Material Composition and Laser Cutting Quality: What Is Their Relationship?

At present, laser cutting technology has gradually replaced traditional metal processing methods with its low processing cost, rapid production efficiency, accurate cutting accuracy and perfect processing quality, including in the processing field of carbon steel, stainless steel, copper, aluminum and other non-ferrous metals, laser cutting occupies an irreplaceable position;

With the popularization of laser cutting technology, problems in its processing have gradually emerged, among which the more serious problem is that when cutting carbon steel plates of the same thickness and different components with the same laser cutting machine of the same power and the same cutting process parameters, the surface quality of the cut samples has obvious differences.

Therefore, in order to explore the influence of plate composition on cutting quality, under the background of using fiber lasers with different power ranges (6-30KW), carbon steel plates with different thicknesses and different compositions were tested and compared with oxygen and air respectively.

Related reading: The Application of Air as Auxiliary Gas in Laser Cutting

1. Experimental equipment and methods

1.1 Experimental equipment

The equipment used in this experiment is the 30KW fiber laser, which is a multimode continuous output laser with a beam wavelength of 1080nm, and its core diameter is 150 μm;

The laser head used in this experiment is the Genius 30 (30KW) laser head.

Its quasi-diameter focal length is 100mm, and the focal length of the focusing mirror is 200mm, which can realize automatic focusing.

To ensure the stable operation of the 30KW fiber laser and ensure good operation status, the auxiliary equipment used is a water cooler with a cooling capacity of 70.0KW.

Material Composition and Laser Cutting Quality: What Is Their Relationship? 1
Material Composition and Laser Cutting Quality: What Is Their Relationship? 2

Fig. 1 – Experimental laser, laser head

1.2 Auxiliary materials

In order to ensure the accuracy, efficiency and obvious contrast of the experimental data, the test materials used in this experiment are Q235 carbon steel, Q345 carbon steel, Q460 carbon steel and other carbon steel plates with different thicknesses.

See the data sheet of experimental plates for details;

The auxiliary gas used is 99.9% oxygen with the air supply pressure of 5bar.

To ensure the sufficient number of nozzles in this experiment, prepare the nozzles shown in the experimental nozzle data sheet.

Table 1 Data Sheet of Experimental Plate

Material type

Q235

Q345

Q460

Q690

NM400

45#

T10

Size/mm (L/W: 500/500)

12

12

12

12

12

12

20

16

20

20

20

16

16

30

20

30

30

30

20

20

40

30

/

/

/

30

30

/

Table 2 Data Sheet of Experimental Nozzle

Nozzle type

Double jet

Single jet

Nozzle model

B-1

B-2

B-3

D-4

D-7

D-9

Quantity

5

5

5

5

5

5

1.3 Experimental methods

Under the condition that the spot quality of the equipment (the optical fiber and the laser head are clean and undamaged through the photo paper), the air pressure of the air source (the oxygen is stable at 5 bar, and the air is stable at 11 bar), and the internal lenses of the laser head (the lenses are clean, free of dirt and burning points) are normal, the internal modules are controlled through the laser internal control software, and the maximum output power is respectively adjusted to 12KW, 20KW and 30KW.

Under the above three power states, cutting experiments were carried out on different types and thicknesses of plates described in Table 1.

The perimeter of the cut sample is 205.6mm as shown in Fig. 2.

The cut samples were analyzed and compared by comparing the porosity density, roughness and corresponding process parameters on the surface of the cut samples.

Material Composition and Laser Cutting Quality: What Is Their Relationship? 3

Fig. 2 – Schematic Diagram of Cutting Sample

2. Test results

2.1 Analysis of cutting parameters

During the experiment, according to the five influencing factors of cutting speed, laser power, cutting gas pressure, focus and nozzle aperture, in order to ensure that the samples after cutting can automatically fall off, have no slag, have no burning, have no droplet, and have a high surface finish, adjust the process parameters to achieve the best cutting effect debugging of different materials and different thicknesses.

Related reading: Laser Cutting Thickness & Speed Chart

See Table 3 for specific parameters.

