Main process parameters setting of laser welding in Suzhou

1. Principle of laser welding

Laser welding can be realized by continuous or pulsed laser beam. The principle of laser welding 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; When the power density is greater than 105 ~ 107 w / cm2, the metal surface is concave into "holes" under the action of heating, forming deep penetration welding, which has the characteristics of fast welding speed and large depth width ratio.

The principle of heat conduction laser welding is: laser radiation heats the surface to be processed, surface heat is guided to internal diffusion through heat transfer, 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.

The laser welding machine used for gear welding and metallurgical sheet welding mainly involves laser deep penetration welding. The following focuses on the principle of laser deep penetration welding.

Laser deep penetration welding generally uses continuous laser beam to complete the connection of materials, and its metallurgical physical process is very similar to electron beam welding, that is, the energy conversion mechanism is completed through the "key hole" structure. When the laser power density is high enough, the material will evaporate and form small holes. The vapor filled hole is like a black body, which absorbs almost all the energy of the incident light beam. The equilibrium temperature in the hole cavity is about 2500 C. the heat is transferred from the outer wall of the high temperature hole cavity to melt the metal surrounding the hole cavity. The keyhole is filled with high-temperature steam generated by continuous evaporation of wall material under the irradiation of light beam. The four walls of the keyhole are surrounded by molten metal, and the liquid metal is surrounded by solid material (in most conventional welding processes and laser conduction welding, the energy is deposited on the surface of the workpiece first, and then transferred to the interior by transmission). The liquid flow outside the hole wall and the surface tension of the wall are in dynamic equilibrium with the continuous vapor pressure in the hole cavity. The material outside the hole is flowing continuously. With the movement of the beam, the hole is always in a stable state of flow. That is to say, the keyhole 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 by the keyhole moving away and condenses, and the weld is formed. All of this happens so fast that the welding speed can easily reach several meters per minute.

2. Main process parameters of laser deep penetration welding

laser power 

There is a threshold value of laser energy density in laser welding. If it is lower than 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 the material), the plasma will be generated, which marks the stable deep penetration welding. If the laser power is lower than this threshold, only the surface of the workpiece is melted, that is, the welding is carried out in a stable heat conduction mode. 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. In laser deep penetration welding, laser power controls 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.

Focal spot

Spot size is one of the most important variables in laser welding, because it determines the power density. But for high power laser, its measurement is a difficult problem, although there are many indirect measurement techniques.

The diffraction limit spot size can be calculated according to the diffraction theory, but the actual spot size is larger than the calculated value due to the aberration of the focusing lens. The simplest measurement method is equal temperature profile method, which is to measure the diameter of focal spot and perforation after burning and penetrating polypropylene plate with thick paper. This method needs to master the laser power and the time of beam action through measurement practice.

Material absorption value

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

There are two factors that affect the absorptivity of laser beam: the first is the resistance coefficient of the material. After measuring the absorptivity of the polished surface, it is found that the absorptivity of the material is proportional to the square root of the resistance coefficient, and the resistance coefficient changes with temperature; Secondly, the surface state (or finish) of the material has an important influence on the beam absorptivity, which has a significant effect on the welding effect.

The output wavelength of CO2 laser is usually 10.6 μ m. The absorption rate of non-metal 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 materials are melted or even gasified, the absorption rate increases sharply. It is very effective to improve the absorption of light beam by coating or forming oxide film on the surface.

welding speed 

The welding speed has a great influence on the penetration. Increasing the welding speed will make the penetration shallow, but too low the welding speed will lead to excessive melting of the material and penetration of the workpiece. Therefore, for a certain laser power and a certain thickness of a specific material, there is a suitable welding speed range, and the maximum penetration can be obtained at the corresponding speed value. Figure 10-2 shows the relationship between welding speed and penetration of 1018 steel.

Shielding gas

Inert gas is often used to protect the molten pool in laser welding. When some materials are welded without surface oxidation, the protection can also be ignored. However, helium, argon, nitrogen and other gases are often used to protect the workpiece from oxidation in most applications.

Helium is not easy to ionize (the ionization energy is high), so the laser can pass through smoothly, and the beam energy can reach the surface of the workpiece without obstruction. This is the most effective shielding gas for laser welding, but the price is relatively expensive.

Argon gas is cheaper and has higher density, so the protection effect is better. But it is easy to be ionized by high temperature metal plasma. As a result, part of the beam is shielded from the workpiece, the effective laser power is reduced, and the welding speed and penetration are also damaged. The surface of the weldment protected by argon is smoother than that protected by helium.

Nitrogen is the cheapest shielding gas, but it is not suitable for some types of stainless steel welding, mainly due to metallurgical problems, such as absorption, 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, because the ejecta becomes very powerful, it is more necessary to protect the lens.

The third function of shielding gas is to disperse the plasma shield produced by high power laser welding. The metal vapor is ionized into plasma cloud by absorbing laser beam, and the protective gas around the metal vapor is ionized by heating. If there is too much plasma, the laser beam is consumed by the plasma to some extent. As the second kind of energy, plasma exists on the working surface, which makes the weld penetration shallow and the weld pool surface wide. In order to reduce the electron density in the plasma, the recombination rate of electrons is increased by increasing the collision of electrons with ions and neutral atoms. The lighter the neutral atom is, the higher the collision frequency is and the higher the recombination rate is; 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.

