Laser Deep Penetration Welding: Things You Should Know

Laser welding process (mainly for sheet metal welding) can be divided into fiber continuous laser welding or YAG pulse laser welding according to the laser category.

According to the principle of laser welding, it can be divided into heat conduction welding and laser deep penetration welding.

When the power density is less than 104 ~ 105 W/cm², it is heat conduction welding. At this time, the penetration is shallow and the welding speed is slow;

When the power density is greater than 105 ~ 107 W/cm², the metal surface will be concave into “holes” under the action of heating to form deep penetration welding, which has the characteristics of fast welding speed and large depth width ratio.

The principle of heat conduction laser welding is: the surface to be processed is heated by laser radiation, the surface heat is guided by heat transfer and diffused internally, and the workpiece is melted to form a specific molten pool by controlling laser parameters such as laser pulse width, energy, peak power and repetition frequency, which is suitable for thin plate welding.

The laser welding machine used for gear welding and metallurgical sheet welding mainly relates to laser deep penetration welding.

The following focuses on the principle of laser deep penetration welding.

Aluminum shell lithium battery top cover welding - fiber continuous laser (new energy vehicle cell, mostly 3-Series aluminum)

Aluminum shell lithium battery top cover welding – fiber continuous laser (new energy vehicle cell, mostly 3-Series aluminum)

Principle of laser deep penetration welding

Laser deep penetration welding generally uses optical fiber continuous laser beam to connect materials.

Its metallurgical physical process is very similar to electron beam welding, that is, the energy conversion mechanism is completed through the “key hole” structure.

Under the irradiation of sufficiently high power density laser, the material evaporates and forms small holes.

The small hole filled with steam is like a blackbody, which absorbs almost all the incident beam energy.

The equilibrium temperature in the hole cavity reaches about 2500 ℃, and the heat is transmitted from the outer wall of the high-temperature hole cavity to melt the metal surrounding the hole cavity.

The small hole is filled with high-temperature steam generated by the continuous evaporation of wall material under the irradiation of light beam.

The four walls of the small hole are surrounded by molten metal and the liquid metal is surrounded by solid materials (in most conventional welding processes and laser conduction welding, energy is first deposited on the surface of the workpiece and then transmitted to the interior by transmission).

The liquid flow and wall surface tension outside the hole wall are in dynamic equilibrium with the continuous steam pressure in the hole cavity.

The light beam continuously enters the small hole, and the material outside the small hole is flowing continuously.

With the movement of the light beam, the small hole is always in a stable state of flow.

That is, the small hole and the molten metal surrounding the hole wall move forward with the forward speed of the leading beam, the molten metal fills the gap left after the small hole is removed and condenses, and the weld is formed.

All of the above processes happen so fast that the welding speed can easily reach several meters per minute.

6 series aluminum fiber CW laser welding (this is the high-speed rail floor)

6 series aluminum fiber CW laser welding (this is the high-speed rail floor)

Main process parameters of laser deep penetration welding

(1) Laser power

There is a laser energy density threshold in laser welding.

Below this value, the penetration is very shallow. Once it reaches or exceeds this value, the penetration will be greatly improved.

Only when the laser power density on the workpiece exceeds the threshold (related to material), plasma will be generated, which marks the progress of stable deep penetration welding.

If the laser power is lower than this threshold, only surface melting of the workpiece occurs, that is, the welding is carried out in a stable heat conduction type.

When the laser power density is near the critical condition of keyhole formation, deep penetration welding and conduction welding are carried out alternately, which becomes an unstable welding process, resulting in great fluctuation of penetration depth.

During laser deep penetration welding, the laser power controls the penetration depth and welding speed at the same time.

The weld penetration is directly related to the beam power density and is a function of the incident beam power and the beam focal spot.

Generally speaking, for a certain diameter laser beam, the penetration increases with the increase of beam power.

