ALUMINUM ALLOY STRUCTURAL COMPONENTS, STARTING MATERIAL AND METHOD OF MANUFACTURE

Description

The invention relates to structural components made of an aluminum alloy, in particular structural components for automobiles, a starting material for such structural components and associated methods for manufacturing the same.

It is known to produce aluminum alloy bars, which are used, for example, to manufacture structural components for vehicles, using the continuous casting process.

In a continuous casting process, a material—e.g. an aluminum alloy—is transferred from a molten to a solidified state by continuously feeding the alloyed or unalloyed melt into a short, water-cooled ring mold. The ring mold is closed off by a lowerable foot block on a casting table. The casted bar is produced by lowering the casting table and continuously feeding the mold chamber. Water is used to cool the ring mold and the solidified cast product. Besides ring molds, molds with a ceramic structure on the mold (“hot top”) are also used, which reduces the temperature gradient in the solidifying melt, which can lead to a more uniform structure in the bar cross-section and a thinner edge shell with increased surface quality. Casting lengths are usually between 3 and 7 meters. (F. Ostermann “Anwendungstechnologie Aluminium”, Springer, 2007, 2nd edition, p. 419f.).

Prior to the step of forming, the final contour pre-material produced in continuous casting is usually brought into suitable preliminary dimensions by means of extrusion before the actual forming step to the final contour. In addition to changing the dimensions of the forged pre-material produced in this way, the grain structure of the material also changes. If a more globulitic grain structure is present in the as-cast state, the extrusion process changes this to a fiber structure oriented in the direction of extrusion (F. Ostermann “Anwendungstechnologie Aluminium”, Springer, 2007, 2nd edition, p. 482).

Extrusion and forging are two of the most economical forming processes for aluminum. The design limits of extrusion and forging are influenced by the alloy and the available process forces, among other factors. In addition to the machine settings and tool design, the quality of a formed part depends to a large extent on the alloy system selected. AlMn(Cu) and AlMgSi alloy systems in particular are widely used for extruded products. (F. Ostermann “Application Technology Aluminum”, Springer, 2007, 2nd edition, pp. 435-444). In principle, all wrought aluminum alloys and casting alloys can be used for hot forming by forging. For technical and economic reasons, however, mainly selected wrought alloys of the 2xxx, 5xxx, 6xxx and 7xxx series according to DIN EN 573-4 and DIN EN 586-3 are used (F. Ostermann “Anwendungstechnologie Aluminium”, Springer, 2007, 2nd edition, p. 481f.).

The general heat treatment cycle for age-hardenable wrought aluminum alloys is shown in FIG. 1 and consists of the steps of solution annealing, quenching and age-hardening.

As the hot forming temperatures during forming are in the range of the solution annealing temperature, solution annealing can be dispensed with for low to medium-strength materials. In the case of high-strength materials, the forging material is subjected to solution annealing after forming in order to exploit the full potential of the alloys. Solution annealing that is too short results in a reduced strength level and reduced ductility after hardening. Re-annealing after forming serves to homogenize the forging structure. In the case of high-alloy materials, it is important to precisely control the quenching rate after the solution annealing process in order to counteract premature segregation processes. The C-curve of a pressed profile made of EN AW-6060 is shown as an example in FIG. 2.

Following quenching of the structural component after hot forming, an alloy-specific single or multi-stage cold and/or artificial ageing process is usually carried out on the formed component. This serves to set the required mechanical properties (source: F. Ostermann “Anwendungstechnologie Aluminium”, Springer, 2007, 2nd edition, p. 157-166).

In the past, increasing attention has been paid to the development of aluminum alloys with improved corrosion resistance and simultaneously increased mechanical properties. This is mainly due to the fact that there is an increasing demand in the automotive industry for materials that enable an advantage in the CO₂ balance during a vehicle's life cycle by reducing weight through wall thickness.

There are also requirements for wear resistance, low density and thermal expansion as well as good formability. To date, these requirements have mainly been met by alloys from the 6xxx series in accordance with DIN EN 573-4, for example 6082 alloys.

Until now, the requirements for maximum mechanical properties could only be met by using precipitation sequences with copper additions (Cu additions) to the alloy. However, this is increasingly leading to a conflict of objectives, as Cu-containing 6xxx alloys are reaching their limits in terms of corrosion resistance, particularly with regard to intercrystalline corrosion.

