WO2007134345A1

WO2007134345A1 - Magnesium-based alloy

Info

Publication number: WO2007134345A1
Application number: PCT/AT2007/000181
Authority: WO
Inventors: Peter J. Uggowitzer; Jörg F. LÖFFLER; Franz Riemelmoser; Maria KÜHLEIN; Michael Kettner; Helmut Kilian
Original assignee: Arc Leichtmetallkompetenzzentrum Ranshofen Gmbh
Priority date: 2006-05-19
Filing date: 2007-04-19
Publication date: 2007-11-29
Also published as: US20090291015A1; EP2021521B1; EP2021521A1; ES2615127T3; AT503854B1; SI2021521T1; AT503854A4

Abstract

The invention relates to a magnesium-based alloy and to a semifinished part having a uniformly small grain size and a good cold forming capability which is produced therefrom. To produce a fine-grained slug whose material is highly formable (deep-drawable) at elevated temperature and at room temperature and has desired corrosion properties from a cast block, the invention provides a magnesium-based alloy (L1, L2) containing, in percent by weight, zinc (Zn) more than 0.8, but less than 6.2, zirconium (Zr) traces, but less than 1.0, manganese (Mn) more than 0.04, but less than 0.6, calcium (Ca) more than 0.04, but less than 2.0, silicon (Si) traces, but less than 1.0, antimony (Sb) traces, but less than 0.5, silver (Ag) more than 0.1, but less than 2.0, with magnesium and impurities due to production as balance.

Description

Magnesium-based alloy

The invention relates to a magnesium-based alloy and a semifinished product produced therefrom.

In more detail, the invention relates to a magnesium-based alloy with uniformly small grain size and high in particular Kaltumformvermögen the material.

Magnesium is an alkaline earth metal, crystallized in hexagonal closest packing of the atoms, has a density of 1, 7 kg / dm ³ , a modulus of elasticity of 44 kN / mm ² and a tensile strength of 150 to 200 N / mm ² . A hexagonal close-packed lattice has only a limited set of slip planes, so magnesium can only be deformed to a small extent at room temperature.

Alkaline earth metals are generally very reactive. Magnesium is coated in air or water with a thin, adherent, oxide / hydroxide topcoat and is at least partially resistant to especially water. However, despite the protective surface layer, the high reactivity of magnesium may cause corrosion.

To increase strength, reduce notch sensitivity and improve corrosion resistance, magnesium may be predominantly alloyed with the elements aluminum (Al), zinc (Zn), manganese (Mn), these alloys generally being multiphase in the form of mixed crystals and intermetallic phases at room temperature available.

Solution heat treatment followed by quenching can influence the toughness or ductility and, by slow cooling or precipitation hardening, the strength of the material consisting of these alloys.

The most important currently used magnesium alloys have a name and chemical composition listed in Tab. 1. However, known magnesium alloys have the disadvantages of an inhomogeneous structural adjustment in the bolt during extrusion at elevated temperature and a limited ductility of the material at room temperature.

One of the major advantages of magnesium is the low density of the metal, so that experts have long been confronted with the desire for magnesium-based wrought alloys.

From a publication "The Effect of Ca Addition on Age Hardening Behavior and Mechanical Properties in Mg-Zn Alloy" (Materials Science Forum VoIs., 419-422 (2003) pp. 307-312), for example, an alloy of magnesium and magnesium has become known 6 wt .-% zinc 0.1 to 0.5 wt .-% calcium to increase the mechanical properties and to improve the curing parameters.

With the same objective of increasing the firmness and improving the creep resistance, according to the document "Microstructure and mechanical properties of Mg-Zn-Si-based alloys" (Materials Science and Engineering A357 (2003) 314-320), alloying of 1% by weight was carried out. Si and optionally 0.25 wt .-% Ca a magnesium base material with 6 wt .-% Zn.

In order to provide high strength and deformed magnesium base alloys, an attempt has also been made, as disclosed in the document "Microstructure and Mechanical Properties of Mg-Zn-Ag Alloys" (Materials Science Forum VoI., 419-422 (2003) pp.159-164) to add silver (Ag) to a Z6 alloy, wherein with a content of 3% by weight Ag, a remarkable grain refinement and a hardness increase could be achieved.

