Aluminum-magnesium-silicon alloy

Aluminum-magnesium-silicon alloys ( AlMgSi ) are aluminum alloys - alloys that mainly consist of aluminum - which, in terms of quantity, contain both magnesium and silicon as the most important alloying elements. Both together make up less than 2 percent by mass . The magnesium content is higher than that of silicon, otherwise they belong to the aluminum-silicon-magnesium alloys (AlSiMg).

AlMgSi is one of the hardenable aluminum alloys, i.e. those that can become stronger and harder through heat treatment . This hardening is largely based on the precipitation of magnesium silicide (Mg ₂ Si). The AlMgSi alloys are therefore regarded as a separate group in the standards (6000 series) and not as a subgroup of the aluminum-magnesium alloys that cannot be hardened.

AlMgSi is one of the aluminum alloys with medium to high strength , high fracture toughness , good weldability , corrosion resistance and formability . They can be processed excellently by extrusion and are therefore particularly often processed into construction profiles by this process . They are usually heated to make processing easier; as a side effect, they can be quenched immediately afterwards, so that a separate subsequent heat treatment can be omitted.

Applications

They are used, among other things, for bumpers , car bodies and for large profiles in rail vehicle construction . In the case of the latter, they were largely responsible for the changed design of rail vehicles in the 1970s: previously, riveted tubular structures were used. Thanks to the good extrudability of AlMgSi, large profiles can now be produced, which are then welded . Also in the aircraft they are used, there are but AlCu and AlZnMg preferred but difficult or impossible to weld are. The weldable high-strength AlMgSiCu alloys (AA6013 and AA6056) are in the Airbus models A318 and A380 for ribbed plates in the aircraft fuselage used, where by the laser welding are possible weight and cost savings. (Welding is more cost-effective than riveting that is otherwise common in aircraft construction ; the overlaps required for riveting can be omitted when welding, which saves component mass.)

Alloy constitution

Phases and equilibria

The Al-Mg ₂ Si system forms a eutectic at 13.9% Mg ₂ Si and 594 ° C. The maximum solubility is 583.5 ° C and 1.9% Mg ₂ Si, which is why the sum of both elements in the common alloys is below this value. The stoichiometric composition of magnesium to silicon of 2: 1 corresponds to a mass ratio of 1.73: 1. The solubility drops very quickly with falling temperature and is only 0.08 percent by mass at 200 ° C. Alloys without further alloying elements or impurities are then in two phases prior to the - mixed crystal , and the phase (Mg ₂ Si). The latter has a melting point of 1085 ° C and is therefore thermally stable. Even accumulations (clusters) of magnesium and silicon atoms that are only metastable dissolve only slowly because of the high binding energy of the two elements. ${\displaystyle \alpha }$ ${\displaystyle \beta }$

Many standardized alloys have an excess of silicon. It has hardly any influence on the solubility of magnesium silicide, increases the strength of the material more than an excess of Mg or an increase in the Mg ₂ Si proportion, increases the volume and number of precipitates and accelerates the precipitation during cold and hot hardening. It also binds unwanted impurities; especially iron. On the other hand, an excess of magnesium reduces the solubility of magnesium silicide.

Alloy elements

In addition to magnesium and silicon, the standardized grades contain other elements.

• Copper is used in quantities of 0.2–1% to improve strength and artificial aging. It forms the Q phase (Al ₄ Mg ₈ Si ₇ Cu ₂ ). Copper leads to a denser dispersion of the needle-shaped, partially coherent precipitate (cluster of magnesium and silicon). In addition, there is the phase that is typical for aluminum-copper alloys . Alloys with higher copper contents (alloys 6061, 6056, 6013) are mainly used in aviation. ${\displaystyle \beta ''}$ ${\displaystyle \theta }$
• Iron occurs as an impurity in all aluminum alloys in amounts of 0.05–0.5%. It forms the phases Al ₈ Fe ₂ Si, Al ₅ FeSi and Al ₈ FeMg ₃ Si _6, all of which are thermally stable, but are undesirable because they make the material brittle. Excess silicon also serves to bind iron.
• Manganese (0.2–1%) and chromium (0.05–0.35%) are intentionally added. If both are added at the same time, the sum of the two elements is less than 0.5%. After annealing at at least 400 ° C, they form a dispersion of precipitates and thus improve strength. Chromium is particularly effective in combination with iron.
• As dispersion former come zirconium and vanadium are used.

