New surface treatment technology of the hottest di

New technology of surface treatment of die-casting dies

die casting dies are a large category of dies. 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 service life of the die-casting die. It is still difficult to meet the needs of continuously improving performance only by relying on the application of new mold materials. Various surface treatment technologies must be applied to the surface treatment of die-casting molds in order to achieve the high efficiency, high precision and high life of die-casting molds. Among all kinds of molds, the working conditions of die-casting molds are relatively harsh. Pressure casting is to fill the mold cavity with molten metal under high pressure and greatly strengthen the function of fatigue testing machine. It is formed by die casting and repeatedly contacts with hot metal in the working process. Therefore, the rapid realization of plastic film microchannel networking and its microfluidic utilization require that the die casting mold has high heat fatigue resistance, thermal conductivity, wear resistance, corrosion resistance, impact toughness, red hardness, good demoulding performance, etc. Therefore, the requirements for surface treatment technology of die-casting dies are high. In recent years, various new surface treatment technologies of die-casting dies have been emerging, but in general, they can be divided into the following three categories: (1) improved technology of traditional heat treatment process; (2) Surface modification technology, including surface thermal diffusion treatment, surface phase transformation strengthening, EDM strengthening technology, etc; (3) Coating technology, including electroless plating, etc

1 improved technology of traditional heat treatment process

the traditional heat treatment process of die-casting mold is quenching tempering, and later developed surface treatment technology. Due to the variety of materials that can be used as die-casting dies, the same surface treatment technology and process will produce different effects on different materials. Shikov recently proposed the substrate pretreatment technology for mold substrate and surface treatment technology. On the basis of traditional technology, he proposed suitable processing technology for different mold materials, so as to improve mold performance and improve mold life. Another development direction of the improvement of heat treatment technology is to combine the traditional heat treatment process with the advanced surface treatment process to improve the service life of die-casting dies. For example, the combination of chemical heat treatment method carbonitriding and NQN (i.e. carbonitriding quenching carbonitriding) combined with conventional quenching and tempering process can not only obtain higher surface hardness, but also increase the depth of effective hardening layer, reasonable hardness gradient distribution of carburizing layer, and improve the return fire stability and corrosion resistance, so that the surface quality and performance of die-casting mold can be greatly improved while obtaining good core performance

2 surface modification technology

2 1 surface thermal diffusion technology

this type includes carburizing, nitriding, boriding, carbonitriding, sulfur carbonitriding, etc

211 carburizing and carbonitriding

carburizing process applied to cold and hot work and surface strengthening of plastic molds can improve the service life of molds. For example, the surface hardness of 3Cr2W8V Steel die-casting mold can reach hrc56 ~ 61 After Carburizing, quenching at 1140 ~ 1150 ℃ and tempering twice at 550 ℃, which increases the service life of die-casting non-ferrous metals and their alloys by 1.5% 8 ~ 3.0 times. When carburizing, the main process methods include solid powder carburizing, gas carburizing, vacuum carburizing, ion carburizing and carbonitriding formed by adding nitrogen element in carburizing atmosphere. Among them, vacuum carburizing and ion carburizing are technologies developed in recent 20 years. This technology has the characteristics of fast carburizing speed, uniform carburizing layer, gentle carbon concentration gradient and small workpiece deformation. It will play an increasingly important role in the surface treatment of mold, especially precision mold

212 nitriding and related low-temperature thermal diffusion technology

this type includes nitriding, ion nitriding, carbonitriding, oxynitriding, sulfur nitriding, and sulfur carbon nitrogen, oxygen nitrogen sulfur ternary nitriding. These methods have the advantages of simple treatment process, strong adaptability, low diffusion temperature (generally 480 ~ 600 ℃), small workpiece deformation, especially suitable for the surface strengthening of precision molds, and high hardness, good wear resistance and good anti sticking performance of nitriding layer. After tempering and nitriding at 520 ~ 540 ℃, the service life of 3Cr2W8V Steel die-casting die is 2 ~ 3 times longer than that of non nitriding die. Many die-casting molds made of H13 steel in the United States need nitriding treatment, and nitriding is used instead of one-time tempering. The surface hardness is as high as HRC65 ~ 70, while the core hardness of the mold is low and the toughness is good, so excellent comprehensive mechanical properties are obtained. Nitriding process is a commonly used process for surface treatment of die-casting dies, but when a thin and brittle white bright layer appears in the nitriding layer, it cannot resist the effect of alternating thermal stress, and it is very easy to produce microcracks and reduce the thermal fatigue resistance. Therefore, in the nitriding process, the process should be strictly controlled to avoid the formation of brittle layers. Recently, foreign countries have proposed to adopt secondary and multiple nitriding processes. The method of repeated nitriding can decompose the nitride white layer that is easy to produce microcracks in the service process, increase the thickness of the nitriding layer, and at the same time, there is a very thick residual stress layer on the surface of the die, so that the service life of the die can be significantly improved. In addition, salt bath carbonitriding and salt bath sulfur nitrogen carbonitriding are also used. These processes are widely used abroad and rare in China. For example, tfi+abi process is to impregnate in alkaline oxidizing salt bath after nitrocarburizing in salt bath. The surface of the workpiece is oxidized and black, and its wear resistance, corrosion resistance and heat resistance have been improved. The service life of aluminum alloy die casting die treated by this method is increased by hundreds of hours. Another example is the oxynit process developed in France, which is nitriding after sulfur nitrocarburizing. It has more characteristics when applied to non-ferrous metal die-casting molds

