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Process Analysis Of Laser Welding


Jan 02.2018
1. The process parameters of laser welding
1. Power density. Power density is one of the most important parameters in laser processing. With higher power density, the surface can be heated to the boiling point in the range of microsecond time, producing a large amount of vaporization. Therefore, high power density is beneficial to material removal, such as drilling, cutting, and carving. For low power density, the surface temperature reaches a boiling point and needs a few milliseconds. Before the surface vaporization, the bottom layer reaches melting point and easy to form good fusion welding. Therefore, in the conductive laser welding, the power density is in the range of 104~106W/cm2.

2. Laser pulse waveform. Laser pulse waveform is an important problem in laser welding, especially for thin sheet welding. When the high intensity laser beam is applied to the surface of the material, the surface of the metal will be reflected by the laser energy of 60~98%, and the reflectivity varies with the surface temperature. During the period of the action of a laser pulse, the reflectance of the metal varies greatly.

3. The width of the laser pulse. Pulse width is one of the important parameters of pulsed laser welding. It is not only an important parameter which is different from material removal and material melting, but also a key parameter to decide the cost and volume of processing equipment.

4. The effect of the amount of defocus on the welding quality. Laser welding usually requires a certain amount of defocus, because the power density of the center of the laser spot is too high and easily evaporates into the hole. The distribution of power density is relatively uniform on each plane leaving the focus of the laser. There are two ways of defocus: positive defocus and negative defocus. The focal plane is located at the top of the workpiece as a positive defocus, and vice versa. According to the geometrical optics theory, when the positive and negative is equal to the article, the power density on the corresponding plane is approximately the same, but actually the shape of the molten pool is different. At the time of negative defocus, a greater depth of melting can be obtained, which is related to the formation of the molten pool. Experiments show that laser heating of 50~200us material begins to melt, forming liquid phase metal and vaporization, forming a pressurized steam, and emitting at a very high speed, emitting brilliant white light. At the same time, the high concentration vapour moves the liquid metal to the edge of the molten pool and forms a depression at the center of the molten pool. In the case of negative defocus, the internal power density of the material is higher than the surface, and it is easy to form a stronger melting and vaporization, so that the light energy is transferred to the deeper of the material. Therefore, in the practical application, when the melting depth is large, the negative defocus is adopted; when the thin material is welded, it is appropriate to use the positive defocus.

Two. Laser welding technology:

1, the welding between pieces and pieces. It includes 4 methods, such as welding, end welding, center penetration and melting welding, central perforation melting and so on.

2, the welding of silk and silk. It includes 4 methods, which are wire and silk welding, cross welding, parallel lap welding and T welding.

3, the welding of the wire and the block element. The connection between the wire and the block element can be successfully realized by laser welding, and the size of the block element can be arbitrary. The geometrical size of the filamentous element should be paid attention to in welding.

4. Welding of different metals. Welding of different types of metal to solve the weldability and weldable parameters range. Laser welding between different materials is possible only with certain combination of materials. It is not suitable for laser brazing to connect some components with laser welding. However, laser can be used as heat source for soft brazing and brazing, and also has the advantage of laser fusion. There are many kinds of brazing, of which laser soldering is mainly used in the welding of printed circuit board, especially for the assembly of the bar element.

Two. Laser welding technology:

1, the welding between pieces and pieces. It includes 4 methods, such as welding, end welding, center penetration and melting welding, central perforation melting and so on.

2, the welding of silk and silk. It includes 4 methods, which are wire and silk welding, cross welding, parallel lap welding and T welding.

3, the welding of the wire and the block element. The connection between the wire and the block element can be successfully realized by laser welding, and the size of the block element can be arbitrary. The geometrical size of the filamentous element should be paid attention to in welding.

4. Welding of different metals. Welding of different types of metal to solve the weldability and weldable parameters range. Laser welding between different materials is possible only with certain combination of materials. It is not suitable for laser brazing to connect some components with laser welding. However, laser can be used as heat source for soft brazing and brazing, and also has the advantage of laser fusion. There are many kinds of brazing, of which laser soldering is mainly used in the welding of printed circuit board, especially for the assembly of the bar element.

