PAPER CODE NO. MECH607 EXAMINER: Dr G Dearden DEPARTMENT: Engineering TEL No: 0151 794 4584 SUMMER 2014 EXAMINATIONS ADVANCED MANUFACTURING WITH LASERS TIME ALLOWED: Three Hours INSTRUCTIONS TO CANDIDATES FOUR questions to be answered. All questions carry equal weight PAPER CODE MECH 607 Page 1 of 4 Continued/ 1. a) Give a concise account of Richard Feynman’s ideas on microprocessing as set out in his lecture ‘There’s Plenty of Room At the Bottom – An Invitation to Enter a New Field of Physics’. [10 marks] b) Discuss how scaling affects the functionality of small components, giving as examples: i. a column supporting a weight ii. the effect of surface area/volume ratio [10 marks] 2. Discuss the principles of, and recent developments in, the following laser micro joining processes. Use sketches to illustrate your answer. a) Laser micro welding (various techniques of) [14 Marks] b) Selective laser soldering [6 Marks] PAPER CODE MECH 607 Page 2 of 4 Continued/ 3. a) Briefly outline the attributes of pulsed lasers for micro cutting applications and describe how these are used in the manufacture of medical ‘stent’ devices. [6 marks] b) Briefly describe with the use of sketches how lasers are used for trimming electronic printed resistors. [4 marks] c) Give a detailed description of the Laser Micro-Jet process, which involves a laser beam guided by a fine water jet, and how this is being used in the electronics industry. Use sketches to illustrate your answer. [10 marks] 4. a) Briefly describe, with the aid of sketches, how the Temperature Gradient Mechanism (TGM) operates in two-dimensional (2D) laser forming. What are the key advantages and disadvantages of TGM compared to the buckling and upsetting mechanisms? [10 marks] b) Sketch a graph to illustrate how, in 2D laser forming by the TGM, the bend angle varies with the number of irradiation passes and briefly discuss the theories that have been researched and put forward to explain why such a characteristic variation is obtained. Use further sketches to illustrate your discussion, where appropriate. [6 marks] c) What factors can lead to both distortion from design shape and errors in degree of deformation when laser forming a 3-dimensional part from flat sheet? [4 marks] PAPER CODE MECH 607 Page 3 of 4 Continued/ 5. a) Describe using sketches the principles and balance of forces for optical trapping with a laser under the following conditions. i) The wavelength of the laser light is much less than the size of the trapped particle ii) The wavelength of the laser light is larger than the size of the trapped particle [10 Marks] b) Describe in detail the methods for the accurate measurement of very small forces that can be realised by optically trapped objects. [10 Marks] 6. a) In laser micromachining, given that thermal effects decrease as pulse duration is decreased, discuss and compare the absorption mechanisms and process results for 'long pulse' and 'Ultra-Short pulse' laser ablation. [10 marks] b) Spatial Light Modulators (SLMs) can be employed for advanced control of the wavefront and polarisation state of a laser beam. Describe, with the aid of sketches, the principle, structure and operation of an SLM based on Liquid Crystal on Silicon (LCoS) technology to produce multiple laser beams for parallel processing in UltraShort Pulsed laser micromachining. [10 Marks] PAPER CODE MECH 607 Page 4 of 4 End PAPER CODE NO. MECH607 EXAMINER: Dr G Dearden DEPARTMENT: Engineering TEL No: 0151 794 4584 SUMMER 2015 EXAMINATIONS ADVANCED MANUFACTURING WITH LASERS TIME ALLOWED: Three Hours INSTRUCTIONS TO CANDIDATES FOUR questions to be answered. All questions carry equal weight PAPER CODE MECH 607 Page 1 of 4 Continued/ 1. a) Briefly describe the five commonly used definitions for the measured beam diameter of a so called real laser beam. [5 marks] b) Which beam diameter definition is used as the international standard in ISO11146 and how are other definitions accounted for? [2 marks] c) A 1.5kW CO2 laser (wavelength 10.6 µm) with an M2 of 3 has a collimated measured beam diameter of 20mm incident on a 50mm diameter ZnSe processing lens of focal length 127mm. What is the minimum possible focussed spot diameter and depth of focus? [8 marks] d) For the conditions in c) and without replacing any of the components, how could the minimum beam spot diameter be decreased to 100μm using additional standard optical components? Illustrate your answer with a worked numerical example deriving the required optical properties of these additional components in terms of focal length, separation and outer diameter. [5 marks] 2. a) Briefly describe, with the aid of sketches, how the Temperature Gradient Mechanism (TGM) operates in two-dimensional (2D) laser forming. What are the key advantages and disadvantages of TGM compared to the buckling and upsetting mechanisms? [10 marks] b) Discuss in detail the dependency of 2D laser forming by TGM on the