Efforts to substitute Cu additives by using alloying elements from the group of rare earth metals are showing positive results in some cases. However, the poor availability of alloying elements from the group of rare earth metals and the associated price situation make such variants less attractive in an industrial environment.

Aluminum alloys for the manufacture of structural components are described in EP 2 644 727 A2 or EP 2 811 042 B1.

Known aluminum alloys according to AA 6182 have the following composition:

	Aluminum, Al	95-97.85%
	Chromium, Cr	<=0.25%
	Copper, Cu	<=0.10%
	Iron, Fe	<=0.50%
	Magnesium, Mg	0.70-1.2%
	Manganese, Mn	0.50-1.0%
	Silicon, Si	0.90-1.3%
	Titanium, Ti	<=0.10%
	Zinc, Zn	<=0.20%
	Zirconium, Zr	0.05-0.20%
	other, each	<=0.05%
	other, total	<=0.15%

The invention is based on the object of creating a structural component which has both high strength and good corrosion resistance and can be manufactured economically, as well as an associated manufacturing process.

According to a first aspect of the invention, this object is solved by a method for producing starting material for forged structural components made of aluminum, which comprises the following steps:

• • Preparation of a melt with the components.
    • 1.2 to 1.4 wt.-% silicon,
    • 0.8 to 1.2 wt.-% magnesium,
    • 0.3 to 1.2 wt.-% manganese,
    • 0.1 to 0.2 wt.-% zirconium,
    • 0.01 to 0.05 wt.-% titanium,
    • 0.08 to 0.3 wt.-% iron,
    • 0.08 to 0.2 wt.-% chromium
    • max. 0.04 wt.-% zinc,
    • max. 0.02 wt.-% copper and
    • Residual aluminum and unavoidable impurities
    • wherein the ratio of silicon to magnesium is at least 1:0.8, and
    • wherein the melt contains less than 0.5 and preferably less than 0.25 ml H₂ per 100 g aluminum, and
  • Continuous casting of the molten aluminum alloy, in which the molten metal is guided in such a way that it has no contact with a solid surface during solidification, for producing a bar which preferably has an electrical conductivity of between 16 MS/m and 18 MS/m.

Preferably, the melt is liquid-cooled as it passes through a mold so that it solidifies without touching the outer solid boundary. Preferably, the coolant temperature and the mass flow rate of the coolant (e.g. water) are set so that the highest cooling rate of the melt is more than −25 K/s. Suitable cooling rates are, for example, between −15 K/s and −35 K/s for a billet with a diameter of between 90 mm and 100 mm, measured at a lateral distance of approximately 20 mm from a central longitudinal axis of the billet. The coolant is preferably water with a maximum temperature of 80° C. At a casting speed of between 100 mm/min and 500 mm/min, the cooling water flow rate for a billet with a diameter of between 90 mm and 100 mm is preferably between 40 l/min and 80 l/min. Preferably, the cooling water flow rate is 20 to 50 times greater than the flow rate of the cast aluminum alloy, wherein the cooling water volume flow for billet diameters in the order of 50 mm is preferably about 25 times greater than the volume flow of the cast aluminum alloy and for billet diameters in the order of 100 mm is preferably about 40 times greater than the volume flow of the cast aluminum alloy. The cooling capacity can be determined and adjusted as a function of flow/return temperature and volume flow.

A water-cooled ring mold is preferably used as the mold, which is designed in such a way that the molten aluminum alloy is separated from the solid components of the ring mold by a film of water and therefore does not touch any solid components of the mold during solidification. The water is used for the described liquid cooling of the casting strand.

Preferably, the melt passing through the mold is fully exposed to an oxygen-containing gas mixture from the outside, preferably pure oxygen, in order to promote the formation of a solidified surface layer of oxidized aluminium alloy. Oxygen is applied before water is applied for liquid cooling. Only after the solidified surface layer has formed is the casting strand (bar), which is initially still liquid inside, cooled with water as mentioned above. The aforementioned water film therefore does not come into contact with the molten metal, but with the already solidified surface layer of the casting strand. Prior to this, the melt is separated from the mold by the gas mixture during the solidification of the surface layer.

The gas mixture preferably contains between 10% and 80% or between 20% and 70% oxygen (O₂).

