A Report O n Laser w - PowerPoint Presentation

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A Report On Laser welding

Submitted to:

Mr: Qamar Tanveer

Submitted by:

Saurabh

Gupta

IntroductionLASER stands for “Light Amplification by Stimulated

Emission Of Radiation”. Lasers are devices that produce intense beams of light which

are monochromatic, coherent, and highly collimated. The wavelength (color) of laser light is extremely pure monochromatic. It can travel over great distances or can be focusedto a very small spot with a brightness which exceeds that of the sun.

Introduction…The basic operating principles

of the laser were put forth by Charles Townes and Arthur

Schalow from the Bell Telephone Laboratories in 1958, and the first actual laser, based on a pink ruby crystal,was demonstrated in 1960 by Theodor Maiman

at

Hughes Research Laboratories.

Principle Of LASER Welding

Explanation…The gain medium is excited by an external source of energy into an excited state. In most lasers this medium consists of population of atoms which have been excited into such a state by means of an outside light source, or an electrical field which supplies energy for atoms to absorb and be transformed into their excited states.

The gain medium of a laser is normally a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission described above. This material can be of any 

state: gas, liquid, solid, or plasma. The gain medium absorbs pump energy, which raises some electrons into higher-energy ("excited

") 

quantum states

. Particles can interact with light by either absorbing or emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, 

population inversion

 is achieved and the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. By itself, this makes an 

optical amplifier

. When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser oscillator.

In a few situations it is possible to obtain lasing with only a single pass of EM radiation through the gain medium, and this produces a laser beam without any need for a resonant or reflective cavity (see for example 

nitrogen laser

). Thus, reflection in a resonant cavity is usually required for a laser, but is not absolutely necessary.

The optical 

resonator

 is sometimes referred to as an "optical cavity", but this is a misnomer: lasers use open resonators as opposed to the literal cavity that would be employed at microwave frequencies in a 

maser

. The resonator typically consists of two mirrors between which a coherent beam of light travels in both directions, reflecting back on itself so that an average photon will pass through the gain medium repeatedly before it is emitted from the output aperture or lost to diffraction or absorption. If the gain (amplification) in the medium is larger than the resonator losses, then the power of the

recirculating

light can rise 

exponentially

. But each stimulated emission event returns an atom from its excited state to the ground state, reducing the gain of the medium. With increasing beam power the net gain (gain minus loss) reduces to unity and the gain medium is said to be saturated. In a continuous wave (CW) laser, the balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser. If the applied pump power is too small, the gain will never be sufficient to overcome the resonator losses, and laser light will not be produced. The minimum pump power needed to begin laser action is called the 

lasing threshold

. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons in a 

spatial mode

supported by the resonator will pass more than once through the medium and receive substantial amplification.

LASER Welding Process

Process…Like electron beam welding (EBW), laser beam welding has high power density (on the order of 1 MW/cm

2) resulting in small 

heat-affected zones and high heating and cooling rates. The spot size of the laser can vary between 0.2 mm and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the workpiece.

A continuous or pulsed laser beam may be used depending upon the application. Milliseconds long pulses are used to weld thin materials such as razor blades while continuous laser systems are employed for deep welds.

LBW is a versatile process, capable of welding 

carbon steels

, 

HSLA steels

, 

stainless steel

, 

aluminum

, and 

titanium

. Due to high cooling rates, cracking is a concern when welding high-carbon steels. The weld quality is high, similar to that of electron beam welding. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the

workpieces

. The high power capability of 

gas lasers

 make them especially suitable for high volume applications. LBW is particularly dominant in the automotive industry.

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Some of the advantages of LBW in comparison to EBW are as follows:

- the laser beam can be transmitted through air rather than requiring a vacuum,

- the process is easily automated with robotic machinery,

- x-rays are not generated, and

- LBW results in higher quality welds.

A derivative of LBW, 

laser-hybrid welding

, combines the laser of LBW with an arc welding method such as 

gas metal arc welding

. This combination allows for greater positioning flexibility, since GMAW supplies molten metal to fill the joint, and due to the use of a laser, increases the welding speed over what is normally possible with GMAW. Weld quality tends to be higher as well, since the potential for undercutting is reduced

Lasers Beam Welding:

photographic view of laser welding unit

Specimen

Laser Head

Shielding Gas Nozzle

Specimen Holder

Advantages

Single pass weld penetration up to 3/4” in steel

High Travel speedMaterials need not be conductiveNo filler metal requiredLow heat input produces low distortion

Does not require a vacuum

Limitations

High initial start-up costs Not portable

Metals such as copper and aluminum have high reflectivity and are difficult to laser weldHigh cooling rates may lead to materials problems.

Facts about lasers for welding

Wavelengths of some important laser sources for materials processing

Far

infrared

Why do we need laser for welding?

Traditional welding:

Natural limitations to speed and productivity Thicker sections need multi-pass welds

A large heat input

Results in large and unpredictable distortions

Very difficult to robotize

Laser beam welding:

High energy density input process

single pass weld penetration up to ¾ inch

Precisely controllable (close tolerence: ± 0.002 in.)

Low heat input produces low distortion

Does not require a vacuum (welds at atmospheric pressure)

No X-rays generated and no beam wander in magnetic field.

No filler metal required (

autogenous

weld and no flux cleaning)

Materials need not be conductive

Applications Of Laser Welding

Applications…

Applications…

Energy density is frequently used as process parameter in energetic term:

LP : laser power describing the thermal source,

WS : welding speed determining the interaction time

φ

Spot

: focal spot diameter defining the area through which energy flows into the material

Results

and Discussion:

ReferencesManufacturing Science II by K. M. MoeedManufacturing Science by Asthana & Kumar & Dahotre

Manufacturing Science by Amitabha

Ghosh