Analyze laser welding technology

1. Process characteristics of laser welding

According to the mechanism of weld pool formation, there are two basic modes of laser welding: thermal conductivity welding and deep penetration welding. The former uses a lower laser power density (105～106W/cm2). After the workpiece absorbs the laser, it only reaches the surface melting. Then rely on heat conduction to transfer heat to the inside of the workpiece to form a molten pool. This welding mode has shallow penetration and relatively small depth and width. The latter laser train has a high density (106～107W/cm2), and the workpiece is quickly melted and even vaporized after absorbing the laser. The molten metal forms a small hole under the action of steam pressure. The laser beam can directly illuminate the bottom of the hole, so that the small hole continues to extend until The vapor pressure in the pores balances with the surface tension and gravity of the liquid metal. When the small hole moves along the welding direction with the laser beam, the molten metal in front of the small hole bypasses the small hole and flows to the back, forming a weld after solidification. This welding mode has a large penetration depth and a large aspect ratio. In the field of mechanical manufacturing, in addition to those meager parts, deep penetration welding should generally be used.

　　The metal vapor and shielding gas generated during the deep penetration welding process are ionized under the action of the laser, thereby forming plasma inside and above the small hole. Plasma absorbs, refracts and scatters laser light, so in general, the plasma above the molten pool will weaken the laser energy reaching the workpiece. It also affects the focusing effect of the beam and is unfavorable for welding. Usually can be supplemented with side blowing to drive or weaken the plasma. The formation of small holes and the plasma effect make the welding process accompanied by characteristic sound, light and electric charge generation, study their relationship with welding specifications and weld quality, and use these characteristic signals to improve the laser welding process and quality Monitoring has very important theoretical significance and practical value.

Because the focused laser beam has a small spot (0.1～0.3mm), high power density, several orders of magnitude higher than arc welding (5×102～104W/cm2), laser welding has significant advantages that traditional welding methods cannot match: Small heating range, narrow welding seam and heat-affected zone, excellent joint performance; small residual stress and welding deformation, high-precision welding can be achieved; high melting point, high thermal conductivity, heat sensitive materials and non-metals can be welded; fast welding speed , High productivity; highly flexible, easy to realize automation.

　　 There are many similarities between laser welding and electron beam welding, but it does not require a vacuum chamber and does not generate X-rays, so it is more suitable for promotion and application in production. Laser welding has actually achieved the position of electron beam welding 20 years ago and has become the mainstream of the development of high-energy beam welding technology.

  2. Laser welding equipment

　　Laser welding equipment is mainly composed of laser, light guide system, welding machine and control system.

  1. Laser

　　The lasers used for laser welding mainly include CO2 gas lasers and YAG solid-state lasers.

　　The most important performance of a laser is the output power and beam quality. Considering these two aspects, CO2 laser has great advantages over YAG laser. It is currently the main laser used in deep penetration welding. Most production applications are still in the range of 1.5-6kW, but the largest CO2 laser in the world has reached 50kW. . However, YAG lasers have been difficult to increase power for a long time in the past. Generally, the power is less than 1kW and is used for micro-connection of thin and small parts. However, in recent years, foreign countries have made breakthroughs in the development and production of high-power YAG lasers. The maximum power has reached 5kW and has been put on the market. Because of its short wavelength, only 1/10 of CO2 laser, it is good for metal surface absorption and can be transmitted by optical fiber, which greatly simplifies the light guide system. It can be expected that high-power YAG laser welding technology will develop rapidly in the next period of time and become a strong competitor of CO2 laser welding.

2. Light guide and focusing system

　　The light guide focusing system is composed of a circular polarizer, a beam expander, a mirror or an optical fiber, a focusing mirror, etc. It realizes the functions of changing the polarization state and direction of the beam, transmitting the beam and focusing. The condition of these optical parts has an extremely important influence on the quality of laser welding. Under the action of high-power laser, the performance of optical components, especially the lens, will deteriorate and the transmittance will decrease; thermal lens effect (the focal length of the lens will be shortened by thermal expansion); surface pollution will also increase the transmission loss. Therefore, the quality of optical components, maintenance and monitoring of working conditions are very important to ensure the quality of welding.

　　3. Laser welding machine

　　Its function is to realize the relative movement between the beam and the workpiece, and complete the laser welding. It is divided into two types: special welding machine and general welding machine. The latter often uses numerical control systems, with rectangular coordinate two-dimensional and three-dimensional welding machines or articulated laser welding robots.

　　3. Improve and develop new technologies for laser welding

　　 The following technologies can help expand the application range of laser welding and improve the level of automatic control of laser welding

　　L. Filler wire laser welding

　　 Laser welding generally does not fill the welding wire, but the assembly gap of the weldment is very high, and it is sometimes difficult to guarantee in actual production, which limits its application range. The use of filler wire laser welding can greatly reduce the requirements for assembly clearance. For example, for aluminum alloy plates with a thickness of 2mm, if filler wires are not used, the gap between the plates must be zero to obtain good forming. For example, if φ1.6mm welding wires are used as filler metals, the weld can be guaranteed even if the gap is increased to 1.0mm. Good shape.

　　 In addition, the filler wire can also adjust the chemical composition or perform multi-layer welding of thick plates.

　　2. Beam rotating laser welding

　　The method of rotating the laser beam for welding can also greatly reduce the requirements for the assembly of the weldment and the beam alignment. For example, when the 2mm thick high-strength alloy 　　 steel plate is butted, the allowable seam assembly gap is increased from 0.14mm to 0.25mm; while for the 4mm thick plate, it is increased from 0.23mm to 0.30mm. The allowable error of the alignment between the center of the beam and the center of the weld is increased from 0.25mm to 0.5mm.

　　3. On-line inspection and control of laser welding quality

　　Using the plasma light, sound, and charge signals to detect the laser welding process has become a research hotspot at home and abroad in recent years, and a few research results have reached the level of closed-loop control.