Laser processing of materials. Laser technological processes

Transcription

1 Laser processing of materials Laser technological processes Prof. Dr. Frank Mücklich Dr. Andrés Lasagni Lehrstuhl für Funktionswerkstoffe Sommersemester 2007 Contents: Laser technological processes 1. Worldwide laser sales 2. Laser-Based Equipment for Microtechnology 3. Methods of cutting 1. Vaporization cutting 2. Fusion cutting 3. Reactive fusion cutting 4. Practical performance 4. Laser welding 1. Joining efficiency 2. Main characteristics of laser welding 3. Process arrangement 4. Conduction and keyhole welding 5. Effect of continuous and pulsed power 6. Shielding gas 5. Direct Laser Writing 6. Other laser processes 1

2 Laser technological processes Micromachining Laser power flux (W/cm 2 ) Liquid Shock processing Vapor Drilling Melting Vaporization Cutting Welding Vaporization and melting Melting Heating Interaction time (s) Stereolithography 10-1 Worldwide nondiode-laser sales by application 2

3 Worldwide nondiode-laser sales by type Total (2005): 2300 millions Laser-Based Equipment for Microtechnology Power supply Laser parameters control system Visual control Laser Laser beam Optical head Microprocessor Technological process-control system Work piece Scanning table 3

4 Methods of cutting The gas is needed both to aid the cutting operation and to protect the optic from spatter Methods of cutting The gas is needed both to aid the cutting operation and to protect the optic from spatter 4

5 Methods of cutting Cutting schema Arrangement for transmissive cutting Arrangement for reflective cutting Laser cutting advantages Methods of cutting Cutting edges are tight and parallel The cut edge can be smooth and clean. The cut is reckoned to be a finished cut, requiring no further cleaning or treatment. The cut can have a very narrow kerf width giving a substantial saving in material Reduced Heat Affected Zone Possibility to operate on complex profiles and reduced curving radius Absence of mechanical distortion of the laser worked piece No influence of the hardness of the material No problems to cut materials prevoiusly coated It is one of the faster cutting processes Tool wear is zero since the process is a non-contact process The noise level is low Nearly all materials can be cut 5

6 Methods of cutting Method Concept Relative energy 1- Vaporization Fusion cutting 20 Melt and blow 3- Reactive fusion 10 cutting 4- Cold cutting UV-LASERS 100 O 2 5- Laser assisted 5 oxygen cutting Vaporization cutting Vaporization cutting The focus beam must first heats the surface to boiling point and so generates a keyhole. The keyhole causes a sudden increase in the absorptivity due to multiple reflections and the hole deepens quickly. The generated vapour escapes blowing eject out of the hole and establishing the molten walls of the hole Penetration rate: Considerations: No heat conduction (this is true if the penetration rate is similar to or faster than the rate of conduction 6

7 Penetration rate: Vap v = F 0 / v v = d/t Vaporization cutting Vap v = vaporization energy (J/m 3 ) F 0 = absorbed power density (W/m²) v = penetration velocity (m/s) Vap v = ρ[l+c p (T v -T 0 )] v = F 0 / {ρ[l+c p (T v -T 0 )]} (approximation!) 1 s 2 s 3 s 4 s d How fast is the boiling point reached? (t v ) T(t) = (2 F 0 /K)[(α.t)/π] 1/2 t v = π/α [(T v K)/(2 F 0 )]² K = thermal conductivity (W m -1 K -1 ) α = thermal diffisivity (K/ρC p ) Vaporization cutting Considering a laser of 2000 W focused to 0.2 mm beam diameter F 0 = 2000 W x 10 6 / [π(0.1)²] F 0 = 6.3 x10 10 W m -2 Laser type Tp (s) Laser pulse energy Laser power (W) Power density (diametar 1 mm) Tv Nd:Yag 1.00E mj 50 MW 6.40E E-13 Nd:Yag 1.00E mj 900 KW 1.15E E-09 CO2 (CW) 1 25 KJ 25 KW 3.18E E-06 CO2 (CW) 1 27 J 27 W 3.44E E+00 7

8 Fusion cutting Melt and blow Considerations: 1- Firstly it is necessary to produce the penetration hole, or to start to cut from the edge. 2- A strong gas jet is necessary to blow the molten metal. No vaporization energy necessary is about 10 times lower 3- Thermal conduction is negligible (what is true: HAZ ~ 1-5 µm) ηp = w t V ρ [L f + m L v + C p T] P/ t V= w ρ / η [L f + m L v + C p T] = f(material) P = incident power (W) w = average kerf width (m) t = thickness (m) V = cutting speed (m/s) m = fraction of melt vaporized L f, L v = latent heat of melting and vap., respectively T = temperature rise to cause melting η = coupling coefficient Fusion cutting Melt and blow P/ t V= w ρ / η [L f + m L v + C p T] P = α t V, with α = f(material) 8

9 Reactive fusion cutting 1- Reactive gas which is able to react exothermically with the work piece 2- In some case up to 90 % of the energy is supplied by the chemical reaction (Steel: 60%; Ti: 90%) 3- Disadvantages: formation of oxides (changes in the surface chemical composition) 60% 90% Practical performance (cutting) Beam properties: Spot size and mode, power, pulsed or CW, polarization, wavelength The effect of polarization on the cutting performance with the cut direction CO 2 : 10.6 µm CO: 5.5 µm 9

