Mechanism And Control Technology Principle And Characteristics Of Laser Welding

The development trend and characteristics of modern means of delivery are high-speed operation and lightweight structure. As a result, higher requirements have been put forward for the manufacturing of its key structures, such as lightweight, monolithic, high-reliability, long-life and low-cost green manufacturing. To this end, high strength and lightweight titanium alloy, aluminum alloy and other typical structural materials, high energy density beam current as the welding heat source of high-energy beam welding manufacturing of the overall structure, one of the results of advanced manufacturing technology to comply with the needs of this development. High-energy beam welding manufacturing with many advantages, known as materials processing and advanced manufacturing technology with revolutionary changes in the new technology, especially in the new lightweight alloys in the key structure of the welding manufacturing has broad prospects for application, on behalf of one of the large complex titanium alloys, aluminum alloys.
Reinforced wall plate or cavity structure of the laser high-speed and efficient welding manufacturing. Laser is a high-intensity coherent light based on the principle of excited radiation of atoms, which produces a high intensity light by exciting the working substance. In addition to obeying all the laws of optics as ordinary light does, the laser has a number of other characteristics not found in any other light source such as good directionality, high brightness and good monochromaticity. It is the laser's good direction and high brightness constitutes a high concentration of energy in space and time, can be transmitted over long distances and has a high energy or high intensity, in the field of materials processing (including welding) can be regarded as an ideal heat source. The application of laser as a new energy greatly broadens the application field of material processing laser welding is one of the important aspects of laser application.
Laser welding is a kind of use after focusing with high energy density (10 ^ 6 ~ 10 ^ 12 W / cm) of the laser beam as a heat source to heat the melting of the workpiece special melting welding method. It is a melt welding based on the photothermal effect, which presupposes that the laser is absorbed by the material and converted into the heat energy required for welding. Usually, different intensities of laser light on the surface of the material leads to different physical phenomena, including surface temperature increase, melting, vaporization, the formation of small holes, and the generation of photo-plasma, etc. These physical phenomena determine the welding process thermal action mechanism, so that the laser welding of the existence of heat conduction welding and deep melting welding welding mode of the two kinds of welding. The transition between the two modes depends mainly on the power density of the laser spot acting on the material.
For a given material, there is a specific power density threshold (0.5x10^6 to 10^7 W/cm for most steels). When the laser power density applied to the material is below this threshold, the laser energy is absorbed by the surface of the material and is quickly transferred to the interior of the material, forming a thermally conductive weld with a relatively large width and depth. When the laser power density applied to the material is higher than the threshold, the surface of the workpiece is too late to transfer heat to the material inside, the laser energy will make the material surface rapidly warming, melting and vaporization. And as the laser energy continues to be fed, small holes are formed in the direction of the penetration thickness. The hole is surrounded by a liquid metal melt pool, and the hole is filled with high-temperature metal vapor and plasma. The expansion force of the high-temperature metal vapor and plasma acts in conjunction with the gravity and surface tension of the liquid metal around the hole to maintain the stable existence of the hole. The small holes move along the welding direction, the rear molten pool rapidly cools and solidifies, and a deep molten weld with a relatively large depth and width is formed. Therefore, the laser welding mode is related to the laser power density and welding line energy that determine the thermal action mechanism.
When the laser power density is lower than 10 ^ 6 W / cm, laser heating is limited to the metal surface, can reach the melting threshold of most metals, but no vaporization, this time the metal in the continuous action of the laser (enough line energy), to thermal conduction welding mode to form a weld, weld formation mechanism and conventional melt welding. Laser thermal conduction welding is generally used in electronic components sealing welding and ultra-thin material welding. When the laser power density is higher than 10^6W/cm, the laser makes the metal melt and vaporize instantly, if the line energy is enough, the metal vapor force produces tiny holes in the melted metal, and the welding process forms the weld in the mode of deep-melt welding with the effect of small holes. Deep melt welding hole is surrounded by molten metal, filled with high temperature metal vapor and plasma, welding process hole by metal vapor force and liquid metal gravity and surface tension balance to maintain the small hole in the laser and the thermal coupling of the material is the thermal action of the laser deep melt welding mode of the thermal action of the mechanism, mainly used in the degree of greater than 1mm structure of the welding.
Compared with traditional arc welding methods, laser welding technology has unique advantages and is an advanced welding technology.
Laser welding technology has developed rapidly in the past few decades, gradually evolving from pulsed-wave welding to continuous-wave, high-power thick plate, multi-station welding, and has been widely used in aviation, aerospace, automotive, high-speed railroads and other fields. In military aircraft manufacturing, there has been a significant growth in the application of laser welding of aluminum alloys and titanium alloys, which account for more than 60% and 20% of the structural weight of advanced fighter jets, respectively. Laser welding technology can replace the traditional riveting method, significant weight reduction, cost reduction and improve material utilization. For example, Airbus A380 fuselage wall plate through laser welding can reduce weight by 15%, 15% cost reduction.
In China, large-scale structural welding of reinforced wall plate is becoming more and more popular in aircraft and naval applications. Compared with mechanical processing and riveting methods, laser welding not only saves materials, but also improves component lightweight and manufacturability, shortens the processing cycle and reduces production costs. However, the laser welding process is complex, involving rapid heating, cooling, material phase changes, especially for aluminum alloys and titanium alloys, due to high reflectivity, high thermal conductivity and surface tension and other characteristics, bringing technical challenges such as weld quality control and stability, welding process stability, defect control is not mature, structural shape does not meet the standards and other issues are prominent. The root cause of these problems lies in the lack of basic research on the weldability and mechanical properties of titanium alloys and aluminum alloys, resulting in joint fatigue life, stress and deformation control, etc. It is difficult to meet the high reliability requirements of lightweight alloy components, and there is a large gap with the international level.