Types of castings. casting methods. Rare and precious metals

10.05.2023

The ability of metal in molten form to fill any cavities has been used by man for many years for the manufacture of various products.

Nowadays there are various metal casting methods, which differ from each other in technology, since certain conditions must be created for each material so that it can fill all the cavities of a pre-prepared form. This is due to the different fluidity of metals - a parameter that characterizes the ability of the melt to spread rapidly.

Let us consider in detail what methods of casting metals are now used in industry, and what parts or blanks can be obtained with their help.

The most popular metal casting methods:

casting into the ground;

Die casting;

centrifugal casting;

electroslag casting;

Injection molding;

Static fill.

To understand what the features of each of the methods are, let's get acquainted with their technologies.

Casting metals into the ground

This process has been famous since ancient times, today it is used mainly for the manufacture of single castings.

The most important advantage of the technology of casting metal into the ground is its low cost, and the disadvantage is its high labor intensity.

The process begins with the fact that a model of the future casting is made in a special workshop, wood and other materials are used for this.

Then the molding sand is prepared, it contains earth and other additives. After that, a mold is made into which the melt is poured.

After cooling, the workpiece is removed from the mold and sent for further processing, it is cleaned by sandblasting or polished to completely remove the remnants of the molding earth.

Cast iron is best suited for this type of casting because it has excellent fluidity, and other metals are also used.

Metal casting in a chill mold

This casting method consists in the fact that a mold (chip mold), consisting of two parts, one of which contains a core, is connected before starting the process.

Liquid metal is poured into the mold, where it quickly cools, and after a few minutes a finished casting is obtained, which can be removed.

This method also uses only those materials that have good fluidity, while injection molding is suitable for other types.

Die casting

Filling the mold with metal in this case is carried out under high pressure of air or a piston. The application of pressure helps the material to take even the most complex shape configuration, fill its finest grooves and repeat all the bends.

Such metal casting methods require particularly strong molds, which are made of steel.

centrifugal casting

For this casting method, sand or metal molds are used. The peculiarity lies in the fact that they rotate around the axis vertically or horizontally during the process.

The melt is poured into the mold and fills its periphery under the action of centrifugal forces, then it solidifies.

This method is most expedient to produce pipes, rings and similar elements.

Electroslag casting

Liquid metal for this type of casting is obtained by electroslag remelting.

As a mold, a water-cooled copper mold is used, and the metal enters it after melting, without coming into contact with air.

Static pouring of metal

This is the simplest method, in which the melt is poured into a fixed mold until it is completely filled. Then it freezes and is removed.

Metal casting methods of this type make it possible to produce castings of the simplest form.

Advantages and disadvantages of metal casting technology

The manufacture of metal products by casting has its advantages and disadvantages.

The advantages include the relative simplicity of technology and high productivity, as well as the good quality of the resulting castings.

Obvious disadvantages are: the need to use special melting furnaces, the high energy intensity of the processes, the impossibility of applying the method to certain types of metal.

Despite this, many industrial enterprises use technology for the manufacture of a wide variety of parts.

In addition, technologies have recently appeared that allow to automate all processes as much as possible, which has made them less labor-intensive.

Presentation of equipment and technologies for metal casting at a specialized exhibition

It will be held at the Expocentre Fairgrounds in the spring.

At the international class event, exhibitors from around the world will present the latest metal casting methods and other processing technologies, demonstrate equipment and tools, and introduce guests to their latest developments.

You can order e-tickets right now so as not to miss the most important metalworking event of the year.

In recent years, special casting methods have been introduced everywhere in the foundry industry, which have a number of advantages compared to traditional casting in disposable sand-clay molds. The proportion of castings obtained by special methods is steadily increasing.

Special methods include casting: a) into permanent metal molds (chill molds), b) centrifugal, c) under pressure, d) into thin-walled one-time molds, e) according to investment models, f) cortical, or shell, g) electroslag casting.

Special casting methods make it possible to obtain castings of more accurate dimensions with good surface quality, which helps to reduce metal consumption and the laboriousness of machining; improve the mechanical properties of castings and reduce losses from marriage; significantly reduce or eliminate the consumption of molding materials; reduce production space; improve sanitary and hygienic conditions and increase labor productivity.

Most operations with special casting methods can be easily mechanized and automated.

The economic feasibility of replacing casting in disposable sand-clay molds by one or another special method depends on the scale of production, the shape and size of the castings, the casting alloys used, etc. It is determined on the basis of a thorough technical and economic analysis of all costs associated with a new technological process.

One of the most common is mold casting. A chill mold is a solid or split metal mold made of cast iron or steel.

Chill molds are designed to produce a large number of identical castings from non-ferrous or iron-carbon alloys. The resistance of molds depends on the material and dimensions of the casting and the mold itself, as well as on compliance with its operation mode. Approximately the resistance of cast iron molds is 200,000 tin-lead, 150,000 zinc, 50,000 aluminum or 100.5000 iron castings. It is expedient to use chill molds both in mass and serial production (with a batch of castings of at least 300,500 pieces).

Before pouring the metal, the molds are heated to a temperature of 100-300 °C, and the working surfaces in contact with the molten metal are covered with protective coatings. The coating provides an increase in the service life of the mold, prevention of welding of metal to the walls of the mold and facilitating the extraction of castings. Heating protects the mold from cracking and facilitates the filling of the mold with metal. During operation, the required temperature of the mold is maintained due to the heat released by the poured metal. After hardening, the casting is removed by shaking or using an ejector.

Chill casting makes it possible to reduce the consumption of metal for risers and risers, to obtain castings of higher accuracy and surface finish, and to improve their physical and mechanical properties. However, this casting method also has disadvantages. Rapid cooling of the metal makes it difficult to obtain thin-walled castings of complex shape, and causes the danger of the appearance of hard-to-cut surfaces in cast-iron castings.

Injection molding- one of the most productive methods for obtaining precise shaped castings from non-ferrous metals. The essence of the method lies in the fact that liquid or mushy metal fills the mold and crystallizes under excessive pressure, after which the mold is opened and the casting is removed.

According to the method of creating pressure, they are distinguished: casting under piston and gas pressure, vacuum suction, liquid stamping.

The most common shaping of castings under piston pressure is in machines with a hot or cold compression chamber. Alloys used for injection molding must have sufficient fluidity, a narrow temperature-time interval of crystallization, and not chemically interact with the mold material. To obtain castings by the considered method, zinc, magnesium, aluminum alloys and alloys based on copper (brass) are used.

Injection molding produces parts of devices: drums of counting machines, camera bodies and body parts weighing up to 50 kg, cylinder heads of motorcycle engines. Holes, inscriptions, external and internal threads can be obtained in castings.

Fig.5 Special casting methods

a - under pressure; b - centrifugal.

Figure 5, a shows the sequence of obtaining a casting on a piston machine (with a cold vertical compression chamber). The molten metal is fed in portions into the vertical pressing chamber 2. When moving down, the piston 1 presses on the metal, moves the heel 4 down, as a result of which the feed channel 3 opens and the metal enters the mold cavity 5. After filling the mold and holding for 3-30 s, the piston and the heel rise, while the heel cuts off the sprue and pushes out the press residue b. The movable part of the mold 8 moves to the right, and the casting 7 is easily removed. Internal cavities and holes in castings are made using metal rods.

Before starting work, the mold is heated and lubricated. During operation, the required temperature is maintained and the mold is periodically lubricated.

Molds are made from alloyed tool steels (3Kh2V8, KhVG, Kh12M, etc.) and subjected to hardening with high tempering. The cost of the mold is 3.5 times the cost of the mold.

The durability of molds, depending on the size and shape of the castings, is 300.500 thousand castings from zinc alloys, 30.50 thousand castings from aluminum, and 5.20 thousand castings from copper. The productivity of piston machines reaches 500 castings per hour.

Under conditions of mass production, the use of injection molding is economically justified, since this method makes it possible to reduce the labor intensity of obtaining castings by 10–12 times, and the labor intensity of mechanical processing, by 5–8 times.

