Introduction to powder metallurgy forming

Die forming (die) is a widely used forming method in powder metallurgy production. Similar powder metallurgy processes were used in the second half of the 18th century and the first half of the 19th century in Spain, Russia and England to make platinum products. At that time, Russian Sobolevsky (P., C, breakwebs daredaredobech) used steel die and screw press. W.h. Wollaston in Britain used a pull-rod press with greater pressure and a higher purity of platinum powder to produce a compact platinum material with few remaining pores. Later, the die forming method was gradually perfected and used to manufacture various shapes of copper base oil bearing and other products. Since the 1930s, in the process of powder metallurgy parts industrial production, press equipment, die design and other aspects of continuous improvement, die forming method has been greater development, mechanization and automation has reached a higher degree. In order to enlarge the size and shape range of products, especially in order to increase the density of products and improve the uniformity of density, a variety of forming methods have been developed. Early appearance of powder rolling, cold static pressing, extrusion, hot pressing, etc.; Since the 1950s, hot forming methods such as hot static pressing, hot extrusion and hot forging have appeared. These methods have promoted the production of fully dense, high-performance powder metal materials.

Die forming Metal powder is loaded into the cavity of a steel die, and the powder is pressed into shape by die punching.

The powder installed in the mold cavity during the molding process will produce the so-called "arch bridge" phenomenon due to the friction and mechanical mesh between the particles, forming many pores of different sizes. When pressed, the volume of the powder body is compressed, and the process is generally represented by the curve of relative density of the billet - pressing pressure. At the beginning, the powder particles move and redistribute, and the pores are filled, so that the compact density increases sharply and the filling density reaches a large size. At this time, the powder particles have been pressed together, so when the pressing pressure increases, the density of the billet is almost unchanged, and the curve is flat. Then the pressing pressure continues to increase, the powder particles will have elastic, plastic deformation or brittle fracture, so that the compact is further densified. Due to the mechanical meshing between the particles and the attraction between the metal atoms on the contact surface, the pressed powder becomes a compact with certain strength. From 1923, when E.E.Walker published his thesis, dozens of theoretical and empirical formulae have appeared. Among them, the formulae of L.F.Athy (1930), S. Barishen (South. However, these theories are still being explored. In the process of molding, the pressing pressure is mainly consumed in the following two parts: (1) to overcome the friction force between powder particles (called internal friction force) and the deformation resistance of powder particles; (2) Overcome the friction force of powder particles on the mold wall (known as the external friction force). Because of the external friction force, the density distribution of the blank is actually uneven. For example, in unidirectional pressing, the density of the part closer to the punch of the pressing die is higher, and the density of the part farther away is lower. In the bidirectional pressing (actually is the combination of two unidirectional pressing), the density of the two ends of the blank along the direction parallel to the pressure is higher, and the central part is lower. Adding lubricant to powder or coating on mold wall can improve the inhomogeneity of compact density.

After the pressure is removed or the mold is demoulded, the volume of the pressed blank will expand elastic due to the relaxation of the internal stress. This phenomenon is called elastic aftereffect. Elastic aftereffect is an important parameter in die design.

The main equipment of die and press forming is die and press. The principle of die design is: give full play to the process characteristics of less cutting and no cutting of powder metallurgy, ensure to meet the three requirements of billet quality (namely, geometric shape, dimensional accuracy and uniformity of finish and density); Select the die material and die structure reasonably, put forward the die processing requirements. The press is divided into mechanical press and hydraulic press. Mechanical press is characterized by high speed and high productivity; Its disadvantages are smaller pressure, short stroke, stamping is not smooth enough, pressure retention is difficult, not suitable for pressing larger and longer products. Compared with the mechanical press, the hydraulic press (FIG. 2) is characterized by large pressure, long stroke, relatively stable, can achieve stepless speed regulation and pressure preservation, suitable for pressing larger and longer products; Its disadvantages are slow speed and low productivity. Hot pressing is a forming method that combines molding with sintering. Because metals and alloy powders are plastic and prone to deformation at high temperatures, hot-pressed products are generally denser and stronger than cold-pressed sintered products. Hot pressing can be carried out under atmospheric, protective atmosphere or real conditions. There are three main heating methods: conduction, induction and resistance heating. The density of the product is related to hot pressing temperature, pressure and time. However, when the hot pressing temperature is high enough that there is a liquid phase in the material, the pressure cannot be too great. Otherwise, the liquid phase will be extruded, which can not only cause material composition changes, but also seriously damage the mold. Hot pressing as long as equipped with a heating system of the press and high temperature resistant mold. The common mold material is graphite. Because the pressure required by hot pressing is small, the product is dense, the size is jing, so it is often used in the production of hard alloy roll, hammer and other large parts. Hot pressing is also suitable for the production of poor sintering properties of cermet and other materials. The disadvantage of hot pressing is low productivity and higher cost than die pressing.

Isostatic pressure through liquid or gas pressure transfer to achieve densification of powder body under uniform pressure in all directions, known as isostatic pressing, isostatic pressure for short (see isostatic pressure processing). Isostatic pressure can be divided into cold isostatic pressure and hot isostatic pressure.

