Aluminium–manganese alloys

Aluminium–manganese alloys (AlMn alloys) are aluminium alloys that contain manganese (Mn) as the main alloying element. They consist mainly of aluminium (Al); in addition to manganese, which accounts for the largest proportion of about 1% of the alloying elements, but they may also contain small amounts of iron (Fe), silicon (Si), magnesium (Mg), or copper (Cu). AlMn is almost only used as a wrought alloy and is processed into sheets or profiles by rolling or extrusion presses. These alloys are corrosion-resistant, have low strengths for aluminium alloys, and are not hardenable (by heat treatment). They are standard in the 3000 series.

Aluminium–manganese alloys are used in applications with low strength requirements and also in chemical and food-related environments due to their corrosion resistance. AlMn is therefore used more commonly as a functional material than a construction material.

AlMn is processed into beverage cans and is generally referred to as the packaging material used. It is used for apparatus and pipes in the chemical industry, for roof cladding, wall coverings, pressure vessels, roller shutters, roller doors, and heat exchangers.^[1]

Manganese combines with aluminium to form intermetallic phases—phases which contain a different crystallographic structure than either manganese or alumunium by itself. Compared to conventional alloys, the increased metallic bonding within the intermetallic phase of AlMn increases its strength and chemical resistance. Each percent of manganese increases its strength by about 42 MPa. Iron and silicon are usually unwanted accompanying elements that cannot be completely removed. Magnesium and copper are more effective in enhancing strength, providing an increase of 70–85 MPa per % of Mg when added to the alloy.^[2]

Aluminium and manganese are partially miscible in the solid state, meaning they can mix to some extent in the solid phase. However, their complete solubility in each other is limited, and they tend to form various intermetallic phases. The eutectic, between aluminium and Al₆Mn is 1.3% manganese and 660 °C, while pure aluminium melts at 660.2 °C. Values of 1.8% and 657 °C or 658 °C can also be found in older literature.

Above 710 °C, Al₄Mn is formed with a manganese content of at least 4%. However, such high levels are not typically used. Below 510 or 511 °C, Al₁₂Mn forms.

The solubility of manganese in the Mn-Al solid solution falls rapidly with decreasing temperature and is close to zero at room temperature.

Some of the AlMn materials also contain iron (Fe) or silicon (Si) additives. These form the phases Al₃Fe, Al₈Fe₂Si, Al₅FeSi, Al₁₅Si₂(Mn,Fe)₃. Mixed crystals also occur in the form of Al₁₂(Mn, Fe)₃Si.

• Aluminium and Al₁₅Si₂(Mn,Fe)₃ are formed from the melt, Al₃Fe and Al₆(Mn,Fe) at 648 °C.
• At temperatures below 630 °C, aluminium, Al₁₅Si₂(Mn,Fe)₃ and Al₈Fe₂Si are formed from the melt, Al₃Fe.
• Aluminium, Al₅FeSi and Al₁₅Si₂(Mn,Fe)₃ are formed from the melt and Al₈Fe₂Si at around 600 °C.
• Aluminium, silicon and Al₁₅Si₂(Mn,Fe)₃ are formed from the melt and Al₅FeSi at around 565 °C.

The structure resulting from casting into bars or slabs consists of the main mass, which is an oversaturated mixed crystal, along with areas containing manganese-containing phases that have an average size of about 100 μm. A significant portion of the manganese (approximately 0.7 to 0.9%) remains dissolved in aluminium because the cooling rates after casting are too rapid for all the manganese to undergo diffusion and precipitate. This is further exacerbated by the very low diffusion speed of manganese in aluminium.

Through processes such as homogenization and forming (rolling, forging), the structure of the material undergoes changes. During this transformation, various phases that were previously present within the basic aluminium crystal structure, each having a size of less than one micron, are eliminated. These particles contribute to an approximately 25% increase in strength compared to pure aluminium. They exhibit thermal stability and are challenging to dissolve.

In the formed and homogenized state, the material exhibits a very fine structure, and the larger manganese-containing areas observed in the initial casting state are no longer present. These finely dispersed particles also impede grain growth, further enhancing the material's strength. However, it's important to note that this improvement is relatively modest, as the influence of grain size on the strength of aluminium materials is generally limited.

The presence of silicon accelerates the excretion of Al₁₂(Mn, Fe)₃Si.^[3] If there is enough silicon, the Al₆(Mn,Fe) is converted to Al₁₂(Mn,Fe)₃Si during homogenization.

3000 series are alloyed with manganese and can be work hardened.

• Friedrich Ostermann: Application technology aluminium. 3. Edition. Springer, 2014, ISBN 978-3-662-43806-0, p. 100-102.
• Aluminium paperback. Volume 1: Fundamentals and materials. 16. Edition. Beuth-Verlag, Berlin/ Vienna/ Zurich 2002, ISBN 3-87017-274-6, p. 104 f, 122.
• George E. Totten, D. Scott MacKenzie: Handbook of Aluminum. Volume 1: Physical Metallurgy and Processes. Marcel Dekker, New York/ Basel 2003, ISBN 0-8247-0494-0, p. 159f.