Introduction
In the process of laser cutting carbon steel with air under negative focal conditions, slag adhesion is a prevalent issue. This problem not only significantly affects the quality of the cutting surface, such as causing roughness and unevenness, but also reduces production efficiency by necessitating additional post – processing steps to remove the slag. In industrial production scenarios, especially in manufacturing processes that demand high – precision and high – efficiency cutting of carbon steel, the presence of slag adhesion can lead to increased costs and decreased product competitiveness. Therefore, understanding and addressing this problem are of great significance. This article will explore two common types of slag adhesion, aiming to provide in – depth insights and practical solutions for manufacturers and operators in the field.
Type 1: Continuous, Dripping Burs (Dross) at the Bottom
Characteristics
This type of dross is characterized by its relatively large and continuous form. It adheres firmly to the bottom edge of the cutting section, presenting as a string of molten metal beads. The diameter of these beads can range from several millimeters to even larger, depending on the specific cutting conditions. This continuous and drooping dross not only affects the appearance of the cut – edge but also has a significant impact on the subsequent processing of the workpiece. For example, in precision manufacturing, such dross can interfere with the assembly of parts, reducing the overall accuracy of the product.
Causes
- Insufficient energy: This is the most fundamental reason. Negative – focal cutting means that the focal point is below the surface of the plate, causing the spot diameter to increase and the energy density to decrease. If the energy is not sufficient to completely vaporize or melt the material, the remaining liquid metal cannot be completely blown away by the auxiliary gas, and thus condenses at the bottom to form dross. For instance, when cutting a thick – gauge carbon steel plate with an inappropriate laser power setting, the energy reaching the bottom of the cut is far from enough to handle the volume of material, leading to dross formation.
- Insufficient or unstable air pressure: The air compressor may not provide enough pressure, or there may be significant fluctuations in air pressure. This results in an inability to generate sufficient momentum to blow the molten metal liquid away from the slit. Since air, as an auxiliary gas, has much less kinetic energy compared to nitrogen, it has higher requirements for air pressure. In some industrial production lines with aging air – supply systems, the unstable air pressure can cause inconsistent cutting quality, with dross frequently appearing due to the ineffective removal of molten metal.
- Excessive cutting speed: When the plate moves too quickly relative to the laser beam, the energy absorbed per unit length of the material is insufficient. As a result, the material is not completely cut through, and the molten material does not have enough time to be blown away. In high – speed cutting attempts without proper parameter optimization, the rapid movement of the workpiece can cause the laser to miss some areas, leaving behind uncut or partially – cut material that solidifies into dross.
- Improper focal position: If the negative – focal amount is set too large, the energy becomes overly dispersed. Consequently, the energy at the bottom of the incision is severely insufficient, making it difficult to fully process the material at the bottom, and eventually leading to dross formation.
Solutions
- Appropriately increase power: Increasing the laser power is the most direct and effective way to increase energy input. It is crucial to ensure that the power matches the thickness of the plate and the cutting speed. For a 5 – mm – thick carbon steel plate, after a series of tests, it may be found that increasing the laser power from 1000W to 1200W can significantly reduce the amount of dross.
- Adjust the focal position:
- Reduce the negative – focal amount: Try to adjust the focal point upward (towards the surface of the plate) to reduce the negative – focal amount. This can increase the energy density at the bottom of the incision. A focal – position process test can be carried out, cutting with different parameters from – 1mm to – 3mm of negative – focal amount, to find the focal point with the least dross. For example, in a test of cutting a 3 – mm – thick carbon steel plate, it is discovered that a negative – focal amount of – 1.5mm results in the cleanest cut with minimal dross.
- Increase air pressure: Ensure that the air pressure is high enough. For thin plates, a pressure of 0.8 – 1.2MPa may be required, and for thick plates, an even higher pressure is needed. Regularly check the air compressor and the drying and filtering system to ensure that the gas source is pure, dry, and the pressure is stable. Contaminants such as oil and moisture in the air can seriously affect the cutting effect. In a manufacturing workshop, after replacing a malfunctioning air – pressure regulator and installing a high – efficiency air – drying filter, the cutting quality improved significantly, with much less dross.
