EP3370900B1 - Light metal casting part and method of its production - Google Patents

Description

The invention relates to a method for producing a light metal cast component, in particular for a motor vehicle, and a light metal cast component which is produced from a hypoeutectic aluminum cast alloy using such a method.

The trend towards lightweight construction and occupant protection, which is particularly present in the motor vehicle industry, leads to the increased development of high-strength and ultra-high-strength components that are lighter than conventional components with at least the same strength properties. It is known that light alloy wheels for motor vehicles can be produced by casting or forging. The requirements for the molds and the alloy used are different for forging and casting.

Forged light-alloy wheels have an extraordinary strength that allows for a slimmer and lighter design than comparable steel rims. Due to the high strength, relatively thin walls and spokes can also be constructed, which leads to a low weight. They are usually manufactured by chill casting from a wrought alloy. The mold is usually flat and only roughly corresponds to the diameter of the end product. After casting, the blank is gradually pressed into a mold at approx. 500°C with up to two thousand tons of pressure. The actual wheel rim dish is now complete. Afterward the rim base is produced by rolling and machining takes place. Compared to cast wheels, forged wheels are much more heavily alloyed with strength-enhancing alloying elements such as magnesium, silicon and titanium.

During casting, the shape of the mold is designed to be close to the final shape of the component to be manufactured. According to one possibility, the casting can take place in low-pressure casting with about 1 bar from bottom to top. As an alternative to this, a pressure die-casting process can also be used, in which the liquid melt is pressed under high pressure of around 10 to 200 MPa into a preheated mold, where it then solidifies. The melt displaces the air in the mold and is kept under pressure during the solidification process. After leaving the mold, the component is machined. Compared to forged wheels, cast wheels usually have only a very small proportion of foreign metals such as titanium.

In the case of components manufactured using the casting process, the casting properties of metal alloys and the mechanical properties of the finished component depend essentially on the grain size. A grain-refining treatment of the melt can improve the static and dynamic strength values in castings and the feeding capacity of the melt in the mold as well as its flowability. The solidification of many metallic alloys begins with the formation of crystals, which grow in all directions starting from the nucleation sites until they abut the neighboring grain or the mold wall.

For high strength of the component to be produced, it is desirable to set the size of the grains as uniformly or as finely as possible. For this purpose, what is known as grain refinement is often carried out, with as many nucleating agents (foreign nuclei) as possible being offered to the solidifying melt.

From the EP 2 848 333 A1 a method for producing a metallic component by means of a casting and molding tool is known, with the steps: pouring a melt into the casting and molding tool at a first pressure, pressurizing the solidifying melt in the tool with a higher second pressure, and compressing the melt solidified component in the tool at a greater third pressure.

From the JP 2001 288547 A is a cast aluminum having a composition, by weight, of 2.0 to 6.0% silicon, 0.15 to 0.34% magnesium, up to 0.2% iron, 0.0003-0.01% Strontium, the remainder aluminum and unavoidable impurities, and optionally with 0.01 to 0.25% titanium and 0.0001 to 0.001% boron subjected to 60 minutes and quenched.

From the publication " Microstructural, mechanical and tribological properties of Al-7Si-(0-5)Zn alloys" by Yasin Alemdag and Murat Beder in "Materials and Design", Vol. 63, 2014, XP002764402 , the production of a series of Al-7Si-(0-5)Zn alloys in the chill casting process is known, the microstructure and mechanical and tribological properties of which were examined in the cast state. Thereafter, the addition of zinc to the Al-7Si alloy leads to an increase in density, strength and hardness, while the elongation at break and impact work show an inverse trend.

From the publication " Grain refinement" in Rheinfelden Aluminium: "Hut aluminum casting alloys" January 2010, XP002756661, pages 90 to 92 , the influence of grain refinement during casting is known. A larger number of nuclei in a melt produces many small aluminum grains from dendrites.

From the JP H11 293430A a method for producing a high-strength aluminum casting is known. After casting, the aluminum casting has a composition, based on weight, of 3.5 to 5.0% silicon, 0.15 to 0.4% magnesium, up to 1.0% copper, up to 0.2% Iron, treatment agent and the balance aluminum. After casting, the cast component is heated at 550°C to 575°C for a period of two to four hours, then rapidly cooled and then subjected to a further heat treatment at 160°C to 180°C for a period of one to three hours.

From the JP H05 171327 A an aluminum casting alloy for high-pressure casting is known which has a composition, in each case by weight, of 4.0 to 6.0% silicon, 0.3 to 0.6% magnesium, up to up to 0.5% iron, 0. 05 to 0.2% titanium included. The alloy can be used to cast vehicle wheels.

From the EP 0 488 670 A1 a high strength aluminum casting is known with, in each case by weight, 2.4 to 4.4% silicon, 1.5 to 2.5% copper, 0.2 to 0.5% magnesium, and the remainder aluminum, wherein the matrix of the aluminum casting includes dendrites having a grain size of 30 microns or less.

