Deformable and cast aluminum alloys

Aluminum is silver-white metal. Melting point 650 ° C. Aluminum has a crystalline fcc lattice. Aluminum has an electrical conductivity of 65% of the electrical conductivity of copper. Aluminum takes the third place in the distribution in the earth’s crust after oxygen and silicon. Aluminum is resistant to atmospheric corrosion due to the formation of a dense oxide film on its surface. The most important feature of aluminum is a low density of 2.7 g / cm3 versus 7.8 g / cm3 for iron and 8.94 g / cm3 for copper. Has good heat and electrical conductivity. Well processed by pressure.
It is marked with the letter A and with a number indicating the aluminum content. Aluminum of special purity has the mark A999 – the content of Al in this brand is 99.999%. Aluminum of high purity – A99, A95 contain Al 99.99% and 99.95% respectively. Technical aluminum – A85, A8, A7, etc.

It is used in the electrical industry for the manufacture of current conductors, in the food and chemical industries. Aluminum is not stable in acidic and alkaline environment, therefore aluminum dishes are not used for marinades, pickles, fermented milk products. It is used as a deoxidizer in the production of steel, for aluminizing parts in order to increase their heat resistance. In its pure form is used rarely because of low strength – 50 MPa.

Deformable aluminum alloys

Depending on the possibility of thermal hardening, deformable aluminum alloys are divided into not hardened and hardened by thermal treatment.
Alloys that are not refractory t / o include alloys Al c Mn (AMc1), and alloys Al c Mg (AMg 2, AMg3). The figure is the conventional number of the brand.

These alloys are well welded, have high plastic properties and corrosion resistance, but low strength. These alloys harden. Alloys of this group have found application as a sheet material used for the manufacture of complex products in the form of cold and hot stamping and rolling. Products obtained by deep drawing, rivets, frames, etc.

Alloys, hardened t / o, are widely used in mechanical engineering, especially in aircraft construction, because Have a low specific gravity at sufficiently high mechanical properties.

These include:

Duralumin – the main alloying components – copper and magnesium:

D1 – blades of propellers, D16 – plating, frames, aircraft spars, D17 – the main riveted alloy.
High-strength alloys – B95, B96 along with copper and magnesium still contain a significant amount of zinc. Used for highly loaded structures.
Alloys of increased plasticity and corrosion resistance – АВ, АД31, АД33. Helicopters blades, forged and forged parts of complex configuration.

Foundry aluminum alloys

The most common alloys of the system are Al-Si silumin.
Silumin has a combination of high foundry and mechanical properties, low specific gravity. A typical silumin alloy AL2 (AK12) contains 10-13% Si, is subjected to hardening and aging (AK7 (AL9), AK9 (AL4)).

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Copper alloys, properties, application

Copper as gold and silver is found in native form, and therefore in ancient times a person who did not know metallurgy (restoration of metal from ores) could already find and use copper. Currently, copper is produced metallurgically, separating it from oxygen and sulfur. Despite the fact that the copper content in the earth’s crust is small (0.01%), it is not scattered metal and is concentrated in copper ores, where its content is of the order of 5%. By properties, copper is close to silver and gold. The latter are not oxidized in air and are therefore called noble metals; Copper is oxidized poorly, so it is called a semi-noble metal. Pure copper has a number of valuable technical properties. High plasticity, high electrical and thermal conductivity, low oxidizability – all this has caused widespread use of copper. In addition, copper is the basis of the most important alloys – brass and bronze. The high electrical conductivity of copper causes its primary use in electrical engineering as a conductive metal. After silver, copper is in second place in terms of electrical conductivity. All impurities reduce the electrical conductivity of copper, hardening also reduces its electrical conductivity. Therefore, if the wires should not be particularly strong, then use annealed copper. For hanging wires, where strength is required, hardened copper or copper is used with small additives of active hardeners.

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Alloys of copper with zinc (brass)

Practical applications are copper alloys with a zinc content of up to 45%, which are called brasses. At room temperature, practically used brass either consists of alpha crystals alone, or is a mixture of alpha and beta crystals.
Zinc increases the strength and plasticity of the alloy. An alloy with a 30% zinc content has the maximum plasticity. The casting properties of brass are determined by the mutual arrangement of the liquidus and solidus lines. Brass easily lends itself to plastic deformation, so the rolled semi-finished products (sheets, ribbons, profiles) are made of brass. Brasses are marked with the letter L. followed by a figure showing the average copper content in the alloy. Since zinc is cheaper than copper, the more zinc in brass, the cheaper it is.
In addition to simple brass – copper and zinc alloys, special brass is used, in which various elements are added to impart certain properties: lead to improve workability, tin to increase corrosion resistance in seawater, aluminum and nickel to improve mechanical properties.

