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Laser Cutting Defects: Quality Control Tips

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The process of laser cutting involves absorbing light energy and converting it into heat energy, which causes the material to melt and vaporize.

The high energy density laser beam is output by the laser generator. The beam is then focused through the focusing lens, resulting in a highly concentrated energy source. The focused beam passes through the center of the nozzle, which ejects an auxiliary cutting gas along the same axis as the light path. The combination of the laser beam and cutting gas rapidly heats, oxidizes, and evaporates the cutting material to achieve the desired cutting effect.

The basic principle behind laser cutting is the interaction between the laser and matter. This interaction encompasses both complex microscopic quantum processes and macroscopic phenomena, such as the absorption, reflection, refraction, energy conversion, and transmission of the material to the laser, as well as the material’s state and the composition of the ambient gas.

These macroscopic phenomena, along with other factors such as the tissue effect of a light beam on a material surface, make the factors affecting the quality of laser cutting very complex.

In addition to the material being processed, other factors that influence the quality of laser cutting include the characteristics of the light beam, laser power, cutting speed, type (aperture) and height of the nozzle, focus position, and type and pressure of the auxiliary gas.

Laser Cutting Working Principle Drawing

The effect of the beam on cutting quality

The width of laser cutting is closely tied to the beam mode and focal spot diameter. The power and energy density of laser irradiation are related to the diameter of the laser spot, so it is desirable to have a smaller spot diameter in order to achieve greater power and energy density in laser cutting. The size of the spot diameter is determined by the diameter of the laser beam output by the oscillator and its divergence angle, as well as the focal length of the focusing lens.

For the common use of ZnSe flat convex focusing lenses in laser cutting, the relationship between the spot diameter (d), the focal length (ƒ), the divergence angle (θ), and the diameter (D) of the incident laser beam can be calculated using the following formula:

spot diameter formula

(1.1)

As seen in the equation above, a smaller divergence angle in the laser beam will result in a smaller spot diameter, thereby improving the cutting effect. Reducing the lens focal length (ƒ) is beneficial in reducing the spot diameter, but doing so also shortens the focal depth and makes it difficult to achieve an equal width of the incision on both the top and bottom sections when cutting thicker plates, which affects the quality of the cut.

At the same time, reducing the lens focal length also reduces the distance between the lens and the workpiece. During cutting, slag may splash onto the surface of the lens, affecting normal cutting operation and the lens’s service life.

A short focal length lens has a high power density but a limited focal depth, making it suitable for high-speed cutting of thin plates as long as the spacing between the lens and workpiece remains constant. In contrast, a long focal length lens has a low power density but a large focal depth and is suitable for cutting thick sections of material.

As a general rule, the shorter the focal length, the smaller the focal spot and the shallower the focal depth; conversely, the longer the focal length, the larger the focal spot and the deeper the focal depth. For example, when the lens focal length is doubled, the focal spot size will also double (from Y to 2Y), and the focal depth will increase fourfold (from X to 4X).

The focus of the focusing lens

Fig.1 The focus of the focusing lens

The pattern of the laser beam is related to its focusing capability, which is similar to the sharpness of a mechanical tool. The lowest order mode is TEM00, and the energy in the spot is distributed in a gaussian-like manner. This mode is capable of focusing the beam to a theoretical minimum size, such as a few microns in diameter, resulting in a highly concentrated energy density. The laser mode is depicted in the figure.

In contrast, high-order or multi-mode beams have a more widespread energy distribution, resulting in a larger focused light spot with lower energy density. Using this type of beam for cutting is like cutting with a dull knife.

Beam energy distribution pattern

Fig.2 Beam energy distribution pattern

The quality of laser cutting is directly related to the mode of the beam. The lower the mode, the smaller the spot size, the higher the power density and energy density, and the better the cutting performance.

For example, when cutting low carbon steel, a TEM00 mode beam cuts 10% faster and produces a surface with a lower roughness (10μm less Rz) compared to a TEM01 mode beam. In optimal cutting parameters, the roughness of the cutting surface can be as low as 0.8μm.