In further embodiments, the molten aluminum alloy may also optionally comprises one or more of the following elements in the proportions specified below:

• • max. 0.2% wt.-% chromium
  • max. 0.04% wt.-% zinc
  • max. 0.02% wt.-% copper

In comparison with standard continuous casting processes, the use of a direct quenching continuous casting process, which avoids contact between the melt and solid surfaces (e.g. the mold), makes it possible to achieve a pre-material quality that can be solid formed directly and is characterized by sufficiently homogeneous surface quality, sufficiently fine microstructure and sufficiently low recrystallization tendency.

The pre-material according to the invention has a significantly reduced tendency to secondary recrystallization compared to the prior art, so that, for example, the circumferential coarse grain seam typical of forged parts is eliminated (see also FIGS. 3 and 4). The electrical conductivity is significantly lower than in comparable extruded material, namely around 16 to 18 megasiemens per meter (MS/m) compared to around 25 MS/m in the current state of art. This is advantageous when the pre-material according to the invention is used to manufacture components of electrical machines, because the lower conductivity reduces harmful eddy currents.

The starting material according to the invention has increased corrosion resistance compared to the prior art.

In addition, products manufactured from the starting material according to the invention offer, for example, a higher pressure tightness compared to cast aluminum, which results from the absence of porosity.

The aluminum alloy used according to the invention is easy to cast and form and has good mechanical properties. The mechanical properties achieved are, for example, a tensile strength R_m>375 MPa, a yield strength R_p02>345 MPa, an elongation A>10% and a hardness>100 HB. In addition, the aluminum alloy has good machinability and high corrosion resistance.

Accordingly, the components made from the aluminum alloy are characterized by high strength and elongation properties combined with excellent corrosion resistance.

The aluminum alloy used according to the invention is particularly suitable for manufacturing structural components, especially automotive components, by means of forming processes.

Preferably, the weight ratio of titanium to zirconium (Ti:Zr) in the melt of the aluminum alloy is between 1:4 and 1:6, particularly preferably at least approximately 1:5.

Preferably, the weight ratio of iron to chromium to manganese (Fe:Cr:Mn) in the molten aluminum alloy is at least approximately 1:1:4.

Preferably, the aluminum alloy contains up to max. 0.3 wt.-% of one or more elements of the so-called rare earths, preferably Sc, Er, La, Ce, Y and/or Yb, to form further finely dispersed dispersoids.

Preferably, grain refinement is carried out on the aluminum alloy according to the invention.

For this purpose, the alloy can contain Ti and B for grain refinement, with titanium and boron being added to the melt via a master alloy containing 2.7 to 3.2 wt.-% Ti and 0.6 to 1.1 wt.-% B, the remainder being aluminum. According to one variant, the aluminum master alloy contains 2.9 to 3.1 wt.-% Ti and 0.8 to 0.9 wt.-% B and has a Ti:B weight ratio of about 3:1.

A master alloy with a Ti:B weight ratio of between approximately 5:1 and 5:0.6 is preferred. In this case, the titanium content is preferably between 4.5 and 5.5 wt.-%, for example between 4.8 and 5.2 wt.-%.

The content of the master alloy in the alloy according to the invention is preferably adjusted to 0.02 to 0.3 wt.-%.

Accordingly, the manufacturing method according to the invention preferably additionally comprises the following steps:

• • Preparation of an aluminum master alloy with 4.8 to 5.2 wt.-% Ti and 0.8 to 1.2 wt.-% B and the remainder aluminum and unavoidable impurities and
  • Adding the aluminum master alloy to the melt of the aluminum alloy with the components:
    • 1.2 to 1.4 wt.-% silicon,
    • 0.8 to 1.2 wt.-% magnesium,
    • 0.3 to 1.2 wt.-% manganese,
    • 0.1 to 0.2 wt.-% zirconium,
    • 0.01 to 0.05 wt.-% titanium,
    • 0.08 to 0.3 wt.-% iron,
    • 0.08 to 0.2 wt.-% chromium,
    • max. 0.04 wt.-% zinc,
    • max. 0.02 wt.-% copper and
    • Remainder aluminum and unavoidable impurities.

Ti and B are added to the aluminum alloy for grain refinement by means of the aluminum master alloy.

In the preferred manufacturing process for an aluminum alloy pre-material, the aluminum prealloy containing titanium and boron is preferably added to the melt in the form of a wire immediately before the mold.

The primary material produced by the process according to the invention is preferably forged into a structural component immediately after continuous casting.

A further aspect of the invention is a manufacturing process for the aluminum alloy according to the invention, by means of which the required component properties are obtained.