Table 3 Table of Experimental Parameters

Plate

Thickness/mm

Power

Speed m/min

Air pressure/bar

Power/KW

Focus/mm

Q345B

12

12KW

1.7

0.8

9600

9.8

20

1.4

0.6

12000

11.6

30

0.9

0.7

12000

12.5

45#

12

1.9

1.2

10000

9.8

20

1.6

1.5

12000

9.5

30

1

0.6

12000

12.3

NM400

12

1.6

1

9000

9.6

20

1.5

0.45

12000

12

30

1

1

12000

12

Q345B

12

20KW

1.7

0.8

9600

9.8

20

1.6

1.3

16000

12

30

1.2

1

17000

12.5

45#

12

1.9

1.2

10000

9.8

20

1.6

1.3

14000

12

30

1.2

1.5

16000

11.5

T10

20

1

1.4

15000

11

30

0.8

1.6

18000

11.5

40

0.7

1.7

18000

11

NM400

12

1.6

1

9000

9.6

16

1.8

0.55

14000

12

20

1.5

0.6

14000

12.5

30

1.1

0.85

17000

12.5

Q345B

35

30KW

1.3

1.8

30000

12

40

0.85

1.2

24000

12.5

45#

30

1

3

26000

13

40

0.8

1.7

30000

12.5

T10

35

0.8

1.6

18000

11.5

40

0.7

1.7

18000

11

NM400

40

1.3

1.6

23000

13

30

0.8

1.7

30000

12.5

2.1.1 Cutting speed analysis

It can be seen from the data in Table 3 that, with the laser power of 20KW as the test background, the cutting speeds of the plates with the thickness of 20mm and 30mm of Q345, 45 # steel, NM400 and T10 are compared respectively, as shown in Fig. 3.

With the same laser power, plate thickness and oxygen as the auxiliary gas background, the cutting speed of the T10 plate is the slowest, while that of NM400 is the slowest, while that of Q345 and 45 # steel is no significant difference.

It can be inferred that:

The carbon content in the cutting material has the most obvious influence on the cutting speed.

With the increase of the carbon content in the plate, the cutting speed of the equal thickness plate decreases gradually;

And with the increase of rare elements (Cr, Ni, etc.) content in the plate, the cutting speed will gradually decrease.

Material Composition and Laser Cutting Quality: What Is Their Relationship? 4

Fig. 3-20KW Cutting Speed Comparison

2.1.2 Auxiliary air pressure analysis

It is known that the principle of laser cutting carbon steel with oxygen as the auxiliary gas is to cut the plates with the energy generated by the laser light source and the oxidation reaction in the cutting process.

It can be seen that the oxygen pressure has an obvious effect on different types of plates;

According to Table 4 of the drawing of the process data of cutting 20mm and 30mm Q345, 45 # steel and T10 with 20KW laser provided in Table 3:

For different kinds of plates with the same thickness, the auxiliary gas pressure is gradually increasing with the increase of the carbon content in the plates for the purpose of the best cutting effect of the samples.

Material Composition and Laser Cutting Quality: What Is Their Relationship? 5

Fig. 4-20KW Auxiliary Air Pressure Comparison Diagram

2.1.3 Cutting focus analysis

It can be seen from the data provided in the previous test that when cutting Q235, Q345, 45 # steel and T10 with the same thickness under the same power, oxygen is used as the cutting auxiliary gas.

It is known that 45 # steel and T10 steel have more carbon content inside than Q235 and Q345.

During the cutting process, a large number of carbon dioxide pores will be generated on the surface, resulting in a rough surface.

When the cutting focus changes within ± 1, the cutting effect has no obvious change, and the focus can be reduced appropriately to improve the cutting speed;

The cutting effect of Q235 and Q345 is obviously affected by the cutting focus, so they have no such advantage.

2.2 Analysis of commissioning effect

The samples of different types and thicknesses of plates cut by different types of auxiliary gases under different powers are shown in the following table.

It can be clearly seen from Table 4 that different types of plates with the same thickness are cut under the same cutting power to compare their best cutting effects;

The surface roughness of samples cut from different materials has a significant difference, among which the Q345B sample has the best cutting effect, the surface oxide skin is thin, and the surface roughness is the smallest;

The oxide skin on the cutting surface of NM400 sample has obvious stratification, of which the oxide skin on the upper side of the cutting surface is smooth, and the thickness of the oxide skin on the lower side is obviously higher than that on the upper side, resulting in a large surface roughness of the sample;

The 45 # sample was roughly cut, and the oxide skin on the lower side showed obvious protrusion;

T10 sample has the worst cutting effect, with rough surface, full of pores, and obvious oxide skin on the lower side;

Compared with the oxide skin surface roughness of Q345B, NM400 and 45 # cutting surfaces, T10 plate has the worst cutting effect.

