PDPM IIITDM JABALPUR - IIITDM JABALPUR LASER Beam Machining Advancements ME 306 ADVANCED...

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  • PDPM IIITDM JABALPUR

    LASER Beam Machining Advancements

    ME 306

    ADVANCED MANUFACTURING PROCESSES

    Submitted To Dr. TVK Gupta

    Submitted By: G11

    Sandeep Singh 2009105

    Santosh Kr. Maurya 2009106

    Satyendra Singh 2009107

    Saurabh Rathi 2009108

  • 1

    ACKNOWLEDGEMENTS

    We express our sincere thanks to Dr. TVK Gupta for his immense help and guidance during completion

    of this term paper.

    Sandeep Singh 2009105

    Santosh Kr. Maurya 2009106

    Satyendra Singh 2009107

    Saurabh Rathi 2009108

  • 2

    INDEX

    S.No. TOPIC PAGE NO.

    1. Abstract 3

    2. Introduction 3

    3. Experimental Setup 4

    4. Mechanism of Material Removal 5

    5. Material Removal 6

    6. Improvisations and Advancements in LBM Process 7

    7. Applications of LBM 9

    8. Conclusions 11

    9. References 11

  • 3

    ABSTRACT:

    The high intensity which can be obtained by focusing the pulsed energy emitted by a LASER can offer

    potential as a tool for nearly forceless machining. The method can be used on any material, regardless

    of thermal properties, which can be evaporated without decomposition, including almost all ceramics

    and metals.

    With most substances, almost all of the material removed by LASER machining leaves in the liquid state.

    Only a small fraction is vaporized, and the high rate of the vaporization exerts forces which expel the

    liquid metal.

    All features of LASER beam machining improve with increased intensity. The higher the intensity, the

    less heat is resonant in the uncut material, an important consideration with materials which are

    sensitive to heat shock, and the more efficient the process is in terms of volume of material removed

    per unit of energy. The intensities which are available with the LASER are high enough so that the heat

    affected zone (HAZ) on a cut surface is too small to be detected and there is no solidified liquid film

    residue on the cut surface.

    INTRODUCTION:

    LASER BEAM MACHINING (LBM) is a valuable tool for drilling, cutting and milling of almost any material.

    The mechanism by which a LASER beam removes material from the surface being worked usually

    involves a combination of melting and evaporation, although with some materials, such as carbon and

    certain ceramics, the mechanism is purely one of evaporation. Any solid material which can be melted

    without decomposition can be cut with the LASER beam.

    Advances in nanotechnology motivate the extension of LASER machining of microstructures to the

    smaller dimensions of interest. Optical LASERs such as RUBY LASERs and CO2 LASERs are widely used for

    micro-milling and micro-hole drilling over a wide range of materials. The size of the smallest features

    that can be created focusing intense LASER beams onto materials is limited mainly by the LASER

    wavelength and by the diffusion of heat.

    A variety of different techniques have been developed to overcome the limitations imposed by the

    diffraction limit in order to produce ablation craters of sub-wavelength size using optical and UV-LASERs.

    Nowadays, there have been several experiments over a wide range of LASER applications for material

    removal and cutting in which UV-LASERs and femto-second LASERs are the most popular for industrial

    use. Also efforts have been made to minimize the tapers and HAZ which result due to high temperature

    of the LASER beam.

  • 4

    EXPERIMENTAL SETUP:

    Shown below is the experimental setup of Excimer LASER beam system.

    Figure 1: Experimental setup of an Excimer LASER beam system.

    (1) LASER unit (2) LASER beam

    (3) LASER shutter (4) Attenuator

    (5) & (6) LV1, LV2 (vertical lens) (7) Mirror 1

    (8) & (10) LH1, LH2 (horizontal lens) (9) Mirror 2

    (11) Scanning system (12) Mirror 3

    (13) Field lens (14) Mask plane

    (15) Projection lens (16) Photo diode detector

    (17) Diode LASER (18) Z-axis

    (19) X-axis (20) Y-axis.

