Application of Laser Cutting in Sheet Metal Industry

Traditional processing methods in sheet metal workshops include processes such as shearing, punching and bending.

The punching process requires a large number of dies and is characterized by a low or no-cutting process.

When a product is processed, dozens of sets of molds are usually equipped, and some products may even need hundreds of sets of molds.

From the perspective of the economy, it needs to be equipped with a large number of dies, the cost of the product increases accordingly, resulting in a waste of money.

In order to adapt to modern sheet metal processing, it reduces production costs and improves the processing technology, so laser processing technology came into being.

With the application of laser cutting machines, sheet metal processing technology has been developed rapidly, which is brought a revolutionary concept to sheet metal fabrication and processing.

Laser cutting process and laser cutting machine equipment are being familiar with and accepted by the majority of sheet metal processing enterprises, and with its many advantages such as high processing efficiency, high processing accuracy, good cutting section quality, and three-dimensional cutting processing, it gradually replaces traditional sheet metal cutting equipment (mainly CNC equipment, including traditional plate processing equipment such as shears, punches, flame cutting, plasma cutting and high-pressure water cutting, etc.).

Laser processing technology has a very important position in sheet metal processing technology, which improves labor productivity and promotes the development of sheet metal technology.

Laser cutting machine with a high degree of flexibility, can greatly reduce the processing cycle, increase the cutting speed with high production efficiency, improve processing accuracy and speed up the development of products.

These advantages are paid attention to by many manufacturing companies.

A laser cutter is a laser that emits a laser beam, which is focused on a high power density laser beam by an optical path system.

The laser beam illuminates the surface of the workpiece, causing it to reach its melting or boiling point, while a high-pressure gas coaxial with the beam blows away the molten or vaporized metal.

As the beam moves in relation to the workpiece, it eventually forms a slit in the material for cutting purposes.

The laser cutting process is the use of an invisible beam instead of the traditional mechanical knife, with high precision, fast cutting, not limited by cutting pattern and automatic nesting to save materials, smooth kerf and low processing costs, etc., which will gradually improve or replace the traditional metal cutting equipment.

The mechanical part of the laser cutter head is not in contact with the workpiece, and will not cause scratches on the surface of the workpiece during the work;

Laser cutting is fast, with a smooth and even kerf, it generally does not need subsequent processing;

Small heat-affected zone for cutting, small deformation of the plate, and narrow slit (0.1 to 0.3mm);

No mechanical stress in the incision, no shearing burr;

Good repeatability, no damage to the surface of the material, CNC programming, which can process any plan.

It is economical and time-saving for cutting large-format whole boards without developing molds.

In general, the carbon steel plate within 12mm and stainless steel plate within 10mm are to recommended for the use of the laser cutting machine.

Laser cutting machines without cutting force, processing without deformation and no tool wear make it have good material adaptability.

Whether it is simple or complex parts, can be used to laser a precise rapid-forming cutting.

Automatic nesting can be achieved by cutting and nesting to improve the utilization of materials with economic benefits.

Types of laser cutting machine

According to the different laser generators, the current market can be broadly divided into three kinds of laser cutting machine: CO2 laser cutting machine, YAG (solid-state) laser cutting machine, optical fiber laser cutting machine.

CO2 laser cutting machine

CO₂ laser cutting machine: it can stably cut carbon steel within 20mm, stainless steel within 10mm and aluminum alloy below 8mm.

The wavelength of CO₂ laser is 10.6μm, which is easy to be absorbed by non-metal.

It can cut wood, acrylic, PP, plexiglass and other non-metal materials with high quality, but the photoelectric conversion rate of CO₂ laser is only about 10%.

The CO₂ laser cutting machine is equipped with a nozzle for blowing oxygen, compressed air or inert gas N₂ at the beam exit to increase the cutting speed and ensure the smoothness of the cut.

In order to improve the stability and lifetime of the power supply, the CO₂ gas laser has to address the discharge stability of high-power lasers.

