Three details of the new surface treatment technology for die casting molds in China

With the rapid development of China's automobile and other related industries, the die-casting mold industry has also been driven up. However, according to the present development status of China's die-casting mold industry, its current status does not meet the development of the automotive industry. The backward production technology is the most fundamental s reason.

Die-casting molds are a large category of molds. 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 for the comprehensive mechanical properties and life of die-casting molds. According to Luo Baihui, the secretary-general of the International Association for Modeling, it is still difficult to satisfy the need for continuous improvement in performance. It is necessary to apply various surface treatment technologies to the surface treatment of die casting molds to achieve high efficiency in die casting molds. , high precision and long life requirements. In various molds, the working conditions of die-casting molds are harsh. Pressure casting is to make the molten metal filled with die cavity at high pressure and high speed and die-casted. It is in contact with hot metal repeatedly during the work process. Therefore, the die casting die is required to have high thermal fatigue resistance, thermal conductivity abrasion resistance and corrosion resistance. , impact toughness, red hardness, good mold release and so on. Therefore, the technical requirements for surface treatment of die casting molds are relatively high.

In recent years, a variety of new techniques for surface treatment of die casting molds have emerged, but in general can be divided into the following three categories: (1) improved technologies for traditional heat treatment processes; (2) surface modification technologies, including surface thermal expansion Seepage treatment, surface transformation strengthening, EDM strengthening technology, etc.; (3) Plating technology, including electroless plating.

1 The traditional heat treatment process improvement technology Traditional die casting mold heat treatment process is quenching - tempering, and later developed a surface treatment technology. Due to the variety of materials that can be used as die casting molds, the same surface treatment techniques and processes applied to different materials will produce different results. Shi Kefu recently proposed the substrate pretreatment technology for mold substrate and surface treatment technology, based on the traditional process, put forward suitable processing technology for different mold materials, thereby improving mold performance and improving mold life. Another development direction of the heat treatment technology improvement is to combine the traditional heat treatment process with the advanced surface treatment process to increase the service life of the die casting mold. Such as the carbonitriding method of chemical heat treatment, combined with the conventional quenching and tempering process NQN (ie carbonitriding - quenching - carbonitriding) composite strengthening, not only get a higher surface hardness, but also effective hardening The depth of the layer increases, the hardness gradient of the penetration layer is reasonably distributed, and the tempering stability and the corrosion resistance are improved, so that the die casting mold has a good core performance, and the surface quality and performance are greatly improved.

2 surface modification technology 21 surface thermal diffusion technology This type includes carburizing, nitriding, boronizing and carbonitriding, sulfur carbonitriding and so on.

211 carburizing and carbonitriding carburizing process can be applied to cold, hot work and plastic mold surface strengthening, can increase the die life. Such as 3Cr2W8V steel die-casting mold, the first carburizing, and then quenched by 1140 ~ 1150 °C, 550 °C tempered twice, the surface hardness of up to HRC56 ~ 61, die-casting non-ferrous metals and their alloys die life increased by 1.8 to 3.0 times . Carburizing treatment, the main process methods are solid powder carburizing, gas carburizing, and vacuum carburizing, ion carburizing and carbonitriding in the carburizing atmosphere by adding nitrogen. Among them, vacuum carburizing and ion carburizing are technologies developed in the past 20 years. The technology has the characteristics of fast infiltration speed, uniform layer penetration, smooth carbon concentration gradient, and small deformation of workpieces. It will be in the mold surface, especially precision molds. Surface treatment plays an increasingly important role.

212 nitriding and related low-temperature thermal diffusion technology This type includes nitriding, ion nitriding, carbonitriding, oxynitriding, nitriding and sulfur-carbon-nitrogen, oxygen-nitrogen-sulfur ternary etc. method. These methods have simple processing techniques, strong adaptability, low diffusion temperature (usually 480-600°C), small workpiece deformation, and are especially suitable for surface hardening of precision molds. Moreover, the nitrided layers have high hardness and good wear resistance. Good anti-sticking performance.

