"Drops of water pierce the stone" reflects the potential power of water itself in people's eyes. However, as an independent and complete processing technology, high-pressure water jet and abrasive water jet have only been produced in recent 30 years. The use of high-pressure water to serve people's production began in the 1970s, when it was used to mine gold and peel bark. Until World War II, the radar cabin was eroded by rain during the operation of the aircraft. It was not until 1950s that the possibility of high-pressure water jet cutting originated in the Soviet Union, but the first patent of cutting technology was produced in the United States, that is, 1968 was obtained by Dr. Norman Franz, a professor of forestry at the University of Missouri. In recent ten years, great progress has been made in water jet (wj, awj) cutting technology and equipment, and its application covers all aspects of industrial production and people's life. Many universities, companies and factories are competing for research and development, and new ideas, theories and technologies are constantly emerging, forming a momentum of catching up. At present, more than 3,000 sets of water jet cutting equipment have been applied in dozens of industries in dozens of countries, especially in high-tech, sophisticated and difficult technical fields such as aerospace, shipbuilding, military industry and nuclear energy. It can cut more than 500 kinds of materials, and the annual growth rate of its equipment exceeds 20%.
2. Composition and pressurization principle of high-pressure water jet machining system.
The basic principle of high-pressure water jet is: using the principle of liquid pressurization, the mechanical energy of power source (motor) is converted into pressure energy through a specific device (pressurization port or high-pressure pump), and the water with great pressure energy passes through a small hole nozzle (another energy conversion device), and then the pressure energy is converted into kinetic energy, thus forming a high-speed jet (wj). So it is often called high-speed water jet.
As shown in figure 1, the high-pressure water jet system is mainly composed of pressurization system, water supply system, pressurization and constant pressure system, nozzle pipeline system, numerical control workbench system, water collection system and water circulation treatment system. The low pressure oil (10 ~ 30 MPa) in the oil pressure system pushes the big piston to move back and forth, and its direction is automatically controlled by the reversing valve. The water supply system first purifies the water and adds antirust additives. Then the low-pressure water is pumped by the water supply pump and enters the high-pressure cylinder from the one-way valve. The pressurization and constant pressure system consists of supercharger and accumulator. The principle of obtaining high pressure by supercharger is shown in Figure 2, that is, it is realized by using the area difference between large piston and small piston. Theoretically, large P oil = small P water, and P water = large/small P oil. The pressure increasing ratio is the ratio of the area of the big piston to the small piston, which is usually 10: 1 ~ 25. Because the compression ratio of water reaches 12% at 400mpa, the piston rod will not output high-pressure water until one-eighth of the total stroke. When the piston reaches the end of the stroke, the reversing valve automatically changes the direction of the oil path (as shown by the dotted arrow in the figure), thus pushing the big piston to travel in the opposite direction, and at this time, the other end outputs high-pressure water. If this high-pressure water is directly sent to the nozzle, the injection pressure from the nozzle will be pulsating (dotted line in fig. 3), and this will periodically oscillate the piping system. In order to obtain a stable high-pressure water jet, an energy storage (constant pressure) device is often installed between the supercharger and the nozzle loop to eliminate the water pressure pulsation and achieve the purpose of constant pressure, and the pulsation can often be controlled within 5% (solid line in Figure 3).
Cutting and cutting mechanism
High-speed jet has high rigidity, which produces extremely high impact dynamic pressure (p=ρvc) and vortex formation when it collides with the target. Microscopically, there are ultra-high speed zone and low speed zone (sometimes negative) relative to the average jet velocity, so although the high-pressure water jet is a cylindrical model on the surface, there are actually high and low stiffness parts inside, and the impact dynamic pressure generated by the high stiffness part reduces the propagation time and increases the impact strength. Macroscopically, it acts as a quick wedge, while the low-rigidity part forms a flexible space relative to the high-rigidity part, which plays the role of chip absorption and chip discharge. The combination of the two seems to only make it work like an axial "saw knife" when cutting materials.
The damage process of high-speed water jet to materials is a dynamic fracture process, and brittle materials (such as rocks) are mainly crack damage and diffusion. For plastic materials, it conforms to the instantaneous fracture criterion of maximum tensile stress, that is, once the normal tensile stress of a point in the material reaches or exceeds a critical value σy, the point will fracture. According to elastic-plastic mechanics, the dynamic fracture strength is about one order of magnitude higher than the static fracture strength, mainly because the dynamic stress action time is short and the cracks in the material have no time to develop, so this dynamic fracture is not only related to the stress, but also related to the tensile stress action time.
4. Application scope of high-pressure water jet cutting
High-pressure water jet cutting is cutting by high-speed jet with high kinetic energy (sometimes called high-speed water jet machining), which belongs to the category of high-energy beam machining like laser, ion beam and electron beam. As a high-tech, high-pressure water jet cutting is a revolution in the cutting field in a sense, which has a very broad application prospect. With the maturity of technology and the overcoming of some limitations, it is a perfect complement to other cutting technologies. At present, its uses and advantages are mainly reflected in difficult-to-machine materials, such as ceramics, cemented carbide, high-speed steel, die steel, quenched steel, white cast iron, tungsten-molybdenum-cobalt alloy, heat-resistant alloy, titanium alloy, corrosion-resistant alloy and composite materials (frm, frp, etc. ), calcined ceramics, high-speed steel (below hrc30), stainless steel, high manganese steel, die steel and martensitic steel (HRC-30), high-silicon cast iron, malleable cast iron and other general engineering materials. Besides cutting, high-pressure water jet can also be used for cleaning, crushing, surface roughening and strengthening by slightly reducing the pressure or increasing the target distance and flow. In the United States, almost all automobile and aircraft manufacturers have applications. At present, it has been successfully used in the following industries: automobile manufacturing and repair, aerospace, machining, national defense, military industry, weapons, electronic power, petroleum, mining, light industry, building materials, nuclear industry, chemical industry, shipbuilding, food, medical care, forestry, agriculture, municipal engineering and so on.
2. Laser cutting is to scan the surface of material with high power density laser beam, heat the material to thousands to tens of thousands of degrees Celsius in a very short time to melt or vaporize the material, and then blow the melted or vaporized material away from the slit with high pressure gas to achieve the purpose of cutting the material.