Due to the high precision of manufacturing and the provision of increased mechanical properties of castings produced under pressure, savings of up to 30.50% of metal are achieved compared to casting in single molds. It creates the possibility of complete automation of the process.

Centrifugal casting method It is mainly used to produce hollow castings such as bodies of revolution (bushings, shells for piston rings, pipes, liners) from non-ferrous and iron-carbon alloys, as well as bimetals. The essence of the method consists in pouring liquid metal into a rotating metal or ceramic mold (mould). Liquid metal due to centrifugal forces is thrown to the mold walls, spreads along them and hardens.

With the casting method under consideration, castings are obtained dense, free of gases and non-metallic inclusions, with a fine-grained structure.

Centrifugal casting is highly productive (40 .50 cast iron pipes with a diameter of 200 .300 mm can be cast in 1 hour), makes it possible to obtain hollow castings without the use of rods and bimetallic castings by sequential pouring of two alloys (for example, steel and bronze).

Along with high productivity and simplicity of the process, the centrifugal casting method, in comparison with casting into stationary sand-clay and metal molds, provides a higher quality of castings, almost eliminates metal consumption for risers and risers, and increases the yield of good casting by 20.60%.

The disadvantages of the method include the high cost of molds and equipment and the limited range of castings.

Investment casting consists of the following. The metal is poured into a one-time thin-walled ceramic mold, made according to models (also one-time) from a low-melting model composition. In this way, precise castings from any alloys weighing from a few grams to 100 kg are obtained, which practically do not require machining.

The dimensional accuracy and surface finish of the resulting castings are such that they make it possible to reduce the amount of machining or to abandon it, which is especially important in the manufacture of parts from hard-to-machine alloys;

The technology for the production of castings according to the models being performed includes the following stages: production of molds for models; obtaining wax models by pressing the model composition into molds; assembly of a block of models on a common feeder (in the case of small castings); applying a refractory coating to the surface of a single model or block; melting models from refractory (ceramic) mold shells; annealing molds; pouring metal into hot molds.

Split molds are made of steel or other alloys according to the drawing of the part or its standard, taking into account the shrinkage of the model mass and the casting metal.

The model composition (for example, from paraffin with additives of ceresin, petroleum bitumen, rosin, polyethylene) in a pasty state is pressed with a syringe or on a pressing machine.

The resulting models are removed from the molds and lined in several layers with a refractory coating, dipped several times in a binder and sprinkled with quartz sand. Each layer of the coating is dried. The model of small castings is assembled into blocks before coating, connecting them (soldering) to a common gating system, and then lining the block.

Melting models from ceramic shells is done with hot air or hot water. The model material is collected for reuse, and the resulting ceramic mold with a smooth working surface is fed to the calcination. The latter is necessary to give the form mechanical strength and the final removal of the model material. The mold is placed in a steel box, covered with quartz sand, leaving the gating cup available for pouring metal, and calcined at a temperature of 850-900 °C.

The metal is poured into a hot mold, which improves the fluidity of the metal and makes it possible to obtain the most complex thin-walled castings.

After cooling, the casting is cleaned from the layer of refractory coating by hand blows or on pneumatic vibrators. In cavities and holes, mold residues are removed by leaching in a boiling solution of caustic soda, then the casting is washed in warm water with the addition of soda.

Separation of the gating system from castings can be carried out on lathes and milling machines, vulcanite abrasive wheels and vibration machines.

Investment casting produces a variety of complex castings for automotive and tractor construction, instrument making, for the manufacture of aircraft parts, turbine blades, cutting and measuring tools.

The cost of 1 ton of investment castings is higher than those produced by other methods, and depends on many factors (serial production of parts, the level of mechanization and automation of foundry processes and casting machining processes).

In most cases, reducing the labor intensity of machining, the consumption of metal and metal-cutting tools when using precision castings instead of forgings or castings obtained by other methods, gives a significant economic effect. The greatest effect is achieved when transferring to investment casting of parts, in the cost structure of which a large share is the cost of metal and milling, especially when using difficult-to-machine structural and tool materials.

Much attention is paid to the introduction of investment casting, since most of the operations can be easily mechanized and automated. Through the joint efforts of employees of scientific research institutes and advanced factories, high-performance automatic lines and automated workshops for investment casting are being created.

Shell casting it is used to obtain castings weighing up to 100 kg from cast iron, steel and non-ferrous metals. Thin-walled (wall thickness 6.10 mm) molds are made from a sand-resin mixture: fine-grained quartz sand and thermosetting synthetic resin (3.7%). The sand-resin mixture is prepared by mixing sand and crushed powdered resin with the addition of a solvent (cold method) or at a temperature of 100-120 ° C (hot method), as a result of which the resin envelops (clads) the sand grains. Then the mixture is further crushed to obtain individual grains, clad with resin, and loaded into the hopper. Molding is made on metal models.

The model in the gating system is fixed on the under-model plate, heated to a temperature of 200-250 ° C and a thin layer of release agent is applied to their working surface. After that, the mouth of the bunker is closed with a model plate (the model is inside) and it is rotated by 180°. The mixture falls on the heated model, the resin melts and after 15.25 s a shell (half-mould) of the required thickness is formed on the model. The hopper is again rotated by 180°, the remaining mixture is poured to the bottom of the hopper, and the model board with a semi-hard shell is placed in an oven for final hardening at a temperature of 300.400 °C for 40.60 s. With the help of special ejectors, the half-mould can be easily removed from the model.

Fastening (assembly) of half-forms is carried out with metal brackets, clamps or quick-hardening glue. Sand-resin cores for hollow castings are produced in a similar way.

The assembled shell molds are placed in flasks to give them greater rigidity, they are covered with iron shot or dry sand from the outside and poured with metal. After the casting has hardened, the shell mold is easily destroyed.

Castings made in shell molds are distinguished by high accuracy and surface cleanliness, which makes it possible to reduce the mass of castings by 20–40% and the labor intensity of their machining by 40–60%. Compared to casting in sand-clay molds, the complexity of manufacturing castings is reduced by several times. In this way, critical machine parts are obtained - crankshafts and camshafts, connecting rods, ribbed cylinders, etc. Shell manufacturing processes can be easily automated.

Despite the high cost of the sand-resin mixture, in comparison with the sand-clay mixture, a significant economic effect is achieved in the mass and serial production of castings.

Shell mold casting is used to manufacture parts mainly from iron-based alloys (cast iron, carbon steel and stainless steel), as well as from copper and special alloys.

At the Kiev Motorcycle Plant, ribbed cylinders are cast from modified chromium-nickel cast iron in this way; at the Gorky Automobile Plant, crankshaft halls from high-strength cast iron are obtained in shell molds.

3. Manufacture of products by pressure

METAL PRESSURE PROCESSING - shaping of metal materials by mechanical means without chip removal.

Processing of metals by pressure is a group of technological processes, as a result of which the shape of a metal workpiece changes without violating its continuity due to the relative displacement of its individual parts, i.e., by plastic deformation. The main types of metal fabrication are: rolling, pressing, drawing, forging, and stamping. Omd is also used to improve surface quality.

The introduction of technological processes based on metalworking, compared with other types of metalworking (casting, cutting), is steadily expanding, which is explained by a decrease in metal losses and the possibility of ensuring a high level of mechanization and automation of technological processes.

O.m.d. products can be obtained with a constant or periodically changing cross-section (rolling, drawing, pressing) and piece products of various shapes (forging, stamping), corresponding in shape and size to finished parts or slightly different from them. Piece products are usually machined. The volume of metal removed in this case depends on the degree of approximation of the shape and dimensions of the forging or stamping to the shape and dimensions of the finished part. In a number of cases, metalworking products are obtained that do not require cutting (bolts, screws, most sheet stamping products).

Along with shaping, pressure treatment can improve the quality and mechanical properties of the metal. Processing of metals by pressure is carried out either in a "hot" (heated) or "cold" (corresponding to room temperature) state. For many metals and alloys, the forming process is first hot worked to take advantage of the increased ductility of the heated material, followed by cold finishing to ensure high surface quality and accurate dimensions. Basic metal forming methods- forging, stamping, rolling, pressing.