- Reduce the cutting speed:
- Appropriately reduce the speed: Reducing the cutting speed allows the material more time to absorb energy, ensuring that it is completely melted and blown away. The speed and power need to be adjusted in coordination to find the best balance. When cutting a 8 – mm – thick carbon steel plate, reducing the cutting speed from 1000mm/min to 800mm/min while maintaining a suitable power can effectively eliminate the continuous dross at the bottom.
Type 2: Fine, Hard Granular Dross at the Bottom
Characteristics
This type of dross is characterized by its fine and hard texture. It appears as small granules or powder, firmly adhering to the cutting surface. These granular dross particles are usually much smaller in size compared to the continuous dross of the first type, typically in the range of micrometers to sub – millimeters. Their hardness makes them difficult to remove during post – processing, often requiring more aggressive mechanical or chemical methods. For example, when using a simple wire – brush for cleaning, the granular dross may still remain on the surface, affecting the overall finish and quality of the workpiece.
Causes
- Excessive material oxidation: This is an inherent characteristic of air – assisted cutting. In the high – temperature environment during cutting, the oxygen in the air reacts violently with the carbon steel. The chemical reactions mainly include Fe + O₂ → FeO/Fe₃O₄/Fe₂O₃. The oxides produced (mainly iron oxides) have high melting points and large viscosities. As a result, they are not easily blown away by the auxiliary gas and condense to form hard dross. In the cutting of thick – walled carbon steel pipes with air as the auxiliary gas, a large amount of high – melting – point iron oxides are generated, which are difficult to expel from the cutting area, leading to the formation of granular dross.
- Excessive heat input: While insufficient energy leads to the first type of dross, an excessive amount of heat input, usually caused by a combination of high power and low speed, can cause the plate to burn excessively. This results in the production of a large amount of high – melting – point oxides, exacerbating the dross problem. When attempting to cut a thin carbon steel sheet with an overly high laser power and a very slow cutting speed, the material not only undergoes excessive oxidation but also melts and vaporizes unevenly, leaving behind granular dross on the cut surface.
- Nozzle wear or misalignment: Worn – out nozzles can cause the air flow to become disordered, preventing it from entering the slit symmetrically and vertically. If the nozzle center is not coaxial with the laser beam, the blowing ability of the gas is weakened, and the molten slag cannot be effectively removed. In a production line where nozzles are used for an extended period without replacement, the worn – out nozzles cause the air flow to deviate from the optimal direction, resulting in the accumulation of granular dross on the cutting surface due to ineffective slag removal.
Solutions
- Optimize the matching of cutting speed and power:
- Adopt a “high – speed, moderate – power” strategy: On the premise of ensuring complete penetration, appropriately increasing the speed and reducing the power can reduce the residence time of the material in the high – temperature zone, thereby alleviating the excessive oxidation reaction. This approach is contrary to the solution for the first type of dross and requires careful debugging. For a 2 – mm – thick carbon steel sheet, increasing the cutting speed from 1500mm/min to 1800mm/min while reducing the power from 800W to 700W can effectively reduce the formation of granular dross.
- Adjust gas parameters (strategic changes):
- For thin plates, try slightly reducing the air pressure: Excessively high air pressure may exacerbate the oxidation reaction rather than blowing away the slag. It is advisable to use a lower air pressure while ensuring that the slag can be blown away. In a test of cutting a 1 – mm – thick carbon steel plate, reducing the air pressure from 1.0MPa to 0.8MPa, while maintaining other parameters stable, results in a cleaner cut with less granular dross.
- Ensure gas purity: Only dry and oil – free compressed air should be used. Moisture can rapidly cool the molten metal and promote oxidation, while oil contamination can dirty the lenses and affect the cutting quality. Installing high – efficiency air – drying and oil – filtering devices in the air – supply system can ensure the purity of the air, significantly improving the cutting quality and reducing dross.
- Inspect and replace the nozzle:
- Check the nozzle condition: Regularly inspect the nozzle for wear, deformation, or slag blockage. Worn – out nozzles must be replaced immediately. In a manufacturing workshop, a scheduled inspection of nozzles every 50 hours of operation can prevent the occurrence of dross caused by nozzle problems.
- Calibrate the nozzle center: Use alignment paper or specialized alignment tools to ensure that the center of the nozzle hole coincides completely with the laser beam. This is a crucial step in ensuring the correct direction of the air flow. After using a professional nozzle – alignment tool, the cutting quality improves remarkably, with a significant reduction in granular dross due to the optimized air – flow direction.
