Process Analysis of Laser Welding
Power density is one of the most critical parameters in laser processing. Using a higher power density, in the microsecond time range, the surface can be heated to the boiling point, resulting in a large number of vaporization. Therefore, the high power density for material removal processing, such as drilling, cutting, sculpting favorable. For lower power densities, the surface temperature reaches the boiling point needs to go through a few milliseconds, before the surface vaporization, the bottom to reach the melting point, easy to form a good fusion welding. Therefore, in the conduction type high power laser pointer welding, the power density in the range of 104 ~ 106W / cm2.
Laser pulse waveform in laser welding is an important problem, especially for sheet welding is more important. When the high-intensity laser beam to the surface of the material, the metal surface will be 60-98% of the laser energy reflected and lost, and the reflectivity with the surface temperature. During a laser pulse, the metal reflectivity changes greatly. Pulse width is one of the important parameters of pulse laser welding. It is not only an important parameter different from material removal and material melting, but also a key parameter to determine the cost and volume of processing equipment.
Laser welding usually requires a certain degree of defocus, because the green laser pointer focal spot at the center of the power density is too high, easy to evaporate into holes. The power density distribution is relatively uniform across the planes away from the laser focus. There are two defocus modes: positive defocus and negative defocus. Focal plane is located above the workpiece is positive from the focus, otherwise negative out of focus. According to the theory of geometric optics, when positive and negative do an article equal, the corresponding power density on the plane is approximately the same, but in fact the shape of the pool is different. In the case of negative defocusing, a greater penetration depth is obtained, which is related to the formation of the bath. Experiments show that the laser heating 50 ~ 200us material began to melt, the formation of liquid metal and the emergence of sub-vaporization, the formation of municipal pressure steam, and high speed jet, emitting dazzling white. At the same time, the high concentration vapor moves the liquid metal to the edge of the molten pool, forming a depression in the center of the bath. When the negative focus, the internal power density of the material is higher than the surface, easy to form a stronger melting, vaporization, so that the light energy to the deeper transmission material. Therefore, in practical applications, when the required depth of penetration is large, the use of negative out of focus; welding thin material, it is appropriate to use is out of focus.
The welding between the chip and the chip including butt welding, end welding, center penetration melting welding, central perforation melting welding four kinds of process; wire and wire welding. Including wire and wire butt welding, cross welding, parallel lap welding, T-type welding and other four kinds of methods; wire and the welding of block components. The use of burning laser pointer welding can be successfully achieved wire and block-like components of the connection, the size of block-like components can be arbitrary. In the welding should pay attention to the geometric size of filament components; different metal welding. Soldering different types of metal to solve the weldability and weldability parameters range. Laser welding between different materials is only possible with certain combinations of materials. Laser brazing some components of the connection should not be laser welding, but can use the laser as a heat source, the implementation of soldering and brazing, the same laser welding has the advantage. There are many ways to use brazing, in which laser soldering is mainly used for printed circuit board welding, especially for chip components assembly technology.
The use of laser soldering compared with other methods have the following advantages: As the local heating, components are not easy to produce thermal damage, heat affected zone is small, so the implementation of the soldering in the vicinity of the thermal element; with non-contact heating, melting bandwidth, Does not require any auxiliary tools, double-sided printed circuit board in the double-sided components after processing equipment; repeated operation of good stability. The 5mw laser pointer beam is easy to realize the spectrophotometry, and can be divided into time and space by optical elements such as half mirror, mirror, prism, scanning mirror, etc. The laser beam is easy to be divided into two parts: the laser beam is easy to control, Can be multi-point simultaneous symmetrical welding; laser brazing multi-purpose wavelength of 1.06um laser as a heat source can be used optical fiber transmission, it can be welded in the conventional way difficult to deal with parts of the processing, flexibility is good; focus, and easy to achieve multi-station device Of automation.
