From atomization powder making to 3D printing, boron nitride nozzle as a solution to the problem of molten metal
Release time:
2023-01-04
Powder metallurgy has core process advantages such as high material utilization rate, low unit energy consumption, environmental protection, etc. It is a technology in line with the direction of carbon neutrality in the future. In recent years, with the maturity of powder metallurgy technology and the trend of miniaturization of components, two emerging process routes, metal injection molding (MIM) and 3D printing (AM), have rapidly emerged. At the same time, the supply of high-quality powder raw materials has become a major factor restricting the development of the industry.
Powder metallurgy has core process advantages such as high material utilization rate, low unit energy consumption, environmental protection, etc. It is a technology in line with the direction of carbon neutrality in the future. In recent years, with the maturity of powder metallurgy technology and the trend of miniaturization of components, two emerging process routes, metal injection molding (MIM) and 3D printing (AM), have rapidly emerged. At the same time, the supply of high-quality powder raw materials has become a major factor restricting the development of the industry.
powder metallurgy
The traditional compression molding process is suitable for the production of large volume, medium to low density, and simple shaped products, with relatively low requirements for metal and alloy powders (particle size 100 μ Around m); But small and complex parts are undoubtedly more suitable for injection molding and 3D printing (particle size 20 μ M or even smaller), and has been increasingly applied in high-end fields such as aerospace, medical, electronics, military, etc. Therefore, the preparation of metal powders with high purity, good sphericity, small particle size and narrow distribution, and low oxygen content has become a new industry focus. These parameters have a crucial impact on the quality of metal products.
1. Atomization powder making and nozzle
The earliest iron and nickel powders used for MIM were produced using the carbonyl method. Although this method is mature, it is difficult to prepare alloy powders containing more than two elements, with limited variety and high production toxicity. Therefore, later water atomization, gas atomization, oil atomization, gas-water linkage atomization, and plasma atomization were developed and replaced by carbonyl method as the mainstream. These processes are collectively referred to as atomization powder making, and the key component of atomization powder making - the nozzle, largely determines the atomization rate (fine powder recovery rate), and then also determines production efficiency and powder quality. The industry is constantly exploring improvements to nozzles, such as changing gas, melt, and liquid flow fields through design to increase gas-liquid ratio and control oxygen content. However, nozzles face harsh working conditions such as erosion, wear, high temperature, and severe thermal shock, and their materials determine process stability and component life.
The most common zirconia ceramic nozzle
Among metal oxide materials, zirconia has the best high-temperature stability and insulation performance, while zirconia ceramics have the characteristics of good wear resistance and high hardness. The preparation process is relatively simple, the cost of the body is low, and it is easy to market application. Therefore, zirconia ceramic nozzles were first popularized in the application of metal atomization powder making.
However, the traditional zirconia nozzle lacks strict tolerance and clean edges, which leads to uneven and stable atomization and wide distribution of powder particle size; In addition, the friction between zirconia and metal melt is too high, which leads to nozzle blockage. High purity boron nitride ceramics have excellent high-temperature resistance, while composite boron nitride ceramics sacrifice slightly their high-temperature resistance in exchange for improved abilities in different directions such as corrosion resistance, wear resistance, and thermal shock resistance. The composite boron nitride nozzle can minimize blockage and metal creep, thereby reducing the frequency of nozzle replacement. Due to the low friction coefficient of boron nitride, a smooth surface finish and tighter tolerances provide a predictable particle size distribution between batches. In addition, the extremely strong thermal shock resistance allows the boron nitride nozzle to be used without the need for extensive preheating.
2. 3D printing and nozzles
The biggest difference between 3D printing and injection molding is that 3D printing does not require molds, which is more conducive to personalized and diversified production. Due to the lack of constraints and auxiliary effects of molds, the production process naturally relies more on the performance of printing equipment and powder raw materials. The nozzle is a key component that determines the quality of the finished product. Only by selecting the nozzle according to the requirements can satisfactory results be obtained - the best understanding is that in pursuit of speed, one must give up precision and choose a large nozzle, while in pursuit of precision, one must give up speed and choose a small nozzle.
Brass nozzle
Brass nozzles are the main force of conventional polymer materials such as plastic, resin, wax, etc. They are widely used and have high cost-effectiveness. The Mohs hardness of brass is 3, the maximum printing temperature is not more than 300 ℃, and the coefficient of thermal expansion is as high as 18m/m.K. These material characteristics determine that brass is easy to process, but cannot be used for high temperature consumables and high-precision printing, such as metal and alloy materials.
With the development of metal 3D printing technology, the benefits brought by boron nitride for metal atomization are increasingly related to these new 3D printing technologies. For example, some 3D printing manufacturers are currently searching for ways to cope with molten metal at high temperatures - high temperatures can cause significant thermal stress on mechanical components, posing new challenges in printer design; In addition, there are also requirements for non sticking and non wetting molten metal...... The high thermal shock resistance and low thermal expansion coefficient of boron nitride ceramics enable them to withstand high thermal gradient, and their high thermal conductivity is conducive to rapid solidification of deposited metal.
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