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Which process parameters determine the welding quality?

Time: 2020-07-02 10:01 Reading: seven hundred and eighty-four second
Principle of laser welding
Laser welding machine 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, at this time, the penetration depth 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" by 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 as follows: the surface to be processed is heated by laser radiation, the surface heat is guided to internal diffusion by heat transfer, and the workpiece is melted by controlling the laser parameters such as laser pulse width, energy, peak power and repetition frequency to form a specific molten pool. The laser welding machine 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 material connection, and its metallurgical physical process is very similar to that of electron beam welding, that is, the energy conversion mechanism is completed through "key hole" structure. Under the irradiation of high power density laser, the material will evaporate and form small holes. The vapor filled hole is like a black body, absorbing almost all the energy of the incident beam. The equilibrium temperature in the cavity is about 2500 C. heat is transferred from the outer wall of the high-temperature cavity to melt the metal surrounding the 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 hole are surrounded by molten metal, and the liquid metal is surrounded by solid materials (while 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 outside the hole and the wall surface tension are in dynamic equilibrium with the continuous steam pressure in the cavity. With the beam moving, 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, and the molten metal fills the gap left by the removal of the small hole, and then condenses, and the weld is formed. All of this happens so quickly that the welding speed can easily reach several meters per minute. Which process parameters determine the welding quality?
Main process parameters of laser deep penetration welding

laser power

There is a threshold value of laser energy density in laser welding, below which the penetration is very shallow. Once it reaches or exceeds this value, the penetration will be greatly increased. 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 surface melting occurs, that is to say, welding is carried out with stable heat conduction. When the laser power density is near the critical condition of keyhole formation, deep penetration welding and conduction welding alternately become unstable welding process, which leads to great fluctuation of penetration. In laser deep penetration welding, laser power controls penetration depth and welding speed at the same time. The welding 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 beam

Laser spot density is one of the most important variables in determining laser spot size. But for high power laser, its measurement is a difficult problem, although there are many indirect measurement techniques.

However, due to the aberration of the focusing lens, the actual spot size is larger than the calculated value. The simplest measurement method is the isotherm profile method, which is to measure the diameter of focal spot and perforation after coking with thick paper and penetrating polypropylene plate. This method needs to master the laser power and the time of beam action through the measurement practice.  

Material absorption value

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

Second, the resistance of the material is directly proportional to the material's surface resistance, which is found to be directly related to the material's surface resistance The welding effect has obvious effect.  

The output wavelength of CO2 laser is usually 10.6 μ M. the absorption rate of non-metallic materials such as ceramics, glass, rubber and plastics is very high at room temperature, but the absorption of metal materials is very poor at room temperature. It is not until the material melts and even gasifies that its absorption 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 shallower, but if the speed is too low, it will lead to excessive melting of the material and welding penetration of the workpiece. Therefore, there is a suitable welding speed range for a certain material with a certain laser power and a certain thickness, 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.

Protective gas

Inert gas is often used to protect the molten pool during laser welding. When some materials are welded, the surface oxidation can not be considered. 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 (with high ionization energy), so the laser can pass through smoothly, and the beam energy can reach the surface of the workpiece unimpeded. This is the most effective shielding gas for laser welding, but the price is relatively expensive.  

Argon is cheaper and more dense, so the protection effect is better. However, it is easy to be ionized by high temperature metal plasma. As a result, part of the beam is shielded from the workpiece, which reduces the effective laser power and damages the welding speed and penetration. The surface of weldments protected by argon is smoother than that with 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 generate pores in the lap joint.

The second function of using shielding gas is to protect the focus lens from metal vapor contamination 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 absorbs the laser beam and ionizes into plasma cloud, and the protective gas around the metal vapor will also be ionized due to heating. If there is too much plasma, the laser beam is consumed by the plasma to some extent. As the second energy, plasma exists on the working surface, which makes the penetration shallower and the surface of welding pool wider. The electron recombination rate is increased by increasing the electron collision with ions and neutral atoms 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.  

