Laser Cutting Process - Stainless Steel Material (Solving Burr Defects)
Methods to Resolve Burr Defects in Stainless Steel Cutting
1.1 Characteristics of Oxygen-Free Cutting of Stainless Steel
Phenomenon
In stainless steel cutting, oxygen, nitrogen, or air is generally used as the assist gas. The choice of gas depends on the specific processing application. Among these gases, oxygen has the lowest consumption rate, while nitrogen and air consume relatively more. When it comes to cutting speed, air or nitrogen results in faster cutting, while oxygen leads to a slower process. The degree of oxidation on the cutting surface increases in the order of nitrogen, air, and oxygen, with the burden of removing the oxide layer increasing accordingly.
Principle
(1) Cutting Speed
Figure 4.1-1 illustrates the relationship between plate thickness and cutting speed when cutting SUS304 material with a 3 kW power laser. When the thickness is below 3 mm, using oxygen for cutting allows the oxidation reaction to achieve higher speeds. However, when the thickness exceeds 3 mm, the molten metal's flowability is better when cutting with nitrogen, resulting in faster oxygen-free cutting. Air cutting can achieve a cutting speed similar to nitrogen, but the surface roughness and dross attachment significantly degrade the processing quality compared to nitrogen cutting.
(2) Surface Treatment of the Cutting Edge
The hardness of the surface layer on the cutting edge will vary significantly depending on whether oxygen or nitrogen is used for cutting (as shown in Figure 4.1-2). The oxidized cutting surface has a hardness approximately twice that of the base material, while the oxygen-free cutting surface has a lower hardness and a smoother surface, making subsequent grinding and polishing processes relatively easier. The cutting surface processed with oxygen tends to develop a stubborn oxide layer, which increases the burden on subsequent processing steps.
(3) Corrosion Resistance of the Cutting Edge
Figure 4.1-3 shows the results of a salt spray corrosion resistance test on SUS304 material that was laser-cut using different types of assist gases. The cutting edges processed with oxygen and air showed signs of rust, whereas the oxygen-free cutting edge processed with nitrogen did not rust. The type of assist gas used during cutting has a significant impact on the corrosion resistance of the cutting edge.
(4) Welding Quality of the Cutting Edge
When welding the cutting edge after laser cutting, if the edge has been oxidized, pores may form in the weld seam, leading to reduced welding strength. However, if the cutting edge is not oxidized, a high-quality weld can be achieved.
1.2 Methods to Address Defects Caused by Whisker-Like Burrs in Stainless Steel Cutting
Phenomenon:
During the piercing process of stainless steel, as soon as the laser beam hits the metal, the metal begins to melt. As shown in Figure 4.2-1, the molten material is expelled onto the surface of the material, splattering around the small hole and forming whisker-like burrs. These burrs can cause scratches on the cutting surface and may also interfere with the profiling action of the capacitive sensor.
Cause
When oxygen is used as an assist gas, the molten metal oxidizes during the piercing process, preventing the formation of whisker-like protrusions and reducing its adhesion to the stainless steel surface. However, when nitrogen is used as the assist gas, the molten metal does not oxidize, resulting in lower viscosity. This allows the molten metal to stretch into whisker-like forms and adhere strongly to the material's surface, leading to accumulation around the small holes.
Solutions
To prevent the splashing and adhesion of molten metal, the following methods can be employed: (a) reduce the amount generated; (b) prevent adhesion; (c) remove the adhesion after it occurs (as shown in Figure 4.2-2).
(1) Reducing the Amount of Molten Metal
① Adjust Piercing Conditions: Increasing the frequency while lowering the output power of individual pulses can effectively reduce the amount of molten metal produced. Figure 4.2-3 shows the results of processing at frequencies of 200Hz and 1500Hz. It is important to note that using these processing conditions will also increase heat input, making them unsuitable for cutting thick plates.
② Using Assist Gas or Side Blowing to Disperse Molten Metal
Blowing the molten metal ejected from the piercing hole away using assist gas or side blowing can reduce adhesion. Figure 4.2-4 shows the results of processing with assist gas at pressures of 0.05 MPa and 0.7 MPa. It can be observed that using high-pressure gas results in less slag adhering to the surface.
(2) Preventing Adhesion
Applying a protective coating on the material's surface can also help prevent molten metal from adhering. When a protective coating is applied, the molten metal produced during piercing accumulates on the coating rather than directly adhering to the material's surface. The protective coating can be a slag prevention agent or a surfactant that is easy to remove in subsequent processes (as shown in Figure 4.2-5).
(3) Removal
There are two methods for removing burrs: One method is to cut very small circular holes near the perforation; while cutting the circular hole, the molten metal is also removed. The other method is to move the focal point upwards after perforation to remelt the accumulated material and blow it away with gas (see Fig. 4.2-2(3)).
1.3 Methods for Solving Processing Defects in Thick Plates After Perforation
[Phenomenon]
As shown in Fig. 4.3-1, in oxygen-free cutting of thick stainless steel plates, the molten metal generated during perforation will accumulate above the perforation hole, leading to poor processing quality when the processing head passes over. Switching the auxiliary gas used during perforation to oxygen can reduce the accumulation of molten metal. However, when using oxygen during perforation, it is important to ensure that any remaining oxygen in the auxiliary gas line is completely removed before proceeding to the next cutting step. Otherwise, the remaining oxygen may mix with nitrogen and cause oxidation on the cutting surface.