Table 4 Experimental Effect Table

Material Composition and Laser Cutting Quality: What Is Their Relationship? 6

2.2.1 Material melting point analysis

It is known that the materials selected for this test are Q235, Q345B, NM400 and 45 # carbon steel plates, and their carbon content is: 0.22%, 0.20%, 0.25%, 0.47%.

By comparing the carbon content of the four materials with the iron carbon phase diagram in Figure 5, it can be seen that the melting point temperature of the four materials is about 1500 ℃;

It is known that laser cutting carbon steel uses the laser as preheating heat source, and oxygen as auxiliary gas to generate a high exothermic oxidation reaction with materials, releasing a large amount of oxidation energy (see the following formula).

Fe+O→FeO+heat(257.58kJ/mol)2Fe+1.5O2→Fe2O3+heat(826.72kJ/mol)

It is known that the temperature of the plate area at the processing location has exceeded 1726.85 ℃ due to the energy released by the laser itself and the oxidation process during the laser processing, far exceeding the melting points of Q235, Q345B, NM400 and 45 #.

From the analysis, it can be seen that the melting points of the above materials have little influence on the effect of the oxide scale on the surface after cutting.

Material Composition and Laser Cutting Quality: What Is Their Relationship? 7

Fig. 5-Fe-C Phase Diagram

2.2.2 Chemical composition analysis of materials

The content of chemical elements in different kinds of steel plates was obtained by using spectrum analyzer for the materials in this experiment, as shown in Table 5 below.

Table 5 Chemical Element Analysis

Chemical element/%/plate type

Q345

Q235

Q460

NM400

Q690

45#

T10

C

0.2

0.22

0.2

25

0.18

47

1

Mn

1.7

0.65

1.8

1.6

2

0.65

0.4

Si

0.5

30

0.6

0.7

0.6

27

0.35

S

0.035

0.05

0.03

0.01

0.02

/

0.02

P

0.035

0.045

0.03

0.025

0.025

/

0.03

Cr

30

0.3

0.3

14

1

0.25

0.25

Ni

0.5

0.3

0.8

1

0.8

0.3

0.2

Cu

/

0.3

/

/

 

25

0.3

Mo

0.1

/

/

0.5

0.3

/

/

Nb

0.07

/

0.11

/

0.11

/

/

V

0.15

/

0.2

/

0.12

/

/

Ti

200

/

0.2

/

/

/

/

AI

0.015

/

/

/

/

/

/

B

/

/

/

0.004

0.004

/

/

1) Mn element content analysis

According to the comparison of Q235 and Q345B elements in Table 5, the two materials belong to low carbon steel, and the content of other elements in the material has no significant difference except for the content of manganese (0.65% for the former and 1.70% for the latter), which serves as the exploration benchmark for the relationship between the laser cutting quality and the content of manganese in the material;

In the actual cutting test process, the cutting surface effect of the two materials is shown in Fig. 6.

The surface is clean and bright, the surface roughness is similar, and the experimental parameters are the same.

Therefore, it can be determined that in conventional low-carbon steel, Mn element has a slight impact on its laser cutting effect.

Material Composition and Laser Cutting Quality: What Is Their Relationship? 8

Q235-20kw-20mm

Material Composition and Laser Cutting Quality: What Is Their Relationship? 9

Q345B-20kw-20mm

Fig. 6

2) S element content analysis

It can be seen from the sheet element content provided in the following table that the maximum difference of S element content of different sheets is 0.05%.

This data cannot analyze the influence of S element content on cutting quality;

Later, it can be seen from the query data that when the content of Mn and S in the plate is about 0.5% and 0.25% respectively, the slag at the bottom of the cutting surface increases with the increase of the plate thickness, and the cutting quality gradually decreases.

Table 6 Comparison of S and Mn Elements

Sheet/Element%Q345Q235Q460NM400Q69045#T10
Mn1.70.651.81.62.00.650.4
S0.0350.050.030.010.02 0.02

3) Si element content analysis

It is known that when the content of Si element in the metal plate is lower than 0.25% and higher than 0.25% respectively, the cutting speed of carbon steel plate with the content of Si element greater than 0.25% is lower than that of carbon steel plate with the content of Si element less than 0.25% by more than 20%, and a large amount of slag will be generated at the bottom of the plate.