Laser deep melting of metallurgical physical process and electron beam welding is very similar, that energy conversion mechanism is through the "small hole" structure to complete. In the high enough power density beam irradiation, the material evaporates to form small holes. The steam hole is like a black body, almost all of the energy absorbed by the incident light, the cavity temperature balance of 25,000 degrees. Heat is transferred from the outer wall of the high-temperature cavity, causing the metal surrounding the cavity to melt. The holes are filled with high-temperature steam produced by continuous evaporation of the wall material under the irradiation of the light beam. The small holes surround the molten metal, and the liquid metal surrounds the solid material. The liquid flow and the surface tension of the wall surface are in constant equilibrium with the pressure of steam generated continuously in the cavity. The beam continues to enter the hole, the small hole in the continuous flow of material, as the beam moves, the hole is always in a steady state of flow. That is, the small holes and the molten metal surrounding the cell walls move forward with the advancing speed of the leading beam, and the molten metal is filled with voids left after the small holes are removed and then condensed, and the weld is formed.
Laser power, laser beam diameter, material absorption rate, welding speed, shielding gas, lens focal length, focal position, laser beam position, laser power at the beginning and end of laser welding are gradually increased. , Gradual control.
Since the molten metal is formed around the cylindrical high temperature steam chamber and extends toward the workpiece, the weld becomes deep and narrow; the minimum heat input. Because the source cavity temperature is high, the melting process occurs very fast, very low input heat of the workpiece, heat distortion and heat affected zone is small; high density. Because the hole is full of high-temperature steam is conducive to welding pool and the gas to escape mixing, resulting in the formation of non-porous welding penetration. Welding high cooling rate and easy to make the weld microstructure; strong weld; precise control; non-contact, atmospheric welding process.
Since the focused laser pointer beam has a much higher power density than conventional methods, resulting in fast welding speed, less heat affected zone and deformation, it is also possible to weld difficult-to-weld materials such as titanium and quartz; since the beam is easily transmitted and controlled, Due to the purification effect and high cooling rate, the weld is strong, the comprehensive performance is high; Because the balanced heat input is low, the processing precision is high, the welding efficiency is high, and the welding efficiency is high, Reduced rework costs. In addition, the laser welding of the moving costs are relatively low, can reduce production costs; easy to automate, the beam intensity and fine positioning can be effectively controlled.
Laser deep-melting welding usually use continuous-wave CO2 lasers, these lasers can maintain a high enough output power, resulting in "small hole" effect, penetration of the entire workpiece cross-section, forming a strong welded joints. As far as the laser itself is concerned, it is simply a device capable of producing a parallel beam of good directivity as a heat source. If it is directed and effectively processed and fired at the workpiece, the input power to have a strong compatibility, so that it can better adapt to the automation process. In order to effectively implement welding, lasers and other necessary optical, mechanical and control components together constitute a large welding system. The system comprises a 50mw laser pointer, a beam transport assembly, a workpiece handling and movement device, and a control device. This system can be simply by the operator simply hand handling and fix the workpiece, can also include the workpiece can automatically install, unload, fixed, welding, inspection. The overall design and implementation requirements of this system are to achieve satisfactory welding quality and high production efficiency.
In general, carbon steel laser welding is good, the quality of its welding depends on the impurity content. Like other welding processes, sulfur and phosphorus are sensitive to weld cracking. Preheating is required when the carbon content exceeds 0.25% in order to obtain satisfactory welding quality. When steels with different carbon contents are welded to each other, the torch can be slightly biased towards the low carbon material side to ensure joint quality. Low carbon boiling steel due to sulfur and phosphorus content is high, not suitable for laser welding. Low-carbon killed steel due to low impurity content, the welding effect is very good. Medium, high carbon steel and ordinary alloy steel can be a good laser welding, but the need for preheating and post-weld treatment to eliminate stress, to avoid the formation of cracks.
Laser welding of stainless steel. In general, stainless steel 2000mw laser pointer welding is easier than conventional welding to obtain high quality joints. Sensitization is not an important issue due to the small heat-affected zone at high welding speeds. Compared with carbon steel, the low coefficient of thermal conductivity of stainless steel is easier to obtain deep-melting narrow weld.
Laser welding Extremely high cooling rates and very small heat affected zones create favorable conditions for the compatibility of materials with different structures after the melting of many different metal welds. The following metals have been demonstrated to be successfully laser deep-fused: stainless steel to low carbon steel, 416 stainless steel to 310 stainless steel, 347 to HASTALLY nickel alloy, nickel to cold forged steel, and nickel.
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