Physical properties of metals. Melting point and density of metals and alloys

The melting point of metals, which varies from the smallest (-39 ° C for mercury) to the highest (3400 ° C for tungsten), as well as the density of metals in the solid state at 20 ° C and the density of liquid metals at the melting point, are given in the table of melting non-ferrous metals .

Table 1. Melting of non-ferrous metals

Welding and melting of non-ferrous metals

Copper welding . The melting temperature of Cu metal is almost six times higher than the melting temperature of steel, copper intensively absorbs and dissolves various gases, forming oxides with oxygen. Copper oxide II with copper forms a eutectic, the melting point of which (1064°C) is lower than the melting point of copper (1083°C). When liquid copper solidifies, the eutectic is located along the grain boundaries, making copper brittle and prone to cracking. Therefore, the main task in welding copper is to protect it from oxidation and active deoxidation of the weld pool.

The most common gas welding of copper with an oxy-acetylene flame using burners that are 1.5 ... 2 times more powerful than a burner for welding steels. The filler metal is copper rods containing phosphorus and silicon. If the thickness of the products is more than 5...6 mm, they are first heated to a temperature of 250...300°C. Fluxes in welding are roasted borax or a mixture consisting of 70% borax and 30% boric acid. To boost mechanical properties and improve the structure of the deposited metal, copper after welding is forged at a temperature of about 200 ... 300 ° C. Then it is again heated to 500-550°C and cooled in water. Copper is also welded by the electric arc method with electrodes, in a stream of protective gases, under a layer of flux, on capacitor machines, by the friction method.

brass welding . Brass is an alloy of copper and zinc (up to 50%). The main pollution in this case is the evaporation of zinc, as a result of which the seam loses its qualities, pores appear in it. Brass, like copper, is mainly welded with an acetylene oxidizing flame, which creates a film of refractory zinc oxide on the surface of the bath, which reduces further burnout and evaporation of zinc. Fluxes are used the same as for welding copper. They create slags on the surface of the bath, which bind zinc oxides and make it difficult for vapors to escape from the weld pool. Brass is also welded in protective gases and on contact machines.

bronze welding . In most cases, bronze is a casting material, so

welding is used when correcting defects or during repairs. The most commonly used metal electrode welding. The filler metal is rods of the same composition as the base metal, and the fluxes or electrode coating are chloride and fluoride compounds of potassium and sodium.

. The main factors hindering the welding of aluminum are its low melting point (658°C), high thermal conductivity (about 3 times higher than the thermal conductivity of steel), the formation of refractory aluminum oxides, which have a melting point of 2050°C, so the technology of melting non-ferrous metals , such as copper or bronze is not suitable for aluminum smelting. In addition, these oxides react poorly with both acidic and basic fluxes, so they are poorly removed from the weld.

The most commonly used gas welding aluminum acetylene flame. AT last years Submerged arc and argon-based automatic arc welding with metal electrodes has also become widespread. For all welding methods, except for argon-arc, fluxes or electrode coatings are used, which include fluoride and chloride compounds of lithium, potassium, sodium and other elements. As a filler metal for all welding methods, wire or rods of the same composition as the base metal are used.

Aluminum is well welded by an electron beam in a vacuum, on contact machines, by electroslag and other methods.

Aluminum alloy welding . Aluminum alloys with magnesium and zinc are welded without

special complications, as well as aluminum. An exception is duralumin - alloys of aluminum with copper. These alloys are thermally hardened after quenching and subsequent aging. When the melting temperature of non-ferrous metals is above 350°C, a decrease in strength occurs in them, which is not restored by heat treatment. Therefore, when welding duralumin in the heat-affected zone, the strength decreases by 40 ... 50%. If duralumin is welded in protective gases, then such a decrease can be restored by heat treatment up to 80 ... 90% in relation to the strength of the base metal.

