A Brief Analysis of the Laser Cutting Process

As early as the 1970s, lasers were already being used for cutting. In modern industrial production, laser cutting is widely used in the processing of sheet metal, as well as plastics, glass, ceramics, semiconductors, textiles, wood and paper.

Over the next few years, laser cutting于precision machiningApplications in the fields of micro-fabrication and micro-machining will also see tangible growth.

laser cutting

When the focused laser beam strikes the workpiece, the irradiated area heats up rapidly, causing the material to melt or vaporise. Once the laser beam penetrates the workpiece, the cutting process begins: the laser beam travels along the contour line whilst melting the material. Typically, a jet of gas is used to blow the molten material away from the cut, leaving a narrow gap between the cut section and the backing plate; this gap is almost equal to the width of the focused laser beam.

Flame cutting

A standard process used for cutting low-carbon steel is flame cutting, which utilises oxygen as the cutting gas. The oxygen is pressurised to as high as 6 bar and blown into the cut; at the cut, the heated metal reacts with the oxygen, initiating combustion and oxidation. This chemical reaction releases a significant amount of energy—up to five times that of a laser—which assists the laser beam in the cutting process.

Figure 1: The laser beam melts the workpiece, whilst the cutting gas blows away the molten material and slag from the cut

Melting cutting

Another standard process used for cutting metal is fusion cutting, which can also be used to cut other fusible materials, such as ceramics.

In this scenario, nitrogen or argon is used as the cutting gas, with the gas blown across the cut at a pressure of between 2 and 20 bar. Argon and nitrogen are inert gases, meaning they do not react with the molten metal in the cut. They simply blow the molten metal away from the base. Furthermore, inert gases protect the cut edges from oxidation by the air.

Compressed air cutting

Compressed air can also be used for cutting thin sheets; increasing the air pressure to between 5 and 6 bar is sufficient to blow away the molten metal from the cut, Given that approximately 80–90 per cent of the composition of air is nitrogen, cutting using compressed air essentially falls under the category of fusion cutting.

Plasma-assisted cutting

If the parameters are selected appropriately, a plasma cloud will form in the cut produced by plasma-assisted melting cutting, The plasma cloud consists of ionised metal vapour and ionised cutting gas; the plasma cloud absorbs the energy of the CO₂ laser and transfers it to the workpiece, causing more energy to be coupled into the workpiece, causing the material to melt more rapidly and thereby increasing the cutting speed. Consequently, this cutting process is also known as high-speed plasma cutting.

In fact, plasma clouds are transparent compared to solid-state lasers, so plasma-assisted melting cutting can only be performed using CO₂ lasers.

Oxy-fuel cutting

Vapour-phase cutting, which involves the evaporation of the material, minimising the thermal impact on the surrounding material as much as possible. By using continuous CO₂ laser processing to vaporise this type of material, which has low heat input and high absorption, the aforementioned results can be achieved. Examples include thin plastic films, as well as non-melting materials such as wood, paper and foam.

Thanks to ultrashort laser pulses, this technology can be applied to other materials; the free electrons in the metal absorb the laser energy and heat up rapidly. As the laser pulse does not interact with molten particles or plasma, the material sublimates directly. There is no time for energy to be transferred to the surrounding material in the form of heat; consequently, when picosecond pulses ablate the material, there is neither a significant thermal effect nor any melting or burr formation.

Figure 3 illustrates gasification cutting, in which the laser causes the material to evaporate, whilst combustion also occurs. The pressure generated by the vapour forces the molten slag out of the cut.

Parameters: Adjusting the machining process

The laser cutting process is influenced by numerous parameters, some of which depend on the technical performance of the laser and the machine tool, whilst others are subject to fluctuation.

Polarisation

There is a quantity known as the degree of polarisation, which indicates the percentage of the laser that is converted; the typical degree of polarisation is usually around 90:1:3, which is sufficient for high-quality cutting.

Focal diameter

The focal diameter influences the width of the incision; by adjusting the focal length of the focusing lens, the focal diameter can be altered, with a smaller focal diameter resulting in a narrower incision.

Focus position

The beam diameter on the workpiece surface is determined by the focal position, as is the power density; moreover, the shape of the cut is also determined in this way.

Figure 4: Focus positions: inside the workpiece, on the surface of the workpiece and above the workpiece

Laser power

The laser power must be appropriate for the type of processing, suited to the material type, and commensurate with the thickness. The power must be sufficiently high—high enough that the power density on the workpiece exceeds the processing threshold.

Figure 5: Higher laser power allows for the cutting of thicker materials

Operating mode

Firstly, continuous mode is used for cutting standard metal and plastic profiles; this cutting process is suitable for dimensions ranging from millimetres to centimetres. Subsequently, a low-frequency pulsed laser is employed for melting perforations or creating precision profiles.

Cutting speed

The laser power and cutting speed must be properly matched. If the cutting speed is too fast or too slow, this will result in increased surface roughness and the formation of burrs.

Figure 6: Cutting speed decreases as sheet thickness increases

Nozzle diameter

The flow rate of the gas emerging from the nozzle, as well as the shape of the jet, is determined by the diameter of the nozzle. The thicker the material, the larger the diameter of the gas jet must be; correspondingly, the diameter of the nozzle orifice must also be increased.

Gas purity and pressure

Oxygen and nitrogen are frequently used as cutting gases. The purity and pressure of the gases affect the quality of the cut.

When using an oxy-acetylene cutting torch, the purity of the gas must be 99.95 per cent. The thicker the steel plate, the lower the gas pressure required.

When using nitrogen for flame cutting, the gas must be 99.995 per cent pure (ideally 99.999 per cent); higher gas pressure is required when cutting thick steel plates.

Technical Specifications

In the early days of laser cutting, users had to determine the processing parameters themselves through trial runs. Nowadays, established processing parameters are stored within the cutting system’s control unit. Corresponding data exists for every type of material and thickness. These technical parameter tables enable even those unfamiliar with the technology to operate laser cutting equipment with ease.

Factors affecting the quality of laser cutting

There are numerous criteria used to assess the quality of laser-cut edges; whilst factors such as the form of burrs, indentations and surface patterns can be assessed with the naked eye, parameters such as perpendicularity, surface roughness and kerf width require specialised instruments for measurement. Material deposition, corrosion, the heat-affected zone and distortion are also key factors in assessing the quality of laser cutting.

Figure 7: Good cut, bad cut. Criteria for assessing the quality of cut edges

Bright prospects

When it comes to laser cutting, its sustained success is on a scale that most other processing methods struggle to match; this trend continues to this day, and in the future, the prospects for laser cutting are set to become even broader.