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LASER WELDING

 

Laser welding (LW) is a welding technique used to join multiple pieces of metal through the use of a laser. The Laser Spot provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications, such as in the automotive industry.

 

Operation

 

Like electron welding (EW), laser 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 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 work piece.

 

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

 

LW 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 welding. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the work pieces. The high power capability of gas lasers make them especially suitable for high volume applications. LW 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
  • LW results in higher quality welds

 

A derivative of LW, laser-hybrid welding, combines the laser of LW 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.

 

The Advantages of Laser Welding over Traditional Welding Methods

 

Laser Welding

 

TIG and MIG welding have long been recognized as good choices for welding small components because of their excellent finish. However, such welding requires skill and dexterity, and despite their controllability, suffer from several disadvantages. Laser welding is an excellent substitute that frequently outperforms arc welding processes, and its tightly focused beam limits heating effects. Laser welding is capable of welding tasks beyond the capability of traditional welding methods.

 

Traditional Welding Methods

 

TIG and MIG processes use a shielding gas to create an inert atmosphere around the welding head. With TIG, the arc is created by a tungsten electrode, and a hand held filler material used, whereas with MIG welding the electrode is the filler wire. These welders can be adjusted to permit welding of delicate components and the final weld quality is high. Another frequently used method is spot-welding which works by clamping parts between a pair of electrodes and passing an electric current. All arc and spot-welding processes transfer a significant amount of heat to the work piece, affecting the metallurgical structure around the weld.

 

Laser Welding

 

The heat required for welding is supplied by a tightly focused light beam with a diameter as small as two-thousandths of an inch. Welding is conducted by firing a series of short pulses that melt the metal to create a high-quality weld. Depending upon the particular welding task, filler material may be required as with TIG welding. Because the laser beam is tightly focused, heat input is minimized and parts can be handled almost immediately.

 

How does laser welding work?

 

Laser welding operates in two fundamentally different modes: conduction limited welding and keyhole welding. The mode in which the laser beam will interact with the material it is welding will depend on the power density of the focused laser spot on the workpiece.

 

Conduction limited welding occurs when the power density is typically less than 105W/cm2. The laser radiation is absorbed only at the surface of the material and does not penetrate into the material. Therefore, conduction limited welds exhibit a high width to depth ratio.

 

Laser welding is more usually accomplished using higher power densities, by a keyhole mechanism. When the laser beam is focused to a small enough spot to produce a power density typically > 106-107 W/cm2, the workpiece surface vaporises before significant quantities of heat can be removed by conduction. The focused laser beam penetrates the workpiece and forms a cavity called a 'keyhole', which is filled with metal vapour or ionised metal vapour (plasma). This expanding vapour or plasma contributes to the prevention of the collapse of the molten walls of the keyhole in to this cavity. Furthermore, the coupling of the laser beam to the workpiece is improved dramatically by the formation of the keyhole. Deep penetration welding is then achieved by traversing the keyhole along the joint to be made (or moving the joint with respect to the laser beam) and results in welds with a high depth to width. Under the action of vapour pressure and surface tension, the molten material at the leading edge of the keyhole flows around the cavity created by the beam to the back, and solidifies to form the weld. This action leaves a top bead with a chevron pattern, which points towards the start of the weld.

 

Equipment

 

  • 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 (CO2 laser) as a medium.
  • Regardless of type, however, when the medium is excited, it emits photons and forms the laser beam

 

Solid state laser

 

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 laser

 

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

 

In fiber lasers, 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.

 


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