From the DE 10 2006 039 684 B4 an aluminum safety component for automobile construction is known, which is made from an aluminum-silicon die-cast alloy. The die-cast alloy has 1.0 to 5.0 percent by weight silicon, 0.05 to 1.2 percent by weight chromium and the balance aluminum and unavoidable impurities. The chromium is said to improve castability and formability. The die-casting alloy may also contain titanium in an amount of 0.01 to 0.15% by weight, which titanium acts as a grain refiner, especially when used together with boron.

From the EP 0 601 972 A1 a hypoeutectic aluminium-silicon cast alloy is known which contains a master alloy as a grain refiner. The cast alloy includes a silicon content of 5 to 13 percent by weight and may also include magnesium in an amount of 0.05 to 0.6 percent by weight. The master alloy contains 1.0 to 2.0 percent by weight titanium and 1.0 to 2.0 percent by weight boron. The aluminium-silicon cast alloy is used for the production of wheel rims for motor vehicles by low-pressure die casting. The master alloy is added in an amount of 0.05 to 0.5 percent by weight, based on the total amount of the melt.

From the DE 692 33 286 T2 For example, a method for grain refinement of aluminum and aluminum alloys is known in which a solid silicon-boron alloy is added to molten aluminum or molten aluminum alloy. The resulting melt contains about 9.6 percent by weight silicon and at least 50 ppm boron. The component produced from the melt has grain sizes in the range of 300 microns.

From the EP 1 244 820 B1 discloses a process for grain refining high strength cast aluminum alloys to achieve a cast product with a grain size of less than 125 microns. For this purpose, for example, an alloy with more than 3.8 percent by weight copper, a maximum of 0.1 percent by weight silicon and 0.25 to 0.55 percent by weight magnesium, or an alloy with more than 4.5 and less than 6.5 percent by weight zinc, maximum 0.3% by weight silicon and 0.2 to 0.8% by weight magnesium. For grain refinement, dissolved titanium with a grain size of less than 125 microns in an amount of 0.005 to 0.1 percent by weight and borides are added to the melt.

From the WO 2001 042521 A1 discloses a method for producing a grain refiner based on an aluminum-titanium-boron master alloy by introducing starting materials containing titanium and boron into an aluminum melt to form TiB2 particles and allowing this master alloy melt to solidify. A source cited there describes a theory on the course of the processes involved in grain refinement of aluminum alloys by adding an Al-Ti-B master alloy, for example A)Ti5B1. According to this, the best grain refinement results are achieved when the TiB2 particles that are insoluble in the aluminum melt are at least partially covered with a layer of Al3Ti phase on their surface. The nucleation of the alpha aluminum phase takes place on the Al3Ti layers, the effect of which increases with decreasing layer thickness.

The present invention is based on the object of proposing an improved method for producing a light-metal cast component with which a light-metal cast component with a fine-grain structure and good strength properties can be produced in a simple manner. The object is also to propose a corresponding light metal cast component produced with such a method.

• Strontium (Sr) mit 100 bis 150 ppm,
• Zinn (Sn) mit weniger als 250 ppm,
• Kupfer (Cu) mit weniger als 1,0 Gewichtsprozent, insbesondere weniger als 550 ppm, Nickel (Ni) mit weniger als 550 ppm,
• Titanborid (TiBor) mit weniger als 30 ppm,
• Zink (Zn) mit weniger als 550 ppm,
• Chrom (Cr) mit weniger als 500 ppm,
• Eisen (Fe) mit weniger als 0,7 Gewichtsprozent und
• Mangan (Mn) mit weniger als 0,15 Gewichtsprozent.

To solve a method for producing a light metal casting according to claim 1 is proposed with the steps: providing a melt from a Aluminum cast alloy containing silicon with 3.5 to 5.0 percent by weight, magnesium with 0.2 to 0.7 percent by weight, titanium with 0.07 to 0.12 percent by weight, boron with a maximum of 0.012 percent by weight, optionally other alloying elements together with less than 1.5 percent by weight, the remainder aluminum and unavoidable impurities, the melt being produced from a base melt containing aluminum, a first grain refiner made from an aluminium-silicon alloy which contains a maximum silicon content of 12.5 percent by weight and aluminium , and a second grain refiner made of an aluminum-titanium alloy, which contains at least titanium, boron and aluminum as alloying elements, wherein the melt, based on the total weight, contains a total of 0.1 to 5.0 percent by weight of the grain refiner from the aluminum contains silicon alloy and the aluminum-titanium alloy grain refiner; Pouring the melt into a casting and molding tool in the low-pressure process at a low first pressure (P1), in particular by means of gravity casting or low-pressure casting, after the casting and molding tool has been completely filled, pressurizing the solidifying melt in the casting and molding tool at a second pressure (P2), which is greater than the first pressure (P1), and when the melt has at least largely solidified to form the component, compacting the component, which has at least largely solidified from the melt, in the casting and molding tool at a third pressure ( P3) that is greater than the second pressure (P2), wherein the other alloying elements are at least one of the following:

