Powder metallurgy, as an advanced material preparation and forming technology that is both ancient and dynamic, originated from ancient ceramic preparation technology and iron smelting technology. Until 1909, the advent of ductile tungsten by powder metallurgy marked the advent of the modern powder metallurgy era. Over the past 100 years, powder metallurgy technology has flourished, and various important new materials and key products have continued to emerge, becoming one of the important engineering technologies that are indispensable to today's national economy and science and technology. Characteristics and advantages of powder metallurgy
Powder metallurgy is a technology that uses metal powder (or a mixture of metal and non-metal powder) as raw materials to manufacture metal materials, composite materials and various types of products through processes such as molding and sintering.
Compared with traditional smelting and casting processes, powder metallurgy has many advantages. On the one hand, it can effectively avoid the possible component segregation during the smelting process, ensure the uniformity of the material composition, and thus obtain more stable and excellent performance. On the other hand, powder metallurgy can achieve near-net forming, greatly reducing subsequent processing procedures and material waste. According to relevant statistics, the material utilization rate of parts manufactured by powder metallurgy can reach more than 90%, while the material utilization rate of traditional mechanical processing methods is usually only 30%-50%, which not only reduces production costs, but also improves production efficiency, which is in line with the development concept of green and environmental protection in modern manufacturing. In addition, by adjusting the powder composition, particle size and preparation process, the material properties can be precisely controlled to meet the needs of different fields for special material properties, such as high strength, high hardness, high temperature resistance, corrosion resistance, etc.
Main process of powder metallurgy process
(I) Powder preparation
Mechanical crushing method: Mechanical force is used to crush block metal or alloy into powder. The equipment is simple, the cost is low, and the output is large, but the powder shape is irregular, the particle size distribution is wide, and impurities are easily introduced.
Atomization method: The molten metal liquid is sprayed into small droplets with high-pressure gas (nitrogen, argon) or high-speed water flow, and cooled and solidified into powder. The gas atomization method has high sphericity and good fluidity, which is suitable for making high-performance parts; the water atomization method has low cost and high efficiency, and the powder shape is irregular. It is often used for ordinary steel powder and products with low performance requirements.
Reduction method: Use reducing agents such as hydrogen and carbon monoxide to reduce metal oxides into powders with high purity, high activity, high sintering activity, and low-temperature densification. However, production requires high temperature and a specific atmosphere, and the equipment investment is large and the cost is high.
Electrolysis method: Electrolyze metal salt solutions or molten salts to precipitate metal ions into powders at the cathode. The powders are extremely pure, fine and uniform in particle size. They are suitable for fields with high requirements for purity and particle size, such as electronic materials, but they have low production efficiency, high energy consumption and high cost.
(II) Molding
Compression molding: Put the pretreated metal powder into the mold and press it into shape. The steps include powder filling, pressing and demolding. It is suitable for products with simple shapes and high precision requirements, such as gears. The advantages are simple equipment, high efficiency, low cost, and large-scale production; the disadvantages are that it is difficult to design and manufacture molds for complex products, and it is difficult to ensure uniform density.
Isostatic pressing: Use liquid to uniformly transmit pressure, and put the powder into an elastic mold and press it into a high-pressure container. Cold isostatic pressing is carried out at room temperature and is suitable for products with complex shapes and high density requirements; hot isostatic pressing uses high temperature and high pressure at the same time and is used for high-performance aerospace materials, etc. The advantage is that the product has uniform density in all directions and is suitable for large and complex products; the disadvantage is that the equipment is expensive, the cycle is long, and the cost is high.
Injection molding: Mix metal powder and binder into injection material, and use an injection machine to inject it into the mold cavity for molding. It is suitable for manufacturing high-precision complex small parts, such as electronic components. The advantage is high molding efficiency and precision, and it is suitable for large-scale production; the disadvantage is that the selection and removal of binders are difficult, and improper handling affects the performance of the product.
(III) Sintering
Conventional sintering: Heat the molded body at a suitable temperature and atmosphere (hydrogen, nitrogen, vacuum, etc.) to combine the powder particles and increase the density and strength. Hydrogen atmosphere removes impurities, nitrogen prevents oxidation, and vacuum is suitable for materials with high oxygen content requirements.
Hot pressing sintering: Pressure is applied during sintering, and it is carried out in special equipment. The mold is made of materials such as graphite. It can reduce the sintering temperature, shorten the time, and obtain products with higher density and performance. It is often used in the preparation of high-performance ceramics and other materials.
Spark plasma sintering (SPS): Rapid heating and sintering by generating discharge plasma and Joule heat through pulse current. It can remove impurities on the surface of particles, activate the surface, heat up quickly (100-1000℃/min), take a short time (several minutes to tens of minutes), and inhibit grain growth. It is used to prepare nanomaterials, etc.
