTEIN COEFFICIENTS ▪ If we now compare Eqs. (7) and (11), it derives as EL 𝐵12 = 𝐵21 (12) ▪ This result shows that the absorption and stimulated emission processes are equivalent. ▪ Moreover, we obtain that PT (13) N ▪ Asserting that absorption and spontaneous emission are proportional to each other THE TWO-LEVEL SYSTEM EL For stimulated emission to produce a laser beam or amplify light, the stimulated emission rate must be higher than spontaneous emission and absorption. PT This can be expressed by the following equation: (14) N Since B12=B21 (Eq. 12), (14 only holds if N2 > N1, meaning more atoms are in the excited state than the ground state. THE TWO-LEVEL SYSTEM EL Population inversion causes this. According to Eq. (10), At thermal equilibrium, the population density relation between levels 2 and 1 is given by the Boltzmann distribution, and N2 must be lower than N1 (N2 < N1) PT because hυ12/σBT is positive. In thermal equilibrium, population inversion cannot occur. N The rate equation for this type of system is given by (15)  THE TWO-LEVEL SYSTEM EL Since B12=B21, we omitted the subscripts for the Einstein coefficient B, and since the spontaneous emission takes place only from level 2 to 1, we can omit the subscripts. PT N Two-level system scheme E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177. THE TWO-LEVEL SYSTEM At t = 0, all atoms are in ground state 1, so N1 = Ntotal and N2 EL = 0. Rewriting (15) PT (16) N If in Eq. (16) we put t ➔ ∞, we obtain (17) THE TWO-LEVEL SYSTEM EL Since A > 0, from Eq. (17) for each given value of t, we obtain PT (18) At thermal equilibrium, the number of excited atoms or electrons will not exceed the number in the ground state, so population inversion N cannot occur in a two-level system.  THE THREE-LEVEL SYSTEM EL A three-level system, with level 1 representing the ground level and levels 2 and 3 representing excited states with energies E2 and E3, respectively. PT N In a three-level system, population inversion is possible. This system will lase when N3 > N2, due to a population inversion A three-level system scheme between excited states 2 and 3. E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177.  EL THE THREE-LEVEL SYSTEM Since Bij =Bji, B indicates absorption and stimulated emission between two PT states. Before atoms interact with radiation, N1(0) = Ntotal. When the system interacts with electromagnetic radiation with radiation energy density ρυ (υ31) where hυ31 = E3 − E1,, N some atoms transition from ground state 1 to excited state 3. Pump source radiation excites atoms. THE THREE-LEVEL SYSTEM EL Atoms in excited state 3 can relax by stimulated or spontaneous emission to level 1 or 2. Total atoms in a three-level system equal the sum of energy PT level populations; therefore: (19) N THE THREE-LEVEL SYSTEM EL Similar to the two-level system, rate equations can be written and solved for each of the population states 1, 2, and 3 due to pumping. Each level's population is constant at equilibrium. PT N THE THREE-LEVEL SYSTEM EL ▪ Consider state 2's rate equation. Population N2 is given by spontaneous emission from state 3 to 2 and state 2 to 1, stimulated emission from state 3 to 2, and absorption from state 2 to 3. ▪ PT State 2's rate equation when equilibrium is maintained is: N (20) (21) THE THREE-LEVEL SYSTEM EL ▪ where it can be rewritten as (22) ▪ PT When A21 > A32 then N3 > N2, atoms in state 2 decay to state 1 N faster than atoms in state 3 decay to state 2. This system can now lase after a population inversion between states 3 and 2. Three- level (or more than two-level) systems gain medium.  THE FOUR-LEVEL SYSTEM EL The ground level has an energy equal to E1 and is indicated 1. Levels 2, 3, and 4 are excited states, and their energies are E2, E3, and E4, respectively. PT N Scheme of a four-level system E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177. THE FOUR-LEVEL SYSTEM EL The ground level has an energy equal to E1 and is indicated 1. Levels 2, 3, and 4 are excited states, and their energies are E2, E3, and E4, respectively. PT When level 1 atoms are excited to level 4, they relax to level 3, a metastable level with a long lifetime. The transition from level 3 to level 2 causes lasing in a carefully N designed cavity during population inversion. THE FOUR-LEVEL SYSTEM EL To maintain lasing (population inversion), level 2's lifetime must be short so atoms can quickly relax to the ground state and be pumped to level 4. PT Level 2 must be above level 1 and unpopulated at normal