The surface quality of plasma arc cutting is between oxygen-acetylene cutting and band saw cutting. Compared with mechanical cutting, plasma arc cutting has a larger tolerance. When the plate thickness is above 100mm, more metal will be melted due to the lower cutting speed, often forming rough cuts.
The standard for a good cut is: the width should be narrow, the cross-section of the cut is rectangular, the surface of the cut is smooth, and there is no slag or slag, and the surface hardness of the cut should not hinder the machining after cutting. For detailed requirements, please refer to the article Evaluation Criteria for Cutting Quality of CNC Plasma Cutter.
1. Incision width and flatness
The cut width refers to the distance between the two cutting surfaces on the upper edge of the cut caused by the cutting beam. When the upper edge of the incision is melted, it refers to the distance between the two cutting surfaces immediately below the melted layer.
Plasma arc often cuts more metal from the upper part of the incision than the lower part, so that the end face of the incision is slightly inclined. The upper edge is generally square, but sometimes slightly round. The cut width of plasma arc cutting is 1.5 to 2.0 times wider than that of oxygen-acetylene cutting. As the plate thickness increases, the cut width also increases. For stainless steel or aluminum with a plate thickness of 25mm or less, low-current plasma arc cutting can be used, and the incision has high straightness, especially for cutting plates with a thickness of 8mm or less, small edges and corners can be cut, even without processing. Welding, which is difficult to obtain with high current plasma arc cutting. This provides convenience for cutting irregular curves of thin plates and cutting irregular holes.
The flatness of the cutting surface refers to the distance between the highest point and the lowest point on the cutting surface of the measured part and the distance between two parallel lines in the inclination direction of the cutting surface.
There is a molten layer about 0.25 to 3.80 mm thick on the surface of the plasma arc incision, but its chemical composition has not changed. For example, when cutting an aluminum alloy containing 5% w(Mg), although there is a 0.25mm thick melted layer, the composition remains unchanged and no oxide appears. If the cut surface is used for welding directly, a dense weld can also be obtained. When cutting stainless steel, because the heated zone quickly passes the critical temperature of 649°C, chromium carbide will not precipitate along the grain boundaries. Therefore, cutting stainless steel with a plasma arc will not affect its corrosion resistance.
2. Elimination method of incision fusion
The gaps with irregular width, depth, and shape formed on the cutting surface interrupt the uniform cutting surface. The iron oxide slag attached to the lower edge of the cutting surface after cutting is called dross.
Taking stainless steel as an example, due to the poor fluidity of the molten metal of stainless steel, it is not easy to blow off all the molten metal from the incision during the cutting process. Stainless steel has poor thermal conductivity, and the bottom of the incision is prone to overheating so that the molten metal remaining in the incision that has not been blown off will be fused with the lower part of the incision to form a so-called nodule or slag after cooling and solidification. The toughness of stainless steel is good, and these welds are very tough and difficult to remove, which brings great difficulties to machining. Therefore, it is a more critical issue to remove the weld tumor of stainless steel plasma arc cutting.
When cutting copper, aluminum and their alloys, due to their good thermal conductivity, the bottom of the incision is not easy to re-fusion with molten metal. Although these fuses are "hanging" under the incision, they are easy to remove. When the plasma arc cutting process is used, the specific measures to remove the weld tumor are as follows:
(1) Ensure the concentricity of the tungsten electrode and the nozzle. Poor alignment of the tungsten electrode and the nozzle will cause the symmetry of the gas and the arc to be destroyed so that the plasma arc cannot be compressed well or the arc is blown, the cutting ability is reduced, the incision is asymmetrical, and the welded edge increases. Causes double arcs, so that the cutting process cannot proceed smoothly.
(2) Ensure that the plasma arc has sufficient power. The plasma arc power increases, that is, the plasma arc energy increases, and the arc column elongates so that the temperature of the molten metal during the cutting process is increased and the fluidity is good. At this time, under the action of the high-speed airflow blowing force, the molten metal is easily blown off, increasing the arc The column power can increase the cutting speed and the stability of the cutting process, making it possible to use larger airflow to increase the blowing force of the airflow, which is very beneficial to eliminate the incision weld.
(3) Choose the appropriate gas flow and cutting speed. If the gas flow is too small, the blowing force is not enough, and it is easy to produce melted nodules. When other conditions remain unchanged, as the gas flow rate increases, the quality of the incision is improved, and an incision without fusibility can be obtained. However, an excessive gas flow rate will cause the plasma arc to become shorter and make the melting ability of the plasma arc to the lower part of the workpiece worse. , The drag after slitting increases, and the incision is V-shaped, but it is easy to form a fusion tumor.
3. Avoid double arc
The occurrence of the double arc phenomenon of the transferred plasma arc is related to the specific process conditions. In plasma arc cutting, the existence of double arcs will inevitably lead to rapid nozzle burning. The lighter changes the geometry of the nozzle channel destroys the stable conditions of the arc, and affects the cutting quality; the severer causes the nozzle to be burned and leaks, forcing the cutting process to be interrupted. . For this reason, plasma arc cutting is the same as plasma arc welding. The factors that affect the formation of double arcs must be taken to avoid the appearance of double arcs.
4. Large thickness cutting quality
In the production, the plasma arc can be used to cut stainless steel with a thickness of 100-200mm. To ensure the cutting quality of large thickness plates, the following process characteristics should be noted.
(1) As the cutting thickness increases, the amount of metal to be melted also increases, so the required plasma arc power is relatively large. When cutting plates with a thickness of 80mm or more, the plasma arc power is 50-100kW. To reduce the burning loss of the nozzle and the tungsten electrode, it is advisable to increase the cutting voltage of the plasma arc at the same power. For this reason, the no-load voltage of the cutting power supply is required to be above 220V.
(2) The plasma arc is required to have a slender shape, good stiffness, and a long-distance for the arc column to maintain a high temperature. That is, the axial temperature gradient should be small, and the temperature distribution on the arc column should be uniform. In this way, the bottom of the incision can get enough heat to ensure the cut through. It is better to use nitrogen and hydrogen mixed gas with larger enthalpy value and high thermal conductivity.
(3) When arcing, due to the large current sudden change, it will often cause arc interruption and nozzle burnout during the arcing process. Therefore, the equipment is required to adopt the method of increasing the current arc or graded arc. Generally, a current-limiting resistor (about 0.4Ω) can be connected in series in the cutting circuit to reduce the current value when turning the arc, and then short-circuit the resistor.
(4) Preheating is required at the beginning of cutting, and the preheating time is determined according to the properties and thickness of the material being cut. For stainless steel, when the thickness of the workpiece is 200mm, it needs to be preheated for 8-20s; when the thickness of the workpiece is 50mm, it needs to be preheated for 2.5 to 3.5s. After the large-thickness workpiece is cut, it is necessary to wait until the workpiece is cut through in the thickness direction before moving the cutting torch to achieve continuous cutting, otherwise, the workpiece will not be cut through. In the end, the arc must be completely cut off.