For a welding application, the laser is finely focused as a high-collimated eam of photons that is referred to as a coherrent beam. This monochromatic beam is capable of delivering up to 30,000 W/in2. It allows deep penetration welds without affecting the base metal because the energy bond is small and because broad heating does not take place. Thus, either thin or thick materials are welded successfully. Joint designs for laser welds are similar that except that tolerancs are tighter and the surface finish is better because of the laser beam. This type of welding makes excellent, precse welds, usually no more that 0.001 in (0.025 mm) wide, on all metal, and the base metal temparature rises almost imperceptibly. The most common laser in welding is the CO2 type, which can weld 1/32 in0thich stainless steel. New gas dynamic lasers can weld upto 3/4 in. thick stainless.
Description and Operation of Laser Welding:
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.
Advantages of Laser Welding:
Similar and dissimilar metals can be welded easily.
Laser beam can be controlled to a great precision and hence, the welding spots could also be located precisely.
Certain locations in the material that are difficult to reach can be welded easily by this process.
Heating and cooling rates are much higher in this process. Also, head affected zone is very small. Hence, the process is ideal for a location which is surrounded by heat sensitive components.
Clean weld joints can be obtained by this process.
Disadvantages of Laser Welding:
Slow welding speeds (25-250 mm/min).
Rapid cooling rate cause problems such as cracking in high carbon steels.
High equipment costs.
Applications of Laser Welding:
Used in electronics industry for applications such as connecting wire leads to small electronic components, to weld medical equipments, transmission components in automobiles and in cladding process.
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