temperature, which is sometimes accomplished by lowering the system temperature. N  ▪ The main component of Laser system are Gain medium, pumping sources, and optical resonance systems. EL PT N Main component of Laser system ▪ Active medium or gain medium and mirrors that reflect emitted radiation from the laser cavity. To excite the gain medium's atoms or molecules, a pump is needed. E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177. ▪ GAIN MEDIUM EL Gain medium (also known as active laser medium) is made of atoms, ions, molecules, or electrons. Depending on the gain medium, lasers can have different PT wavelengths. Solid, liquid, gas, dye, or semiconductor are gain mediums. N The first laser was a solid ruby (Al2O3) with Cr3+ ions replacing Al3+ ions. Lasing ions can achieve population inversion in Al2O3 due to their electronic energy levels. PUMPING SOURCE Pumping source excites gain medium atoms from the EL ground state to upper excited states, creating the population inversion necessary for lasing. The gain medium can be optically or electrically excited. PT First, the active laser medium is excited by a high- intensity light source, such as flash lamps and lasers. N The laser medium's excitation by an intense electrical discharge (i.e. nonradiative) is known as the electrical excitation approach. Gas lasers pump this way. RESONANT OPTICAL CAVITY Once the population inversion in the gain medium is EL reached, light intensity is amplified by forcing stimulated photons back and forth across it. Laser medium is placed in a resonator cavity to amplify light. PT The cavity's two parallel mirrors direct light through the laser medium back and forth. N One mirror is 100% reflective, while the other is less than 100% reflective, so the amplified light wave can be removed as an output beam.  Lasers are categorized as continuous waves (CW) and EL pulsed waves (PW). First-generation lasers produce a continuous beam whose characteristics are determined by the gain medium. wavelength. PT First CW laser was a helium–neon laser with 1153 nm N Since then, gas lasers, solid-state lasers, and dye lasers have been developed. Different types of PW lasers are available depending on pulse duration, energy, repetition rate, and wavelength. Q-switching and mode-locking generate nanosecond and femtosecond pulses, respectively. ▪ Lasers can be classified by different parameters, but most are categorized by the active medium. EL ▪ Lasers can be grouped as follows: 1. Solid-State Laser 2. 3. Gas Lasers PT Liquid Dye Lasers N 4. Semiconductor Diode Lasers 5. Fiber Lasers ▪ These lasers can produce coherent light at different wavelengths, and the output beam can be continuous (CW) or pulsed. SOLID-STATE LASERS: In such lasers, the gain medium is solid at room EL temperature. In 1960, a solid-state ruby laser (Cr3+- Al2O3) was invented. PT The first laser used Cr3+ impurities. Most solid-state lasers embed a dopant in a host material. N Neodymium (Nd3+) is a common dopant in commercial lasers, while yttrium orthovanadate (YVO4), and yttrium lithium fluoride (YLF), and yttrium aluminum garnet are popular host materials (YAG).  SOLID-STATE LASERS: EL PT N Solid-state Laser scheme E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/10.1201/9781420039177. ▪ SOLID-STATE LASERS: EL Solid-state lasers operate at different wavelengths depending on dopant and host materials. Doped YAG and YLF produce laser beams with 1064.1 PT and 1054.3 nm wavelengths. Ti3+ dopant ions in Al2O3 produce a 780 nm laser, while the ruby laser is 694.3 nm. N Nd:YAG SOLID-STATE LASER: Neodymium (Nd) and CO2 lasers are the most common in AM. EL We'll discuss the current trend later. This solid-state laser uses Nd-doped host crystals as gain media. PT Host materials include yttrium aluminum garnet (YAG, Y3Al5O12) and glass. Host crystal determines laser emission wavelengths. N Nd: YAG crystal output wavelength is 1064 nm, while Nd: glass is 1054-1062 nm. These lasers can be continuous or pulsed, with a few kW or 20 kW peak power. Nd:YAG Solid-State Laser: The laser medium in Nd: YAG lasers are pumped by EL krypton or xenon flashlamps; however, some systems use diode lasers. These lasers can be continuous or pulsed, with a few kW PT or 20 kW peak power. The Nd laser has four energy levels, E1, E2, E3, and E4, where E1 is the ground state and E4 is the highest. N Nd:YAG SOLID-STATE LASER: When the flashlamp excites Nd ions, electrons reach the EL highest energy level (E4), which has a 250 μs lifetime. Electrons