Working Principle Of Laser Welding, Laser Welding Process

Laser welding is the use of laser radiation energy to realize the effective welding technology, its working principle is: through the specific way to motivate the laser active medium (such as a gas mixture of CO2 and other gases, YAG yttrium aluminum garnet crystals, etc.), the reciprocating oscillation in the cavity, thus forming a stimulated radiation beam, when the beam is in contact with the workpiece, the energy absorbed by the workpiece when it in the temperature of the material melting point welding can be carried out.

Laser welding can be divided into heat conduction welding and deep fusion welding, the former heat diffusion through the heat transfer to the workpiece interior, only in the weld surface melting phenomenon, the workpiece interior is not completely molten, basically, no vaporization phenomenon, mostly used for low-speed thin wall material welding; The latter not only completely melts through the material, but also vaporizes the material, forming a large amount of plasma. Due to the large heat, keyholes will appear at the front end of the molten pool. Deep fusion welding can completely weld through the workpiece, and the input energy is large, welding speed is fast, is the most widely used laser welding mode.

Advantages of laser welding

1. Laser welding can be used to obtain high-quality joint strength and larger depth to width ratio, and welding speed is faster.

2. Because laser welding does not need a vacuum environment, through the lens and optical fiber, it can achieve remote control and automatic production.

3.  the laser has a large power density, difficult to weld materials such as titanium, quartz has a good welding effect, and can be applied to different properties of materials.

Micro welding can be carried out. The laser beam can be focused to obtain a small spot and can be accurately positioned, can be used in the mass automatic production of micro, small workpiece welding.

Disadvantages of laser welding

The price of laser and welding system accessories is relatively expensive, so the initial investment and maintenance costs are higher than the traditional welding technology, economic benefits are poor.

Due to the low absorption rate of solid materials to laser, especially after the emergence of plasma (plasma has an absorption effect on laser), so the conversion efficiency of laser welding is generally lower (usually 5% ~ 30%).

Because the laser welding focus spot is small, the workpiece joint precision requirements are higher, a small equipment deviation will produce a large machining error.

Is laser welding harmful to people?

Welding machine laser invisible and energy is too high, non-professionals do not go to contact the laser source, otherwise, it is dangerous. In addition, the laser also belongs to the electromagnetic wave, but the laser wavelength used by the welding machine is very large, so there is no radiation hazard of short-wavelength light waves such as ultraviolet.

Welding process will produce a lot of gas, but mostly inert gas, no toxicity, but also to see the different treatment of welding materials, it is best to take protective measures to reduce gas inhalation.

The laser from the welding machine has almost no radiation hazard, but there will be ionizing radiation and stimulated radiation in the welding process. It is better to stay away from the welding site during the welding process. This kind of induced radiation there is no lack of short wave, and on the eyes, the body is not small, it is best to stay away from the solder joint. For close operation, protective measures should be taken as far as possible, such as wearing respiratory protective equipment, wearing radiation protective clothing, and wearing an eye mask.

Important parameters of laser welding

Power density:

Power density is one of the most important parameters in laser machining. With higher power density, the surface layer can be heated to the boiling point in the microsecond time range, producing a large amount of vaporization. Therefore, high power density for material removal processing, such as drilling, cutting, the carving is very advantageous. For lower power density, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface vaporizes, the bottom layer reaches the melting point, which is easy to form a good melting welding.

Laser pulse waveform:

When the high-intensity laser beam hits the material surface, 60-98% of the laser energy will be reflected off the metal surface and lost, especially gold, silver, copper, aluminum, titanium and other materials with strong reflection and fast heat transfer. The reflectivity of a metal changes with time during a laser pulse. When the surface temperature of the material rises to the melting point, the reflectance will decrease rapidly. When the surface is in the melting state, the reflection will be stable at a certain value.

Laser pulse width:

Pulse width is an important parameter of pulsed laser welding. The pulse width is determined by the fusion depth and thermal influence zone. The longer the pulse width is, the larger the thermal influence zone is, and the fusion depth increases with the 1/2 power of the pulse width. However, the increase of pulse width will reduce the peak power, so the increase of pulse width is generally used for heat conduction welding, and the resulting weld is wide and shallow, especially suitable for lap welding of thin plate and thick plate.

However, the lower peak power leads to redundant heat input, and each material has an optimal pulse width that maximizes the penetration depth.

From the coke quantity:

Laser welding usually requires a certain amount of defocusing, because the power density in the center of the spot at the laser focus is too high, which can easily evaporate into holes. The power density distribution is relatively uniform on the plane leaving the laser focus.

There are two ways to defocus:

Positive defocus and negative defocus. The focal plane above the workpiece is positive defocus, and vice versa. According to the geometrical optics theory, when the distance between the positive and negative out-of-focus plane and the welding plane is equal, the power density on the corresponding plane is approximately the same, but in fact, the shape of the molten pool obtained is somewhat different. When negative defocusing, larger melting depth can be obtained, which is related to the formation process of the melting pool.

Welding speed:

The welding speed has a great influence on the penetration depth. If the welding speed is increased, the penetration depth will become shallow, but if the welding speed is too low, the material will be over melted and the workpiece will be welded through. Therefore, there is an appropriate welding speed range for a particular material of certain laser power and a certain thickness, in which the maximum penetration can be obtained at the corresponding speed value.

Protective gas:

Inert gases are often used to protect the pool during laser welding, and gases such as helium, argon, and nitrogen are often used in most applications. The second effect of shielding gas is to protect the focusing lens of metal vapor contamination and liquid sputtering. This is especially necessary in high-power laser welding, where the ejecta is very strong. A third effect of the shielding gas is to dissipate the plasma shielding produced by high-power laser welding. If there is too much plasma, the laser beam will be consumed by the plasma to some extent.