Kettle horse - YAG pulse laser welding (can directly make the appearance surface)

Kettle horse – YAG pulse laser welding (can directly make the appearance surface)

(2) Beam focal spot

Beam spot size is one of the most important variables in laser welding because it determines the power density.

However, for high-power laser, its measurement is a difficult problem, although there are many indirect measurement technologies.

The limit spot size of beam focus diffraction can be calculated according to the light diffraction theory, but the actual spot is larger than the calculated value due to the existence of focusing lens aberration.

The simplest measurement method is equal temperature profile method, that is, the focal spot and perforation diameter are measured after burning thick paper and penetrating polypropylene plate.

This method needs to master the laser power and the action time of the beam through measurement practice.

(3) Material absorption value

The laser absorption of materials depends on some important properties of materials, such as absorptivity, reflectivity, thermal conductivity, melting temperature, evaporation temperature and so on.

The factors affecting the absorptivity of the material to the laser beam include two aspects:

First, the resistance coefficient of the material. After measuring the absorptivity of the polished surface of the material, it is found that the absorptivity of the material is directly proportional to the square root of the resistance coefficient, and the resistance coefficient changes with temperature;

Second, the surface state (or finish) of the material .It has an important influence on the beam absorptivity, which has an obvious effect on the welding effect.

Material is a difficult problem.

According to the above, stainless steel and nickel are the materials with high purity and general conductivity, and the weldability is the best.

Copper, aluminum and other high conductivity materials are not easy to weld.

At present, it is verified that they can be welded within 6 series aluminum (3 Series, pure aluminum, etc.)

Yes, the welding of 6 series aluminum and above is easy to have cracks and pores.

The welding of copper generally depends on the application requirements. It can be welded with YAG pulse laser and fiber continuous laser.

The welding of gold and silver is generally spot welding in the jewelry industry. There are some applications, but there are few industrial applications. We focus on industrial applications.

The output wavelength of CO2 laser is usually 10.6 μm. The absorption rate of non-metallic materials such as ceramics, glass, rubber and plastics is very high at room temperature, while the absorption rate of metal materials is very poor at room temperature.

Once the material is melted or even gasified, its absorption increases sharply.

The method of surface coating or forming oxide film on the surface is very effective to improve the absorption of light beam.

(4) Welding speed

The welding speed has a great influence on the penetration.

Increasing the speed will make the penetration shallower, but too low speed will lead to excessive melting of materials and welding penetration of workpieces.

Therefore, there is a suitable welding speed range for a specific material with a certain laser power and thickness, and the maximum penetration can be obtained at the corresponding speed value.

Stainless steel YGA pulse laser wire filling welding (it can overcome the problem of large butt joint and appearance surface treatment in the later stage)

Stainless steel YGA pulse laser wire filling welding (it can overcome the problem of large butt joint and appearance surface treatment in the later stage)

(5) Shielding gas

Inert gas is often used to protect the molten pool in the process of laser welding.

When some materials can be welded without considering surface oxidation, protection can also be ignored.

However, helium, argon, nitrogen and other gases are often used for protection in most applications to protect the workpiece from oxidation during welding.

Helium is not easy to ionize (high ionization energy), which allows the laser to pass smoothly, and the beam energy can reach the workpiece surface unimpeded.

This is the most effective shielding gas used in laser welding, but the price is relatively expensive.

Argon is relatively cheap and has high density, so it has good protection effect.

However, it is easy to be ionized by high-temperature metal plasma.

As a result, it shields part of the beam from the workpiece, reduces the effective laser power of welding, and also damages the welding speed and penetration.

The surface of the weldment protected by argon is smoother than that protected by helium.

Nitrogen is the cheapest as a shielding gas, but it is not suitable for some types of stainless steel welding.

It is mainly due to metallurgical problems, such as absorption, and sometimes pores will be generated in the lap area.

The second function of using shielding gas is to protect the focusing lens from metal vapor pollution and liquid droplet sputtering.

Especially in high-power laser welding, it is more necessary to protect the lens because its ejecta becomes very powerful.

