Unalloyed Aluminium

For wrought aluminium, the mass percentage of other elements present shall not exceed the following limits  

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Within the BS series the corrosion resistance of unalloyed aluminium increases with increasing metal purity. 

The ultimate tensile strength of pure aluminium increases markedly with increasing amounts of alloying or impurity additions, as shown in Fig. 3.1-10. Unalloyed aluminium is soft (tensile strength 10—30 MPa, Table 3.1-11) and, like all fee metals, shows a low rate of work hardening. 

The properties of aluminium strongly depend on the concentration of alloying additions and impurities. Even the low residual contents of Fe and Si in unalloyed aluminium (A199 to A199.9) have a marked effect. 

Al—Fe—Si alloys, and of unalloyed aluminium, are strongly influenced by the elements which are in solid solution and the binary and higher phases that form. Increasing amounts of alloying additions lead to a marked increase in strength hut there is a decrease in electrical conductivity since transition elements have a high effective scattering power for electrons. 

Manganese additions increase the strength of unalloyed aluminium (Fig. 3.1-23). The chemical resistance is not impaired. These alloys have very good forming properties, when the Mn content is below the maximum solubility of Mn in the Al-rich a-phase, i. e., practically below 1.5 wt%. At higher Mn content, brittle AlgMn crystals form and impair workability. 

Fig. 3.1-80 Schematic representation of the structure of the oxide film formed on unalloyed aluminium in dry air, the total thickness is typically 0.005 to 0.02mm [1.50]. A1 = Aluminium 1 = Surface layer 2 = Mixed oxides 3 = Pores 4 = Barrier layer 5 = Heterogeneous components...

Aluminium alloys have heterogeneous microstrac-tural components such as intermetallic phases and resulting oxides in the surface and barrier layers. This explains why unalloyed aluminium and aluminium alloys have lower corrosion resistance than high-purity aluminium. 

Unalloyed aluminium is an excellent heat conductor, with roughly 60% of the thermal conductivity of copper, the optimum performer among common metals. The thermal conductivity of aluminium alloys depends on their composition and metallurgical temper (Tables A.3.5 and A.3.9). 

The principal physical properties of unalloyed aluminium are listed in Table A.2.1. 

Iron and silicon are the two main impurities of unalloyed aluminium in the 1000 series their total concentration determines the purity of the metal. The iron/silicon ratio is close to two, for most grades, unless it is deliberately modified, as with the 8000 series. The concentration of impurities can vary, depending on the alloy, from a few ppm (parts per million) in refined aluminium (1199) up to 1000-2000 ppm in most wrought alloys. The impurity level of casting alloys can be higher for alloys based on secondary aluminium. 

Intercrystalline corrosion is caused by a difference in the electrochemical potentials between the bulk of the grain and the grain boundaries where intermetallic phases precipitate. The grain (also called the matrix) comprises a solid solution and dispersed intermetallic compounds. At room temperature, the solubility of iron, nickel, or magnesium in aluminium is so low that the solid solution has a potential very close to that of unalloyed aluminium 1050A. However, when the solid solution is supersaturated or enriched at ambient temperature, the potential depends on the concentration of the alloying element (Figure B.1.8). 

The dissolution potentials of alloys of the 5000 and 6000 series as well as those of magnesium and silicon-containing casting alloys of the series 40000 and 50000 are very close to each other, and very similar to that of unalloyed aluminium of the 1000 series (Table B.1.4). Therefore, there is no risk of galvanic corrosion between these materials. 

The 1.3-mm-thick aluminium sheets were manufactured in Neuhausen (Switzerland) in 1895. The composition of the metal corresponds to what was commonly achieved when the production of aluminium by molten salt electrolysis started (Table C.5.1). Very high in iron and siUcon, the metal contained 98.3% aluminium. In the last 50 years, when unalloyed aluminium (1000 series) has been used for this type of application, it contains at least 99.5% aluminium. 

In the 1000 series, the aluminium content has no important influence on the dissolution rate, which is the same for 1050, 1080 and 1199, as shown in Table E.5.10. The dissolution rate of unalloyed aluminium (1000 series) in nitric acid has been the subject of numerous... 

Due to the rather low mechanical strength (/ = 90 MPa) of this material, alloys have been considered for this use. In general, the resistance of aluminium alloys at room temperature is slightly less good than that of unalloyed aluminium. Alloying elements such as silicon, copper, zinc or magnesium slightly increase the dissolution rate, especially at low acid concentrations. 

It is catalysed by aluminium chloride [8], which, the higher the temperature, the more soluble it is in carbon tetrachloride. Once initiated, this reaction will not stop. The lower the iron content of the aluminium, the more severe the attack is, because insoluble iron chloride forms a layer on the metal, thus impeding the attack. Alloys such as 3003 and 5052 should, therefore, be preferred to unalloyed aluminium 1050A and, of course, to refined aluminium 1199. 

It is customary and convenient to refer to aluminium although in most cases what is meant is aluminium alloys. It should be remembered that unalloyed aluminium amounts to not more than 10% of the annual worldwide consumption, all uses and products combined. 

Etching is carried out as for unalloyed aluminium, but the duration is increased to 2 minutes. The typical residual surface oxygen concentration is 0.2 - 0.4 Mg/cm. ... 

These are not considered here as sufficiently reliable calibration samples are needed, which at this time are not available. The boron concentrations to be expected in unalloyed aluminium are in the 0.5 to 10 ig/g range. 

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