A further aspect is a structural component made of the aluminum alloy according to the invention, in particular a structural component for a vehicle, for example an aircraft or an automobile.

A further aspect is a method for producing a structural component, in which the starting material produced as described above is hot-formed, in particular forged, into a structural component immediately after continuous casting, without any treatment for homogenizing of the pre-material taking place between the continuous casting and the hot forming.

The forged structural component can be quenched directly after hot forming without the hot-formed structural component having to be solution-annealed again beforehand. The forged structural component can then be artificially aged.

Alternatively, the forged structural component can be solution annealed, quenched and artificially aged after hot forming. The temperature during solution annealing corresponds approximately to the forming temperature for forging, i.e. between 400° C. and 570° C.

Preferably, the structural component is subjected to artificial ageing and subsequent air cooling after hot forming, in particular after forging. The artificial ageing is preferably carried out for 2 to 7 hours at 180° C. to 210° C.

Due to the material properties, crash or deformation-relevant parts or fail-safe parts made from the pre-material according to the invention are particularly preferred. Examples of preferred structural components are the following vehicle parts: sliding wedges, battery casings, structure nodes, engine mounts, steering knuckles and control arms.

Accordingly, a further aspect is the use of the aluminum alloy according to the invention for the manufacture of a structural component for a vehicle, for example an aircraft or an automobile, in particular for the manufacture of deformation-relevant parts or fail-safe parts, in particular for the manufacture of sliding wedges, battery casings, structure nodes, engine mounts, steering knuckles and control arms for vehicles.

The invention includes the realization that in order to achieve the required mechanical properties, in particular a high yield strength R_p0.2, copper or zinc must usually be added to a 6xxx alloy. However, this procedure conflicts with the required corrosion resistance. In order to achieve this, the permissible proportions of Cu and Zn in the alloy must be severely limited, as otherwise precipitation will occur, particularly at the grain boundaries, which will have a negative effect on the corrosion resistance of the alloy. The required properties must therefore be realized by other suitable measures, such as setting the Si:Mg ratio to at least 1:0.8 and preferably the Fe:Mn:Cr ratio to at least approximately 1:1:4 or the addition of dispersoid formers. In addition, a suitable heat treatment strategy is an effective means of achieving the required objectives. This involves forming phases that counteract dislocation movements when force is applied.

The invention also includes the realization that known EN AW-6082 aluminum alloys do not provide sufficient strength with sufficient corrosion resistance. On the other hand, known EN AW-6056 aluminum alloys provide sufficient mechanical properties, but fail in the required corrosion properties. Other EN AW-6xxx alloys and the other wrought alloy groups also do not meet the required properties.

Furthermore, the invention includes the realization that only a few wrought alloys are actually used for forging, although in principle all aluminium and wrought alloys can be formed by hot forging. The reason for this is the high demands in terms of strength and component safety in the typical areas of application for forged aluminum parts, namely vehicle construction and mechanical engineering. A frequent field of application for forged aluminum parts are chassis safety parts, which can be permanently exposed to corrosion due to their exposed position; cf. F. Ostermann, Anwendungstechnologie Aluminium, London, Heidelberg: Springer, 2007.