Welding of magnesium alloys . In gas welding, fluoride fluxes are necessarily used, which, unlike chloride fluxes, do not cause corrosion of welded joints. Arc welding of magnesium alloys with metal electrodes through the poor quality of welds has not yet been used. When welding magnesium alloys, a significant grain growth is observed in near-weld areas and strong development columnar crystals in the weld. Therefore, the tensile strength of welded joints is 55 ... 60% of the tensile strength of the base metal.

Table 2. Physical properties of industrial non-ferrous metals

Crucible melting

An integral part of the production of metal and metal products is the use during production process crucibles for the production, smelting and remelting of both ferrous and non-ferrous metals. Crucibles are an integral part of metallurgical equipment for casting various metals, alloys, and the like.

Ceramic crucible for melting non-ferrous metals has been used for melting metals (copper, bronze) since ancient times.

After crystallization, it is necessary to make sure that the substance is sufficiently pure. The simplest and most effective method for identifying and determining the measure of purity of a substance is to determine its melting point ( T pl). The melting point is the temperature range at which a solid becomes liquid. All pure chemical compounds have a narrow temperature range of transition from solid to liquid. This temperature range for pure substances is a maximum of 1-2 o C. The use of the melting point as a measure of the purity of a substance is based on the fact that the presence of impurities (1) lowers the melting point and (2) expands the melting temperature range. For example, a pure sample of benzoic acid melts in the range of 120–122°C, while a slightly contaminated sample melts at 114–119°C.

The use of melting point for identification is obviously subject to great uncertainty, since there are several million organic compounds, and inevitably the melting points of many of them coincide. However, firstly, T the mp of the substance obtained in the synthesis almost always differs from T pl starting compounds. Secondly, the technique of "determining the melting point of a mixed sample" can be used. If a T mp of a mixture of equal amounts of the test substance and a known sample does not differ from T pl of the latter, then both samples are the same substance.

METHOD FOR DETERMINING THE MELTING TEMPERATURE. Thoroughly triturate the test substance into a fine powder. The capillary is filled with the substance (by 3–5 mm in height; the capillary should be thin-walled, sealed on one side, with an inner diameter of 0.8–1 mm and a height of 3–4 cm). To do this, carefully press the capillary with its open end into the powder of the substance and periodically hit its sealed end against the table surface 5–10 times. For complete displacement of the powder to the sealed end of the capillary, it is thrown into a vertical glass tube (30–40 cm long and 0.5–1 cm in diameter) on a hard surface. Insert the capillary into a metal cassette fixed on the thermometer nose (Fig. 3.5), and place the thermometer with the cassette into the device for determining the melting point.

In the device, a thermometer with capillaries is heated by an electric coil, the voltage to which is supplied through a transformer, and the heating rate is determined by the applied voltage. First, the device is heated at a rate of 4–6 ° C per minute, and 10 ° C before the expected T pl is heated at a rate of 1–2 o C per minute. The melting temperature is taken as the interval from the softening of the crystals (wetting of the substance) to their complete melting.

The obtained data is recorded in the laboratory journal.

1. Distillation

Distillation is an important and widely used method for purifying organic liquids and separating liquid mixtures. This method consists in boiling and evaporating the liquid and then condensing the vapors into a distillate. The separation of two liquids with a boiling point difference of 50–70 ° C or more can be carried out by simple distillation. If the difference is smaller, fractional distillation must be used on a more sophisticated apparatus. Some liquids with high boiling points decompose during distillation. However, when the pressure is reduced, the boiling point decreases, which makes it possible to distill high-boiling liquids without decomposition in a vacuum.

At which the crystal lattice of the metal is destroyed and it passes from the solid state to the liquid state.

The melting point of metals is an indicator of the temperature of the heated metal, upon reaching which the process (melting) begins. The process itself is the reverse of crystallization and is inextricably linked with it. To melt metal? must be heated using external source heat to the melting point, and then continue the supply of heat to overcome the energy of the phase transition. The fact is that the very value of the melting point of metals indicates the temperature at which the material will be in phase equilibrium, at the boundary between the liquid and the solid. At this temperature, a pure metal can exist simultaneously in both solid and liquid states. To carry out the melting process, it is necessary to overheat the metal slightly above the equilibrium temperature in order to provide a positive thermodynamic potential. Give a boost to the process.