- If processing conditions permit, use coated steel plates such as galvanized plates. The coating can sometimes play a certain “flux – aiding” role during the cutting process or change the properties of the slag, making the dross easier to remove. However, this is not a fundamental solution. When cutting galvanized carbon steel plates, the zinc coating can react with the molten metal and the oxidation products in a way that modifies the slag’s characteristics, facilitating its removal from the cutting surface.
Quick Troubleshooting Steps
Hardware Check
- Protective lens: Regularly inspect whether the protective lens is clean and free from damage. A dirty or damaged lens can significantly attenuate the laser energy, leading to inconsistent cutting quality and increased slag adhesion. If contaminants are detected, clean the lens carefully using appropriate cleaning agents and tools.
- Nozzle: Examine the nozzle for any signs of wear, deformation, or clogging. A worn – out nozzle with an enlarged or irregular inner diameter can cause the air flow to deviate from the optimal direction, weakening the slag – blowing ability. Replace the nozzle immediately if any issues are found. Additionally, ensure that the nozzle size is correct for the specific cutting task, as an improper – sized nozzle can also contribute to slag problems.
- Air pressure: Verify that the air pressure reaches the set value and remains stable throughout the cutting process. Install a reliable pressure gauge to monitor the air pressure accurately. Fluctuations in air pressure can disrupt the stable removal of molten metal, resulting in slag formation. If the pressure is insufficient or unstable, check the air – supply system, including the air compressor, pipelines, and valves, to identify and resolve the source of the problem.
- Gas source: Ensure that the gas source is dry and clean. Moisture in the air can rapidly cool the molten metal, promoting oxidation and the formation of hard slag. Oil contamination can not only dirty the lenses but also affect the chemical reactions during cutting, leading to poor – quality cuts. Install high – efficiency air – drying and oil – filtering devices in the air – supply system to guarantee the purity of the air.
Focus Optimization
Conducting a focus – position process test is of utmost importance. This test helps to determine the optimal focus position for the specific material and thickness being cut. Different materials and thicknesses require different focus settings to achieve the best – quality cuts with minimal slag adhesion.
The method involves performing a series of test cuts with varying negative – focal amounts, typically ranging from – 1mm to – 3mm for carbon – steel cutting with air under negative – focal conditions. During the test, carefully observe the cutting quality, especially the amount and type of slag adhesion. The focus position that results in the least amount of slag, a smooth cutting surface, and complete penetration of the material is considered the optimal setting. Record these optimal focus – position values for future reference when cutting similar materials and thicknesses.
Power and Speed Adjustment
- For continuous, dripping dross: If the slag is in the form of continuous, dripping burs at the bottom, the priority is to increase the laser power or reduce the cutting speed. Increasing the power directly adds more energy to the cutting process, ensuring that the material is fully melted and vaporized. Reducing the speed allows the material more time to absorb the laser energy, facilitating complete cutting and effective removal of molten metal. However, when adjusting these parameters, it is crucial to maintain a balance to avoid other potential issues such as over – heating or excessive oxidation.
- For fine, hard granular dross: When dealing with fine, hard granular dross, the strategy is to increase the cutting speed or appropriately reduce the power. Increasing the speed shortens the time the material spends in the high – temperature zone, reducing the extent of oxidation. Reducing the power helps to prevent excessive heat input, which can lead to the production of large amounts of high – melting – point oxides. By optimizing the combination of speed and power, the formation of granular dross can be effectively minimized.
Conclusion
The problem of slag adhesion in air negative – focal cutting of carbon steel is complex, influenced by multiple factors such as energy input, gas parameters, and hardware conditions. For the two common types of slag adhesion, continuous, dripping burs and fine, hard granular dross, different causes and corresponding solutions have been explored. In practical production, it is essential to continuously test and optimize parameters. Regularly checking hardware components, such as the protective lens and nozzle, and ensuring the stability and purity of the gas source, are the basic steps. Through focus – position tests, an optimal focus setting can be determined. Additionally, the coordinated adjustment of laser power and cutting speed according to the type of slag adhesion is crucial. By achieving a balance between “utilizing the oxidation reaction to increase heat” and “preventing excessive oxidation from generating high – melting – point slag,” high – quality and efficient carbon – steel cutting can be realized, meeting the requirements of modern industrial production.