With the development of science and technology, laser welding has appeared in recent years. So what is laser welding? What are the characteristics and advantages of laser welding?
First, what is a laser? The world's first laser beam was produced by using a flash bulb to excite the ruby crystal in 1960. Due to the limited heat capacity of the crystal, it can only produce a very short pulse beam with a low frequency. Although the peak pulse energy can be as high as 106 watts, it is still a low energy output.
The laser technology uses a polarizer to reflect the laser beam and concentrate it in the focusing device to generate a beam of huge energy. If the focus is close to the workpiece, the workpiece will melt and evaporate in a few milliseconds. This effect can be used in the welding process of high power CO2 and The emergence of high-power YAG laser has opened up a new field of laser welding. The key to laser welding equipment is high-power lasers. There are two main types. One is solid-state lasers, also known as Nd:YAG lasers. Nd (neodymium) is a rare earth element, YAG stands for yttrium aluminum garnet, and its crystal structure is similar to ruby. The wavelength of the Nd:YAG laser is 1.06μm. The main advantage is that the generated beam can be transmitted through the optical fiber, so it can be saved to a complex beam transmission system. It is suitable for flexible manufacturing systems or remote processing. It is usually used for workpieces with high welding accuracy requirements. The Nd:YAG laser with an output power of 3-4 kW is commonly used in the automotive industry. The other type is a gas laser, also known as a CO2 laser. Molecular gas is used as a working medium to produce an infrared laser with a uniform 10.6 μm. It can work continuously and output high power. The standard laser power is between 2-5 kilowatts.
Compared with other traditional welding techniques, the main advantages of laser welding are:
1. Fast speed, large depth and small deformation.
2. Welding can be performed at room temperature or under special conditions, and the welding equipment is simple. For example, the laser beam passes through an electromagnetic field, and the beam does not shift; the laser can be welded in a vacuum, air, and a certain gas environment, and can be welded through glass or materials transparent to the beam.
3. Weldable refractory materials such as titanium, quartz, etc., and can be welded on heterogeneous materials, the effect is good.
4. After laser focusing, the power density is high. When welding high-power devices, the aspect ratio can reach 5:1, and the highest can reach 10:1.
5. Micro welding is possible. After focusing, the laser beam can obtain a very small light spot, and can be accurately positioned. It can be used in the assembly welding of micro and small workpieces produced by large-scale automated production.
6. Welding the hard-to-access parts, non-contact remote interval welding has great flexibility. Especially in recent years, the fiber transmission technology has been adopted in the YAG laser processing technology, so that the laser welding technology has been more widely promoted and applied.
7. The laser beam is easy to realize the beam splitting in time and space, and can process multiple beams at the same time and multi-station processing, which provides conditions for more precise welding.
However, laser welding also has certain limitations:
1. High precision welding assembly is required, and the position of the beam on the workpiece must not be significantly shifted. This is due to the small spot size and the narrow weld seam after laser focusing, which is filled with metal material. If the workpiece assembly accuracy or beam positioning accuracy does not meet the requirements, welding defects are easily caused.
2. The cost of lasers and related systems is relatively high, and the one-time investment is relatively large.
WXS welding head
Process parameters of laser welding
(1) Power density
Power density is one of the most critical parameters in laser processing. With a higher power density, the surface layer can be heated to the boiling point within a microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is beneficial for material removal processing, such as drilling, cutting, and engraving. For lower power densities, it takes a few milliseconds for the surface temperature to reach the boiling point. Before the surface layer vaporizes, the bottom layer reaches the melting point, and it is easy to form a good fusion weld. Therefore, in conductive laser welding, the power density ranges from 104 to 106 W/cm2.
(2) Laser pulse waveform
Laser pulse waveform is an important issue in laser welding, especially for sheet welding. When the high-intensity laser beam hits the surface of the material, 60~98% of the laser energy will be reflected and lost on the metal surface, and the reflectivity changes with the surface temperature. During a laser pulse, the reflectivity of the metal changes greatly.
(3) Laser pulse width
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 that determines the cost and volume of processing equipment.
(4) Influence of defocusing amount on welding quality
Because the power density at the center of the spot at the laser focus is too high, it easily evaporates into a hole. On each plane away from the laser focus, the power density distribution is relatively uniform. There are two ways to defocus: positive defocus and negative defocus. The focal plane above the workpiece is positive defocus, otherwise it is negative defocus. According to the theory of geometric optics, when the distance between the positive and negative defocusing planes and the welding plane is equal, the power density on the corresponding plane is approximately the same, but in fact the shape of the molten pool obtained is different. With negative defocusing, a greater penetration depth can be obtained, which is related to the formation process of the molten pool.
Application areas of laser welding
Laser welding has a wide range of applications in the manufacturing industry, powder metallurgy, automotive industry, electronics industry, and other fields.
In particular, the editor has collected the development status of laser welding in the automobile manufacturing industry, and the reduction is as follows:
At present, Volkswagen of Germany uses laser welding on the roof of AudiA6, GolfA4, Passat and other brands, BMW and GM also use laser welding on the top of the frame, and German Mercedes-Benz uses laser welding of transmission parts.
In addition to laser welding, other laser technologies have also been widely used: Volkswagen, General Motors, Mercedes-Benz, Nissan applied laser technology to cut the cover, Fiat and Toyota applied laser coating engine exhaust valves, and Volkswagen applied the engine camshaft. Laser surface hardening.
From the current domestic situation, the localized models of international brands: Passat, Polo, Touran, Audi, Dongfeng Peugeot, Fox, etc. have all adopted laser welding technology, among which FAW Volkswagen Audi A6 top cover and Bora rear cover are laser welded. , Sagitar and Touran's body laser weld length reached 30 and 40m, respectively. In addition, domestic auto brands Brilliance, Chery and Geely have also applied laser welding technology to their new models.
Improve and develop new technologies for laser welding
With the progress of the times, the technology of laser welding is also constantly developing. The following technologies can help expand the application range of laser welding and improve the automatic control level of laser welding.
1. Filler wire laser welding
Laser welding is generally not filled with welding wire, but the assembly gap of welding parts is very high, and it is sometimes difficult to guarantee in actual production, which limits its application range. The use of wire-filled laser welding can greatly reduce the requirements for assembly gaps. For example, the aluminum alloy plate with a thickness of 2mm, if no filler wire is used, the gap between the plates must be zero to obtain a good shape. If the φ1.6mm welding wire is used as the filler metal, even if the gap is increased to 1.0mm, the weld can be guaranteed Good shape. In addition, the filler wire can also adjust the chemical composition or perform multi-layer welding of thick plates.
2. Beam rotation laser welding
The method of rotating the laser beam for welding can also greatly reduce the requirements for weldment assembly and beam alignment. For example, when the 2mm thick high-strength alloy steel plate is butted, the gap between the seam assembly is allowed to increase from 0.14mm to 0.25mm; and for the 4mm thick plate, it is increased from 0.23mm to 0.30mm. The alignment of the beam center and the weld center allows the error to increase from 0.25mm to 0.5mm.
3. Online inspection and control of laser welding quality
The use of plasma light, sound, and charge signals to detect the laser welding process has become a hotspot of research at home and abroad in recent years, and a few research results have reached the level of closed-loop control.