The melting point of metals is constant only for pure substances. The presence of impurities will shift the equilibrium potential in one direction or another. This is because the metal with impurities forms a different crystal lattice, and the interaction forces of atoms in them will differ from those present in pure materials. Depending on the melting point, metals are divided into fusible (up to 600 ° C, such as gallium , mercury), medium-melting (600-1600°С, copper, aluminum) and refractory (>1600°С, tungsten, molybdenum).

AT modern world pure metals are rarely used due to the fact that they have a limited range physical properties. The industry has long and densely used various combinations metals - alloys, varieties and characteristics of which are much larger. The melting point of the metals that make up the various alloys will also differ from the melting point of their alloy. Different concentrations of substances determine the order of their melting or crystallization. But there are equilibrium concentrations at which the metals that make up the alloy solidify or melt simultaneously, that is, they behave like a homogeneous material. Such alloys are called eutectic.

Knowing the melting temperature is very important when working with metal, this value is necessary both in production, for calculating the parameters of alloys, and in the operation of metal products, when the phase transition temperature of the material from which the product is made determines the limitations in its use. For convenience, these data are summarized in a single melting of metals - a summary result physical research characteristics of various metals. There are also similar tables for alloys. The melting point of metals also significantly depends on pressure, so the data in the table are relevant for a specific pressure value (usually this is normal conditions when the pressure is 101.325 kPa). The higher the pressure, the higher the melting point, and vice versa.

In the metallurgical industry, one of the main areas is the casting of metals and their alloys due to the cheapness and relative simplicity of the process. Molds with any outlines of various dimensions, from small to large, can be cast; it is suitable for both mass production and customized production.

Casting is one of the oldest areas of work with metals, and begins around the Bronze Age: 7-3 millennium BC. e. Since then, many materials have been discovered, leading to advances in technology and increased demands on the foundry industry.

Nowadays, there are many directions and types of casting, differing in technological process. One thing remains unchanged - the physical property of metals to go from solid to liquid, and it is important to know at what temperature melting begins different types metals and their alloys.

metal melting process

This process refers to the transition of a substance from a solid to a liquid state. When the melting point is reached, the metal can be in both a solid and a liquid state, a further increase will lead to a complete transition of the material into a liquid.

The same thing happens during solidification - when the melting limit is reached, the substance will begin to pass from a liquid state to a solid state, and the temperature will not change until complete crystallization.

At the same time, it should be remembered that this rule only applicable to bare metal. Alloys do not have a clear temperature boundary and make a transition of states in a certain range:

1. Solidus - the temperature line at which the most fusible component of the alloy begins to melt.
2. Liquidus is the final melting point of all components, below which the first crystals of the alloy begin to appear.

It is impossible to accurately measure the melting point of such substances; the transition point of the states indicates the numerical interval.

Depending on the temperature at which the melting of metals begins, they are usually divided into:

• Fusible, up to 600 °C. These include zinc, lead and others.
• Medium-melting, up to 1600 °C. Most common alloys, and metals such as gold, silver, copper, iron, aluminum.
• Refractory, over 1600 °C. Titanium, molybdenum, tungsten, chromium.

There is also a boiling point - the point at which the molten metal begins to transition into a gaseous state. This is very heat, typically 2 times the melting point.

Pressure influence

The melting temperature and the solidification temperature equal to it depend on the pressure, increasing with its increase. This is due to the fact that as the pressure increases, the atoms approach each other, and in order to destroy the crystal lattice, they must be moved away. At high blood pressure more energy of thermal motion is required and the melting temperature corresponding to it increases.

There are exceptions when the temperature required to go into a liquid state decreases with increased pressure. Such substances include ice, bismuth, germanium and antimony.

Melting point table

It is important for anyone involved in the steel industry, whether a welder, foundry worker, smelter or jeweler, to know the temperatures at which the materials they work with melt. The table below lists the melting points of the most common substances.