Forging and stamping. Hand forging was historically the first method of forming metal processing still used today. The first steam hammer, which appeared in 1843, deformed the metal by the force of a falling load, and the steam served to lift the latter. Following such a single-action hammer, in 1888 a double-action hammer appeared, the upper "woman" of which, when moving down, is additionally accelerated by steam power. Forging and forging can be done with a hammer or a press. Forging is free and in stamps. Die forging hammer and for hot stamping presses consist of the upper (fixed on the upper head of the hammer or press) and lower parts, on the contact surfaces of which there are streams for consistent shaping of products. Dies for sheet stamping (cutting, punching, bending, etc.) consist of two main parts - a matrix and a punch included in it, and sometimes the same part of the stamp serves as both a punch and a matrix.

Rolling. Roll reduction is the most common metal forming process. Although the "father" of modern rolling methods is considered to be G. Kort, whose first rolling mill dates back to about 1783, historical documents indicate that gold and silver for minting coins were rolled into sheets in France as early as 1753. There are many different types of rolling mills, but in almost all such installations, the reduction is carried out by two rolls rotating towards each other. Rolls capture the workpiece, and it comes out of them, decreasing in thickness and increasing in length. The resulting lateral, or transverse, broadening is in most cases insignificant. The names of the rolling mill usually indicate the type of product produced: blooming, slab, plate, strip, plate. In accordance with the temperature of the rolled metal, hot and cold rolling mills are distinguished.

Pressing. Many metals and alloys at elevated temperatures are so ductile that they can be squeezed out under pressure through a die hole, like toothpaste from a tube. This method of pressing by extrusion, or extrusion, can produce products of complex cross-section. By extrusion, for example, rods, pipes, shaped products are obtained, lead-sheathed cables. By pressing without expiration, in particular, deep drawing operations are carried out - the transformation of a flat workpiece into a sleeve.

Firmware. The piercing operation is used in the manufacture of seamless pipes from cast cylindrical billets and extruded bars. The heated workpiece is captured by two oblique (conical) rolls of the piercing mill, rotating towards each other, and is advanced in the process of helical (helicoidal) rolling onto a mandrel fixed in the middle between the rolls. Of the various devices for the production of seamless pipes, the most famous is the Mannesmann piercing mill. Not all metals and alloys can be pierced, but steel, copper and some copper-based alloys are ductile enough for such processing, which requires very large deformation.

Drawing. Rods and wire. The diameter of a bar obtained by extrusion or rolling can be reduced by pulling it through a hole in a drawing board (wolves or dies). By drawing through a series of dies with progressively smaller holes, a small diameter rod can be obtained. In the same way, a wire can be obtained from a bar of the smallest diameter. The compression of a wire, especially a very thin one, is often carried out by continuously pulling it through a series of dies, the number of which can reach 12.

Pipes. Pipe drawing is typically used to reduce the outside diameter of a pipe or its wall thickness, or both. Cold drawing provides a smooth pipe surface, accurate dimensions and improved mechanical properties. Such "reduction" when sizing pipes is carried out by drawing through a die with a slightly reduced hole, in the center of which a mandrel is fixed. The reduction in pipe wall thickness is determined by the diameter of the mandrel.

extrusion. A thin metal is formed by extrusion on a lathe, pressing it against a rotating mandrel. This method is suitable only for the manufacture of symmetrical products with a circular cross section. For extrusion of products with a diameter that varies along the axis, collapsible mandrels are required that allow the removal of the finished product.

4. Making non-separable joints

Automatic submerged arc welding. The essence of the process is that the welding arc 2 burns between the electrode wire 1 and the product to be welded 9 under a layer of loose flux 6. The heat of the arc melts the base metal, welding wire and flux. The wire is mechanically fed into the arc burning zone, and the machine moves along the welded edges with the help of an electric motor, such a welding process is called automatic; if only wire feeding is mechanized, then this is mechanized submerged arc welding. Melting, the flux forms a flux-gas bubble 3 and liquid slag 5. Molten metal 4 crystallizes during cooling to form a weld 8. Almost simultaneously with the crystallization of the molten metal, the molten flux solidifies - liquid slag, forming a slag crust 7 (Fig. 1). The pressure in the gas bubble is 5-9 g/cm3 (0.5-0.9 kPa). If the arc breaks out during welding, this indicates an insufficient flux layer. Varieties of submerged arc welding are shown in fig. 2, with this type of welding, high labor productivity is achieved and an equal-strength weld with the base metal is obtained.

Rice. 1. Flow chart of automatic submerged arc welding process:

1 - electrode, 2 - welding arc, 3 - flux gas bubble, 4 - molten metal, 5 - liquid slag, 6 - flux, 7 - slag crust, 8 - weld, 9 - welded product

Rice. 2. Varieties of submerged arc welding:

a - single-arc, b - single-arc with a split electrode, c - two-arc, d - three-phase arc;

1 - workpiece to be welded, 2 - flux, 3 - welding wires supplying welding current from the power source to the welding arc, 4 - electrode

Electroslag welding. The essence of the process is as follows. In the initial period, a welding arc occurs under the flux, due to the heat of the arc, the flux melts and an electrically conductive slag is formed, which must have a significant ohmic resistance. The welding arc after melting the flux with the formation of an electrically conductive slag dies out - it is shunted, and the current passing through the electrically conductive molten slag releases such an amount of heat that is sufficient to melt the subsequent portion of the flux, base metal and wire. The molten metal of the weld pool, crystallizing, forms a weld (Fig. 3, b).

Rice. 3. Scheme of electroslag welding:

1 - electrode, 2 - welded metal, 3 - molten flux - electrically conductive slag, 4 - molten metal, 5 - copper sliders, 6 - water supply for cooling the sliders, 7 - weld, 8 - flux; Vsv - welding speed

In practice, this process (Fig. 3, a) occurs between the edges of the base metal 2, which are located vertically with a large gap. To form a seam, i.e., to hold the molten metal of the weld pool, copper sliders 5, cooled by water, are installed on both sides of the joint. Electrode wire 1 is fed into the welding zone, which, under a layer of flux 8, excites the burning of the welding arc.

Advantages of this type of welding:

the possibility of welding in one pass of metal of great thickness;

no need to remove slag and adjust the welding mode for the next pass, as is done with other types of welding;

the ability to perform welding without cutting edges and the elimination of metal spatter;

the possibility of using an almost unlimited number of electrodes (wires) for welding;

exclusion of heat treatment of the weld when welding steels prone to the formation of shrinkage cracks;

high productivity and flux savings.

Disadvantages of this type of welding:

the possibility of welding metal with a thickness of at least 16 mm;

welding is practically possible only in a vertical position;

the formation of unfavorable structures due to heat treatment of the seam and the heat-affected zone is possible.

According to the type of electrode, electroslag welding is divided into welding with a wire, plate electrode and consumable mouthpiece; by the presence of oscillations of the electrode - without oscillations and with oscillations of the electrode; by the number of electrodes with a common supply of welding current - into single-electrode, two-electrode and multi-electrode.

electron beam welding. This type of welding is performed in chambers with a vacuum of up to 10-4-10-6 mm Hg. Art. Pa. Heat is generated due to the bombardment of the metal surface with high-velocity electrons, the anode is the workpiece to be welded, and the cathode is a tungsten spiral.

Electron beam welding can be performed without oscillations and with oscillations of the electron beam. In the direction of oscillations, electron beam welding is distinguished with longitudinal, transverse, vertical and complex oscillations of the electron beam.

Gas welding is based on the melting of the welded and filler metals by a high-temperature gas-oxygen flame. As a fuel for combustion in oxygen, acetylene, hydrogen, propane-butane mixture, vapors of kerosene, gasoline, urban, natural, light, oil, coke and other gases are used.

Light welding According to the type of light source, it is divided into solar, laser and artificial light sources. In practice, so far only laser welding has been mainly used. This type of welding is based on the use of a special light beam that melts the metal. To obtain a strong light beam, laser installations are used.

