LASER BEAM MACHINING LBM LASER BEAM MACHINING LBM

LASER BEAM MACHINING LBM

LASER BEAM MACHINING (LBM) • Capable of very high power density • Highly coherent , logical and consistent (in space and time) – Of electromagnetic radiation – Wavelength varying • – Due to operational restrictions, usable wavelength • • 0. 1 – 70 μm 0. 4 – 0. 6 μm Laser beam are – Parallel – Monochromatic – Can be focussed to a very small diameter – With power density • 107 W/mm 2 – Normally pulsed ruby is used – CO 2 -N 2 laser is also employed 2

Schematic diagram of LBM (Using typical pulse ruby laser) 3

• Figure shows: – A typical pulsed ruby laser – A coiled xenon flash tube placed around • – The ruby rod Internal surface is highly reflecting • Maximum light falls on the ruby rod – The capacitor is charged – Very high voltage is applied to triggering electrode – For initiation of flash – Beam emitted is focussed by a lens system – Meets the work surface – Removal of work material by – • Vaporisation • High speed ablation (mechanical means) • A very small portion of metal vaporises very quickly • Substantial mechanical impulse is generated throw out a large portion of liquid metal. Energy released by flash tube is much more than the energy emitted by laser head • The system must be properly cooled 4

Efficiency of LBM • Very low – 0. 3 % to 0. 5 % • Energy output of laser – Approximately 20 J with a pulse duration of 1 millisecond • Peak power value – Approximately 20, 000 W • Divergence of the beam – 2 x 10 -3 radian using a lens with focal length of 25 mm • Spot diameter – 50 μm 5

Utility of LBM • Drilling micro-holes – Up to 250 μm diameter • Cutting narrow slots • Dimensional accuracy – ± 0. 025 mm • Taper – 0. 05 mm/mm if work-piece thickness is more than 0. 25 mm 6

MECHANISM OF LBM • • • Interaction of laser beam with work material Heat conduction and temperature rise Melting, vaporisation and ablation Accurate analysis is difficult Considering only the temperature rise of work material: 7

Conti. . • Interaction of laser beam with work – Machining depends upon thermo-optic interaction • Beam • Solid work material – Should not reflect back the incident beam energy – Absorbed light propagates into the medium – Energy as heat is gradually transferred to the atoms the absorption can be given by Lambert’s law as: • I(z) = I(O) e-λz Where I(z) denotes light intensity at depth z λ is the absorption coefficient 8

Figure. Incident laser beam that hits the surface with diffuse and specular reflections. The diffuse–specular reflection ratio depends on the optical properties of the measured surface. In the case of transparent materials, some light can also penetrate into the material 9

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Figure. Characteristics of focused laser beam used for surface texturing. (a) Schematic of the beam shape during fabrication process. (b) Calculated beam diameter determined as 1/e 2 of maximum intensity profile and the corresponding pulse fluence as function of the defocus distance z. 11

Cont. . . – Most of the energy is absorbed in a thin layer at the surface ( ~ 0. 01 μm) – Absorbed light energy is converted into heat • Can be considered as heat flux • Heat conduction and temperature rise – Re-radiation from the surface at 3000 K • ~ 600 W/cm 2 • It is negligible • Total input flux 105 to 107 W/cm 2 12

Assumptions • To make analysis one-dimensional, we assume: – Diameter of beam spot > depth of penetration – Thermal properties; conductivity and specific heat remains unaffected by temperature change – Equivalent heat conduction has uniform heat flux H(t) at surface of a semi-finite body 13

The equation for heat conduction, For z > 0 is Where • α is thermal diffusivity • θ is the temperature • At the surface, z = 0 • k is thermal conductivity 14

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Solution of the equation • At t =0, temperature of the body is zero When the laser pulse shape is known, the temperature θ(z, t) can be determined urther, if the laser beam pulse is a step function, H is constant, the equation will be 16

Hence, the surface temperature is given as: Where θm is the melting temperature of work material and tm is the time required to reach θm 17

Realistic approach • Heat flux is considered to be on a circular spot • Diameter of spot = diameter of focussed beam, d • Heat flux is uniform, both in space and time 18

Then the solution appears as: At z=0: 19

Using the expression for ierfc (ζ) as similar to the problems in EDM, this equation can be solved. The values of the error function {probability integral erf (ζ)} are available in standard tables. If power intensity is much higher, ierfc – term is very small as compare with 1/√π. Equation is same as that for tm. As power intensity decreases, time required for melting increases. 20

For large value of time Since the smallest possible value of erf (ζ) =0, H to attain melting temperature given as: If power intensity < this critical value, The melting temperature will never be reached 21

Steady state hole penetration Determination of dimensions of molten pit • If the molten pit is deep and shallow – Major heat conduction through side walls • When the heat input rate = heat loss – Shape and size is maintained • At steady state, rate of heat loss is given as: 22

From experience D ≈ 55 d Equating the heat input rate to heat loss rate 23

• When the beam intensity is very high >107 W/cm 2 • Heating is rapid • Incident beam heats up – The surface quickly vaporises • The beam falls recedes as the material vaporises • Let v =velocity with which the surface recedes H ≈ v. L • L = the energy to vaporises unit volume of material. 24

THANKS 25