Laser welding main process parameters

1) Laser power. There is a laser energy density threshold in laser welding, below which the depth of melt is shallow, and once this value is reached or exceeded, the depth of melt increases substantially. Only when the laser power density on the workpiece exceeds the threshold (material dependent), plasma is generated, which marks the stabilization of deep fusion welding. If the laser power is below this threshold, the workpiece only undergoes surface melting, i.e. the welding proceeds in a stable heat transfer type. When the laser power density is near the critical condition of small hole formation, deep fusion welding and conduction welding alternate and become unstable welding processes, resulting in large fluctuations in the melt depth. In laser deep fusion welding, the laser power controls both the depth of penetration and the welding speed, as shown in Figure 1. The welding depth of melt is directly related to the beam power density and is a function of the incident beam power and beam focal spot. In general, for a certain diameter of the laser beam, the depth of melt increases as the beam power increases.

2) Beam focal spot. Beam spot size is one of the most important variables in laser welding, as it determines the power density. However, its measurement is a challenge for high power lasers, although many indirect measurement techniques are already available.

The beam focal diffraction limit spot size can be calculated from light diffraction theory, but the actual spot is larger than the calculated value due to the presence of focusing lens aberration. The simplest real measurement method is the isothermal profile method, which is to measure the focal spot and perforation diameter after burning and penetrating a polypropylene plate with thick paper. This method should be measured by practice, mastering the size of the laser power and the time of the beam action.

3) Material absorption value. The absorption of laser by the material depends on some important properties of the material, such as absorption rate, reflectivity, thermal conductivity, melting temperature, evaporation temperature, etc. The most important one is the absorption rate.

Factors affecting the absorption rate of the material to the laser beam include two aspects: firstly, the resistivity of the material. After measuring the absorption rate of the polished surface of the material, it is found that the material absorption rate is proportional to the square root of the resistivity coefficient, which in turn varies with temperature; secondly, the surface state (or finish) of the material has a more important effect on the absorption rate of the beam, thus having a significant effect on the welding effect.

CO2 laser output wavelength is usually 10.6μm, ceramics, glass, rubber, plastic and other non-metals on its absorption rate at room temperature is very high, while metal materials at room temperature on its absorption is very poor, until the material once melted or even vaporized, its absorption increased sharply. The use of surface coating or surface generation of oxide film method to improve the absorption of the material to the beam is very effective.

4) welding speed. Welding speed has a large impact on the depth of melt, increase the speed will make the depth of melt shallow, but the speed is too low and will lead to excessive melting of the material, the workpiece weld through. Therefore, a certain laser power and a certain thickness of a particular material has a suitable range of welding speed, and in which the corresponding speed value can be obtained when the maximum depth of melt. Figure 2 gives the relationship between welding speed and melt depth of 1018 steel.

5) Protective gas. Laser welding process often use inert gas to protect the melt pool, when some materials welded regardless of surface oxidation, then also do not consider protection, but for most applications are often used helium, argon, nitrogen and other gases for protection, so that the workpiece from oxidation during the welding process.

Helium is not easily ionized (ionization energy is high), allowing the laser to pass through and the beam energy to reach the surface of the workpiece unimpeded. It is the most effective shielding gas used in laser welding, but is more expensive.

Argon is cheaper and more dense, so it protects better. However, it is susceptible to high temperature metal plasma ionization, which results in shielding part of the beam to the workpiece, reducing the effective laser power for welding and also impairing the welding speed and depth of melt. The surface of the welded part is smoother with argon protection than with helium protection.

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, which sometimes produces porosity in the lap zone.

The second role of using a shielding gas is to protect the focusing lens from metal vapor contamination and sputtering of liquid molten droplets. This is especially necessary in high power laser welding, where the ejecta become very powerful.

A third function of the shielding gas is that it is effective in dispersing the plasma shielding produced by high-power laser welding. The metal vapor absorbs the laser beam and ionizes into a plasma cloud, and the shielding gas around the metal vapor is also ionized by the heat. If too much plasma is present, the laser beam is consumed by the plasma to some extent. The presence of plasma as a second energy on the working surface makes the depth of melt shallower and the weld pool surface wider. The rate of electron complexation is increased by increasing the number of electron-ion and neutral-atom three-body collisions to reduce the electron density in the plasma. The lighter the neutral atom, the higher the collision frequency, the higher the compound rate; on the other hand, only the high ionization energy of the shielding gas, so as not to increase the electron density due to the ionization of the gas itself.

