Thermal metal cutting is an essential process in most metal fabrication, such as sheet metal processing, where we need to first cut the steel plate into the rough shape we require, and then perform precision machining or welding to create the desired component.
In industrial production, thermal metal cutting generally includes gas cutting, plasma cutting, and laser cutting, among others.
Compared to gas cutting, plasma cutting has a wider cutting range and higher efficiency.
Fine plasma cutting technology has reached the lower limit of laser cutting in terms of the surface quality of the cut material and is much cheaper than laser cutting. Therefore, it has been widely used in royal cutting.
1. Understanding Metal Thermal Cutting
Flame cutting, plasma cutting and laser cutting are classified based on the type of cutting thermal energy and the shape of cutting.
1.1 Flame Cutting (Gas Cutting)
The cut is formed by fusing the metal with a flame produced by burning a mixture of flammable gas and oxygen and then blowing it away.
Acetylene gas is usually used, but petroleum gas, natural gas or coal gas may also be used.
Due to factors such as gas pressure, cutting nozzle height and preheating time, the overall deformation range of the cut material is relatively large in flame cutting, making it unable to meet the needs of high-precision cutting, and the cutting speed is relatively low.
Additionally, preheating is required before cutting, which takes time and makes it difficult to adapt to unmanned operations.
1.1.1 Advantages:
(1) Flame cutting can cut very thick carbon steel, and its cutting range is wide, capable of cutting steel plates with a thickness of 6mm to 200mm;
(2) The price of flame cutting equipment is relatively low and the initial investment cost is also low.
1.1.2 Disadvantages:
(1) Long preheating and drilling time required for cutting, slow cutting speed;
(2) Significant thermal deformation during cutting, especially when cutting thin sheets (0.5-6mm), low cutting accuracy;
(3) Cannot cut colored metals such as copper and aluminum and stainless steel;
(4) The fuel burning method is highly polluting to the environment and is not environmentally friendly.
Numerically controlled flame cutting of thin sheets (0.5-6mm) has gradually been replaced by plasma cutting in the cutting area, but in terms of cutting thick and medium sheets, numerically controlled flame cutting is still irreplaceable, and flame cutting still occupies a certain market share due to its price advantage in cutting thin sheets.
1.2 Plasma Arc Cutting
Using plasma arc as a heat source and relying on high-speed thermal plasma gas (such as nitrogen, argon and nitrogen-argon, argon-hydrogen mixture gas, etc.) to melt the metal and blast it to form a seam cutting.
Under the same conditions, the cutting speed of plasma arc is higher than gas cutting, and the range of materials cut is also wider than gas cutting.
There are three common types: small current plasma arc cutting, large current plasma arc cutting and waterjet plasma arc cutting.
1.2.1 Advantages:
(1) Wide cutting field, can cut all metal sheets;
(2) Fast cutting speed, high efficiency, cutting speed can reach 10m/min or higher;
(3) The cutting accuracy is higher than flame cutting, underwater cutting has no deformation, and the accuracy of fine plasma cutting is even higher.
1.2.2 Disadvantages:
(1) It is difficult to cut steel sheets over 20mm, requiring a high-power and high-cost plasma source.
(2) When cutting thick plates, the verticality of the cut is poor and the cut becomes V-shaped.
Underwater plasma cutting can eliminate noise, dust, harmful gases and arc pollution generated during cutting, effectively improving the working environment.
Related Reading: CNC Plasma Cutting Dust Removal: Explained
With the use of fine plasma cutting, the cutting quality has approached the level of laser cutting, and with the maturity of high-power plasma cutting technology, the cutting thickness has exceeded 150mm, expanding the cutting range of CNC plasma cutting machines.
1.3 Laser Cutting
Cut using a laser beam as a heat source (laser source).
Its temperature exceeds 11,000 ℃, enough to vaporize any material. Laser cutting has a narrow, precise cutting edge, a smooth surface and a higher quality than any other thermal cutting method.
The laser source is generally a CO 2 laser beam with working power of 500 ~ 2500 watts, the laser beam is concentrated into a very small area through reflective lenses and mirrors.