• Strontium (Sr) with 100 to 150 ppm,
• tin (Sn) with less than 250 ppm,
• Copper (Cu) with less than 1.0 percent by weight, in particular less than 550 ppm, nickel (Ni) with less than 550 ppm,
• Titanium boride (TiBor) with less than 30 ppm,
• zinc (Zn) with less than 550 ppm,
• chromium (Cr) with less than 500 ppm,
• Iron (Fe) with less than 0.7% by weight and
• Manganese (Mn) with less than 0.15% by weight.

According to the independent claim 8, a corresponding light metal cast component is also proposed, which is produced using the method according to the invention is, wherein the light metal casting component is 3.5 to 5.0 percent by weight silicon and 0.2 to 0.7 percent by weight magnesium 0.07 to 0.12 percent by weight titanium, maximum 0.012 percent by weight boron, optionally further alloying elements as defined in claim 1 together with less than 1.5 percent by weight, the remainder aluminum and unavoidable impurities, and the light metal cast component has a structure with an average grain size of at most 500 micrometers.

One advantage of the light metal cast component is that it can be produced by low-pressure casting due to the relatively low silicon content and has good mechanical properties, particularly with regard to strength, ductility, elongation at break and porosity, due to the fine-grain structure.

The tensile strength (Rm) of the light metal cast component is preferably at least 270 N/mm ² , in particular at least 300 N/mm ² , or at least 320 N/mm ² .

The relatively low silicon content of less than 5 percent by weight results in a hypoeutectic aluminium-silicon alloy. The light metal cast component produced from this has high ductility and elongation at break. The elongation at break (A5) of the light metal cast component is at least 5%, in particular at least 8%. The elongation at break can be below the elongation at break that is usual for a forged part, in particular below 12%.

The light metal cast component preferably has a yield point (Rp0.2) of at least 220 N/mm ² , in particular at least 250 N/mm ² , or at least 280 N/mm ² .

The light metal cast component preferably has a maximum porosity of less than 0.5%, in particular less than 0.1%. The low porosity also contributes to good strength properties and toughness. The light metal cast component can have a surface roughness of less than 50 micrometers, in particular less than 20 micrometers.

The low surface roughness of less than 50 microns contributes to the particularly good mechanical characteristics of the surface quality of the component. According to a preferred embodiment, the light metal cast component has a yield point (Rp0.2) of at least 280 N/mm ² in a raw cast surface area, an elongation at break (A5) of at least 8% and a tensile strength (Rm) of at least 320 N/mm ² . The raw cast surface area means an area of the raw cast component unmachined after casting with a depth of up to 1.0 mm from the component surface.

After solidification, the light metal cast component can be subjected to heat treatment, in particular solution annealing and subsequent aging. The heat treatment contributes to the improvement of the known material properties, in particular to the increase in strength. The material characteristics mentioned above relate in particular to a condition after heat treatment has taken place.

The main alloying elements of the cast alloy used to manufacture the light metal cast component are aluminum and silicon. In this respect, the cast alloy can also be referred to as an aluminium-silicon cast alloy.

In addition to aluminium, silicon and manganese, the cast alloy can also contain other alloying elements or unavoidable impurities. The proportion of other alloying elements and unavoidable impurities is less than 1.5 percent by weight based on the total weight of the light metal cast component, preferably less than 1.0 percent by weight. Accordingly, the aluminum-silicon cast alloy has at least 93 percent by weight, preferably at least 95 percent by weight, aluminum.

In principle, it is desirable for the light metal cast component to be produced to have good mechanical properties, in particular high strength. On the other hand, strength-enhancing alloying elements can lead to an increased tendency to corrosion, which in turn is undesirable.

Provision is therefore made in particular for the proportion of strength-increasing alloying elements to be as low as possible so that the light-metal cast component has high corrosion resistance. The corrosion resistance should be so high that the relevant corrosion tests for the respective light metal cast component are met. Standardized corrosion tests are described in EN ISO 9227 or ASTM B117, for example. Depending on the component, corrosion tests relating to the external stresses of motor vehicles, such as the CASS test (copper accelerated salt spray test) or the Filiform test for vehicle wheels, should also be passed. The CASS test is carried out in particular on coated or painted components. The components to be tested are permanently exposed to various, highly corrosive salt sprays in a chest-like facility. Filiform corrosion can be tested, for example, in accordance with DIN EN 3665 or a comparable standard.

