Kunshan Shengyufeng New Material Technology Co., Ltd.
Contact: Mr. Yang
Address: No. 1911, Shuixiu Road, Kunshan City, Jiangsu Province
Compression (steel mold) forming is the most widely used forming method in powder metallurgy production. In the second half of the 18th century and the first half of the 19th century, Spain, Russia, and the United Kingdom used similar powder metallurgy processes to manufacture platinum products. At that time, Russia Sobolevsky (П.Г.Соболевсκий) used steel molds and screw presses. W.H. Wollaston in the United Kingdom used a tie-bar press with greater pressure and higher purity platinum powder to produce a dense platinum material with almost no residual pores. Later, the compression molding method was gradually improved and used to manufacture various shapes of copper-based oil-impregnated bearings and other products. Since the 1930s, in the industrial production process of powder metallurgy parts, press equipment, mold design and other aspects have been continuously improved, compression molding methods have been more developed, and mechanization and automation have reached a higher level. In order to expand the size and shape range of products, especially to increase the density of products and improve the uniformity of density, various forming methods have appeared and developed. The early emergence of powder rolling, cold isostatic pressing, extrusion, hot pressing, etc.; since the 1950s, hot isostatic pressing, hot extrusion, hot forging and other hot forming methods have appeared. These methods promote the production of fully dense, high-performance powder metal materials.
Compression molding inserts metal powder into a steel mold cavity, and presses the powder through a die to form it.
Molding process The powder in the mold cavity will produce a so-called "arch bridge" phenomenon due to the friction between particles and mechanical meshing, forming many pores of different sizes. When pressurized, the volume of the powder is compressed, and the process is generally represented by the relative density of the green compact-compression pressure curve. In the initial stage, the powder particles move and redistribute relatively, and the pores are filled, so that the density of the green compact increases sharply to reach the maximum packing density; at this time, the powder particles have been compressed with each other, so when the pressing pressure increases, the green compact density is almost No change, the curve appears flat. Then continue to increase the pressing pressure, and the powder particles will undergo elastic or plastic deformation or brittle fracture, which will further densify the compact. Due to the mechanical engagement between the particles and the attraction between the metal atoms on the contact surface, the compacted powder body becomes a compact with a certain strength. Regarding the powder compaction theory, since EWalker published his thesis in 1923, dozens of theories and empirical formulas have appeared, including Aji (LFAthy, 1930) and Balishin (Μ.Ю.Бальшин, 1938), Chuanbei Gongfu (1963) and others' formulas have certain practical significance; nevertheless, these theories are still in the exploratory stage. The compression pressure and the density distribution of the compacts are mainly consumed in the following two parts during the molding process: ①Overcoming the friction between the powder particles (called internal friction) and the deformation resistance of the powder particles; ②Overcoming the powder particles against the mold wall The friction (called external friction). Due to the presence of external friction, the density distribution of the molded compacts is actually uneven. For example, in unidirectional pressing, the part closer to the punch of the pressing die has a higher density, and the part farther away has a lower density. In the two-way pressing (actually a combination of two one-way pressing), the two ends of the compact along the direction parallel to the pressure have higher density and lower center. Adding lubricant to the powder or coating on the mold wall can improve the unevenness of the green compact density.
After the elastic after-effect compact is removed from the pressure or demold, the volume of the compact expands elastically due to internal stress relaxation. This phenomenon is called elastic after-effect. Elastic aftereffect is an important parameter for designing the die.
Compression molds and presses The main equipment for compression molding is compression molds and presses. The principle of die design is: give full play to the process characteristics of powder metallurgy with less cutting and no cutting to ensure that the three requirements of green compact quality (ie geometric shape, dimensional accuracy, smoothness, and uniformity of density) are met; mold materials are selected reasonably And the mold structure, put forward the mold processing requirements. Presses are divided into mechanical presses and hydraulic presses. The mechanical press is characterized by fast speed and high productivity; its disadvantages are that the pressure is small, the stroke is short, the stamping is not stable enough, the pressure is difficult to hold, and it is not suitable for pressing larger and longer products. Compared with the mechanical press, the hydraulic press (Figure 2) is characterized by high pressure, long stroke, relatively stable, can realize stepless speed regulation and pressure holding, and is suitable for pressing larger and longer products; its disadvantage is slow speed , Low productivity. Hot pressing is a forming method that combines molding and sintering. Because metal and alloy powders have good plasticity at high temperatures and are easy to deform, hot-pressed products are usually denser and stronger than cold-pressed sintered products. Hot pressing can be carried out in the atmosphere, protective atmosphere or real conditions. There are three main heating methods: conduction, induction and resistance heating. The density of the product is related to the hot pressing temperature, pressure and time. However, when the hot pressing temperature is so high that a liquid phase appears in the material, the pressure cannot be too high. Otherwise, the liquid components will be squeezed out, which not only can cause changes in the material composition, but also severely damage the mold. Hot pressing only needs a press equipped with a heating system and a high temperature resistant mold. The commonly used mold material is graphite. Because the pressure required for hot pressing is small, the product is compact, and the size is accurate, it is often used to produce large parts such as cemented carbide rolls and top hammers. Hot pressing is also suitable for the production of materials such as cermets with poor sinterability. The disadvantage of hot pressing is low productivity and higher cost than compression molding.