The invention is now the goal of creating a magnesium-based alloy, which provides a fine-grain billet in a hot pressing an optionally conditioned continuous casting bolt, the material of which is highly deformable at elevated temperature and at room temperature. It is a further object of the invention to improve or to influence the corrosion resistance of the material.

This goal is greater than 0.8 but less than 6.2 for a magnesium-based alloy containing in wt% zinc (Zn)

Zirconium (Zr) traces, but less than 1, 0 Manganese (Mn) more than 0.04, but less than 0.6

Calcium (Ca) more than 0.04, but less than 2.0

Silicon (Si) traces, but less than 1, 0

Antimony (Sb) traces but less than 0.5 aluminum (Al) traces, but less than 0.5

Silver (Ag) reaches more than 0.1, but less than 2.0, magnesium and manufacturing impurities as the remainder.

The advantages achieved with the magnesium base alloy according to the invention are essentially in a strictly balanced element concentration and a microalloying technology, in which the interaction of all alloying elements and the reaction kinetics and Komwachstumskriterien are taken into account, the advantages in particular a homogeneous fine grain structure of the material, high cold workability and an improvement in the corrosion resistance thereof.

Zinc at levels greater than 0.8 to less than 6.2 weight percent in the alloy significantly affects the solidification interval and prevents formation of very coarse columnar crystals upon solidification. Lower concentrations than 0.8% by weight of Zn lead to a disproportionately decreasing effect, whereas contents of more than 6.2% by weight result in a disadvantageously acting eutectic solidification of the melt.

Zirconium is fine-grained by precipitations from the melt and enrichment at the crystallization front. Contents of more than 1, 0 wt .-% Zr coarsen the precipitates in for the crack initiation of the material under adverse conditions.

Manganese at levels greater than 0.04 but less than 0.6 weight percent has a multiple effect in the alloy. On the one hand, Mn binds in the melt Fe, which compound precipitates, on the other hand Mn forms with zirconium even at a higher temperature in the melt phases, which can have a fine grain.

Calcium at levels greater than 0.04 but less than 2.0 weight percent in the metal will cause phase formation in the solid alloy, which phases

Grain boundary stabilizer effectively hinder crystal growth. This Ca ₂ Mg6Zn ₃ - Phase, which presupposes Zn in the above-mentioned contents in the alloy, is produced in the range of 0.1 to 1% by volume particularly finely and homogeneously distributed in the material, whereby an excellent fine grain structure is retained in the material.

Corn border stabilizing precipitates are in conventional

Magnesium alloys are usually more noble than the magnesium matrix, so that the corrosion resistance is impaired by galvanic effects. In the alloy according to the invention, the non-noble Ca ₂ MgeZn ₃ phase precipitates, so that a galvanic corrosion mechanism is significantly reduced. The result is improved corrosion resistance.

The alloying element silicon is soluble in magnesium only to a very small extent or in traces and forms the phase Mg ₂ Si. About 1, 0 wt .-% Si, the phase content in the material of the alloy is large and deteriorates its mechanical properties.

Antimony is essentially related to silicon because antimony can provide a modification of the Mg ₂ Si phase, with a required Sb concentration in the alloying metal being about half that of Si.

Although magnesium-based alloys, which may contain up to 8 wt .-% aluminum and above, also have an application potential in terms of increased material strength and creep resistance, aluminum is an undesirable element in the material according to the invention. By contents of greater than 0.5 wt .-%, brittle grain boundary phases of the type Mg ^ Al ₁₂ can arise, which also have a corrosive effect on corrosion. Furthermore, during the extrusion of the material under about 230 ⁰ C cracks, which can lead to a brittle pressing, which may also have significant grain size differences over the cross section and the longitudinal direction.