decay nonradiative to metastable energy level E3 with a longer lifetime than E4. Therefore, Population PT inversion results. The electron from E3 will decay after some time in the lower energy level E2 through spontaneous emission, N emitting a photon of wavelength 1064 nm. E2 has a short lifetime, and electrons quickly decay nonradiative to E1. ▪ Nd:YAG SOLID-STATE LASER: EL PT N Energy-level diagram for Nd3+ doped in YAG W. T. Silfvast, Laser fundamentals, 2nd edition, Cambridge University Press, Cambridge, MA, 2004. DISK LASER: Disk lasers or active mirrors are solid-state lasers with a EL heat sink and a laser beam released on the other side of an active gain medium. The beam is not necessarily circular. Optically pumped PT semiconductors are active media. The disc lasers are very efficient and small, but the power is limited because we must transfer the heat out N of the setup. Disk lasers are popular in AM, especially LDED, due to their high wall-plug efficiency. GAS LASERS : In gas lasers, the gain medium is gas or a gas mixture and EL depending on the gas used, they can be divided into atomic and molecular gas lasers. Helium-neon and argon–ion lasers are atomic gas lasers. PT CO2 and excimer lasers are second-class gas lasers. In gas lasers, low-pressure gas is contained in a cylinder N with two electrodes. An electric discharge inside the gas tube creates a current that ionizes gas atoms to form free electrons that travel from the cathode to the anode. After colliding with free electrons, some atoms decay to lower energy levels. When laser theory conditions are met, a population inversion causes lasing. CO2 LASERS : Carbon dioxide lasers operate at 11 μm and 9 μm wavelengths EL with 100 kW or 10 kJ CW or pulsed powers. Most AM waves are 10.6 μm. PT The molecular gas laser's laser structures are longitudinally excited, waveguide, and transversely excited. N In longitudinally excited lasers, the gaseous laser medium is enclosed in long narrow cylinder glass.  CO2 LASERS : EL PT N Longitudinally excited CO2 Laser E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177.  CO2 LASERS : EL PT N Transversely excited CO2 laser E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177. CO2 LASERS : EL The electrical discharge current is applied through the glass tube's electrodes. In sealed laser systems, the tube must be changed breakdown. PT periodically to prevent electrode corrosion from CO2 Recirculating gas through a tube preserves gas in other N systems. Waveguide lasers with a waveguide gain medium can poduce CW CO2 lasers. CO2 LASERS : In this type of laser, the bore region is between radio EL frequency (RF) electrodes. Connecting the electrodes to an 80–100 MHz RF power supply creates a high-frequency alternating field. PT Small bore dimensions ensure efficient laser gas cooling and high-pressure operation, leading to high gain and 100 kW power output. N Transversely excited lasers use gas at 1 atm or higher and parallel electrodes a few centimeters apart. CO2 LASERS : Pre-ionizing the gas between the electrodes ensures a EL uniform discharge when high voltage is applied. This configuration has been used in some excimer lasers to produce high-energy pulsed lasers. PT These lasers have the same excitation mechanism. In these systems, the laser gas is a 0.8:1 mixture of CO2 N and N2. CO2 LASERS : N2 boosts laser efficiency 30%. The excited energy EL level of nitrogen is close to the vibrational level of CO2. The N2 excited level is metastable to radiative decay, PT and collisions transfer energy to the CO2 vibrational level (0, 0, 1), causing population inversion. The possible laser transitions in CO2 lasers are of two N types: (0, 0, 1) ➔ (1,0,0) and (0, 0, 1) ➔ (0,2,0). These transitions emit 10.6 and 9.4 m laser beams.  LIQUID DYE LASERS EL Organic dye lasers and dye lasers use systems in which the lasing medium is a liquid at room temperature. PT N Liquid dye laser schematic E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177. LIQUID DYE LASERS EL Typically, the gain medium is an organic dye solution with strong absorption and emission properties. According to the type of dye used, a wide range of wavelengths PT can be achieved. Using various dyes sequentially, a tunable laser output extending from 320 to 1200 nm can be created. N LIQUID DYE LASERS EL Such lasers need a high-voltage, low-current power source as well as a sizable storage capacitor. A frequent dye used in this kind of laser is rhodamine B. PT Dye lasers can be pumped by other lasers and come in pulsed, continuous, and mode-locked forms. N Medical and material texturing are two fields in which dye lasers are used.  