Laser Beam Welding

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Laser beam welding (LBW) is a welding technique used to join multiple pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequentl
y used in high volume applications using automation, such as in the automotive i ndustry. It is based on keyhole or Penetration mode welding.

Like electron beam welding (EBW), laser beam welding has high power density (on the order of 1 MW/cm2) 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 prop ortional 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. Mi llisecond-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, stain less steel, aluminum, and titanium. Due to high cooling rates, cracking is a con cern 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.


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
- 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 great er 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 potentia l for undercutting is reduced.


Although laser beam welding can be accomplished by hand, most systems are automa ted use a system of computer aided manufacturing based off of computer aided designs. Laser welding can also be coupled with milling to form a finished part
Recently the RepRap project, which historically worked on fused filament fabrica tion, expanded to development of open source laser welding systems. Such systems have been fully characterized and can be used in a wide scale of applications while reducing conventional manufacturing costs.

The two types of lasers commonly used are solid-state lasers (especially ruby lasers and Nd:YAG lasers) and gas lasers. The first type uses one of several solid media, including synthetic ruby (chromium in aluminum oxide), neodymium in glass (Nd:glass), and the most common type, neodymium in yttrium aluminum garnet (Nd:YAG). Gas lasers use mixtures of gases such as helium, nitrogen, and carbon dioxide (C O2 laser) as a medium. Regardless of type, however, when the medium is excited, it emits photons and fo
rms the laser beam.

Solid-state lasers operate at wavelengths on the order of 1 micrometer, much shorter than gas lasers, and as a result require that operators wear special eyewear or use special screens to prevent retina damage. Nd:YAG lasers can operate in both pulsed and continuous mode, but the other types are limited to pulsed mode. The original and still popular solid-state design is a single crystal shaped as a rod approximately 20 mm in diameter and 200 mm long, and the ends are ground flat. This rod is surrounded by a flash tube containing xenon or krypton. When flashed, a pulse of light lasting about two milliseconds is emitted by the laser. Disk shaped crystals are growing in popularity in the industry, and flashlamps are giving way to diodes due to their high efficiency. Typical power output for
ruby lasers is 10 20 W, while the Nd:YAG laser outputs between 0.04 6,000 W. To deliver the laser beam to the weld area, fiber optics are usually employed.

Gas lasers use high-voltage, low-current power sources to supply the energy needed to excite the gas mixture used as a lasing medium. These lasers can operate in both continuous and pulsed mode, and the wavelength of the CO2 gas laser beam is 10.6 µm, deep infrared, i.e. 'heat'. Fiber optic cable absorbs and is destroyed by this wavelength, so a rigid lens and mirror delivery system is used. Power outputs for gas lasers can be much higher than solid-state lasers, reaching 25 kW

Fiber laser, the gain medium is the optical fiber itself. They are capable of power up to 50 kW and are increasingly being used for robotic industrial welding.

Laser beam delivery
Modern laser beam welding machines can be grouped into two types. In the traditional type, the laser output is moved to follow the seam. This is usually achieved with a robot. In many modern applications, remote laser beam welding is used. In this method, the laser beam is moved along the seam with the help of a laser scanner, so that the robotic arm does not need to follow the seam any more. The advantages of remote laser welding are the higher speed and the higher precision of the welding process.

 

LASER BEAM WELDING
PRINCIPLE

  • Laser beam welding (LBW) is a welding technique used to join multiple pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates.

SET UP FOR ‘LBW’

  • Laser beam equipment consists of a cylindrical ruby crystal with both the ends made absolutely parallel to each other. Ruby is aluminum oxide (AlO2) with chromium dispersed throughout it.
  •  One of the end faces of the ruby crystal highly silvered so that it reflects nearly 96% of the incident light. In order to tap the laser output, the other end face of the crystal is partially silvered and contains a small hole through which the laser beam emerges.
  •  The ruby crystal is surrounded by a helical flash tube containing inert gas ‘xenon’ which itself in turn is surrounded by a ‘reflector’ to maximize the intensity of the incident light on the ruby crystal. The flash tube converts electrical energy into light energy.
  • Cooling system, either gas or liquid is provided to protect the ruby crystal from the enormous amount of heat generated.
  •  When the flash tube is connected to a pulsed high voltage source, xenon transforms the electrical energy into while light flashes (light energy).
  •  As the ruby is exposed to the intense light flashes, the chromium atoms of the crystal are excited and pumped to a high energy level. These chromium atoms immediately drop to an intermediate energy level with the evolution of discrete quantity of radiation in the form of red fluorescent light.
  •  As the red light emitted by one excited atom hits another excited atom, the second atom gives off red light which is in phase with the colliding red light wave. The effect is enhanced as the silvered ends of the ruby crystal cause the red light to reflect back and forth along the length of the crystal.
  •  The chain reaction collisions between the red light wave and the chromium atoms becomes so numerous that, finally the total energy bursts and escapes through the tiny hole as a ‘LASER BEAM’.
  •  The laser beam is focused by an optical focusing lens on to the spot to be welded. Optical energy as it impacts the workpiece is converted into heat energy.
  •  Due to the heat generated, the material melts over a tiny area and upon cooling, the material within becomes homogeneous solid structure to make a stronger joint.

Laser Beam Welding Seminar Reports

APPLICATIONS

  • Laser is a high energy light beam that can both weld and cut the metals.
  • For connecting leads on small electronic components and in integrated circuitry in the electronic industry.
  • To weld lead wires having polyurethane insulation without removing the insulation. The laser evaporates the insulation and completes the weld.
  • To join hard high melting point metal alloys.
  • In space and aircraft industry for welding light gauge materials.

Lbw advantages and disadvantages :

  • Works with high alloy metals without difficulty
  • Can be used in open air
  • Can be transmitted over long distances with a minimal loss of power
  • Narrow heat affected zone
  • Low total thermal input
  • Welds dissimilar metals
  • No filler metals necessary
  • No secondary finishing necessary
  • Extremely accurate
  • Produces deep and narrow welds
  • Low distortion in welds
  • High quality welds
  • Can weld small, thin components
  • No contact with materials

Limitations

  • Rapid cooling rate may cause cracking in some metals
  • High capital cost for equipment
  • Optical surfaces of the laser are easily damaged
  • High maintenance costs

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