Thermite welding consists in the fact that the parts to be welded are placed in a refractory mold, and thermite, a powdered mixture of aluminum with iron scale, is poured into the crucible mounted on top. During the burning of thermite, a high temperature develops (more than 2000ºС), a liquid metal is formed, which, when filling the mold, melts the edges of the welded products and fills the gap, forming a weld.

contact welding. With this type of welding, the junction is heated and melted by the heat released during the passage of electric current through the contact points of the parts to be welded; when a compressive force is applied in this place, a welded joint is formed. According to the shape of the welded joint, spot, seam, butt, relief, seam-butt contact welding and welding according to the Ignatiev method are distinguished. Spot welding, in turn, is divided into one-, two- and multi-point.

Butt welding according to the nature of the process, it is divided into welding with intermittent and continuous flashing and resistance welding. Contact welding can be performed with direct, alternating and pulsating current. According to the type of energy source, resistance welding is divided into capacitor, battery, energy stored in a magnetic field and in a motor-generator system.

Diffusion welding is carried out due to the mutual diffusion of atoms of the contacting parts with a relatively long exposure to elevated temperature and slight plastic deformation.

Gas pressure welding is based on heating the ends of rods or pipes along the entire circumference by multi-flame burners to a plastic state or melting and subsequent compression of the rods by an external force.

ultrasonic welding It is based on the combined effect of mechanical vibrations of ultrasonic frequency and small compressive forces on the parts to be welded.

Friction welding. When one of the rods rotates and its end comes into contact with the end of the fixed rod, the ends of the rods are heated and welded with the application of axial force.

Cold welding is based on the ability of metal crystals to coalesce under significant pressure.

Induction pressure welding. This type of welding is based on high-frequency current heating of the ends of the joined rods or pipes to a plastic state, followed by the application of axial forces to obtain a permanent connection.

5. Metal processing technologies

Most machine parts are made by machining. The blanks of such parts are rolled products, castings, forgings, stampings, etc.

The process of machining parts by cutting is based on the formation of new surfaces by deformation and subsequent separation of the surface layers of the material with the formation of chips. The part of the metal that is removed during processing is called the allowance. Or, in other words, the allowance is an excess (in excess of the drawing size) layer of the workpiece left for removal by a cutting tool during cutting operations.

After removing the allowance on metal-cutting machines, the workpiece acquires the shape and dimensions corresponding to the working drawing of the part. To reduce the labor intensity and cost of manufacturing the part, as well as for the sake of saving metal, the size of the allowance should be minimal, but at the same time sufficient to obtain a good quality part and with the necessary surface roughness.

In modern mechanical engineering, there is a tendency to reduce the volume of metal cutting by increasing the accuracy of the original workpieces.

Basic methods of metal cutting. Depending on the nature of the work performed and the type of cutting tool, the following metal cutting methods are distinguished: turning, milling, drilling, countersinking, slotting, broaching, reaming, etc. (Fig. 12).

Turning- the operation of processing bodies of revolution, helical and spiral surfaces by cutting with cutters on machines of the turning group. When turning (Fig. 12.1), the workpiece is given a rotational movement (main movement), and the cutting tool (cutter) is given a slow translational movement in the longitudinal or transverse direction (feed movement).

Milling- a high-performance and widespread process of processing materials by cutting, performed on milling machines. The cutter receives the main (rotational) movement, and the workpiece receives the feed movement in the longitudinal direction (Fig. 12.2).

drilling- the operation of processing the material by cutting to obtain a hole. The cutting tool is a drill that performs a rotational movement (main movement) of cutting and an axial movement of the feed. Drilling is carried out on drilling machines (Fig. 12.3).

Planing- a method of machining planes or ruled surfaces. The main movement (rectilinear reciprocating) is performed by a curved planer, and the feed movement (rectilinear, perpendicular to the main movement, intermittent) is the workpiece. Planing is carried out on planing machines (Fig. 12.4).

chiselling- a method of processing planes or shaped surfaces with a cutter. The main movement (rectilinear reciprocating) is performed by the cutter, and the feed movement (rectilinear, perpendicular to the main movement, intermittent) is performed by the workpiece. Slotting is carried out on slotting machines (Fig. 12.5).

grinding- the process of finishing and finishing of machine parts and tools by removing a thin layer of metal from their surface with grinding wheels, on the surface of which abrasive grains are located.

The main movement is rotational, which is carried out by a grinding wheel. With circular grinding (Fig. 12.6), the workpiece rotates at the same time. With flat grinding, the longitudinal feed is usually carried out by a workpiece, and the transverse feed is carried out by a grinding wheel or a workpiece (Fig. 12.7).

Stretching- a process, the productivity of which is several times greater than that of planing and even milling. The main movement is rectilinear and less often rotational (Fig. 12.8).

In the manufacture of parts from hard-to-cut materials by cutting, an increasing place is occupied by electrical and chemical processing methods. This is due to the special physical and mechanical properties of these materials, primarily high strength and hardness, which reach or even exceed these indicators for modern tool materials, which makes it impossible in some cases to use the conventional cutting method cost-effectively. In addition, electrical and chemical methods make it possible to produce surfaces of complex shapes, provide higher processing accuracy and surface quality, which increases the performance of manufactured parts.

In instrumentation, electron-ion processing methods (Aelionics), that is, the use of electron and ion beams for the manufacture of integrated circuits and semiconductor devices, are of particular importance. Electron diffraction makes it possible to obtain submicroscopic structures.

electrical called processing methods that use electrical energy directly for technological purposes by supplying it to the processing zone without intermediate conversion into other types of energy. The conversion of electrical energy into another type of energy (thermal, chemical, etc.) occurs directly in the material being processed. In accordance with this, electrical processing methods are divided into

electrothermal, using mainly the thermal effect of electric current

electrochemical, using its chemical action,

electroerosive, using the erosive action of the current,

electromechanical using its mechanical action.

Electrochemical processing (ECM) is carried out using a low voltage direct current in an environment of moving conductive liquids - electrolytes. The removal of the material of the removed layer occurs due to its anodic dissolution, i.e., the conversion of electrical energy into the energy of chemical bonds; as a result of this, the material of the removed layer is converted into chemical compounds that are easily removed from the treatment zone.

Electroerosive machining (EDM) is carried out by means of a pulsed electric gas discharge, which causes erosion destruction of the material of the removed layer.

Electromechanical machining (EMT) uses the mechanical action of an electric current; so, electrohydraulic processing uses the action of shock waves resulting from a pulsed breakdown of a liquid medium, electromagnetic molding - pulsed shaping by the forces of interaction between the magnetic current of the conductor and the magnetic field induced in the workpiece.

Beam processing methods (JIMO) are based on the use of a focused beam with a high energy density to remove material from the impact; material is removed by evaporation due to the conversion of electrical energy directly into heat.

Chemical processing methods are those that use chemical energy directly for technological purposes; in this case, the processing of the part, i.e., the removal of a certain layer of metal, is carried out in a chemically active environment. This includes, for example, chemical milling.

Chemical methods of processing materials are called, in which the removal of a layer of material occurs due to chemical reactions in the processing zone. Advantages of chemical processing methods:

a) high productivity, provided by relatively high reaction rates, primarily the absence of productivity dependence on the size of the treated surface area and its shape;

b) the possibility of processing especially hard or viscous materials;

c) extremely low mechanical and thermal effects during processing, which makes it possible to process parts of low rigidity with a sufficiently high accuracy and surface quality.

Dimensional deep etching (chemical milling) is the most common chemical processing method. It is advisable to use this method for processing surfaces of complex shapes on thin-walled parts, obtaining tubular parts or sheets with a smooth change in thickness along the length, as well as when processing a significant number of small parts or round blanks with large; the number of processed places (perforation of cylindrical surfaces of pipes). By local removal by this method from excess material in unloaded or lightly loaded aircraft and missiles, the overall weight can be reduced without reducing their strength and rigidity. In the United States, the use of chemical milling has reduced the weight of a supersonic bomber wing by 270 kg. This method allows you to create new structural elements, such as sheets 1 of variable thickness. Chemical milling is also used in the manufacture of printed circuits for electronic equipment. In this case, the sections specified by the scheme are removed from the panel of insulating material, covered on one or both sides with copper foil, by etching.

