Aluminum: production and application

Aluminum and steel are the most common metallic materials. However, only at the end of 19 Century Aluminum was able to economically compete with steel in the engineering of industrial projects. What makes aluminum such a popular material?

Figure 1.1 – The content of aluminum in the earth's crust [5]

Everything about aluminum: Encyclopedia of Aluminum and Its Alloys, Two-Volume Set (2019)

The most important properties

Aluminum offers a wide range of properties, which are easy to find a use for it in many specific technical projects. It provides a wide variety of alloys, their states and manufacturing technology. The properties of aluminum and its alloys, which give them widespread use include the following:

Easy. Their density is only one third that of steel..
Available in a wide range of strengths. From ductile and soft commercially pure aluminum to high-strength alloys with a tensile strength of up to 690 MPa.
High specific strength, that is, the strength per unit weight.
Retains its strength at low temperatures and is often used in cryogenic conditions.
Has high corrosion resistance under most operating conditions. Wherein, Unlike steel, it does not form corrosion products, that spoil the appearance of the products and designs
Good conductor of heat and electricity.
High reflectivity.
non-magnetic. This property is important in the electrical and electronics industries.
Not self-igniting. This is important when working with flammable or explosive materials.
non-toxic. In large amounts, it is used as containers for foods and beverages.
Attractive appearance in its natural form. Its natural surface may be blasted to matte, shiny or mirror. It is easily the application of protective and decorative coatings, for example, anodising or powder coating.
Easily and repeatedly recyclable, that is, smelting and manufacturing of new finished products. This gives great economic and environmental benefits.
Easy to handle. Aluminum can be molded and processed by all known methods of metals processing, as well as ways of their connection.

Strength

Tensile strength

Industrial pure aluminum has tensile strength of about 90 MPa. Therefore the usefulness unalloyed aluminum as the material of construction is largely limited. After metalworking, for example, cold rolling, its strength can be increased in 1,5-2 times (figure 1.2).

Figure 1.2 – Impact of cold deformation (autofrettage)
on the strength and plastic properties of aluminum grade 1050 [5]

A much greater increase in strength can be obtained by doping with aluminum is not large amount of one and a few other elements, such as, manganese, silicon, copper, magnesium or zinc. Like pure aluminum alloys can also be some hardening by cold plastic working. Other alloys obtain their high strength by thermal hardening (Figure 1.2).

Figure 1.2 – Comparison of the strength of typical aluminum alloys and low carbon steel [5]

The figure 1.3 shows levels of strength characteristics of aluminum alloys as compared with steels, titanium alloys and magnesium alloys.

Figure 1.3 – Comparison of tensile strength
aluminum and aluminum alloy
with major competing metals [1]

specific strength

Aluminum has a density of only 2,7 grams per cubic centimeter, compared with 7,9 for steel, 8,93 for copper and 8, 53 for brass. An important characteristic for structural materials is their specific strength, that is, the ratio of their strength to mass (figure 2).

Figure 2 – Comparison of specific strength
aluminum and aluminum alloy
with major competing structural metals [1]

industrial metal

The revolutionary technology for the electrolytic reduction of aluminum oxide (Al₂O₃), dissolved in molten cryolite, It was independently discovered in 1886 by American Charles Hall and Frenchman Paul Heroult. This event coincided with three equally revolutionary breakthroughs in technology [1]:

Just at this time it appeared the first craft on internal combustion engines and aluminum value, as the structural material, has increased dramatically.
Secondly, Electrification has demanded a huge amount of light conductive material for long-distance transmission of electricity and the construction of supports for supporting electric cables.
Thirdly, Wright brothers, at that same time gave impetus to the development of a new industry – aircraft construction, where aluminum was not simply replace. Because aluminum produced aircraft frame, engines and other parts and units. Following the aircraft appeared missiles, and then the spacecraft, in which aluminum is the major lightweight structural material.

Primary aluminum

Process

The modern process of production of primary aluminum ingots is shown in Figure 3.1. Bayer method is used to convert bauxite to alumina, which in the aluminum industry is called alumina (upper part of the figure 1). The alumina is then reduced to metallic aluminum by the Hall-Heroult method (lower part of the figure 1).

