Various new technologies for surface treatment of die-c […]
Various new technologies for surface treatment of die-casting molds are emerging, but in general they can be divided into the following three categories:
(1) Improved techniques for conventional heat treatment processes;
(2) Surface modification technology, including surface thermal diffusion treatment, surface phase transformation strengthening, and electric spark strengthening technology;
(3) Coating technology, including electroless plating.
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 die-casting molds. Luo Baihui, secretary-general of the International Model Association, believes that it is still difficult to meet the ever-increasing use performance requirements by relying only on new mold materials. Various surface treatment techniques must be applied 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 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 the die-casting mold is highly demanded.
1. Improved technology of traditional heat treatment process
The traditional heat treatment process of die-casting molds is quenching-tempering, and later surface treatment technology has been developed. 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 proposes 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). The 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
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 at 1140 ~ 1150 °C, tempered twice at 550 °C, the surface hardness can reach HRC56 ~ 61, so that the die life of die-casting non-ferrous metals and their alloys increased 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, uniform infiltration layer, 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.
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. These methods are simple in process, strong in adaptability, low in expansion temperature, generally 480-600 ° C, small deformation of the workpiece, especially suitable for surface strengthening of precision molds, and high hardness and good wear resistance of nitrided layer, and 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, many die-casting molds made of H13 steel are subjected to nitriding treatment, and tempering is used instead of tempering. The surface hardness is as high as HRC65-70, while the hardness of the core of the mold is low and the toughness is good, thus obtaining excellent synthesis. Mechanical properties. The nitriding process is a commonly used process for 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. 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 rarely seen in China. 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 of nitriding after sulphur-nitrogen carbon co-infiltration developed in France. It is more suitable for non-ferrous metal die-casting molds.