Isostatic pressing is a method in which the powder body is uniformly compressed in all directions through liquid or gas transmission pressure to achieve densification, called isostatic pressing, or isostatic pressing for short (see isostatic pressing). Isostatic pressing can be divided into cold isostatic pressing and hot isostatic pressing.
Cold isostatic pressing is usually to seal the powder in a soft sleeve, and then put it into the liquid medium in a high-pressure container, and apply pressure to the liquid to uniformly compress the powder in all directions to obtain the required compact. The liquid medium can be oil, water or glycerin. The covering material is an elastoplastic material such as rubber. The metal powder can be sleeved directly or after being molded. Because the powder is uniformly compressed in all directions in the sheath, a compact with a relatively uniform density can be obtained, and thus it is not easy to deform and crack during sintering. The disadvantage is that the green compact has poor dimensional accuracy and requires mechanical processing. Cold isostatic pressing has been widely used in the forming of cemented carbide, refractory metals and other various powder materials.
Hot isostatic pressing This is a new technology that appeared in the 1950s. Put the metal powder into a sheath that is easy to deform at high temperature, and then place it in a sealable cylinder (a high-pressure vessel with a heating body on the inner wall). After the cylinder is closed, the compressor is used to inject gas and be electrically heated. As the temperature increases, the gas pressure in the cylinder increases. The powder becomes a product with a certain shape under the action of this uniform pressure and temperature. The pressurized medium is generally argon. Commonly used sheathing materials are metals (low carbon steel, stainless steel, titanium), and glass and ceramics can also be used. Due to the simultaneous action of temperature and isostatic pressure, many kinds of materials that are difficult to form can reach or approach the theoretical density, and have fine grains, uniform structure, isotropy and excellent performance. The hot isostatic pressing method is most suitable for the production of materials and products such as cemented carbide, powdered superalloys, powdered high-speed steel and metal beryllium; it can also be used for secondary processing of molten cast products to eliminate pores and micro-cracks; it can also be used to manufacture different materials Tightly bonded multilayer or composite materials and products.
Powder forging presses metal powder into a preform, sinters it and then heats it for forging (see die forging) to reduce or even completely eliminate the residual pores, which is called powder forging. There are three forging methods: (1) Hot repressing. The shape of the preform is close to the shape of the finished product, and the outer diameter is slightly smaller than the inner diameter of the forging die cavity. Because the material does not flow laterally during forging, the forging has a residual porosity of 0 to 2%. ②Forging without flash. This forging is carried out in a limited die, the material flows laterally, and the forging does not produce flash. ③Closed die forging. The shape of the preform is relatively simple, and the outer diameter is much smaller than the inner diameter of the forging die. Flash occurs during forging, which is a method similar to conventional forging. Flashless forging and closed die forging are often used to produce parts that require high density. The design and manufacture of the preform is one of the key steps in powder forging. In addition, the hot forging preforms must be protected to prevent the oxidized and fallen oxide scales from sinking into the forgings and causing forging waste. The density of powder forgings can reach more than 98% of the theoretical density. Compared with conventional forging, powder forging has low pressure, low temperature, high material utilization rate, simple process and precise size; the performance of forgings can be close to ordinary forgings, and the directionality is small. Powder forgings are widely used in the automotive industry and transportation machinery.
Powder extrusion The advantages of powder extrusion are that the length of the extrusion is not limited, the product density is uniform, the production can be continuous, the efficiency is high, the flexibility is large, the equipment is simple, and the operation is convenient. Powder extrusion is divided into direct extrusion of metal powder and hot extrusion after packaging (see extrusion processing).
Direct extrusion mixes the organic and metal powder with good plasticity and puts it into the extrusion die. Under the action of external force, the plasticized powder is extruded through a certain geometric extrusion nozzle to become various pipes, bars and other special shapes. Of semi-finished products. The main factors affecting the extrusion process are the content of plasticizer, pre-pressing pressure, extrusion temperature and extrusion speed.
Sleeve extrusion Hot extrusion can combine hot pressing and thermoplastic processing to obtain fully dense high-quality materials; but in order to prevent the powder or compacts from oxidizing, they need to be packed into the envelope for hot extrusion. The material of the sheath must meet the following requirements: the rigidity of the sheath material at the extrusion temperature should be as close as possible to the powder being extruded, it will not react with the powder and can be removed by pickling or mechanical processing.
Powder rolling is a method in which metal powder is fed into the gap of a pair of rotating rolls, and the powder is continuously compressed and formed by the rolls due to frictional force. It is the main process for producing strip-shaped powder metallurgy materials. Generally include powder direct rolling, powder bonding rolling and powder hot rolling. The characteristics of powder rolling are: it can produce materials with special structure and performance, high yield rate, few working procedures, small equipment investment and low production cost.
Other methods ① loose sintering. Used in the manufacture of various porous materials and products, such as filters. ②Pour the slurry. Various products with complex shapes can be manufactured, such as tubes, crucibles, spherical vessels and hollow products. ③High-energy and high-speed forming and explosive forming. It can manufacture large and complex shaped products, such as turbine blades. In recent years, it has attracted widespread attention for use in forming to stimulate solidified powder. ④ Soft mold forming. It can form compacts that are difficult to form by ordinary pressing methods such as spheres, cones, and multi-step bodies. ⑤ Wedge suppression. It is suitable for making endless long products and thicker strips. ⑥ Discharge forming. Used for forming small and medium-sized products with complex shapes.
Contact: Mr. Yang
Address: No. 1911, Shuixiu Road, Kunshan City, Jiangsu Province