The sub-critical amount of strength-enhancing alloying elements depends on the respective alloy composition and the corrosion test used, and can therefore not be specified in an absolute or concrete manner. For this reason, it is only stated here as an example that the proportion of strength-enhancing alloying elements such as copper (Cu), zinc (Zn) and titanium (Ti) can be less than one percent by weight based on the total weight of the component.

In one configuration, the cast aluminum alloy can have copper (Cu) with a maximum content of 1.0 percent by weight, in particular a maximum of 0.5 percent by weight, in particular up to 550 ppm (parts per million). It can also be provided that the cast alloy or the component produced from it contains less than 250 ppm or no copper at all.

In one configuration, the cast aluminum alloy can have zinc (Zn) with a maximum content of 550 ppm (parts per million). It can also be provided that the cast alloy or the component produced from it contains less than 250 ppm or no zinc at all.

• ist in der Gusslegierung beziehungsweise im
• hieraus hergestellten Bauteil enthalten.

A proportion of 0.07 to 0.12% by weight titanium

• is in the cast alloy or in the
• component made from it included.

The cast aluminum alloy has boron (B) with a content of at most 0.012 percent by weight, in particular at most 0.06 percent by weight.

According to one embodiment, the titanium and the boron can also be provided in the form of titanium boride in the cast aluminum alloy or in the component produced from it. In particular, the aluminum casting alloy can have titanium boride (TiBor) with a proportion of less than 30 ppm.

According to one configuration, the cast aluminum alloy can have strontium (Sr) with a proportion of 100 ppm to 150 ppm.

According to one configuration, the cast aluminum alloy can contain tin (Sn) with a proportion of less than 250 ppm.

According to one configuration, the cast aluminum alloy can contain nickel (Ni) with a proportion of less than 550 ppm.

According to one embodiment, the cast aluminum alloy can contain manganese (Mn) in a proportion of less than 0.5 percent by weight.

According to one configuration, the cast aluminum alloy can contain chromium (Cr) in a proportion of less than 500 ppm, preferably less than 200 ppm. In particular, this also includes the possibility that no chromium is contained in the cast aluminum alloy or in the component produced from it. This also applies to the other alloying elements mentioned above.

According to one configuration, the cast aluminum alloy can contain iron (Fe) with a proportion of less than 0.7 percent by weight.

According to one embodiment, the cast aluminum alloy can contain manganese (Mn) in a proportion of less than 0.15 percent by weight.

It goes without saying that all of the alloying elements mentioned can be provided either individually or in combination with one or more other elements. The remainder of the cast aluminum alloy consists of aluminum, silicon, magnesium, and in particular titanium and boron, and unavoidable impurities. The proportion by weight of the other alloying elements, ie the alloying elements present in addition to aluminum, silicon, magnesium, titanium and boron, is less than 1.5, in particular less than 1.0 percent by weight.

A further advantage of the light metal cast components according to the invention is that they have greater design freedom than conventional light metal cast components and forged light metal components. In this way, smaller cross-sections of the components can be realized, or complex post-forming processing can be omitted. According to one embodiment, the light metal cast component in the finished state can have sections that are mechanically unprocessed, in particular mechanically unhardened, after casting. The mechanically unmachined sections can have a wall thickness of less than 3.0 millimeters, at least in some areas.

According to one possible embodiment, the cast light metal component can be a safety or structural component, in particular a vehicle wheel or a vehicle rim for a motor vehicle or the like. It goes without saying that the light metal cast component can also be designed in a different form or for applications other than motor vehicles, for example for the construction industry. The safety or structural component preferably has a weight of at least 500 grams, in particular at least 3000 grams.

Another advantage of the casting process described is that it can be used to produce components with particularly high strength and a particularly fine structure in a short time. In particular, the method can be used to produce light metal cast components with an average grain size of less than 500 micrometers, in particular from 200 to 500 micrometers. In this respect, the advantages of the method and the advantages of the component produced according to the method intertwine here. In this context, it is understood that all the characteristics and advantages mentioned in relation to the product also apply to the process, and vice versa.

Another advantage of the process is that the components produced have a near-net-shape shape as a result of the compaction, which leads to excellent material utilization. Furthermore, the products manufactured using the method mentioned have a high level of dimensional accuracy and surface quality. The tool costs are low because different process steps are carried out with one tool. The method is particularly suitable for the production of wheel rims for motor vehicles, although the production of other components is of course not excluded.

According to a preferred procedure, the melt is poured at a temperature significantly above the liquidus temperature, in particular at a pouring temperature which is at least 10% above the liquidus temperature. For example, the melt consisting of cast aluminum alloy can be cast at a temperature of 620°C to 800°C, in particular at a temperature of 650°C to 780°C. The casting tool, which is also referred to as a casting mold or mold, can have a comparatively low temperature of, for example, below 300°C.

