Laser Elite Collection 2024-03-11 09:00 Shanghai Yangtze River Delta G60 Laser Alliance Chen Changjun reprinted!
Preamble:
The stability of the welding process can affect the quality of the weld, especially in high-power welding processes, where the laser beam hitting the surface of the material causes the material to evaporate rapidly to form a keyhole that locks the laser energy inside and converts it into heat. The process involves large variations and complex couplings of various physical elements, which makes it extremely challenging to achieve a stable welding process.
Since energy coupling and energy deviation are the key factors affecting welding stability, it is of great significance to reveal the law of energy coupling and deviation in the laser welding process. The keyhole effect is essential for energy coupling for three main reasons:
1. Multiple reflection and absorption of laser energy in the keyhole;
2. The energy distribution in the keyhole is affected by the metal plume;
3. Laser energy transmission is affected by the metal plume above
Ref. [1] The X-ray fluoroscopy-based laser penetration fusion welding process has been studied. They pointed out that the absorption rate of laser energy by the anterior wall of the small hole, especially the position of the evaporation point on the anterior wall of the keyhole, changes frequently during the continuous laser welding process with stable power, resulting in continuous instability of the keyhole.
[1] Matsunawa A, Kim J, Seto N, Mizutani M, Katayama S (1998) Dynamics of keyhole and molten pool in laser welding.
The research group of Professor Gao Xiangdong from Guangdong University of Technology has done a lot of research on this, and the experiment uses a high-power disk laser (Trumpf 16002) with a maximum power of 16 kW. THE TRUMPF PFO LASER SCANNER WITH A FOCUSED BEAM DIAMETER OF 200 MICRONS AND A FOCAL LENGTH OF 300 MM IS CONNECTED TO A MOTOMAN SIX-AXIS ROBOTIC ARM. The detection system consists of a photoelectric sensor module (mainly including photodiodes) and an industrial camera module, which is designed to observe and obtain the physical phenomena and signals generated during the welding process in an all-round way. Specifically, the scanning laser head collects the optical radiation signal, which is then transmitted to the photoelectric sensor module via optical fibers by a unidirectional mirror and a focusing lens. In the photoelectric sensor module, the signal is divided into visible and reflected light signals by means of binary mirrors and filters. In the industrial camera module, a monochrome high-speed camera (BW camera for short) is set above the welding area, which works together with the low-power diode laser light source and filter to obtain the signal melt pool and keyhole on the surface of the workpiece. In the center of the field is the keyhole location. Images are taken from the side with a color camera in a direction parallel to the workpiece, with the center of the field as the base plate, from which the metal plume and plasma generated on the surface and back of the workpiece can be obtained.
The sheet is 6 mm thick SUS 304 stainless steel, welded flat plate, laser power set to 6 kW, welding speed 2 m/min, shielding gas Ar (flow rate: 30 L/min pointing in the direction of welding), and the focal position varies from -4 to 4 mm.
a. The surface (beam side) and the back side of the weld. bTop row, metal plume, middle row, splash/melt pool surface, and bottom row, weld profile cross-section
As the focal position moves further towards the internal molten pool, the keyhole absorbs more laser energy, resulting in an increase in the upper metal plume, and the main area of metal plume evaporation is also gradually shifted downward, and the huge recoil and shear forces of the metal plume to expand the opening at the root of the keyhole, resulting in more volume of metal plume being released from the root of the keyhole. As the metal plume is released from the keyhole opening, the shear force generated at the keyhole edge overcomes the liquid surface tension and gravity, causing the liquid to flow away, which ultimately leads to the occurrence of splashing As more metal plume is released, the recoil and shear forces are strengthened, creating more spatter. This is the cause of the splash. At the same time, it is also observed that the metal plume volume and the splash volume change synchronously, which can indirectly control the splash and metal plume.
When the focal position decreases in the range of -1 to -3 mm, the energy absorption rate of the pore increases, and as the focus deepens, the metal melts and evaporates. As a result, the plume recoil pressure increases. Since the recoil pressure from the upper metal plume drives the liquid from the front wall of the orifice to flow downward, the faster the liquid from the front wall of the orifice flies downward, the more splashing of the lower part is generated.
