Aluminum-magnesium alloy

Aluminum-magnesium alloys ( AlMg ) are aluminum alloys - i.e. alloys that mainly consist of aluminum - which contain magnesium as the main alloying element . Most standardized alloys still contain small amounts of manganese ( AlMg (Mn) ).

This article only deals with the pure AlMg alloys and the AlMg (Mn) alloys, which are among the medium-strength, naturally hard alloys (which cannot be hardened by heat treatment) and are standardized in the 5000 series. This does not deal with the hardenable aluminum alloys (by heat treatment) with magnesium as the main alloy element:

• Aluminum-magnesium-copper alloys (AlMgCu)
• Aluminum-magnesium-silicon alloys (AlMgSi, 6000 series)

Applications and processing

The AlMg alloys are among the most important construction materials for aluminum alloys. They can be easily cold formed , i.e. processed by rolling and forging , and are easy to weld with Mg contents of at least 3%. AlMg is only rarely processed by extrusion , since subsequent changes in strength must be avoided in extruded profiles.

The majority of AlMg alloys are processed into rolled products as well as tubes, rods, wires and open-form or drop forged parts . A part is also processed into extruded profiles with simple cross-sections.

Because of its good corrosion resistance and high strength at low temperatures, AlMg is used in shipbuilding , in apparatus engineering for chemical apparatus and pipelines, for refrigeration technology and automobiles . The good weldability is decisive for the use in aircraft construction , there also with additives of scandium and zircon for better weldability.

Solubility of Magnesium and Phases

The solubility of magnesium in aluminum is very high and reaches a maximum of 14% to 17% at 450 ° C, depending on the literature. At 34.5% there is a eutectic with Al ₈ Mg ₅ (sometimes also referred to as Al ₃ Mg ₂ ), an intermetallic phase ( phase). The solubility of Mg decreases sharply with falling temperature: at 100 ° C it is still 2%, at room temperature 0.2%. ${\displaystyle \beta }$

With pure AlMg alloys, the separation of the phase takes place according to a four-stage process; with technically used alloys with additional alloying elements and impurities, the process is much more complicated: ${\displaystyle \beta }$

• First of all, clusters are formed, referred to as GP zones for aluminum . These are local accumulations of magnesium atoms in the aluminum lattice, which do not yet form a phase of their own and also do not have a regular arrangement.
• Formation of the coherent phase. Their crystals have the same spatial orientation as those of the aluminum mixed crystal. ${\displaystyle \beta ''}$
• Formation of the partially coherent phase. It is only partially oriented towards the lattice of the Al mixed crystal. ${\displaystyle \beta '}$
• Formation of incoherent phase. It has no spatial orientation with the Al mixed crystal. ${\displaystyle \beta }$

In the case of technical alloys, the separation differs from this for the following reasons:

• Low diffusion of magnesium in aluminum
• A high supersaturation of 7% Mg and more is required for the formation of the GP zones and phase, which is not achieved in most alloys. No GP zones or phases were found in AlMg4.5Mn0.7 even after prolonged annealing at temperatures up to 250 ° C , although the phase is already present after a few days . ${\displaystyle \beta ''}$ ${\displaystyle \beta ''}$ ${\displaystyle \beta }$
• Dislocations are not sufficient seeds for the formation of phase, phase, or phase. The reason is the small volume difference between these phases and the matrix. ${\displaystyle \beta ''}$ ${\displaystyle \beta '}$ ${\displaystyle \beta }$

structure

round test body made of AlMg3, without grain refinement (2 ×)

round test specimen made of AlMg3, grain refined

The diffusion of magnesium in aluminum is very low. The reason is the large difference in size between the radius of the aluminum atoms and that of the magnesium atoms ( ). Therefore, after casting, only part of the magnesium is precipitated from the solid solution, while most of the magnesium is present as a supersaturated solution in the aluminum. This condition cannot be eliminated even by prolonged annealing. ${\displaystyle r_{Al}:r_{Mg}=1,43:1,6}$ ${\displaystyle \alpha }$

Excess magnesium is mainly excreted at the grain boundaries and on dispersion particles in the grain. The speed of the process depends on the Mg content and the temperature and increases with both. At the grain boundaries, so-called plaques are initially deposited, thin plates that are not connected, i.e. do not yet form a continuous layer around the grain. At 70 ° C they form after 3 months, at 100 ° C after 3 days and at 150 ° C after one to nine hours. As time passes at elevated temperature, the plaques grow together to form a cohesive film. This has a negative effect on the corrosion resistance, but can be resolved again by heat treatment. Annealing at 420 ° C for one hour with subsequent slow cooling of 20 ° C / h or tempering at 200 ° C to 240 ° C is suitable. The plaques of the phase transform into numerous small particles, referred to in the specialist literature as "pearl-like". They no longer form a coherent film. ${\displaystyle \beta }$