The pressure required to pour the melt into the mold depends on the casting process, for example gravity casting or low pressure casting. For example, when using gravity pouring, the first pressure may be ambient pressure, ie about 0.1 MPa (1 bar). On the other hand, the first pressure when using low-pressure casting is correspondingly like this high that the melt can rise through the riser pipe into the mold cavity of the casting tool. For example, the pressure in low-pressure casting can be between 0.3 MPa and 0.8 MPa (corresponding to 3 to 8 bar). The first pressure is at most as high as required for low-pressure casting and should preferably be less than 1 MPa.

The pressurization provided after the casting tool has been filled is carried out at a higher second pressure, which can be, for example, greater than 5 MPa (50 bar), in particular more than 9 MPa (90 bar). The application of pressure with the second pressure begins after the casting mold is completely filled with melt, in particular while the melt is initially solidifying into the component or when the melt is beginning to transition into the semi-solid state. In the low-pressure process, the completely filled state of the casting mold can be sensed, for example, by a pressure surge on the filling piston.

The pressurizing of the solidifying melt can take place, for example, at a component edge shell temperature below the liquidus line and/or above the solidus line of the light metal alloy. However, it is also conceivable that the process starts even before the liquidus line is reached, for example at 3% above the liquidus line. In this context, the component edge shell temperature is understood to mean a temperature which the component has in an edge layer region, or in an edge shell which is solidifying or has solidified from the melt. The solidification takes place from the outside to the inside, so that the temperature of the solidifying component is higher on the inside than on the edge Melt can be exercised.

For compression, an even higher third pressure is built up and exerted on the workpiece, which can preferably be more than 15 MPa (150 bar). The compression preferably takes place at a component edge shell temperature that is lower than the second temperature of the already partly or mostly solidified light alloy. A lower limit of the third temperature for performing compaction is preferably half the solidus temperature of the metal alloy. Parts of the component can also be outside of the temperature range. During compaction, the temperature of the component or the lower part and/or upper part of the tool can be monitored by means of appropriate temperature sensors. The end of the forming process can be defined by reaching an end position of the relative movement of the upper part to the lower part or by reaching a specific temperature.

The melt is produced from a base melt containing at least aluminum and grain refiners. The grain refiners act as nucleating agents when the light metal melt crystallizes. These nucleating agents have a higher melting point than the light metal melt to be poured and therefore solidify first on cooling. The crystals formed from the melt easily accumulate on the grain refiners. As many crystals as possible are formed, which then prevent each other from growing, resulting in a fine, even structure overall. The grain refiners include a first grain refiner made of an aluminum-silicon alloy containing a maximum silicon content of 12.5 percent by weight and aluminum, and a second grain refiner made of an aluminum-titanium alloy containing at least titanium, boron and aluminum as alloying elements. In particular, it is provided that the two grain refiners are composed of different alloys. The use of the two grain refiners leads to a significant improvement in castability and the strength of the component made from them.

The melt contains, based on the total weight of the ready-to-cast melt or the component produced from it, a quantity of 0.1 to 5.0 percent by weight of the first grain refiner made of the aluminum-silicon alloy and the second grain refiner made of the aluminum-titanium alloy.

The melt of the aluminum casting alloy, or the light metal casting component produced from it, contains silicon at 3.5 to 5.0 percent by weight, magnesium at 0.2 to 0.7 percent by weight, titanium at 0.07 to 0.12 percent by weight, boron with a maximum of 0.012 percent by weight, optionally further alloying elements with a total of less than 1.5 percent by weight, the rest aluminum and unavoidable impurities.

If alloying elements such as silicon, titanium, boron or others are mentioned, this should be understood within the scope of the present disclosure in such a way that not only the pure alloying elements can be used, but also compounds that contain the alloying elements mentioned in each case. The stated maximum silicon content of 12.5 percent by weight refers to the total weight of the first grain refiner.

In one embodiment, the first grain refiner can contain silicon at 3.0 to 7.0 percent by weight, magnesium at 0.2 to 0.7 percent by weight, titanium at 0.07 to 0.12 percent by weight, boron at a maximum of 0.012 percent by weight, optionally with other alloying elements together less than 1.5 percent by weight, the remainder aluminum and unavoidable impurities. The values mentioned relate to the total weight of the first grain refiner. The first grain refiner can have the same or a different alloy composition than the base melt. According to one possible embodiment, the first grain refiner is treated with ultrasound in the molten state, so that a globulitically shaped mixed crystal is formed during solidification. This means that the portion of the silicon dissolved in the aluminum forms a globulitically formed mixed crystal. The grain refiner is heated in particular up to the transition temperature between solid and liquid (semi-solid) or above. Another effect of the ultrasonic treatment is that the boron or borides contained in the grain-refining melt serve as nuclei on which Al3Ti accumulates. On cooling, the Al3Ti particles formed in this way solidify in the globular structure. The first grain-refining melt is preferably solidified as quickly as possible, ie within up to 10 seconds, for example. Nucleation takes place later on the Al3Ti particles when they are stirred into the base melt.

