1. Principle of laser welding: interaction between materials
The laser processing of metal materials is mainly based on the thermal processing of the photothermal effect. When the laser irradiates the surface of the material, various different changes will occur in the surface area of the material under different power densities. These changes include increasing surface temperature, melting, vaporization, keyhole formation, and photo-induced plasma generation. Moreover, changes in the physical state of the surface area of the material greatly affect the absorption of the laser by the material. In general, the higher the temperature, the higher the absorption rate of the material to the laser. With the increase of power density and action time, metal materials will undergo the following changes in physical states.
The core of laser welding is two: heat transfer and heat conduction. Heat transfer is related to heat source, power density and line energy. Heat conduction is mainly related to the heat dissipation and heat transfer speed of the material, which belongs to the inherent properties of the material, and can generally be fine-tuned by water cooling fixtures and protective gas flow. In the welding process, the heat source, power density and line energy are mainly adjusted. The process parameters involved are: laser core diameter, power, speed, defocus and so on.
There are two main types of laser welding according to the absorption rate of the welding process. One is thermal conductive welding (depth-to-width ratio < 1, laser absorption rate of red light within 20%, different wavelengths have differences). The other is deep penetration welding (depth-to-width ratio > 1, the absorption rate is greater than the absorption rate of the material melt pool, more than 60%, mainly the laser multiple reflection absorption in the keyhole).
Laser thermal conduction welding:
Different laser irradiance will cause different changes in the material state, which is reflected in the welding process as two typical welding modes: laser thermal conductivity welding and laser deep penetration welding. The heat transfer process, weld formation mechanism, technological characteristics and application range of the two are very different.
Laser thermal conduction welding mode:
During thermal conductive welding, the laser irradiance on the surface of the workpiece is in the range of 10E4~10E6W/cm. The laser energy is absorbed by the thin layer of the surface layer of 10 ~100m, and the laser energy of the surface layer is transmitted to the interior of the material by thermal conduction, and the laser can not be directly touched. After a certain period of laser irradiation, the surface reaches melting, and this melting isotherm propagates deep into the material, and the surface temperature continues to rise. However, the highest can only reach the boiling point of the material, and the higher the temperature, the material will vaporize to form a pit. The stable thermal conduction welding process will be damaged, the weld pool will oscillate, and the material will be burned. Generally, thermal conductive welding is mostly used in thin sheets, and this situation needs to be eliminated. With the relative movement of the laser beam and the workpiece, a shallow and wide weld is formed, as shown in the figure below. The depth-to-width ratio of the weld is small, and the width of the weld is generally more than 2 times the depth of penetration. The following figure shows the profile of a typical laser thermal conductivity welding weld, and the weld shape is approximately hemispherical.
laser deep penetration welding:
When the irradiance is greater than 10E7W/cm, the surface of the material melts and vaporizes under the action of the laser, and the generated vapor recoil pressure impacts the molten pool downward to form the keyhole. The light beam acts directly on the bottom of the keyhole, further melting and vaporizing the metal. High-pressure gas is continuously generated from the inside of the keyhole and continuously erupts outward, thus deepening the hole further. The beam also goes deeper and deeper, and the laser heat source also acts inside the material, transferring heat from the inside of the keyhole to the material to form a deeper heat-affected zone. At the same time, the keyhole is filled with plasma partially ionized by high-temperature vapor, and a certain range of plasma cloud is formed above the exit of the keyhole.
The keyhole effect plays an important role in the absorption of laser in the laser welding process. The laser beam entering the keyhole is almost completely absorbed through multiple reflections of the hole wall. As shown in the figure, if the keyhole is a conical surface (with an Angle of ∅), a light beam incident along the axis of the conical is reflected straight through the cone toward the bottom of the keyhole and is reflected, reflecting a total of 180°/∅. The absorption of steel is about 13% per reflection. When P=10° is set, the total absorptivity reaches 92% in 18 reflections, which is greatly improved compared with the thermal conductivity of 13%. The distinction between thermal conductivity and deep melting is generally according to the metallurgical melting depth: the melting width is greater than 1, which can be considered deep melting, because the appearance of keyholes improves the absorption rate. This simple method is suitable for single laser welding. Composite is not suitable for this judgment, which is generally deep penetration welding, and the center beam has a keyhole effect.