The highly concentrated energy can quickly heat a local area, causing the stainless steel to evaporate.
Furthermore, because the energy is highly concentrated, only a small amount of heat is transmitted to other parts of the steel, causing minimal or no deformation.
The laser can accurately cut complex shapes from the raw material, and the cut raw material does not need additional processing.
1.3.1 Advantages:
(1) Good cutting quality, narrow cutting width, high precision, good cutting surface roughness, and generally no need for additional processing;
(2) It can be welded directly after processing;
(3) Fast cutting speed, small heat affected zone, minimum deformation;
(4) Clean, safe and pollution-free.
1.3.2 Disadvantages:
(1) Currently, laser cutting is only suitable for cutting thin plates (it generally takes a long time to drill holes in thick plates);
(2) The price of laser cutting equipment is quite expensive, about 1.5 million yuan or more.
From the current situation of laser cutting application, more and more companies will adopt CO2 laser cutting technology.
CO 2 laser cutting is widely used in 12mm thick low carbon steel plates, 6mm thick stainless steel plates and 20mm thick non-metallic materials.
For cutting three-dimensional curved surfaces, it also began to be applied in the automobile and aerospace industries.
2. Comparison of the technical-economic characteristics of three hot cutting methods”
Flame cutting, plasma cutting and laser cutting are currently in high demand, each with a certain market share depending on cutting requirements and market prices.
A comparison of their technical-economic characteristics can be seen in Tables 1 and 2.
Table 1: Comparison of one of the three cutting methods
| Technical and economic comparison of three cutting methods | |||||
| Cutting method | Flame cutting | plasma cutting | Laser cut | ||
| Heat source energy | small | average | big | ||
| Suitable materials for cutting | Carbon steel, low alloy steel | Low carbon steel, low alloy steel, stainless steel, steel, aluminum and its alloys and other non-ferrous metals. | Low carbon steel, low alloy steel, stainless steel, non-ferrous metals and non-metals. | ||
| Maximum cutting thickness/mm | =4000 carbon steel) | =200 (stainless steel) | =20 (steel) |
||
| Cutting speed/mm.min-1 | <1mm | – | 3,000 | >5000 | |
| two | – | 4000 | 3500 | ||
| 6 | 600 (equal pressure nozzles) |
3700 | 1000 | ||
| 12 | 500 | 2700 (200A oxygen plasma cutting) | 360 | ||
| 25 | 450 | 1200 | – | ||
| 50 | 300 | 250 | – | ||
| >100 | <150 | – | – | ||
Table 2: Comparison of two of the three cutting methods
| Comparison of Technoeconomic Characteristics of the Three-Cut Method | |||
| Cutting method | Flame cutting | plasma cutting | Laser cut |
| Cutting width/mm | 1.5-2.5 | 2.5-5.0 | 0.2-0.8 |
| Cutting deformation | big | small | Minimum |
| Cutting Dimension Accuracy/mm | Weak 1-2 | Usually 0.5-1 (0.2) | Very good 0.2 |
| Perpendicularity of the cutting surface | good | Poor | good |
| Cutting surface roughness | commonly | very good | preferably |
| Degree of fusion of the upper edge of the cutting surface | not big | Big (not big) | Tiny |
| Heat affected zone (heat absorbed by the unit)/J.mm-1 | Large (22.53) | Medium (small) (7.33) | Small (4.09) |
| environment pollution | commonly | Large underwater cuts (dust, noise, arc light, etc.) can be greatly reduced | rarely |
| Investment in equipment completed/defined | Low | Lower | High |
Plasma cutting, when combined with different working gases, can cut various metals that are difficult to cut with oxygen cutting, especially non-ferrous metals (stainless steel, aluminum, copper, titanium, nickel), with even better cutting results .
Its main advantage is that when cutting thin metals, plasma cutting is fast, especially when cutting common carbon steel sheets, with a speed that can reach 5 to 6 times that of gas cutting, with a cutting surface smooth, minimal thermal deformation. , and almost no thermal impact zone.
At present, with the maturity of high-power plasma cutting technology, the cutting thickness can reach 150mm, and the use of waterjet technology in high-power plasma cutting has made the cutting quality close to the lower limit precision (±0.2mm) laser cutting.