It is generally known that pure aluminum is highly resistant to corrosion. The susceptibility only arises through the addition of corresponding alloying elements. F. Uyma writes that increasing proportions of magnesium improve the corrosion properties. Manganese influences the recrystallization and precipitation behaviour, which leads to the increased occurrence of the so-called “Chinese script” (binary eutectic of Al and Mg₂ Si), which in turn causes intercrystalline corrosion. Zinc and scandium additions are said to increase resistance to intergranular corrosion; cf. F. Uyma, Untersuchungen auf dem Gebiet der Al-Mg-Si- und Al/Mg₂ Si-in-situ Legierungen, Freiberg: Technische Universität Bergakademie Freiberg, 2007. J. P. Wloka states that a continuous corrosion-active path through the metal is responsible for the occurrence of intergranular corrosion. This occurs primarily in artificially aged materials that exhibit bead-like grain boundary precipitates. This phenomenon is not observed in overaged structures; cf. J. P. Wloka, Korrosionsuntersuchungen an scandiumhaltigen AlZnMgCu-Legierungen unter besonderer Berücksichtigung des Einflusses intermetallischer Phasen, Erlangen: Universität Erlangen-Nurnberg, 2007. It is also known that the sensitivity to intergranular corrosion is increased by insufficient quenching rates after solution heat treatment. This applies in particular to age-hardenable alloys. Intergranular corrosion can therefore hardly be avoided in thick-walled semi-finished products; cf. F. Ostermann, Anwendungstechnologie Aluminium, London, Heidelberg: Springer, 2007. G. Svenningsen et al. have investigated the influence of copper content in AlMgSi alloys. The susceptibility to intercrystalline corrosion is therefore largely based on the copper content of an alloy. In addition, the quenching rate has a significant influence on the intercrystalline corrosion resistance of an alloy. Cooling with water is therefore preferable to cooling in air. Finally, as also described in J. P. Wloka, Korrosionsuntersuchungen an scandiumhaltigen AlZnMgCu-Legierungen unter besonderer Berücksichtigung des Einflusses intermetallischer Phasen, Erlangen: Universität Erlangen-Nurnberg, 2007, it can be shown that overaging has a positive influence on resistance with regard to intergranular corrosion. The so-called Q phase (Al4Mg8Si7Cu2) was identified as the main phase responsible for intergranular corrosion (G. Svenningsen et al., “Effect of low copper content and heat treatment on intergranular corrosion of model AlMgSi alloys,” Corrosion Science, vol. 1, no. 48, p. 226-242, 2006). In this context, the ratio of magnesium to silicon is much debated. However, there is no consensus on the best ratio, partly because no direct evidence of its influence has yet been provided. G. Svennigsen et al. set the ratio at Mg/Si˜0.87. Altenpohl suggests a ratio of Mg/Si˜1 (G. Svenningsen et al., “Effect of low copper content and heat treatment on intergranular corrosion of model AlMgSi alloys,” Corrosion Science, vol. 1, no. 48, p. 226-242, 2006), (D. Altenpohl, Aluminium und Aluminiumlegierungen, London, Heidelberg: Springer, 1965).

With the alloy composition according to the invention, significantly improved corrosion properties in solid-formed components can be achieved with at least the same high strength values and at least the same low recrystallization tendency. Surprisingly, it has been shown that, in addition, a more stable casting process and a more moderate use of grain refiners is made possible.

The invention will now be explained in more detail with reference to examples of embodiments and in comparison with other aluminum alloys. The figures show:

FIG. 1: the general heat treatment cycle for age-hardenable wrought aluminum alloys;

FIG. 2: C-curves for profiles made of alloy EN AW-6060 (AlMgSi);

FIG. 3: a schematic representation of the coarse grain development on a forged part using the usual extruded material;

FIG. 4: a schematic representation of the coarse grain development on a forging using the material according to the invention; and

FIG. 5: a schematic sketch illustrating the manufacture of the pre-material according to the invention.

FIG. 1 (source: F. Ostermann “Anwendungstechnologie Aluminium”, Springer, 2007, 2nd edition, p. 174), already mentioned in the introduction to the description, shows the diagram of the general heat treatment cycle for age-hardenable aluminum alloys. I illustrates the non-critical heating rate during solution annealing, II illustrates the critical cooling rate during quenching after solution annealing, III illustrates the critical heating rate during artificial ageing (this depends on the alloy) and IV illustrates the non-critical cooling rate after artificial ageing. “RT” indicates the room temperature.

The process presented in this description differs from the general illustration in FIG. 1 in that it does not involve explicit cold ageing. Instead, there is only intermediate storage until artificial ageing. The artificial ageing process is single-stage. It is precisely one advantage of the process described here that the billets produced in the continuous casting process can be cold-formed after continuous casting without the need for extended ageing. After continuous casting, the billets can be artificially aged at 180° C. to 210° C. for a period of 2 to 7 hours after cooling.

The C-curves shown in FIG. 2 illustrate the influence of the quenching speed on the formation of phases or unwanted precipitates during extrusion. The quenching speed must be selected in such a way that no undesirable precipitation occurs. The principles that apply to extrusion are intended to illustrate the influence that a quenching speed has on the formation of precipitates. According to the process described here, the billets are preferably forged rather than extruded. After forging, solution annealing can be carried out if necessary.

An advantage of the aluminum alloy according to the invention described in more detail below is that it does not require any heat treatment or only a short artificial ageing if a preliminary product is produced in the manner described here using the direct quenching continuous casting process.