3Cr2W8V steel die casting mold, after quenched and tempered, 520 ~ 540 °C after nitriding, the service life is not nitrided mold increase 2 to 3 times. In the United States, many die-casting molds made of H13 steel are subjected to nitriding treatment, and nitriding is used instead of one tempering. The surface hardness is as high as HRC65 to 70, and the hardness of the core of the die is low and the toughness is good, thereby obtaining excellent comprehensiveness. Mechanical properties. Nitriding process is a commonly used process for die casting mold surface treatment, but when the nitride layer appears thin and brittle white bright layer, can not resist the effect of alternating thermal stress, easily produce micro-cracks, reduce thermal fatigue resistance. Therefore, during the nitriding process, the process must be strictly controlled to avoid the formation of brittle layers. Recently, foreign countries have proposed the use of secondary and multiple nitriding processes. The method of repetitive nitriding can decompose the white bright layer of nitride that is easy to produce micro-cracks during service, increase the thickness of nitrided layer, and at the same time make the surface of the mold have a very thick residual stress layer, so that the life of the mold can be significantly improved. There are also methods such as salt bath carbonitriding and salt bath sulfur nitrocarburizing. These processes are widely used in foreign countries and are rarely seen in China. For example, the TFI ABI process is immersed in a basic oxidizing salt bath after nitrocarburizing in a salt bath. Oxidation of the workpiece surface appears black, and its wear resistance, corrosion resistance, and heat resistance are improved. The aluminum alloy die-casting die treated by this method has an increased lifetime of several hundred hours. Another example is the oxynit process for nitriding after nitriding and carbonitriding developed in France. It is more characteristic when applied to non-ferrous metal die-casting molds.

213 Boron Boron Boron technology is obtained in the mold industry 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 adhesion resistance. Better application effect. However, due to the severe working conditions of die casting molds, the boronizing process is rarely used in the surface treatment of die casting molds. However, in recent years, an improved boronizing method has emerged to solve the above problems and can be applied to the surface treatment of die casting molds. Such as multiple, paint powder seepage. Boron coating powder boronizing method is to mix the boron compound and other infiltrating agent coated on the surface of the die casting mold, until the liquid volatilizes, then in accordance with the general method of powder boronization sealed, 920 °C heating and insulation 8h, followed by Air cooling. This method can obtain a dense, uniform layer of penetration, the hardness of the surface layer of the mold, wear resistance and bending strength are improved, the average service life of the mold is increased by more than 2 times.

214 Surface Hardening of Rare Earths In recent years, the method of adding rare earth elements in mold surface strengthening has been widely respected. This is because rare earth elements have many functions such as increasing the infiltration speed, strengthening the surface, and purifying the surface [13]. It has a great influence on improving the surface structure, surface physical, chemical and mechanical properties of the mold, and can increase the infiltration speed and strength. Surface, generate rare earth compounds. At the same time, the deleterious effect of trace impurities distributed on the grain boundary can be eliminated, and it plays a role of strengthening and stabilizing the grain boundary of the mold cavity surface. In addition, rare earth elements react with harmful elements in the steel to form high melting point compounds, which in turn can inhibit the segregation of these harmful elements on the grain boundaries, thereby reducing the deep brittleness and the like. The addition of rare earth elements in the surface hardening process of the die casting mold can significantly increase the thickness of the infiltration layer and improve the surface hardness. At the same time, the fineness of the infiltrated layer can be dispersed and the gradient of hardness can be reduced, thus making the wear resistance and resistance of the mold. The cold and thermal fatigue properties are significantly improved, which greatly increases the life of the mold. Currently used in die casting mold cavity surface treatment methods are: rare earth carbon infiltration, rare earth carbonitriding, rare earth boron co-infiltration, rare earth borax co-infiltration, rare earth soft nitriding, rare earth sulfur nitrocarburizing and so on.

