How can the impact of plasma clouds on laser metal cutting be reduced?
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2021-09-08
In recent years, as the demands for sheet metal cutting have grown increasingly stringent, traditional cutting methods have struggled to meet production needs effectively due to their limitations, such as low precision and large heat-affected zones. In contrast, fiber laser cutting—boasting advantages like a small heat-affected zone, high precision, rapid processing speeds, and non-contact machining—is steadily replacing conventional cutting techniques.
As shown in Figure 1, the principle of laser cutting metal involves using a laser beam as a heat source to irradiate the surface of the metal material, raising its temperature until it reaches the melting (or boiling) point. Simultaneously, a cutting gas—emitted from the nozzle and directed parallel to the laser beam—is blown out to carry away the molten (or vaporized) material. (When the cutting gas is an active gas like oxygen, it also reacts with the metal material through oxidation, generating additional thermal energy.) By precisely controlling the motion system, the cutting head moves along a predetermined path, enabling the creation of workpieces with various intricate shapes.
Figure 1: Principle of Laser Cutting Metal
During laser cutting of metal, the changes occurring on the metal surface vary depending on the power density of the incident laser beam. Generally speaking, when the laser power density at the metal surface reaches around 10 MW/cm², the material rapidly heats up to its boiling point, causing intense vaporization into metallic steam. However, if the laser power density exceeds 100 MW/cm², the metal vapor—which doesn’t have enough time to dissipate—can be reheated by the laser energy itself, leading to the formation of a plasma cloud.
Most of the plasma cloud generated during laser cutting of metal materials is blown away by the cutting gas, while the small remaining portion forms plasma clusters that can still affect the metal-cutting process.
1) The plasma cloud remains trapped on the surface of the metal material, obstructing the transmission of laser energy and reducing the cutting speed.
2) The plasma cloud trapped beneath the nozzle not only alters the capacitive medium between the nozzle and the metal material but also heats up the nozzle, affecting its capacitive performance parameters. This interference can distort the detection results of the capacitive height controller, reducing the accuracy of the follow-up control mechanism and ultimately degrading the cutting quality.
Take, for instance, the widely used 2000W laser currently on the market. When paired with a cutting head featuring a 100/125 (collimating lens focal length/focusing lens focal length) configuration—as shown in Table 1—once the core diameter of the output fiber falls below 40 μm, the average power density at the focal point of the beam spot rises to around 100 MW/cm². This is particularly true when cutting thin metal sheets, as it significantly increases the likelihood of plasma cloud formation.
Table 1: Average Power Density of the Spot at the Zero Focus
Note: Formula 1: Cutting head zoom ratio: M = d₁/d₂ (where d₁ = focal length of the focusing lens, and d₂ = focal length of the collimating lens)
Formula 2: Spot radius at the focal point: R = 0.5 × D × M (D: Core diameter of the laser fiber cable)
Formula 3: Average Power Density of the Light Spot: P = P₁ / (π * R²) (P₁: Laser Power)
In recent years, the advantages of single-mode lasers have gained market recognition. As optical designs continue to evolve and optimize, the output power of single-mode lasers has been steadily increasing. As shown in Table 1, higher beam quality—typically associated with thinner fiber core diameters—and greater output power have led to progressively higher average power densities at the focal point. This, in turn, makes it easier to generate plasma clouds when cutting thin metal sheets.
To address this challenge, the following cutting process can effectively reduce the impact of the plasma cloud on the cutting operation:
1. Employ pulsed cutting. This pulsed cutting method ensures both the laser's peak power and minimizes the exposure time of the laser on the metal material, thereby reducing plasma cloud formation.
2. Appropriately reduce the laser cutting power. By lowering the cutting power without altering other conditions, you can decrease the average power density at the focal point and minimize plasma plume formation. A major domestic laser company once conducted tests using a single-mode 2000W laser to perform zero-focus cuts on 1mm-thick stainless steel at full power. However, due to the interference caused by the plasma plume, the cutting speed was less than ideal. When the cutting power was reduced to 1800W, the cutting speed actually improved by 50%.
3. Appropriately widen the cutting kerf. Widening the cutting kerf not only provides a broader pathway for the plasma plume to disperse downward, reducing its interference with the cutting process, but also helps accelerate the removal of molten slag from the kerf, thereby enhancing the overall cutting performance.
4. Appropriately reduce the cutting height. The cutting height not only directly determines the thickness of the plasma plume between the nozzle and the metal surface (the shorter the distance, the thinner the plume), but also ensures that the cutting gas pressure emitted from the center of the nozzle increases as you get closer to the cutting nozzle (see Figure 2). Higher cutting gas pressure helps accelerate the dispersion of the plasma plume beneath the nozzle, thereby reducing the plasma's shielding effect on the incident laser beam. Therefore, while maintaining the safety of the cutting head, it’s best to keep the standoff distance as close as possible.
Figure 2: Gas Pressure Distribution at Various Distances Below the Nozzle
5. Use the appropriate cutting nozzle. A suitable nozzle can increase gas flow velocity without enlarging the nozzle diameter, thereby accelerating the dissipation of the metal plasma cloud.
6. Add a side-blowing air device and a nozzle cooling system to the cutting head. The side-blowing air device helps disperse part of the plasma plume, reducing its accumulation beneath the nozzle, while the nozzle cooling system minimizes the thermal impact of the plasma plume on the nozzle, preventing any adverse effects on the nozzle's capacitive performance parameters.
7. Utilizes a high-sampling-rate capacitive height controller. This advanced device not only ensures precise follow-up accuracy but also allows the system to monitor changes in capacitance values, enabling it to detect variations in the plasma cloud beneath the nozzle. By continuously tracking these plasma cloud dynamics, the machine tool can implement adaptive measures—such as slowing down, pausing, or switching to pulse-cutting mode—to minimize the impact of the plasma cloud on the cutting process.
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