Ratio of the Temperatures changes with depth are t

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1 Melting Depths Consider light pulse on surface Will get melting to some depth Eventually also raise surface to vaporization point This is the laser welding situation Can estimate the depth of melt front after some time t Recall temperature distribution ΔT( z,t ) = 2H k α t ierfc 2 z α t Ratio of the Temperatures changes with depth are t ΔT( z,t ) z = π ierfc ΔT(0,t ) 2 α t Eventually surface raises to vaporization point It cannot rise higher without vapourization thus stays at T v Hence can calculate the melt pool depth with this.

2 Melt Depth Estimate Estimate the depth of melt front for that after some time t Bottom of melt is at melting point, Top at vapourization point Assume base temperature is near 0 o C (ie ~room temp) ( z,t) = Tm ΔT ( 0,t) Tv Δ T = T T Recall that at the surface m v = π ierfc 2 Δ T (0,t ) = 2H k z α t α t π Thus time can be eliminate by solving for Depth of melt is given by T v k π = 2H α t zmh T ierfc = Tvk π Tv Note: for a given material Hz m is fixed Thus large welding depths given by low heat intensities applied for long time provided that there is sufficient energy in the beam m π

3 Example of Melting Calculations What is the heat flow required for weld depth of 0.1 mm in copper From the table for copper T m = 1060 o C T v = 2570 o C K = 400 W/m o C Thus z H ierfc Tv k π From the graph or calculation Thus T k π T = T π v π m m = = ierfc(x=0.44) = ( 2570) π (0.44 ) 9 2 v H = = = zm x10 W / m

4 Vaporization of Material When material removed by vaporization Get a melt front and a heated front liquid front moves with velocity v s From the heat balance, assume that all power goes into heating Then the melt front should be s ( CT L ) H ν ρ + where H is power density per square area Note this is the minimum power value Good rule of thumb is actual power required twice this loss about the same by heat flow to substrate Can calculate depth d v of holes by knowing laser pulse duration t p and front velocity v d = ν t ν s p v

5 Example Depth of Hole with Vaporization Heat pulse of H = W/m 2 and t = 500 microsec hits copper. What will be the resulting max hole depth From the tables T v = 2570 o C ρ= 8960 kg/m 3 C = 385 J/kg o C L v = 4.75x10 6 J/kg d ν = ν t s p = ρ = 0.95x10 3 Ht ( CT + L ) v p v = m = 0.95mm ( 5x10 ) 6 ( 385[ 2570] x10 )

6 Keyholes and Increased Welding/Cutting Depths When the laser forms hole in material Beam penetrates to much greater depth Creates a large deep melt pool behind moving beam Melt fills in hole behind moving beam If not true welding limited 1 mm in steel

7 Keyholes Formulas Modeled by Swift, Hook and Gick, 1973 Assume linear heat source power P (W) Note P is total power while H is unreflected power because keyhole absorbs all the power (reflections do no escape) Extends into metal depth a Moving forward with velocity v (weld speed) in direction y direction across weld is x (centred on the heating point) Temperature distribution becomes 2 2 P vx vα x + y T = exp K 0 2π ak 2α 2 where K 0 is the Bessel function 2nd kind order 0 Width w of the weld is given by the point where T=melting α P w v akt m

8 Laser Machining Processes Laser heat processing divided into 3 regions Heating Melting Vaporization

9 Laser Surface Treatment Annealing or Transformation Hardening Surface hardness Surface Melting Homogenization, recrystallization Alloying Changing surface composition Improves corrosion, wear or cosmetic properties Cladding Applying a different material to surface Improves corrosion, wear or cosmetic properties Texturing Changing surface appearance Plating By Chemical Vapor Deposition

10 Laser Annealing Uses the rapid, local, high temperature, heating and cooling (quenching) Materials where heating with quenching changes characteristics Best examples: Iron/Steel With laser can make local changes in material parameters Increase hardness, strength Temper (make more ductile) Laser annealing of shaft

11 Comparison of Material Heating Processes

12 Laser Hardening of Steel Steel (Iron/Carbon combination) Many different crystal phases with different hardness/ductility tradeoffs At 800 o C 4% carbon steel changes to Austenite form Causes carbon to redistribute If quenched < 1 sec get Martensite very hard but brittle layer Called Case Hardening Can be applied by laser to just local areas Laser heat treating a transmission shaft. Saw tooth hardened by laser

13 Laser Parameters and Case Hardening Can tradeoff laser power and time Recall from uniform heating the surface temperature Δ T (0,t ) = 2H k α t π Thus for the same material T is proportional to H Thus doubling power cuts time by 4 However depth of heat zone is changed because T m z = π ierfc T v 2 α t When α t is small z change is large t

14 Spreading the Laser Beam To evenly heat the surface Defocus the laser beam Transport surface (rastor scan the beam) either straight, zig zag or circular Mirrors can spread the beam to wider areas

15 Modification of Surface Reflectivity Reducing reflectivity important for lower power requirements Basic level set by material reflectivity But can modify this by surface treatment Roughen surface: increases scatter Oxidize surface (very good for iron) Coat surface Most useful for Annealing, not useful if melting occurs melt destroys these processes