The productivity of chemical milling is determined by the rate of material removal in depth. The rate of etching increases with an increase in the temperature of the solution by about 50-60% for every 10 ° C, and also depends on the type of solution, its concentration and purity. Mixing of the solution during the pickling process can be done with compressed air. The etching process is determined by an exothermic reaction, so the supply of compressed air cools it somewhat, but basically the constancy of temperature is ensured by placing water coils in the bath.

Etching by immersion has a number of disadvantages - the use of manual labor, partial breakdown of protective films on untreated surfaces. When processing a number of parts, the jet etching method is more promising, in which alkali is supplied by nozzles.

A means of increasing the productivity of chemical milling is the use of ultrasonic vibrations with a frequency of 15-40 kHz; in this case, the processing productivity increases by 1.5-2.5 times - up to 10 mm/h. The process of chemical treatment is also greatly accelerated by the influence of infrared radiation of directional action. Under these conditions, there is no need to apply protective coatings, since the metal is subjected to strong heating along a given heating circuit, the remaining areas, being cold, practically do not dissolve.

Combined cutting methods are used to remove a given metal layer by the simultaneous action of several phenomena that are different in their physical essence or by combining different methods of energy supply. Examples of combined processing methods are the processing methods discussed above, based on thermomechanical effects - cutting with heated workpieces; processing methods based on the simultaneous mechanical and chemical action on the cut layer, for example, mechanical processing with the supply of active cutting fluids into the cutting zone. This also includes the below electrocontact processing (ECO), which is carried out by removing the material of the cut layer as a result of a combination of electrothermal, electroerosive and mechanical effects. Another example is anode-mechanical processing (AMO) - it uses electrochemical, electroerosive and mechanical effects on the workpiece being processed. Currently, the method of anode-mechanical processing of hard-to-machine materials with the imposition of vibrations of low and ultrasonic frequencies, the method of vibration drilling with the introduction of direct current into the cutting zone, electroerosive and electrochemical processing with ultrasonic oscillations of the electrode are being worked out.

Foundry is the process of obtaining shaped products (castings) by pouring molten metal into a hollow mold that reproduces the shape and dimensions of the future part. After solidification of the metal in the mold, a casting is obtained - a workpiece or part. Castings are widely used in mechanical engineering, metallurgy and construction.

With all the variety of casting techniques that have developed over a long period of development of its technology, the basic scheme of the technological process of casting has practically not changed over more than 70 centuries of its development and includes four main stages: melting metal, making a mold, pouring liquid metal into a mold, removing a hardened casting from a mold.

Special methods include casting:

a) into permanent metal molds (chill molds),

b) centrifugal,

c) under pressure

d) in thin-walled one-time forms,

e) investment models,

e) cortical, or sheath,

g) electroslag casting.

One of the most common is die casting. A chill mold is a solid or split metal mold made of cast iron or steel.

Before pouring the metal, the molds are heated to a temperature of 100...300°C, and the working surfaces in contact with the molten metal are coated with protective coatings. The coating provides an increase in the service life of the mold, prevention of welding of metal to the walls of the mold and facilitating the extraction of castings. Heating protects the mold from cracking and facilitates the filling of the mold with metal. During operation, the required temperature of the mold is maintained due to the heat released by the poured metal. After hardening, the casting is removed by shaking or using an ejector.

Injection molding is one of the most productive methods for obtaining precise shaped castings from non-ferrous metals. The essence of the method lies in the fact that liquid or mushy metal fills the mold and crystallizes under excessive pressure, after which the mold is opened and the casting is removed.

According to the method of creating pressure, they are distinguished: casting under piston and gas pressure, vacuum suction, liquid stamping.

Rice. 1 - Special casting methods: a - under pressure; b - centrifugal

The centrifugal casting method is mainly used to produce hollow castings such as bodies of revolution (bushings, shells for piston rings, pipes, liners) from non-ferrous and iron-carbon alloys, as well as bimetals. The essence of the method consists in pouring liquid metal into a rotating metal or ceramic mold (mould). Liquid metal due to centrifugal forces is thrown to the mold walls, spreads along them and hardens.

Long pipes and sleeves are cast on machines with a horizontal axis of rotation, short bushings, crowns of large diameter - on machines with a vertical axis of rotation.

Along with high productivity and simplicity of the process, the centrifugal casting method, in comparison with casting into stationary sand-clay and metal molds, provides a higher quality of castings, almost eliminates metal consumption for risers and uplifts, and increases the yield of good casting by 20 ... 60%. The disadvantages of the method include the high cost of molds and equipment and the limited range of castings.

Casting, according to smelted (melted) models, consists in the following. The metal is poured into a one-time thin-walled ceramic mold, made according to models (also one-time) from a low-melting model composition. In this way, precise castings from any alloys weighing from a few grams to 100 kg are obtained, which practically do not require machining.

Casting in shell molds is used to obtain castings weighing up to 100 kg from cast iron, steel and non-ferrous metals.

Thin-walled (wall thickness 6 ... 10 mm) molds are made from a sand-resin mixture: fine-grained quartz sand and thermosetting synthetic resin (3 ... 7%). The sand-and-resin mixture is prepared by mixing sand and crushed powdered resin with the addition of a solvent (cold method) or at a temperature of 100 ... 120 ° C (hot method), as a result of which the resin envelops (clads) the sand grains. Then the mixture is further crushed to obtain individual grains, clad with resin, and loaded into the hopper. Molding is made on metal models.

The model in the gating system is fixed on a model plate, heated to a temperature of 200 ... 250 ° C and a thin layer of a release agent is applied to their working surface. After that, the mouth of the bunker is closed with a model plate (the model is inside) and it is rotated by 180°. The mixture falls on the heated model, the resin is corrected and after 15 ... 25 s a shell (half-mould) of the required thickness is formed on the model. The bunker is turned again by 180°, the remaining mixture falls to the bottom of the bunker, and the model plate with a semi-solid shell is placed in an oven for final hardening at a temperature of 300 ...

Fastening (assembly) of half-forms is carried out with metal brackets, clamps or quick-hardening glue. Sand-resin cores for hollow castings are produced in a similar way.

The assembled shell molds are placed in flasks to make them more rigid, covered with cast-iron shot or dry sand from the outside, and poured with metal. After the casting hardens, the shell mold is easily destroyed.

Castings made in shell molds are distinguished by high accuracy and surface cleanliness, which makes it possible to reduce the mass of castings by 20...40% and the labor intensity of their machining by 40...60%. Compared to casting in sand-clay molds, the complexity of manufacturing castings is reduced by several times. In this way, critical machine parts are obtained - crankshafts and camshafts, connecting rods, ribbed cylinders, etc. Shell manufacturing processes are easy to automate.

Despite the high cost of the sand-resin mixture compared to the sand-clay mixture, a significant economic effect is achieved in the mass and serial production of castings.

Of the special casting methods, metal mold casting, centrifugal casting, injection molding, precision investment casting, vacuum suction casting and shell mold casting are currently common.

The improvement and introduction of special types of casting makes it possible to obtain castings so close to the final form of the product that machining can be limited only to finishing and grinding.

Casting in metal molds (die casting)

When casting into metal molds, castings with good mechanical properties are obtained due to the fine-grained structure of the metal due to rapid cooling. Castings have fairly accurate outlines that require almost no processing, and if they provide for a processing allowance, then several times less than when casting in sand. When casting into metal molds, land management, flasks, drying ovens disappear, and working conditions become more hygienic (no dust from molding earth). Due to the massiveness of the metal mold, the weight of the cast parts is limited.

Currently, automatic casting machines are successfully used, in which the closing and opening of a metal mold is mechanized. The removal of gases from gas-tight molds is carried out through vents, through trihedral slots and ventilation filament channels in the plane of the parting of the mold, sufficient in cross section for the release of gases, but insufficient for metal leakage.