The second grain refiner based on an aluminum-titanium alloy can in particular be a commercially available grain refiner, such as Al5Ti1B.

The first and second grain refiners can be introduced into the base melt individually or as a composite grain refiner system, with the nucleating first grain refiner and the nucleating second grain refiner being completely melted in the melt. The resulting melt, which consists of the base melt with the grain refiners dissolved therein, is then poured into the casting or molding tool.

According to one possible procedure, the first and second grain refiners are added to the base melt immediately before the casting of the respective cast component. In concrete terms, provision can be made in particular for the melt to be poured into the casting tool within a maximum of five minutes in particular after the first grain refiner and/or the second grain refiner has been stirred into the base melt. In this way, the Al3Ti particles of the grain refiner that is stirred in are present at least essentially in a solid form, so that the grain refinement effects are increased.

Figur 1

ein erfindungsgemäßes Verfahren zur Herstellung eines Leichtmetallgussbauteils mittels eines Gieß- und Formwerkzeugs mit den Verfahrensschritten S10 bis S50;

Figur 2

ein Zustandsdiagramm (Phasendiagramm) für eine Metalllegierung zur Herstellung eines Bauteils gemäß dem Verfahren nach Figur 1.

A preferred procedure is explained below with reference to the drawing figures. It shows:

figure 1

a method according to the invention for producing a light metal cast component by means of a casting and molding tool with the method steps S10 to S50;

figure 2

a state diagram (phase diagram) for a metal alloy for the production of a component according to the method figure 1 .

The Figures 1 and 2 are described together below. figure 1 shows a method for producing a light metal cast component by means of a casting and molding tool in several method steps S10 to S50.

A light metal casting alloy is used as the material, which contains at least the following alloy components: 3.5 to 5.0 percent by weight silicon, 0.2 to 0.7 percent by weight magnesium, 0.07 to 0.12 percent by weight titanium, a measurable proportion of boron of up to 0.012 percent by weight, at least 93.0 percent by weight aluminum and unavoidable impurities. The alloy can also contain small amounts of traces of other elements such as copper, manganese, nickel, zinc, tin and/or strontium.

Specifically, an exemplary alloy may include 4.0 wt.% silicon, 0.4 wt.% magnesium, 0.08 wt.% titanium, 0.012 wt.% boron, about 400 ppm copper (Cu), about 400 ppm zinc (Zn), about 100 ppm strontium (Sr) , about 200 ppm tin (Sn), about 400 ppm nickel (Ni), about 400 ppm manganese (Mn), also unavoidable impurities and the remainder aluminum (Al).

In the first method step S10, the melt for producing the light metal cast component is produced. For this purpose, a base melt is produced from a base alloy. At least one grain refiner can be introduced into the base alloy, which acts as a nucleating agent during crystallization. Specifically, in one example, a first grain refiner made from an aluminum-silicon alloy can be used, which contains a silicon content of at most 12.5 percent by weight, based on the total weight of the first grain refiner alloy. In addition, a second grain refiner made of an aluminum-titanium alloy can be used, which contains aluminum as the main component and at least titanium and boron as additional alloying elements. The grain refiners are introduced into the melt of the base alloy, with the grain refiners being melted. With regard to the proportions, it is provided in particular that a total of 0.1 to 5.0 percent by weight of the first and second grain refiner, based on the total weight of the component to be produced, is introduced.

In the second method step S20, the melt of the light metal casting alloy is poured into a casting and molding tool at a low first pressure (P1). The casting can be done by gravity casting or low pressure casting, the first pressure (P1) preferably being below 1.0 MPa. The melt is cast at a temperature (T1) above the liquidus temperature, in particular at a temperature of 650°C to 780°C. The casting tool, which is also referred to as a casting mold or mold, can have a comparatively low temperature of, for example, below 300°C.

In the subsequent method step S30, the light metal alloy located in the mold cavity is subjected to pressure. For this purpose, a pressure P2 that is greater than 5 MPa (50 bar) is built up between a lower part and an upper part of the casting tool. This pressure can be generated, for example, by the weight of the upper part. Before applying pressure, all openings of the casting and molding tool must be closed so that no material is pressed out of the mold. The melt can be pressurized in a component edge shell temperature range T2 from around the liquidus line TL to above the solidus line TS of the metal alloy, ie TS<T2<TL. Before the pressure is applied, the material is still liquid. At the end of the pressurization, the material is at least partially solidified, that is, it is in a semi-solid state.