The third function of shielding gas is to disperse the plasma shielding produced by high-power laser welding.

The metal vapor absorbs the laser beam to ionize into a plasma cloud, and the shielding gas around the metal vapor will also ionize due to heating.

If there is too much plasma, the laser beam will be consumed by the plasma to some extent.

Plasma exists on the working surface as the second energy, which makes the penetration shallower and the surface of welding pool wider.

The recombination rate of electrons is increased by increasing the collision of electrons with ions and neutral atoms, so as to reduce the electron density in the plasma.

The lighter the neutral atom, the higher the collision frequency and the higher the recombination rate; on the other hand, only the shielding gas with high ionization energy will not increase the electron density due to the ionization of the gas itself.

Atomic (molecular) weight and ionization energy of common gases and metals

Material He Ar N Al Mg Fe
Atomic (molecular) weight 4 40 28 27 24 56
Ionization energy (eV) 24.46 15.68 14.5 5.96 7.61 7.83

It can be seen from the table that the size of plasma cloud varies with the shielding gas used.

Helium is the smallest, nitrogen is the second, and argon is the largest.

The larger the plasma size, the shallower the penetration.

The reason for this difference is not only the different ionization degree of gas molecules, but also the difference of metal vapor diffusion caused by different density of shielding gas.

Helium has the smallest ionization and density. It can quickly drive away the rising metal vapor generated from the metal molten pool.

Therefore, using helium as shielding gas can inhibit plasma to the greatest extent, so as to increase penetration and welding speed;

It can escape due to its light weight and is not easy to cause pores. Of course, from our actual welding effect, the effect of argon protection is good.

The effect of plasma cloud on penetration is most obvious in the low welding speed region.

When the welding speed increases, its influence will be weakened.

The shielding gas is emitted to the workpiece surface through the nozzle mouth at a certain pressure.

The hydrodynamic shape of the nozzle and the diameter of the outlet are very important.

It must be large enough to drive the sprayed shielding gas to cover the welding surface, but in order to effectively protect the lens and prevent metal vapor pollution or metal splash from damaging the lens, the size of the nozzle must also be limited.

The flow rate should also be controlled, otherwise the laminar flow of protective gas will become turbulent, the atmosphere will be drawn into the molten pool, and finally form pores.

In order to improve the protection effect, additional lateral blowing can also be used, that is, the shielding gas can be directly injected into the small hole of deep penetration welding at a certain angle through a small diameter nozzle.

The shielding gas not only suppresses the plasma cloud on the workpiece surface, but also affects the plasma in the hole and the formation of small holes.

The penetration depth is further increased and an ideal weld with depth width ratio is obtained.

However, this method requires accurate control of the size and direction of gas flow, otherwise it is easy to produce turbulence and damage the molten pool, resulting in the difficulty of stabilizing the welding process.

(6) Lens focal length

The laser is usually focused during welding, and the lens with focal length of 63 ~ 254mm (2.5 “~ 10”) is generally selected.

The focus spot size is directly proportional to the focal length. The shorter the focal length, the smaller the spot.

However, the focal length also affects the focal depth, that is, the focal depth increases synchronously with the focal length, so the short focal length can improve the power density.

However, due to the small focal depth, the distance between the lens and the workpiece must be accurately maintained, and the penetration is not large.

Due to the influence of spatter and laser mode during welding, the shortest focal depth used in actual welding is mostly 126mm (5 “).

When the joint is large or the weld needs to be increased by increasing the spot size, a lens with a focal length of 254mm (10 “) can be selected. In this case, in order to achieve the deep penetration keyhole effect, higher laser output power (power density) is required.

When the laser power exceeds 2kW, especially for 10.6 μm CO2 laser beam, due to the use of special optical materials to form the optical system, in order to avoid the risk of optical damage to the focusing lens, the reflection focusing method is often used, and the polished copper mirror is generally used as the mirror.

Because it can be cooled effectively, it is often recommended for high-power laser beam focusing.

