Metallurgy: metal casting
Obtaining goods by means of casting metal alloys yavlis Busy one of the oldest methods of metalworking. In the historical literature to date is an ongoing debate on the question of what methods of manufacturing metal objects man possessed before - or forging, ie who had appeared - caster or blacksmith. It should be noted, however, that pure metals in the crust yesAlso in those days is extremely rare. Their production is associated with processing ores in which the result is obtained in the liquid metal, molten form. They are embedded in special molds, and after hardening of the casting or ingot is obtained. Casting is a finished product, and bar must also be subjected to plastic GRAINheel, such as forging. Thus, the molding process is usually preceded by any method of metal working.
Production of metals and alloys
Currently almost all products from metal alloys prepared from the cast, t. E. A crystallized from a liquid state, withmelt. Production of metals and alloys in the liquid, molten state called pyro. It is the primary metallurgical process. In connection with this casting processes occurring in the preparation of the liquid metal casting or ingot, largely determine the quality characteristics of metal and performance of any mAshin from the car to a space rocket.
Furthermore pyrometallurgy hydrometallurgy intensively developed methods in which the metals are obtained from solutions or melts of salts. The metals precipitated in the solid form. These methods, for example electroplating obtaineds copper cathode, based on chemical and electrochemical processes. However, the volume of metal produced hydrometallurgical methods, does not go to any comparison with the volume of pyrometallurgical production. This situation will continue for the foreseeable future.
currently intensively developed powder metallurgy, i.e. to provide articles by compacting and sintering metal powders. However, methods for obtaining basic powders based on crystallization pulverized in various environments (such as water) of molten metal, i.e. There are also place casting process.
The mainthe process of the formation of the casting or ingot is solidifying alloy t. e. the transition of the alloy from the liquid to the solid state. Typically, there are two aspects of solidification.
The first aspect
The first aspect — heat. As is known, the melting of the substance requires an expenditure of energy toThoraya respect to unit mass of a substance is called the specific heat of fusion. Tion during the solidification of the equivalent amount of heat must be removed from the alloy. If the molten metal at a temperature in crystallizing place ideal insulating sheath, it will be in liquid form indefinitely. Greater speed than thto be discharged from the melt in the mold wall, the faster hardened alloy. Therefore liteyshik should be able to control the thermal processes during cooling and solidification and heat form.
The second aspect
The second aspect is related to the formation of the crystal structure of the casting,ie crystallization. Curing is usually accompanied by a phase transition from liquid to solid crystalline state. This transition is associated with the birth and growth of crystallization centers, called nuclei. Performance and physical properties of castings depends on the size and shape of the crystallites. Nucleation and crystal growth - withfalse physical and chemical processes that, according to the academician LV Shubnikov, can be compared only with the birth and development of biological organisms.
The crystal structure of the casting
The crystal structure of the cast is largely determined by the structure and properties of liquid alloys. Therefore, great importance is the control of the liquid state of the alloy by changing its chemical composition, the introduction of various impurities, the use of physical and mechanical effects (pressure, ultrasound, electromagnetic effects, radiation, X-rays, gamma rays, and so on. N.).
As you know, the body expands when heated and contracts when cooled. During the solidification of almost all metals reduce its volume. The exception is bismuth, gallium n. Reduction of the volume on cooling and solidification of the alloy is called shrinkage. Shrinking processes lead to the formation of castings pazlichnyh defects such as shrinkage cavities (shells or pores). Due to thermal shrinkage or mechanical braking time in castings having residual stresses that reduce its strength and sometimes leading to warping or casting to rise cracks therein. Questions of formation of stress go far beyond purely casting probleat and have a great scientific importance.
Completing the alloy
In the context of the Earth as a planetary formation casting processes involve not only the action of gravitational forces, but also the influence of the gas atmosphere. Questions dissolved gases and separate them from the metal melts is an important section of the casting technology. During the preparation of the alloy, its casting and solidification occur complex physical and chemical phenomena accompanying the dissolution of gases in liquid and solid metal, the formation of various products of the chemical interaction of gases with alloy components (oxides, carbides, nitrides, and so on. D.). In aboutprocess of casting and alloy casting and thermal interaction forms can be captured gas flow of the alloy, the appearance of oxide captive gas formation in the amount of sand molds, leading to an increase in gas pressure in the mold cavity and its walls. If you do not ensure compliance with the conditions imposed on gas-dynamic modeforms that occur in the casting defects in the form of a gas or pores shells. The increase in gas pressure die casting with high and low gas permeability gazotvornostyu may even lead to the release of metal from the mold.
The most important process in casting technology is a form filling alloy. Therefore engineer lit.eyschik should be a specialist in the field of fluid mechanics, especially fluid mechanics melts. You must be able to control the speed of the metal in the form and its channels, the flow pattern can capture various non-metallic inclusions, caught in the melt, as well as a smooth, shock-free motion of the melt in different parts of the mold cavity.
The theory of foundry processes
The theory of foundry processes includes a wide range of issues related to the shaping, forming mechanical and technological properties of molds, forms interaction with the melt determines the quality of the surfaces produced castings. Addressing these issues is based on the extensive use of the latest achievements of physical and inorganic chemistry. You can talk about the development of special chemistry - chemistry foundry molding materials.
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