EP0969111B1 - Metal Bolt having thixotropic properties and process for ist production - Google Patents

Abstract

The cross section of a metal bolt differs from a circular cross section. During heating of the bolt, for attaining a thixotropic state, the difference between its lowest core temperature and its highest surface temperature is smaller than the corresponding difference of a round bolt with the same cross-sectional area. Producing the bolt includes casting of a continuous billet with a noncircular cross section.

Description

The invention relates to a metal bolt according to the preamble of claim 1. Within the scope of the invention also a process for the production of metal bolts and their use.

Bolts with thixotropic properties are known by vertical or horizontal continuous casting of a metal alloy. For generation of the fine-grained structure required for the thixotropic behavior the molten metal in the solidification area is stirred vigorously, whereby above all have proven electromagnetic stirring devices. Through the stirring process the dendrites that form are sheared off or developed back so that these primarily solidifying solid particles are essentially globulitic Take shape. Show such bolts serving as primary material after they have warmed up to a temperature between solidus and liquidus used metal alloy lying temperature thixotropic properties. With bolts heated in this way, the metal alloy contains in the thixotropic state the reverse-engineered dendritic, primarily solidified particles in one surrounding matrix of liquid metal.

Bolts made of continuous cast material, for further processing are provided in the thixotropic state, according to the state of the Technique a circular cross section. When heating the bolts to the processing temperature required for setting the thixotropic state results from the limited thermal conductivity of the metal alloy a temperature gradient between the stud surface and the stud core, which is more pronounced the higher the heating rate is chosen.

From US-A-4 434 837 is the manufacture of a thixotropic metal strand known, which also has a cross-section deviating from the circular shape can have. A tech-nical cross section is explicitly stated. Also the EP-A-0 483 943 discloses the production of a thixotropic metal strand Technical cross section.

To maintain high quality standards for products that are processed of materials in the thixotropic state is essential Meaning that the liquid metal portion over the bolt cross section at the processing temperature within specified tolerance values lies, for example, between 30 and 50%. In particular, there is a sharp increase of the liquid metal portion in the area of the bolt surface as a result of avoid high temperatures. In practice, therefore Energy input in the bolt in the area of the solid / liquid transition between Solidus and liquidus temperature of the metal alloy so slow that at the The maximum permissible temperature is not exceeded and the temperature in the bolt core is not significantly lower, so that the liquid metal portion over the bolt cross section in the specified tolerance range lies. It should be noted that with increasing dwell time in the area between solidus and liquidus temperature a grain growth of the primarily solidified Solid particles in the surrounding matrix of liquid metal entry. This coarsening of particles has a negative effect on those on the end product desired fine-grained structure. A long dwell leads to a low production speed of those heated for further processing Bolt. The production speed can be increased the number of heating devices must be increased, but increased investment and production costs are accepted.

The invention is therefore based on the object, metal bolt of the beginning mentioned type so that in comparison to conventional round bolts faster heating up for further processing in the required temperature is possible in the thixotropic state.

A metal bolt with the features of claim 1 leads to the achievement of the object according to the invention. The one when heating up of the bolt to the thixotropic state between the lowest temperature in the core and the highest temperature on the surface of the bolt temperature difference is compared to a bolt with a circular Cross-section with the same cross-sectional area under the same heating conditions kept smaller or reached after a shorter heating-up time.

The essential essence of the invention is that if there is a deviation from the circular cross-sectional shape with the same cross-sectional area the bolt surface enlarged and at the same time the distance between the pin surface and core is downsized. Both measures mean that the Heating up a bolt designed according to the invention in comparison to a round bolt according to the prior art with the same cross-sectional area under the same heating conditions, smaller temperature differences between the bolt surface and the bolt core. On the other hand, this means that the thixotropic state required for further processing the metal alloy with bolts designed according to the invention in a shorter length Time can be reached because the larger stud surface when heating allows a higher energy input in the bolt.

Basically, each bolt has a different shape from the circular one Cross section the above-mentioned advantage over round bolts on. In addition, the bolt shape can be made by special cross-sectional shapes for example, better adapted to molded parts to be produced by thixoforming become. The shapes for the bolt cross section are dumbbell, cross, star, H or double cross shape.

To produce the metal bolts according to the invention, all are in the state of the Technique for the production of metal bolts with thixotropic properties for the Suitable for further processing in the thixotropic state, in which a strand with a thixotropic structure is made from a molten metal cast, the cast strand cut to bolts and the bolts for adjustment of the thixotropic state are heated before further processing. According to the invention, the molten metal becomes a strand with one described above, cross section deviating from the circular shape. When heating up the bolt to the thixotropic state will be located between the deepest Temperature in the core and the highest temperature on the surface of the bolt Setting temperature difference compared to a bolt with a circular Cross-section with the same cross-sectional area under the same heating conditions kept smaller or reached after a shorter heating-up time.

