Main Characteristics of Laser Welding
Laser welding is a kind of laser processing technology, which is mainly used for welding thin-walled materials. In the heat conduction welding process, the surface of the workpiece is heated by laser radiation, and the surface heat is diffused to the inside through heat conduction. The components are melted by controlling the width, energy, peak power, and repetition frequency of the fiber laser pulse to form a specific molten pool. Because of its unique advantages, laser welding has been applied to precision welding of micro and small parts.
The emergence of high-power CO2 lasers and high-power YAG lasers brought laser welding into a new era. Deep penetration welding based on the keyhole effect has been widely used in machinery, automobiles, steel, and other industrial fields.
Advantages of Laser Welding
1. Fast speed, large depth, and small deformation.
2. Only simple welding equipment is needed, and laser welding can be welded at room temperature or in special conditions. For example, the laser beam will not be deflected by the electromagnetic field. Lasers can be used to weld in a vacuum, air, and certain gas environments, and it can also be used for welding glass or transparent materials.
3. It can weld refractory materials such as titanium and quartz, etc., and it also can weld opposite materials with good effect.
4. After laser focusing, the power density is high. In the welding of high-power devices, the depth-width ratio can be up to 5:1 and the highest can be up to 10:1.
5. Micro welding can be carried out. The laser beam can get a small spot after focusing and can be precisely positioned, which can be used in the welding of micro and small workpieces produced automatically in large quantities.
6. Laser welding is flexible, which can carry out non-contact remote welding of workpieces. Especially in recent years, the fiber optic transmission technology in YAG laser processing technology has made laser welding technology more widely promoted and applied.
7.Laser beam is easy to realize light beam splitting according to time and space and can be processed simultaneously with multiple beams and multiple stations, providing conditions for more precise welding.
However, Laser Welding Also Has Some Limitations
1. The assembly precision of welding parts is high, and the workpiece beam position should be stable. It is because the laser focusing spot after the rain inch small, narrow weld, to add filler metal material. If the workpiece assembly precision or beam positioning precision does not meet the requirements, it is easy to cause welding defects.
2. High cost of laser and its related systems, which is a large one-time investment.
Laser Welding Heat Conduction
Laser welding is to radiate a high-intensity laser beam to the metal surface, and the high temperature melts the metal to form a weld. Metal melting is just one of the physical phenomena of laser-metal interaction. Sometimes the energy is not the only way to weld metal, there are other ways, such as vaporization, plasma, etc. However, to weld the workpiece perfectly, laser welding is still the first choice. Therefore, it is necessary to understand the various physical phenomena of laser interaction with the medium.
Technological Parameters of Laser Welding
1. Power density
Power density is one of the most important parameters in laser machining. With a high power density, the surface can be heated to the boiling point in the microsecond time range, producing a large amount of vaporization. Therefore, high power density is beneficial for material removal processing, such as drilling, cutting, and engraving. For low power density, it takes several milliseconds for the surface temperature to reach the boiling point. Before the surface vaporizes, the bottom layer reaches the melting point, which is easy to melt welding. Therefore, the power density in conductive laser welding is in the range of 104~106W/CM2.
2. Laser pulse waveform
The laser pulse waveform is an important problem in laser welding, especially for wafer welding. When the high-intensity laser beam is shot to the material surface, the metal surface will have 60~98% of the laser energy reflected and lost, and its reflectivity changes with the surface temperature. The reflectivity of the metal varies greatly during a laser pulse.
3. Laser pulse width
Pulse width is an important parameter of pulse laser welding, and it is different from material removal and material melting. It also is the key parameter to determine the cost and volume of processing equipment.
4. Influence of defocus amount on the welding quality
Laser welding usually requires a certain amount of separation, because the laser focus at the spot center of the power density is too high, easy to evaporate into the hole. The power density distribution is relatively uniform in each plane away from the laser focus.
There are two ways of defocusing: positive defocusing and negative defocusing. If the focal plane is above the workpiece, it is positive defocusing, otherwise, it is negative defocusing. According to the geometric optics theory, when the positive and negative are equal, the power density is approximately the same, but the molten pool is different. When negative defocusing, a larger depth of fusion can be obtained, which is related to the fusion pool formation process. The experiment shows that the laser heating 50~ 200US material began to melt, forming liquid phase metal and evaporation, forming market pressure steam, and at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor makes the liquid metal move to the edge of the molten pool, forming a depression in the center of the molten pool. When negative defocusing, the material’s internal power density is higher than the surface, which is easy to form stronger melting and vaporization so that the light energy is transferred to the material deeper. So in practical application, negative defocus is used when the depth of fusion is required to be large. When welding thin materials, it is advisable to use positive defocus.
Laser Welding Process
1. Welding between sheets
It includes butt welding, end welding, center penetration fusion welding, and center perforation fusion welding.
2. Wire to wire welding
It includes the wire to wire butt welding, cross welding, parallel lap welding, T welding, and so on.
3. Welding of metal wire and block components
The connection between metal wire and block element can be realized successfully by laser welding, and the size of the block element can be arbitrary. Attention should be paid to the geometry of the filaments in welding.