Due to the high price of laser cutting machines and their current suitability only for cutting thin sheets (generally with a long drilling time for thick sheets) and with thin plasma cutting machines with a cutting precision that can reach the limit lower than laser cutting and a similar cutting surface quality, but with a much lower cutting cost than laser cutting, about 1/3, with a maximum cutting thickness of 25 mm, it is advantageous to replace expensive machines laser cutting by fine plasma cutting machines, to perform fine and high-speed cuts of medium and thin sheets in the most economical way.
3. Classification and Application of Plasma Cutting
3.1.1 Plasma cutting method can be classified into oxygen plasma cutting, nitrogen plasma cutting, air plasma cutting and argon-hydrogen plasma cutting based on the plasma working gas.
Different cutting methods have different applications due to differences in the physical and chemical properties of the working media used.
(1) Plasma oxygen cutting has the characteristics of high cutting speed, small workpiece deformation and fast electrode consumption, due to the high dissociation heat, good heat transport and active chemical properties of oxygen as the working gas, and is generally only used for cutting carbon steel;
(2) Plasma nitrogen cutting uses nitrogen as the working gas, and due to the presence of nitrogen, it is easy to produce a nitrided layer on the cutting surface, resulting in poor surface quality, but because nitrogen is cheap, this method It is generally used for cutting stainless steel that is not directly used as welding material and with low surface quality requirements;
(3) Plasma cutting uses air as the working medium, which has the above two cutting methods in common and is also used to cut carbon steel with low surface quality requirements.
In recent years, China has vigorously developed small current air plasma cutting machines, and its use is becoming more and more widespread, and the development of inverter air plasma cutting machines has created conditions for energy conservation ;
(4) The argon-hydrogen plasma cutting method uses easily ionizable argon and hydrogen with good thermal conductivity as the working gas, and the combination of the two can form a stable, high-energy density arc column and beam plasma with strong cutting capacity.
However, due to its high price, it is generally used for cutting stainless steel and aluminum with high cutting quality requirements.
3.1.2 Classification of Plasma Cutting and its Applications
Based on the plasma cutting medium, plasma jet cooling method and cutting quality, plasma cutting methods can be divided into traditional plasma cutting, dual gas plasma cutting, jet plasma cutting of water and fine plasma cutting.
(1) Traditional plasma cutting (Figure 1) generally uses the same gas (usually air or nitrogen) to cool and generate the plasma arc.
Most systems have a current rating of less than 100 A and can cut materials less than 16 mm thick, mainly used for manual cutting situations.
(2) Dual gas plasma cutting (Figure 2), uses two gases; one to form the plasma and the other for protection. Shielding gas is used to isolate the cutting area from air, resulting in a smoother cutting edge.
This is also the most popular cutting process because different combinations of gases can be used to obtain the best cut quality for a given material.
(3) Water-protected plasma cutting (Figure 3) evolved from the dual-gas process, using water instead of the shielding gas. It improves the cooling effect of the nozzle and workpiece and can achieve better cutting quality when cutting stainless steel.
This process is for industrial cutting applications only.
(4) Water Jet Plasma Cutting (Figure 4) uses a gas to generate plasma and injects water directly into the arc in a radial or vortex manner, greatly increasing the degree of compression of the arc and, therefore, the density and temperature of the arc.
The current range of waterjet plasma cutting is 260 to 750A and is used for high-quality cutting of various materials of different thickness.
This process is also only for industrial cutting applications.
(5) Fine plasma cutting (Figure 5) is a process that has a high plasma arc current density, typically several times greater than the current density of a conventional plasma arc.
Arc stability has also been improved by the introduction of techniques such as rotating magnetic fields, leading to very high cutting precision.
Excellent cut quality can be achieved when cutting thin materials (less than 16 mm) at lower speeds. The improvement in quality is due to the use of cutting-edge technology to compress the arc very effectively, thus greatly increasing energy density.
The requirement for operating at a lower speed is to allow the motion equipment to move more accurately along the specified contour. This process is only used in industrial cutting applications.