The bolts according to the invention and their manufacture are not limited to a specific one Limited material. However, preferred materials are aluminum, Magnesium, copper, steel and their alloys, whereby in particular for light metals and very particularly preferably for aluminum and magnesium alloys, which, for example, can also be fiber or particle reinforced can achieve excellent results.

The bolts are preferred on a horizontal continuous casting machine with a electromagnetic stirring device arranged in the area of the mold carried out. Such systems for producing a metal strand with Thixotropic properties are well known in the prior art. The The bolts are heated for further processing in the thixotropic state usually inductive. In principle, however, other heating methods can also be used or devices are used with which bolts quickly and can be heated evenly.

A particularly preferred area of application for the inventive Metal stud is the production of molded parts by thixoforming. At the Thixoforming is converted into the thixotropic state by heating Metal bolts placed in a filling chamber and by pressurization introduced or shot into a mold cavity by means of a piston, in which the thixotropic metal alloy solidifies as a molded part.

Another preferred area of application is the production of a profile through thixoextrusion. In this process one becomes a thixotropic state heated bolt pressed into a profile strand by a cooled mold.

- Fig. 1

den Flüssigmetallanteil einer Aluminiumlegierung als Funktion der Temperatur;

- Fig. 2

einen Temperatur-Zeitverlauf beim Aufheizen eines Metallbolzens aus einer Aluminiumlegierung zum thixotropen Zustand;

- Fig. 3

das Temperaturgefälle über dem Querschnitt eines Metallbolzens aus einer Aluminiumlegierung im thixotropen Zustand;

- Fig. 4 - 9

verschiedene Querschnittsformen von Metallbolzen.

Further advantages, features and details of the invention result from the following description of preferred exemplary embodiments and from the drawing; this shows schematically in

- Fig. 1

the liquid metal content of an aluminum alloy as a function of temperature;

- Fig. 2

a temperature-time curve when heating a metal bolt made of an aluminum alloy to the thixotropic state;

- Fig. 3

the temperature gradient across the cross section of a metal bolt made of an aluminum alloy in the thixotropic state;

- Fig. 4 - 9

different cross-sectional shapes of metal bolts.

Using the example of the alloy AlSi7Mg, FIG. 1 shows that the liquid metal portion of the alloy rises from about 10 to 50% in the range of the binary eutectic within a narrow temperature range of about 2 ° C. The diagram clearly shows that the temperature gradient across the cross-section of a metal bolt in the thixotropic state must be kept small in order to produce a liquid metal component that is within narrow tolerance limits. The diagrams shown in FIGS. 2 and 3 were determined on round bolts with a diameter of 60 mm from the above-mentioned alloy. The bolts were heated inductively in two stages. In a first stage the heating was carried out with 100% heating output for 95 seconds and then in a second stage for a time of 200 to 350 seconds with a heating output of 23.6%. An induction furnace with an inner diameter of 80 mm was used, the frequency was set to 1 KHz. The essential results of FIG. 3 are summarized in Table 1. Here, T _{O is} the temperature at the stud surface, T _{K is} the temperature in the stud core, fl _{O is} the liquid metal portion on the stud surface, fl _{K is} the liquid metal portion in the stud core, and t _{2 is} the heating time of the second stage. t ₂ [s] T _K [° C] fl _K [%] T _O [° C] fl _O [%] 200 569.2 26.1 571.5 52.0 250 569.5 31.7 572.3 53.0 300 569.8 40.3 574.1 53.9 350 570.1 51.2 575.8 55.1

Model calculations on metal bolts with deviations from the circular shape Cross-sections have shown that the heating-up times to achieve comparable Temperature profiles with comparable liquid metal components up to 50% and more can be reduced.

4 to 9 show examples of possible cross-sectional shapes of metal bolts. Depending on the application, there may be a specific cross-sectional shape turn out to be cheap. Essentially oval, dumbbell, cross or star-shaped and H or double cross-shaped bolt cross sections according to FIGS. 4 to 9 are suitable, for example, for the simultaneous production of several parts from a bolt or for large parts. Especially in combination with one or more slit-shaped gate cross sections Expand the range of parts that can be manufactured using thixoforms. slotted Gate cross sections increase the shear rate and subsequently lead to one lower pressing force and better mold filling capacity, which the Production of complex shaped parts allowed.

Claims (4)

1. Metal bolt with thixotropic properties for processing further in the thixotropic state, where the bolt has a cross-section deviating from the circular round form,
   characterised in that
   the bolt cross-section is shaped as a dumbbell, cross, star, H or double cross.
2. Process for production of metal bolts with thixotropic properties for further processing in the thixotropic state, where from a metal melt is cast a bead with a thixotropic structure and a cross-section deviating from the circular round form, the cast bead is cut into lengths to form bolts and the bolts are heated to establish the thixotropic state for further processing,
   characterised in that
   the bolt cross-section is shaped as a dumbbell, cross, star, H or double cross.
3. Use of a metal bolt according to claim 1 or use of a process according to claim 2 for production of a moulding by thixoforming.
4. Use of a metal bolt according to claim 1 or use of a process according to claim 2 for production of a profile by thixoextrusion.

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