4. Welding of different metals
Weld different types of metals to address weldability and range of weldability parameters. Laser welding between different materials is only possible for certain combinations of materials.
Laser fusion welding is not suitable for the connection of some components, but the laser can be used as a heat source to perform soft brazing and hard brazing, which also is the advantage of laser fusion welding. Laser soft brazing is used in the welding of printed circuit boards, especially in the assembly of chip components. Compared with other methods, laser soft brazing has the following advantages:
1. Due to local heating, the element is not easy to generate heat damage, and the heat affected area is small. Therefore, soft brazing can be carried out near the thermal sensing element.
2. With non-contact heating, melting bandwidth, do not need any auxiliary tools, which can be installed on the double-sided printed circuit board after processing.
3. Good stability in repeated operation. The flux has little pollution to welding tools, and the laser irradiation time and output power are easy to control, and the laser brazing yield is high.
4. The laser beam is easy to realize splitting, and the optical elements are used for time and space segmentation, such as semi-lens, mirror, prism, and scanning mirror, which can realize multi-point symmetric welding at the same time.
5.Laser brazing USES the laser with a wavelength of 1.06um as a heat source, which is transmitted by optical fiber. Therefore, it can be processed in parts that are not easy to be welded conventionally, with good flexibility.
6. Good focus, easy to realize the automation of the multi-station device.
Laser deep fusion welding
Metallurgical process and process theory
The physical process of laser deep fusion welding is very similar to that of electron beam welding, the energy conversion mechanism is accomplished through the “small hole” structure. At a beam of high enough power density, the material evaporates to form holes. The small hole filled with steam is like a black body, which absorbs almost all the incident light energy. The equilibrium temperature in the hole cavity reaches about 25,000 degrees. Heat is transferred from the outer wall of the high-temperature cavity, causing the metal surrounding the cavity to melt. The hole is filled with high-temperature steam generated by continuous evaporation of wall material under beam irradiation. The four walls of the hole are surrounded by molten metal, and the liquid metal is surrounded by solid material.
The fluid flow outside the pore wall and the wall layer surface tension are in constant equilibrium with the steam pressure in the pore cavity. The light beam keeps entering the hole, and the material outside the hole flows continuously. As the light beam moves, the hole is always in a stable state of flow. That is, the holes and the molten metal surrounding the walls of the holes move forward with the speed of the leading beam, filling in the gaps left by the holes and condensing as they move, creating a weld.
The factors influencing laser deep fusion welding include laser power, laser beam diameter, material absorption rate, welding speed, protective gas, lens focal length, focus position, laser beam position, laser power increasing, and decreasing control of welding starting and ending points.
Features and advantages of laser deep fusion welding
Features of laser deep fusion welding
(1) high depth-width ratio. As the molten metal is formed in the cylindrical high-temperature steam chamber and extends to the workpiece, the weld becomes deep and narrow.
(2) Minimum heat input. Due to the high temperature of the source cavity, the melting process is rapid, and the low temperature of the input workpiece, so that the thermal deformation and heat-affected area are very small.
(3) High density. Because the holes filled with high-temperature steam are conducive to the mixing and gas escaping from the welding pool, the non-porous penetration welding is formed. The high cooling rate after welding also makes the weld microstructure fine.
(4) Strong weld.
(5) Accurate control.
(6) Non-contact, atmospheric welding process.
(1) Because the focused laser beam has a higher power density than the traditional method, the welding speed is faster, the heat affected area is smaller, and the deformation is smaller. It can also weld refractory materials such as titanium and quartz.
(2) Because the beam is easy to transmit and control, and there is no need to change welding torch and nozzle frequently, the auxiliary time of shutdown is significantly reduced, so the load coefficient and production efficiency are both high.
(3) Due to the purification effect and high cooling rate, the weld is strong and the overall performance is high.
(4) Due to low balance heat input and high processing precision, reprocessing costs can be reduced. Besides, the laser welding switching cost is also relatively low, which can reduce the production cost.
(5) It is easy to realize automation, and can effectively control the beam intensity and precise positioning.
Laser deep fusion welding equipment
Continuous-wave CO2 laser is usually used for deep laser welding. This kind of laser can maintain high enough output power and produce a “small hole” effect, which melts through the whole workpiece cross-section and forms strong welded joints.
As far as the laser itself is concerned, it is simply a device that produces a well-directed parallel beam that can be used as a heat source. If it is directed and effectively processed into the workpiece, its input power has strong compatibility, so that it can better adapt to the automated process.
For more funny welding components, optical, mechanical and control components are indispensable. Together with the laser, they form a large welding system. The system includes a laser, a beam transmission module, a loading, unloading, and moving device for the workpiece, and a control device. The system may be simple manual handling and fixing of the workpiece by the operator, or it may include automatic loading, unloading, fixing, welding, and inspection of the workpiece. The overall requirements for the design and implementation of this system are to achieve satisfactory welding quality and high production efficiency.
Laser welding of steel and iron materials
Generally speaking, the laser welding quality of carbon steel is better. The quality of welding depends on the content of impurities. Like other welding processes, sulfur and phosphorus are sensitive factors for welding cracks.
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