4) Content analysis of element C

When comparing the element content of Q235, 45 # and T10, it is found that the former Q235 belongs to low carbon steel, 45 # belongs to medium carbon steel and T10 belongs to high carbon steel.

According to the above element table, it can be seen that there are significant differences only between C and Mn;

It is known that under the condition of high temperature and sufficient oxygen as auxiliary gas, carbon reacts with oxygen as follows:

C+O2→CO2(g)(393.5KJ/mol)

It can be seen from theoretical analysis that with the increase of carbon content in the material, the amount of carbon dioxide gas generated by oxidation reaction will gradually increase under the background of oxygen as auxiliary gas, and the number of pores on the material cutting surface will also increase accordingly;

As shown in Fig. 4, the internal carbon content of Q235, 45 # steel and T10 is gradually increasing, and the number of pores on the cutting surface is also gradually increasing;

Material Composition and Laser Cutting Quality: What Is Their Relationship? 10

Fig. 7 – Comparison Chart of Carbon Content of Materials

When comparing Q235 and Q345B materials in the early stage, it is known that the content of Mn element has slight influence on the actual cutting effect, which can be ignored;

The actual cutting effect of the three materials with the same thickness is shown in Fig. 8.

Q235 surface is bright, roughness is small, 45 # surface is rough, the thickness of oxide skin at the bottom is significantly thicker, and T10 surface is the roughest and the oxide skin on the surface is the thickest.

It can be judged from the actual test effect:

The carbon content in the material has an obvious influence on the cutting effect, and with the increase of carbon content, the number of pores on the cutting surface gradually increases, the thickness of the surface oxide skin gradually becomes thicker, and the surface roughness becomes larger;

Material Composition and Laser Cutting Quality: What Is Their Relationship? 11

Fig. 8-Q235-30kw-40mm (left), 45 # – 30kw-40mm (middle), T10-30kw (right)

5) Ni element content analysis

The types and contents of chemical elements inside Q235 and Q460 materials are shown in Table 7.

The difference of Ni element content between the two materials is obvious.

Therefore, cutting tests are conducted for the same thickness of the above two kinds of plates. The actual cutting quality is shown in Fig. 10.

There is no obvious difference in the surface stripes, oxide skin thickness, and surface roughness.

Therefore, the inference is as follows: In conventional low-carbon steel, Ni content has no obvious effect on the cutting quality of high power laser;

Material Composition and Laser Cutting Quality: What Is Their Relationship? 12

Fig. 9 – Comparison Chart of Nickel Content in Materials

Table 7 Comparison of Ni Elements

Chemical element/%

Plate type

Q235

Q460

C

0.22

0.2

Mn

0.65

1.8

Si

0.3

0.6

S

0.05

0.03

P

0.045

0.03

Cr

0.3

0.3

Ni

0.3

0.8

Cu

0.3

/

Mo

/

/

Nb

/

0.11

V

/

0.2

Ti

/

0.2

AI

/

/

B

/

/

Material Composition and Laser Cutting Quality: What Is Their Relationship? 13

Q460-20mm-20KW

Material Composition and Laser Cutting Quality: What Is Their Relationship? 14

Q235-20mm-20KW

Fig. 10

6) Cr element content analysis

When comparing the content of other elements in the plate, it is found that the content of Cr element in material NM400 and Q690 is significantly higher than that in other materials, as shown in Fig. 4.2-5.

Material Composition and Laser Cutting Quality: What Is Their Relationship? 15

Fig. 11 Comparison Chart of Chromium Content in Materials

It is known that in the process of laser cutting the plate, most elements in the plate will oxidize with the auxiliary gas oxygen and release a lot of heat when the laser releases a lot of heat.

At this time, an obvious heat affected zone will be generated on the surface of the plate.

In this heat affected zone, Cr in the plate will oxidize with oxygen and generate dense Cr2O3 and other oxides, which will gradually increase with local temperature.

