In previous article, we talk about the principle and characteristics of laser cutting, also compare the different auxiliary gas. Today we will continue to discuss the factors affecting cutting quality, thickness and efficiency of laser cutting. If you want to know what role the compressed air play, let’s check following.
1. Factors affecting the quality of laser cutting
1.1 Factors for evaluating laser cutting quality
There are many standards to judge the edge quality of laser cutting.
Standards such as burr form, depression and grain can be judged by naked eyes;
The perpendicularity, roughness and incision width need to be measured by special instruments.
Material deposition, corrosion, heat affected area and deformation are also important factors to measure the quality of laser cutting.
The most common poor cutting quality is over burning and slag hanging.
The power of laser cutting machine, laser pulse frequency, cutting speed, beam focus, beam polarization direction, nozzle size, auxiliary gas pressure and flow rate will affect the evaluation of laser cutting quality factors.
1) Overburning is caused by the fact that the heat from the melting of the workpiece cannot be carried away by the high-pressure airflow in time when the laser power is too high, cutting holes or cutting speed is too slow.
It can reduce the laser power and increase the gas flow rate to solve the cutting over burning.
2) Adhering slag is also called burr.
The laser power is insufficient, or the auxiliary gas pressure is insufficient, or the cutting speed is too fast, so that the auxiliary gas can not completely blow out the melted or gasified materials produced in the cutting process, resulting in slag adhering to the lower edge of the cutting surface.
1.2 Factors for evaluating laser cutting quality
The focus position of the laser beam has an important influence on cutting quality.
When cutting different materials, different focus positions need to be adjusted.
Sometimes, cutting burr is not due to insufficient auxiliary gas pressure, but the focus is too high.
The influence of beam focus position on cutting quality is shown in Fig. 7.
1.3 Influence of cutting speed on cutting quality
1) The cutting speed is too fast
- It may cause unable to cut with splash sparks;
- Some areas can be cut off, some areas can not be cut off;
- The whole cutting section is rough;
- A diagonal cut pattern is formed and slag is produced in the lower part of the plate (Fig. 8).
2) Cutting speed is too slow
- The results show that the cutting plate is over melted and the cutting section is rough;
- When the cutting seam is widened, the whole area will melt at the small fillet or sharp corner, resulting in overburning;
- Cutting efficiency is reduced.
1.4 Influence of nozzle on laser cutting quality
When the center of the nozzle is different from the center of the laser beam, the high-speed airflow from the nozzle will blow the molten material, which makes it easier to have slag on one side and not on the other side.
This phenomenon is obvious when cutting 3 mm thick plate, and it can’t be cut in serious conditions.
Nozzle size should be appropriate.
Cutting thick plate requires high laser power, and the diameter of the spot projected on the plate by laser focusing is also large, which requires that the diameter of the nozzle passing through the beam is also large.
However, if the diameter of the nozzle is too large.
On the one hand, the velocity of the air jet from the nozzle will be slow, and the ability to blow out the molten material will be weakened, and the cutting section may form burr;
On the other hand, due to the large size of the nozzle and the slow air flow speed, the slag and smoke generated by laser cutting are easy to splash into the nozzle and damage the protective lens.
For high-power laser machine, the reflected laser beam may burn the laser head.
1.5 Influence of compressed air quality on laser cutting quality
The quality of compressed air has a very direct impact on the quality of laser cutting.
The compressed air contains water mist and oil.
If it is not cleaned, high-pressure jet to the protective mirror of the laser cutting head will seriously affect the transmission of the laser beam, disperse the focus, and cause the product to be cut incompletely and produce waste products.
If it is a super high power laser cutting machine, as long as the mirror or nozzle surface adheres with a little tiny oil film or water mist, it may also cause high-energy laser emission to burn the laser head.
As the laser head price of super high power laser cutting machine (above 12000 W) is more than 20000.
Most of the air compressor providers can not handle the oil and moisture in the compressed air well.
Most users of ultra-high power laser cutting machines are not willing to risk using compressed air as auxiliary gas.
Figure 9 and 10 shows uncontaminated lenses and contaminated lenses, respectively.
2. The factors affecting the thickness and efficiency of metal laser cutting
From a large number of practices, the main factors affecting the metal laser cutting thickness and cutting efficiency are the power of the laser cutting machine, the type of auxiliary gas, the pressure and the flow rate of auxiliary gas.
When choosing air compressor for laser cutting machine, these factors should be considered comprehensively.
According to experiment 4 of laser cutting research conducted by Tsinghua University, the influence of laser power and auxiliary gas pressure on laser cutting ability is shown in Fig. 9.
The cutting speed is 3 m/min, the spot diameter is 0.16 mm, the nozzle shape is conical, the diameter of the nozzle bottom is 1.5 mm, and the distance between the nozzle and workpiece is 1 mm.
As can be seen from Fig. 11 (a), if the laser power is lower than 860 W for 2 mm thick low carbon steel plate, no matter how much pressure of auxiliary gas nitrogen is increased, the workpiece can not be cut off because the energy provided by the laser is not enough to penetrate the workpiece.
If the laser power is high enough to penetrate the workpiece, the higher the laser power, the lower the auxiliary gas pressure required.
It can be seen from Fig. 11 (b) that if oxygen is used to cut low carbon steel plate with the same thickness, the influence of laser energy and auxiliary gas pressure on cutting ability is similar to that of nitrogen cutting.
However, the laser power and auxiliary gas pressure required for cutting are greatly reduced.
This is because oxygen reacts with iron during cutting, which releases about four times the laser power.
This also explains why the general medium power laser cutting machines prefer to use oxygen cutting when cutting low carbon steel plate of 2 mm or more.
When cutting carbon steel plate with compressed air, there is about 20% oxygen in the air.
This part of oxygen also reacts with iron element and releases heat during cutting.
Therefore, the efficiency of cutting carbon steel plate with compressed air is slightly higher than that with nitrogen.
In addition, different laser cutting machine manufacturers, due to the different laser and technology, cutting capacity is not the same. This paper extracts the public data of several representative manufacturers for reference.
| Material | 500W | 1000W | 2000W | 3000W |
| Low Mild Steel(mm) | 6 | 10 | 16 | 20 |
| Stainless Steel(mm) | 3 | 5 | 8 | 10 |
| Aluminum Alloy(mm) | 2 | 3 | 5 | 8 |
| Brass(mm) | 2 | 3 | 5 | 8 |
| Material | 1000W | 1500W | 2000W | 2500W | 3000W | 4000W |
| Mild Steel | 10 | 12 | 16 | 20 | 22 | 25 |
| Stainless Steel | 4 | 5 | 8 | 8 | 12 | 16 |
| Aluminum Alloy | 4 | 5 | 6 | 8 | 12 | 16 |
| Copper Alloy | 2 | 3 | 4 | 5 | 6 | 8 |
Due to the decrease in laser cost, the overall cost of the laser cutting machine has been greatly reduced.
At present, the power of laser cutting machines purchased by general metal processing users is 3000-4000W.
A small number of metal processing users around the metal material market mainly buy 12000 -15000W ultra-high power laser cutting machine.
Therefore, in general, laser cutting can be used for low-carbon steel plate with thickness less than 22-25 mm, stainless steel plate, aluminum alloy plate and copper alloy plate less than 10-16 mm.
Among them:
- Low carbon steel plate with a thickness less than 2-2.5mm should be cut with high-pressure compressed air;
- Low carbon steel plate with thickness of 2-2.5mm should be cut by oxygen;
- Stainless steel plate, galvanized sheet, aluminum alloy plate and copper alloy plate should be cut with nitrogen or high pressure compressed air.