laser process parameters, the thermal properties and geometrical features of the work-piece material, and the effect of multiple irradiation scans. Uses sketches where appropriate to illustrate your answer. [10 Marks] PAPER CODE MECH 607 Page 2 of 4 Continued/ 3. a) In laser micromachining, given that thermal effects decrease as pulse duration is decreased, discuss and compare the absorption mechanisms and process results for 'long pulse' and 'Ultra-Short pulse' laser ablation. [10 marks] b) Spatial Light Modulators (SLMs) can be employed for advanced control of the wavefront and polarisation state of a laser beam. Describe, with the aid of sketches, the principle, structure and operation of an SLM based on Liquid Crystal on Silicon (LCoS) technology to produce multiple laser beams for parallel processing in UltraShort Pulsed laser micromachining. [10 Marks] 4. a) In terms of advanced manufacturing, what is direct writing? [5 marks] b) Compare laser based and non-laser based forms of direct writing by discussing relevant examples of each type. Include sketches to help illustrate your description of the examples. [8 marks] c) Give a brief description of the likely application areas of Direct Writing with examples of key components that could be produced. Include sketches to support your description of the cases. [7 marks] PAPER CODE MECH 607 Page 3 of 4 Continued/ 5. a) Give a concise account of Richard Feynman’s ideas on microprocessing as set out in his lecture ‘There’s Plenty of Room At the Bottom – An Invitation to Enter a New Field of Physics’. [10 marks] b) Discuss how scaling affects the functionality of small components, giving as examples: i. a column supporting a weight ii. the effect of surface area/volume ratio [10 marks] 6. a) Briefly outline the attributes of pulsed lasers for micro cutting applications and describe how these are used in the manufacture of medical ‘stent’ devices. [6 marks] b) Briefly describe with the use of sketches how lasers are used for trimming electronic printed resistors. [4 marks] c) For the case of micromachining with a near-Gaussian pulsed laser beam, write down the expression that describes in theory how the diameter of a single pulse ablation feature (crater) varies as a function of the beam waist radius, the peak fluence and the ablation threshold fluence. If a laser with a pulse energy of 100J can be focused to a spot size (twice the beam waist radius) of 30m, first calculate the peak fluence in Jcm-2 generated and then use this in the above expression to estimate the diameter of crater that would be produced by a single pulse on Ti-6Al-4Va (with an ablation threshold fluence 0.24 Jcm-2). [10 marks] PAPER CODE MECH 607 Page 4 of 4 End MECH607 School of Engineering 44584 Prof Geoff Dearden Second Semester Examinations 2015 Master of Engineering Yr 4 (MECH607) Master of Science (MECH607) ADVANCED MANUFACTURING WITH LASERS Time Allowed: THREE HOURS Instructions to candidates: Answer FOUR out of SIX Questions Page 1 of 4 MECH607 School of Engineering 44584 Prof Geoff Dearden 1. A carbon dioxide laser (wavelength 10.6 µm) has a beam quality factor of 5, a beam diameter of 50 mm and a plane wavefront when incident on a thin lens of focal length 300 mm. The maximum beam power is 5kW. For these conditions, use Gaussian beam propagation theory to calculate: a) The minimum focused spot diameter; [4 marks] b) The depth of focus; [4 marks] c) The effect on the depth of focus of changing the lens focal length to 125 mm, while keeping all other factors the same; [6 marks] d) The effect of using a lens focal length of either 300mm or 125mm on the feasibility of laser keyhole welding, if the threshold intensity required for this process is 2 x 105 W/mm2. [6 marks] 2. a) Briefly outline the various factors affecting light absorption in laser-material interaction. [5 marks] b) Describe the stages of the physical process leading to laser beam absorption and the secondary mechanisms leading to keyhole formation, for the case of thermal laser-material interaction. [10 marks] c) Draw a diagram to depict the major energy flows around the interaction zone during laser material processing and write down a simple energy balance relationship to account for all the energy flow mechanisms involved. [5 marks] Page 2 of 4 MECH607 School of Engineering 44584 Prof Geoff Dearden 3. A typical baseline capability for laser cutting in industry is the flat-bed profile cutting with CO2 laser and oxygen assist gas jet of 10-20mm thick mild steel at 2-3kW beam power and 50-100mms-1 cutting speed. Given this, describe in detail various ways in which it may be possible to extend the scales of the process from this baseline position. For each of the parameters that might be scaled, show how recent advances in laser cutting science and technology have already addressed some of these aspects. Use sketches to illustrate your answer. [20 Marks] 4. a) Describe in detail, in