Cold isostatic pressing is usually to seal the powder in a soft envelope, and then put it into the liquid medium in a high-pressure vessel. By applying pressure to the liquid, the powder body is under uniform pressure in all directions, so as to obtain the required billet. The liquid medium may be oil, water, or glycerin. The casing material is elastoplastic material such as rubber. The metal powder can be installed directly or after molding. Because the powder is under uniform pressure in all directions in the envelope, the compact with uniform density can be obtained, so it is not easy to deform and crack during sintering. Its disadvantage is the dimensional precision of the compact is poor, but also mechanical processing. Cold isostatic pressing has been widely used in forming hard alloy refractory metal and other powder materials.

Hot isostatic pressure this was a new technology that came out in the 1950s. The metal powder is packed into a jacket that is easy to deform at high temperature, and then placed in an airtight cylinder block (a high-pressure vessel with a heater on the inner wall). After the cylinder block is shut down, the gas is driven in by a compressor and heated by electricity. As the temperature rises, the gas pressure in the cylinder increases. The powder becomes a product with a certain shape under the action of uniform pressure and temperature in all directions. Argon gas is generally used as the pressurized medium. Commonly used cladding materials for metal (low carbon steel, stainless steel, titanium), but also available glass and ceramic. Due to the simultaneous action of temperature and isostatic pressure, many kinds of hard to form materials can reach or close to the theoretical density, and fine grain, uniform structure, isotropy and excellent performance. Hot isostatic pressing is suitable for the production of cemented carbide, powder superalloy, powder high speed steel, beryllium and other materials and products. It can also be used for secondary treatment to eliminate pores and microcracks. It can also be used to manufacture multiple layers or composite materials and products which are closely bonded with different materials.

Powder forging The process by which metal powder is pressed into preformed billets, sintered and then heated for forging (see die forging) in order to reduce or even completely eliminate the residual pores in them is called powder forging. There are three forging modes: (1) hot repressing. The shape of the preformed blank is close to that of the finished product, and the outside diameter is slightly less than the inside diameter of the die cavity of the forging die. Because no lateral flow occurs during forging, the forgings have a residual porosity of 0 ~ 2%. ② Forging without flying edge. This forging is carried out in limited die, the material has lateral flow, the forging does not produce flying edge. ③ Closed die forging. The shape of the preforming billet is simple, and the outside diameter is much smaller than the inside diameter of the forging die, and the forging produces flying edge, which is a similar method with the conventional forging. Flightless forging and closed die forging are often used to produce parts requiring high density. The design and manufacture of preforming billet is one of the key steps in powder forging. In addition, the hot forging preforming billet must be protected from oxidation and falling oxide skin trapped in the forging 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 the advantages of low pressure, low temperature, high material utilization rate, simple process and accurate size. The performance of forgings is close to that of ordinary forgings, and the directivity is small. Powder forgings are widely used in automobile industry, transportation machinery and so on.

The advantages of powder extrusion are that the length and size of extrusion parts are not limited, the product density is uniform, the production can be continuous, high efficiency, flexibility, simple equipment, convenient operation. Powder extrusion is divided into two kinds: direct extrusion of metal powder and hot extrusion after packaging (see extrusion processing).

After direct extrusion, the organic matter and metal powder with good plasticity are mixed and put into the extrusion mold. Under the action of external force, the plasticized powder is extruded through the extrusion nozzle of certain geometric shape to become a variety of semi-finished products of pipes, bars and other special-shaped shapes. The main factors affecting the extrusion process are the content of plasticizer, precompression pressure, extrusion temperature and extrusion speed.

Hot extrusion can combine hot pressing and thermoplastic machining to obtain full compact high quality materials. But to prevent the powder or billet from oxidizing, they need to be packed into the envelope for hot extrusion. The material of the envelope must meet the following requirements: The rigidity of the envelope material at the extrusion temperature should be as close as possible to the extruded powder, do not react with the powder and can be removed by pickling or machining.

Powder rolling A process in which metal powder is fed into the cracks of a pair of rotating rollers and is continuously compressed by the rollers due to friction. It is the main process of producing strip powder metallurgy materials. It generally includes powder direct rolling, powder bonding rolling and powder hot rolling. The characteristics of powder rolling are: can produce special structure and performance of materials, high yield, less process, equipment investment, low production cost.

Other methods ① loose sintering. Used in the manufacture of various porous materials and products, such as filters. ② slurry casting. Can manufacture a variety of complex shape products, such as tubes, crucible, spherical vessels and hollow products. ③ High energy high speed forming and explosive forming. Large and complex shape products can be manufactured, such as turbine blades. In recent years, the application in forming chilled solidified powder has attracted widespread attention. ④ Soft die forming. Can form such as ball, cone, multi - step body and other ordinary pressing method is difficult to form the billet. ⑤ Wedge pressing. Suitable for making long ring products and thicker strip. ⑥ Discharge forming. Used for medium, small and complex shaped products forming.