Copper alloys with tin (tin bronzes)

The high casting properties of bronze are determined by an exceptionally small shrinkage, which bronze has. The most complex castings are usually made of bronze. The fluid flow of bronze is low due to the large temperature difference between the liquidus and solidus lines. For the same reason, bronze does not give a concentrated shrinkage shell and it is not suitable for casting from high-density bronze.
The effect of tin on the mechanical properties of copper is similar to that of zinc, but it is more pronounced. Even at 5% tin the plasticity begins to fall. Due to the high technical resistance of bronze, they are used to manufacture armature (steam, water, etc.). Thus, the main application of bronze is complex castings, bearing shells, etc. To reduce the price, 5-10% zinc is added to most industrial bronzes. Zinc in these quantities dissolves in copper and does not have a significant effect on the structure. Phosphorus is introduced into bronze as a deoxidizer and it eliminates brittle inclusions of tin oxide. In the presence of about 1% phosphorus, such bronze is called phosphorous.
Bronze is labeled with the initial letters Br, followed by letters showing which alloying elements contain bronze, and then figures showing the number of these elements in whole percentages.

Copper alloys with aluminum, silicon, beryllium and other elements

Alloys of copper with aluminum, silicon, beryllium and other elements are also called bronzes; In contrast to tin, they are called, respectively, aluminum, siliceous, etc. The small amount of shrinkage tin bronze surpasses these bronzes, but they in turn outperform the tin in other respects: mechanical properties, chemical resistance, fluidity. Tin is a scarce element, so these bronzes, except, of course, are beryllium, cheaper than tin. Beryllium bronze differs from the rest with high hardness and elasticity.
Lead-free bronze containing 30% lead is a high-quality antifriction material widely used in engineering. The structure of such an alloy consists of individual grains of copper and lead. High antifriction properties of the alloy are ensured by uniform impregnation of lead in copper.

Hard metal alloys

Hard alloys over the past two decades have become very widespread in the industry. They are used in the mining industry – for drilling, metalworking industry – for cutting, stamping and drawing, as well as for surfacing wear parts.

The wide spread of hard alloys in the industry is explained by the fact that tools equipped with hard alloys allow many times to increase the productivity of existing equipment and reduce the cost of manufactured products and that parts directed by hard alloys work abrasion significantly (sometimes tens of times) longer than non-melted parts.

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The main constituent of all alloys are carbides of metals: tungsten, molybdenum, chromium, titanium, manganese. Carbides impart high hardness and wear resistance to alloys. In addition, the composition of hard alloys include cobalt, nickel, iron.
Solid alloys are divided into cast, powdered and cermet.

Cast and powdered hard alloys.

These alloys are used for surfacing wear parts.

Cast solid alloys – stellite and stellite-like – are distinguished by high corrosion resistance, in particular in sulfuric acid; They retain their resistance at high temperatures (stellites – up to 8000, stellite-like up to – 6000).
Stellites and srirmat are widely used in engineering for surfacing parts and tools that work without impact, and where the part after machining must be smooth and clean (mainly in sliding friction), for example: for bending and drawing matrices, machine centers, measuring Staples, rings for broach. In view of the high heat resistance of these alloys, they are also used for surfacing parts working at high temperatures, for example: for parts of metallurgical equipment, hot-cutting knives, valves of internal combustion engines.
Surfacing of cast hard alloys can be performed on steel (iron) and cast iron parts, regardless of their cross-section and configuration. Covering the working surface of the part with a layer of alloy is produced with the help of a gas burner with an acetylene-oxygen flame.

Powdered solid alloys – VKAR and STEINITE – are used mainly for welding parts that produce rough work, where the maximum number of pores and shells is allowed and treatment of the welded surface is not mandatory (cheeks of crushers, excavator teeth, excavators, etc.).

Vokar contains 86% of tungsten, 9.5 – 10.5% of carbon, up to 0.5% of silicon and up to 2.5% of iron; Stannite – 16-20% chromium, 8-10% carbon, 13-17% manganese to 3% silicon, the rest – iron.

Powder-like hard alloys are welded by the direct-current arc by the method of Benardos (using a carbon electrode). The surface to be welded is installed horizontally, a thin (0.2-0.3 mm) layer of flux (calcined borax) and a layer of powdered hard alloy (batch) with a thickness of 3 to 5 mm are applied to it. The electrode is connected to the negative pole, the part is connected with a positive . The electric arc formed between the electrode and the workpiece melts the burden and adjacent layers of the parent metal, and a small bath of the molten hard alloy and base metal is formed. The electrode is given a progressive zigzag motion, and the arc is continuously transferred over the surface of the hard alloy.

Metal-ceramic hard alloys

These alloys are applied in the form of plates to the cutting tool. Tools with hard alloy plates are now widely used in the factory practice for high-speed cutting of metals.

A characteristic feature of metal-ceramic hard alloys is their high hardness and the ability to maintain cutting properties at temperatures up to 1000 – 11000.

