The composite stamping involved in this paper does not refer to the combination of blanking, drawing and punching, but refers to the combination of stamping technology and other processing technologies, such as stamping and electromagnetic forming, stamping and cold forging, stamping and machining.
Composite technology of stamping and electromagnetic forming
Electromagnetic forming is high-speed forming, which can not only expand the forming range of aluminum alloy, but also improve its formability. The specific method of composite stamping forming of aluminum alloy covering parts is: installing electromagnetic coils at the sharp corners and difficult-to-form contours of aluminum alloy covering parts with a set of convex and concave dies, forming by electromagnetic method, forming the easy-to-form parts of covering parts with a pair of dies on a press, and then deforming the preformed parts with electromagnetic coils at high speed to complete the final forming. Practice has proved that aluminum alloy plate which is difficult to obtain by single stamping method can be obtained by this composite forming method.
The latest research shows that magnesium alloy is a metal with high specific strength, good rigidity and strong electromagnetic interface protection performance, and its application prospect in electronics, automobiles and other industries is very promising, and it has a great tendency to replace traditional ferroalloys, aluminum alloys and even plastic materials. At present, magnesium alloy parts used in automobiles include instrument chassis, seat frame, engine hood and so on. Magnesium alloy tubes are also widely used in cutting-edge industrial fields such as airplanes, missiles and spacecraft. However, the close-packed hexagonal lattice structure of magnesium alloy determines that it cannot be formed by stamping at room temperature. Now people have developed a die which integrates heating and forming to punch magnesium alloy products. The forming process of the product is as follows: in the process of lowering the slider of the punch press, the upper die and the lower die clamp and heat the material, and then the material is formed in a proper motion mode.
This method is also suitable for the connection of molded products and the composite molding of various products in the punch. Many difficult-to-form materials, such as magnesium alloy, titanium alloy and other products, can be formed by stamping in this way. Because this kind of stamping requires the slider of the punch to have the function of stopping in the descending process, so as to provide time for heating materials, people have developed a new concept of punch-CNC crankshaft servo motor punch, which can also realize compound processing including tapping and riveting in the stamping die, thus effectively expanding the stamping range and laying a solid foundation for the wide application of magnesium alloys in the plastic processing industry.
Combination of stamping and cold forging
Generally, sheet metal stamping can only form parts with equal wall thickness, and at most, thin-walled parts with bottom thickness can be drawn by thinning. The limitation of stamping limits its application range. However, in the production of automobile parts, some thin-walled parts with different wall thicknesses are often encountered, and it is easy to form by using a single composite plastic forming method combining stamping and cold forging. Therefore, the combination of stamping and cold forging can expand the range of sheet metal processing. Its method is preformed by stamping and finally formed by cold forging. The advantages of stamping and cold forging composite plastic forming are: first, raw materials are easy to be purchased cheaply, which can reduce production costs; The second is to reduce the forming force required for single cold forging, which is beneficial to improve the die life. The micro-machining we are talking about now refers to the micro-part processing technology. The definition of micro-parts usually means that the size in at least one direction is less than 100μ m, which has incomparable application prospects compared with conventional manufacturing technology. Micro robots, micro airplanes, micro satellites, satellite gyroscopes, micro pumps, micro instruments, micro sensors, integrated circuits, etc. The products made by this technology have excellent applications in many fields of modern science and technology. It can bring new expansion and breakthrough to many fields, and will undoubtedly have a far-reaching impact on China's future science and technology and national defense, and its role in promoting the development of science and technology in the world is immeasurable. For example, micro-robot can complete the leading, bonding and docking of optical fibers, the detection of small pipes and circuits, and also complete complex operations such as the production and assembly of integrated chips, so it is not difficult to see the attractive charm of micro-processing.
Developed industrial countries attach great importance to the research and development of micro-machining and have invested a lot of manpower, material resources and financial resources. Some well-known universities and companies with foresight have also joined the ranks. China has also done a lot of research work in this field, and it is reasonable to think that micromachining will bring great changes and far-reaching influences to the whole world like microelectronics technology in the 2 1 century.
