Plasma cutting is a machining method that uses the heat from a high-temperature plasma arc to cause the metal in the workpiece cut to melt and partially evaporate, and with the momentum of the high-speed plasma, the metal melt is expelled to form a cut.
As it relies on fusion reactions rather than oxidation to cut materials, its application range is much wider than oxyfuel cutting. It can cut virtually all metals, non-metals, multilayer materials and composites.
The cuts are narrow, with good surface quality, fast cutting speed and can reach a thickness of 160 mm.
Furthermore, due to the high temperature and high speed of the plasma arc, there is no deformation when cutting thin sheets.
Especially when cutting stainless steel, titanium alloys and non-ferrous metal materials, excellent cutting quality can be achieved.
Therefore, plasma cutting is widely used in industries such as automobiles, pressure vessels, chemical machinery, nuclear industry, general machinery, construction machinery and steel structures.
1. Operating principle of plasma cutting machine
A plasma cutter ionizes mixed gases through a high-frequency electrical arc, causing some gases to “decompose” or ionize into basic atomic particles, thus generating “plasma.”
When the arc strikes the workpiece, the high-pressure gas blows the plasma out of the torch nozzle at an exit speed of 800-1000 m/s (about 3 Mach).
The temperature of the plasma arc column is extremely high, reaching 10,000°C to 30,000°C, far exceeding the melting point of all metallic or non-metallic materials.
This causes the cut piece to melt quickly and the molten metal to be expelled by the ejected high-pressure air stream.
Therefore, smoke extraction and slag removal equipment is required. Plasma cutting combined with different working gases can cut various metals that are difficult to cut with oxyfuel cutting, especially non-ferrous metals (stainless steel, aluminum, copper, titanium, nickel) with better cutting effects.
Its main advantages are that when cutting metal materials with a not very thick thickness, plasma cutting is fast, especially when cutting thin sheets of common carbon steel, the speed can reach 5 to 6 times that of oxyfuel cutting with a smooth cutting surface , minimal thermal deformation and practically no zones affected by heat.
With the development of plasma cutting, the working gas used (the working gas is the conductive medium of the plasma arc and is the heat carrier and also expels the molten metal from the cut) has a significant effect on the cutting characteristics of the plasma arc. cut. plasma arc, as well as cutting quality and speed.
Commonly used plasma arc working gases include argon, hydrogen, nitrogen, oxygen, air, steam and certain mixed gases.
2. Standards for evaluating the quality of plasma arc cutting
(1) Cutting width
It is one of the most important characteristics for evaluating the quality of operation of a cutter and reflects the minimum radius that the cutter can handle. It is measured at its widest point, with most plasma cutters producing a cutting width between 0.15 and 6.0mm.
Influencing factors include: a. Excessively wide cuts not only waste material, but also reduce cutting speed and increase energy consumption. B. The cutting width is mainly related to the nozzle opening, and is normally 10% to 40% larger than it. w.
As the kerf thickness increases, a larger nozzle opening is often required, which in turn widens the kerf. d. An increased cutting width can lead to greater deformation of the part to be cut.
(2) Surface roughness
This describes the appearance of the cut surface and determines whether additional processing is required after cutting. It is also a measurement of the Ra value at two-thirds of the depth of cut.
The roughness is mainly due to the longitudinal vibrations caused by the cutting air flow in the cutting direction, which results in ripples in the cut.
The general requirement for surface roughness after oxyacetylene cutting is: Class 1 Ra≤30μm, Class 2 Ra≤50μm, Class 3 Ra≤100μm.
Plasma arc cutting typically produces a higher Ra value than flame cutting but smaller than laser cutting (less than 50 μm).
(3) Squaring of the cutting edge
This is another important parameter that reflects the quality of the cut and is related to the degree of additional machining required after cutting. This index is often represented by U verticality or angular tolerance.
For plasma arc cutting, the U value is closely related to the plate thickness and process parameters, generally U≤(1%~4%)δ (δ being the plate thickness).
(4) Width of heat affected zone
This metric is crucial for low-alloy steels or hardenable or heat-treatable alloy steels, as a wide heat-affected zone can significantly alter near-cut properties.
Air plasma arc cutting has a heat-affected zone width of about 0.3 mm, which can be narrower in underwater plasma arc cutting.
(5) Amount of slag
This describes the amount of oxidized slag or recrystallized material adhered to the lower edge of the cut after thermal cutting. The degree of slagging is usually determined by visual inspection, often described as none, mild, moderate or severe.
In addition, there must be specific requirements for cutting straightness, top edge fusion, and notches.