213 boronizing

due to the high hardness (feb:hv1800 ~ 2300, fe2b:hv1300 ~ 1500), wear resistance and red hardness of boronizing layer, as well as certain corrosion resistance and adhesion resistance, boronizing technology has achieved good application results in mold industry. However, because the working conditions of die-casting dies are very harsh, boronizing process is rarely used in the surface treatment of die-casting dies. However, in recent years, improved boronizing methods have emerged to solve the above problems, and can be applied to the surface treatment of die-casting dies, such as multicomponent, coating powder boronizing, etc. The method of coating powder boronizing is to mix boron compound and other boronizing agents and apply them to the surface of die-casting die. After the liquid volatilizes, it is packed and sealed according to the general method of powder boronizing, heated at 920 ℃ and kept warm for 8h, followed by air cooling. This method can obtain a dense and uniform infiltration layer, improve the hardness, wear resistance and bending strength of the infiltration layer on the mold surface, and increase the service life of the mold by more than 2 times on average

214 rare earth surface strengthening

in recent years, the method of adding rare earth elements to the surface strengthening of molds has been widely respected. This is because rare earth elements have many functions, such as improving the infiltration rate, strengthening the surface and purifying the surface [13]. It has a great impact on improving the surface structure, surface physical, chemical and mechanical properties of the mold, and can improve the infiltration rate, strengthen the surface and generate rare earth compounds. At the same time, it can eliminate the harmful effect of trace impurities distributed on the grain boundary, and play a role in strengthening and stabilizing the grain boundary on the surface of the mold cavity. In addition, rare earth elements interact with harmful elements in steel to form high melting point compounds, which can inhibit the segregation of these harmful elements on the grain boundary, thereby reducing the brittleness of deep layers. The addition of rare earth elements in the surface strengthening treatment process of die-casting dies can significantly improve the thickness of the infiltration layer of various infiltration methods, improve the surface hardness, and make the microstructure of the infiltration layer fine and dispersed, and reduce the hardness gradient, so as to significantly improve the wear resistance, cold resistance, thermal fatigue performance of the die, and thus greatly improve the service life of the die. At present, the treatment methods applied to the surface of die-casting die cavity are: rare earth carbonitriding, rare earth carbonitriding, rare earth boriding, rare earth boron aluminizing, rare earth soft nitriding, rare earth sulfur nitrogen carbonitriding, etc

2 laser surface treatment

laser surface treatment is to use laser beam to heat, so that the surface of the workpiece can quickly melt a certain depth of thin layer. At the same time, vacuum evaporation, electroplating, ion implantation and other methods are used to coat the alloy elements on the surface of the workpiece, which are fully fused with the base metal under laser irradiation, and the thickness of 10 ~ 1000 is obtained on the surface of the mold after condensation μ M alloy layer with special properties, the cooling rate is equivalent to quenching in low-cost sales of Jinan assaying machine. If the H13 steel surface is treated by laser rapid melting process, the melting zone has high hardness, good thermal stability, high resistance to plastic deformation, and obvious inhibition on the initiation and propagation of fatigue cracks. Recently, Saha and dahotero have adopted the method of laser cladding VC layer on H13 substrate. The research shows that the obtained mold surface is a continuous, dense and non porous VC steel composite coating, which not only has strong oxidation resistance at 600 ℃, but also has strong resistance to molten metal reduction [19]. 23. With the continuous development of surface modification technology, an electro spark deposition process has emerged. Under the action of electric field, the process produces instantaneous high temperature and high pressure zones on the surface of the base metal, which can be caught up in a short time. At the same time, it infiltrates the ionic cermet materials to form metallurgical bonding on the surface, while the surface of the base metal also has instantaneous transformation, forming martensite and fine austenite structures [20]. This process is different from welding, spraying or element infiltration. It should be a process between the two. It makes good use of the characteristics of high wear resistance, high temperature resistance and corrosion resistance of cermet materials, and has simple process and low cost. It is a new way for surface treatment of die-casting dies

3 coating technology

coating technology, as a kind of mold strengthening technology, is mainly used in mold surface treatment with relatively simple working environment, such as plastic mold, glass mold, rubber mold, stamping mold, etc. Die casting dies need to withstand the harsh environment of alternating cold and heat stress, so coating technology is generally not used to strengthen the surface of die casting dies. However, in recent years, it has been reported that chemical composite plating is used to strengthen the surface of die-casting die to improve the adhesion resistance and demoulding property of die surface. In this method, polytetrafluoroethylene particles are infiltrated on the aluminum base die-casting die and then (NIP) - polytetrafluoroethylene composite plating is carried out. Experiments show that this method is feasible in both technology and performance, and greatly reduces the friction coefficient of the die surface

4 conclusion

die pressure machining is an important part of mechanical manufacturing, and the level, quality and life of dies are closely related to die surface strengthening technology. With the progress of science and technology, various mold surface treatment technologies have made great progress in recent years. It is manifested in: ① the improvement of traditional heat treatment process and its combination with other new processes; ② Surface modification technology, including carburization, low temperature thermal diffusion (various nitriding, carbonitriding, ion nitriding, ternary co nitriding, etc.), salt bath thermal diffusion, boriding, rare earth surface strengthening, laser surface treatment and EDM deposition of cermets; ③ Coating technology, etc. However, for die-casting molds with extremely harsh working conditions, the existing new surface treatment process can not meet the growing requirements. It can be expected that more advanced technology is also expected to be applied to the surface treatment of die-casting molds. Since surface treatment is one of the important means to improve the service life of die-casting dies, surface treatment technology will play an important role in improving the overall level of die-casting die production in China