The main cutting component of metal-ceramic hard alloys is tungsten carbides; Some grades of alloys contain, in addition, titanium carbides. Cobalt is used as the binding metal.
For the manufacture of plates of metal-ceramic hard alloys, the powdery components are thoroughly mixed and the mixture is pressed under a pressure of 1000 to 4200 kg / cm2. Semi-finished products obtained in molds are placed in an electric furnace, where their sintering takes place at a temperature of 1400-15000. During sintering, the binding metal (cobalt) melts and, wrapping the carbide grains, binds them. In the production of hard alloys, pressing and sintering operations are often replaced by a single operation – hot pressing.
Plates of hard alloys serve to equip tools, drills, cutters and other tools. The equipment is made by soldering the plates on the holders or by mechanically attaching the plates to the holders.

Light metals and their alloys

Alloys of aluminum with silicon

Called also silumin, in the technique find use of silumin, close to the eutectic composition (from 6 to 13%). These alloys have good casting properties (high fluidity and low shrinkage), high density and increased mechanical properties compared to aluminum. Increased mechanical properties are achieved by modifying, consisting in processing molten silumin with a modifier (metallic sodium or a mixture of fluorine salts of sodium and potassium). A small amount of modifier (about 0.01% by weight) dramatically changes the structure of silumin: the crystals become shallow, and the fracture acquires a velvety appearance. Silumin, not subject to modification, have a coarse-grained structure and inferior mechanical properties.
When introducing a small amount of magnesium and manganese into the composition of silumin, their mechanical properties are further improved.

Metal castings from magnesium alloys

For the production of shaped castings, three groups of magnesium alloys are used: alloys of magnesium with aluminum and zinc, magnesium alloys with zinc and zirconium, magnesium alloys doped with rare earth metals.

The alloys of the 1 st group are intended for the production of highly loaded castings operating in an atmosphere with high humidity. To increase the corrosion resistance, 0.1-0.5% manganese is introduced into the alloys, and 0.001-0.002% beryllium or 0.5-0.1% calcium is used to reduce the oxidizability. The alloys of this group are considered to be high-strength. The main hardener in them is aluminum, whose solubility in magnesium at a eutectic temperature is 17.4%, and at normal – 2.8%. Zinc also strengthens magnesium, but is less effective than aluminum.

The main structural components of the alloys of this group are the primary crystals of aMg solid solution of aluminum and zinc in magnesium, phases g (Mg17Al12), h (Mn, Al) and manganese phase. Phase g is a hardener of alloys during heat treatment.

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The alloys of the second group are also classed as high-strength. They differ from magnesium alloys of other groups with increased mechanical properties and good machinability by cutting. Doping them with lanthanum improves casting properties, slightly improves heat resistance and weldability, but reduces strength and ductility at normal temperature. These alloys have satisfactory casting properties, have zirconium grains, can be hardened by heat treatment. From them, it is possible to obtain castings with homogeneous properties in different cross-sections. They are used to make castings operating at 200-250 ° C and high loads. The main structural components are the solid solution of zinc and zirconium in magnesium (aMg) and the inclusion of intermetallides Mg2Zn3 and ZrZn2, which are hardeners during heat treatment.

Alloys of the third group have high heat resistance and good corrosion resistance. They are designed for continuous operation at 250-350 ° C and short-term at 400 ° C. These alloys have good casting properties, high tightness, low propensity to form micro-flies and shrinkage cracks, high and uniform mechanical properties in sections of different thicknesses. Alloys with rare-earth elements are used for making castings that work under the influence of static and fatigue loads. Their main structural constituents are the solid solution of neodymium and zirconium in magnesium and the inclusion of the phases Mg3Nd, Mg9Nd, Mg2Zr. For the production of castings, alloys of the first group are more often used.

Features of melting and casting of magnesium alloys

Melting of magnesium alloys is associated with a number of difficulties, associated primarily with their light oxidizability. On the surface of magnesium melts, unlike aluminum, a loose oxide film is formed, which does not protect the metal from further oxidation. With a slight overheating, magnesium melts easily ignite. In the process of melting, magnesium and its alloys interact with nitrogen to form nitrides, and intensively absorb hydrogen (up to 30 cm3 per 100 g of melt). Oxides and nitrides, when suspended, cause a decrease in the mechanical properties of the alloy and the formation of microporosity in the castings.

To prevent intense interaction with furnace gases, the smelting of magnesium alloys is conducted under fluxes or in a protective gas environment. When most of the magnesium alloys are melted, fluxes are used, the basis of which is carnallite. Cover fluxes for alloys with rare-earth elements should not contain magnesium chloride, since it interacts with the REM with the formation of chlorides, increasing their losses to 20%.

The use of fluxes causes a number of undesirable phenomena. The ingress of flux into the casting body leads to the formation of foci of intense corrosion due to their high hygroscopicity; The working conditions deteriorate substantially. Therefore, nowadays, flux-free melting is widely used, using gas mixtures to protect magnesium melts. In industrial conditions, the most common mixture is air with 0.1% sulfur hexafluoride.

Depending on the scale of production and the weight of the castings, three methods of melting casting magnesium alloys are used: in stationary crucibles, withdrawable crucibles and a duplex process (in an induction crucible furnace).