First, the process parameters of laser welding
1. Power density
Power density is one of the most critical parameters in laser processing. With higher power density, the surface layer can be heated to the boiling point in the microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is beneficial for material removal processes such as punching, cutting, and engraving. For lower 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 form a good fusion welding. Therefore, in conduction laser welding, the power density is in the range of 104~106W/cm2.
2. Laser pulse waveform
Laser pulse waveform is an important issue in laser welding, especially for thin sheet welding. When the high-intensity laser beam hits the surface of the material, 60-98% of the laser energy on the metal surface will be reflected and lost, and the reflectivity varies with the surface temperature. During the action of a laser pulse, the reflectivity of metals varies greatly.
3. Laser pulse width
Pulse width is one of the important parameters of pulsed laser welding. It is not only an important parameter that is different from material removal and material melting, but also a key parameter that determines the cost and volume of processing equipment.
4. Influence of defocus amount on welding quality
Laser welding usually requires a certain degree of defocusing, because the power density in the center of the spot at the laser focus is too high, and it is easy to evaporate into a hole. On the planes away from the laser focus, the power density distribution is relatively uniform.
There are two ways of defocusing: positive defocusing and negative defocusing. The focal plane is located on the workpiece. The above is positive defocusing, otherwise it is negative defocusing. According to the geometrical optics theory, when the positive and negative distances are equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different.
When the defocus is negative, a larger penetration depth can be obtained, which is related to the formation process of the molten pool. Experiments show that when the laser is heated for 50~200us, the material begins to melt, forming a liquid phase metal and vaporizing it, forming a market-pressure steam, which is ejected at a very high speed, emitting a 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 the defocus is negative, the internal power density of the material is higher than that of the surface, and it is easy to form stronger melting and vaporization, so that the light energy can be transmitted to the deeper part of the material. Therefore, in practical applications, when the penetration depth is required to be large, when using negative defocus to weld thin materials, positive defocus should be used.
2. Laser welding process method
1. Welding between islands
Including butt welding, end welding, center penetration fusion welding, center penetration fusion welding and other 4 process methods.
2. Wire to wire welding
Including wire-to-wire butt welding, cross welding, parallel lap welding, T-welding and other 4 process methods.
3. Welding of wire and block components
The connection between the metal wire and the block element can be successfully realized by laser welding, and the size of the block element can be arbitrary. During welding, attention should be paid to the geometrical dimensions of the filamentary elements.
4. Welding of Tong metal
Welding different types of metals addresses weldability and weldability parameter ranges. Laser welding between different materials is only possible with certain material combinations.
Laser brazing The connection of some components is not suitable for laser welding, but the laser can be used as a heat source to implement soft soldering and brazing, which also has the advantages of laser welding. There are many ways to use brazing. Laser soldering is mainly used for the welding of printed circuit boards, especially for chip component assembly technology.
3. Compared with other methods, the use of laser soldering has the following advantages
1. Due to local heating, the components are not prone to thermal damage and the heat-affected zone is small, so soldering can be performed near the heat-sensitive components.
2. Using non-contact heating, melting the bandwidth, without any auxiliary tools, it can be processed after the double-sided components are equipped on the double-sided printed circuit board.
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 yield of laser brazing is high.
4. The laser beam is easy to achieve beam splitting, and optical elements such as half mirrors, mirrors, prisms, scanning mirrors and other optical elements can be used for time and space division, which can realize multi-point simultaneous symmetrical welding.
5. Laser brazing mostly uses a laser with a wavelength of 1.06um as a heat source, which can be transmitted by optical fiber, so it can be processed in parts that are not easy to be welded by conventional methods, with good flexibility.
6. Good focus, easy to realize the automation of multi-station device.
4. Laser deep penetration welding
1. Metallurgical process and process theory
The metallurgical physical process of laser deep penetration welding is very similar to that of electron beam welding, that is, the energy conversion mechanism is completed through the "pinhole" structure. Under the irradiation of a sufficiently high power density beam, the material evaporates to form small holes. This small hole filled with steam is like a black body, which absorbs almost all the energy of the incident light, and the equilibrium temperature in the hole is about 25,000 degrees. Heat is transferred from the outer wall of the high-temperature cavity, melting the metal surrounding the cavity.