Table atomic (molecular) weight and ionization energy of common gases and metals

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Material helium argon nitrogen aluminum magnesium iron

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Atomic (molecular) weight 4 40 28 27 24 56

Ionization energy (EV) 24.4615.6814.55.967.617.83

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It can be seen from the table that the size of plasma cloud varies with the protective gas used. Helium is the smallest, followed by nitrogen, and argon is the largest. The larger the plasma size is, the shallower the penetration is. The reason for this difference is not only the different ionization degree of gas molecules, but also the different diffusion of metal vapor due to the different density of shielding gas.

Helium has the smallest ionization and density, which can quickly remove the rising metal vapor from the molten pool. Therefore, using helium as shielding gas can suppress plasma to the greatest extent, so as to increase penetration and welding speed; Because it is light and can escape, it is not easy to cause pores. Of course, from our actual welding effect, the effect of argon protection is not bad.

The effect of plasma cloud on penetration is most obvious in low welding speed zone. When the welding speed increases, its influence will be weakened.

The protective gas is injected to the surface of the workpiece through the nozzle with 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 protective gas to cover the welding surface, but in order to effectively protect the lens and prevent metal vapor pollution or metal spatter from damaging the lens, the nozzle size should also be limited. Otherwise, the laminar flow of shielding gas will become turbulent, and the atmosphere will be involved in the molten pool and eventually form pores.

In order to improve the protection effect, additional lateral blowing can be used, that is, the protective gas can be directly injected into the 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 surface of the workpiece, but also exerts an influence on the plasma in the hole and the formation of the small hole, and further increases the penetration, so as to obtain an ideal weld with depth width ratio. However, this method requires accurate control of gas flow size and direction, otherwise it is easy to produce turbulence and damage the molten pool, which makes the welding process difficult to be stable.

Lens focal length

When welding, the laser is usually focused, and the lens with focal length of 63 ~ 254MM (2.5 "~ 10") is generally selected. The focus spot size is proportional to the focal length. The shorter the focal length is, the smaller the spot is. But the length of focal length also affects the depth of focus, that is, the depth of focus increases with the focal length, so the short focal length can improve the power density, but because the focal depth is small, 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, the shortest focal depth used in welding is 126mm (5 "). When the seam is large or the spot size needs to be increased to increase the weld, the lens with 254MM (10 ") focal length can be selected. In this case, in order to achieve the keyhole effect, higher laser output power (power density) is required.

When the laser power exceeds 2kW, especially for 10.6kw μ In order to avoid the risk of optical damage to the focusing lens, the reflective focusing method is often used, and the polished copper mirror is usually used as the mirror. Due to its effective cooling, it is often recommended for high power laser beam focusing.

Focus position

In order to keep enough power density, the focus position is very important. The change of the relative position between the focus and the workpiece surface directly affects the width and depth of the weld. Figure 2-6 shows the effect of focus position on the penetration and seam width of 1018 steel.

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

Laser beam position

When laser welding different materials, the position of laser beam controls the final quality of 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 mainly composed of low carbon components, which has good crack resistance. In some applications, the geometry of the workpiece to be welded needs a laser beam deflection angle. 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.

Control of laser power increasing and decreasing at the beginning and end of welding

In laser deep penetration welding, no matter how deep or shallow the weld is, small holes always exist. 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, there will be excessive absorption of the laser beam, resulting in overheating or porosity of the weldment.

In order to prevent the above phenomenon, we can program the starting and ending point of power to make the starting and ending time of power adjustable, that is, the starting power is raised from zero to the set power value in a short time by electronic method, and the welding time is adjusted, and finally the power is gradually reduced from the set power to zero at the end of welding.

3. Characteristics, advantages and disadvantages of laser deep penetration welding

Characteristics of laser deep penetration welding

1) High aspect ratio. Because the molten metal forms around the 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 hole is very high, the melting process occurs very fast, the heat input into the workpiece is very low, and the thermal deformation and heat affected zone are very small.

3) High density. Because the keyhole filled with high temperature vapor is conducive to the weld pool stirring and gas escape, resulting in the formation of the penetration weld without pores. The high cooling rate after welding makes the weld microstructure fine.

4) Strengthen the weld. Because of the hot heat source and the full absorption of non-metallic components, the impurity content is reduced, the size of inclusions and their distribution in the molten pool are changed. The welding process does not need electrode or filler wire, and the melting zone is less polluted, which makes the weld strength and toughness at least equal to or even exceed the parent metal.

5) Precise control. Because the focus spot is very small, the weld can be positioned with high accuracy. Laser output has no "inertia", it can stop and restart at high speed, and complex workpiece can be welded with numerical control beam moving technology.

6) Non contact atmosphere welding process. Because the energy comes from the photon beam, there is no physical contact with the workpiece, so there is no external force on the workpiece. In addition, magnetism and air have no effect on the laser.

Advantages of laser deep penetration welding

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

2) Because the beam is easy to transmit and control, and there is no need to change the welding gun and nozzle frequently, and there is no vacuum required for electron beam welding, which significantly reduces the downtime, 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) Because of 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 lower, which can reduce the processing cost of the workpiece.

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

Disadvantages of laser deep penetration welding

1) The depth of welding is limited.

2) The requirement of workpiece assembly is high.

3) One time investment of laser system is high