Die casting molds are a large class in the mold. With the rapid development of China's automobile and motorcycle industry, the die-casting industry has ushered in a new era of development. At the same time, it also puts forward higher requirements on the comprehensive mechanical properties and life of the die-casting mold. To meet the ever-increasing performance requirements, it is still difficult to meet the application of new mold materials. Various surface treatment techniques must be applied to the surface treatment of die-casting molds to achieve high efficiency, high precision and long life of die-casting molds. . In various molds, the working conditions of the die-casting mold are more demanding. Pressure Casting is to make the molten metal fill the mold cavity under high pressure and high speed and die-casting. It is repeatedly in contact with the hot metal during the working process. Therefore, the die-casting mold is required to have high heat fatigue resistance, thermal conductivity wear resistance and corrosion resistance. , impact toughness, red hardness, good release properties, etc. Therefore, the surface treatment technology of die-casting molds is relatively high. In recent years, various new techniques for surface treatment of die-casting molds have emerged, but in general they can be divided into the following three categories: (1) Improvement techniques of traditional heat treatment processes; (2) Surface modification technology, including surface thermal diffusion treatment, surface phase transformation strengthening, electric spark strengthening technology, etc.; (3) coating technology, including electroless plating.
1. Improved technology of traditional heat treatment process
The traditional heat treatment process for die-casting molds is quenching-tempering, and surface treatment techniques have been developed in the future. Because of the variety of materials that can be used as die-casting molds, the same surface treatment techniques and process applications can have different effects on different materials. Schkov recently proposed a substrate pretreatment technology for mold substrate and surface treatment technology. Based on the traditional process, a suitable processing technology is proposed for different mold materials to improve mold performance and improve mold life. Another development direction of heat treatment technology improvement is to combine the traditional heat treatment process with advanced surface treatment technology to improve the service life of the die casting mold. For example, carbonitriding by chemical heat treatment, combined with conventional quenching and tempering processes, NQN (ie, carbonitriding-quenching-carbonitriding) composite strengthening, not only obtains high surface hardness, but also effectively hardens. The layer depth is increased, the hardness gradient distribution of the layer is reasonable, the tempering stability and the corrosion resistance are improved, so that the surface quality and performance of the die-casting mold are greatly improved while obtaining good core performance.
2, surface modification technology
2.1. Surface thermal diffusion technology This type includes carburizing, nitriding, boronizing, carbonitriding, and sulfur-carbonitriding.
2.2, carburizing and carbonitriding carburizing process applied to cold, hot work and plastic mold surface strengthening, can improve the life of the mold. For example, 3Cr2W8V steel die-casting mold, first carburized, then quenched at 1140 ~ 1150 °C, tempered twice at 550 °C, surface hardness can reach HRC56 ~ 61, so that the die life of die-casting non-ferrous metals and their alloys increased by 1. 8 ~ 3.0 times. When carburizing is carried out, the main processes are solid powder carburizing, gas carburizing, vacuum carburizing, ion carburizing, and carbonitriding formed by adding nitrogen in a carburizing atmosphere. Among them, vacuum carburizing and ion carburizing are technologies developed in the past 20 years. The technology has the characteristics of fast percolation speed, uniform permeability, gentle carbon concentration gradient and small deformation of the workpiece, which will be on the mold surface, especially the precision mold. Surface treatment plays an increasingly important role.
2.3, nitriding and related low-temperature thermal expansion technology This type includes nitriding, ion nitriding, carbonitriding, oxygen-nitrogen osmosis, sulfur-nitrogen osmosis and ternary osmosis of sulfur, carbon, nitrogen, oxygen, nitrogen and sulfur And other methods. These methods are simple in process, strong in adaptability, low infiltration temperature (generally 480-600 ° C), small deformation of the workpiece, especially suitable for surface strengthening of precision molds, and high hardness and wear resistance of the nitride layer. Good anti-sticking performance. 3Cr2W8V steel die-casting mold, after quenching and tempering, nitriding at 520 ~ 540 °C, the service life is 2 to 3 times higher than that of non-nitriding mold.
In the United States, the die-casting mold made of H13 steel is subjected to nitriding treatment, and tempering is used instead of tempering. The surface hardness is as high as HRC65-70, and the hardness of the core of the mold is low and the toughness is good, so that excellent synthesis is obtained. Mechanical properties. The nitriding process is a common process for the surface treatment of die-casting molds. However, when a thin and brittle white bright layer is present in the nitrided layer, it cannot resist the effect of alternating thermal stress, and it is easy to generate microcracks and reduce thermal fatigue resistance. Therefore, in the nitriding process, the process must be strictly controlled to avoid the generation of a brittle layer. Recently, foreign countries have proposed the use of secondary and multiple nitriding processes. The repeated nitriding method can decompose the nitride white bright layer which is easy to generate microcracks during the service process, increase the thickness of the nitriding layer, and at the same time, the surface of the mold has a thick residual stress layer, so that the life of the mold can be significantly improved. In addition, there are methods such as salt bath carbonitriding and salt bath sulfur-nitrogen carbonitriding. These processes are widely used in foreign countries and are more common in China.
Rare. For example, the TFI+ABI process is immersed in a basic oxidizing salt bath after nitrocarburizing in a salt bath. The surface of the workpiece is oxidized and black, and its wear resistance, corrosion resistance and heat resistance are improved. The life of the aluminum alloy die-casting mold treated by this method is increased by several hundred hours. Another example is the oxynit process for nitriding after sulphur-nitrogen carbon co-infiltration developed in France. It is more suitable for non-Ferrous Metal die-casting molds.