The small hole is filled with high-temperature steam generated by the continuous evaporation of the wall material under the irradiation of the light beam, the four walls of the small hole surround the molten metal, and the liquid metal surrounds the solid material. The liquid flow outside the pore wall and the surface tension of the wall layer maintain a dynamic equilibrium with the steam pressure continuously generated in the pore cavity. The light continuously enters the small hole, and the material outside the small hole is continuously flowing. As the light beam moves, the small hole is always in a stable state of flow. That is, the small hole and the molten metal surrounding the hole wall move forward with the advancing speed of the leading beam, the molten metal fills the gap left after the small hole is removed and condenses, and the weld is formed.
2. Influencing factors
Factors affecting laser deep penetration welding include: laser power, laser beam diameter, material absorption rate, welding speed, shielding gas, lens focal length, focus position, laser beam position, welding start and end points. The laser power gradually increases. Up and down control.
3. Characteristics of laser deep penetration welding
(1) High aspect ratio. As the molten metal forms around the cylindrical high-temperature steam chamber and extends toward the workpiece, the weld becomes deep and narrow.
(2) Heat input. Because the source cavity temperature is very high, the melting process occurs very fast, the heat input to the workpiece is extremely low, and the thermal deformation and heat affected zone are small.
(3) High density. Because the small holes filled with high temperature steam are conducive to the agitation of the welding pool and the escape of gas, resulting in the formation of non-porous penetration welding. The high cooling rate after welding is easy to make the weld microstructure.
(4) Strengthen the weld.
(5) Precise control.
(6) Non-contact, atmospheric welding process.
4. The advantages of laser deep penetration welding
(1) Due to the much higher power density of the focused laser beam than the conventional method, the welding speed is fast, the heat-affected zone and the deformation are small, and the difficult-to-weld materials such as titanium and quartz can also be welded.
(2) Because the beam is easy to transmit and control, and there is no need to replace the welding torch and nozzle frequently, the auxiliary time of downtime is greatly reduced, so the load factor and production efficiency are 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 equilibrium heat input and high machining accuracy, reprocessing costs can be reduced. In addition, the moving cost of laser welding is relatively low, which can reduce production costs.
(5) It is easy to realize automation, and can effectively control the beam intensity and fine positioning.
5. Laser deep penetration welding equipment
Continuous-wave CO2 lasers are usually used in laser deep penetration welding, which can maintain a sufficiently high output power, produce a "pinhole" effect, penetrate the entire workpiece section, and form a strong welded joint. As far as the laser itself is concerned, it is just a device that can be used as a heat source and has a well-directed parallel beam.
If it is guided and effectively processed and then shot to the workpiece, its input power has strong compatibility, making it better suited to automated processes. In order to carry out welding effectively, the laser and some other necessary optical, mechanical and control components together form a large welding system. The system includes lasers, beam delivery components, workpiece handling and movement devices, and controls. This system can be simply manual handling and fixing of the workpiece by the operator, or it can include automatic loading and unloading, fixing, welding and inspection of the workpiece. The overall requirement for the design and implementation of this system is to obtain satisfactory welding quality and high production efficiency.
5. Laser welding of steel materials
1. Laser welding of carbon steel and ordinary alloy steel
In general, the laser welding effect of carbon steel is good, and its welding quality depends on the impurity content. As with other welding processes, sulfur and phosphorus are sensitive factors for welding cracks. In order to obtain satisfactory welding quality, preheating is required when the carbon content exceeds 0.25%. When steels with different carbon content are welded to each other, the welding torch can be slightly biased towards the low carbon material-side to ensure the quality of the joint. Low carbon boiling steel is not suitable for laser welding due to its high content of sulfur and phosphorus. Low carbon killed steel has good welding effect due to low impurity content. Both medium and high carbon steels and common alloy steels can be laser welded well, but require preheating and post-weld treatment to relieve stress and avoid crack formation.
2. Laser welding of stainless steel
In general, stainless steel laser welding is easier to obtain high-quality joints than conventional welding. Due to the small heat-affected zone at high welding speeds, sensitization is not a significant issue. Compared with carbon steel, the low thermal conductivity of stainless steel makes it easier to obtain deep penetration and narrow welds.
3. Laser welding between metals
The extremely high cooling rate and small heat-affected zone of laser welding create favorable conditions for the compatibility of materials with different structures after welding of many different metals. It has been proved that the following metals can be successfully laser deep penetration welding: stainless steel ~ low carbon steel, 416 stainless steel ~ 310 stainless steel, 347 stainless steel ~ HASTALLY nickel alloy, nickel electrode ~ cold forged steel, bimetallic strips with different nickel content.