For the mold industry, due to the miniaturization of stamping parts and the continuous improvement of precision requirements, higher requirements are put forward for mold technology. The reason is that micro parts are more difficult to form than traditional parts. The reasons are as follows: ① The smaller the parts, the faster the surface area to volume ratio increases; ② The adhesion and surface tension between the workpiece and the tool are significantly increased; (3) the particle size has a significant influence, and it is no longer an isotropic homogeneous continuum; ④ It is relatively difficult to store lubricant on the surface of workpiece. An important aspect of micro-stamping is punching small holes. For example, there are many small holes that need to be punched in micro-machinery and micro-instruments. Therefore, the study of small hole stamping should be an extremely important topic in micro-stamping. The research on punching holes mainly focuses on: first, how to reduce the size of punch; The second is how to increase the strength and rigidity of the micro-punch (this aspect not only involves materials and processing technology, but also involves the guidance and protection of the micro-punch). Although there are still many problems to be studied in drilling small holes, many gratifying results have been achieved. According to some data, the micro-stamping machine tool developed abroad is 1 1mm long, 62mm wide and 170mm high, equipped with AC servo motor, which can generate 3kN pressure. The press is equipped with a continuous stamping die, which can realize punching and bending.
The University of Tokyo, Japan used a WFDG technology to make micro-stamping punches and dies. With this die, 40μ m wide micropores can be punched on a 50μ m thick polyamide plastic plate. Tsinghua University has made a good start in deep drawing of ultra-thin wall metal cylindrical parts. The key of ultra-thin wall drawing technology is to have a high-precision forming machine. They developed a precision forming testing machine with microcomputer control function in the forming of ultra-thin wall metal tube with a wall thickness of 0.00 1 mm ~ 0. 1 mm, which enabled the centering accuracy of punch and die to reach 1μ m during the machining process, effectively solving the problem that the ultra-thin wall is prone to wrinkling and cracking and cannot be operated normally. Brass and pure aluminum with initial wall thickness of 0.3mm were thinned and deeply processed by this machine tool, and a series of ultra-thin wall metal cylinders with inner diameter of 16mm, wall thickness of 0.0 15 mm ~ 0.08 mm and length of 30mm were processed. After testing, the wall thickness difference of the formed ultra-thin wall tube is less than 2μ m, and the surface roughness ra is Ra0.057μ m, which greatly improves the accuracy of the ultra-thin wall tube instrument and correspondingly improves the performance of the whole instrument. Green manufacturing is a modern manufacturing mode that comprehensively considers environmental impact and resource efficiency, so is green stamping, which is essentially the concrete embodiment of human sustainable development strategy in modern stamping. It should be included in the mold design, manufacture, maintenance and production application.
1, green design The so-called green design refers to integrating measures such as environmental protection and reducing resource consumption into product design at the mold design stage, and taking disassembly, recyclability and manufacturability as design objectives in parallel to ensure product function, quality life and economy. With the development of die industry, the requirements for sheet metal forming quality and die design efficiency are getting higher and higher, and the traditional empirical design method can no longer adapt to the development of modern industry. In recent years, computer simulation of sheet metal forming process by finite element method is a revolution in the field of die design. The distribution of displacement, stress and strain in the forming process can be obtained by computer numerical simulation. By observing the deformed shape of the workpiece after displacement, the possible wrinkling can be predicted. According to the position of principal strain at discrete points on the limit curve of sheet metal forming, or using the damage mechanics model, the possible fracture in the forming process can be predicted. By removing the binding force or part of the external force, the springback process can be simulated and the shape and residual stress distribution of the workpiece after springback can be obtained. All these provide scientific basis for optimizing stamping process and die design, which is a real green die design.