2. The welding metallographic comparison of laser with different core diameters
After understanding the basic power density, thermal conductivity welding, deep penetration welding concepts, then the power density of different core diameters and metallographic comparison analysis. This welding experiment is conducted for the common laser core diameter in the market.
From the power density point of view, under the same power, the finer the core diameter, the higher the laser brightness, the more concentrated the energy. If the laser is compared to a sharp knife, the smaller the core diameter of the laser, the sharper. The power density of 14um core diameter is more than 50 times that of 100um core diameter laser, and the processing capacity is stronger. At the same time, the power density calculated here is simply the average density. The actual energy distribution is approximately Gaussian, and the central energy will be several times the average power density.
Different core diameter laser comparison:
(1) The speed of the experiment is 150mm/s, the focus position is welded, the material is 1 series aluminum, 2mm thick.
(2) The larger the core diameter, the larger the melting width, the larger the heat-affected zone, and the smaller the unit power density. When the core diameter exceeds 200um, it is not easy to play deep penetration on high reverse alloys such as aluminum and copper, and higher power is required to achieve deep penetration welding.
(3) The small core laser has high power density, can quickly punch keyholes on the surface of the material at high energy, and the heat affected zone is small, but at the same time, the weld surface is rough, and the probability of keyhole collapse is high at low-speed welding. The welding cycle keyhole closing cycle is long, and is easy to produce defects, porosity and other defects, which is suitable for high-speed machining or processing with swing trajectory.
(4) Large core laser due to the large spot, and the energy is more dispersed, which is more suitable for laser surface remelting, cladding, annealing and other processes.
3. Simple application analysis
Advantages and Applications of Small Core Laser (< 100um)
(1) High inverse materials: aluminum, copper, stainless steel, nickel, molybdenum, etc.
High inverse materials need to choose a small core diameter laser. The high-power density laser beam is used to quickly heat the material to the liquefied or vaporized state, improve the laser absorption rate of the material, and realize efficient and rapid processing. Choose a laser with a large core diameter, which can easily lead to high reaction, resulting in virtual welding and even burning of the laser.
(2) Crack-sensitive materials: nickel, nickel-plated copper, aluminum, stainless steel, titanium alloy, etc.
This material generally requires strict control of the heat-affected zone, the need for a small molten pool, and the choice of a small core diameter laser is more appropriate.
(3) High-speed laser processing:
Deep penetration welding requires high-speed laser processing. It is necessary to choose a laser with high energy density to ensure that the line energy is enough to melt the material at high speed, especially for overlap welding, penetration welding, and other small core lasers with higher penetration requirements.
Advantages and Applications of Large Core Laser (> 100um)
Large core diameter, large light spot, large heat coverage area, wide application surface, and only to achieve the material surface micro-melting. It is very suitable for application in laser cladding, laser remelting, laser annealing, laser hardening and so on. In these areas, large spots mean higher production efficiency and lower defects (thermal conductive welding has almost no defects).
In welding, the large spot is mainly used for composite welding, which is used for composite laser with small core diameters. Large light spots make the surface of the material slightly melt, from solid to liquid, so that the absorption rate of the material to the laser is greatly improved. Then use a small core diameter to punch the keyhole and punch the penetration depth. In this process, due to the preheating and post-processing of large light spots and the large temperature gradient of the molten pool, the material is not easy to appear cracks caused by fast heating and fast cooling, and the appearance of the weld is smoother. The splash is lower than that of the single laser solution.
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