Figure 3.1 – Primary aluminum production scheme:
from original bauxite to finished aluminum [2]

From initial ore – bauxites – get upgraded feedstock – alumina.
Alumina is dissolved in a bath of cryolite and fluoride salts of various additives, to control the temperature of the bath, density, electrical resistance and solubility alumina.
Then passed through a bath of electric current, to divide by electrolysis of aluminum oxide – alumina – oxygen and aluminum.
The resulting oxygen reacts with the graphite anodes, and the molten aluminum collects at the bottom of the electrolysis unit, which serves as a cathode in this process (figures 4.1 and 4.2).
Liquid aluminum periodically pumped through siphon assemblies or special vacuum furnace to the accumulating and transmitting further casting ingots.

Figure 3.2 – alumina production – Bayer method [5]

Figure 4.1 – Functional diagram of primary aluminum smelting [3]

Figure 4.2 – Cross section of an electrolytic cell [5]

inevitable impurities

Smelted main impurities of primary aluminum are iron and silicon, however, zinc, gallium, titanium and vanadium is usually always present in a different number of go. The purity of aluminum is estimated the maximum allowable amount of impurities. for instance, aluminum 99,70 % It contains not more than 0,30 % impurities.

aluminum refining

To obtain a higher degree of purity aluminum using special technology. purity 99,99 % achieved by zone melting or processing of liquid aluminum using the Hoopes method.

Cleaning method Hoopes

method Hupsa – This three-layer electrolytic process, wherein the molten salt is used having a density higher, than liquid aluminum (figure 5). The combination of these two methods of purification may be achieved purity alumina 99,999 %.

Figure 5 – Furnace for refining aluminum according to the Hoops method [3]

The lower layer serves as an anode,. It consists of a refined (purified) aluminum alloy with copper. Copper is introduced in order, to increase the density of the lower layer. Middle layer – This molten electrolyte. Its density is lower, than the density of the anode and the alloy above, than density already refined aluminum, which the “floats” electrolyte top.

Purification of aluminum is due to dissolution of impurities on the anode as a result of electrochemical reactions.

Purification by zone melting aluminum

The principle of zone melting is repeated passes along the zone melting of aluminum ingot. impurities, which lower the melting point of aluminum, They accumulate in the melting zone and gradually move towards the end of the ingot. These contaminants include, for example, lead, beryllium, calcium, iron, cobalt, nickel, magnesium,, copper, silicon, zinc. impurities, which increase the melting point, concentrated at the beginning of the ingot. These contaminants include, for example, chromium, titanium, Molybdenum, vanadium. Manganese does not change the melting point and therefore does not move under the influence of the melting zone. Zone melting reach purity alumina 99,9999 % [2].

Secondary aluminum

Source for secondary aluminum is aluminum scrap and aluminum waste in all forms and types of products, as well as slag and other waste aluminum foundries. Primary and secondary aluminum production are closely linked. Many aluminum alloys, wrought and cast alloys, suggest the presence of various impurities, which may be present in the aluminum scrap and aluminum waste processing. In the last decade, the use of aluminum waste is increasing steadily in the manufacture of various aluminum products. An example of this is the production of aluminum sheet for the production of cans for the packaging of beer and soft drinks.

Figure 6 – Aluminum scrap: cans for packaging of beer and drinks

Details of scrap aluminum and secondary aluminum:

Handbook of aluminium recycling /Ch. Schmitz – 2014

Aluminium recycling /M. Schlesinger – 2017

Aluminum alloys

The aluminum doped

Pure aluminum has a very low strength and its use as a structural material is very limited.

When aluminum is added to the other elements - alloying elements - it increases its strength due to different hardening mechanisms.

Aluminum, basically, possibly doped with a majority of the metallic elements. However, only some of them have a sufficient solubility in the solid state, to be the major alloying elements.

The most important alloying elements of aluminum are:

copper;
manganese;
magnesium;
silicon and
zinc.

At the same time, a considerable number of other elements have a marked effect on improving the properties of aluminum alloys. They are added in small quantities. These elements include chromium, the same manganese and zirconium, which are used mainly for the control of grain structure.

The maximum solubility of alloying elements in aluminum are usually, but not always, It is achieved at the eutectic temperature. The solubility of alloying elements in solid aluminum decreases with decreasing temperature. This change in solubility in solid aluminum is the basis for strengthening aluminum alloys due to the aging mechanism.

