Sheet Metal Bending is one of the most common process methods in actual industrial production, which usually needs die to complete. But the mold design cycle is long and the manufacturing cost is high. For some complex shapes, high precision requirements, small batch products, not economic, and difficult to ensure quality and construction period. The thin plate flexible forming technology developed in recent years has been paid more and more attention by many experts and scholars at home and abroad because it uses thermal stress instead of mechanical force to form the workpiece and does not use mold, which greatly reduces the processing cost and saves the processing time. It is suitable for small batch production of single pieces and online repair.
The advantages of plasma laser forming are energy concentration and high precision. But there are the following problems: laser processing equipment cost and operation cost is high; laser equipment energy conversion rate is low, energy loss is large; for many metal and alloy materials, surface pretreatment is required to increase the energy absorption rate. This limits the flexibility of laser bending forming application to some extent.
Therefore, from the economic point of view, laser forming technology is more suitable for the forming of small and medium-sized parts or precision parts, which limits the forming application of large parts or thick plates. These limitations are essentially caused by the laser beam acting as a heat source, so these problems can be solved by finding alternative heat sources. It is found that non-transfer plasma arc can meet the basic requirements of heat source for thin plate flexible forming. Plasma arc has high energy density and good controllability.
The energy conversion rate of plasma arc heating is about 85%, much higher than that of laser 5%-10%. The cost of plasma arc machining is only 5%~10% of that of laser machining. Moreover, in the process of plasma arc machining, the condition of thin plate surface has little influence on the energy absorption, so compared with laser machining, there is no need for pre-coating, and the energy absorption of thin plate can be controlled more accurately. All these show that plasma arc has advantages as a heat source for forming metal plate.
Next, we will focus on analyzing the bending mechanism of plasma arc.
When a material is unevenly heated, thermal stress will develop inside the material. If the thermal stress exceeds the yield limit of the material, permanent plastic deformation may occur. Plasma arc flexible forming technology is a method to realize forming by using plasma arc to scan the sheet surface and control the thermal stress generated in the uneven temperature field along the thickness direction.
The flexible forming of plasma arc is a very complicated process of elastic-plastic deformation and its machining mechanism is very complex. It is generally believed that there are two basic forms of plasma arc flexible forming: forward bending and reverse bending.
Forward bending consists of two processes: heating and cooling. During the heating process, the plasma arc with high energy density acts on the plate to be bent, making the material temperature on the upper surface of the affected part rise sharply in a short time. However, since the material far away from the upper surface is not directly irradiated, the nearby temperature does not change much in this short period of time, so that the affected part forms a large temperature gradient in the direction of plate thickness. Due to the high temperature, large thermal expansion and low yield limit of the materials in the above table, the compression plastic deformation is not uniform and the formation of material stacking: the lower surface material has low temperature and high yield limit, and basically does not produce or only produces small compression plastic deformation: therefore, the heating result makes the plate bend in the opposite direction of the plasma arc.