Compared with traditional welding technology, laser welding has unparalleled advantages in welding accuracy, efficiency, reliability, automation and other aspects. In recent years, it has developed rapidly in the fields of automobiles, energy, electronics and other fields, and is considered to be one of the most promising manufacturing technologies in the 21st century.
1. Overview of double-beam laser welding
Double-beam laser welding is to use optical methods to separate the same laser into two separate beams of light for welding, or to use two different types of lasers to combine, such as CO2 laser, Nd: YAG laser and high-power semiconductor laser. All can be combined. It was proposed mainly to solve the adaptability of laser welding to assembly accuracy, improve the stability of the welding process, and improve the quality of the weld. Double-beam laser welding can conveniently and flexibly adjust the welding temperature field by changing the beam energy ratio, beam spacing, and even the energy distribution pattern of the two laser beams, changing the existence pattern of the keyhole and the flow pattern of liquid metal in the molten pool. Provides a wider choice of welding processes. It not only has the advantages of large laser welding penetration, fast speed and high precision, but is also suitable for materials and joints that are difficult to weld with conventional laser welding.
For double-beam laser welding, we first discuss the implementation methods of double-beam laser. Comprehensive literature shows that there are two main ways to achieve double-beam welding: transmission focusing and reflection focusing. Specifically, one is achieved by adjusting the angle and spacing of two lasers through focusing mirrors and collimating mirrors. The other is achieved by using a laser source and then focusing through reflecting mirrors, transmissive mirrors and wedge-shaped mirrors to achieve dual beams. For the first method, there are mainly three forms. The first form is to couple two lasers through optical fibers and split them into two different beams under the same collimating mirror and focusing mirror. The second is that two lasers output laser beams through their respective welding heads, and a double beam is formed by adjusting the spatial position of the welding heads. The third method is that the laser beam is first split through two mirrors 1 and 2, and then focused by two focusing mirrors 3 and 4 respectively. The position and distance between the two focal spots can be adjusted by adjusting the angles of the two focusing mirrors 3 and 4. The second method is to use a solid-state laser to split the light to achieve dual beams, and adjust the angle and spacing through a perspective mirror and a focusing mirror. The last two pictures in the first row below show the spectroscopic system of a CO2 laser. The flat mirror is replaced with a wedge-shaped mirror and placed in front of the focusing mirror to split the light to achieve dual beam parallel light.
After understanding the implementation of double beams, let’s briefly introduce the welding principles and methods. In the double-beam laser welding process, there are three common beam arrangements, namely serial arrangement, parallel arrangement and hybrid arrangement. cloth, that is, there is a distance in both the welding direction and the welding vertical direction. As shown in the last row of the figure, according to the different shapes of small holes and molten pools that appear under different spot spacing during the serial welding process, they can be further divided into single melts. There are three states: pool, common molten pool and separated molten pool. The characteristics of single molten pool and separated molten pool are similar to those of single laser welding, as shown in the numerical simulation diagram. There are different process effects for different types.
Type 1: Under a certain spot spacing, two beam keyholes form a common large keyhole in the same molten pool; for type 1, it is reported that one beam of light is used to create a small hole, and the other beam of light is used for welding heat treatment, which can Effectively improve the structural properties of high carbon steel and alloy steel.
Type 2: Increase the spot spacing in the same molten pool, separate the two beams into two independent keyholes, and change the flow pattern of the molten pool; for type 2, its function is equivalent to two electron beam welding, Reduces weld spatter and irregular welds at the appropriate focal length.
Type 3: Further increase the spot spacing and change the energy ratio of the two beams, so that one of the two beams is used as a heat source to perform pre-welding or post-welding processing during the welding process, and the other beam is used to generate small holes. For type 3, the study found that the two beams form a keyhole, the small hole is not easy to collapse, and the weld is not easy to produce pores.
2. The influence of welding process on welding quality
Effect of serial beam-energy ratio on welding seam formation
When the laser power is 2kW, the welding speed is 45 mm/s, the defocus amount is 0mm, and the beam spacing is 3 mm, the weld surface shape when changing RS (RS= 0.50, 0.67, 1.50, 2.00) is as shown in the figure. When RS=0.50 and 2.00, the weld is dented to a greater extent, and there is more spatter on the edge of the weld, without forming regular fish scale patterns. This is because when the beam energy ratio is too small or too large, the laser energy is too concentrated, causing the laser pinhole to oscillate more seriously during the welding process, and the recoil pressure of the steam causes the ejection and splashing of the molten pool metal in the molten pool; Excessive heat input causes the penetration depth of the molten pool on the aluminum alloy side to be too large, causing a depression under the action of gravity. When RS=0.67 and 1.50, the fish scale pattern on the weld surface is uniform, the weld shape is more beautiful, and there are no visible welding hot cracks, pores and other welding defects on the weld surface. The cross-section shapes of the welds with different beam energy ratios RS are as shown in the figure. The cross-section of the welds is in a typical “wine glass shape”, indicating that the welding process is carried out in laser deep penetration welding mode. RS has an important influence on the penetration depth P2 of the weld on the aluminum alloy side. When the beam energy ratio RS=0.5, P2 is 1203.2 microns. When the beam energy ratio is RS=0.67 and 1.5, P2 is significantly reduced, which are 403.3 microns and 93.6 microns respectively. When the beam energy ratio is RS=2, the weld penetration depth of the joint cross section is 1151.6 microns.