SEMICONDUCTOR DIODE LASERS EL PT N Diode laser scheme E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/10.1201/9781420039177. SEMICONDUCTOR DIODE LASERS EL Diode lasers are small, efficient, and require less power than other lasers. In this type of laser, gallium arsenide or semiconductor chips are used. PT These lasers can be electrically or optically pumped, and the lasing medium is a p–n junction made of III–V and II–VI semiconductor compounds. N When current flows through a p–n junction, electrons are excited to a higher conduction band state, then decay to a lower state. SEMICONDUCTOR DIODE LASERS EL The valence band holes move up at the same time. The recombined electrons and holes emit photons near the material's bandgap. PT The process can be spontaneous or stimulated, leading to optical amplification. The beam is not round. This laser can achieve up to 40 half-angle beam divergence. N Nonsymmetric beam distribution is possible. Low energy per area is a feature of these lasers. Depending on the material, energy bandgaps and emission wavelengths can vary. FIBER OPTIC LASERS This type of laser uses optical fiber doped with rare-earth EL ions, such as erbium, neodymium, germanium, or ytterbium, and continuous semiconductor diode lasers to pump light to the core section. PT This laser creates short pulses. Fiber lasers use the fiber as a resonant cavity to create a beam. A 2 kW CW fiber laser with a 50 μm spot and 100 MW/cm2 power density. N Small dimensions and scalable power make this fiber laser a suitable replacement for solid-state and molecular lasers, especially for industrial processes. FIBER OPTIC LASERS EL PT N Scheme of a typical fiber laser E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177. Fiber Optic Lasers Due to different indices of refraction for the cladding and core EL segments, a laser beam pumped into the cladding will bounce and rebound inside the core. through. PT The core absorbs pumping light each time the beam passes Two fiber Bragg gratings (FBG) act as wavelength mirrors in N fiber lasers. The atom levels of earth elements have extremely effective energy levels, allowing the use of an inexpensive diode-laser pump source to provide high output energy. It can also use multiple pumping fibers connected to a coupler. Fiber Optic Lasers EL PT N Schematic of fiber lasers that include FBGs and beam coupler Source: Redrawn and adapted from general concept available on the internet. Yb- and Er-FIBER LASER EL Most industries use ytterbium and erbium fibre lasers. Single- mode Ytterbium lasers are popular in AM. PT N Energy-level diagram of the erbium-doped fiber. W. T. Silfvast, Laser fundamentals, 2nd edition, Cambridge University Press, Cambridge, MA, 2004. Yb- and Er-FIBER LASER EL Single-mode lasers distribute energy perfectly Gaussian. In fiber lasers, the gain medium is a fiber optic connected to a pump source and an optical resonator. PT A CW diode laser pumps the fiber and amplifies the pulses passing through the system. N This type of fiber laser's excitation mechanism is explained by its energy-level diagram. Yb- and ER-FIBER LASER Erbium-doped fiber laser is a three-level laser; optical EL pumping excites electrons from the ground state (4I15/2) to the excited state 4I11/2 at 908 nm. PT Nonradiative electron decay to upper laser level 4I13/2 causes population inversion between N 4I13/2 and ground state (4I15/2). Photons with a 1550 nm wavelength are emitted and amplified in this condition. W. T. Silfvast, Laser fundamentals, 2nd edition, Cambridge University Press, Cambridge, MA, 2004.  EL PT N Laser employed in laser-based AM processes (i.e., laser powder bed fusion [LPBF] and laser directed energy deposition [LDED]) D. Gu, Laser additive manufacturing of high-performance materials, no. Lm, Springer Berlin Heidelberg, Berlin, Heidelberg, 2015.  A laser beam is coherent, mono-color, and collimated (extremely parallel rays). EL Laser medium is placed in a cavity or resonator with two parallel mirrors at each end to amplify light. PT Presence of these mirrors creates transverse modes superimposed on the beam, known as transverse electromagnetic modes (TEM). Gaussian–Laguerre modes are N TEMpl in a laser. p and l indicate the number of zero-intensity nodes transverse to the beam axis in the radial and tangential directions, respectively.  