The material for the manufacture of a metal mold is taken depending on the alloy poured into it; usually used gray cast iron, less often - mild steel. The temperature of the mold before pouring must be at least 200 o C for steel; for cast iron - 200-300 o C; for aluminum alloys - 250-350 o C; for copper alloys - 150-200 o C (with massive castings - 120-150 o C).

Forms to extend their service life are lubricated with one of the following refractory materials: SiO 2 (quartz flour or marshalite), MgO (magnesite), Al 2 O 3 (alumina, refractory clay or bentonite). FeO Cr 2 O 3 (chromium iron ore). The binder in this case is usually liquid glass.

Before pouring copper alloys, the metal mold is not coated, but painted with a special paint made from boiled oil with graphite (4%) or simply lubricating oil with paraffin (50% each), etc. For aluminum alloys, the molds are lubricated with a composition of 30 g of zinc oxide and 30 g liquid glass per 1 liter of water or 200 g of chalk and 30 g of liquid glass per 1 liter of water.

centrifugal casting

In centrifugal casting, molten metal is poured into a rotating mold, which, under the action of centrifugal forces, presses it against the walls and, solidifying, takes the desired shape. Castings are dense, since foreign inclusions, as well as gases, being lighter than metal, are pushed aside by centrifugal force to the inner surface of the mold, and the main body of the casting acquires a dense healthy structure.

In centrifugal casting, molds are made from cast iron and chromium-nickel steel. From the inside of the surface, I lubricate with a quiet layer of refractory material.

Elongated parts (cylinders, bushings) are cast on a machine with a horizontal axis, and gears, circles, rings, combs, screws and fittings are cast on a centrifugal machine with a vertical axis.

With centrifugal casting, it is possible to obtain castings of any shape, and not just bodies of revolution. With the so-called semi-centrifugal casting, the configuration of the cast parts is formed not only by centrifugal force, but also with the help of rods. The axis of rotation of the mold in this case coincides with the axis of symmetry of the casting. During centrifugation, the metal is fed into the mold through a riser in the center, and it enters the cavity of the molds located on a horizontal table through the sprue channels. In this way, it is possible to obtain castings that do not have an axis of symmetry. Any configuration.

Injection molding

In pressure casting, the molten metal is forcibly, under the pressure of a piston or compressed air, fills steel molds and solidifies in them. the finished casting taken out of the mold does not require further processing.

With the help of injection molding, very thin-walled parts (up to 0.1 mm) with threads, holes and complex shapes can be obtained. Dimensional accuracy of injection molded parts is very high (0.1-0.01 mm). All castings are completely identical and interchangeable. Products have a very fine-grained structure, which provides improved mechanical properties.

The productivity of one machine reaches 4000 or more castings per shift.

Recently, using the injection molding method, not only parts from low-melting metals and light alloys, but also from copper alloys - bronze, brass, are very successfully cast. Die casting is also used for reinforced products, for example, from zinc and aluminum alloys with steel, brass and bronze bushings, cores, etc. filled into them.

For fusible lead and tin alloys, the molds are made of carbon steel, which can withstand up to 50 thousand. castings. For zinc alloys, chromium-nickel steel is used, which can withstand up to 100 thousand castings. For castings made of aluminum alloys, chromium tungsten steel is the best material for molds.

The disadvantages of injection molding are the need for expensive steel molds and a special installation for compressed air, as well as the limited dimensions and weight of the castings. The pressure casting of steel parts presents great difficulties.

Casting in shell (crust) molds

Shell-mould casting is one of the advanced casting technologies that allow producing the most accurate castings with minimal machining, with a decrease in metal consumption per chip.

To obtain casting into shell molds, a layer of sand-bakelite mixture is applied to heated metal plates with metal models fixed on them and a gating system. Heated to 150-200 o C model equipment melts bakelite. Which wets the grains of the molding material adhering to the model. The excess of the mixture that did not stick to the model is removed, and the model plate with the mixture crust 7-10 mm thick is placed in an oven heated to 300-350 o C, where the crust hardens on the model quickly (1-3 minutes). The hard shell removed from the model (half-mould) is paired with another shell half-mould corresponding to it and filled with metal.

The material for shell molds poured with cast iron or non-ferrous metals and alloys is fine-grained quartz sand with 10% bakelite resin. In order to improve the surface of steel castings, chromium iron ore, chromium magnesite, magnesite and other additives are sometimes used that increase refractoriness, but increase the cost of the sand-resin mixture.

Replacing a conventional sand mold with only a shell (crust) reduces the consumption of molding sands by 50-90%, improves the dimensional accuracy and cleanliness of the casting surface, increases the removal per square meter of production area, and reduces the cost of casting.

Investment casting

In this method of casting, models are made from an easily melted material - paraffin with stearin, etc. A strong shell is applied to models made with great accuracy, which ensures the operations of melting models, annealing and pouring with liquid metal without the use of fillers and flasks, which impede the production of accurate investment casting. Several layers (2-5 layers) consisting of quartz flour and a hydrolyzed ethyl silicate solution (or their substitutes) are applied to the investment model. The last layer is applied from a mass that gives the ceramic shell the necessary strength after the model has been melted and the shell has been annealed. Good results are provided by a composition of: 40-45% liquid glass solution with a specific gravity of 1.32 and 60-65% by weight of quartz flour (marshalite, ground quartz sand or fused quartz), sifted through a No. 100 sieve. applied layers sprinkled with sand , are subjected to air drying at a temperature of 20-25 o C for at least 4 hours. Or electric dryer (10 min).

During electric drying, the model is heated at the same time, and during air drying, the model is heated for 20-40 minutes. In a thermostat heated to 150-180 o C. When melting, the model sets are placed with the gating cup down.

After the model has been rendered, the shell is heated in a calciner heated to a temperature of 600-650°C. The temperature is then raised to 900°C at a rate of about 100-150°C per hour. Upon reaching 900 o C in the oven, the calcination ends, the shell is removed from the oven and fed to the filling.

In order to avoid the formation of scale on the casting due to air access through the shell and in order to ensure safety, the shell is placed in a thin iron casing on a pallet before being poured with metal and the gap is filled with dry sand (and, if necessary, rapid cooling - with metal shot), covered with a conical lid casting bowl. The cover is removed before pouring the metal.

The castings are obtained without seams (the molds do not have connectors), the dimensions of the castings are more accurate than when casting into the ground, since here the causes of loss of accuracy from splitting the mold by the model during its extraction, distortion of the halves of the mold, lifting of the upper flask and swelling of the mold under liquid pressure are excluded. metal, etc. The accuracy of castings obtained by investment patterns reaches ± 0.05 mm per 25 mm of casting length, and the surface finish is obtained within the 4-6th grade according to GOST 2789-51.

In this way, products from steel, cast iron and non-ferrous metals are cast from a few grams to 50 kg, and artistic castings - up to 100 kg and a size of up to 1.5 m.

The use of precision casting is advisable for the manufacture of parts; 1) from steel and alloys that are difficult or not machinable (a cutting tool that needs only sharpening of its cutting edge on an emery wheel); 2) a complex configuration that requires long and complex machining, a large number of fixtures and special cutting tools, with the inevitable loss of valuable metal in the form of chips during processing (blade turbines, parts of the mechanism of sewing machines, hunting rifles, calculating machines); 3) artistic casting from ferrous and non-ferrous alloys.

There are many other applications for precision investment casting.

Vacuum suction casting

The essence of casting by the vacuum suction method lies in the fact that a thin-walled, continuously water-cooled mold - mold, connected to a vacuum system, is immersed in a bath of molten metal.

Vacuum suction fills the cavity of the crystallizer, the walls of which, due to cooling with water, provide intensive crystallization from the walls to the center.

The required wall thickness of the casting is controlled by the duration of exposure of the mold under vacuum.

Obtaining castings by vacuum suction is carried out on a special installation. The duration of holding the mold under vacuum can be adjusted with an accuracy of 0.1 sec. with automatic setting of switching on and off the vacuum.

After removing the vacuum, the part of the broom that did not have time to crystallize flows back into the bath. The cast billet falls out by itself due to the shrinkage of the metal and the taper of the mold.

