The melting point of aluminum

The melting point of pure aluminum

Aluminum melting, as well as other substances, It takes place with supply of heat energy thereto, outside or directly in its volume, How does this happen, for example, in induction heating.

The melting point of aluminum depends on its purity:

  • The melting temperature of ultrapure aluminum 99,996 %: 660,37 ° C.
  • When the content of aluminum 99,5 % Melting begins at 657 ° C.
  • When the content of aluminum 99,0 % Melting begins at 643 ° C.

Melting point of metals

Metals and non-metals

Any piece of metal, for example, aluminum, millions of individual crystals, called grains. Each grain has a unique orientation of the crystal lattice, but together grain oriented within this piece of randomly. Such a structure is called a polycrystalline.

amorphous materials, for example, glass, different from crystalline materials, for example, aluminum, for two important differences, which are related to each other:

  • absence of long-range order of the molecular structure
  • differences in the nature of melting and thermal expansion.

molecular structure difference can be seen in Figure 1. On the left is shown tightly packed and ordered crystalline structure. Amorphous material shown on the right: less dense structure with random arrangement of atoms.

The difference between the structure of crystalline and amorphous materials

Figure 1 - The structure of crystalline (a) and amorphous (b) materials.
Crystal structure: ordered, repetitive and dense,
amorphous structure – more loosely packed
a disorderly arrangement of atoms.

metal melting

This difference in the structure is manifested in melting metals, including, melting of aluminum of various purities and its alloys. Less densely packed atoms give an increase in volume (decrease in density) compared to the same metal in the solid crystalline state.

Metals with melting experience an increase in volume. In pure metals, this volume change occurs very rapidly and at a constant temperature - temperature melting, as shown in Figure 2. This change is the gap between the sloping lines on either side of the melting point. Both these oblique lines characterize the thermal expansion of the metal, which is usually a variety of liquid and solid state.

Heating curves of pure crystalline material (pure aluminum) and amorphous material

Figure 2 - The characteristic change of the volume of pure metal
compared with the change in the volume of the amorphous material [4]:
Tg – glass transition temperature (transition of a liquid state to a solid);
Tm – the melting temperature

The heat of fusion

With this dramatic increase in the volume of metal at the transition from the solid to the liquid state due a certain amount of heat, which is called the latent heat of fusion. This heat causes the atoms lose an ordered and dense crystal structure. This process is reversible, He works in both directions - and upon heating, and cooling.

The equilibrium melting temperature

As shown above, pure crystalline substance, for example, pure metals, have a characteristic melting temperature, often referred to as "melting point". At this temperature, it is a pure crystalline solid is melted and becomes a liquid. The transition between the solid and liquid state for small specimens of pure metals is so small, that can be measured to an accuracy 0,1 oC.

Liquids have a characteristic temperature, where they are converted into solid. This temperature is called the solidification temperature or solidification point. In theory – under equilibrium conditions – the equilibrium solid melting temperature is the same, and that the equilibrium temperature of solidification. In practice, small differences can be observed between these values ​​(Figure 3).

Melting / solidification at the cooling and heating curves of pure metal, for example, pure aluminumFigure 3 – Cooling and heating curves of pure metal.
Visible phenomena of supercooling when cooled, and overheating during heating.
In the early solidification observed depression in the cooling curve,
which is explained by the delayed onset of crystallization [4]

Temperature liquidus and solidus

  • Temperature beginning melting is called the solidus temperature (or solidus point)
  • Temperature end of melting - liquidus temperature (or liquidus point).

"Solidus" means, understandably, solid, and "liquidus" – liquid: at solidus temperature the entire alloy more solid, and at liquidus temperature - the whole already liquid.

When this alloy solidifies from a liquid state, the temperature of the onset of crystallization (solidification) will be the same liquidus temperature, a closure crystallization - the same solidus temperature. When temperature alloy between its solidus and liquidus temperatures it is in semi-solid semi-, mushy state.

Aluminum melting

The influence of alloying elements and impurities

Adding other elements to aluminum, including alloying, lowers its melting temperature, more precisely – starts its melting. So, some casting aluminum alloys with a high content of silicon and magnesium melting start temperature is reduced to almost 500 ° C. At all, the term "melting temperature" only applies to pure metals and other crystalline substances. Alloys, on the other hand, do not have a specific melting point: the process of their melting (and solidification) occurs in a certain temperature range.