16 Laser Recrystallization of Poly Silicon Poly Silicon is common conductor in IC's Grain size very sensitive to production temperature But want low temperatures to reduce effects on substrate Poly Si's electrical parameters much poorer than single crystal Si Thus attempts at laser recrystallization to get near single crystal behavior Some attempts to build stacked transistors one transistor above another recrystallized poly Si used as the 2nd layer

17 Laser Recrystallization of Poly Silicon Successfully recrystallized silicion However do not need laser to do this: Used just regular light sources Called Rapid Thermal Annealing

18 Laser Surface Melting Want to melt the surface locally Melt & rapid solidification get fine homogeneous structures (recrystallize) Little thermal penetration thus small thermal distortion for sensitive materials Melt gives surface finishes within 25 microns reducing finishing work Process flexibility: easy to software control

19 Case Iron Laser Surface Melting Melting causes carbon redistribution Forms Martensite & Cementite phases significantly increase hardness of Cast Iron

20 Titanium Laser heating creates very fine crystal structure Must be done in inert atmosphere

21 Laser Surface Alloying Coat surface with film of another material Melt layer with laser locally (may inject material into melt pool also) Rapid quenching Alloyed layer has fine microstructure, nearly homogeneous Many materials alloyed into substrates Some materials only possible with rapid quench of laser Thickness form 1 to 2000 microns Applications Cast Iron: Cr, Si, C makes expensive steel surface on cheap iron mass Steel: Cr, N, Mo, B Aluminium: Si, C, N, Ni alloying

22 Laser Cladding Overlay one material with another Usually powders or Chemical Vapours are sources Most common industrial is powder process Powder blown onto surface Laser melts power to surface cladding

23 Laser Cladding Powder could be pre-placed Melt goes rapidly through powder Powder has little thermal contact with substrate When molten heat load increase due to good thermal contact Then melt into substrate and fuse with it Use reflective dome above powder Recovers powder

24 Laser Cladding Shows big improvement when surface roughness changes Power sensitive to cladding thickness

25 Laser Welding Laser welding involves melting two surfaces together Generally two types Conduction weld: just melt but do not vaporize Keyhole weld, some vaporization and deep weld

26 Keyhole weld May need to melt two or more layers Melt pole stabilized by vapor

27 Power and Laser welding types

28 Keyhole Welding of Lasers May need to melt two or more layers Use keyhole melt pool stabilized by vapour

29 Plasma Absorption If get too much vapor can form plasma Laser ionizes vapor Plasma heavily adsorbs beam Get a pulse effect as plasma comes off

30 Gas shielding Use inert gas flow to shield weld also reduces plasma effect if high flow

31 Laser Welding Setup Many laser parameters affect welding Affected also by type of weld Gap between materials important All the classic weld joints can be done by laser

32 Microelectronic Applications Use laser to make microwelds on circuits Used for bonding wires rather ball bonder Widely used in production: make many welds at once 20% stronger and can be done closer together

33 Laser Cutting Advantage Accounts for 82% of CO 2 laser work 43% of Nd:Yag (1986) Cut width (Kerf) very narrow Edges square, not rounded Cut edge very smooth, can be welded directly Little edge burr compared to others methods Heat Affected Zone (HAZ) thin, hence little distortion Cutting done in hidden areas Cut depth is limited (1-2 cm) and controllable Very fast cut, no clamping needed No noise Wide range of materials (including brittle & fragile)

34 Vaporization Cutting Laser heats surface to vaporization Forms keyhole (hole where the beam penetrates) Now light highly absorbed in hole (light reflects with the hole until absorbed) Vapor pressure from boiling material stabilizes the molten walls Material gets ejected from hole (as vapour) can condense and form Dross at bottom and top In materials that do not melt, just the vapor escapes eg Wood, carbon, some plastics

35 Vaporization Cutting Formulas Recall the velocity of melt front formulas v s = ρ H ( CT + L ) where H is power density absorbed per square area The temperature at the surface from the uniform illumination formulas for the vaporization point ( 0,t) T = Thus the time for vaporization is v 2H k v αt π t v π Tvk = α 2H 2

36 Vaporization Cutting Values If we had a 2 KW laser focused to 0.2 mm Then average power is H Can estimate v s and t v 2000 = π r 10 2 = Wm 2

37 Fusion Cutting: Melt and Blow Once melt is formed use gas flow to blow away materials Do not need to vaporize, thus power reduced by factor of about 10

38 Fusion Melting Estimates Can use the heat balance type relationship c c [ C ( T T ) + L + m L ] H = wt V ρ H = effective power input from laser C s = specific heat of solid phase L f = Latent Heat of Fusion: energy for melting L v = Latent Heat of Vaporization: energy to vaporize m' = fraction of the melt vaporized T m = is the melting point, T starting temp. t c = material thicknes w = width of cut (kerf) ρ = density of material Rearranging for a common cutting parameter s m 2 [ C ( T T ) + L + m L ] Jm H fm = = wρ s m f V tcvc f m is generally a function of cutting speed and gas velocity Note there is a small cooling effect caused by the gas flow f V

39 Fusion Cutting CO 2 & Materials