For TEM00 , p and l are zero, representing the lowest order with a Gaussian beam shape. Different TEM modes have different energies and patterns. EL Laguerre equation describes the intensity distribution of a TEMpl at a point (r, φ) (in polar coordinates) from the mode Centre. PT N where I0 is the intensity scale factor (W/m2), rl is the radius of 𝑝 the laser beam profile, M2 is the beam quality factor, and 𝐿𝑙 is the associated Laguerre polynomial of order p and index l.  I0 is presented by: EL PT N Pl is laser net power. Laguerre polynomials can be expressed by one of two equations based on indices.  EL PT N Mode patterns for different TEMs. E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177.  EL PT N Mode patterns for different TEMs. E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/ 10.1201/9781420039177.  The radius of the laser beam, rl , depends on the propagation axis and can be expressed as EL PT where the waist's beam radius is r20𝑙 , its location along the propagation axis is z0, and the far-field divergence angle is θ. N The beam quality factor M2, or the beam propagation factor Q, describes the propagation, and they are related as  The medium's laser wavelength λ and the reflection index is n. Define k as EL If k= M2 = 1, the beam is Gaussian, whereas it is not Gaussian if M2 > PT 1. The larger the M2, the beam shape will become similar to a flat hat. N There is a “depth of focus "parameter in a parabolic beam profile, which refers to a segment of the beam that can be assumed to have a cylindrical shape. The depth of focus for a 28° angle of divergence is reported to be approximately 1.05r0l EL PT N Laser beam profile E. Toyserkani, A. Khajepour, and S. Corbin, Laser cladding, 1st edition, CRC Press, 2004. https://doi.org/10.1201/9781420039177. ▪ Sub systems used in metal AM EL ▪ Types of laser and its components. ▪ Difference between continuous and pulsed laser PT ▪ Properties and characteristics of the laser beam ▪ Important Parameters Needed to be Known for AM N 1. Make a list of different types of laser used in metal additive manufacturing EL 2. Google and find post-processing done in metal additive manufacturing with the help of lasers. PT N N PT EL  EL PT N Dr. J. Ramkumar Professor Department of Mechanical Engineering and Design IIT Kanpur ▪ Basics of electron beam EL ▪ Electron beam powder bed fusion ▪ Electron beam mechanism PT N ▪ In addition to the laser beam, the electron beam (EB or EBM) is a typical heat source for thermal metal powder bed additive EL manufacturing. ▪ EB, a collimated stream of free electrons, is a helpful heating PT and welding technique. ▪ EBs are used in micro- and nanoscopy, machining, and spectroscopy. N ▪ With enough power, EB can be used for cutting, machining, and welding.  EL ▪ EBs are very coherent and have low numerical intensity sources of high kinetic energy, while lasers have high numerical intensity sources of low energy photons. PT ▪ Both energy sources are collimated and can be concentrated on a tiny region while achieving great thermal intensity. These considerations make fine-featured items manufacturable. N ▪ The difference between these energy sources is heat transfer to the powder bed. ▪ Lasers heat by absorbing atomic photons. Heat transfer depends on laser color spectrum and material electrical structure. ▪ Laser absorption depends on feed material finish and substrate EL finish since reflection limits available energy. ▪ EB transports heat by inelastic electron-substrate collisions. Electrons are accelerated to high kinetic energies to impact the PT substance and embed themselves, delaying the process. ▪ Material finish and electronic structure don't affect heat transfer efficiency, unlike laser beams. N ▪ Magnetic fields can move EB at (>>1 km/s). The scanner's mechanical limits limit laser beam speed to 5–7 m/s. N PT EL  EL ▪ EPBF (electron beam powder bed fusion) is also known as EBM. ▪ It is similar to LPBF but uses an electron beam as energy. PT N Schematic of a typical “Arcam.”