4. Selection of plasma cutting process parameters
There are many parameters of plasma cutting process, including cutting current, cutting speed, arc voltage, working gas and flow rate, nozzle height, etc.
Different parameters have different degrees of impact on the stability and quality of the cutting process, and the parameters should be selected according to the type of cutting material, the thickness of the workpiece and the specific requirements during cutting.
4.1 Cutting Current
The cutting current is the most important parameter of the cutting process, which directly determines the thickness and cutting speed, that is, the cutting capacity.
The impact of cutting current on cutting is as follows:
(1) Increasing the cutting current increases the arc energy, improves the cutting capacity and increases the cutting speed accordingly.
(2) Increasing the cutting current increases the arc diameter, making the arc thicker and the cut wider.
(3) Cutting current that is too large increases the thermal load on the nozzle, causing the nozzle to be damaged too early and reducing the cutting quality, or even making normal cutting impossible.
Therefore, the correct cutting current and corresponding nozzle must be selected based on the thickness of the material before cutting.
Overloading the nozzle (i.e. exceeding the nozzle's working current) will quickly damage the nozzle. The current intensity should generally be 95% of the nozzle working current.
4.2 Cutting speed
The ideal cutting speed range can be determined by equipment instructions or by experimentation. Due to factors such as material thickness, material, melting point, thermal conductivity and surface tension after melting, the cutting speed also changes correspondingly.
The impact of cutting speed on cutting is mainly reflected in the following aspects:
(1) Moderately increasing the cutting speed can improve the cutting quality, that is, the cutting is a little narrower and the cutting surface is smoother, and at the same time it can reduce deformation.
(2) If the cutting speed is too fast, the cutting line energy will be less than the required value, and the cutting slag cannot be quickly removed by the jet, resulting in a greater amount of drag, accompanied by suspended slag, and the surface the cutting quality decreases.
(3) When the cutting speed is very low, because the cutting is the cathode of the plasma arc, in order to maintain the stability of the arc itself, the cathode points or cathode region must find a place to conduct the current close to the nearest cutting seam , at the same time, will transfer more heat to the radial direction of the jet, making the cut wider.
The molten material on both sides of the cut accumulates and solidifies at the lower edge, forming slag that is difficult to remove, and the upper edge of the cut forms a rounding due to excessive heating and melting.
(4) When the speed is extremely low, the arc will even be extinguished due to the wide cut. It can be seen that good cutting quality and cutting speed are inseparable.
The cutting speed must be determined based on the power of the plasma arc, the thickness of the part and the material. Under the same cutting power, the cutting speed should be faster for aluminum due to its low melting point, slower for steel due to its high melting point, and slower for copper due to its good thermal conductivity and rapid heat dissipation.
4.3 Arc Voltage
It is generally considered that the normal output voltage of the power supply is the cut-off voltage.
Plasma cutting machines generally have high no-load voltage and working voltage, and when using high ionization gases such as nitrogen, hydrogen or air, the voltage required to stabilize the plasma arc will be higher.
When the current is constant, increasing voltage means increasing arc enthalpy and cutting capacity.
If at the same time as the enthalpy increases, the jet diameter is reduced and the gas flow rate increases, faster cutting speeds and better cutting quality can often be obtained.
4.4 Gas and Workflow
Working gas includes cutting gas and auxiliary gas, and some equipment also requires arc starting gas.
The appropriate working gas should generally be chosen based on the type, thickness and cutting method of the cutting material.
The cutting gas must ensure the formation of the plasma jet and also the removal of molten metal and oxides from the cut.
Too high a gas flow can carry more heat away from the arc, shorten the jet length, reduce cutting capacity and cause arc instability, while too low a gas flow can cause the plasma arc to lose its straightness, making the shallower cutting and easily producing slag.
Therefore, the gas flow must be well coordinated with the cutting current and speed.
At present, plasma arc cutting machines mainly control the gas flow through gas pressure, because when the diameter of the gun body is fixed, the flow is controlled by controlling the gas pressure.
The gas pressure used to cut a given thickness of material should generally be selected in accordance with data provided by the equipment manufacturer.