Therefore, for metal cutting, the TEM00 mode laser is often used to achieve faster cutting speeds and better cutting quality.

Effect of laser power on cutting quality.

The size of the laser power directly affects the thickness of the steel plate that can be cut. The higher the energy, the thicker the material can be cut.

Additionally, it influences the dimensional precision of the workpiece, the width of the cut, the roughness of the cut surface, and the width of the heat-affected zone.

The laser power density (P0, measured in W/cm²) and energy density (E0, measured in J/cm²) that is illuminated on the workpiece during the laser cutting process have a significant impact on the laser cutting process.

As the laser power density increases, the roughness decreases. However, when the power density (P0) reaches a certain value (approximately 3 x 106 W/cm²), the roughness (Rz) value stops decreasing.

The larger the laser power, the thicker the material can be cut. However, for the same laser power, the maximum thickness that can be cut will differ for different materials.

Table 1 shows the maximum thickness for CO2 laser cutting of various metals for different laser powers.

Table 1  Laser power and metal maximum cutting thickness

CO2 LaserMax Cutting Thickness /mm
Power/WMild SteelStainless SteelAluminum AlloyCopperBrass
1500129312
150012634
3000221255
400025141058

For a laser generator with continuous-wave output, the size and mode of the laser power will have a significant impact on the cutting quality. In practice, the maximum power is often set to achieve the fastest cutting speed, increase production efficiency, or cut thicker materials. In theory, the larger the output, the better.

However, when considering the cost of the laser generator, the output power should only be set close to the maximum output power of the cutting machine. The figure below illustrates the problems that arise when cutting low-carbon steel plates with insufficient laser power, such as not cutting through (a), producing a lot of slag on the lower part (b), and producing a rough section (c).

Effect of laser power on cutting quality of low carbon steel
Effect of laser power on cutting quality of low carbon steel

Fig.3 Effect of laser power on cutting quality of low carbon steel

The effect of cutting speed on cutting quality

The cutting speed plays a significant role in determining the quality of the cut on a stainless steel plate. The optimal cutting speed produces a smooth cutting surface and eliminates slag on the bottom.

If the cutting speed is too fast, it may result in an inability to fully cut through the steel plate, leading to sparks and slag on the bottom half, and even damaging the lens. This occurs because the fast cutting speed reduces the energy per unit area and the metal is not fully melted.

Conversely, if the cutting speed is too slow, it may cause excessive melting, a wider cutting seam, an enlarged heat-affected zone, and even burning of the workpiece. This is because the slow cutting speed allows energy to accumulate at the cutting edge, causing the slit to widen, the melted metal to be unable to discharge quickly, and slag to form on the bottom of the steel plate.

These defects are illustrated in Figure 4.

The effect of cutting speed on cutting quality

Fig.4 The effect of cutting speed on cutting quality

The cutting speed and laser output power have a direct impact on the input heat of the workpiece. This means that the relationship between changes in input heat and processing quality due to changes in cutting speed is the same as that between changes in output power and processing quality.

Typically, when the processing conditions are adjusted, only one side (either the output power or the cutting speed) will be changed to alter the processing quality, rather than changing both at the same time.

The type (shape) of the nozzle and the height of the nozzle (the distance between the nozzle outlet and the surface of the workpiece) can also impact the quality of the cutting.

Nozzle function

Control the gas diffusion area to control the cutting quality.

Gas ejection from the nozzle

Fig.5 Gas ejection from the nozzle

The relationship between nozzle and cutting quality

The coaxiality between the center of the nozzle outlet hole and the laser beam is a crucial factor affecting the cutting quality. The effect becomes greater as the thickness of the workpiece increases. If the nozzle becomes deformed or melted, it will directly impact the coaxiality. The nozzle shape and dimensional precision are high requirements, so it’s important to take care of the nozzle and avoid collisions that may cause deformation. If the cutting conditions change due to a damaged nozzle, it is advisable to replace it with a new one.

If the nozzle and laser are not coaxial, the cutting quality can be impacted as follows:

a) Effect on the cutting section

As illustrated in the figure, if the auxiliary gas is blown out of the nozzle unevenly, there can be melting on one side and no melting on the other. This has limited impact on cutting thin plates less than 3mm, but when cutting plates thicker than 3mm, the effect can be significant and may result in the plate not being cut through.

