Core tip: The most widely used materials for die casting molds are H13,SKD61,8407, etc. However, due to the large-scale die casting, the required molds are also developing to large-scale. In order to suppress quenching deformation and cracks of the molds, it is necessary to develop a low-deformation rapid cooling technology to improve the toughness and improve the toughness level of the mold materials.

One. Material of Die Casting Mould
The most widely used materials for die-casting molds are H13,SKD61,8407, etc. However, due to the large-scale die-casting molds, the required molds are also developing large-scale. In order to suppress the quenching deformation and cracks of the molds, it is necessary to develop a low-deformation rapid cooling technology to improve the toughness and improve the toughness level of the mold materials. To improve the production efficiency and reduce the cost of die-casting requires high cycle operation. Compared with the past mold, the high cycle operation increases the thermal load due to insufficient cooling, so that the surface temperature of the mold rises and the thermal cracking occurs early. Due to the increase of internal cooling holes and closer to the cavity, the temperature difference between the internal cooling holes and the cavity surface increases the tensile thermal stress load and reduces the rigidity, which is easy to produce stress corrosion cracking, which makes water leakage connected to the cavity and reduces the life of the mold. Therefore, it is necessary to develop new materials that can improve the life of SKD6l. DAC is similar to SKD61 material is also widely used, but its toughness is better than SKD61. The DAC-S developed to improve its toughness is 60% higher than DAC. In order to evaluate the toughness level of die casting mold materials, the North American Die Casting Association (NADCA) has set the Charpy impact value of AISI H13 (equivalent to SKD61). In order to stabilize the mold life, it has set the Charpy impact value of Prenium H13 and Superion H13 with higher grade. DAC-P is equivalent to the former and DAC-S to the latter. The thermal cracking countermeasures of die-casting molds need to have high temperature strength. Generally speaking, toughness and strength are opposite. DAC-P and DAC-S have the same strength as DAC at room temperature and high temperature due to their high-definition purification and densification of tissues. They have 0.2 higher endurance than DAC at room temperature of 20 degrees and high temperature of 600 degrees. Such as the aforementioned to have a higher grade than SKD61, greatly improve the life, high temperature strength and toughness are greatly improved mold materials. As high-performance die materials for die-casting molds, there are: DACIO, DAC55, DAC45 and the latest development ZHD435. DACI0: Excellent strength and toughness equivalent to DAC in the 600-degree temperature region. It is suitable for precision die-casting parts of high-precision shape complex parts for electrical appliances and communication machines. DAC45: It has higher strength than other materials at a high temperature of 700 degrees, and has the characteristics of resistance to softening, but it has poor toughness compared with DAC. Special attention should be paid to crushing fire cooling. Suitable for small and medium size die casting mold. DAC55; It is characterized by high toughness, initial hardness can be improved, toughness and strength can be balanced material, its service life is 1.4 times higher than SKD6l. It can be applied to the cylinder block, cylinder head and drive system components of automobiles. ZHD435. It has the same toughness as DAC55 and can be used in large parts, has higher high temperature strength than DAC55, and is more advantageous than DAC55 in terms of thermal cracking and life.

Machining of 2. Die Casting Mould
Die-casting mold processing is an important aspect to ensure quality and reduce costs, the use of CAD/CAM/CAE production system, five-axis processing machine, the latest cutting tools and other processing molds, so that mold quality and production efficiency in continuous progress. Its trend has three aspects: based on the three-dimensional model of the processing method, with the machining center instead of EDM machining, hole processing automation and other three aspects. In the three-dimensional model, the main mold design is the thorough use of three-dimensional model, NC digital preparation of simple, after processing quality assurance system construction. EDM with machining center to replace, the main reason is because the electrode machining production than cutting to a greater workload, so the use of EDM mainly because the mold has a rib-like deep groove shape, deep groove shape may also be solved by cutting, so that the mold production efficiency is more improved. Automation of hole processing: the mold has many kinds of holes, which can be roughly divided into shallow holes with a depth of less than 100mm but with accuracy requirements of H7 ~ H8 and deep holes of 100 ~ 600nun for cooling and deflation, but the accuracy does not have the same requirements as shallow holes. Hole processing takes up a large part of the time in mold processing. Coupled with the mold material hard (HRC48 degree) more to the hole processing on behalf of the difficult. The use of quality engineering evaluation techniques in the machining center is to find the best target based on the study of hole processing conditions and the best data used in the machining center. As a result, the surface roughness is increased by 1/3, the deviation of hole diameter is increased by 1/5, and the life of cutting tools is increased by more than 5 times. At the same time, the hole processing on the curved surface and inclined surface is also automated.