2.15 Surface Coating Strengthening In recent years, due to the development of cold welding technology, the surface treatment technology has been greatly improved. In particular, after the ESD-05 is on the market, it is possible to use carbides and other different materials to treat the surface. This method is convenient. Simple, low cost, easy to use. At the same time, the effect is good, and gradually has become the main choice of the industry.

2.2 Surface laser coating 221 Laser surface treatment Laser surface treatment is the use of laser beam heating, the workpiece surface quickly melted to a certain depth of the thin layer, while the use of vacuum evaporation, electroplating, ion implantation and other methods to apply the alloy elements on the surface of the workpiece Under laser irradiation, it is fully fused with the matrix metal, and after the condensation, a layer of alloy with a thickness of 10 to 1000 m with special properties is obtained at a cooling rate equivalent to chill quenching. If the surface of H13 steel is processed by laser rapid melting process, the melting zone has high hardness and good thermal stability, high resistance to plastic deformation, and has obvious inhibitory effect on the initiation and propagation of fatigue cracks. Recently, Saha and Dahort used the method of laser cladding 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. It is not only very strong. Oxidation resistance at 600 °C, and there is a strong ability to resist the reduction of molten metal [19]. 23 EDM deposition cermet technology In the continuous development of surface modification technology, an EDM deposition process has emerged. Under the action of an electric field, the process generates a transient high-temperature, high-pressure zone on the surface of the base metal, and simultaneously penetrates into the ionic cermet material to form a metallurgical bond of the surface. At the same time, the surface of the base metal also undergoes instantaneous phase transition to form martensite and microfine. Austenite [20]. This process is different from welding, and it is also different from spraying or element infiltration. It should be a process between the two. It makes good use of the high wear resistance, high temperature resistance, and corrosion resistance of the cermet materials. The process is simple and the cost is low. It is a new way for the surface treatment of die casting moulds.

The 2.22WS welder handles the same principle of the WS welder and the laser welder, both of which are performed by pulse spot welding. Compared with laser welding, it is more convenient and more flexible. The diameter of the welding wire is 0.1-2.0mm. At the same time, the function of argon arc welding is built in above, which is more convenient and flexible.

3 coating technology coating technology as a mold strengthening technology, mainly used in plastic molds, glass molds, rubber molds, stamping molds and other work environment is relatively simple mold surface treatment. Die-casting molds need to withstand the harsh environment of alternating cold and hot stress, so generally do not use plating technology to strengthen the die casting mold surface. However, in recent years, it has been reported that the method of chemical composite plating is used to strengthen the surface of a die casting mold so as to improve the surface resistance of the mold and mold release properties. In this method, polytetrafluoroethylene particles are impregnated on an aluminum die-casting mold and subjected to (NiP)-polytetrafluoroethylene composite plating. Experiments show that this method is feasible in terms of process and performance, which greatly reduces the friction coefficient of the mold surface.

4 Conclusion Mould pressure processing is an important part of the machinery manufacturing, and mold level, quality and life are closely related to the mold surface strengthening technology. With the advancement of science and technology, in recent years, various mold surface treatment technologies have made great progress. The performances are as follows: 1 The improvement of the traditional heat treatment process and its combination with other new processes; 2 Surface modification technologies, including carburizing, low-temperature thermal diffusion (various nitriding, carbonitriding, ion nitriding, ternary Co-permeation, etc.), salt bath thermal diffusion, boronizing, rare earth surface enhancement, laser surface treatment and EDM deposition of cermets; 3 coating technology and so on. However, for die-casting molds whose working conditions are extremely demanding, the existing new surface treatment processes cannot meet the ever-increasing demands, and more advanced technologies can be expected, and it is also expected to be applied to the surface treatment of die-casting molds. In view of the fact that surface treatment is one of the important means for improving die casting die life, the surface treatment technology will play a decisive role in improving the overall level of die casting mould production in China.

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