The influence of coaxial degree on the cutting section

Fig. 6 The influence of coaxial degree on the cutting section

b) Impact on the sharp angle

If the workpiece has a sharp angle or small angle, it is more susceptible to over-melting and thick plates may not be able to be cut.

c) Impact on perforation

Perforation can be unstable and difficult to control, especially for thick plates, which can cause over-melting and the penetration condition may be difficult to control. This has little effect on thin plates.

The influence of focus position on cutting quality

The focal position refers to the distance between the focal point and the surface of the workpiece, with values being considered positive if the focal point is above the surface and negative if it is below it.

Focal position

Fig.7 Focal position

The focal position plays a critical role in determining the width of the incision, slope, roughness of the cutting surface, and amount of slag attachment. The focal position affects the beam diameter and focal depth of the processed object, resulting in changes to the shape of the groove and the flow of the processing gas and molten metal. To produce a narrow slit, it’s important to minimize the focal spot diameter (d), which is proportional to 4/πd^2 and the focal length of the lens. A smaller focal depth results in a smaller d.

However, cutting can cause spatter, and the lens can easily be damaged if it is too close to the workpiece. As such, the widely used focal length in the industrial application of high-power laser cutting is between 5 inches (127 mm) to 7.5 inches (190 mm), with the actual focal spot diameter ranging between 0.1 to 0.4mm. It is crucial to control the focal position to achieve optimal results.

Considering the factors such as cutting quality and cutting speed, in principle:

  • For the metal with thickness < 6 mm, the focal position is on the surface;
  • For the metal with thickness > 6 mm, the focal position is above the surface;
  • When cutting the stainless steel plate, the focus position is generally below the surface.
  • When the cutting thickness < 4 mm, choose 5″focus lens.

The length of the optical path is different when cutting the proximal and distal ends with a flight path cutting machine, leading to a difference in the size of the beam before focusing.

The larger the diameter of the incident beam, the smaller the focal spot.

To minimize the change in focal spot size due to changes in the size of the beam before focusing, an optical path compensation system can be installed to maintain consistent optical paths at the proximal and distal ends.

The laser beam is shown passing through the focusing lens in Figure 8.

The focal point of a beam passing through the lens

Fig.8 The focal point of a beam passing through the lens

The spot diameter is calculated by the following formula:

spot diameter calculation formula

(2)

Among them:

  • D——beam diameter before focusing;
  • K——beam quality factor

In addition, focus depth is another factor that influences the quality of cutting. Its calculation formula is as follows:

focus depth calculation formula

(3)

It can be seen from the above analysis that the closer the focal position is to the middle of the steel plate, the smoother the cutting surface will be in the absence of slag. The choice of focus position plays a crucial role in determining the quality of the cutting for the stainless steel plate.

When the focal position is appropriate, the material being cut is melted and the material along the edge is not melted, resulting in a clean, non-stick cut seam, as illustrated in Figure (a).

When the focal position lags, the amount of energy absorbed by the cutting material per unit area decreases, causing the cutting energy to weaken and the material to not completely melt and be blown away by the auxiliary gas. This results in the partially melted material being attached to the surface of the cutting plate and forming a sharp, short slag tail, as shown in Figure (b).

When the focal position is advanced, the average energy absorbed by the cutting material per unit area increases, causing both the material being cut and the material along the edge to melt and flow in liquid form. In this case, due to the constant pressure and cutting speed, the molten material forms a spherical shape and adheres to the surface of the material, as illustrated in Figure (c).

Therefore, the focus position can be adjusted by observing the shape of the slag during the cutting process to ensure the cutting quality.

The influence of focus position on slag

Fig.9 The influence of focus position on slag

The influence of different focus positions on cutting quality

Fig.10 The influence of different focus positions on cutting quality

In actual production, when cutting stainless steel plates with a laser cutter, the focus position is selected on or within the surface of the material. This is done to increase the fluidity of the cutting gas and molten material and improve the cutting quality by enlarging the smooth surface area. The focus position will vary depending on the thickness of the steel plate and must be determined through experimentation.