The oxide gradually grows up and presents cluster granular structure, as shown in Fig. 12;

With the extension of time, Cr2O3 oxide skin with high surface stress and not easy to crack is gradually generated on the metal cutting surface, which prevents the oxidation reaction between elements on the subcutaneous side of Cr2O3 oxidation and O2 (as shown in Fig. 13), resulting in significantly poor surface roughness on the underside of NM400 and Q690 cutting surfaces (see Fig. 14).

It can be determined that the cutting effect becomes worse with the increase of Cr content in the material, and the oxide skin at the bottom of the sample becomes thicker.

Material Composition and Laser Cutting Quality: What Is Their Relationship? 16
Material Composition and Laser Cutting Quality: What Is Their Relationship? 17

Fig. 12 – Phase diagram of clustered particles

Material Composition and Laser Cutting Quality: What Is Their Relationship? 18

Fig. 13 – Analysis Diagram of Laser Cutting Surface Oxide Layer

Material Composition and Laser Cutting Quality: What Is Their Relationship? 19

Cutting effect display 20mm NM400

Material Composition and Laser Cutting Quality: What Is Their Relationship? 20

Cutting effect display 20mm Q690

Fig. 14

2.3 Analysis of heat affected zone

It is known that the laser cutting quality is related to the heat affected zone of the cutting slit on the surface of the plate to be cut.

When the heat affected zone is not in place, it may cause distortion, cracks, embrittlement, etc. on the surface of the cutting plate.

It is known that the data comparison fig. 15 provides that the laser cutting power is the main factor affecting the cutting slit width, and the cutting speed is the main factor affecting the stripes and roughness of the cutting surface.

Therefore, in laser cutting, appropriate process parameters should be adjusted as far as possible to reduce the area of heat affected zone on the plate surface; (Deformation and enrichment of components)

Material Composition and Laser Cutting Quality: What Is Their Relationship? 21
Material Composition and Laser Cutting Quality: What Is Their Relationship? 22

Fig. 15 – Influence of power and speed on kerf and cutting surface

In the actual test process, in order to ensure the smooth surface of cutting samples of different types and thicknesses, and free fall after cutting, the cutting process parameters debugged have been the current best effect parameters.

Under the same cutting power, there is no significant difference in the width of slits compared with different types and the same thickness;

Therefore, the heat affected zone area of different materials with the same thickness is similar under the same power, which has a slight impact on the actual surface roughness and can be ignored.

3. Conclusion

The factors affecting the cutting quality of carbon steel by oxygen cutting include alloy composition, material microstructure, thermal conductivity, melting point and boiling point.

Metals with high carbon content mostly belong to metals with high melting point.

Because it is difficult to melt, the cutting and piercing time is increased.

On the one hand, it broadens the kerf and expands the surface heat affected zone, resulting in unstable cutting quality;

Related reading: Things You Should Know About Laser Cut Kerf

On the other hand, the high content of alloy composition increases the viscosity of liquid metal and increases the ratio of splash and slagging, which puts forward higher requirements for the adjustment of laser power and air blowing pressure during processing.

Related reading: How to Select the Power of Fiber Laser Cutting Machine?

It can be seen from the above tests that when the auxiliary gas is oxygen, with the increase of the content of C and Cr elements in the material, the cutting surface effect becomes worse and the surface roughness increases significantly;

When the auxiliary gas is air, the cutting effect will not be significantly affected under the same thickness and power.

Therefore, on the premise of ensuring the quality and efficiency of cutting samples, the types of auxiliary gas recommended for cutting at different cutting powers and materials are shown in the following table:

1. Influence of carbon content:

Under the same laser power, with the increase of carbon content, the cutting speed gradually decreases, while the sample surface gradually becomes rough, the oxide skin gradually becomes thicker, and the effect gradually becomes worse, which will also reduce the thickness limit of laser cutting plates;

2. The influence of chromium content:

With the increase of chromium content, the oxide skin at the bottom of the sample surface accumulates and thickens obviously, and the cutting surface gradually becomes rough from top to bottom;

3. Influence of silicon content:

When the silicon content in the material is higher than 0.25%, the cutting speed decreases significantly with the increase of silicon content, and slag appears at the bottom of the cutting sample;

4. The nickel content has little effect on the quality of high power laser cutting;

5. Influence of manganese content and sulfur content:

When the content of manganese and sulfur in the material is 0.5% and 0.04% respectively, the slag at the bottom of cutting increases gradually with the increase of plate thickness;

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