Vacuum suction bronze castings have better structure and higher mechanical properties than castings made by other casting methods.

The manufacture of castings by vacuum suction is successfully used, for example, in the production of blanks for non-ferrous metal bushings. This method eliminates defects in gas shells and porosity.

Knocking out, shoeing, cleaning and casting control

In individual production, the casting from the strawberry mold is removed manually by knocking out the molding sand from the flasks, loosening it with a crowbar and hitting the surface of the mold and the walls of the flask.

In modern foundries, castings and cores are knocked out of castings mechanized on knockout grates.

The knocked-out earth falls through the lattice from the form, installed on the supports. The vibrators are actuated by compressed air, which is supplied through a pipe by pressing the foot on the pedal.

The cores of the castings are removed manually or with the help of pneumatic vibrating machines, or with a jet of water in a hydraulic chamber. The casting in the chamber is placed on a rotary grating table and a jet of water at a pressure of 25-100 atm is directed onto it from a nozzle with a diameter of 4-8 mm. Water with the core mixture is drained through the slatted floor of the chamber into a sump.

Castings are knocked out on grates at a temperature of about 1000 o , and they are transported to the cleaning and trimming department by cooling conveyors.

Sprues and profits on steel castings are removed with a circular saw and on castings from other ductile metals with band saws. Gas cutting is also used to remove profits.

Manual cutting of the sprues is done with a hammer and a chisel. For small and medium castings, sprues are removed on bran presses; for very small castings, in order to avoid breaking them - with a band saw. Bays and other irregularities are leveled with manual or pneumatic chisels.

The surface of small castings is successfully cleaned of sand in rotating drums, in which sprockets of white cast iron are loaded together with castings; in addition, the products are cleaned by sandblasting machines.

Cleaning with sandblasters is carried out with a jet of compressed air carrying quartz sand with it. Sand grains, hitting the surface of the casting with force, remove the burnt earth from it, and the surface becomes clean, matte.

Recently, instead of sand, white cast iron shot has been used, made by spraying a jet of liquid iron with a jet of water or air. Small drops of cast iron, quickly cooled by water, get the hardness of white cast iron. They are screened out in the form of pellets 0.5-2 mm in size, and larger ones are crushed, and acute-angled fragments are added to the shot. With cast iron shot, there is less dust and the work proceeds in more hygienic conditions. Shot consumption 2.4-3.5 kg per 1 ton of casting (25-35 times less than sand consumption) at air pressure up to 5-6 at.

To clean massive castings of complex configuration, hydraulic cleaning of water jets under pressure up to 150 at. Cleaning is carried out quickly and in the absence of dust, which is very important from the point of view of the safety of workers. During hydraulic cleaning, the rods are also washed out of the castings.

The mechanization of the removal of cores from castings by introducing a hydro-cleaning device and removing the cores together with cleaning the surface of the castings from the burnt mixture (sand-hydraulic cleaning) reduces the complexity of cleaning by about 10 times.

Before sprue cutting and cleaning, the casting is inspected to find out if there are any gross defects in the castings, as a result of which it would be impractical to transfer the casting to cleaning and cutting. There are various marriage classifiers (tables and instructions). They are used not only in the control of castings, but also in the fight against marriage and to prevent it.

The tasks of technical control are the analysis of the marriage of the foundry. Identification of various types and causes of marriage and taking measures to combat it together with the administration of the foundry.

The control of raw materials and materials entering the foundry, model and flask inventory, verification of technological processes, finished products on the basis of existing technical conditions are carried out. The control department reports directly to the director of the plant.

All metals can be cast. But not all metals have the same casting properties, in particular fluidity - the ability to fill a mold of any configuration. Casting properties depend mainly on the chemical composition and structure of the metal. Melting temperature is important. Metals with a low melting point are easy to industrial casting. Of the common metals, steel has the highest melting point. Metals are divided into ferrous and non-ferrous. Ferrous metals are steel, ductile iron and cast iron. Non-ferrous metals include all other metals that do not contain significant amounts of iron. For casting, in particular, alloys based on copper, nickel, aluminum, magnesium, lead and zinc are used. ALLOYS.

Black metals.

Become.

There are five classes of steels for industrial casting: 1) low-carbon (with a carbon content of less than 0.2%); 2) medium carbon (0.2–0.5% carbon); 3) high-carbon (more than 0.5% carbon); 4) low-alloyed (less than 8% of alloying elements) and 5) high-alloyed (more than 8% of alloying elements). Medium-carbon steels account for the bulk of ferrous metal castings; such castings are, as a rule, industrial products of a standardized grade. Various types of alloy steels are designed to achieve high strength, ductility, toughness, corrosion resistance, heat resistance and fatigue resistance. Cast steels are similar in properties to forged steels. The tensile strength of such steel is from 400 to 1500 MPa. The mass of castings can vary in a wide range - from 100 g to 200 tons or more, the thickness in the section - from 5 mm to 1.5 m. The length of the casting can exceed 30 m. Steel is a universal material for casting. Due to its high strength and ductility, it is an excellent material for mechanical engineering.

malleable cast iron.

There are two main grades of ductile iron: regular quality and pearlitic. Castings are also made from some alloyed ductile irons. The tensile strength of ductile iron is 250–550 MPa. Due to its fatigue resistance, high rigidity and good machinability, it is ideal for machine tools and many other mass productions. The mass of castings ranges from 100 g to several hundred kilograms, the thickness in the section is usually no more than 5 cm.

Cast iron.

Cast irons include a wide range of iron-carbon-silicon alloys containing 2–4% carbon. Four main types of cast iron are used for casting: gray, white, chilled and half. The tensile strength of cast iron is 140-420 MPa, and some alloyed cast irons are up to 550 MPa. Cast iron is characterized by low ductility and low impact strength; for designers, it is considered a fragile material. Weight of castings - from 100 g to several tons. Cast iron castings are used in almost all industries. Their cost is low and they are easy to machine.

Cast iron with nodular graphite.

Spherical graphite inclusions give cast iron plasticity and other properties that distinguish it favorably from gray cast iron. The sphericity of graphite inclusions is achieved by treating cast iron with magnesium or cerium immediately before casting. The tensile strength of cast iron with nodular graphite is 400–850 MPa, ductility is from 20 to 1%. True, for cast iron with nodular graphite, a low impact strength of a notched sample is characteristic. Castings can have both large and small thickness in cross section, weight - from 0.5 kg to several tons.

Nonferrous metals.

Copper, brass and bronze.

There are many different copper-based alloys available for casting. Copper is used in cases where high thermal and electrical conductivity is required. Brass (an alloy of copper and zinc) is used when an inexpensive, moderately corrosion-resistant material is desired for a variety of general applications. The tensile strength of cast brass is 180–300 MPa. Bronze (an alloy of copper and tin, to which zinc and nickel can be added) is used in cases where increased strength is required. The tensile strength of cast bronzes is 250–850 MPa.

Nickel.

Copper-nickel alloys (such as monel metal) have high corrosion resistance. Nickel-chromium alloys (such as inconel and nichrome) are characterized by high thermal resistance. Molybdenum-nickel alloys are highly resistant to hydrochloric acid and oxidizing acids at elevated temperatures.

Aluminum.

Cast products made of aluminum alloys have recently been used more and more widely due to their lightness and strength. Such alloys have a fairly high corrosion resistance, good thermal and electrical conductivity. The tensile strength of cast aluminum alloys ranges from 150 to 350 MPa.

Magnesium.

Magnesium alloys are used where lightness is in the first place. The tensile strength of cast magnesium alloys is 170–260 MPa.

Titanium.

Titanium, a strong and lightweight material, is vacuum melted and cast into graphite moulds. The fact is that during the cooling process, the titanium surface can become contaminated due to reaction with the mold material. Therefore, titanium cast into any other forms, except for forms from mechanically processed and pressed powdered graphite, turns out to be heavily contaminated from the surface, which manifests itself in increased hardness and low ductility in bending. Titanium casting is mainly used in the aerospace industry. The tensile strength of cast titanium is over 1000 MPa with a relative elongation of 5%.