Effect of parallel beam-energy ratio on welding seam formation
When the laser power is 2.8kW, the welding speed is 33mm/s, the defocus amount is 0mm, and the beam spacing is 1mm, the weld surface is obtained by changing the beam energy ratio (RS=0.25, 0.5, 0.67, 1.5, 2, 4) The appearance is shown in the figure. When RS=2, the fish scale pattern on the surface of the weld is relatively irregular. The surface of the weld obtained by the other five different beam energy ratios is well formed, and there are no visible defects such as pores and spatter. Therefore, compared with serial dual-beam laser welding, the weld surface using parallel dual-beams is more uniform and beautiful. When RS=0.25, there is a slight depression in the weld; as the beam energy ratio gradually increases (RS=0.5, 0.67 and 1.5), the surface of the weld is uniform and no depression is formed; however, when the beam energy ratio further increases ( RS=1.50, 2.00), but there are depressions on the surface of the weld. When the beam energy ratio RS=0.25, 1.5 and 2, the cross-sectional shape of the weld is “wine glass-shaped”; when RS=0.50, 0.67 and 1, the cross-sectional shape of the weld is “funnel-shaped”. When RS=4, not only cracks are generated at the bottom of the weld, but also some pores are generated in the middle and lower part of the weld. When RS=2, large process pores appear inside the weld, but no cracks appear. When RS=0.5, 0.67 and 1.5, the penetration depth P2 of the weld on the aluminum alloy side is smaller, and the cross-section of the weld is well formed and no obvious welding defects are formed. These show that the beam energy ratio during parallel dual-beam laser welding also has an important impact on weld penetration and welding defects.
Parallel beam – the effect of beam spacing on welding seam formation
When the laser power is 2.8kW, the welding speed is 33mm/s, the defocus amount is 0mm, and the beam energy ratio RS=0.67, change the beam spacing (d=0.5mm, 1mm, 1.5mm, 2mm) to obtain the weld surface morphology as the picture shows. When d=0.5mm, 1mm, 1.5mm, 2mm, the surface of the weld is smooth and flat, and the shape is beautiful; the fish scale pattern of the weld is regular and beautiful, and there are no visible pores, cracks and other defects. Therefore, under the four beam spacing conditions, the weld surface is well formed. In addition, when d=2 mm, two different welds are formed, which shows that the two parallel laser beams no longer act on a molten pool, and cannot form an effective dual-beam laser hybrid welding. When the beam spacing is 0.5mm, the weld is “funnel-shaped”, the penetration depth P2 of the weld on the aluminum alloy side is 712.9 microns, and there are no cracks, pores and other defects inside the weld. As the beam spacing continues to increase, the penetration depth P2 of the weld on the aluminum alloy side decreases significantly. When the beam spacing is 1 mm, the penetration depth of the weld on the aluminum alloy side is only 94.2 microns. As the beam spacing further increases, the weld does not form effective penetration on the aluminum alloy side. Therefore, when the beam spacing is 0.5mm, the double-beam recombination effect is the best. As the beam spacing increases, the welding heat input decreases sharply, and the two-beam laser recombination effect gradually becomes worse.
The difference in weld morphology is caused by the different flow and cooling solidification of the molten pool during the welding process. The numerical simulation method can not only make the stress analysis of the molten pool more intuitive, but also reduce the experimental cost. The picture below shows the changes in the side melt pool with a single beam, different arrangements and spot spacing. The main conclusions include: (1) During the single-beam laser welding process, the depth of the molten pool hole is the deepest, there is a phenomenon of hole collapse, the hole wall is irregular, and the flow field distribution near the hole wall is uneven; near the rear surface of the molten pool The reflow is strong, and there is upward reflow at the bottom of the molten pool; the flow field distribution of the surface molten pool is relatively uniform and slow, and the width of the molten pool is uneven along the depth direction. There is disturbance caused by wall recoil pressure in the molten pool between the small holes in double-beam laser welding, and it always exists along the depth direction of the small holes. As the distance between the two beams continues to increase, the energy density of the beam gradually transitions from a single peak to a double peak state. There is a minimum value between the two peaks, and the energy density gradually decreases. (2) For double-beam laser welding, when the spot spacing is 0-0.5mm, the depth of the molten pool small holes decreases slightly, and the overall molten pool flow behavior is similar to that of single-beam laser welding; when the spot spacing is above 1mm, the small holes are completely separated, and during the welding process There is almost no interaction between the two lasers, which is equivalent to two consecutive/two parallel single-beam laser weldings with a power of 1750W. There is almost no preheating effect, and the molten pool flow behavior is similar to that of single-beam laser welding. (3) When the spot spacing is 0.5-1mm, the wall surface of the small holes is flatter in the two arrangements, the depth of the small holes gradually decreases, and the bottom gradually separates. The disturbance between the small holes and the flow of the surface molten pool are at 0.8mm. The strongest. For serial welding, the length of the molten pool gradually increases, the width is the largest when the spot spacing is 0.8mm, and the preheating effect is most obvious when the spot spacing is 0.8mm. The effect of Marangoni force gradually weakens, and more metal liquid flows to both sides of the molten pool. Make the melt width distribution more uniform. For parallel welding, the width of the molten pool gradually increases, and the length is maximum at 0.8mm, but there is no preheating effect; the reflow near the surface caused by the Marangoni force always exists, and the downward reflow at the bottom of the small hole gradually disappears; the cross-sectional flow field is not as good as It is strong in series, the disturbance hardly affects the flow on both sides of the molten pool, and the molten width is unevenly distributed.
Post time: Oct-12-2023
