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.
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 mechanical function and life of the integrated casting die. It is still difficult to meet the needs of the continuous use of new mold materials. It is necessary to apply various surface treatment techniques to the surface treatment of die-casting molds in order to achieve high efficiency, high precision and longevity of die-casting molds. . Among various molds, the working conditions of the die-casting mold are relatively harsh. Pressure forging is to make the molten metal fill the mold cavity under high pressure and high speed and die-casting. It is repeatedly contacted with hot metal during the working process. Therefore, the die-casting mold is required to have high heat-resistance, thermal conductivity, wear resistance and corrosion resistance. Properties, 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.
Improvements in traditional heat treatment processes Traditional heat treatment processes for die casting molds are quenching-tempering, and surface treatment techniques have been developed in the future. Because the materials available as die-casting molds are varied, 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 the mold function and improve the mold life. Another development direction of heat treatment technology improvement is to combine the traditional heat treatment process with the improvement of the surface treatment process of the predecessors 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 depth of the layer is increased, the hardness gradient of the layer is fair, the tempering is not disordered, and the corrosion resistance is improved, so that the die-casting mold achieves good core function while the surface quality and function are greatly improved.
Surface technology surface thermal diffusion technology
This type includes carburizing, nitriding, boronizing, carbonitriding, and sulfur-carbonitriding.
Carburizing and carbonitriding
The carburizing process is applied to cold, hot work and plastic mold surface strengthening to improve mold life. For example, 3Cr2W8V steel die-casting mold, first carburized, then quenched by 1140 ~ 1150 °C, tempered twice at 550 °C, the surface hardness can reach HRC56 ~ 61, so that the life of die-casting non-ferrous metals and their alloys is 1.8 to 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, average layer thickness, gentle carbon concentration gradient and small deformation of the workpiece, which will be on the mold surface, especially the precision mold. The role of surface treatment is playing an increasingly important role.
Nitriding and related low temperature thermal expansion technology
This type includes nitriding, ion nitriding, carbonitriding, oxygen-nitrogen osmosis, sulfur-nitrogen osmosis, and sulfur-carbon nitrogen, oxygen-nitrogen ternary osmosis. 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 function. 3Cr2W8V steel die-casting mold, after quenching and tempering, nitriding at 520 ~ 540 °C, the service life is 2 to 3 times better than the non-nitriding mold. In the United States, the die-casting mold made of H13 steel has to be nitrided, and the tempering is replaced by nitriding. 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, thus obtaining excellent comprehensive Coordinated mechanical function. The nitriding process is a commonly used process for the surface treatment of die-casting molds. However, when the nitrided layer has a thin and brittle white bright layer, it cannot withstand the effect of alternating thermal stress, and it is easy to generate micro-cracks and reduce the heat 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 easily generates 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 abroad and are rare in the sea. 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 has a black color, and its wear resistance, corrosion resistance and heat resistance are all improved. The life of the aluminum alloy die-casting mold treated by this method is improved 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.
Boronizing
Because of 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 is better in the mold industry. Apply the effect. However, because the working conditions of die-casting molds are very demanding, the boronizing process is less used in the surface treatment of die-casting molds. However, in recent years, an improved boronizing method has been developed to solve the above problems, and it has been applied to the surface treatment of die-casting molds. 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 average 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 uniformly improved by more than 2 times.
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 improving the rate of permeation, strengthening the surface and purifying the surface. It has a great influence on improving the surface structure of the mold, surface physical, chemical and mechanical functions, and can improve the seepage speed and strengthening. Surface, natural rare earth compound. At the same time, the harmful effects of trace impurities distributed on the grain boundary can be eliminated, and the effect of strengthening and not arranging the grain boundary of the mold cavity surface is played. In addition, rare earth elements interact with harmful elements in steel, and natural high melting point compounds can be segregated at the grain boundaries according to these harmful elements, thereby reducing the brittleness of the deep layers. The addition of rare earth element components in the surface strengthening treatment process of the die-casting mold can significantly improve the thickness of the infiltration layer and the surface hardness of various infiltration methods, and at the same time, the microstructure of the infiltrated layer is finely dispersed and the hardness gradient is lowered, thereby making the mold wear resistance and resistance. The cold and hot exhaustion functions have been significantly improved, thus greatly improving 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.
Laser surface treatment
Laser surface treatment is a thin layer that is heated by a laser beam to rapidly melt the surface of the workpiece to a certain depth, and is coated on the surface of the workpiece by vacuum alloying, electroplating, ion implantation, etc., and is irradiated with the base metal under laser irradiation. Fully fused, after condensation, an alloy layer with a special function of 10 to 1000 μm in thickness 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 heat non-disruption, and has high resistance to plastic deformation. It has a significant pressing effect on the initiation and expansion of fatigue cracks. Recently, Saha and Dahot used the method of laser cladding VC layer on the length of H13 substrate. The research shows that the surface of the obtained mold is solid, 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.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 penetrates the ionic cermet material to form the metallurgical bond of the surface, and the surface of the base metal also undergoes instantaneous phase transformation to form martensite and fine. Austenitic organization. This process differs from soldering, as well as sputtering or elemental infiltration, and should be a process in between. It makes good use of the high wear resistance, high temperature resistance and erosion resistance of the cermet material, and the process is simple and the cost is relatively low. It is a new way of surface treatment of die-casting molds.
Coating Technology Coating technology is a kind of mold strengthening technology, which is mainly applied to the surface 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 show that this method is feasible both in terms of process and function, which greatly reduces the friction coefficient of the mold surface.
Conclusion Mold pressure processing is an important component of mechanical manufacturing, and the level, quality and longevity of the mold are related to mold surface strengthening technology. With the improvement 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 strict working conditions, the existing new surface treatment process can not meet the ever-increasing requirements, and it is expected to further improve the technology of the predecessors, and it is also 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, it is necessary to improve the overall level of production of die-casting molds in China, and surface treatment technology will play a pivotal role.