Application fields of powder metallurgy technology
(I) Aerospace field
Aerospace has strict requirements on material performance, and powder metallurgy technology just meets the needs. Powder metallurgy high-temperature alloys are used to manufacture key components such as aircraft engine turbine disks and blades. For example, the turbine disk of the F119 engine of Pratt & Whitney in the United States uses powder metallurgy nickel-based high-temperature alloys to improve engine performance and reliability. Powder metallurgy titanium alloys are used to manufacture aircraft wing beams, fuselage frames and other structural parts with low density, high strength and corrosion resistance, reducing aircraft weight and improving fuel efficiency and flight performance.
(II) Automobile manufacturing field
Powder metallurgy parts are widely used in automobile engines, transmissions, and braking systems. The valve seat rings, guide tubes, and piston rings in the engine are made of copper-based or iron-based alloys, which can withstand high temperature and high pressure, and improve engine performance and life; the gears and synchronizer hubs of the transmission are of high precision and good strength, making gear shifting smoother and improving transmission efficiency; the brake pads and brake discs of the brake system are added with special friction materials, which have good friction and wear resistance to ensure braking safety.
(III) Electronic Information Field
As electronic equipment develops towards small, light, and high performance, powder metallurgy technology is more widely used. Soft magnetic powder metallurgy materials are used to manufacture electronic components such as transformers and inductors; powder metallurgy metal-based composite materials such as copper-tungsten and copper-molybdenum are used for heat dissipation substrates and packaging shells of high-power electronic devices; powder metallurgy contact materials are used for electrical switches and relays to ensure safe circuit switching.
Iron-silicon-nickel magnetic powder core (KNF)
(IV) Mechanical Manufacturing Field
Powder metallurgy technology is used to manufacture mechanical parts such as gears and bearings. Powder metallurgy gears have high precision, smooth transmission and high material utilization rate; powder metallurgy bearings are self-lubricating and wear-resistant, suitable for low-speed, heavy-load and low-noise occasions. Under special working conditions, oil-containing bearings can maintain good performance and improve equipment reliability and service life.
(V) Medical device field
In terms of implants, powder metallurgy titanium alloys are used to manufacture artificial joints, etc. Their porous structure can promote bone cell growth and reduce the risk of implant loosening. Surgical instruments are made of powder metallurgy high-speed steel and stainless steel, which have higher hardness, wear resistance and corrosion resistance, and can also manufacture complex-shaped instruments. Among dental materials, dentures have good strength, toughness and aesthetics. Dental implants use powder metallurgy titanium or titanium alloys, which can improve the success rate of implantation. Orthodontic brackets use powder metallurgy stainless steel or nickel-titanium alloys, which can accurately apply force.
(VI) New energy field
In terms of lithium-ion batteries, positive electrode materials such as lithium iron phosphate and ternary materials prepared by powder metallurgy technology can improve battery energy density and charging and discharging efficiency. In the field of fuel cells, metal bipolar plates and catalyst carriers with high specific surface area manufactured by powder metallurgy process have improved the performance of fuel cells and reduced costs. In wind power generation, gearboxes, bearings and other parts manufactured by powder metallurgy can maintain stable performance in harsh environments and extend the service life of equipment.
Progress in Powder Metallurgy Technology
(I) Fusion of Metal Additive Manufacturing (3D Printing) and Powder Metallurgy
Metal additive manufacturing technology has developed rapidly in recent years. Its combination with powder metallurgy has brought new breakthroughs in the manufacture of complex parts. Through 3D printing technology, metal powders can be directly stacked layer by layer to produce parts with complex internal structures and personalized designs. This technology not only reduces material waste and processing procedures, but also realizes the manufacture of parts that are difficult to manufacture by traditional processing methods, such as complex blades of aircraft engines.
(II) Nanopowder Metallurgy Technology
With the development of nanotechnology, nanopowder metallurgy technology has emerged. Nano-scale metal powders have the characteristics of large specific surface area, high activity, and large sintering driving force, and can prepare nanostructured materials with excellent mechanical properties, electrical properties and magnetic properties. At present, nano powder metallurgy technology has made significant progress in the preparation of high-performance magnetic materials, superconducting materials and high-strength alloys.
Xi'an University of Technology, schematic diagram of the process of preparing spherical nano Ti-TiBw composite powder
(III) Innovation of powder metallurgy composite materials
By adding various reinforcing phases (such as ceramic particles, fibers, etc.) to metal powders, powder metallurgy composite materials with excellent performance are prepared. These composite materials combine the advantages of metals and reinforcing phases, and have the characteristics of high strength, high hardness, good wear resistance, high temperature resistance, etc., and are widely used in aerospace, automobile manufacturing, mechanical engineering and other fields. For example, the aluminum-based composite material prepared by adding silicon carbide particles to aluminum alloy powder has significantly improved strength and hardness, while maintaining the low density characteristics of aluminum alloy.