Where iron in aluminum

All industrial alloys contain about 0,1 to 0,4 % iron (by weight). Typically, the iron in the aluminum alloy is considered an impurity. Its content depends on the initial ore and electrolysis technology when smelting. Sometimes iron intentionally added to impart special properties of the material, for example, to 1 % in alloys for the manufacture of aluminum foil.

For what additives in aluminum

In combination with one or more basic alloying elements, additional elements are often used:

bismuth,
brown,
chromium,
lead,
titanium and
zircon.

These elements are typically used in small amounts, usually, to 0,1 %. However, in some aluminum alloys boron content, lead and chromium can reach 0,5 %. With these alloys produced small corrections necessary properties for specific conditions, such as , good fluidity at casting, high quality machining, heat endurance, corrosion resistance, high strength.

Categories aluminum alloys

It is convenient to divide aluminum alloys into two main categories:

casting alloys and
wrought alloys.

In each of these categories further division based main mechanism, which is responsible for the formation of their properties - heat-hardening alloys, and alloys of thermally neuprochnyaemye. The alloys of the latter group are getting their final properties as a result of the deformation processing - peening. Therefore, sometimes referred to as a positive - deformation of the reinforcement or even "nagartovyvaemye".

At alloys 6060, 6063, AD31

"Rule" in world production aluminum profiles 6xxx series alloys - aluminum alloys doped with magnesium and silicon - each of about one percent. The European standard EN 573-3 It consists of approximately 30 pcs. Of these thirty alloys, aluminum alloys are the most widely used:

6060 and
6063, and
6005A,
6061 and
6082.

Of these five alloys in the world is made more 90 % all pressed aluminum profiles.

Figure 7 – Popular aluminum alloy 6xxx series

Foreign aluminum alloys

In currently recognized is a system of aluminum alloy designations, which was introduced by the American Aluminum Association (AA). This system shared by the international ISO standards, and European EN standards.

Each deformable alloy denoted by a combination of four digits, for example, 2024. The first digit refers to a series of alloys. Each of the seven series alloys have one or two main alloying elements. for instance, in the case of alloy 2024 from the series 2xxx - a copper.

Designations cast alloys also consists of four digits, however, between the third and fourth digits should point, for example, 380.0.

In Russia and other CIS countries, along with international designation system is widely used and the traditional system of alphanumeric symbols aluminum alloys, for example, AD31.

(Very detailed and understandable about aluminum alloys: Aluminum and Aluminum Alloys / ed. J.R. Davis - ASM International, 1993)

some properties

Mechanical properties

manufacture of aluminum products technology determines not only its form, but also its material microstructure. In turn, microstructure determines the properties of the product.

Some aluminum properties slightly dependent on the chemical composition and manufacturing technology. Examples of such characteristics are:

Young's modulus (70 GPa),
density (2700 kg / m3) and
coefficient of linear thermal expansion (24 × 10-6 m / (m K).

Most of the other properties are very sensitive to material microstructure and chemical composition. These properties naturally fall into four categories:

strength, plasticity and formability - bulk properties;
fatigue strength and fracture toughness - local properties;
high temperature resistance and creep resistance - thermomechanical properties;
corrosion resistance, wear resistance and surface quality - surface properties.

The chemical composition of the alloy, method of forming the product (casting, hot rolling, cold rolling, pressing, forging) and heat treatment all together determine the microstructure, and the microstructure, in turn, depend above properties.

Designer aluminum product or part must be familiar with the laws governing these relationships. It must consider the microstructure of the material goods as an important part of design. This will give him the opportunity to "order" in the most appropriate metallurgical aluminum alloy with optimum microstructure.

Table 1 – Density and modulus of elasticity of various industrial metals [4]

Table 2 – Comparison of physical properties of wrought aluminum alloys
with the corresponding properties of pure aluminum 99,99 %
(in square brackets – digital designations of alloys) [4]

The melting temperature

The melting point of aluminum I am very sensitive to its purity. The melting temperature of ultrapure aluminum 99,996 % is 660,37 ° C. When the content of aluminum 99,5 % melting begins at 657 ° C, and when the content of aluminum 99,0 % - when 643 ° C.

aluminum corrosion

aluminum resists corrosion in the form of direct oxidation, which have called rusting steels. Fresh aluminum surface immediately reacts with oxygen to form aluminum item inert solid film of a few nanometers thickness of. This film blocks the further oxidation of aluminum. Moreover, Unlike the layer of rust on steel, This film does not exfoliate flakes exposing a fresh surface for oxidation. in front of, any scratch on the aluminum instantly heals itself.

aluminum products

types of products

Aluminum and its alloys can be cast or molded into finished products and semi-finished products in almost any of the known processes, applied to metal. According to their shape of the product is divided into standard and "according to plan".