    An Excimer LASER operates at 248 nm with 400 mJ maximum output pulse energy, an average power of

    100 Watt and 200 Hz maximum repetition rate. The beam exiting the LASER is rectangular in shape and

    not of homogeneous intensity. To correct the beam, the optics train made of cylindrical lenses (LV1-LV2

    and LH1-LH2) force the beam and makes it parallel with the square cross section in the vertical and

    horizontal directions. Mirror 3 scans the beam across the mask plane to make it homogeneous. The

    beam further passes through the mask plane/ aperture with a maximum area of 15X15 mm2 before

    finally going through a projection lens that gives 15 times linear reduction at the work piece. By

    changing the mask aperture, beam spots of different size and shape can be generated at the work piece.

    Automated or manual focus control is achieved using a diode LASER beam reflected from the work piece

    surface and a photo-diode array detector to provide positional measurement.

  • 5

    MECHANISM OF MATERIAL REMOVAL:

    The material removal by LBM process and vaporized energies are shown in the figure below.

    When LASER hits the material surface, it will have some recoil force. It can drive the liquid away from

    the sides. Short pulsed LASERs generate higher recoil and it results in farther liquid removal.

    UV LASER will generate high temperature on material, and removed material gets ionized. This will form

    plasma in the hole. Plasma can absorb further incoming LASER energy. Part of it gets reemitted in wide

    spectrum and wide angle. It help the LASER energy coupling to material and also resulting in a larger

    "heat affected zone".

    Figure 3: LASER induced effects in the LBM process.

  • 6

    LASER beam machining is a thermal process with emphasis laid on heat requirements and heat

    utilization. It is also important to determine physical properties of the work piece material and their

    relationship to the operating characteristics of optical LASERs.

    The following factors have to be taken into account while LASER machining:

    1. Part of the energy (Large part in case of highly reflective metal surfaces) is reflected and lost.

    2. Most of the energy which is not reflected is used for material removal.

    3. A very small part of the energy is used to evaporate the liquid material.

    4. Another small part of energy is conducted into the converted base material.

    The relative magnitudes of these four avenues of heat consumption depend strongly upon the thermal

    and optical properties of the material being worked and the intensity and pulse duration of the LASER

    beam. Time distribution of energy also plays an important role.

    The most prominent misconception in LBM is that the entire material being removed is evaporated. But

    the large quantity of energy which would be required for this to happen be not actually consumed which

    substantiates the argument. Most of the material leaves the work piece surface in the liquid state and

    relatively high velocity.

    MATERIAL REMOVAL:

    The basic assumptions to analyze the material removal process are:

    1. The intensity of LASER beam does not vary with time.

    2. LASER beam is uniform over the entire area of the hotspot.

    3. The material being removed is both melting and evaporating.

    4. The steady state ablation is characterized by constant rate of material removal and by the

    establishment of a steady temperature distribution.

    According to the above assumptions, the steady temperature distribution is given by,

    (T To)/ (Tm To) = e-Vx/

    (1)

    Where,

    T = temperature at distance x below the ablating surface,

    To = initial uniform temperature of the work piece,

    Tm = melting point of the work piece

  • 7

    V = steady ablation velocity,

    a = thermal diffusivity of work piece, i.e., (K/ Cp

    K, , Cp= thermal conductivity, density, and specific heat, respectively, of the work piece.

    It can be seen that the exponential distribution represented by (1) confirms with boundary conditions

    that T= To when x is very large and T = Tm when x=0.

    The depth at which heat penetrates the ablating surface is of considerable practical importance. It is

    reflected in the depth of the HAZ which will be left when the ablation process is over. It is desirable to

    keep the HAZ as shallow as possible.

    A simple way to identify the depth of heated layer is to define a characteristic depth x:

    xc

    (2)

    The characteristic depth Xc is the depth during steady ablation which has experienced a temperature

    rise 1/2.718 of the way from To to Tm. The characteristic heated depth X. decreases with increasing

    ablation velocity and increases with increasing thermal diffusivity.

    During the initial transient period when ablation is just beginning, part of the heat delivered to the wor