As can be seen from the table, the plasma cloud size varies with the protective gas used, with helium being the smallest, followed by nitrogen, and the largest when argon is used. The larger the plasma size, the shallower the melting depth. The reason for this difference is firstly due to the different degree of ionization of the gas molecules and also due to the difference in the diffusion of the metal vapor caused by the different densities of the protective gases.

Helium is the least ionized and the least dense, and it quickly dispels the rising metal vapor from the molten metal pool. Therefore, the use of helium as a shielding gas can maximize the suppression of plasma, thereby increasing the depth of melt and improving the welding speed; it is not easy to cause porosity because of its light weight and ability to escape. Of course, from our actual welding results, the effect of protection with argon gas is not bad.

Plasma cloud on the depth of melt in the low welding speed zone is the most obvious. When the welding speed increases, its influence will be weakened.

The shielding gas is ejected through the nozzle opening at a certain pressure to reach the workpiece surface. The hydrodynamic shape of the nozzle and the size of 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 contamination or metal spatter damage to the lens, the nozzle size should also be limited. The flow rate should also be controlled, otherwise the laminar flow of shielding gas becomes turbulent and the atmosphere becomes involved in the molten pool, eventually forming porosity.

In order to improve the protection effect, also available additional lateral blowing way, that is, through a smaller diameter nozzle will be the protective gas to a certain angle directly into the deep molten weld hole. 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, further increasing the depth of fusion and obtaining a deeper and wider weld seam than is desirable. However, this method requires precise control of gas flow size and direction, otherwise it is easy to produce turbulence and damage the melt pool, resulting in the welding process is difficult to stabilize.

6) Lens focal length. Welding is usually used to focus the way the laser convergence, the general choice of 63 ~ 254mm (2.5 "~ 10") focal length of the lens. Focused spot size is proportional to the focal length, the shorter the focal length, the smaller the spot. But the focal length also affects the focal depth, that is, the focal depth increases simultaneously with the focal length, so the short focal length can improve the power density, but because of the small focal depth, the distance between the lens and the workpiece must be accurately maintained, and the melting depth is not large. Due to the influence of the spatter generated during the welding process and the laser mode, the actual welding using the shortest depth of focus more focal length 126mm (5"). When the seam is large or the weld seam needs to be increased by increasing the spot size, a lens with a focal length of 254mm (10") can be selected, in which case a higher laser output power (power density) is required to achieve a deep melt small hole effect.

When the laser power exceeds 2kW, especially for the 10.6μm 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, often choose the reflection focusing method, generally using polished copper mirror for the reflector. Due to the effective cooling, it is often recommended for high power laser beam focusing.

7) focal point position. Welding, in order to maintain sufficient power density, the focal point position is critical. Changes in the position of the focal point relative to the workpiece surface directly affect the weld width and depth. Figure 3 shows the effect of focal point position on the depth of melt and seam width of 1018 steel. In most laser welding applications, the focal point is typically positioned approximately 1/4 of the desired depth of melt 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 in the case of butt joints which are more sensitive to this than lap joints. For example, when hardened steel gears are welded to mild steel drums, proper control of the laser beam position will facilitate the production of a weld with a predominantly low carbon component, which has better crack resistance. In some applications, the geometry of the workpiece to be welded requires the laser beam to be deflected by an 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.

9) Welding start and end point of the laser power gradual rise, gradual decline control. Laser deep fusion welding, regardless of the depth of the weld, the phenomenon of small holes always exist. When the welding process is terminated and the power switch is turned off, a crater 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 weld.

In order to prevent the above phenomena, the power start and stop points can be programmed so that the power start and stop times become adjustable, i.e. the starting power is electronically increased from zero to the set power value in a short period of time and the welding time is adjusted, and finally the power is gradually reduced from the set power to the zero value when the welding is terminated.