Hot-and cold-formed aluminum alloy

Abstract

A component or semi-finished piece is made from a hot-form aluminium alloy of the following composition in wt. %: silicon 0.9-1.3, magnesium 0.7-1.2, manganese 0.5-1.0, copper less than 0.1, iron less than 0.5, chromium less than 0.25, titanium less than 0.1, zinc less than 0.2, zirconium and/or hafnium 0.05-0.2 and further unavoidable impurities, whereby the total amount of chromium and manganese and zirconium and/or hafnium is at least 0.4 by weight. The aluminium/silicon mixed crystals are present in addition to magnesium silicide precipitates.

Description

This is a divisional of U.S. patent Ser. No. 10/499,755, filed Jun. 18, 2004, which claims priority to German Patent Application No. 101 63 039, filed Dec. 21, 2001 through International Patent Application Serial No. PCT/EP02/14452, filed Dec. 18, 2002.

The present invention relates to a hot- and cold-workable aluminum alloy and to a method for manufacturing an aluminum component.

BACKGROUND

High-strength Cu- (for example, Al Mg Si 1 Cu 0.5) or Zn-containing, heat-treated Al semi-finished products and Al forgings do have high static strength levels, but their elongation at break is low. Therefore, in the case of a notch effect (for example, stone impact), this results in a low dynamic strength. Moreover, these alloys are susceptible to corrosion so that expensive corrosion protection is required to avoid corrosion pits having a notch effect. Since, for example, highly stressed, forged Al suspension components are always exposed to stone impact (notching) and corrosion, Cu-/Zn-containing Al materials are used in these areas only in exceptional cases. Al Mg Si 1 alloys with higher ductility or lower notch sensitivity, such as EN-AW 6082, are, in fact, corrosion-resistant because of their low Cu- and Zn-content; however, these alloys do not reach adequate strength levels.

Another disadvantage of such alloys is that during forming and subsequent heat treatment, highly worked zones of forgings and semi-finished products recrystallize, forming coarse grains. A coarse-grained or brittle and less stable grain structure leads to premature failure of the Al component.

This is especially true when multiple forming operations are required during forging, for example, to achieve high material yield. In the case of multiple forming operations, the highest degree of deformation usually occurs only at the end of the forming process, and thus at temperatures between 390° C. and 450° C. so that the grain structure recrystallizes during subsequent heat treatment. Even more problematic is the recrystallization behavior of cold-formed Al semi-finished products that are subsequently heat-treated. For example, to produce high-strength Al screws, cold-drawn wire or rods are used, which is/are then cold-formed into a screw blank by upsetting and pressing. During subsequent heat treatment, the grain structure is therefore highly susceptible to recrystallization. The same is true for cold-forged Al wheels.

Unexamined German Laid-Open Patent Applications DE-OS 2 103 614 and DE OS 2 213 136 each describe an aluminum-silicon-magnesium alloy that reacts in a recrystallization-inhibiting manner; however, these alloys have insufficient strength, and the tendency of this alloy to recrystallize is still too high for cold-formed components or components undergoing multiple forming operations. The same is true for the known alloy according to EN-AW 6082.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a component and a method for manufacturing a component whose recrystallization-inhibiting activity is improved over the prior art and which lead to higher strength and corrosion resistance of the components.

The component or semi-finished product according to the present invention is made of an aluminum alloy having the following composition:

silicon                  0.9-1.3,
magnesium                0.7-1.2,
manganese                0.5-1.0,
copper                   less than 0.1,
iron                     less than 0.5,
chromium                 less than 0.25,
circonium and/or hafnium 0.05-0.2.

Advantageously, certain alloying constituents are present in the following proportions:

copper   less than 0.05,
iron     0.1-0.5,
chromium 0.05-0.2,
zinc     less than 0.05.

Moreover, the alloy may contain the elements

zinc     less than 0.2
titanium less than 0.1.

Here, titanium is used for grain refinement, zinc may contribute to an increase in strength. In addition, the alloy contains unavoidable impurities that are attributable to the manufacturing process.

In an advantageous embodiment, the alloy has a silicon content of between 0.9 and 1.7 percent by weight.

It is a further feature of the present invention that the alloying elements manganese, chromium and circonium and/or hafnium all together represent a proportion of at least 0.4 percent by weight. Preferably, the proportion of these elements is higher than 0.6 percent by weight. These elements act as recrystallization inhibitors.

During homogenizing annealing, these elements, together with aluminum, form intermetallic dispersoids which anchor the grain boundaries and do not dissolve, or dissolve only to a small extent, even during further heat treatments. Because the dispersoids are anchored at their grain boundaries, the grains are prevented from growing to coarse grains, thus effectively suppressing recrystallization. Circonium- and hafnium-containing dispersoids are particularly temperature-resistant, which has an inhibiting effect on the recrystallization at high temperatures.

The alloy has a silicon content of from 0.9 to 1.3%. It has turned out that a lower silicon content does not lead to the required strength levels. The silicon acts in combination with the magnesium in the form of precipitation hardening (heat treatment) which develops in the form of Mg2Si precipitates. Higher contents of manganese and chromium also lead to precipitation hardening and an increase in strength.

Moreover, for solid solution hardening, i.e., the formation of an AlSi solid solution, it is expedient that there be an excess of silicon that is not bound in Mg2Si precipitates. Therefore, the ratio of silicon to manganese is preferably between 1.1:1 and 1.3:1, more preferably between 1.16:1 to 1.24:1.