The first ones include:

sheets,
plate,
foil,
rods,
wire,
pipe and
standard profiles (corners, Tauris, I-beams and the like).

Products "according to the customer's drawings" (in English they are called "engineered products") are developed for some special application and include:

extruded profiles,
forgings,
castings,

as well as in much smaller quantities of the product:

powder metallurgy,
impact compaction and other.

About half of them in sheets, plate and foil, near 20 % - on the extruded profiles and pipe.

Aluminum extrusion

Extrusion (or, more formal, in Russian and familiar, "Pressing") of aluminum and its alloys is a process of plastic deformation, wherein the preform, usually part of a round ingot ("pillar"), is forced or extruded through one or more openings matrix – special pressing tool. To do this, use the special equipment - extrusion presses, usually, hydraulic, which provide on the rod (ram, punch), which is directly "presses, presses the "blank, effort by 500 to 4000 tonnes, and sometimes more, depending on the application and performance of the press.

Details of Aluminum Extrusion:

Aluminum Extrusion Technology / P. Saha

Extrusion of Aluminium Alloys / T. Sheppard

aluminum castings

Aluminum castings are usually produced by the following methods:

injection molding;
permanent mold casting (chill casting);
sand casting;
casting in plaster molds;
a meltable casting mold.

These processes include various embodiments and variations, such as vacuum technology, low-pressure casting, spun casting.

Figure 8 – Casting of aluminum wheels

All about casting aluminum and aluminum alloys:
Aluminum Alloy Castings: Properties, Processes And Applications – J. Gilbert Kaufman, Elwin L. Rooy

aluminum forgings

Aluminum forgings produced creation plastic flow of metal by the application of kinetic therein, mechanical or hydraulic forces in the open or closed matrix. forgings, performed manually, have simple geometric shapes – rectangles, cylinders, drives. More complex forms of hammer in closed molds

Processing the aluminum surface

Natural aluminum metal surface is aesthetically appealing to many articles and without further processing. This natural protective oxide coating is transparent and can be made thicker by anodizing. This additional surface protection is achieved without compromising the appearance of the product.

Anodizing

Anodizing gives the opportunity to a natural matte finish or color. Technology of Anodizing Aluminum includes the use of various electrolytes and electrical parameters - voltage and current (figure 9).

Figure 9 – Principle aluminum anodizing

color

Various methods of painting are widely used for aluminum: from applying "wet" paint to powder coating (Figure 10) and electrolytic coating of other metals.

Figure 10 – Vertical powder coating aluminum profiles

Application

The main markets of aluminum products

Construction of buildings
transport sector
Home Furnishings and Sports
electrical industry
Engineering
A lot of aluminum is used for packaging various liquid and bulk products; this is, including, foil, aluminum cans and bottles
Production of frames. Aluminum bicycle frames are made mainly from aluminum alloys 6061 and 7005. Less commonly used some other aluminum alloys, for example, 7075 and 2014, stronger, than both alloys 6061 and 7005.

marine aluminum

In the constructions of ships, as well as facilities on the beach and on the high seas, the following types of wrought aluminum alloy products are used:

sheets,
plate,
profiles,
pipe and
rods.

Figure 10 – Aluminum yacht Steve Jobs

Special alloys are used in these products - often called the sea of beautiful aluminum. "Sea", aluminum sheets and plates made by methods like cold, so the hot rolling. Aluminum profiles, rods and tubes can be produced by compression methods, rolling or drawing.

Sources:

Corrosion of Aluminum and Aluminum Alloys – ASM Speciality Handbook /ed. J.R. Davis, 1999
Aluminum Electrical Conductor Manual – Aluminum Association, 1989.
Handbook of Aluminum, Volume 1 – Physical Metallurgy and Processes / ed. George E. Totten, D. Scott MacKenzie, 2003.
Materials Aluminum Association Germany
TALAT 1101