Surface Treatment of 3. Die Casting Mould
There are several methods such as diffusion treatment, coating and nitriding coating. Although the diffusion treatment method has a variety of trade names, mainly N, C, S, 0 alone or compound diffusion of nitriding, soft nitriding, immersion sulfur nitriding, oxidation, nitriding oxidation treatment, etc., a representative nitriding treatment, the formation of a compound layer (white layer) of its resistance to melting loss, resistance to burning, adhesion resistance, and crack resistance is poor. On the other hand, the compound-free layer only forms a diffusion layer, which is good in heat cracking resistance, while the melting loss resistance, overburning resistance and adhesion resistance are poor. The recent diffusion treatment trend, oxidation treatment or nitride oxide composite treatment is the mainstream, suitable for various surface treatment methods of die casting molds.

The coating has PVD method, CVD method and ID process method. The ceramic magnetic coating film formed by the hard film coating method has excellent dissolution resistance, overburning resistance, and adhesion resistance, but there is little improvement in heat cracking resistance. Such a hard film coating method CVD method and ID process method are better in the adhesion of the film, but in the treatment to 1000 degrees high temperature mold is easy to produce deformation and inch. PVD method can inhibit mold deformation due to low temperature treatment, but it is worse than CVD method in film adhesion. Nitriding coating, the representative method is nitriding method. Only the nitrided diffusion layer is formed in the nitriding furnace, and after removal, the various coating modes of the mold are carried out in a further PVD apparatus. Such nitride PVD has the most surface layer film with resistance to dissolution, burn resistance, adhesion resistance, nitrogen diffusion layer has heat cracking resistance. However, the film formed by the PVD method has a certain range of limits when it is suitable for the mold of complex shape. (Nitriding Oxidation) Compound Treatment. The surface treatment of die-casting molds has a worldwide tendency to develop from the past nitriding and immersion sulfonitriding treatment to the application of oxidation treatment. Since an iron oxide having high stability at high temperature is more effective than an iron nitride formed by a nitriding treatment, its effects on dissolution loss, overburning, and adhesion can be exhibited when applied to a die casting mold. (Nitriding Oxidation) The composite treatment can be carried out continuously in one apparatus, and the time can be greatly shortened. In addition, the continuous oxidation treatment in which the compound layer having no nitrided diffusion layer is formed in a controlled atmosphere can improve the dissolution resistance, the overburning resistance, the sticking resistance and the heat cracking resistance. Plasma CVD (PCVD). The raw material is full of gas, because it is a film formed by plasma chemical reaction, it is a film with excellent low-temperature adhesion and compactness, which can cover complex products well. In addition, the PCVD method can not damage the vacuum one-time project in a device will be nitriding and other diffusion hardening treatment of the hard film of the so-called composite processing simple. The PCVD can thus face the processing of the die-casting mold of the three-dimensional solid shape. PCVD of boron-containing gas system is a new boron diffusion treatment (boronizing) boron-containing film, such as TiB, TiBN, TiBCN, TiBIBN, TiAIBCN0, TiAISiBCN0, BN, BCN and other hard films. For example, TiAIBN film has a hardness of more than 4000Hv, which is called super advanced coating. The combination of different elements can be selected to obtain a high degree of wear resistance, oxidation resistance, mold release, and lubricity that have not been available in the past, so that the performance of die-casting molds that meet various requirements can be improved by leaps and bounds. The examples that have been applied now are: magnesium alloy die-casting molds for mobile phone parts used release agents in the past. After using release agents for about 30,000 times, welding repairs were required due to corrosion. Moreover, the environment was polluted by the smoke scattered by the release agents, and the products were difficult to maintain accuracy due to burrs. After the PCVD nitriding diffusion hardening layer TiN/TiAN/TiAISiCN0 multilayer film is applied to the mold, it can be used for die casting 300000 times without mold release agent, which prolongs the life of the mold, improves the environment, suppresses the generation of flash burrs, shortens the die casting beat and improves the production efficiency. In the past, mold release agent was applied to the molding of camera parts. The fluidity of the melt was poor, wrinkled skin was generated, problems occurred in the processing of products, and the secondary reject rate was high. After the nitride diffusion hardening layer TiN/TiAN/TiAISiCN0 multilayer film is used for this mold, the amount of release agent is only less than 1/8 of that in the past, which improves the fluidity, no longer wrinkles, and processes beautiful products, thus reducing consumption and improving production efficiency. In the past, the molding of automobile parts was formed by spraying a large amount of mold release agent, about 60000 times of welding repair, and there were many casting defects and high defect rate. After the nitride diffusion hardening layer TiN/TiAN/TiAISiCN0 multilayer film is used, the stripping agent is reduced to less than 1/2, and the casting is increased to more than 120000 times. It not only prolongs the life of the mold, but also suppresses casting defects, reduces consumption and improves the production environment. Due to the combined treatment effect of the method, the heat cracking resistance, overburning resistance and adhesion resistance are improved. In aluminum die-casting, there are die-casting molds that have been cast more than 1 million times without mold release agent.

