Laser Welding

In laser welding, heating and melting of the metal by laser beam optical maser (laser).

Common designations of laser welding
LBW - Laser Beam Welding - Welding with a laser beam

The essence of laser welding
The laser beam compared to conventional light beam has a number of characteristics - directivity, monochromaticity, and coherence.

Due to the direction of the laser beam, its energy is concentrated on a relatively small area. For example, the orientation of the laser beam can be several thousand times the focus of the spotlight.

If an ordinary "white" light consists of rays of different frequencies, the laser beam is monochromatic - has a certain frequency and wavelength. Due to this, it is perfectly focused optical lenses because the angle of refraction in the lens is constant.

Coherence - a coordinated course of time some wave processes. Incoherent light beam oscillations have different phases, with the result that can pay off each other. Coherent oscillations cause the same response, which increases the radiation power.

Thanks to the above properties, the laser beam can be focused on a very small surface of the metal and create her energy density of 10 8 W / cm 2 - enough to melt the metal and, consequently, welding.

For laser welding usually uses the following types of lasers:

• Solid-state and
• gas - a longitudinal or transverse gas flow, gas dynamics.

Laser welding is a solid-state laser
The scheme of solid-state laser is shown in the figure below. As an active body uses a rod of ruby ​​glass doped with neodymium (Nd-Glass), or aluminum-yttrium garnet doped with neodymium (Nd-YAG) or ytterbium (Yb-YAG). It is located in the lighting chamber. To excite the atoms of the active body of the pump light is used, creating a powerful flash of light.

 The solid-state laser process

The edges of the active body has a mirror - reflecting and partly transparent. The laser beam goes through a partially transparent mirror, after repeatedly being reflected inside the ruby ​​rod and thus amplified. Power solid-state lasers is relatively small and typically less than 1.6 kW.

Solid-state lasers due to their small capacity of only small parts are welded to a small thickness, typically objects of microelectronics. For example, findings from the finest welded wire 0.01-0.1 mm in diameter, made of tantalum, gold, nichrome. Can be spot-welding products from the point of the foil with a diameter of 0.5-0.9 mm. Laser welding is performed leakproof seam cathodes of CRT televisions today.

The cathode is a tube length of 2 mm, a diameter of 1.8 mm, wall thickness of 0.04 mm. Welded to the tube bottom thickness of 0.12 mm, the material products - chromium-nickel alloy. Welding of these small details can be high due to the focusing of the beam and precise dosage of energy by adjusting the pulse duration in the range 10 -2 -10 -7 s.

Welding gas laser
Are more powerful gas lasers in which the body is used as the active mixture of gases, usually CO 2 + N 2 + He. Diagram of a gas laser with a longitudinal gas flow is shown in the figure below. Gas is pumped from the tanks through the pump discharge tube. For the energy of excitation of the gas used an electric discharge between the electrodes. The edges of the gas discharge tubes arranged mirrors. The electrodes are connected to a power source. Laser cooling water system.

Gas laser with a longitudinal gas flow

The drawback of lasers with longitudinal pumping gas are their large dimensions.

A more compact lasers with transverse flow of gas (see figure below).

Gas laser with a transverse gas flow

They allow you to reach a total capacity of 20 kW or more, which makes it possible to weld metals up to 20 mm with a sufficiently high speed, about 60 m / h

The most powerful are the gas-dynamic lasers (shown below). For use of gas, heated to a temperature of 1000-3000 K. The gas will expire at supersonic speed through a Laval nozzle, resulting in its adiabatic expansion and cooling in the area of the cavity. Upon cooling, the excited molecules of CO 2 is a transition to a lower energy level with the emission of coherent radiation. Can be used to pump another laser or other powerful sources of energy. Such a laser power of N = 100 KW, for example, welded steel, 35 mm thick with a very high speed, about 200 m / h

Gasdynamic laser

Scheme of laser welding is shown in the figure below.

Laser welding process

Laser welding is performed at atmospheric conditions, without creating a vacuum, molten metal must be protected from the air. Typically used to protect the gases, particularly argon. Feature of the process of laser welding is that due to the high thermal capacity of the beam on the surface of the workpiece is evaporating metal. Couples are ionized, which leads to dissipation and shielding of the laser beam. In this regard, the use of high power lasers in the welding zone must be submitted, except for the security, the so-called plazmopodavlyayuschy gas. As plazmopodavlyayuschego gas commonly used helium, argon, which is much easier and does not dissipate the laser beam. To simplify the process appropriate to the use of mixtures of 50% Ar + 50% He, who perform plazmopodavlyayuschuyu and protective functions. In this case, the torch should ensure the supply of gas so that it blew the ionized vapor.

Structures burner nozzle for laser welding

When the laser beam welding gradually deepens into a part, pushing the molten metal weld pool to the rear wall of the crater. This allows you to get a "dagger" at large penetration depth and small width of the seam.

The high concentration of energy in the laser beam can reach high speeds welding, while ensuring a favorable thermal cycle and high technological strength of the weld metal.

The advantages of laser welding
The major advantage of laser welding of solid-state lasers is the possibility of a precise dosage of energy, and therefore can not provide high-quality connections to obtain the production of very small parts.

For high-power gas lasers, the advantage is to obtain a large depth of penetration in the small width of the seam. This reduces the heat-affected zone, to reduce the welding deformation and stress.

In addition, laser welding has several advantages not inherent in other methods of welding. The laser can be located far enough away from the welding area, which in some cases provides a significant economic effect. For example, a known system for laser welding in the repair of pipelines laid on the bottom of the pond. Inside the tube is moving truck with a rotating mirror. The laser is located at the end of the pipe section and sends the beam inside the pipe. This allows laser welding without removing the pipe from the ballast and not raising it to the surface.

Easy to control a laser beam using mirrors and fiber optics allows for welding in tight, and sometimes are not in line of sight locations. It is also possible laser welding a few parts from a single laser split-beam with prisms.

Disadvantages of laser welding
The disadvantages of laser welding is the high complexity and cost of equipment, the low efficiency of the lasers. With the development of laser technology, these disadvantages are eliminated.

See also:
laser cutting technology

Welding technology:

manual arc welding semi-automatic welding argon arc welding machine welding electroslag welding plasma welding laser welding electron beam welding

gas welding resistance welding spot welding seam welding projection welding butt welding welding underwater welding in space