A preferred aluminum alloy for this purpose has the following components:

The preferred silicon content is 0.7 to 1.8 wt.-%, particularly preferably 1.0 to 1.6 wt.-%, in particular 1.2 to 1.4 wt.-%.

The preferred magnesium content is 0.5 to 1.4 wt.-%, particularly preferably 0.6 to 1.2 wt.-%, especially 0.8 to 1.0 wt.-%.

The preferred copper content is max. 0.1 wt.-%.

The preferred manganese content is 0.4 to 0.8 wt.-%, particularly preferably 0.5 to 0.7 wt.-%.

The preferred zirconium content is 0.005 to 0.5 wt.-%, particularly preferably 0.005 to 0.1 wt.-%.

In addition, the alloy may optionally contain one or more of the following elements in the proportions specified below:

The preferred chromium content is max. 0.2 wt.-%.

The preferred iron content is max. 0.3 wt.-%.

The preferred titanium content is 0.001 to 0.05 wt.-%.

The preferred zinc content is max. 0.04 wt.-%, preferably max. 0.01 wt.-%.

The following ratios of the alloy components to each other are preferred:

• • Si:Mg—between 1:0.5 and 1:0.75
  • An appropriately adjusted surplus of silicon causes the strength to increase significantly
  • Fe:Cr:Mn—between about 1:1:4 and 1:1:5
  • wherein the ratio of iron to chromium Fe:Cr is preferably 1:1 and lies between 0.8:1 and 1:0.8. The preferred ratio of iron to chromium to manganese serves to avoid acicular AlFeSi phases, which have a negative effect on ductility. Manganese forms these phases into less problematic AlFeMnSi phases. The chromium content is limited due to the high affinity to free silicon, which can also have a negative effect on the mechanical properties.
  • Ti:Zr—1:5 (TiZr compound/theme recrystallization, also RE-Metals see below, grain refinement)
  • The ratio of titanium to zirconium serves to form a sufficient number of Al3Zr phases with an L12 structure, which as dispersoids have a positive influence on the mechanical properties and recrystallization behavior.
  • Cu<0.02, preferably Cu<0.01
  • Copper is used to inhibit corrosion tendencies, particularly with regard to intercrystalline corrosion
  • Zn<0.04, preferably Zn<0.01
  • Zinc is used to inhibit the tendency to corrode, particularly with regard to stress corrosion cracking
  • Rare earths
  • Rare earths act as additional dispersoid formers to increase the mechanical properties, especially the hardness, and to influence the tendency to recrystallize during hot forming.

DESCRIPTION OF AN EMBODIMENT EXAMPLE

The aluminum alloy can be produced by means of processes known to the skilled person, usually by producing a melt which has a composition corresponding to the alloy composition specified above.

The alloying elements Ti and B are preferably added in the form of a master alloy during the production of the alloy, as explained above.

The starting material made from the aluminum alloy according to the invention is preferably produced using a horizontal continuous moulding (HCM) process. Prior gas treatment of the melt with a suitable inert gas or several inert gases ensures sufficient melt quality and produces a low-H₂ cast product, which is an important step in achieving the required properties in the final product. Methods for treating molten metals with inert gases are known to those skilled in the art. In order to obtain a low-H₂ cast product, the maximum value for hydrogen (H₂) in the melt is preferably between 0.05 and 0.5 ml H₂ per 100 g of aluminum. A maximum value for hydrogen of less than 0.25 ml H₂/100 g Al and in particular less than 0.15 ml H₂/100 g Al is particularly preferred.

The production of structural components using the aluminum alloy according to the invention is carried out by hot forming, preferably by drop forging of starting material preferably produced as described here. Heating to a forming temperature of between 400° C. and 570° C. can be carried out as desired, preferably by means of a heating furnace or induction. Intermediate heating steps can be carried out between individual forming steps.

The forged structural component can be quenched directly from the forming heat—i.e. immediately after hot forming. The forged structural component can then be artificially aged.

Alternatively, the forged structural component can be solution annealed, quenched and artificially aged. The temperature during solution annealing corresponds approximately to the forming temperature for forging, i.e. between 400° C. and 570° C.