The angle between the front wall of the small hole and the welding direction β begins to increase due to the surface tension of the fast-flowing liquid, while the angle between the main ejection direction of the plume and the front wall of the small hole does not change significantly γ. It can be inferred that the recoil and shear forces of the metal plume lead to an increase in the angular α during the first laser reflection. With the increase of the angular α, the metal plume produces a greater recoil and shear force on the liquid near the back wall of the hole, so that the surface liquid flows faster to the back of the melt pool, and the opening in the upper part of the hole is larger.
Schematic diagram of keyholes passing through the same cross-section at different times. Three-dimensional vision. Enlarged view of the droplet-shaped microstructure above b. c. Enlarged view of the middle part. Enlarged view of the droplet-shaped microstructure in the lower part of d
In the process of disc laser welding, the reflected light signal mainly comes from the process of multiple reflections of the incident laser in the small hole, and the reflected light is transmitted to the pad because the amount of laser absorbed and reflected by the metal plume is kept at a small level. The laser head is opened through the upper part of the keyhole. Therefore, as the opening area of the upper part of the hole increases, the laser reflection signal becomes stronger, which also explains why the laser reflection volume changes synchronously with the opening area on the hole, and both the data and the good appearance prove that the welding process is basically stable at the focal position at 0 mm.
Since the focal point is above the surface of the workpiece, the laser beam is more attenuated when it reaches the surface of the workpiece or inside the hole, absorbing less laser energy. The laser beam in the keyhole is concentrated in the upper part of the keyhole after multiple reflections, so that the main energy absorption area in the keyhole is offset upwards with the upward shift of the focal position. As the curvature of the bottom of the keyhole increases, so does the surface tension of the liquid at the bottom of the keyhole, which prevents the bottom of the keyhole from opening more widely. In addition, the metal liquid at the bottom of the keyhole has a high viscosity. This results in the keyhole root being more difficult to open than the keyhole top, and it takes more energy to open the keyhole root than the keyhole top. As the main energy-absorbing region shifts upward, the small hole root is less likely to open at the focal position of -4 mm, which is why the lower metal plume volume and lower splash.
In the process of high-power fiber laser welding, the keyhole maintains a high absorption rate of laser energy, and the transient evolution of the melt pool and keyhole in the quasi-steady-state stage of laser full penetration welding has obvious periodic characteristics.
The conversion of endothermic solid metals to liquid and gaseous states and the creation of "quasi-focus reduction".
A multi-sensory fusion system was established to collect various signals in the process of stainless steel high-power laser welding, and comparative analysis was carried out. Energy deviations are discussed in depth by integrating multiple signals, weld shaping, and microstructure. It affects different physical phenomena in the welding process, such as molten pools, small holes, metal plumes, and spatter. The conclusions are as follows:
1. Due to the high energy density and drastic energy conversion, energy deviation is prone to occur, which affects the welding stability.
2. As the welding process progresses, the preheating effect of the rear becomes more obvious, which leads to the occurrence of "quasi-focus reduction". The preheating effect increases the amount of evaporation, which is another reason for the instability of the weld. Therefore, effective control of laser-induced evaporation is considered to be the key to adjusting the distribution and preventing welding instability. It inspired us to take steps to achieve weld stability to counteract the effects of preheating in industrial applications. For example, after detecting a signal signature that deviates from energy, you can use Raise the focus position.
3. Due to the large curvature of the small hole when it is not penetrated, delamination often occurs in the microstructure of the section, and then the "quasi-heat-affected zone" is generated.
4. Therefore, according to the volume of the metal plume and the size of the keyhole opening, the keyhole shape is judged in real time, and the laser power is automatically adjusted at the same time to ensure the stability of the keyhole, which can effectively improve the stability of laser welding and ensure the welding quality to a certain extent.
参考文献：Ding, S., You, D., Cai, F. et al. Research on laser welding process and molding effect under energy deviation. Int J Adv Manuf Technol 108, 1863–1874 (2020).
Yangtze River Delta G60 Laser Alliance Chen Changjun reprinted!
At the same time, welcome to the 2nd Conference on the Application of Laser Intelligent Manufacturing in the Energy Storage Industry held by the Yangtze River Delta G60 Laser Alliance in Nanjing (Nanjing, April 23-25, 2024)