10 Practical performance (cutting) Transport properties: Speed, focal position Gas properties: Jet velocity, shape, alignment, gas composition Dual focus lens Practical performance (cutting) Gas properties: Jet velocity, shape, alignment, gas composition Material properties: Thermal and optical properties: 1- Can sufficient power be absorbed? 2- Will this power cut successfully or damage the material? 10

11 Practical performance (cutting) Material properties: Thermal and 1- Can sufficient power be absorbed? optical properties: 2- Will this power cut successfully or damage the material? 43%! Laser welding Joining efficiency: 1- Is not a true efficiency (units in mm² joined/kj) 2- It is defined as: V t / P (the reciprocal of the specific energy used in the section cutting ) V = traverse speed (m/s), t is the thickness welded (mm) P = Incident power The higher the joining efficiency, the less energy is spent in unnecessary heating (lower HAZ) 11

12 Laser welding Main characteristics of laser welding: Most important characteristics: high speed, high quality welding control Laser welding Process arrangement: When welding pieces with high reflectivity, target is tilted 5 to avoid back reflections (damage of optic, o-rings, laser system, etc) 12

13 Laser welding Conduction welding power density insufficient to cause boiling strong stirring forces driven by Marangonitype forces out of focus beam Keyhole welding evaporation is caused and hence a hole in the melt pool the hole is stabilized by the pressure from the vapor generated the keyhole behaves be a black body (multiple reflections) nearly all the beam is absorbed (good for high reflectivity metals) Power: effect of continuous power Laser welding Most important problems: lack of penetration and excessive penetration (drop up): These are the boundaries for a good weld for a given power For more power the operation window is large 13

14 Laser welding Analytical model for the maximum welding speed: 0.25 P (1 r f )= V w t ρ C p T m W = weld width (m); t =thickness (m); P = power (W) r f = reflectivity; ρ = density (Kg/m³); C p = specific heat (J/Kg K) T m = melting point (K); V = welding speed (m/s) Assumptions: 52% of the energy is lost by conduction (from analytical solution) and a further 25% is used for the latent heat ( = 0.75 = ) Penetration depth and welding speed: Inversely proportional to laser power and welding speed Power: effect of pulsed power Laser welding Two more variables: pulse repetition frequency (PRF) and % overlap to be considered Welding speed = spot size x PRF x (1-% overlap) Advantages: increased peak power better welding of reflective material (keyhole is initiated quicker) e.g. Nd:YAG Laser (500 Hz, 2kW) 3 times faster than CO2 (CW, 1kW) and 60% improvement of penetration pulsed laser less energy deposited in the work piece (diffusion length) reduction of pores (the flow of molten metal can be better controlled) 14

15 Power: effect of pulsed power For welding, the pulse is usually longer than from drilling: Laser welding Laser welding Shroud Gas : The vapor in the keyhole consist of very hot vapor from the material being welded together with shroud gas (shielding has). The vapor can be partially ionized (plasma formation) and absorbing energy of the laser: Ionization potential (ev): He: N2: O2:

16 Laser welding Copper Hastelloy-X Molybdenum Inconel 625 Monel Nickel Tantalum Titanium Tungsten Zirconium Material Aluminium 1100 Aluminium 2219 Aluminium 2024/5052/6061 Cu-Zn Brasses Beryllium Copper Steel, Carbon Steel, Galvanized Steel, 300 Stainless Steel, 400 Stainless Steel, 17-4PH Stainless Welds well; no cracking problem or transformation No cracks; no filler metal required Requires filler metal of 4047 Al to make hermetic, crack-free welds Out-gassing of Zn prevents good welds Alloys containing higher percentages of alloying agents weld better due to lower reflectivity High reflectivity may crease uneven welds; for material less than 0.01" thick, coating may enhance weldability Requires high pulse rates to prevent hot-short cracking Usually welds brittle; welds may be acceptable where high strength is not required Some tendency for porosity in deep welds Good ductile welds; good penetration Must be cleaned; good ductile welds and penetration Good welds with carbon content under 0.25%; for greater carbon content, may be brittle and crack Severe Zn boil-off causes porosity Welds well, except 3030 and 303SE, which crack Generally welds somewhat brittle; may require pre- and post-weld heat treating Needs post-weld heat treating to strengthen Ductile welds; special precautions against oxidation required Ductile welds; special precautions against oxidation required Brittle welds; requires high energy Comments Ductile welds; special precautions against oxidation required. Table 1. Laser Welding Applications Guideline Direct laser writing (DLW) Laser writing in two dimensions (2D) Applications: writing a CD 16

17 Direct laser writing (DLW) RED-LASER BLUE-LASER DVD HD-DVD BLU-RAY Capacity 4.7GB 20GB 27GB 780 nm 650 nm NA ~ 0.45 NA ~ 0.60 Direct laser writing (DLW) DLW: writing in three dimensions (3D) 17

18 Direct laser writing (DLW) Direct laser writing (DLW) Three-dimensional Photonic Crystals 18

19 Other processes Laser cladding: The goal of cladding is to deposit a material of desired physical properties over a substrate. Cladding materials are chosen based on whether they need to provide corrosion resistance, wear resistance or higher hardness, depending upon theuseof thebasepartbeingclad. Laser hardening: Selective areas can be hardened without affecting the surrounding material Minimal heat input causes limited distortion and reduces the need for additional machining Treatment depth is accurately controlled and highly reproducible Superior hardness can be obtained compared to conventional processes No external quenching is required Quick turn-around time Down and upstream cost saving Can be automated and integrated with other inline production processes 19