Depending on the area of application or requirement profile, the pre-material before hot forming or the structural component obtained by hot forming can be subjected to heat treatment. However, heat treatment is not absolutely necessary because the continuously cast starting material already has excellent properties for the production of structural components by forging. The aluminum alloy according to the invention can, for example, be heat-treated for 0.25 to 24 hours at 150° C. to 260° C. and then quenched in a suitable gaseous medium, e.g. air or inert gas, or in a suitable liquid medium, e.g. water or oil. The preferred heat treatment takes place at 1 h to 10 h and 170° C. to 220° C., particularly preferably 2 h to 7 h at 180° C. to 210° C. with subsequent air cooling.

However, homogenization and/or extrusion of the primary material is not necessary. In particular, the starting material can be hot-formed, for example forged, immediately after continuous casting. Otherwise conventional process steps such as homogenization following continuous casting and extrusion following homogenization can be omitted

The heat-treated primary material is suitable for the production of structural components that are used in highly stressed and safety-relevant areas, e.g. vehicle construction. In addition to high strength, high ductility is required.

The production of the primary material comprises the following process steps, for example:

• • Melting the alloy components
  • Setting the alloy
  • Melt cleaning
  • Direct quenching continuous casting of a bar
  • Production of billets as primary material by sawing the casted bar

The subsequent forging of a structural component includes the following process steps, for example:

• • Heating the billet
  • Preforming
  • Optional: Intermediate heating
  • Forging
  • optional: solution annealing
  • Quenching
  • Artificial aging

It has been shown that forged parts made from the starting material according to the invention exhibit significantly reduced coarse grain formation. This is shown in FIGS. 3 and 4.

FIG. 3 shows a schematic cross-section of a hot-formed structural component 30 made from a conventional primary material. A pronounced coarse grain seam 32 and an equally pronounced flash area 34 can be seen.

FIG. 3 illustrates the typical coarse grain development on a forged part using the usual extruded material.

In contrast to this, FIG. 4 shows the significantly lower coarse grain development on a forged part 30′ using the starting material according to the invention, because the tendency to secondary recrystallization is significantly reduced with the starting material according to the invention. The circumferential coarse grain seam 32 (see FIG. 3) typical of forged parts is eliminated (see FIG. 4). This results in a high surface quality of the forgings.

The following tables show examples of different alloys tested for comparison with the alloy under stress.

	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Zr

6082 BFATmod.	0.91	0.12	0.45	0.46	0.78	0.17	<0.01	0.03	0.02
6082 Standard	0.98	0.17	0.08	0.52	0.83	0.16	<0.01	0.03	<0.01
6182 Standard	inf.	inf.	inf.	inf.	inf.	inf	inf.	inf	inf.
V0a	0.94	0.15	<0.01	0.29	1.19	0.09	<0.01	0.03	0.08
V0b	0.95	0.14	0.09	0.29	1.18	0.09	<0.01	0.04	0.07
V0c	1.34	0.07	<0.01	0.49	1.19	0.11	<0.01	0.03	0.12
V0d	1.06	0.10	<0.01	0.58	1.23	0.15	<0.01	0.03	0.12
V0e	1.07	0.11	<0.01	0.82	1.25	0.15	<0.01	0.03	0.12
6182-A	1.27	0.11	<0.01	0.72	0.94	0.13	0.02	0.03	0.15
6182-B	1.28	0.12	<0.01	0.71	0.93	0.12	0.12	0.03	0.16
6182-C	1.26	0.27	<0.01	0.59	0.94	0.11	<0.01	0.03	0.14
6182-D	1.27	0.11	<0.01	0.62	0.97	0.13	<0.01	0.03	0.14
6182-E	1.27	0.12	<0.01	0.61	0.96	0.08	<0.01	0.04	0.15

	Hot-formed gravity die casting	HCM cast hot-formed

	HBW5/250 T6	Rm Mpa	Rp02 Mpa	A %	Rm Mpa	Rp02 Mpa	A %

6082 BFAT mod.	125	387	359	7.5	406	380	12.8
6082 Standard	101
V0a	101	311	278	5.1
V0b	110	307	291	2.5	342	320	12.5
V0c	117
V0d	114
V0e	114
6182-A	121
6182-B	121
6182-C	121
6182-D	125	367	342	6.2	391	366	11.4
6182-E	125

										other,	other,
Al	Cr	Cu	Fe	Mg	Mn	Si	Ti	Zn	Zr	each	total
95-	<=0.25%	<=0.10%	<=0.50%	0.70-	0.50-	0.90-	<=0.10%	<=0.20%	0.05-	<=0.05%	<=0.15%
97.85%				1.2%	1.0%	1.3%			0.20%