[Online]. Available: http://www.arcam.com/technology/electron-beam-melting/hardware. EBM apparatus. ▪ EBM melts powder using a scanning high-energy electron EL beam cannon. ▪ EBM methods have greater resolution, surface roughness, and layer thickness than LPBF. ▪ PT Larger melt pools create a larger heat-affected zone. The kinetic energy of the beam electrons heats the metal powder, which accumulates a negative charge as it absorbs N the incoming electrons. ▪ Because negatively charged particles repel one other, this may EL cause powder bed expulsion. ▪ This causes a powder cloud and/or a more diffuse beam, which increases the melt pool. ▪ PT Electron beam guns are more effective than lasers because the bulk of the electrical energy is converted into a beam, while the rest is dissipated as heat. N  ▪ Under the influence of electric and magnetic fields, free EL electrons in a vacuum can be accelerated, creating high- energy, narrow electron beams. ▪ The conversion of this kinetic energy into thermal energy is the fundamental principle of an electron beam. PT N Electron beam formation schematic. “World Scientific.”[Online]. Available: http://www.worldscientific.com/worldscibooks/10.1142/7745 ▪ When electrons collide in a vacuum, their kinetic energy is converted to heat. When electron beams accelerate and collide EL with powerful electric fields and magnetic lenses, their power densities can reach up to 106 W/mm2 and the localized temperatures can rise at a rate of up to 109 K/s. ▪ ▪ Power supply PT Basic required equipment to generate an electron beam N ▪ Electron gun/source ▪ An anode ▪ Magnetic lenses ▪ Electromagnetic lenses and deflection coils ▪ A power source is needed for an electron beam. Low- or EL high-voltage equipment from 5 to 30 to 70–150 kV is available depending on the process. ▪ The electron gun's cathode is heated by a low-voltage ▪ PT filament, evaporating electrons from it. As a cathode, tungsten is preferred. Evaporated electrons are accelerated toward the positively N charged anode and placed after the electron gun.  An anode aperture creates an electron jet that moves toward magnetic lenses. EL These components block divergent electrons and provide a strong electron beam. PT The so-formed beam is focused and deflected by electromagnetic lenses and deflection coils to precisely position the part to be printed. N ELECTRON BEAM SOURCES: EL ▪ Gun electrode type was used for AM. Three types are common: Tungsten (W) filament, Lanthanum Hexaboride (LaB6), and field-emission gun (FEG) PT N Gun electrode types: (a) Tungsten (W) filament, (b) Lanthanum Hexaboride (LaB6), and (c) field-emission gun (FEG). ELECTRON BEAM OPTICS AND POSITIONING EL Electrons accelerated by beam power lack focus and control. Focusing, shaping, and positioning the beam provides the required control. ELECTROMAGNETIC LENS ▪ PT EBs are focused using a lens, albeit an electromagnetic one made by magnetic fields, much like optical devices. N ▪ The lens is constructed by winding wires around a coil, much like a solenoid. on the other hand, is wound across a very short length, making the magnetic field less dense towards the center of the lens than a solenoid, which generates a uniform magnetic field. ELECTROMAGNETIC LENS EL ▪ An electron will initially encounter a clockwise force around the entrance as it travels through the lens. The force starts to point in the direction of the opening’s center as soon as the electron gains some velocity in this ▪ direction. PT The windings and other components must be built with great radial symmetry to prevent defects in the lens's N focusing abilities. ▪ An electromagnetic lens' generated picture is rotated and inverted by the force acting clockwise. Nevertheless, because the spot form is typically circular, this is not an issue for the majority of heating applications.  EL PT N Electromagnetic Lens V. Adam, U. Clauß, D. Dobeneck, T. Krüssel, and T. Löwer, Electron beam welding –The fundamentals of a fascinating technology. pro-beam AG & Co. KGaA, 2011. ELECTRON BEAM OPTICS AND POSITIONING EL STIGMATORS ▪ Due to the electromagnetic lens's non-uniformity and the filament's geometry, the beam may initially be elliptical ▪ PT rather than circular. Astigmatism is the term for this condition, which stigmators can treat. Four solenoids with magnetic quadruple make up a stigmator, which is a device. According to which quadrant of N the quadruple field an electron is in, it will either experience force toward or away from the center of the field. ▪ The beam can be made to be more or less elliptic by varying the strength of the current in the opposing pairs of solenoids. ▪ A well-collimated EB that can focus down to tiny spots EL Electrons scatter when they hit gas molecules in a fluid medium. ▪ ▪ PT This scattering reduces beam energy at the heating location and increases spot size, reducing beam intensity. EBs must operate in a vacuum for these reasons. N ▪ Basics of electron beam and its mechanism EL ▪ Similarities and differences between electron beam and laser beam PT N N PT EL