Composition of standardized varieties

The compositions of some standardized types are shown in the table below. Proportions of alloying elements in percent by mass . There are fine gradations of Mg and Mn contents of the grades available. Mn-free are very rare. Standard alloys are AlMg3Mn, AlMg4.5Mn0.7, as well as AlMg4.5Mn0.4 for bodies. Magnesium contents of up to 5% and manganese contents of up to 1% are used for wrought alloys.

For cast alloys, Mg contents of up to 10% are also possible; However, contents of 7% and more are considered difficult to pour .

Numerically	Abbreviation	Si	Fe	Cu	Mn	Mg	Cr	Zn
EN AW-5005	-	0.3	0.7	0.2	0.2	0.5-1.1	0.1	0.25
EN AW-5052	AlMg2.5	0.25	0.4	0.1	0.1	2.2-2.8	0.15-0.35	0.1
EN AW-5083	AlMg4.5Mn0.7	0.4	0.4	0.1	0.4-1.0	4.0-4.9	0.05-0.25	0.25
EN AW-5454	AlMg3Mn	0.25	0.4	0.1	0.5-1.0	2.4-3.0	0.05-0.2	0.25

corrosion

Aluminum-magnesium alloys are considered to be very resistant to corrosion, but this only applies if the phase is present as a discontinuous phase. Alloys with Mg contents below 3% are therefore always corrosion-resistant; with higher contents, suitable heat treatment must be used to ensure that this phase is not present as a continuous film at the grain boundaries. ${\displaystyle \beta }$

The phase and phase are very ignoble compared to aluminum and have an anodic character. Therefore AlMg prone to intergranular corrosion if ${\displaystyle \beta '}$ ${\displaystyle \beta }$

1. the phase is precipitated as a continuous film at the grain boundaries and at the same time ${\displaystyle \beta }$
2. the material is in an aggressive environment.

Alloys in conditions that are susceptible to intergranular corrosion are annealed at temperatures of 200 ° C to 250 ° C with slow cooling ( heterogenization annealing ). As a result, the -phase film changes into globular -phase and the material is resistant to intergranular corrosion. ${\displaystyle \beta }$ ${\displaystyle \beta }$

Mechanical properties

table

Numerically	Abbreviation	Status	Yield strength [N / mm²]	Tensile strength [N / mm²]	Elongation at break [%]	Brinell hardness	Vickers hardness	Fatigue strength [N / mm²]	Modulus of elasticity [N / mm²]
5005	AlMg1 (B)	O ( annealed ) HX2 (work - hardened, 1/4 hard) HX4 (work-hardened, 1/2 hard)	45 125 145	120 140 160	27 13 12	30th 40 45	46 50 55	80	69500
5052	AlMg2.5	O HX2 HX4	90 175 200	195 225 250	25th 15th 14th	50 65 70	50 70 75	105 110 120	70,000
5083	AlMg4.5Mn0.7	O HX2 HX4	145 240 275	300 330 360	23 17th 16	70 90 100	75 95 105	140	71,000
5454	AlMg3Mn	O HX2 HX4	110 205 235	235 265 290	25th 15th 14th	60 75 80	60 80 85	115 120 130	70,500

Strengths and elongation at break in the tensile test

The strength is increased by adding magnesium. With low Mg contents, the increase in strength is relatively strong; with higher contents it is always weaker. Magnesium increases the strength very efficiently compared to other elements; per% Mg, it is therefore stronger than with alternative elements. Even with medium Mg contents, the increase in strength through the addition of manganese is greater than through additional magnesium, which is also one of the reasons why most AlMg alloys still contain manganese. The reason given for the high increase in strength of magnesium is the high binding energy of vacancies on Mg atoms. These spaces are then no longer present as free spaces. However, these are favorable for plastic deformation.

The yield strength increases linearly with increasing Mg content from about 45 N / mm² with 1% Mg to about 120 N / mm² with 4% Mg. The tensile strength also increases linearly, but with a greater gradient. With 1% Mg it is about 60 N / mm², with 4% Mg 240 N / mm².