(7) Focus position

During welding, in order to maintain sufficient power density, the focus position is very important.

The change of the relative position between the focus and the workpiece surface directly affects the weld width and depth.

In most laser welding applications, the focus is usually set at about 1 / 4 of the required penetration below the workpiece surface.

(8) Laser beam position

When laser welding different materials, the laser beam position controls the final quality of the weld, especially the butt joint is more sensitive than the lap joint.

For example, when the quenched steel gear is welded to the low-carbon steel drum, the correct control of the laser beam position will help to produce the weld composed mainly of low-carbon components, which has good crack resistance.

In some applications, the geometry of the workpiece to be welded requires an angle of laser beam deflection.

When the deflection angle between the beam axis and the joint plane is within 100 degrees, the absorption of laser energy by the workpiece will not be affected.

(9) Control of gradual increase and decrease of laser power at welding start and end points

In laser deep penetration welding, small holes always exist regardless of the depth of the weld.

When the welding process is terminated and the power switch is turned off, pits will appear at the end of the weld.

In addition, when the laser welding layer covers the original weld, excessive absorption of the laser beam will occur, resulting in overheating or porosity of the weldment.

In order to prevent the above phenomena, the power start and end points can be programmed to make the power start and end time adjustable, that is, the starting power rises from zero to the set power value in a short time by electronic method, and the welding time is adjusted.

Finally, the power is gradually reduced from the set power to zero at the end of welding.

Stainless steel optical fiber continuous laser welding (suitable for butt welding of small plates with a thickness of 0.2-3mm)

Stainless steel optical fiber continuous laser welding (suitable for butt welding of small plates with a thickness of 0.2-3mm)

Characteristics, advantages and disadvantages of laser deep penetration welding

(1) Characteristics of laser deep penetration welding

1) High aspect ratio

Because the molten metal is formed around a cylindrical high-temperature vapor cavity and extends to the workpiece, the weld becomes deep and narrow.

2) Minimum heat input

Because the temperature in the small hole is very high, the melting process occurs very fast, the input heat of the workpiece is very low, and the thermal deformation and heat affected zone are very small.

3) High density

Because the small hole filled with high-temperature steam is conducive to the stirring of welding pool and the escape of gas, resulting in the formation of penetration weld without pores.

The high cooling rate after welding is easy to refine the weld structure.

4) Strong weld

Because of the hot heat source and the full absorption of non-metallic components, the impurity content is reduced and the inclusion size and its distribution in the molten pool are changed.

The welding process does not require electrodes or filler wires, and the melting zone is less polluted, so that the strength and toughness of the weld are at least equal to or even higher than the parent metal.

5) Precise control

Because the focus spot is very small, the weld can be positioned with high accuracy.

The laser output has no “inertia” and can be stopped and restarted at high speed.

Complex workpieces can be welded with NC beam movement technology.

6) Non contact atmosphere welding process

Because the energy comes from the photon beam and has no physical contact with the workpiece, no external force is applied to the workpiece.

In addition, magnetism and air have no effect on the laser.

(2) Advantages of laser deep penetration welding

1) Because the focused laser has much higher power density than the conventional method, the welding speed is fast, the heat affected zone and deformation are very small, and it can also weld difficult materials such as titanium.

2) Because the beam is easy to transmit and control, the welding gun and nozzle do not need to be replaced frequently, and there is no vacuum pumping required for electron beam welding, which significantly reduces the shutdown auxiliary time, so the load factor and production efficiency are high.

3) Due to purification and high cooling rate, the weld has high strength, toughness and comprehensive properties.

4) Due to the low average heat input and high machining accuracy, the reprocessing cost can be reduced;

In addition, the operation cost of laser welding is also low, which can reduce the workpiece processing cost.

5) It can effectively control the beam intensity and fine positioning, and it is easy to realize automatic operation.

(3) Disadvantages of laser deep penetration welding

1) Limited welding depth.

2) High requirements for workpiece assembly.

3) The one-time investment of laser system is high

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