[Cause]
(1) Impact of Accumulation
As shown in Fig. 4.3-2, the impact of accumulation on processing includes the tendency to cause laser reflection and disturb the auxiliary airflow during cutting. In oxygen-free cutting, it is common to set the focal point near Z=0 during perforation and then shift it to Z=-T (where T is the thickness of the workpiece) during cutting. However, this approach can reduce the energy density of the laser incident on the material surface during cutting, leading to poor processing quality.
(2) Impact of Gas Switching
When switching between oxygen and nitrogen, it is crucial to efficiently and thoroughly remove the residual gas in the pipeline. The more perforations performed, the more frequent the gas switching, and the longer the purging time required for the residual gas.
[Solution]
(1) Addressing the Impact of Accumulation
As shown in Fig. 4.3-3, use high laser energy density conditions for the initial cutting portion where molten metal accumulation is present. Specifically, during cutting, use the same high energy density and focal point position (Z=0) as during perforation. After passing through the accumulated material, shift the focal point to Z=-T (where T is the thickness of the workpiece). When cutting with the focal point at Z=0, the kerf width will be narrower, and the amount of burrs on the back of the workpiece will increase. Therefore, the perforation line (the line segment where cutting begins) should be set away from the part. Other processing parameters should also be set to high power and low speed conditions. The goal of these settings is to ensure stable processing of the accumulation part.
(2) Reducing Gas Switching Time
As shown in Fig. 4.3-3, first complete all perforation operations using oxygen. Then, return to the starting point of the processing, switch the auxiliary gas to nitrogen, and thoroughly purge any remaining oxygen before starting the cutting process. By using this method, gas only needs to be switched once, which saves the time required for purging residual oxygen from the gas lines (see Fig. 4.3-4).
1.4 Methods to Reduce Burrs at Sharp Corners When Cutting Thin Plates with Air or Nitrogen
[Phenomenon]
In stainless steel cutting, when using air or nitrogen as the auxiliary gas, burrs will appear on the back of the material at sharp corners or at the end of the processing shape, as shown in Fig. 4.4-1.
[Cause]
The processing machine or head moves according to the NC (numerical control) set speed, but at sharp corners or the end of the processing shape, the processing speed slows down due to the characteristics of the machine. Typically, the laser power setting of the processing machine remains constant. As a result, when the processing speed decreases, the balance between laser power and speed is disrupted (with excess power output), leading to the formation of burrs (see Fig. 4.4-2).
[Solutions]
(1) General Processing Conditions
Set the maximum cutting speed as low as possible to minimize the difference between the maximum and minimum cutting speeds throughout the processing path. Whether at maximum or minimum cutting speeds, adjust the output power to conditions that generate fewer burrs. The drawback of this method is that the average speed will decrease, leading to longer overall processing time.
(2) Modify the Trajectory
Design an overrun trajectory to prevent the cutting speed from decreasing at sharp corners or the end of the processing path. For example, program a circular overrun at sharp corners. When the trajectory undergoes a circular overrun, it changes gradually at the transition points, avoiding sudden drops in cutting speed. Using a circular overrun program at the end of internal hole processing can also cut out internal holes without reducing cutting speed. However, this method cannot be used if there are products near the sharp corners or if both the inside and outside of the corner are products.
(3) NC Control
Corresponding control functions have been developed to address these issues. This involves real-time detection of the cutting speed by the processing machine, allowing automatic adjustment of the laser output power to match the cutting speed changes.
As shown in Fig. 4.4-3, when the cutting speed slows down at sharp corners, the laser output power is also reduced accordingly. The same adjustment applies at the end of the processing path, where the output power automatically decreases as the cutting speed slows down.
1.5 Methods to Resolve Burrs in Nitrogen Cutting of Thick Stainless Steel Plates
[Phenomenon]
If molten metal is not efficiently expelled from the kerf, it will adhere to the back of the workpiece and form burrs. If the material used is new and has previously cut well but now exhibits issues, the problem might be due to inappropriate processing conditions that need adjustment.
[Cause]
The smooth expulsion of molten metal from the kerf depends on the appropriate auxiliary gas pressure to push the molten metal downward, as well as the kerf shape and the continuity of molten metal flow. The main causes of burr formation, as shown in Fig. 4.5-1, include:
- Deviation in Kerf Width: The width of the kerf deviates from the initial optimal value, becoming either narrower or wider.
- Impact of Processing Shape: The shape being processed affects the continuity of molten metal flow.
[Solutions]
(1) Adjusting the Kerf
Nitrogen cutting of stainless steel differs from carbon steel cutting in that the focal point should be set inside the material (Z<0) to enhance the laser's melting capability and increase the kerf width. If the focal point is not optimally set, the flow of molten metal within the kerf can be adversely affected. When the focal point is too shallow, burrs tend to be sharp, while a deeper focal point results in ball-shaped burrs. Adjust the focal point based on the shape of the burrs to find the optimal position.
If burrs increase progressively with processing and the kerf width changes, it may be due to the heating of optical elements by the laser, leading to a thermal lensing effect. In such cases, clean the lens or PR mirror.
(2) Impact of Processing Shape
Burrs are likely to form when the auxiliary airflow becomes unstable after the laser passes the sharp corner or when the balance between power and speed is disrupted due to sudden changes in cutting speed. To address this, lower the processing speed setting in the processing conditions (see Fig. 4.5-2). The smaller the angle of the sharp corner, the more effective the low-speed settings are. Additionally, when transitioning from low-speed to high-speed conditions, set the speed transition as a step-by-step process.
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