The choice of auxiliary gas (type and pressure) also plays an important role in determining the cutting quality. The gas type, air pressure, nozzle diameter, and geometric structure can affect the edge roughness and the formation of burrs. The gas consumption is determined by the nozzle diameter and air pressure, with low pressure being below 0.5 MPa and high pressure being above 2 MPa. Coaxial ejection of the auxiliary gas and the laser beam helps protect the focusing lens from contamination and removes any slag from the cutting area. Commonly used gases for laser cutting include oxygen, nitrogen, and air, with different cutting materials requiring different auxiliary gases.

The use of oxygen as an auxiliary gas is primarily for cutting carbon steel, stainless steel, and highly reflective materials through tapping and high-speed cutting, as well as for oxidation cutting. The laser cutting machine uses the heat generated by the oxidation reaction for efficient cutting, however, it also results in the formation of an oxide film on the cutting surface.

Nitrogen is mainly utilized in the cutting of stainless steel plates without oxidation and galvanized sheet metal without slag.

Air is primarily used for cutting aluminum and galvanized steel without slag and for cutting ordinary non-metals.

The auxiliary gas pressure is dependent on the type of gas used, the cutting material, the thickness of the plate, and the form of laser output (continuous wave/pulsed). The pressure of the auxiliary gas affects the attachment of slag, the quality of the cut surface, and the size of the heat-affected area.

The air pressure condition of the nozzle outlet during processing is shown in the following table:

Table 2 The relationship between the cutting process and the auxiliary gas pressure

TappingSheet metal O2 cuttingThick carbon plate O2 cuttingStainless steel N2 cuttingAluminum air cuttingAcrylic resin net surface cutting
(MPa)(MPa)(MPa)(MPa)(MPa)(MPa)
0.02-0.050.1-0.30.05-0.10.6-1.50.6-1.0<0.01

Under the premise of determining the auxiliary gas type, the gas pressure size is an extremely important factor.

If the auxiliary gas pressure is too high, a vortex will form on the surface of the workpiece, which will weaken the ability of the airflow to remove the molten material, causing the cutting surface to become rougher and the slit to widen.

If the auxiliary gas pressure is too low, the melted material of the incision will not be blown away, leading to the formation of slag on the back of the cut material.

Therefore, there is an optimal value for the auxiliary gas pressure. High gas pressure is required when cutting thin materials at high speed to prevent slag from forming on the backside of the incision. Conversely, when the material thickness increases or the cutting speed slows down, the gas pressure should be appropriately reduced.

For example, when laser cutting stainless steel plates, the use of auxiliary gas helps cool the surrounding areas of the cutting seam, reducing the heat-affected zone and preventing lens damage from the heat.

Additionally, using nitrogen as the auxiliary gas enhances the fluidity of the molten metal.

See also:

In actual machining, machining defects can be caused by improper process parameters.

With decades of experience in the laser cutting process, it is important to summarize countermeasures for cutting defects to guide actual production. See the appendix for more information.

See also:

Appendix 1 – Different materials’ laser cutting defects and troubleshooting

Carbon steel: cut with O2

DefectsPossible ReasonsSolution
The traction line at the bottom has a large offset. The burr on the bottom is similar to the slag
 The bottom incision is wider
Too fast feed speed Low laser powerLow laser powerHigh focus positionReduce feed speed Increased laser powerIncrease the pressureLower the focal position
The burr on the bottom is similar to the slag, which is in drip shape and easy to remove.
 The burr on the bottom is similar to the slag
Too fast feed speedReduce feed speed.
Low air pressureIncrease the pressure
High focus position.Lower the focal position
The metal burr can be removed as a block.
 The metal burr can be removed as a block
Too high focal positionLower the focal position
The metal burrs on the bottom are difficult to remove. 
The metal burrs on the bottom are difficult to remove
Too fast feed speedReduce feed speed.
Low air pressureIncrease the pressure
Gas is not pureUse purer gas
Too high focal positionLower the focal position
There is only a burr on one side. 
There is only a burr on one side
The nozzle is not centered;Center the nozzle;
The nozzle has defects.Replace the nozzle.
The material is expelled from above. 
The material is expelled from above
The power is too low;Stop cutting immediately to prevent slashes from splashing into the focus lens. Then increase the power and reduce the feed rate.
Too fast feed speed. 
Two sides are good and two sides are bad for slope cutting. 
Two sides are good and two sides are bad for slope cutting
The polarized reflector is not suitable and the installation is incorrect. Or the defective polarized reflector is installed in the position of the deflection lens.Check the polarized reflector
 Check deflection lens
Blue plasma, not cut through the workpiece.
 Blue plasma, not cut through the workpiece
 Stop cutting immediately to prevent slag splashing into the focus lens.
Processing gas error(N2)Use O2 as the processing gas.
Too fast feed speedReduce feed rate
The power is too low;Increase the power
The cutting surface is not precise. 
The cutting surface is not precise
Air pressure is too highReduce the pressure
The nozzle is damagedReplace the nozzle
The nozzle diameter is too largeInstall the appropriate nozzle
The material is not goodUse a smooth, homogeneous material.
Without burr, the traction line is inclined. The
incision becomes narrower at the bottom. 
The incision becomes narrower at the bottom
The feed rate is too high.Reduce feed rate.
Produce crater 
Produce crater
Air pressure is too highReduce the pressure
The feed rate is too low.Increase feed rate.
The focus is too highReduce the focus
The surface of the plate is rusted.Use better quality materials.
The workpiece is overheating. 
The material is not pure 
Very rough-cutting surfaces. 
Very rough cutting surfaces
The focus is too highReduce the focus
Air pressure is too highReduce the pressure
The feed rate is too low.Increase feed rate.
The material is too hotCooling material

Stainless steel: cut with high pressure N2

DefectsPossible ReasonsSolutions
Produce a drip-like small regular burr.
 Produce a drip-like small regular burr
The focus is too lowRaise the focus
The feed rate is too high.Reduce feed rate.
Both sides produce long irregular filamentous burrs, the surface discoloration of large plates. 
Both sides produce long irregular filamentous burrs
The feed rate is too low.Increase feed rate.
The focus is too highReduce the focus
Air pressure is too lowIncrease the pressure
The material is too hotCooling material
Long irregular burr on the cutting edge. 
Long irregular burr on the cutting edge
Not center the nozzleCenter the nozzle
The focus is too highReduce the focus
Air pressure is too lowIncrease the pressure
The speed is too lowIncrease speed
The cutting edges are yellow.Nitrogen contains oxygen impurities.Use good nitrogen.
Plasma is produced on a straight cross-section. 
Plasma is produced on a straight cross section
The feed rate is too high.Stop cutting immediately to prevent slashes from splashing into the focus lens.
The power is too lowReduce feed rate.
The focus is too lowIncrease the power
 Raise the focus
The beam spreadThe feed rate is too high.Reduce feed rate.
The power is too lowIncrease the power
The focus is too lowRaise the focus
Plasma is generated around the corner.The angle tolerance is too high.Reduce the angle tolerance.
Modulation is too highReduce modulation or acceleration.
The acceleration is too high 
The beam diverges at the beginning.The acceleration is too highReduced acceleration
The focus is too lowRaise the focus
The melted material failed to discharge.Pierce a round hole
The incision is roughThe nozzle is damaged.Replace the nozzle
The lens is dirtyClean the lens and replaces it if necessary.
The material is expelled from the above.
 The material is expelled from above
The power is too lowStop cutting immediately to prevent slashes from splashing into the focus lens.
The feed rate is too high.Increase the power
Air pressure is too highReduce feed rate.
 Reduce the pressure

