The quality of the aluminum melt

An overview Quality aluminum melts, which are used for the manufacture of various types of aluminum products.

Figure 1 – Types of contamination in an aluminum melt [7]

Criteria for the quality of aluminum melt

Aluminum melt is the starting material for the manufacture of any aluminum products. Different types of aluminum products require different levels of purity of aluminum melt. The quality of aluminum melt is one of the most important conditions for ensuring a given level of quality of the final aluminum products. The quality level of the melt is determined by three types of contamination [1, 2]:

dissolved hydrogen;
solid inclusions;
dissolved impurity metals.

Figure 2 - Types of contamination in aluminum melts [3]

Contamination measurement in aluminum

To assess the quality of aluminum melts, the following units of measurement are used:

ppm = parts per million (10⁶) = mg / kg = μg / g
ppb = parts per billion (10⁹) = μg / kg
ppt = parts per trillion (10¹²) = ng / kg

An idea of these units is given by their analogs:

1 ppm = 1 a minute in two years = 1 second in 11,5 day
1 ppb = 1 second in 32 years
1 ppt = 1 second in 32000 years.

To estimate the hydrogen content, the following units are used:

1 ppm = 1 mg / kg = 1 μg / g
milliliter (ml or cm³) hydrogen on 100 g of metal
the relationship between these units: 1 ppm = 1 mg / kg = 1,12 ml / 100 g.

To assess the content of inclusions, the following units are used:

ppm units, ppb и ppt;
PoDFA scale: mm²/kg.

Figure 3 – Method for counting inclusions in an aluminum melt [7]

Quality levels of primary and secondary aluminum

The quality of aluminum melt is one of the critical issues, especially for recycled aluminum, which is obtained by remelting aluminum scrap. Typical quality levels of primary aluminum and secondary aluminum are presented in the table 1.

Table 1 - Typical contamination in primary and secondary aluminum melt [4]

Vodorod Aluminum

The only gas in aluminum

Hydrogen gas is the only, which can dissolve in aluminum melt, since it does not form compounds with aluminum, unlike other gases [5] (figure 2):

at 660 ºС in liquid aluminum the content of dissolved hydrogen is 0,69 ppm;
in solid aluminum - the content of dissolved hydrogen is only 0,039 ppm.

Figure 4 - Solubility of hydrogen in aluminum [5]

Figure 5 -Measurement of hydrogen content [7]

The reaction of aluminum with water

Hydrogen enters molten aluminum as a result of the reaction between the melt and water, which is in the form of moisture in the lining, on instruments, in alloying additives, fluxes and charge, as well as in the atmosphere of the furnace.

The reaction between water and aluminum is:

3H₂O + 2Al → Al₂O₃ + 6H

As a result of this reaction, the melt is oxidized and hydrogen H₂, which can disperse in the environment or enter metal. The oxidation reaction is exothermic., that is, with heat. This reaction is so chemically favorable., that almost all traces of water, which is in contact with aluminum, turn into hydrogen and oxide [2].

Oxidation of the melt surface occurs with the formation of an oxide film on it (Fig. 3). Hydrogen is a “by-product” of aluminum oxidation. He or goes into the surrounding atmosphere in the form of molecular hydrogen H₂, or is absorbed by the melt in the form of atomic hydrogen H [2].

Figure 6 - Reaction of aluminum with water vapor [2]

In gas flame melting furnaces, directed to the surface of the melt, favorable conditions are created for increasing the hydrogen content in liquid aluminum, since when gas is burned, water vapor is formed:

CH₄ + 2O₂ → CO₂ + 2H₂O

Therefore, aluminum melt at temperature 700 ºС at the exit from the gas melting furnace usually has a hydrogen content 0,3-0,4 ml / 100 g [2].

The influence of alloying elements

Alloying elements affect the solubility of hydrogen in aluminum, since they change the average size of the melt atoms and affect the size of the interatomic "cavities", where are the hydrogen atoms. Alloying elements can increase or decrease the solubility of hydrogen in the aluminum melt, depending on the specific element and its amount [1]:

silicon, iron and copper reduce;
magnesium, and lithium increase;
zinc and titanium have little effect.

Traps for hydrogen in aluminum

In molten aluminum, hydrogen is in two forms [2]:

atomic hydrogen, dissolved in aluminum;
molecular hydrogen in the form of bubbles.

In solid aluminum, hydrogen is [2]:

atomic hydrogen in solid solution;
molecular hydrogen in the form of porosity (instead of bubbles);
molecular hydrogen in lattice defects, such as vacancies, dislocations and grain boundaries.

Places, where does hydrogen accumulate, when it comes out of a solid solution, called traps. These are vacancies., dislocation, pore, cavity, grain boundaries. When heated during heat treatment, molecular hydrogen in such traps expands and can deform the surrounding metal with the formation of surface defects, such as "bubbles" (figure 4) [2].

Figure 7 - Diffusion and accumulation of hydrogen in hydrogen traps [2]:
a) grain boundaries and internal cavities (for example, pores);
b) cavities near the surface, causing the formation of "bubbles"

Inclusions in aluminum

The influence of inclusions on the properties of aluminum products

Inclusions are unwanted solid phases., which:

remain in the finished casting or ingot;
reduce the level for the desired properties of the finished product.

Inclusions reduce the mechanical properties of castings and ingots, especially fatigue strength and ductility, since solid particles break down during metal forming processes or act as stress concentrators or crack nuclei during casting.

Figure 8 – Oxides in liquid aluminum [7]

solid particles, for example, lining particles, can lead to microscopic holes in rolled foil, wire breaks, tears in the production of cans for drinks ("beer" cans). They can also cause point defects in thick sheets or surface defects in sheets and pressed products..