2.4, boronizing due to the high hardness of the boronized layer (FeB: HV1800 ~ 2300, Fe2B: HV1300 ~ 1500), wear resistance and red hardness, as well as certain corrosion resistance and anti-adhesion, boronizing technology in the mold industry Get better application results. However, because the working conditions of the die-casting mold are very harsh, the boronizing process is less applied to the surface treatment of the die-casting mold. However, in recent years, an improved boronizing method has appeared, which solves the above problems and can be applied to the surface treatment of the die-casting mold. Such as multi-component, powder coating and so on. The method of boronizing the coating powder is to mix the boron compound and other infiltrating agent and apply it on the surface of the die-casting mold. After the liquid is volatilized, it is sealed and sealed according to the general powder boronizing method, and heated at 920 ° C for 8 hours. Air cold. This method can obtain a dense and uniform infiltration layer, and the hardness, wear resistance and bending strength of the surface of the mold are improved, and the service life of the mold is increased by more than 2 times on average.
2.5. Rare Earth Surface Strengthening In recent years, the method of adding rare earth elements in the surface strengthening of molds has been widely praised. This is because rare earth elements have many functions such as increasing the osmotic speed, strengthening the surface and purifying the surface. [13] Wear-resistant electrode, which has a great influence on improving the surface structure, surface physical, chemical and mechanical properties of the mold, and can improve the seepage. Speed, strengthen the surface, and form rare earth compounds. At the same time, it can eliminate the harmful effects of trace impurities distributed on the grain boundary, and play the role of strengthening and stabilizing the grain boundary of the mold cavity surface. In addition, the rare earth element acts on harmful elements in the steel to form a high melting point compound, and also suppresses the segregation of these harmful elements at the grain boundary, thereby reducing the brittleness of the deep layer and the like. The addition of rare earth elements in the surface strengthening treatment process of the die-casting mold can significantly improve the thickness of the infiltration layer and increase the surface hardness, and at the same time make the microstructure of the infiltration layer finely dispersed and the hardness gradient decreases, thereby making the mold wear resistance and resistance. The cold and hot fatigue properties are significantly improved, which greatly increases the life of the mold. At present, the treatment methods applied to the cavity surface of the die-casting mold include: rare earth carbon co-infiltration, rare earth carbonitriding, rare earth boron co-infiltration, rare earth boron-aluminum co-infiltration, rare earth soft nitriding, rare earth sulfur-nitrogen carbonitriding.
2.6. Laser surface treatment Laser surface treatment is to use a laser beam to heat the surface of the workpiece to rapidly melt a thin layer of a certain depth. At the same time, the alloying elements are applied to the surface of the workpiece by vacuum evaporation, electroplating, ion implantation, etc. It is fully fused with the base metal, and after condensation, an alloy layer having a special property of 10 to 1000 μm is obtained on the surface of the mold, and the cooling rate is equivalent to chill quenching. For example, the surface of H13 steel is treated by laser rapid melting process. The melting zone has high hardness and good thermal stability, and has high resistance to plastic deformation, which has obvious inhibitory effect on the initiation and propagation of fatigue cracks.
Recently, Saha and Dahot used laser cladding of the VC layer on the H13 substrate. Studies have shown that the surface of the obtained mold is essentially a continuous, dense and non-porous VC steel composite coating, which is not only very strong. The oxidation resistance at 600 ° C, and has a strong ability to resist the reduction of molten metal [19]. 23 EDM cermet process In the continuous development of surface modification technology, an EDM process has emerged. Under the action of electric field, the process generates instantaneous high temperature and high pressure zones on the surface of the base metal, and simultaneously infiltrates the ionic cermet material to form metallurgical bonding of the surface, and the surface of the base metal also undergoes instantaneous phase transformation to form martensite and fine. Austenitic organization [20]. This process differs from soldering, as well as by sputtering or elemental infiltration, and should be a process in between. It makes good use of the high wear resistance, high temperature resistance and corrosion resistance of cermet materials, and the process is simple and the cost is low. It is a new way of surface treatment of die-casting molds.
3, coating technology
As a kind of mold strengthening technology, the coating technology is mainly applied to the surface treatment of molds with relatively simple working conditions such as plastic molds, glass molds, rubber molds and stamping molds. Die-casting molds need to withstand the harsh environment where the thermal and thermal stresses alternate, so coating technology is generally not used to strengthen the surface of the die-casting mold. However, in recent years, it has been reported that the surface of the die-casting mold is strengthened by a chemical composite plating method to improve the anti-adhesion and mold release property of the mold surface. The method infiltrates the polytetrafluoroethylene particles on an aluminum-based die-casting mold and then performs (NiP)-polytetrafluoroethylene composite plating. Experiments prove that this method is working
4, both artistic and performance are feasible, greatly reducing the friction coefficient of the mold surface.
Conclusion
Mold press processing is an important part of mechanical manufacturing, and the level, quality and longevity of the mold are related to mold surface strengthening technology. With the advancement of science and technology, various mold surface treatment technologies have made great progress in recent years. It is manifested in: 1 improvement of traditional heat treatment process and its combination with other new processes; 2 surface modification technology, including carburizing, low temperature thermal diffusion (various nitriding, carbonitriding, ion nitriding, ternary Co-infiltration, etc., salt bath thermal diffusion, boronizing, rare earth surface strengthening, laser surface treatment and EDM cermet; 3 coating technology. However, for the die-casting molds with extremely harsh working conditions, the existing new surface treatment process can not meet the ever-increasing requirements, and more advanced technologies can be expected, and it is expected to be applied to the surface treatment of the die-casting molds. In view of the fact that surface treatment is one of the important means to improve the life of die-casting molds, surface treatment technology will play an important role in improving the overall level of die-casting mold production in China.