The starting material suitable for forging, for example, is produced from the aluminum alloy according to the invention by producing a melt 10 with the following components:

• • 1.2 to 1.4 wt.-% silicon,
  • 0.8 to 1.2 wt.-% magnesium,
  • 0.3 to 1.2 wt.-% manganese,
  • 0.1 to 0.2 wt.-% zirconium,
  • 0.01 to 0.05 wt.-% titanium,
  • 0.08 to 0.3 wt.-% iron,
  • 0.08 to 0.2 wt.-% chromium,
  • max. 0.04 wt.-% zinc,
  • max. 0.02 wt.-% copper and
  • Remainder aluminum and unavoidable impurities.

In addition, a wire 12 made of an aluminum master alloy with 4.8 to 5.2 wt. % Ti and 0.8 to 1.2 wt. % B and the remainder aluminum and unavoidable impurities is provided and added to the melt immediately before it passes through a water-cooled ring mold 14; see FIG. 5. The ring mold 14 is designed such that the melt of the aluminum alloy is separated from the solid components of the ring mold by a film of water and thus does not touch any solid components of the mold during solidification. In addition, the melt passing through the annular mold 14 is fully exposed to the purest possible oxygen O₂ from the outside in order to promote the formation of a surface layer of oxidized aluminium alloy. Oxygen is applied before water is added for liquid cooling.

When continuously casting a billet with a diameter of between 90 mm and 100 mm, for example, the casting speed is between 150 mm/min and 200 mm/min.

The cooling water 16 for cooling the melt 10 as it passes through the ring mold 14 is guided in the circuit 18 in such a way that the temperature and the volume flow of the cooling water cause a maximum cooling rate of the melt of more than −25 K/s when it solidifies. Suitable cooling rates are, for example, between −15 K/s and −35 K/s for an bar with a diameter of between 90 mm and 100 mm, measured at a lateral distance of about 20 mm from a central longitudinal axis of the bar.

The water 16 used as a coolant has a temperature of between 20° C. and 80° C. When continuously casting an bar with a diameter of between 90 mm and 100 mm at a casting speed of between 150 mm/min and 200 mm/min, the cooling water flow rate is between 40 l/min and 80 l/min, for example.

Preferably, the cooling water volume flow is 20 to 50 times greater than the volume flow of the cast aluminum alloy, wherein the cooling water volume flow for bar diameters in the order of 50 mm is preferably about 25 times greater than the volume flow of the cast aluminum alloy, than the volume flow of the cast aluminum alloy and for bar diameters in the order of 100 mm preferably about 40 times greater than the volume flow of the cast aluminum alloy. With a ring mold 14, for example, a casting strand 20 for bars 22 with an outer diameter of 60 mm to 110 mm can be produced. At a melt temperature of approx. 650° C. and a continuous casting speed v_G between 150 mm/min and 200 mm/min, the cooling water temperature TK is preferably between 20° C. and 80° C. and the cooling water flow is preferably greater than 30 l/min.

After solidification, the bar 20 is separated into individual billets 22, which can then be cold-formed or forged. Preferably, the diameter and the cross-sectional shape—for example circular or oval—of the bar 20 are selected such that the volume of the billets obtained from the bar 20 is as little as possible greater than the volume of the cold-formed finished part. The diameter and cross-sectional shape of the billet 20 can be adjusted by selecting a suitable ring mold 14.

A billet 22 produced from the aluminum alloy described herein using the continuous casting process also described herein has properties that make the billet 22 suitable as a starting material for forging load-bearing structural components with a high strength, a low tendency to corrode and a high surface quality. Typical values achieved by this pre-material are a tensile strength R_m>375 MPa, a yield strength R_p02>345 MPa, an elongation A>10% and a hardness>100 HB.

Structural components forged from the primary material can be deformation-relevant parts or fail-safe parts for motor vehicles, such as sliding wedges, battery boxes, junctions, engine mounts, swivel mounts or wishbones.

Due to its reduced electrical conductivity, the pre-material is also suitable for components of electrical machines in which eddy currents and the associated losses are to be reduced.

LIST OF REFERENCE SYMBOLS

• • 10 Melt
  • 12 Wire
  • 14 Ring mold
  • 16 Cooling water
  • 18 Circulation
  • 20 Bars
  • 22 Billet
  • 30 Structural component
  • 32 Coarse grain seam
  • 34 Flash area