Dispersions

Dispersion particles have only a minor influence on strength. If magnesium or silicon precipitate on them during cooling after the solution heat treatment and thus do not form magnesium silicide as desired, they even lower their strength. They therefore increase the sensitivity to deterrence. If the cooling speed is insufficient, they also bind excess silicon which would otherwise form coarser precipitates and thus reduce the strength. The dispersion particles activate further slip planes even in the hardened state so that ductility increases and, above all, intergranular fracture can be prevented. The alloys with higher strength therefore contain manganese and chromium and are more sensitive to quenching.

The following applies to the effect of the alloying elements in terms of dispersion formation:

• The strength at room temperature hardly changes. However, the yield point rises sharply at higher temperatures, which limits the formability and is particularly unfavorable in extrusion because it increases the minimum wall thickness.
• The recrystallization is difficult, which prevents the formation of coarse grain and has a favorable effect on the formability.
• Dislocation movements are blocked at low temperatures, which improves the fracture toughness .
• AlMn dispersions bind supersaturated silicon when cooling after solution annealing. This improves crystallization and prevents precipitation-free zones that would otherwise arise at the grain boundaries. This improves the fracture behavior from brittle and intergranular to ductile and transcrystalline.
• The quenching sensitivity increases because precipitated silicon is required for the hardening process. Alloys containing Mn or Cr must therefore be cooled faster than without these elements.

• 

  Ductile fracture of an AlMgSi alloy

• 

  Brittle fracture of an aluminum alloy

Grain boundaries

Silicon is preferentially precipitated at the grain boundaries because it has nucleation problems . Magnesium silicide also precipitates there. The processes are presumably similar to the AlMg alloys, but relatively unexplored for AlMgSi until 2008. The phases precipitated at the grain boundaries lead to the tendency of AlMgSi to break into brittle grain boundaries.

Compositions of standardized varieties

All information in percent by mass . EN stands for European standard , AW for wrought aluminum alloy (English aluminum, wrought ); The number has no further meaning.

Numerically	Chemically	Silicon	iron	copper	manganese	magnesium	chrome	zinc	titanium	Others	Others (single)	Others (total)	aluminum
EN AW-6005	AlSiMg	0.6-0.9	0.35	0.10	0.10	0.40-0.6	0.10	-	-	-	0.05	0.15	rest
EN AW-6005A	AlSiMg (A)	0.50-0.9	0.35	0.3	0.50	0.40-0.7	0.30	0.20	0.10	0.12-0.5 Mn + Cr	0.05	0.15	rest
EN AW-6008	AlSiMgV	0.50-0.9	0.35	0.30	0.30	0.40-0.7	0.30	0.20	0.10	0.05-0.20V	0.05	0.15	rest
EN AW-6013	AlMg1Si0.8CuMn	0.6-1.0	0.5	0.6-1.1	0.20-0.8	0.8-1.2	0.10	0.25	0.10	-	0.05	0.15	rest
EN AW-6056	AlSi1MgCuMn	0.7-1.3	0.50	0.50-1.1	0.40-1.0	0.6-1.2	0.25	0.10-0.7	-	0.20 Ti + Zr	0.05	0.15	rest
EN AW-6060	AlMgSi	0.30-0.6	0.10-0.30	0.10	0.10	0.35-0.6	0.05	0.15	0.10	-	0.05	0.15	rest
EN AW-6061	AlMg1SiCu	0.40-0.8	0.7	0.15-0.40	0.15	0.8-1.2	0.04-0.35	0.25	0.15	-	0.05	0.15	rest
EN AW-6106	AlMgSiMn	0.30-0.6	0.35	0.25	0.05-0.20	0.40-0.8	0.20	0.10	-	-	0.05	0.15	rest

Mechanical properties

Conditions:

• O soft ( soft annealed , also hot-formed with the same strength limit values).
• T1: quenched from the hot forming temperature and artificially aged
• T5: quenched from the thermoforming temperature and artificially aged
• T6: solution annealed, quenched and artificially aged
• T7: solution annealed, quenched, artificially aged and overhardened
• T8: solution annealed, strain hardened and artificially aged