Four. Laser deep fusion welding:

1. The theory of metallurgical process and process. The metallurgical physical process of laser deep fusion welding is similar to that of electron beam welding, that is, the mechanism of energy conversion is accomplished through the "small hole" structure. At a high enough power density beam, the material produces vaporization to form a small hole. This vapour hole is like a black body that almost absorbs the energy of the incident light, and the balance temperature in the cavity is about 25000 degrees. The heat is transferred from the outer wall of the high - temperature cavity to melt the metal that surrounds the cavity. The hole filled with high temperature steam generating material wall continuous beam evaporation in the hole walls surrounded by molten metal, the liquid metal is surrounded around the solid material. The fluid flow outside the hole wall and the surface tension of the wall maintain a dynamic equilibrium with the continuous steam pressure produced in the hole. The beam continues to enter the small hole, and the material is flowing continuously in the small hole. As the beam moves, the hole is always in the steady state of flow. That is to say, the pores and the molten metal around the hole wall move along with the advancing speed of the leading beam. The molten metal fills the gap left after the hole is removed and condenses, and the weld is formed.

2. Influence factors. The factors that influence the laser deep penetration welding include: laser power, laser beam diameter, material absorption rate, welding speed, shielding gas, focal length of lens, focal position, laser beam position, welding power initiation and termination point of laser power gradual increase and decline control.

3. Characteristics of laser deep fusion welding:

Features: (1) high ratio of depth to width. The weld becomes deep and narrow because the molten metal is formed and extended to the workpiece by a cylindrical high temperature steam cavity. (2) minimum heat input. Because the temperature of the source cavity is very high, the melting process takes place very quickly, the heat of the input workpiece is very low, and the thermal deformation and heat affected zone are very small. (3) high density. Because of the small hole filled with high temperature steam, it is beneficial to the stirring of weld pool and gas escaping, resulting in the formation of no hole penetration welding. The high cooling rate after welding is easy to make the microstructure of the weld fine. (4) strong weld. (5) accurate control. (6) non-contact, atmospheric welding process.

4, the advantages of laser deep penetration welding are as follows: (1) because the focused laser beam has much higher power density than the conventional method, the welding speed is fast, the heat affected zone and the deformation are all smaller, and it can also weld hard welding materials such as titanium and quartz. (2) because the beam is easy to transmit and control, and it doesn't need to replace torch and nozzle frequently, it can significantly reduce the auxiliary time of shutdown, so the load factor and production efficiency are all high. (3) because of the purification and high cooling rate, the weld is strong and the comprehensive performance is high. (4) because of low balance heat input and high processing precision, the cost of reprocessing can be reduced. In addition, the cost of laser welding is also low, which can reduce the cost of production. (5) it is easy to realize automation and control the intensity of light beam and the precision of fine positioning.

5, laser deep penetration welding equipment: laser deep penetration welding usually selects continuous wave CO2 laser, which can maintain enough high output power, produce "small hole" effect, penetrate the whole workpiece section, and form strong and tough welded joints. As far as the laser is concerned, it is only a device that produces parallel beams that can be used as a heat source and a good direction. If it is directed and effectively treated to the workpiece, its input power has a strong compatibility, so that it can better adapt to the process of automation. In order to effectively carry out the welding, the laser and other necessary optical, mechanical and control components together form a large welding system. The system includes lasers, beam transmission components, workpiece loading and unloading and mobile devices, and control devices. The system can be simply transported and fixed manually by operators, or it can be automatically loaded, unloaded, fixed, welded and inspected. The general requirements for the design and implementation of this system are satisfactory welding quality and high production efficiency.

Five. Laser welding of steel materials:

1, laser welding of carbon steel and ordinary alloy steel. In general, carbon steel laser welding has a good effect, and its welding quality depends on the content of impurities. Like other welding processes, sulfur and phosphorus are a sensitive factor in producing welding cracks. In order to obtain satisfactory welding quality, it needs to be preheated when the carbon content is over 0.25%. When the steel with different carbon content is welded to each other, the torch can be slightly inclined to the low carbon material to ensure the quality of the joint. Low carbon boiling steel is not suitable for laser welding because of its high content of sulfur and phosphorus. Low carbon sedation steel has a good welding effect because of its low impurity content. In general, high carbon steel and alloy steels are good laser welding, but requires preheating and welding postprocessing, to eliminate stress, avoid crack formation.

2. Laser welding of stainless steel. In general, laser welding of stainless steel is more easy to obtain superior quality joint than conventional welding. Due to the small impact zone of high welding speed, sensitization is not an important problem. Compared with carbon steel, the low thermal conductivity of stainless steel is easier to obtain deep weld narrow weld.

3. Laser welding between different metals. The extremely high cooling rate and small heat affected zone of laser welding create favorable conditions for material compatibility of many different metals after welding. It has been proved that the following metals can be successfully carried out laser deep penetration welding: stainless steel to low carbon steel, 416 stainless steel ~310 stainless steel, 347 stainless steel ~HASTALLY nickel alloy, nickel electrode cold forging steel, nickel content of different bimetal strip.
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