After the application of pressure (S30), the workpiece that has at least largely solidified from the melt is compressed in the subsequent method step S40. Compression is performed by relatively moving the bottom to the top at a third pressure P3 that is greater than the second pressure P2 in step S30. The compaction can be done by pressing the lower part in the direction of the upper part with high forces. Compacting preferably only begins when the metal alloy has at least largely solidified or is in the semi-solid state. The compression can take place at a component edge shell temperature T3, which is lower than the temperature T2 of the metal alloy in the process step pressurizing S30. Half of the solidus temperature TS of the metal alloy is specified as the lower limit of the temperature T3, ie T2>T3>0.5TS. The end of the forming process is defined by reaching an end position of the relative movement of the upper part to the lower part and reaching a specific temperature. During compression, the component only experiences a comparatively low degree of deformation of less than 15%, in particular less than 10% or 5%. When compacting, pores in the component are closed so that the microstructure is improved.

After the component has solidified completely, it is removed from the casting tool. The workpiece, which is also referred to as a raw cast component in this state, is then mechanically reworked in method step S50. The mechanical post-processing can be, for example, machining, such as turning or milling, or reshaping, such as ironing.

The light metal cast component can be subjected to heat treatment after solidification, before or after mechanical finishing. For example, the light metal cast component can be solution annealed and then tempered. In particular, the strength properties of the component can be increased by the heat treatment.

Other customary process steps such as quality control, for example by means of X-rays, and painting can follow.

With the method according to the invention, cast blanks can be produced in several stages in the same lower mold, by casting (S20), subsequent application of pressure (S30) and subsequent compression/forming (S40). Pressurization takes place above the solidus temperature (liquid to semi-solid state) of the particular alloy used.

figure 2 shows a state diagram (phase diagram) for a light metal alloy for the production of a component according to the method according to the invention. The percentage ratio of a metal alloy (W _L ) which contains X _A % of a metal A and Xa% of a metal B is indicated on the X-axis. In the present case, the metal A is aluminum and the metal B is silicon. The light metal alloy formed is hypoeutectic due to the proportions of aluminum and silicon mentioned, ie the proportion of silicon (metal B) is in relation to aluminum (metal A) in the light metal alloy (W _L ) so small that a microstructure develops to the left of the eutectic (W _Eu ).

Temperature (T) is indicated on the Y-axis. Casting takes place at a temperature T1 significantly above the liquidus temperature TL or the liquidus line LL. The temperature range T1 is shown in broken lines. The temperature range T2 for pressurization, which is preferably below the liquidus temperature (TL) and above the solidus temperature TS (TL > T2 > TS), is in figure 2 shown with hatching from bottom left to top right. Depending on the process time during pressurization (S20), a residual degree of deformation of less than 15% remains for subsequent compression. The compression (S30) takes place in particular in a temperature range T3 between the temperature T2 and half the solidus temperature 0.5 TS (T2>T3>0.5 TS). This area is in figure 2 shaded from top left to bottom right. Mechanical post-processing (S40) optionally takes place at a temperature T4 below the solidus temperature (T4<TS).

The light metal cast component produced using the method mentioned has a particularly fine-grained structure with low porosity and good mechanical properties, in particular with regard to strength, ductility and elongation at break. The light metal cast component has a maximum porosity of less than 0.5%, in particular less than 0.1%, and a surface roughness (Ra) of less than 50 microns, in particular less than 20 microns. The tensile strength (Rm) of the light metal cast component after heat treatment is at least 270 N/mm ² , in particular at least 320 N/mm ² . The elongation at break (A5) is at least 5%, in particular at least 8%. The yield point (Rp0.2) is at least 220 N/mm ² , in particular at least 280 N/mm ² .

The cast light metal component can be designed in the form of a safety or structural component for a motor vehicle, in particular as a vehicle wheel or a vehicle rim. The method is particularly suitable for producing safety or structural components with a weight of at least 500 grams, in particular at least 3000 grams, without being restricted to this.

One advantage of the method described is that a component produced with it has a particularly fine-grained structure with few voids. Overall, this leads to increased strength of the component. Experiments have shown that the tensile strength (Rm) of a component produced according to the invention could be increased by more than 20% compared to components produced in a conventional manner. The yield point (Rp0.2) could even be increased by more than 40%. Overall, a component with significantly higher strength can be produced using the same material, or a lighter component can be produced using less material.