The level of inclusions in aluminum melts for various purposes is shown in the figure. 5

Figure 5 - Requirements for the content of inclusions
for various aluminum products [3]

Sources of inclusions in aluminum melts

Sources of inclusions are almost all technological operations of aluminum production (figure 6).

Figure 6 - Production sources of inclusions
in aluminum melts [3]

Inclusion classification

Inclusions may vary [1-3]:

by chemical composition (oxides - "neoxides"; non-metallic - metallic and intermetallic);
by shape (particles - films)
by size (macroscopic - microscopic)
by origin (endogenous - exogenous).

Inclusions can be objects from macro-particles (such, as large particles of refractory lining), which measure several millimeters, to microscopic particles and phases, such as intermetallic particles, the sizes of which range from a few micrometers to 100 m.

General purpose aluminum products can contain a significant number of inclusions without any problems. However, products with critical safety requirements or increased quality levels require more stringent inclusion content requirements (see. Figure 5).

The main problem of aluminum melts is precisely non-metallic inclusions [3]. These include:

oxides (Al₂O₃, MgO and spinel MgAl₂O₄)
refractory lining particles
TiB particle clusters₂ (from grinding grain)
salts (metal chlorides)
carbides

Examples of metallic and intermetallic inclusions are:

Cr, CrMn и Zr (Ti) Al₃
Fe-Si
incompletely dissolved alloying elements

Endogenous inclusions are formed directly in the melt during production as a result of chemical reactions, for example, as a result of the reaction of aluminum with oxygen to form aluminum oxide:

3O₂ + 4Al → 2Al₂O₃

Metallic and intermetallic inclusions, such as Al₃Fe, Al₆(Fe, Mn) can enter the melt due to iron contamination. Moreover, that they reduce corrosion resistance, fatigue strength and toughness, they also cause breaks during rolling and "spoil" the appearance of anodized profiles [1].

Egzogenic inclusions are inclusions, that already existed in the mixture before loading into the melting furnace or entered the melt from the outside, for example, in the form of particles from the destroyed refractory lining of furnaces, gutters and t. P. These include [1-3]:

simple oxides, such as Al₂O₃ и MgO
potassium, calcium and aluminum silicates
sodium, calcium and magnesium aluminates
spinel, such as Al₂O₃ · MgO
TiB clusters₂ from additives for grinding grain.

Impurities in aluminum melt

Primary and secondary aluminum

Impurity elements are called chemical elements., which unintentionally enter aluminum melt. Despite, that their melt content is usually small, some impurity elements can significantly affect the properties of aluminum and technological processes, to which he is exposed. In some cases, the diffusion of these elements can cause segregation. This may produce a local higher concentration of these elements..

The main sources of impurity metals in aluminum are [1, 2]:

raw materials for primary aluminum, such as alumina and coke;
scrap aluminum products, contaminated with various "non-aluminum" components;
technological processes;
production technological equipment.

In the primary aluminum melt:

Sodium is present, lithium and calcium, that get there from the electrolyte
The impurity metals with the highest concentration are iron and silicon from alumina
In small concentrations, primary titanium is also usually present in primary aluminum., vanadium, manganese, copper, magnesium,, brown.

In the secondary aluminum melt:

Iron, copper and zinc are the main metal pollutants;
Lead, chromium, lead, nickel may be contained in small quantities. For most wrought alloys, the limit for them is 0,05 %.
Melt from mixed aluminum scrap may only be suitable for the production of secondary casting alloys. The production of secondary deformable aluminum alloys requires careful sorting of aluminum scrap and removal of foreign metals.

The effect of impurity elements on product quality

Impurity elements in aluminum melt render, usually, negative impact on the quality of the final product, for example [6]:

Lithium. The impurity content of lithium is several ppm (μg / g), but already at a level less 5 ppm cast can promote discoloration ("blue corrosion") of aluminum foil when exposed to humid conditions. Trace lithium concentrations greatly increase the oxidation rate of aluminum melt and have a negative effect on the surface condition of rolled products.
Sodium and lithium can cause edge cracking during aluminum rolling. For most products, sodium levels are kept lower. 10 ppm, and for Al-Mg alloys, up to 5 ppm.
Iron is a common admixture in aluminum. Iron has a high solubility in molten aluminum and is therefore readily soluble at all stages of processing liquid aluminum. The solubility of iron in solid aluminum is very low (about 0,05 %). Therefore, most of the iron is present in aluminum in the form of intermetallic secondary phases in combination with aluminum and other elements.
Lead. In industrial aluminum, only present in trace concentrations. Lead compounds are toxic.
Gallium is contaminated in aluminum and is usually present at a level of 0,001 to 0,02 %. At such concentrations, the effect on mechanical properties is negligible. At level 0,2 % gallium reduces the corrosion resistance of aluminum products.

Sources:

1. Direct-Chill Casting of Light Alloys: Science and Technology /J.F. Grandfield, D.G. Eskin, I.F. Bainbridge – TMS-Wiley – 2013

2. Inclusions and Hydrogen and Their Effects on the Quality of Direct Chill Cast and Flat Rolled Aluminium Alloys for Aerospace Applications /A. J. Gerrard – PhD thesis – The University of Birmingham – 2014

3. Understanding of Inclusions – Characterization, Interactions and Boundaries of Removability with Special Focus on Aluminium melts / Bernd Friedrich - Aachen University - 2015

4. V. Kevorkijan – Materials and technology 47 (2013) 1, 13-23

5. Aluminum and Aluminum Alloys / ed. J.R. Davis - ASM International - 1993

6. Aluminum and Aluminum Alloys /J.R. Davis // Alloying: Understanding the Basic – ASM International – 2001