The alloy is particularly resistant to recrystallization both during hot and cold working, and intrinsically has high strength and a low susceptibility to corrosion, nearly independently of the manufacturing process. The low susceptibility to corrosion is primarily attributable to the low content of copper and zinc.

It is a feature of the method that the cast raw material of the alloy is homogenized at temperatures between 420° C. and 540° C., preferably between 460° C. and 500° C. During this homogenization, the alloying constituents magnesium and silicon are finely distributed in the aluminum matrix and, moreover, the dispersoids form whose composition is based on circonium or hafnium, manganese, chromium and/or iron.

It has turned out to be advantageous to homogenize the raw material for at least 4 hrs, particular preference being given to a homogenization of 12 hrs.

In the further process, the raw material is formed into semi-finished products at a temperature between 450° C. and 560° C. (for example, by extrusion or sheet rolling) and quenched, if necessary. The semi-finished products are preferably formed between 500° C. and 560° C., it being necessary to select, in each case, the highest temperature possible in order to avoid recrystallization nuclei. If necessary, the semi-finished products are cut apart into workpieces that are suitable for forming, and are either cold-formed once or multiple times or hot-formed into components or further semi-finished products, possibly multiple times. The semi-finished products may also be machined in a suitable manner, for example, by turning or milling. Cold- or hot-forming or machining may be carried out within the scope of expert skills and may possibly include usual heat treatments.

The hot-forming of the semi-finished product is carried out at temperatures in the range of the usual solution treatment (between 440° C. and 560° C.). During the forming process, in particular, during multiple forming steps, care must be taken that the workpiece temperature does not fall below the mentioned temperature, which would result in coarse precipitations in the grain structure of the component. Accordingly, the forming process replaces the step of solution treatment, which has a considerable effect on the process costs and process duration.

The forming temperatures according to the present invention, which at the same time imply a solution treatment, are higher than the usual forming temperatures, which results in a lower work hardening and thus in less formation of recrystallization nuclei in the grain structure. Thus, recrystallization is effectively suppressed, resulting in higher strength levels and, above all, in a significantly higher elongation at break in highly worked areas.

After the forming process, the workpiece is preferably quenched in water, thus freezing the grain structure. The desired increase in strength occurs during the subsequent artificial aging between 160° C. und 240° C.

If the composition meets the alloy specifications, the aluminum component according to the present invention has a tensile strength of at least 400 MPa and a minimum breaking strain (A5) of 10%. Components of this kind are preferably used as tension rods or other suspension components, sections, bolts, screws, or wheels.

BRIEF DESCRIPTION OF THE DRAWING

In the following, the present invention is explained in more detail with reference to two examples. The process procedure on which examples 1 and 2 are based is shown in the drawing, in which:

FIG. 1 shows a flow chart of a method of manufacturing a component or semi-finished product according to the present invention.

DETAILED DESCRIPTION

According to FIG. 1, cast raw material, provided at block AA, is homogenized at a temperature between 420° C. and 540° C. at block BB, and formed into semi-finished products, such as by pressing or rolling at a temperature between 450° C. and 560° C. at block CC. Then, the semi-finished product is heated to a solution treatment temperature of 440° C. and 560° C. and hot-formed, possibly multiple times, at this temperature at block DD. The forged semi-finished product is then quenched, for example in air or water, and artificially aged, for example, at a temperature between 160° C. and 240° C. at block EE to arrive at the component or semi-finished product represented at block FF.

EXAMPLE 1

An alloy melt having the composition in percent by weight:

silicon   1.2,
magnesium 1.0,
manganese 0.5,
copper    0.05,
iron      0.2,
chromium  0.2,
titanium  0.05,
zinc      0.1,
circonium 0.2,

is cast in ingots. The ingots are homogenized at a temperature of 480° C. for 12 hrs. In the next process step, the ingots are pressed into round rods (=semi−finished product) at a temperature of 500° C. The round rods are quenched and cut apart into workpieces having a length of about 20 cm.

The workpieces are heated to a temperature of 530° C. and formed into tension rods in several forging operations (=forming process). During forging, the workpiece temperature does not fall below 440° C. The tension rods are quenched in water and artificially aged at 200° C. for 4 hrs. The tension rods have a tensile strength of more than 400 MPa and an elongation at break (A5) of more than 13% both in the region of a central rod and in the region of a large eye which usually has a high degree of recrystallization due to the high degree of deformation.

EXAMPLE 2

Analogously to Example 1, cast ingots are homogenized and subsequently rolled into sheets (=semi−finished product) at a temperature of 500° C. Round workpieces are punched out from the sheets and formed into wheels in several steps.

Claims

1-11. (canceled)

12: A method for manufacturing a component or semi-finished product, the method comprising:

homogenizing a cast raw material at a temperature between 420° C. and 540° C.;

forming the raw material into a shaped part at a temperature between 450° C. and 560° C.;

heating the shaped part to a solution treatment temperature between 440° C. and 560° C.;

hot-forming the shaped part at the solution treatment temperature so as to form a forged part;

quenching the forged part; and

artificially aging the forged part at a temperature between 160° C. and 240° C. so as to create the component or semi-finished product.

13: The method as recited in claim 21, wherein the quenching is performed in at least one of water and air.

14: The method as recited in claim 21, wherein the hot-forming is performed multiple times.

15: The method as recited in claim 21, wherein the homogenizing is performed for at least four hours.

16: The method as recited in claim 21, wherein the homogenizing is performed for twelve hours.