Alloy: Cut with high pressure N2

DefectsPossible ReasonSolution
Both sides produce long irregular filamentous burrs that are difficult to remove.
 Both sides produce long irregular filamentous burrs that are difficult to remove
The focus is too highReduce the focus
Air pressure is too lowIncrease the pressure
The feed rate is too low.Increase the feed rate.
Both sides produce long irregular burrs that can be removed by hand.
 Both sides produce long irregular burrs that can be removed by hand
The feed rate is too low.Increase the feed rate.
The incision is roughThe nozzle diameter is too large.Install the appropriate nozzle.
The nozzle is damaged.Replace the nozzle
Air pressure is too highReduce the pressure
The small regular burrs are difficult to remove. 
The small regular burrs are difficult to remove
The focus is too lowRaise the focus
The feed rate is too high.Reduce feed rate.
Plasma is produced on a straight cross-section.The feed rate is too high.Reduce feed rate.
The focus is too lowRaise the focus
The beam spreadThe feed rate is too high.Reduce feed rate.
Plasma is generated around the corner.The angle tolerance is too high.Reduce angle tolerance.
Modulation is too highReduce modulation or acceleration.
The acceleration is too high 
The beam diverges at the beginning.Approach speed is too highReduced approach speed
The focus is too lowRaise the focus
The incision is roughThe nozzle is damaged.Replace the nozzle
The material is expelled from the above.
 The material is expelled from above
The power is too lowStop cutting immediately to prevent slashes from splashing into the focus lens.
The feed rate is too high.Increase the power
 Reduce feed rate.

Appendix 2 Physical photograph with cutting defect

1.Stainless steel cutting defects

DefectsPossible ReasonSolution
Stainless steel cutting defectsToo fast speedReduce the speed
The focus is too lowIncrease the power
The power is too low 
Stainless steel cutting defectsCenter is not rightInspection center
The hole in the nozzle is not smooth and round.Check nozzle status
The light path is not straightCheck the light path
Stainless steel cutting defectsThe focus is too lowRaise the focus by 0.1-0.2 mm each time.
Stainless steel cutting defectsLow nitrogen pressureIncrease nitrogen pressure
Stainless steel cutting defectsThe focus is too highLower the focus, each time lower by 0.1-0.2mm.
Stainless steel cutting defectsCutting speed too fastCutting speed reduces by 50-200 mm/min each time.
Stainless steel cutting defectsThe focus is too lowThe focus is increased by 0.1-0.2mm each time.
Stainless steel cutting defectsNitrogen is not pureCheck the purity of nitrogen.
There is oxygen or air in the air pipe.Increase the delay to clean the air pipe.
 Check gas path (no leakage)

2. Carbon steel cutting defects

DefectsPossible ReasonSolution
Carbon steel cutting defectsThe center of the lens is not right.Check lens center
The nozzle hole is blocked or not round.Check nozzle state
The light path is not straightCheck the light path and hit the target again.
Carbon steel cutting defectsThe introduction length of line or introduction is incorrect.Correct the introduction method and introduction length.
Linear wrongCheck the line type
The perforation time is too long.The perforation time is less than 2 seconds.
There is too much heat in cutting.Reduce the duty cycle by 2-3% each time.
Carbon steel cutting defectsThe pressure is too highReduce the pressure, 0.1 bar at a time.
The focus is too highReduce the power
Power is too highCheck the focus of the lens.
The material is not good 
Carbon steel cutting defectsLow powerIncrease the power
High speedReduce the speed
The low pressureIncrease the pressure
Carbon steel cutting defectsSpeed is too highReduce speed
Low powerIncrease the duty cycle by 5-10% each time.
The pressure is too lowAdd power, 100W each time.
 Gradually increase the pressure, 0.1-0.2bar each time.
Carbon steel cutting defectsToo much local heatChange the cut order
Material issueChange the material
Carbon steel cutting defectsThe pressure is too highReduce pressure by 0.1-0.2bar each time.
Speed is too highReduce the speed
  
Carbon steel cutting defectsThe focus is too lowIncrease the focus, 0.1-0.2 mm per step.
The pressure is too lowIncrease the pressure, 0.1-0.2 bar per step.

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6 thoughts on “Laser Cutting Defects: Quality Control Tips”

  1. Shane,

    We are struggling with cutting a 0.31 hole using 1/4″ mild steel with our 5K watt CO2 Laser. The top of the hole is clean while the bottom side of the hole is irregular (not a perfect circle).

    Thanks, Nate

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