There are different statements for the elongation at break : Research on alloys on a purest basis shows increasing elongation at break from about 20% elongation at 1% Mg to 30% at 5% Mg. Sometimes a U-shaped curve for the elongation at break can also be found in the literature : At first it drops sharply from 38% elongation and 1% Mg to 34% elongation and around 1.8% Mg, at 3% Mg it reaches a minimum with only 32% elongation and then increases again to around 35% elongation at 5 % Mg.

The flow curves for AlMg show the typical behavior for metallic materials of increasing the flow stress with the true elongation or the degree of deformation. With all alloys, the increase is relatively strong with low elongations and less with higher elongations. However, the curves for higher alloyed grades are always above the lower alloyed ones. With a true elongation of 0.2, AlMg0.5 has a yield stress of about 100 N / mm², AlMg1 has one of 150 N / mm², AlMg3 of 230 N / mm² and AlMg4.5Mn0.4 of about 300 N / mm². The higher the alloy content and the greater the elongation, the greater the PLC effect and the Lüders effect .

Influence of the grain size

In the case of pure aluminum, the grain size has little influence on the strength of metals. In the case of alloys, the influence increases with the alloy content. With 5% Mg, materials with grain sizes of 50 µm achieve uniform elongations of around 0.25, with 250 µm they are around 0.28. AlMg8 already achieves uniform elongations of 0.3 at a grain diameter of 200 µm. With increasing grain size, both the Lüders expansion and the Lüders effect decrease.

Work hardening and heat treatment

At very high degrees of deformation with strongly work-hardened alloys, softening can also occur at room temperature. In a long-term study over 50 years, a decrease in strength could be measured by the end. The higher the degree of deformation and the higher the alloy content, the greater the decrease. The softening itself is very pronounced at the beginning and quickly subsides. The effect can be avoided by a stabilization annealing at around 120 ° C to 170 ° C for several hours.

See also

• Magnesium Alloy - Some grades contain aluminum as the main alloying element

literature

• Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, ISBN 978-3-662-43806-0 , pp. 102-116.
• Aluminum Pocket Book - Volume 1: Fundamentals and Materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, pp. 103, 134-136, 152 f.
• George E. Totten, D. Scott MacKenzie: Handbook of Aluminum Volume 1: Physical Metallurgy and Processes . Marcel Dekker, Yew York, Basel. 2003, 1296 pp. 160-168.

Individual evidence

1. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, ISBN 978-3-662-43806-0 , p. 102 f.
2. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, ISBN 978-3-662-43806-0 , p. 103.
3. ^ Ostermann, p. 104.
4. ^ Ostermann, p. 105.
5. ^ Ostermann, p. 105.
6. ^ Ostermann, p. 105.
7. ^ Ostermann: Application technology aluminum, appendix.
8. ^ Aluminum paperback, appendix.
9. ↑ Andreas Bühring-Polaczek , Walter Michaeli , Günter Spur (ed.): Handbuch Urformen , Hanser, 2014, p. 67.
10. ↑ Ostermann: Application Technology Aluminum, p. 103
11. ↑ Ostermann: Application Technology Aluminum, p. 106
12. ^ Aluminum pocket book, p. 136.
13. ^ Ostermann: Application technology aluminum, appendix
14. ↑ Ostermann: Application Technology Aluminum, p. 106.
15. ^ Ostermann: Application technology aluminum, 3rd edition, pp. 106-108. (Reference is made to the following studies: Falkenstein, H.-P., Gruhl, W., Scharf, G .: Contribution to the forming of aluminum materials. Metall. 37, 1197-1202 (1983); and: Yanagawa, M., Ohie , S., Koga, S., Hino, M .: Controlling factors of ductility in Al-Mg alloys. Kobelco Technol. Rev. 16, 25-30 (1993))
16. ^ Aluminum-Taschenbuch, 16th edition, p. 135. (with reference to Scharf, G; Influence of the chemical composition of AlMgSi wrought materials. Aluminum 58 (1982) 7, p. 391/397)
17. ↑ George E. Totten, D. Scott MacKenzie: Handbook of Aluminum Volume 1: Physical Metallurgy and Processes. Marcel Dekker, Yew York, Basel. 2003, 1296 p. 165.
18. ^ Ostermann: Application technology aluminum, p. 107.
19. ↑ Ostermann: Application Technology Aluminum, p. 109 f.
20. ↑ Ostermann: Application Technology Aluminum, p. 110 f.