Claims (14)

1. Method for producing a light metal cast component with the steps:

   - providing a melt from an aluminium casting alloy that contains silicon with 3.5 to 5.0 weight percent, magnesium with 0.2 to 0.7 weight percent, titanium with 0.07 to 0.12 weight percent, boron with at most 0.012 weight percent, optionally further alloy elements with together less than 1.5 weight percent, the rest aluminium as well as unavoidable impurities, wherein the melt is produced from a base melt that contains aluminium, a first grain refiner of an aluminium-silicon-alloy that contains a proportion of silicon of maximal 12.5 weight percent and aluminium, and a second grain refiner of an aluminium-titanium-alloy that contains as alloy elements at least titanium, boron and aluminium, wherein the melt, in relation to the total weight, contains in total an amount of 0.1 to 5.0 weight percent of the grain refiner of the aluminium-silicon-alloy and of the grain refiner of the aluminium-titanium-alloy;

   - casting the melt into a casting- and forming tool by a low-pressure method at a low first pressure (P1), in particular by means of gravity casting or low-pressure casting,

   - after completely filling the casting- and forming tool, applying a pressure to the solidifying melt in the casting- and forming tool with a second pressure (P2) that is larger than the first pressure (P1), and

   - when the melt is at least mostly solidified to the component, compacting the component, that is at least mostly solidified from the melt, in the casting- and forming tool at a third pressure (P3) that is larger than the second pressure (P2),

   wherein the further alloy elements are at least one of:

   strontium (Sr) with 100 to 150 ppm,

   tin (Sn) with less than 250 ppm,

   copper (Cu) with less than 1.0 weight percent, in particular less than 550 ppm,

   nickel (Ni) with less than 550 ppm,

   titanium boride (TiBor) with less than 30 ppm,

   zinc (Zn) with less than 550 ppm,

   chromium (Cr) with less than 500 ppm,

   iron (Fe) with less than 0.7 weight percent and

   manganese (Mn) with less than 0.15 weight percent.
2. Method according to claim 1,
   characterised in
   that the first grain refiner is produced by producing a grain refinement melt from the aluminium-silicon-alloy and treating the grain refinement melt with ultrasonic sound, such, that after the solidification a globules-like formed-in alpha-mixed crystal is present.
3. Method according to one of claims 1 or 2,
   characterised in
   that the first grain refiner and the second grain refiner are introduced into the base melt by stirring, in particular with at least a partial overlap in time.
4. Method according to anyone of claims 1 to 3,
   characterised in
   that the casting of the melt takes place latest five minutes after introduction of the first grain refiner and/or the second grain refiner.
5. Method according to anyone of claims 1 to 4,
   characterised in
   that the casting takes place at a first temperature (T1) of 620°C to 800°C, in particular at a first temperature of 650° up to 780°C.
6. Method according to anyone of claims 1 to 5,

   characterised in

   that the pressure application with the second pressure (P2) is carried out at a second temperature (T2) that is lower than the first temperature and is below the liquidus line,

   wherein the compacting with the third pressure (P3) is carried out at a third temperature (T3) that is lower than the second temperature (T2) and that is at least half of the solidus temperature of the aluminium casting alloy.
7. Method according to anyone of claims 1 to 6,
   characterised in
   that the light metal cast component is subjected to a heat treatment after the solidification, in particular a solution heat treatment and subsequent aging.
8. Light metal cast component, in particular for a motor vehicle, produced with the method according to anyone of the claims 1 to 7,

   wherein the light metal cast component contains 3.5 to 5.0 weight percent silicon and 0.2 to 0.7 weight percent magnesium, 0.07 to 0.12 weight percent titanium, maximal 0.012 weight percent boron, optionally further alloy elements as defined in claim 1 with together less than 1.5 weight percent, the rest aluminium as well as unavoidable impurities, and

   wherein the light metal cast component has microstructure with an average grain size of 500 micrometres at most.
9. Light metal cast component according to claim 8,
   characterised in
   that the light metal cast component has a maximal porosity of less than 0.5 %, in particular less than 0.1%.
10. Light metal cast component according to claim 8 or 9,
    characterised in
    that the light metal cast component has an elongation at fracture (A₅) of at least 5%, a yield strength (Rp_0.2) of at least 220 N/mm² and/or a tensile strength (Rm) of at least 270 N/mm².
11. Light metal cast component according to anyone of claims 8 to 10,
    characterised in
    that the light metal cast component has a surface roughness [Ra] of less than 50 micrometres, in particular less than 20 micrometres.
12. Light metal cast component according to anyone of claims 8 to 11,
    characterised in
    that the light metal cast component has a yield strength (Rp_0.2) of at least 280 N/mm², an elongation at fracture (A₅) of at least 8% and a tensile strength (Rm) of at least 320 N/mm² in the area of a cast blank surface.
13. Light metal cast component according to anyone of claims 8 to 12,
    characterised in
    that the light metal cast component has partial portions in the finished state that are mechanically unmachined after the casting, in particular mechanically non-compacted, wherein the mechanically unmachined partial portions have a wall thickness of less than 3.0 millimetres.
14. Light metal cast component according to anyone of claims 8 to 13,
    characterised in
    that the light metal cast component is a safety- or structural component, in particular a vehicle wheel of a motor vehicle, wherein the safety- or structural component has a weight of at least 500 grams, in particular at least 3000 grams.

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