Helium has the smallest ionization and density. It can quickly remove the rising metal vapor from the molten metal pool. Therefore, using helium as shielding gas can restrain the plasma to the greatest extent, thus increasing the penetration and increasing the welding speed. Of course, from our actual welding effect, the effect of argon protection is good.  

The influence 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 ejected from the nozzle to the surface of the workpiece 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 ejected shielding 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. The flow rate should be controlled, otherwise the laminar flow of the protective gas will become turbulent flow, and the atmosphere will be drawn into the molten pool and finally form pores.  

In order to improve the protection effect, additional side blowing method can be used, that is, through a small diameter nozzle, the shielding gas is directly injected into the keyhole of deep penetration welding at a certain angle. The shielding gas not only suppresses the plasma cloud on the surface of the workpiece, but also exerts influence on the plasma and the formation of the keyhole in the hole. The penetration depth is further increased, and the ideal weld seam with depth width ratio is obtained. However, this method requires accurate control of gas flow size and direction, otherwise it is easy to produce turbulence and damage the weld pool, which will lead to the welding process difficult to be stable.
Focal length of lens

When welding, the focus mode is usually used to focus the laser, and the lens with 63 ~ 254MM (2.5 "~ 10") focal length is generally selected. The focal spot size is proportional to the focal length. The shorter the focal length, the smaller the spot. But the focal length also affects the depth of focus, that is, the depth of focus 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 fact that the focal length of the object in the welding process is 1265 mm, which is affected by the actual welding process. When the seam is large or the spot size needs to be increased to increase the welding seam, a 254MM (10 ") focal length lens can be selected. In this case, higher laser output power (power density) is required to achieve deep penetration keyhole effect.

When the laser power exceeds 2kW, especially for CO2 laser beam of 10.6 μ m, 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 reflective focusing method is often used, and the polished copper mirror is generally used as the mirror. Due to its effective cooling, it is often recommended for high power laser beam focusing.  

Focal position

In order to maintain 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 weld width and depth. Figure 2-6 shows the influence of focus position on penetration depth and seam width of 1018 steel.  

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

Laser beam position

In laser welding of different materials, the position of laser beam controls the final quality of weld, especially the situation of butt joint is more sensitive than that of lap joint. For example, when the quenched steel gear is welded to a low carbon steel drum, the correct control of the laser beam position will be conducive to the production of welds mainly composed of low carbon components, which have good crack resistance. In some applications, the geometry of the workpiece to be welded requires a laser beam deflection angle. When the deflection angle between the beam axis and the joint plane is within 100 degrees, the laser energy absorption of the workpiece will not be affected.  

Control of laser power increasing and decreasing at the beginning and ending points of welding

In laser deep penetration welding, keyhole always exists regardless of the depth of 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, the laser beam will be absorbed excessively, which will lead to overheating or porosity.  

In order to prevent the above phenomenon, we can program the starting and ending points 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. Finally, the power is gradually reduced from the set power to zero value at the end of welding.  
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 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 welding pool stirring and gas escape, resulting in the formation of penetration weld without pores. The high cooling rate after welding is easy to make the microstructure of weld fine.  

4) Strengthen the weld. Because of the hot heat source and the full absorption of non-metallic components, the impurity content is reduced, and the size and distribution of inclusions 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 higher than the parent metal.  

5) Precise control. Because the focus light spot is very small, the welding seam can be positioned with high precision. Laser output has no "inertia" and can stop and restart at high speed. Complex workpieces can be welded with numerical control beam moving technology.  

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

Advantages of laser deep penetration welding

1) Due to the higher power density of focused laser than conventional methods, 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, there is no need to replace the welding gun and nozzle frequently, and there is no vacuum needed for electron beam welding, which can significantly reduce the auxiliary time of shutdown, so the load factor and production efficiency are high.

3) Due to the 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 low, which can reduce the processing cost of 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 welding depth is limited.

2) High requirements for workpiece assembly.

3) High one-time investment of laser system



Source: welding technology





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