According to international safety standards, the laser hazard level is divided into 4 levels, CO₂ laser belongs to the least hazardous level.

Main advantages: high power, general power is between 2000 ~ 4000W, which can cut the full size within 25mm of stainless steel, carbon steel and other conventional materials, as well as the aluminum within 4mm and acrylic board within 60mm, wood material board, PVC board and cutting sheet etc. very fast.

In addition, because the CO₂ laser outputs a continuous laser, it has the smoothest cutting cross-section of the three types of laser cutting machines.

Main market orientation: 6-25mm medium and heavy plate cutting, mainly for large and medium-sized enterprises and some pure foreign laser cutting enterprises.

Due to insurmountable factors such as large maintenance consumption of its lasers and large power consumption of the main engine, the market has been significantly shrinking due to the huge impact of fiber laser cutting machines in recent years.

YAG (solid-state) laser cutting machine

YAG solid-state laser cutting machine has the characteristics of low price and good stability, but the energy efficiency is generally less than 3%.

Most of the output power of the current products are below 800W, and due to the low output energy, it is mainly used for punching, spot welding and thin plate cutting.

Its green laser beam can be applied in the case of pulsed or continuous wave, with a short wavelength and good condensation characteristics, which is suitable for precision machining especially in the case of pulsed hole processing, but also for cutting, welding and lithography.

The wavelength YAG solid laser cutting machine laser is not easily absorbed by non-metallic, so it can not cut non-metallic materials.

YAG solid laser cutting machine needs to improve power stability and life.

It is necessary to develop a large-capacity, long-life optical pump excitation light source, and the use of semiconductor optical pumps can greatly increase energy efficiency.

The main advantages: It can cut aluminum, copper and most non-ferrous metal materials that other laser cutting machines can not cut.

The machine is cheap to purchase at low cost and simple maintenance, and most of the key technologies have been mastered by domestic enterprises.

The price of accessories and maintenance costs are low, and the operation and maintenance of the machine are simple, and the quality of workers is not high.

Main market orientation: cutting below 8mm.

It is mainly used for self-use small enterprises, medium-sized enterprises and most users in sheet metal manufacturing, home appliance manufacturing, kitchenware manufacturing, decoration, advertising and other industries with low processing requirements.

In the future, it may gradually replace traditional processing equipment like wire cutting, CNC punching, water cutting and low-power plasma etc.

Optical fiber laser cutting machine

Optical fiber laser cutting machine can be transmitted through the optical fiber with an unprecedented degree of flexibility, fewer failure points, easy maintenance and fast speed, which has a great advantage in cutting thin plate within 4mm.

But by the influence of solid laser wavelength, the quality is poorer when cutting thick plates.

The wavelength of the optical fiber laser cutting machine is 1.06μm, which is not easy to be non-metallic absorption, so it can not cut non-metallic materials.

The photoelectric conversion rate of optical fiber laser is as high as 25%.

Optical fiber laser has obvious advantages in terms of electricity consumption and supporting cooling system parameters.

According to international safety standards, optical fiber lasers are the most harmful to the eyes due to their short wavelength.

For safety reasons, optical fiber laser processing needs to be carried out in a fully enclosed environment.

As an emerging laser technology, the optical fiber laser cutting machine is far less popular than the CO₂ laser cutting machine.

The main advantages: high photoelectric conversion rate; less power consumption; can cut stainless steel plate within 12mm, carbon steel plate, which is the laser cutting machine with the fastest cutting speed among the three machines.

It can be used for fine cutting due to its fine slit and good spot quality.

Main market orientation: cutting below 12mm, especially the high precision processing of thin plates.

It is mainly for manufacturers with extremely high requirements for processing accuracy and efficiency.

It is estimated that with the emergence of 5000W and above lasers, fiber laser cutting machines will eventually replace most of the market for CO₂ high-power laser cutting machines.