Table of melting points of metals and alloys

In addition to the melting table, there are many other auxiliary materials. For example, the answer to the question, what is the boiling point of iron lies in the table of boiling substances. In addition to boiling, metals have a number of other physical properties, such as strength.

In addition to the ability to transition from a solid to a liquid state, one of important properties material is its strength - the possibility solid body resistance to fracture and irreversible changes in shape. The main indicator of strength is considered to be the resistance arising from the rupture of the workpiece, pre-annealed. The concept of strength does not apply to mercury, since it is in a liquid state. The designation of strength is accepted in MPa - Mega Pascals.

Exist following groups metal strength:

• Fragile. Their resistance does not exceed 50 MPa. These include tin, lead, soft alkali metals
• Durable, 50-500 MPa. Copper, aluminum, iron, titanium. The materials of this group are the basis of many structural alloys.
• High-strength, over 500 MPa. For example, molybdenum and.

Metal strength table

The most common alloys in everyday life

As can be seen from the table, the melting points of the elements vary greatly even for materials often found in everyday life.

So, minimum temperature Mercury has a melting point of -38.9 °C, so at room temperature it is already in a liquid state. This explains the fact that household thermometers have a lower mark of -39 degrees Celsius: below this indicator, mercury turns into a solid state.

Solders most commonly used in domestic use, have in their composition a significant percentage of the content of tin, which has a melting point of 231.9 ° C, therefore most of solder melts at the operating temperature of the soldering iron 250−400°C.

In addition, there are low-melting solders with a lower melt boundary, up to 30 ° C, and are used when overheating of the soldered materials is dangerous. For these purposes, there are solders with bismuth, and the melting of these materials lies in the range from 29.7 - 120 ° C.

The melting of high-carbon materials, depending on the alloying components, lies in the range from 1100 to 1500 °C.

The melting points of metals and their alloys are in a very wide temperature range, from very low temperatures(mercury) to the limit of several thousand degrees. Knowledge of these indicators, as well as other physical properties, is very important for people who work in the metallurgical field. For example, knowing at what temperature gold and other metals melt will be useful to jewelers, casters, and smelters.

Each metal and alloy has its own unique set of physical and chemical properties, not least of which is the melting point. The process itself means the transition of the body from one state of aggregation to another, in this case, from a solid crystalline state to a liquid one. To melt a metal, it is necessary to supply heat to it until the melting point is reached. With it, it can still remain in a solid state, but with further exposure and an increase in heat, the metal begins to melt. If the temperature is lowered, that is, part of the heat is removed, the element will harden.

Highest melting point among metals belongs to tungsten: it is 3422C o, the lowest is for mercury: the element melts already at - 39C o. As a rule, it is not possible to determine the exact value for alloys: it can fluctuate significantly depending on the percentage of components. They are usually written as a number span.

How is it happening

The melting of all metals occurs in approximately the same way - with the help of external or internal heating. The first is carried out in a thermal furnace, for the second, resistive heating is used when passing electric current or induction heating in a high-frequency electromagnetic field. Both options affect the metal in about the same way.

As the temperature increases, so does amplitude of thermal vibrations of molecules, structural lattice defects appear, which are expressed in the growth of dislocations, hopping of atoms, and other disturbances. This is accompanied by the breaking of interatomic bonds and requires a certain amount of energy. At the same time, a quasi-liquid layer is formed on the surface of the body. The period of destruction of the lattice and the accumulation of defects is called melting.

Depending on the melting point, metals are divided into:

Depending on the melting temperature choose and melting apparatus. The higher the score, the stronger it should be. You can find out the temperature of the element you need from the table.

Another important value is the boiling point. This is the value at which the process of boiling liquids begins, it corresponds to the temperature saturated steam, which forms above a flat surface of a boiling liquid. Usually it is almost twice as high as the melting point.

Both values ​​are usually given at normal pressure. Among themselves they directly proportional.

1. The pressure increases - the amount of melting will increase.
2. The pressure decreases - the amount of melting decreases.

Table of fusible metals and alloys (up to 600C o)

Table of medium-melting metals and alloys (from 600С o to 1600С o)