Numerically	Chemical (CEN)	Status	Modulus of elasticity / MPa	G module / MPa	Yield strength / MPa	Tensile strength / MPa	Elongation at break /%	Brinell hardness	Bending fatigue strength / MPa
EN AW-6005	AlSiMg	T5	69500	26500	255	280	11	85	nb
EN AW-6005A	AlSiMg (A)	T1	69500	26200	100	200	25th	52	nb
		T4	69500	26200	110	210	16	60	nb
		T5	69500	26200	240	270	13	80	nb
		T6	69500	26200	260	285	12	90	nb
EN AW-6008	AlSiMgV	T6	69500	26200	255	285	14th	90	nb
EN AW-6056	AlSi1MgCuMn	T78	69000	25900	330	355	nb	105	nb
EN AW-6060	AlMgSi	0	69000	25900	50	100	27	25th	nb
		T1	69000	25900	90	150	25th	45	nb
		T4	69000	25900	90	160	20th	50	40
		T5	69000	25900	185	220	13	75	nb
		T6	69000	25900	215	245	13	85	65
EN AW-6061	AlMg1SiCu	T4	70000	26300	140	235	21st	65	60
EN AW-6106	AlMgSiMn	T4	69500	26500	80	150	24	45	nb
		T6	69500	26200	240	275	14th	75	<75

Heat treatment and hardening

AlMgSi can be hardened in two different ways by heat treatment , whereby hardness and strength increase, while ductility and elongation at break decrease. Both start with solution heat treatment and can also be combined with mechanical processes ( forging ), with different effects:

1. Solution annealing : Annealing takes place at temperatures of around 510-540 ° C, whereby the alloying elements go into solution.
2. Quenching almost always takes place immediately afterwards . As a result, the alloying elements initially remain in solution even at room temperature, whereas they would form precipitates if cooled down slowly.

• Cold hardening: At room temperature, precipitates gradually form which increase strength and hardness. In the first hours after quenching, the increase is very high, in the next few days less, then only gradually, but not yet complete even after several years.
• Artificial hardening: The materials are reheated in the oven at temperatures of 80–250 ° C (160–150 ° C are common). The curing times are usually around 5–8 hours. As a result, the alloying elements precipitate faster and increase hardness and strength. The higher the temperature, the faster the maximum possible strength for this temperature is reached, but the lower the higher the temperature, the lower it is.

Intermediate storage and stabilization

If time passes after quenching and artificial hardening (so-called intermediate storage), the strength that can be achieved during artificial hardening decreases and occurs later. The reasons lie in the change in the material during the intermediate storage, which is taking place during the cold hardening process. However, the effect only applies to alloys with more than 0.8% Mg ₂ Si (without Mg or Si excesses) and alloys with more than 0.6% Mg ₂ Si if Mg or Si excesses are present.

To prevent these negative effects, AlMgSi can be annealed after quenching at 80 ° C for 5–30 minutes, whereby the material condition stabilizes and temporarily no longer changes. The thermosetting ability is then retained. Alternatively, a step quenching is possible, in which the first step is quenching to temperatures that are used during artificial hardening. The temperatures are maintained for a few minutes to several hours (depending on the temperature and alloy) and then cooled down completely to room temperature. Both variants allow the workpieces to be machined in the quenched state for some time. If the waiting time is longer, cold curing begins. Longer treatment times increase the possible storage time, but reduce the formability . Some of these processes are protected by patents.

The stabilization has further advantages: The material is then in a definable state, which enables repeatable results in the subsequent processing. Otherwise, the time of the temporary storage would, for example, have an impact on the springback during bending , so that a constant bending angle would not be possible over several workpieces.

Influence of cold deformation

Forming ( forging , rolling , bending ) leads to work hardening of metals and alloys , an important form of increasing strength. With AlMgSi, however, it also has an influence on the subsequent artificial hardening. Cold forming in the artificially hardened condition, on the other hand, is not possible because of the low ductility in this condition.

Cold forming directly after quenching increases the strength through work hardening, but reduces the increase in strength through cold hardening and largely prevents it at forming degrees of 10% or more .

Cold forming in the partially or fully cold-hardened state, on the other hand, also increases the strength, so that both effects add up.

If, after cold forming (in the quenched or cold-hardened state), hot forming takes place, this takes place more quickly, but the achievable strengths decrease. The higher the solidification, the higher the yield point , but the tensile strength does not increase. If, on the other hand, cold forming takes place in the stabilized state, the achievable strength values ​​are improved.

literature

• Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, ISBN 978-3-662-43806-0 .

Individual evidence

1. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, pp. 26, 30, 39f, 43, 53.
2. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, Appendix.