terms of basic physical mechanisms, the various stages of the thermal laser drilling process, using sketches to illustrate your answer. [10 Marks] b) Discuss the interaction phenomena that occur in Nd:YAG laser drilling, as the laser intensity is increased to greater than 108 W/cm2, and its effect on the drilling process. [5 Marks] c) For single-pulse drilling of metals with Copper Vapour Lasers, sketch and briefly discuss the typical effect of pulse power on ablation depth and material removal mechanism, using the case of aluminium as an example material. [5 Marks] Page 3 of 4 MECH607 School of Engineering 44584 Prof Geoff Dearden 5. a) Briefly describe, with the aid of sketches, how the Temperature Gradient Mechanism (TGM) operates in two-dimensional (2D) laser forming. What are the key advantages and disadvantages of the TGM compared to the buckling and upsetting mechanisms? [10 marks] b) Sketch a graph to illustrate how, in 2D laser forming by the TGM, the bend angle varies with the number of irradiation passes and briefly discuss the theories that have been researched and put forward to explain why such a characteristic variation is obtained. Use further sketches to illustrate your discussion, where appropriate. [6 marks] c) What factors can lead to both distortion from design shape and errors in degree of deformation when laser forming a 3-dimensional part from flat sheet? [4 marks] 6. Discuss the principles of, and recent developments in, the following laser micro joining processes. Use sketches to illustrate your answer. a) Laser micro welding; [13 Marks] b) Selective laser soldering. [7 Marks] Page 4 of 4 MECH 607 Advanced Manufacturing with Lasers Week 2 Lecture 2 Dr Stuart Edwardson Prof. Geoff Dearden Mechanical, Materials & Aerospace Engineering School of Engineering The University of Liverpool Liverpool, UK www.lasers.org.uk MECH607 – Advanced Manufacturing with Lasers Lecture Moving • Due to clash, Friday Lecture moving to Tuesdays 10am in ELEC202 (E5), 2nd floor Electrical Engineering Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Course Overview Week Date ( WB) Associated Lab (TBC) Lecture Topic 1 30/01/2017 Introduction to Lasers for Manufacturing \ Safe use of Lasers 2 06/02/2017 Optical components for laser engineering \ Laser beam Characteristics 3 13/02/2017 Laser Materials Interaction 4 20/02/2017 Advanced Laser Cutting 5 27/02/2017 Advanced laser based joining techniques 6 06/03/2017 Laser Forming 7 13/03/2017 Laser Drilling 8 20/03/2017 Fabrication at the micro scale 9 27/03/2017 Laser Micro Cutting & Laser Micromachining Easter Break 10 24/04/2017 Advanced Ultra-Short pulsed laser processing 11 01/05/2017 Laser based Direct Write (DW) 12 08/05/2017 Course Review SE GD Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Coherence • Of the various components that make up a propagating lightwave from a source, the relative phases of the waves must remain constant over a period of time for the source to be considered Coherent. • Most light sources maintain the same phase for only a few oscillations, after which the phase may skip randomly. • The distance between these random skips is called the coherence length Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Coherence • Most laboratory light sources have coherence lengths of only a few cm. • Lasers have much longer coherence lengths than most other light sources, ranging from 10m to 100km depending on the laser design. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Coherence - Interferometry • The Michelson interferometer • This produces interference fringes by splitting a beam of monochromatic light, so that one beam strikes a fixed mirror and the other a movable mirror. • When the reflected beams are brought back together, an interference pattern results. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Interferometry Einstein's gravitational waves 'seen' from black holes http://www.bbc.co.uk/news/scie nce-environment-35524440 Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Laser Beam Mode Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Laser Beam Mode • As the laser cavity is an optical oscillator. When it is oscillating there will be resonant (harmonic) standing electromagnetic wave set up within the cavity and defined by the cavity geometry. • Due to constructive/deconstructive interference, the ratio of the length of the cavity to the width of the output aperture determines the number of off-axis directions or modes which fit an exact number of half wavelengths between the two mirrors. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Laser Beam Mode • The off-axis modes describe the transverse electromagnetic mode (TEM) structure of the power distribution across the beam. • Essentially a standing wave across the beam formed from the interference between the various longitudinal standing waves. Transverse Longitudinal Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Laser Beam Mode • The classification of these Transverse Electromagnetic Mode (TEM) patterns is by: TEMplq • Where p is the number of radial zero fields, l is the number of angular zero fields and q is the number of longitudinal zero fields (q not always considered). Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Laser Beam Mode TEMplq • TEM00 is considered a perfect Gaussian beam mode • TEM01 mode is made from an oscillation between two orthogonal TEM01 modes Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Optics Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Properties of a Gaussian beam • The idealised form of the beam emerging from a laser cavity is a Gaussian (or TEM00 mode) beam. • Understanding the properties of a Gaussian beam is the starting point to the understanding of more complex, real beams which can be analysed in comparison with the Gaussian form. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Properties of a Gaussian beam • The boundaries of optical beams are not dearly defined and, in theory at least extend to infinity. • Consequently, the dimensions of a beam cannot be defined as easily as the dimensions of hard physical objects. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Properties of a Gaussian beam • The commonly used definition of beam width is the width at which the beam intensity has fallen to 1/e2 (13.5%) of its peak value when measured in a plane that is orthogonal to the optical axis. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Real Beams • Many lasers, however exhibit a significant amount of beam structure and are non-circular, and applying this simple definition leads to problems. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Real Beams – Diameter Definitions • Five definitions of the beam width/diameter are in common use: • 1/e2 width – as per the Gaussian description • Full width at half maximum (FWHM)- diameter obtained is the full width of the beam at half its maximum intensity. • D4σ or second moment width - the diameter that is 4 times the standard deviation σ of the horizontal or vertical distribution • Moving knife-edge/variable slit width – region containing 90% of power of the clipped beam. • D86 width - the diameter of the circle that is centred at the centroid of the beam profile and contains 86% of the beam power. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Real Beams – ISO Standard • It is crucial for researchers, system designers and laser manufacturers to be able to measure accurately and consistently parameters that define a laser beam. • ISO standard 11146 defines approaches to be used in measuring such real beams. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Real Beams – Diameter • The ISO 11146 standard specifies the beam width as the 1/e2 point of the second moment of intensity (D4σ method) around a calculated centroid. • This value is calculated from the raw intensity data and which reduces to the common definition for a Gaussian beam. • Can account for non-circular beams with structure Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Real Beams – Defined • ISO 11146 • If measurement equipment with a sufficiently high signalto-noise ratio and a simultaneously high spatial resolution is not available, alternative methods, may be used. • Correction factor can be used between the beam propagation ratios determined using one of the alternative methods, and the results of the standard ISO11146 method Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Real Beams – Defined • Typical beam profiling techniques: • • • • • Camera-Based Profilers Scanning Pinhole Profilers Scanning Slit Profilers Scanning Knife-Edge Profilers Tomographic Scanning – Any of the above combined with automated multi-slice and/or angle scanning Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • • • • • All laser beams will diverge eventually Depends on cavity design, gain medium and power level Waist can either be in the cavity or outside The better the laser the lower the divergence This can be used as a laser quality measure Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • A laser (with divergence) can be focused to a beam waist using a lens of a given focal length, the beam then diverges again. • The size of the waist/diameter is critical in laser material processing Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • A perfect Gaussian (TEM00) beam of finite diameter D focused by a thin lens of focal length f will propagate to a diffraction limited minimum beam diameter. f 2 w0  d min  2.44 D • The beam will then diverge with a divergence angle R Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • A non-Gaussian higher order real beam (mixed mode) of the same initial diameter will be focused to a larger minimum spot size and will have a larger divergence angle act Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • The beam quality factor M2 can be defined as the ratio of the divergence of a real beam act compared to that of an ideal Gaussian beam R with the same input diameter.  