Sun Apr 30 14:19:10 CST 2023
Various new technologies for surface treatment of die-casting moulds are emerging, but in general they can be divided into the following three broad categories:
(1) Improvement techniques for traditional heat treatment processes;
(2) surface modification technology, including surface heat expansion infiltration treatment, surface phase change strengthening, EDM strengthening technology, etc.;
(3) coating technology, including chemical plating, etc.
Die-casting moulds are a large category of moulds. With the rapid development of China's automobile and motorbike industry, die-casting industry ushered in a new period of development, at the same time, also on the comprehensive mechanical properties of die-casting moulds, life and other higher requirements. International mold association secretary general LuoBaiHui think, to meet the increasing use performance demand just rely on the application of new mold material is still difficult to meet, must be a variety of surface treatment technology applied to die-casting die surface treatment in order to achieve high efficiency, high precision and high life requirements of die-casting die. In all kinds of moulds, die-casting moulds working conditions are more demanding. Pressure casting is to make molten metal in high pressure, high speed under the mold cavity and die-casting molding, in the process of work repeatedly and hot metal contact, so require die-casting die with high heat fatigue, thermal conductivity wear resistance, corrosion resistance, impact toughness, red hardness, good mold release, etc.. Therefore, the die-casting mold surface treatment technology requirements are high.
1, the traditional heat treatment process improvement technology
The traditional die-casting mould heat treatment process is quenching - tempering, and later developed the surface treatment technology. As the die-casting moulds can be used as a variety of materials, the same surface treatment technology and process applied to different materials will produce different results. Skov proposed for the mould base material and surface treatment technology of the base material pretreatment technology, in the traditional process based on the different mould materials to propose suitable processing process, so as to improve the mould performance, improve mould life. Another development direction of heat treatment technology improvement is to combine the traditional heat treatment process with advanced surface treatment process to improve the service life of die-casting moulds. Such as the chemical heat treatment method of carbon and nitrogen infiltration, and conventional quenching, tempering process combined with NQN (i.e. carbon and nitrogen infiltration - quenching - carbon and nitrogen infiltration compound strengthening, not only to get a higher surface hardness, and effective hardening layer depth increase, permeation layer hardness gradient distribution is reasonable, tempering stability and corrosion resistance to improve, thus making die-casting die in the acquisition of good heart performance at the same time, the surface quality and performance significantly improved The surface quality and performance of die casting moulds are greatly improved.
2、Surface modification technology
Surface thermal expansion infiltration technology
This type includes carburizing, nitriding, boron penetration and carbonitriding, sulfur-carbonitriding, etc.
Carburizing and carbonitriding
Carburising process is applied to cold, hot work and plastic mould surface strengthening, all can improve the life of the mould. Such as 3Cr2W8V steel die-casting mold, first carburizing, and then 1140 ~ 1150 ℃ quenching, 550 ℃ tempering twice, the surface hardness of HRC56 ~ 61, so that die-casting non-ferrous metals and their alloys die life increased by 1.8 ~ 3.0 times. Carburizing treatment, the main process methods are solid powder carburizing, gas carburizing, as well as vacuum carburizing, ion carburizing and carburizing atmosphere with the addition of nitrogen elements to form the carbon and nitrogen carburizing. Among them, vacuum carburizing and ion carburizing is the technology developed in the past 20 years, the technology has fast carburizing speed, uniform carburizing layer, carbon concentration gradient and small workpiece deformation, etc., will play an increasingly important role in mould surface, especially in precision mould surface treatment.
Nitriding and related low-temperature thermal expansion technology
This type includes nitriding, ion nitriding, carbonitriding, oxy-nitriding, sulphur-nitriding and sulphur-carbon-nitriding, oxy-nitriding-sulphur ternary coextrusion methods. These methods are easy to process, adaptable, low expansion infiltration temperature is generally 480 ~ 600 ℃, the workpiece deformation is small, especially adapted to the surface strengthening of precision molds, and nitriding layer hardness, good wear resistance, good resistance to sticky mold performance.
3Cr2W8V steel die-casting moulds, after tempering, 520 ~ 540 ℃ nitriding, service life than the non-nitriding moulds to improve 2 ~ 3 times. The United States with H13 steel die-casting die, many are to nitriding treatment, and nitriding instead of a tempering, surface hardness up to HRC65 ~ 70, while the mold heart hardness is low, good toughness, so as to obtain excellent overall mechanical properties. Nitriding process is a common process for surface treatment of die casting moulds, but when the nitriding layer appears thin and brittle white bright layer, can not resist the effect of alternating thermal stress, very easy to produce micro-cracking, reduce the thermal fatigue resistance. Therefore, in the nitriding process, the process should be strictly controlled to avoid the generation of brittle layers. The use of secondary and multiple nitriding processes is proposed abroad. The use of repeated nitriding method can decompose the nitride white bright layer which is easy to produce micro-cracks during service, increase the thickness of the nitriding layer, and at the same time make the mould surface exists very thick residual stress layer, so that the life of the mould can be significantly improved. In addition there is the use of salt bath carbonitriding and salt bath sulphur carbonitriding and other methods. These processes are more widely used in foreign countries, in the domestic less common. Such as TFI + ABI process, is in the salt bath carbonitriding and then in the alkaline oxidizing salt bath impregnation. Workpiece surface oxidation, black, its wear resistance, corrosion resistance, heat resistance have been improved. By this method of treatment of aluminium alloy die-casting die life increased by hundreds of hours. Such as the French development of sulphur and nitrogen carbon infiltration after the oxynit process of nitriding, applied to non-ferrous metal die-casting moulds are more characteristic.