Rare and precious metals.

Castings from gold, silver, platinum and rare metals are used in jewelry, dental technology (crowns, fillings), some parts of electronic components are also made by casting.

CASTING METHODS

The main casting methods are: static casting, pressure casting, centrifugal casting and vacuum casting.

Static fill.

Most often, static filling is used, i.e. pouring into a fixed mold. With this method, molten metal (or non-metal - plastic, glass, ceramic suspension) is simply poured into the cavity of a fixed mold until it is filled and held until it solidifies.

Injection molding.

The casting machine fills a metal (steel) mold (which is usually called a mold and can be multi-cavity) with molten metal under a pressure of 7 to 700 MPa. The advantages of this method are high productivity, high surface quality, precise dimensions of the cast product and minimal need for machining. Typical metals for injection molding are alloys based on zinc, aluminium, copper and tin-lead. Due to their low melting point, these alloys are highly adaptable and allow for tight dimensional tolerances and excellent casting performance.

The complexity of the configuration of castings in the case of injection molding is limited by the fact that when separated from the mold, the casting can be damaged. In addition, the thickness of the products is somewhat limited; more preferred products are thin sections, in which the melt quickly and uniformly solidifies.

There are two types of injection molding machines - cold chamber and hot chamber. Hot chamber machines are mainly used for zinc alloys. The hot chamber is immersed in molten metal; under a slight pressure of compressed air or under the action of a piston, liquid metal is forced out of the hot pressing chamber into the mold. In cold chamber casting machines, molten aluminium, magnesium or copper alloy fills the mold under pressure from 35 to 700 MPa.

Injection moldings are used in many household appliances (vacuum cleaners, washing machines, telephones, lamps, typewriters) and very widely in the automotive and computer industries. Castings can weigh from a few tens of grams to 50 kg or more.

Centrifugal casting.

In centrifugal casting, molten metal is poured into a sand or metal casting mold that rotates around a horizontal or vertical axis. Under the action of centrifugal forces, the metal is thrown from the central sprue to the periphery of the mold, filling its cavities, and hardens, forming a casting. Centrifugal casting is economical and for some types of products (axisymmetric type of pipes, rings, shells, etc.) is more suitable than static casting.

Vacuum filling.

Metals such as titanium, alloy steels and superalloys are melted under vacuum and poured into multiple molds such as graphite under vacuum. With this method, the content of gases in the metal is significantly reduced. Ingots and castings obtained by vacuum casting weigh no more than a few hundred kilograms. In rare cases, large quantities of steel (100 tons or more), smelted by conventional technology, are poured in a vacuum chamber into molds or casting ladles installed in it for further casting in air. Metallurgical vacuum chambers of large dimensions are pumped out by multi-pump systems. The steel obtained by this method is used for the manufacture of special products by forging or casting; this process is called vacuum degassing.

CASTING MOLDS

Casting molds are divided into multiple and single (sand). Multiple molds are metal (molding molds and molds), or graphite or ceramic refractory.

Multiple forms.

Metal molds (moulds and molds) for steel are usually made of cast iron, sometimes from heat-resistant steel. For casting non-ferrous metals such as brass, zinc and aluminum, cast iron, copper and brass molds are used.

Molds.

This is the most common type of multiple casting molds. Most often, molds are made of cast iron and are used to obtain steel ingots at the initial stage of the production of forged or rolled steel. Molds are open casting molds because the metal fills them from above by gravity. "Through" molds are also used, open both from above and from below. The height of the molds can be 1–4.5 m, the diameter is from 0.3 to 3 m. The wall thickness of the casting depends on the size of the mold. The configuration can be different - from round to rectangular. The cavity of the mold slightly expands upwards, which is necessary to extract the ingot.

Ready for pouring, the mold is located on a thick cast-iron plate. As a rule, molds are filled from above. The mold cavity walls must be smooth and clean; when pouring, you need to ensure that the metal does not splash or splash onto the walls. The poured metal solidifies in the mold, after which the ingot is removed (“the ingot is stripped”). After the mold has cooled, it is cleaned from the inside, sprayed with molding paint and used again. One mold allows you to get 70-100 ingots. For further processing by forging or rolling, the ingot is heated to a high temperature.

Kokili.

These are closed metal casting molds with an internal cavity corresponding to the configuration of the product, and a gating (pouring) system, which are made by machining in a cast iron, bronze, aluminum or steel block. A chill mold consists of two or more parts, after joining which only a small hole remains at the top for pouring molten metal. To form internal cavities, gypsum, sand, glass, metal or ceramic "rods" are placed in the mold. Die casting produces castings from alloys based on aluminum, copper, zinc, magnesium, tin and lead.

Die casting is used only in cases where it is required to obtain at least 1000 castings. The resource of the mold reaches several hundred thousand castings. The mold goes into scrap when (due to gradual burnout from the molten metal) the quality of the surface of the castings begins to decrease unacceptably and the design tolerances for their dimensions cease to be maintained.

Graphite and refractory molds.

Such molds consist of two or more parts, when connected, the required cavity is formed. The form can have a vertical, horizontal or inclined parting surface, or can be disassembled into separate blocks; this makes it easier to remove the casting. Once ejected, the mold can be reassembled and used again. Graphite molds allow hundreds of castings, ceramic molds only a few.

Graphite multiple molds can be made by machining graphite, while ceramic molds are easy to shape and are much cheaper than metal molds. Graphite and refractory molds can be used for recasting in case of unsatisfactory castings obtained by die casting.

Refractory molds are made from china clay (kaolin) and other highly refractory materials. In this case, models made of easily machined metals or plastics are used. Powdered or granular refractory is kneaded with clay in water, the resulting mixture is molded and the mold blank is fired in the same way as bricks or dishes.

Disposable forms.

There are far fewer restrictions on sand casting molds than on any other. They are suitable for producing castings of any size, any configuration, from any alloy; they are the least demanding on the design of the product. Sand molds are made from a plastic refractory material (usually siliceous sand), giving it the desired configuration so that the poured metal, upon solidification, retains this configuration and can be separated from the mold.

The molding sand is obtained by kneading sand with clay and organic binders on water in a special machine.

In the manufacture of a sand mold, it is provided with an upper sprue hole with a “bowl” for pouring metal and an internal sprue system of channels for supplying the casting with molten metal during solidification, otherwise voids (shrinkage cavities) may form in the casting due to solidification shrinkage (typical of most metals).

Shell forms.

These molds are of two types: low melting point material (gypsum) and high melting point material (based on fine silica powder). A gypsum shell mold is made by kneading a gypsum material with a binder (quick-setting polymer) on water to a thin consistency and lining the casting model with such a mixture. After the mold material has hardened, it is cut, processed and dried, and then the two halves of the mold are “paired” and poured. This casting method is suitable only for non-ferrous metals.

Lost wax casting.

This casting method is used for precious metals, steel and other alloys with a high melting point. First, a mold is made to match the part to be cast. It is usually made of low-melting metal or (machined) brass. Then, by filling the mold with paraffin, plastic or mercury (then frozen), a model for one casting is obtained. The model is lined with refractory material. The shell mold material is made from a fine refractory powder (eg silica powder) and a liquid binder. The refractory lining layer is compacted by vibration. After it hardens, the mold is heated, the paraffin or plastic model melts and the liquid flows out of the mold. Then the mold is fired to remove gases and, in a heated state, is poured with liquid metal, which flows by gravity, under pressure from compressed air or under the action of centrifugal forces (in a centrifugal casting machine).

Ceramic forms.

Ceramic molds are made from china clay, sillimanite, mullite (aluminosilicates), or other highly refractory materials. In the manufacture of such molds, models from easily processed metals or plastics are usually used. Powdered or granular refractory materials are mixed with a liquid binder (ethyl silicate) to a gel-like consistency. A freshly made mold is plastic so that the model can be removed from it without damaging the mold cavity. Then the mold is fired at a high temperature and poured with a melt of the desired metal - steel, hard brittle alloy, alloy based on rare metals, etc. This method allows you to make molds of any type and is suitable for both small-scale and large-scale production.