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Application of advanced ceramics in mold materials

update:08-10-2019
abstract:

A mold refers to a tool that makes a blank into a part […]

A mold refers to a tool that makes a blank into a part having a specific shape and size under the action of an external force. It can be said that without the mold, there is no modern industry. The development level of the mold industry is a key indicator to measure the manufacturing level of the whole country, so it is also called the “mother of industry”. In electronics, automobiles, motors, appliances, instruments, meters, home appliances and communications, 60% to 80% of parts and components must rely on mold forming. The high precision, high complexity, high consistency, high productivity and low consumption of the parts produced by the mold are unmatched by other processing methods. For example, in the most common cars, automobile covers formed by stamping dies, automotive interior parts formed by injection molds, and various engine transmission parts formed by precision forging all show the precision and efficiency of the mold industry.
However, the traditional mold processing is generally carried out under harsh environments. The alternating temperature and stress conditions make the mold prone to various fatigue damages; and the severe friction between the metal plastic flow and the mold cavity makes the mold wear an important failure mode. One. Therefore, the development of new high hardness, high wear resistance, high strength mold materials has been receiving much attention.
As early as the 1990s, boron carbide ceramics have been applied to mold materials in China. The boron carbide drawing die developed by Mudanjiang Carbon Boron Research Institute is made of high-purity and ultra-fine-grained boron carbide micropowder. It is pressed by high temperature and high pressure and has high hardness and good wear resistance. It can be used as the main equipment for cable industry. Wear-resistant components, which have a life expectancy of more than three times that of conventional cemented carbides.
However, ceramic materials have the advantages of high hardness, high temperature resistance, and no sticking. The biggest difficulty in application is the brittleness of ceramics. As a brittle material, its fracture toughness is low and it can hardly withstand tensile stress. This disadvantage limits its As an integral mold application. In order to solve this problem, the ceramic material can be compounded with the metal material to ensure high hardness and high wear resistance, and also has good plasticity and toughness of the metal material; and can also be used as a surface coating material to adhere to the surface of the conventional mold material to form "External hard and tough" mold. These two methods of "strengthening and complementing each other" have been widely used.
Cermet mold
In 2017, Xiong Wei of the Jiangxi Institute of Mechanical Science announced a cermet mold using a hot extrusion process. In this study, a new generation of W-Ti cermet heat was developed by blending metal carbides such as W and Ti into cermet mixes through dry pressing, cold isostatic pressing, pressureless sintering, and mechanical processing. The extrusion die breaks through the traditional technology of producing hot extrusion die with high alloy steel. Tests have shown that the service life of W-Ti cermet hot extrusion die is 7.5 times that of conventional alloy steel.
(Small tip: The extrusion process has very high requirements on the mold. The common extrusion force is above 2000MPa. Therefore, the research on new mold materials often involves the development of new materials for extrusion molds. Extrusion has always been considered difficult to achieve, because it must have ultra-high strength mold materials to withstand such large extrusion forces. Cold extrusion is the first large-scale production of steel parts during World War II. Germany used new alloy tool steel and phosphating. The saponification technology is realized. The traditional extrusion is carried out at room temperature. In recent years, some high-strength materials must be extruded at high temperatures, which puts higher requirements on the mold materials.
In addition, early studies generally used hot press sintering to prepare cermets, which used metal particles to toughen alumina ceramic molds to overcome the brittleness of ceramic materials. In addition to the toughening technology of alumina ceramics, Central South University and Chinalco have developed a new mold material that utilizes metal Ti to enhance silicon nitride ceramics, and believes that silicon nitride is more excellent in red hardness and wear resistance than alumina. Sexual and comprehensive mechanical properties are one of the most ideal materials for the manufacture of hot extrusion dies.
"outer hard and tough" ceramic coating
Compared with the development of new cermet materials, the ceramic coating is simpler in theory and the actual operating cost is lower. Current major methods include chemical vapor deposition (CVD) and physical vapor deposition (PVD), with the latter being more widely used. PVD technology uses low-voltage, high-current arc discharge technology under vacuum conditions, using gas discharge to evaporate the target and ionize the vaporized material and gas, and use the acceleration of the electric field to make the evaporated material and its The reaction product is deposited on the workpiece.
Hatebur Switzerland is a world-class supplier of mold equipment, which is the first to apply a titanium nitride coating to the punch of a cold extrusion die by PVD. (small tip: In the extrusion process, the titanium oxide coating of the PVD coating can only be used at 450 ° C. This temperature is the temperature that can be reached by the interface between the material and the mold when the cold extrusion is severely rubbed; In the hot extrusion, in order to prevent the coating from falling off, the CVD method is often used, so that the working temperature can reach above 800 ° C. In addition, the coating is carried out by the CVD method using titanium carbide, and the working temperature can reach 950 ° C to 1100 ° C. )
In November 2017, at the 3rd “Internet +” College Students Innovation and Entrepreneurship Competition, Xi’an Jiaotong University’s “Ingenuity Cloud” project won the gold medal and was considered by the organizing committee to be the most valuable entrepreneurial project. The 10,000-level Pre-A round of financing, the proposed transfer of 10% of the shares, mainly used for foundry construction and professional team building. It is reported that the "Artificial Clouds" project was initiated by the doctoral student Qian Dan of the 85th National People's Congress of Xi'an Jiaotong University. Currently, based on the self-developed coating technology, university expert team and research and development platform, it has developed titanium nitride, titanium carbonitride and chromium nitride. A variety of ceramic PVD coating technologies, such as chromium aluminum nitride, replace the traditional plating method of metal plating or alloy, and put a "soft armor" on the mold.
summary
In general, the application of traditional die steel or carbide mold materials is very mature, but in some of the more severe conditions, performance and life are facing enormous challenges. As pointed out in the tip, the traditional cold extrusion technology is currently developing in the direction of warm extrusion. In the hot extrusion process, due to the higher temperature, the problem of poor red hardness of traditional materials is more exposed, using ceramic coating. After the hard material performance is improved, the life is extended, the action of frequently changing the mold is reduced, and better economic benefits are obtained. In contrast, the use of metal particle reinforced ceramic materials in industrial applications has rarely been reported, mainly because the overall cermet mold price is still very expensive, and the service life prediction is limited to the laboratory. However, the development of the combined die has made the mold more flexible. For the working part of the mold that needs to face high temperature and high pressure, the reinforced ceramic material can be used, and the rest is still made of traditional materials which are relatively inexpensive. In addition, the demand for stainless steel and heat-resistant alloy steel is driving the development of thermoforming technology. Facing the processing environment with high deformation resistance and high temperature, cermet molds are still worth looking forward to.

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