act M  R 2 Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • The M2 propagation factor is an invariant that describes the relationship of a non-Gaussian beam to a Gaussian beam as it passes through an optical system.  act M  R 2 Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • M2 can be 1 (perfect beam) or greater. It is always greater than 1 for a non-Gaussian higher order beam.  act M  R 2 Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • Knowing how a real beam propagates through a system with aberration-free optics when compared to a Gaussian beam allows the calculation of spot size and depth of focus. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • Minimum focused spot diameter dmin by a thin lens with a focal length f, wavelength λ and input beam size DL for a known M2 is given by: d min 4 M 2 f  2 w0  D0  DL Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • Question d min 4 M 2 f  DL • A CO2 laser (10.6µm) has a TEM00 beam with an M2 of 1 has a collimated beam diameter of 15mm incident on a ZnSe focussing optic of focal length 127mm. What is the minimum possible focussed spot diameter? • What is dmin for: f= 100mm and 300mm? Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality d min 4 M 2 f  DL • Answer • dmin (127mm)=(4*1*10.6x10-6*127x10-3)/3.141*15x10-3) • = 114μm • dmin (100mm)= 90μm • dmin (300mm)= 270μm • Shorter the focal length the smaller the spot size Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • Question d min 4 M 2 f  DL • A CO2 laser has a TEM01 beam with an M2 of 2.5 has a collimated beam diameter of 15mm incident on a ZnSe focussing optic of focal length 127mm. What is the minimum possible focussed spot diameter? • What is dmin for: f= 100mm and 300mm? Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality d min 4 M 2 f  DL • Answer for M2 = 2.5 • dmin (127mm)= (4*2.5*10.6x10-6*127x10-3)/3.141*15x10-3) • = 286μm • dmin (100mm)= 225μm • dmin (300mm)= 675μm • Note: 2.5 times the previous result, the beam is 2.5 times less like a Gaussian beam Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • If this were produced by a 1500W laser, would the power density (@100mmFL) be sufficient for laser keyhole welding (105 W/mm2 is required)? – Power density = Power (P)/ area  P w0 2  1500 / 3.142.[0.113]2  1500 / 3.142.[0.0127] = 3.77x104 W/mm2 – This is outside the range to initiate keyhole welding. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • Question d min 4 M 2 f  DL • How could dmin be reduced to increase energy density and reduced processed feature size? • note: without changing laser (M2) and focusing optic (f) Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality d min 4 M 2 f  DL Expansion factor = fo/|fe| Separation ~ fo + fe OR ~ fo - fe • Answer: • Expanding the beam before the focusing optic using a Beam Expander or Collimator would increase DL and hence reduce dmin • Question: • What are the limitations? • What beam expansion factor would be required to give a focused spot size less than 100µm and what lens combination would achieve this? Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality d min 4 M 2 f  DL Expansion factor = fo/|fe| Separation ~ fo + fe OR ~ fo - fe • Answer • There is an upper limited to how large DL can be made due to size of focusing optic, normally DL  input aperture • DL (100µm dmin)= (4*2.5*10.6x10-6*127x10-3) /3.141*100x10-6) • = 42.8mm (or greater) • Required Expansion factor = 42.8/15 = 2.85 • Possible lens focal length selection – fe -25mm fo 70mm (2.8 expansion factor) Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • The Depth of Focus (DOF or zf), the distance over which the focused beam has approximately the same intensity, where the beam diameter D0 (dmin or df) grows by 5% either side of the focus. 2 2 M  f z F  2.56 F 2 M 2   2.56 2 DL • Where F is the F number of the system equal to f/D Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • Question M 2 f 2 z F  2.56 2 DL • For the conditions earlier (M2 = 2.5, λ=10.6x10-6m), what is the depth of focus (DOF) for f= 100mm, 127mm and 300mm? Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality M 2 f 2 z F  2.56 2 • Answer DL M2 =1 • ZF (f 100mm) =2.56*(1*10.6E-6*(100E-3)2)/(15E-3)2 = 1.2mm • ZF (f 127mm) =1.9mm • ZF (f 300mm) = 10.8mm Longer the focal length M2 =2.5 the bigger the DOF – less critical to maintain • ZF (f 100mm) =3mm focal position but lower • ZF (f 127mm) =4.86mm intensity. • ZF (f 300mm) =27mm Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality Linde Gas A small spot diameter (dmin / f ) is favoured by: • Short focal length (f) • Good beam mode • Short wavelength(λ) • A large beam diameter at the lens - telescope (D) The depth of focus Zf, which defines the level of tolerance for variation of the distance from the lens to the workpiece as well as the thickness that can be processed, depends on the same parameters. In general, • a small spot size equals a short depth of focus • A balance is needed for a given process. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • The advantage of a short focal length is offset by the necessary shorter working distance and hence less space to include gas delivery nozzles, etc. • It may also result in the lens / mirror being spattered by debris from the interaction between the laser and the workpiece, with resultant damage or loss of operating efficiency. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Gaussian and Real Beam Propagation M2 Concept of Beam Quality • While power density depends linearly on the laser power, it depends on the square of the beam waist diameter and hence on the square of the M2 factor • Hence for such processing conditions, more advantage may be gained by choosing a laser with a lower M2 factor than by choosing one with a higher power. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers How to measure M2 Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Beam Quality Factor M2 • From ISO11146 • A Laser Beam Analysis (LBA) system based on a CCD video camera is one of a number of methods to analyse the beam geometry and energy distribution • M2 cannot be determined from a single beam profile measurement. Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Beam Quality Factor M2 • To truly characterise a laser beam, more than a single profile has to be measured along the propagation axis of the focused beam under test. • According to the ISO11146 standard, 10 different z positions need to be taken to calculate M2 d min 4 M 2 f  DL Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Beam Quality Factor M2 • New product from Ardent Photonics BQM50 • Built around a variable focus liquid lens • No need to move the camera to measure propagation. d min 4 M 2 f  DL Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Lasers – Application example • Laser Forming • Rapid 2D Bend • http://www.youtube.com/watch?v=FDZhex_ASQE • Cutting and forming • http://www.youtube.com/watch?v=HUM1tAszhjQ Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers In Summary • Laser Beam Characteristics – • Basic construction, Wavelength, Coherence, Coherence length, Mode, TEM, Beam Diameter, ISO11146 • Gaussian optics – • Diffraction Limited Spot Size, Beam Quality M2, Rayleigh Range, Depth of Focus, Methods of determining M2 Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers References for further reading • Silfvast W T Laser Fundamentals (2nd edition) Cambridge University press 2004 IBSN 9780521541053 • Steen WM Laser Material Processing (4th Edition) Springer Verlag 2010 ISBN 9781849960618 • ISO 11146 -1, 2, 3 :2005 – International standard for Lasers and laser-related equipment Test methods for laser beam widths, divergence angles and beam propagation ratios. Available on VITAL in lecture notes for MECH607 Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Course Overview Week Date ( WB) Associated Lab (TBC) Lecture Topic 1 30/01/2017 Introduction to Lasers for Manufacturing \ Safe use of Lasers 2 06/02/2017 Optical components for laser engineering \ Laser beam Characteristics 3 13/02/2017 Laser Materials Interaction 4 20/02/2017 Advanced Laser Cutting 5 27/02/2017 Advanced laser based joining techniques 6 06/03/2017 Laser Forming 7 13/03/2017 Laser Drilling 8 20/03/2017 Fabrication at the micro scale 9 27/03/2017 Laser Micro Cutting & Laser Micromachining Easter Break 10 24/04/2017 Advanced Ultra-Short pulsed laser processing 11 01/05/2017 Laser based Direct Write (DW) 12 08/05/2017 Course Review SE GD Dr Stuart Edwardson / Prof Geoff Dearden MECH607 – Advanced Manufacturing with Lasers Course Overview - Labs MECH607 – Advanced Manufacturing with Lasers Semester 2 - Tuesdays 2pm – 5pm Week 1 2 3 4 5 6 7 8 9 10 11 12 Lab 1 TBC TBC TBC TBC TBC TBC TBC TBC TBC Lab 2 TBC TBC TBC TBC TBC TBC TBC TBC TBC Demo TBC TBC TBC TBC TBC TBC TBC TBC TBC • Labs take place/start in the Brodie Tower Lower Ground (Basement) laser lab, Tuesdays 2pm-5pm from week 4 onwards • lab groups to be posted on VITAL soon (~11 people) • White labs coats are mandatory (Supplied in Lab) Dr Stuart Edwardson / Prof Geoff Dearden Thank You MECH 607 Advanced Manufacturing with Lasers Week 3, Lecture 2 Laser-Material Interactions 2 Prof Geoff Dearden School of Engineering The University of Liverpool g.dearden@liverpool.ac.uk www.lasers.org.uk MECH607 – Advanced Manufacturing with Lasers Course Overview Week Date ( WB) Associated Lab (TBC) Lecture Topic 1 30/01/2017 Introduction to Lasers for Manufacturing \ Safe use of Lasers 2 06/02/2017 Optical components for laser engineering \ Laser beam Characteristics 3 13/02/2017 Laser-Material Interactions 4 20/02/2017 Advanced Laser Cutting 5 27/02/2017 Advanced laser based joining techniques 6 06/03/2017 Laser Forming 7 13/03/2017 Laser Drilling 8 20/03/2017 