2 green manufacturing in mold manufacturing, green manufacturing should be adopted. At present, there is a laser remanufacturing technology, which uses suitable alloy powder as material, and uses computer-controlled laser head to repair the mold with the support of CAD/CAM software with parts prototype. The specific process is that when the powder feeder and the processing machine move according to the specified spatial trajectory, the beam radiation is synchronized with the powder feeding, so that the repaired parts are gradually deposited, and finally a three-dimensional solid similar to the prototype parts is generated, and its performance can reach or even exceed the level of the original substrate. This method is most widely used in die repair, especially in die repair of panels. Because this technology does not aim at consuming a lot of natural resources, it is called green manufacturing. In addition, in stamping production, it is necessary to minimize the stamping process waste and structural waste, maximize the use of materials, and minimize the generation of waste. Reducing process waste is solved by optimizing the layout, such as adopting double row and cross row, and adopting the method of less waste and no waste, thus greatly improving the utilization rate of materials. The so-called optimal layout is to solve two problems: first, how to express it as a mathematical model; Secondly, how to find the optimal solution as soon as possible according to the mathematical model is the key. Modern optimization technology has developed into intelligent optimization algorithm, which mainly includes artificial neural network, genetic algorithm, simulated annealing, tabu search and so on. It is believed that there will be a breakthrough in optimizing layout. For the workpiece with more structural waste, layout method can be used to cut, so as to realize the utilization of waste and turn waste into treasure.
In addition, it is not completely impossible to solve the problem by changing the product structure. For sleeve cutting, we all know that the gasket in sleeve cutting has a big gasket and the small gasket in sleeve cutting has a middle gasket. Nowadays, high-strength steel and ultra-high-strength steel have realized the lightweight of vehicles and improved the collision strength and safety performance of vehicles, so they have become an important development direction of automotive steel. However, with the improvement of plate strength, the traditional cold stamping process is easy to crack in the forming process, which can not meet the processing requirements of high-strength steel plate. At present, under the condition that the forming conditions cannot be met, the hot stamping technology of ultra-high strength steel plate is being studied step by step internationally. This technology is a new technology integrating shape, heat transfer and microstructure transformation. It is mainly a forming process by using the characteristics of increasing plasticity and decreasing yield strength of metal plates in high temperature austenite state. However, hot forming requires in-depth study of process conditions, metal phase transformation and CAE analysis technology. At present, this technology is monopolized by foreign manufacturers, and its domestic development is slow.
When the material is formed by stamping, it will harden. Different steels have different degrees of hardening. Generally, high-strength low-alloy steel only slightly increases by 3 KSI, which is less than 10%. Note: the yield strength of dual-phase steel is increased by 20KSI, exceeding 40%! During the forming process, the metal will become completely different, unlike before the stamping process. The yield strength of these steels has increased a lot after being stressed. The higher yield stress and work hardening of materials are equivalent to a substantial increase in flow stress. -This will lead to the need for more tonnage to manufacture parts-This will increase the deformation temperature of metals (which may burn or destroy unsuitable lubricants), and hard spots will increase the wear of molds-The coating may not be helpful or last as long as expected. To sum up, the high pressure requirement of high-strength steel forming, the increase of springback, the improvement of working hardness and the operation at high forming temperature all pose challenges to die and lubrication.
In the past, when producing deep drawing or heavy stamping workpieces, everyone thought that ep lubricating oil was the best choice to protect the mold. It has a long history to mix sulfur and chlorine extreme pressure additives into pure oil to improve the service life of dies. However, with the appearance of new metal-high strength steel and strict environmental protection requirements, the value of EP oil-based lubricating oil has been reduced and even lost the market.
EP oil-based lubricating oil loses its performance when forming high-strength steel at high temperature, so it cannot provide physical mold protection diaphragm in extreme temperature applications. IRMCO high solid polymer lubricant at extreme temperature can provide necessary protection. With the deformation of metal in stamping die and the increase of temperature, EP oil-based lubricating oil will become thinner, and in some cases it will reach flash point or burn (smoke). The consistency of IRMCO polymer lubricant is usually much lower when spraying. With the increase of temperature in the molding process, it will become thicker and tougher. In fact, polymer extreme temperature lubricants are "heat-seeking" and will stick to metals, forming a diaphragm that can reduce friction. This protective barrier allows the workpiece to extend, and there is no fracture and adhesion in the most demanding workpiece forming process, thus controlling friction and metal flow. Effectively protect the mold, prolong the service life of the mold and improve the stamping strength.