Laser cutting method

Figure 1 shows the three methods of laser cutting.

Fig. 1: Laser cutting method

Laser melting cutting

(1) In laser melting cutting, the workpiece is partially melted and the molten material is ejected by means of an air stream.

Because the transfer of the material occurs only in its liquid state, the process is called laser melting cutting.

(2) Laser beam with high-purity inert cutting gas to promote the melted material away from the slit, the gas itself does not participate in cutting.

(3) Laser melting cutting allows for higher cutting speeds than vaporizing cutting. The energy required to vaporize is usually higher than the energy required to melt the material. In laser melting cutting, the laser beam is only partially absorbed.

(4) The maximum cutting speed increases with increasing laser power and decreases inversely with increasing plate thickness and increasing material melting temperature.

At a given laser power, the limiting factors are the air pressure at the cutting kerf and the thermal conductivity of the material.

(5) Laser melting cuts produce oxidation-free cuts for ferrous materials and titanium.

For steel materials, it produces laser power density that melts but does not vaporize, and the laser power density is between 104 W/cm2 and 105 W/cm2.

Laser flame cutting

Laser flame cutting differs from laser fusion cutting in that it uses oxygen as the cutting gas.

The interaction between the oxygen and the heated metal results in a chemical reaction that further heats the material. For the same thickness of structural steel, a higher cutting rate can be achieved with this method than with fusion cutting.

On the other hand, the quality of the kerf is poorer compared to melt cutting.

In fact, wider kerfs, significant roughness, increased heat affected zone area and poorer edge quality are produced.

(1) Laser flame cutting is flawed when working with precision models and sharp corners (there is a danger of burning off sharp corners). Pulsed mode lasers can be used to limit the heat-affected zone.

(2) The laser power used determines the cutting speed. For a given laser power, the limiting factors are the availability of oxygen and the thermal conductivity of the material.

Laser vaporization cutting

During laser vaporization cutting, the material vaporizes at the cutting edge, which requires a very high laser power.

In order to prevent the material vapor from condensing onto the slit wall, the thickness of the material must not exceed significantly the diameter of the laser beam, and the process is therefore only suitable if there is no discharge of the molten material.

In practice, this process can only be used in the very small field of use of iron-based alloys.

The process cannot be used on wood and certain ceramics, where thicker kerfs are usually achieved.

(1) In laser vaporization cutting, the optimal beam focus depends on the material thickness and beam quality.

(2) Laser power and heat of vaporization have an effect on the optimal focus position.

(3) The maximum cutting speed is inversely proportional to the material vaporization temperature in the case of certain plate thickness.

(4) The required laser power density is greater than 108W/cm2, depending on the material, cutting depth and beam focus position.

(5) In the case of certain plate thickness. Assuming sufficient laser power, the maximum cutting speed is limited by the speed of the gas jet.

Laser cutting process

The process refers to the interaction between the laser beam, the process gas and the workpiece.

Figure 2 shows the processing parameters.

Fig.2 Processing parameters

Cutting process

Before cutting, the laser must heat the workpiece to the temperature required to melt and vaporize the material.

The cutting plane consists of an almost vertical plane that absorbs laser radiation to heat and melt.

In laser flame cutting, a stream of oxygen enters the slit further heats the melting zone to a temperature close to the boiling point, and the resulting vaporization removes the material.

At the same time, with the aid of heated gas, the liquefied material is discharged from the lower part of the workpiece.

In laser melting cutting, the liquefied material is discharged with the gas, which protects the slit against oxidation.

The continuous melting zone gradually slides in the direction of the cut, thus creating a continuous slit.

Many important activities of the laser cutting process take place in this zone and the analysis of these activities gives important information about laser cutting, which allows calculation of the cutting speed and explains the formation of the draw line characteristics.

Material Properties

The result of cutting on the workpiece may result in a neat cut, or on the contrary, there may be rough edges or overburning.

