Aluminum Alloy Structural Components, Starting Material and Method of Manufacture

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 inand 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.

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 CObalance 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:

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:

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 (O).

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

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). 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>375 MPa, a yield strength R>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:

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, 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.