The melting of pure aluminum and aluminum alloy - heating curves

Figure 4- Change in the specific volume of pure metal (aluminum) and
alloy of this metal (aluminum alloy) [4]

Intervals melting temperature

The table below shows the solidus and liquidus temperature of some commercial wrought alloys. It must be borne in mind, that the concepts of the liquidus and solidus temperatures are defined for equilibrium reactions in the liquid phase and a solid back, ie at infinite duration processes. In practice, it is necessary to make adjustments based on the rate of heating or cooling.

Таблица интервалов плавления-затвердевания алюминиевых сплавов

melting silumin

Not all alloys have the spacing between solidus and liquidus temperatures. Such alloys are called eutectic. for instance, the aluminum alloy containing 12,5 % silicon, the liquidus and solidus points are reduced to a point: this alloy, like pure metals, has no interval, a point melting. This point is called eutectic temperature. This alloy belongs to the well-known casting aluminum-silicon alloys - silumin narrow solidus-liquidus interval, which gives them the best casting properties.

The binary alloys Al-Si solidus temperature is constant at 577 ° C. By increasing the content of silicon decreases the liquidus temperature of the maximum value for pure aluminum 660 ° C, and to coincide with the solidus temperature 577 ° C with silicon Content 12,6 %.

Among other alloying elements of aluminum, magnesium lowers the melting point the most: eutectic temperature 450 ° C is achieved when the magnesium content 18,9 %. Copper eutectic temperature gives 548 ° C, and manganese - just 658 ° C! Most alloys are non-double, and triple, and even quadruple. Therefore, when the joint effect of several alloying elements solidus temperature - melting start or end of solidification can be lower.

aluminum solidification

Pure aluminum

pure metals, including, pure aluminum, have a clear melting point - melting point. Solidification or "freezing" pure aluminum also occurs at a constant temperature. When pure molten aluminum is cooled, its temperature falls to freezing point and remains at that temperature, until all of it (liquid aluminum) hardens. In figures 5 and 6 typical cooling curves of pure metal with its transition from liquid to solid are shown.

Curve solidification of pure aluminumFigure 5 - Cooling curve of bare metal (e.g., aluminum) [3]


Figure 6 – Solidification of pure aluminum [5]

Aluminium alloy

During solidification of aluminum alloy, which consists of aluminum dissolved therein and the alloying element, for example, silicon or copper, the cooling curve of the alloy shows, that the beginning of the solidification occurs at a temperature, and the ending is at a different temperature (figure 7).

aluminum alloy solidification curveFigure 7 - Cooling curve of the alloy (for example, aluminum alloy) [3]

Molten aluminum alloy casting

For an aluminum alloy heated to a liquid state temperature, in which you can do the casting operations, used melting furnaces of various types. Thermal energy, which is required in order, to heat the metal to a liquid state temperature, at which it can be poured into molds, consists of the sum of the following components:

  • Heat, to raise the metal temperature to the melting point
  • The heat of fusion, to convert the metal from solid to liquid state
  • The heat for heating the molten metal to a predetermined casting temperature

casting temperature - a temperature of the molten metal, at which it is poured into a mold. An important factor here is the difference in temperature between the casting temperature and, at which solidification starts. This temperature is the melting point (point) for pure aluminum or liquidus temperature for aluminum alloy.. This temperature difference is sometimes called superheating. The term can also be applied to the quantity of heat, which must be away from the liquid metal between the casting and the start of solidification.

Boiling temperature

  • The boiling point of pure aluminum is 2494 ºS [1]

Other thermal properties of aluminum [1]:

  • latent heat of fusion: 397 kJ / g
  • specific heat of vaporization: 1,18 · 10-4 MJ / (g K)
  • calorific value: 31,05 MJ / kg
  • heat capacity: 0,900 kJ / (g K) at 25 ºS;
    1,18 kJ / (g K) at 660,4 ºС (liquid)

The melting temperature of various metals

The melting point of some other net metals is (degrees Celsius) [1]:

  • mercury: minus 39
  • lithium: 181
  • lead: 232
  • lead: 328
  • zinc: 420
  • magnesium: 650
  • copper: 1085
  • nickel: 1455
  • iron: 1538
  • titanium: 1670

Sources:
1. Aluminum and Aluminum Alloys, ASM International, 1993
2. Handbook of Aluminum: Vol. 1, ed. G. E. Totten, D. S. MacKenzie, 2003
3. Groover, Mikell P. Fundamentals of modern manufacturing: materials, processes and systems, 4th ed. – JOHN WILEY & SONS, 2010
4. Introduction to Alloy Phase Diagrams – ASM International, 1992
5. TALAT 1205