The most important factors affecting the cutting quality are:

(1) Alloy composition

Alloy composition affects the strength, specific gravity, weldability, oxidation resistance and acidity of the material to some extent.

Some of the important elements in ferrous alloy materials are carbon, chromium, nickel, magnesium and zinc.

The higher the carbon content, the more difficult the material is to cut (the critical value is considered to be a carbon content of 0.8%).

The following types of carbon steels can be cut well with laser: St 37-2, StW 22, DIN 1.203.

(2) The basic microstructure of the material

In general, the finer the particles that make up the material, the better the quality of the cutting edge.

(3) Surface quality and roughness.

If the surface has areas of rust or oxidation, the cut profile will be irregular and have numerous breakage points.

To cut corrugated board, select the maximum thickness cutting parameter.

(4) Surface treatment

The most common surface treatments are galvanized, painted, anodized or covered with layered plastic film.

Sheets treated with zinc tend to have sluggish on the edges.

In the case of painted boards, the cutting quality depends on the composition of the painted product.

Plates coated with layered material are very suitable for laser cutting.

In order for the capacitive detection to work trouble-free and for optimal adhesion of the layered coating (to avoid blisters), the layered edge must always be on the upper part of the cutting workpiece.

(5) Beam reflection

How the light beam is reflected on the surface of the workpiece depends on the base material, surface roughness and treatment mode.

Some aluminum alloys, copper, brass and stainless steel sheet have high reflectivity characteristics.

When cutting these materials, special care should be taken to adjust the focus position.

(6) Thermal conductivity

When welding, materials with low thermal conductivity require less power than materials with high thermal conductivity.

For example, for chrome-nickel alloy steel, less power is required than for structural steel, and less heat absorption is produced by processing.

Materials such as copper, aluminum and brass lose a large portion of the heat absorbed by the laser when they are welded because the heat is conducted away from the target point of the beam, so the material in the heat-affected zone is more difficult to melt.

(7) Heat-affected zone.

Laser flame cutting and laser melting cutting will cause material variation in the edge area of the cut material.

Regarding the relationship between the range of the heat-affected zone and the thickness of the basic material.

Table 1 lists some reference values.

Table 1 Relationship between material thickness and the heat-affected zone

It can be understood from the table:

(1) When processing low-carbon steel or oxygen-free steel, the quenching phenomenon of the heat-affected zone is reduced;

(2) High carbon steel (such as Ck60) will harden the edge area;

(3) The heat-affected zone of hard-rolled aluminum alloy will be slightly softer than the rest.

Laser cutting incision evaluation analysis

Processability of different materials

(1) Structural steel

Oxygen cutting can be used, but the cutting edge will be slightly oxidized.

For plates with a thickness of 4mm, nitrogen gas is suitable for high-pressure cutting.

When processing complex contours and small holes (the diameter is less than the material thickness), the pulse mode should be used for processing, which is to avoid cutting off sharp corners.

Structural steel: cut with O₂

For cutting structural steel, the following should be noticed:

1) The higher the carbon content, the easier it is to quench the cutting edges, and the easier it is to overcook the corners.

2) Sheets with higher alloy content are harder to cut than those with lower content.

3) An oxidized or sandblasted surface will give a poorer cutting quality.

4) The residual heat on the surface of the plate has a negative effect on the cutting result.

5) For plates thicker than 10 mm, better results can be obtained by using special electrodes for the laser and by oiling the workpiece surface during the process.

6) To eliminate tension, it only cut steel plates after secondary treatment.

7) In order to obtain structural steel with a clean-cut surface, it is necessary to follow:

• Si ≤ 0.04%: laser processing preferred.
• Si < 0.25%:  poor cut quality in some cases.
• Si > 0.25%: not suitable for laser cutting.