If there are other special applications, the gas pressure must be determined through actual cutting tests.
The correct pressure (flow) of the working gas is very important for the useful life of consumables. If the pressure is too high, the electrode life will be greatly reduced, and if the pressure is too low, the nozzle life will be affected.
Plasma cutting systems require clean, dry working gas to function properly. Dirty gas is generally a problem of the gas compression system, which will shorten the service life of wearing parts and cause abnormal damage.
4.5 Nozzle Height
Nozzle height refers to the distance between the end face of the nozzle and the cutting surface and constitutes part of the entire arc length.
Because plasma arc cutting generally uses external constant current or steep drop characteristic power sources, when the nozzle height increases, the current change is small, but the arc length will increase and the arc voltage will increase, resulting in an increase in arc power;
However, at the same time, the length of the arc exposed to the environment will increase and the energy loss of the arc column will increase.
Under the combined effect of these two factors, the former is often completely compensated by the latter and instead the effective cutting energy will decrease, resulting in a reduction in cutting capacity.
This is generally reflected in a decrease in the blowing force of the cutting jet, an increase in residual slag at the bottom of the cut, and rounded edges at the top edge due to excessive melting.
Furthermore, from the perspective of the shape of the plasma jet, the diameter of the jet expands outward after exiting the gun, and the increase in the height of the nozzle will inevitably cause an increase in the cutting width.
Therefore, choosing the lowest nozzle height is beneficial to improve cutting speed and quality, but too low a nozzle height may cause double arc phenomenon.
Using an external ceramic nozzle can set the nozzle height to zero, that is, the end face of the nozzle comes into direct contact with the cutting surface, which can achieve good results.
According to the instructions, use a reasonable nozzle height, when drilling, use a distance of 2 times the normal cutting distance or use the maximum height that the plasma arc can transmit, this can increase the service life of the wearing parts.
4.6 Cutting Power Density
To obtain a high compression plasma cutting arc, the cutting nozzles adopt smaller nozzle diameter, longer nozzle channel length and enhanced cooling effect, which increases the current passing through the effective section of the nozzle, i.e. , the arc power density increases.
At the same time, compression also increases arc power loss. Therefore, the actual effective energy used for cutting is less than the power of the energy source, with a loss rate generally between 25% and 50%.
Some methods, such as water-compressed plasma cutting, have a higher energy loss rate, which must be considered when designing cutting process parameters or calculating cutting costs.
Therefore, the actual diameter of the cutting nozzle must be determined based on the thickness of the cutting part and the selected ionic gas species.
When the cutting thickness is greater, the nozzle diameter should also be increased accordingly.
When using Ar+H2 mixed gas, the nozzle diameter can be a little smaller, while using N2, it should be larger.
5. Conclusion
The 21st century is an era that advocates green environmental protection.
With the rapid rise of the economy and the prosperous development of the manufacturing industry, higher requirements for cutting technology are inevitable.
Plasma cutting technology has many environmentally friendly characteristics, so the development of plasma cutting technology has the advantage of keeping up with the times.
Plasma cutting technology can eliminate noise, dust, harmful gases and arc light generated during cutting through an underwater cutting platform or smoke and dust treatment device, which basically meets environmental protection requirements.
In recent years, plasma cutting technology has developed rapidly, and some precise high-speed cutting technologies are competing with laser cutting.
Numerical control plasma cutting technology is a high-tech field that integrates plasma cutting technology, numerical control technology and inverter power source technology, among others, and has progressed along with the development of computer control, research of plasma arc characteristics and power electronics. .
Furthermore, using numerical control plasma cutting with automatic feeder programming software can increase the material utilization rate by 5% to 10%. With an annual cutting capacity of 20 million tons, 100,000 to 200,000 tons of steel can be saved annually, worth billions of yuan.
In industrialized countries, there is a tendency to replace flame cutting machines and laser cutting machines with numerical control plasma cutting machines.
In China, plasma cutting machines are widely used in various industries, such as automobiles, locomotives, pressure vessels, chemical machinery, nuclear industry, general machinery, engineering machinery and steel structures.