Life and heat treatment of 4. die-casting molds
The reasons for the low life of die-casting molds involve many factors, such as materials, processing, heat treatment, maintenance, repair and other complex factors, which will affect the life of the mold. The thermal fatigue of the mold in the casting process is related to the toughness, ductility, crack progression rate of the mold steel and the characteristics of the material, and the mechanical stress generated during the operation is also the cause. On the other hand, the dissolution phenomenon, due to the mold material and the contact metal reactivity and contact friction are also very different. The reaction morphology also varies depending on the type of die cast metal (AI, Mg, Zn, etc. In addition, the stability and life of the mold during operation are also closely related to its maintenance and repair technology, such as the improvement of the EDM metamorphic layer, the selection of welding conditions, and the adjustment of deformation. The heat treatment of mold steel is a very important basic technology to effectively play the function and performance of the material, and the performance of the heat treatment is significantly different. No matter how good the steel is without proper heat treatment, the mold will produce many faults such as hot cracking, cutting, corrosion resistance and wear resistance reduction during operation. In recent years, the heat treatment of die steel mostly uses vacuum gas pressurized heat treatment furnace, mainly because it can improve the operating environment, can carry out bright heat treatment on the mold, high pressure when cooling, and can manufacture large heat treatment furnace. Vacuum pressurized gas cooling methods, traditionally water quenching, oil quenching, polymer quenching, salt bath quenching, etc., in recent years can generally achieve the same cooling rate, can also be made to the soft point less processing. In recent years, along with the large-scale mold, the development of steel for large molds to improve the quenching performance. Large mold heat treatment due to the surface with the center part of the cooling rate is very different, uneven heat treatment and produce deformation, change inch, split and other problems. Therefore, it is very important to control the temperature difference between inside and outside during the cooling process. For the best steel grade, unstable heat treatment is difficult to exert its characteristics. The North American Die Casting Association has set standards for the quality requirements of die casting mold materials and heat treatment, and its main projects are: 1. Material composition 1, hardness (annealing, quenching-tempering treatment) 3, cleanliness of material is determined by (ASTM, A- 681, Sec6) 4, ultrasonic flaw detection test (internal defect inspection), impact test (ductility and ductility determination) 6, crystal grain size 7, microsegregation 8, annealing structure, etc. Countries, regions, suppliers and production enterprises can have a unified recognition of all aspects of steel, mold processing and heat treatment, which is beneficial to improve the quality and production of die casting molds.