 Several key parameters affecting the process

N1 gas parameters

• Gas type: nitrogen, oxygen and compressed air
• Gas purity: Generally between 99.99% and 99.999% of air pressure.
• The maximum air pressure during low-pressure cutting is 5 bar, and the maximum air pressure during high-pressure cutting is 20 bar between the nozzle and the plate;
• The distance between the nozzle opening and the workpiece surface must be as small as possible.
• The smaller the distance, the greater the actual airflow into the incision.
• The gap is usually between 0.5 and 1.5 mm.

(2) Stainless steel

1) In the case of negligible edge oxidation, oxygen cutting is used.

2) By using a combination of high power and high-pressure nitrogen, a cutting speed equal to or higher than that of oxygen cutting can be achieved.

3) When using nitrogen to process stainless steel over 4mm, it needs to reset the focus position and reduce the speed to reduce the appearance of burrs.

4) For thick plates above 5mm, oxygen cutting is suitable;

At the same time, it needs to reduce the feed speed and use the laser pulse mode.

5) For piercing and cutting, the same nozzle height needs to be used.

Stainless steel: cutting with N₂ high pressure

(3) Aluminum

Aluminum and its alloys are more suitable for cutting in continuous mode.

N2 laser power

There is a choice of continuous or pulsed mode, with the continuous mode being used for regular, fast cutting operations in general.

Pulse mode is used for high-precision cutting operations with strict requirements for the end face, and is significantly slower than the continuous mode.

1) When cutting with oxygen, the cutting surface is rough and hard, which produces only a small flame that is difficult to eliminate.

2) When cutting with nitrogen gas, the cutting surface is smooth.

In addition, when processing plates of less than 3mm, an incision with virtually no burrs can be obtained through optimal adjustments;

while for thicker plates, burrs that are difficult to remove will occur.

3) Pure aluminum is very difficult to cut because of its high purity.

4) The higher the alloy content, the easier the material is to cut.

Note: Aluminum should only be cut if a “reflective absorption” device is installed on the system, otherwise the optical components will be destroyed.

Aluminum alloy: cutting with N₂ high pressure

(4) Titanium

Titanium plates are cut with argon and nitrogen as process gases, other parameters can be found in nickel-chromium steel.

(5) Copper and brass

1) Both materials have high reflectivity and very good thermal conductivity.

2) The thickness of the brass up to 1mm can be cut with nitrogen gas.

3) For copper processing with a thickness of less than 2mm, oxygen gas must be used.

Note: Copper and brass can only be cut when a “reflection absorption” device is installed on the system, otherwise the optical components will be destroyed.

(6) Synthetic materials

Cutting speed

Depending on the thickness of the sheet, the thinner the sheet, the faster the cutting speed.

When processing straight contours, the cutting speed can reach the highest peak of the set value.

When processing arc contours or corners, the cutting speed will automatically be adjusted downwards to ensure the excellent processing quality.

At the same time, it is also related to laser power.

Generally speaking, the higher the laser power, the faster the processing speed.

When cutting synthetic materials, it is important to bear in mind the hazards of cutting and the possible emissions of hazardous substances.

Synthetic materials that can be processed are: thermoplastics, thermosetting materials and artificial rubber.

It is forbidden to use a laser cutter to process PVC or polyethylene because of the released toxic gases.

For these two materials, water cutting is preferable.

Acrylic glass can be cut by laser.

Nitrogen is used as the processing gas, but the pressure must be lower than 0.5bar so that a smooth cutting surface can be obtained.

(7) Organics

Acrylic glass can be cut with a laser and nitrogen is used as a process gas, but the air pressure must be less than 0.5 bar in order to obtain a smooth cutting surface.

There is a fire hazard in all organic cutting (nitrogen is used as the processing gas, and compressed air can also be used as the processing gas).

Wood, leather, cardboard and paper can be laser cut. The cut edges will be burnt (brown).

The higher the feed rate, the less carbonization.

When processing plywood, there is no guarantee that there will be clean cuts, because each layer of glue varies in composition according to its type and type.