Fabrication at the micro scale 9 27/03/2017 Laser Micro Cutting & Laser Micromachining Easter Break 10 24/04/2017 Advanced Ultra-Short pulsed laser processing 11 01/05/2017 Laser based Direct Write (DW) 12 08/05/2017 Course Review SE GD Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Outline • Introduction • Interaction of radiation & matter • Heating processes – – – – L1 Heating by optical absorption Surface heating Energy balance process model Factors affecting absorption • Thermal laser-material interaction – Effect of laser parameters L2 • Pulsed laser material interaction • References for further reading Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption • Dependencies of reflectivity, R – – – – – – – Temperature Wavelength Conductivity Surface effects Intensity Angle of incidence Polarisation R  1/T R  -0.5 R  -0.5 R  1/Ra R  1/I0 Complex (Fresnel*) Complex * Fresnel absorption is that associated with multiple reflections due to angular incidence of laser beam Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption • How temperature affects absorption in Fe & steel (theory & experiment) • How temperature & angle of incidence affect absorption in Al (theory) Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption • Absorption of iron at perpendicular incidence (comparing Hagen-Rubens / Drude theories of electrons in metals with experiment) Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption • Dependence of absorption on the angle of incidence of the laser radiation. • It is worth considering what is the actual angle of incidence in some practical cases of laser processes. • Clue: it may, or may not, be obvious from the images (right) presented earlier. Laser cutting Laser drilling Laser cladding Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption • Dependence of absorption on angle of incidence of laser radiation. • Fusion laser cutting case (‘melt & blow’) Gas flow Laser beam Cut direction Evaporated material Erosion front Material Melt Ejection of liquid material Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption • Dependence of absorption on angle of incidence of laser radiation. • Shown below are characteristic angles of incidence normally associated with popular laser processes. Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption [Powell J, AILU Magazine, 2011] • Dependence of absorption on angle of incidence; practical example Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption • Surface finish and coatings affect absorption • Absorption increases with greater roughness, since light not absorbed by any one particle is more likely to be reflected on to others. • Emissivity  of surfaces relates to absorptivity (emissivity is used when referring to coatings)   1  1  (11) •  is the ratio of energy emitted from the surface to the energy emitted from a ‘black’ surface at the same temperature. Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption • For laser surface treatments with CO2 lasers (e.g. hardening, bending, melting or alloying), it is essential to increase the absorptivity of the surface by pre-treating it. • The application (by aerosol spray or painting) of carbon bearing paints is standard practice. • However, this is not an optimised solution to the problem since: – a separate process step (paint application) is needed; – carbon may be introduced into the surface material composition, especially if surface melting is involved. Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Factors affecting absorption Surface Type Reflectivity % Direct Diffuse Total Sandpaper roughened 90.0 2.7 92.7 Sandblasted (19um) 17.3 14.5 31.8 Sandblasted (50um) 1.8 20 21.8 Oxidised 1.4 9.1 10.5 Graphite 19.1 3.6 22.7 Molybdenum sulphide 5.5 4.5 10.0 Dispersion paint 0.9 0.9 1.8 Plaka paint 0.9 1.8 2.7 Table: Effect of surface pre-treatment on the absorptivity of steel to 10.6m CO2 laser radiation Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers ‘Thermal’ laser-material interaction • For most materials, the absorption length is a few nm; • So, for all practical purposes the laser light is totally absorbed in a thin surface layer and behaves as a surface source. Focused laser beam Liquid layer Heated region Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers ‘Thermal’ laser-material interaction • The absorbed fraction may vary from about 1% in copper to tens of percent for steels. • Without additional mechanisms, even a few tens of percent absorption is insufficient to transfer significant energy into the material,… • …unless an absorptive coating such as colloidal graphite is used (which is only acceptable in laser forming and some laser hardening cases). Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Discussion • Consider / discuss: • What additional effects do you think need to take place for the surface absorption to translate into deeper penetration e.g. for laser welding? (NB assume no absorptive coating is applied) Clue: physical mechanisms discussed earlier Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers ‘Thermal’ laser-material interaction • A second stage of absorption is now needed, but which requires the first stage of absorption to bring the metal surface to its boiling point. • With increasing temperature, a rapid increase in absorption takes place. • Once vaporised, some of the metal electrons become free – a process called ionisation. • These free electrons absorb energy directly from the incoming light (laser beam) by Inverse Bremsstrahlung resulting in higher temperatures, more ionisation and increasing absorption. Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers ‘Thermal’ laser-material interaction • The rapid increase in absorption vaporises the surface, forms plasma and the surface recedes to produce a cavity or ‘keyhole’ through the depth of the material. • Keyhole formation is the basis of deep penetration laser welding. • The keyhole is a liquid-lined cavity with a very hot ionised gas core. • Surface tension and gas pressure hold the liquid in place against gravity and other forces, while light absorption in the central gas core keeps the process going. • The ‘snapshot’ section view (right) is a condition that only occurs at a certain set of parameters (speeds, power, focal conditions etc.). Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers ‘Thermal’ laser-material interaction • Absorption of laser light in the keyhole is very efficient and is where most of the beam power goes. • Light absorption by plasma is also efficient. • In extreme cases, too high an intensity or hot metal vapours escaping from the keyhole can lead to excessive plasma formation, resulting in absorption of the beam above the metal surface; • this causes loss of energy coupling into the process and collapse of the keyhole. Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers ‘Thermal’ laser-material interaction • What happens with heat sources that cannot be focused? • Heat flow is mainly by conduction through the material. • In processes, this can lead to excessive heat input to the part e.g. distortion in welding metal. Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers ‘Thermal’ laser-material interaction • What happens with high power (cw) lasers? • Surface intensity is sufficient to vaporise material, creating plasma by ionisation, so the absorption is increased locally. • The laser beam ‘eats’ through the material (keyhole). Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Effect of laser parameters • Laser beam properties that determine the interaction and coupling effects Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Effect of laser parameters • Surface phenomena at increasing laser power density Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Effect of laser parameters • Keyhole coupling (multiple reflections) Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Effect of laser parameters • Plasma absorption & refraction  At very high temperatures, thermal agitation strips electrons from atoms and the gas becomes conductive, hence absorbing and refractive; absorption proportional to 2. Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Effect of laser parameters • Polarisation Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Effect of laser parameters • Wavelength • In general, the degree of absorption of a material surface increases as the wavelength of the incident radiation decreases (alongside other dependencies including material type, temperature, etc.) Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Pulsed laser material interaction • IR laser pulses of ns-ms duration (as used in thermal laser drilling) initiate rapid surface heating and melting,… • … yet achieve ejection of the molten material without significant heat conduction into the bulk material. Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Discussion • Consider / discuss: • Why does significant heat conduction NOT occur in this case, do you think? Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Pulsed laser material interaction • This is because the pulse duration is much shorter than the thermal response time of the material (for heat flow by conduction). • Other mechanisms take over (vaporisation, plasma formation, pressure gradients) to give rapid vapour & liquid material expulsion. • The effect is that a steady state is never achieved and heating is limited to thin surface layers and light energy absorption in the vapour / plasma within the hole. Laser-material interactions 1 (Geoff Dearden) MECH607 – Advanced Manufacturing with Lasers Ultra short pulse interaction • Recall earlier concept of light-matter interaction X electrons • An intense ultra short pulse will rapidly raise the electron temperature of the solid, while leaving the lattice ions largely unaffected (much cooler). • For 
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