Silver has as a grain growth inhibiting element in the inventive

Alloy high potential in the contents of more than 0.1, but less than 2.0 wt .-% on. Ag is in these concentrations in the warm state of the alloyed material in solution, which was found at levels of about 0.1 wt .-% Ag an increase in concentration at the grain boundaries is formed, which highly effectively precludes grain growth. Furthermore, by Ag

Curing effect of the material over the phase Mg ₄ Ag can be achieved. Higher Ag Contents as 2.0 wt .-% have in particular economic and corrosion chemical disadvantages.

In claims 2 and 3 preferred chemical compositions of the magnesium-based alloys according to the invention are given.

Of particular importance for a homogeneous fine-grained microstructure and a high deformability of an article of the alloy according to the invention in the range of room temperature, it was found, the sum concentration of the micro-alloying elements Mn, Ca and Si of greater than 0.1, but less than 0.65 % By weight in the magnesium base material.

A semi-finished product of a magnesium-based alloy according to the invention, which has been deformed with a cross-sectional area ratio of greater than 1:16, in particular greater than 1:20 of a cast pin to a pressure at a temperature of about 380 ⁰ C, has a Grain size of the structure of less than 10 microns and that with a high degree of isotropy based on the cross section and in the longitudinal direction. Inventive compacts can be further deformed or pressed at temperatures below 200 ⁰ C, in particular at room temperature, with an error-free surface or gloss surface can be achieved.

The invention will be substantiated below with some experimental results.

Tab. 2 gives the chemical composition of the investigated materials.

The figures show:

Fig. 1 Stress-strain behavior of tested alloys Fig. 2 Trial alloy U., cast structure Fig. 3x Trial alloy IJ., Cast structure Fig. 3.1 Magnification scale: 500 μm

Fig. 3.2 Magnification scale: 200 μm

Fig. 3.3 Magnification scale: 50 μm

Fig. 3.4 Magnification scale: 20 μm Fig. 4x Experimental alloy LJ., Deformed Fig. 4.1 Cross-section edge

Fig. 4.2 Cross section center Fig. 4.3 Longitudinal edge

Fig. 4.4 Longitudinal grinding center Fig. 5 Trial alloy L2, cast structure Fig. 6x Trial alloy L2, cast structure Fig. 6.1 Magnification scale: 500 μm

Fig. 6.2 Magnification scale: 200 μm

Fig. 6.3 Magnification scale: 50 μm

Fig. 6.4 Magnification scale: 20 μm Fig. 7x trial alloy L2 Fig. 7.1 Cross-section edge

Fig. 7.2 Cross section center

Fig. 7.3 Longitudinal edge

Fig. 7.4 Longitudinal grinding center Fig. 8 Comparative alloy AZ31, cast state Fig. 9 Comparative alloy ZK31, cast state

Fig. 1 shows the result of the elongation as a function of the tension in the tensile test according to EN 10002-1: 2001 of magnesium-based alloys.

In the following, reference will be made to the alloy designations and alloy compositions given in Tab.

In the tensile test (A50), the sample designated L1 of an alloy according to the invention with deformation by means of indirect pressing and with a compression ratio of 1: 25 gave an elongation of more than 25% at room temperature at a maximum stress of about 260 MPa.

On the sample of a further trial alloy L2 according to the invention, a yield strength of Rpo _{, 2} = 330 MPa of the material was determined at 380 ° C. after room pressure at 380 ° C., whereby an elongation value of greater than 15%, as a measure of the ductility, in the given case of about 19%.

The comparative alloys ZK31, AZ31 and ZM21 had, as is apparent from Fig. 1, consistently lower elongation at break Ac than the materials of the invention. From Fig. 2, the dendritic cast structure of alloy L1 can be seen. An average particle size of 140 μm was determined with a substantially homogeneous structure over the entire cross section of the block.

3 shows the largely homogeneous structure in the cast state of the block of the alloy L1 over the cross section in with different magnifications at a scale of 500 μm (FIG. 3.1), 200 μm (FIG. 3.2), 50 μm (FIG. 3.3). and 20 μm (Figure 3.4) and shows spherical grains with some grain boundary phases.

Fig. 4 shows a deformed with a press ratio of 1:25 at 380 ⁰ C material of the alloy L1 according to the invention in the longitudinal and transverse directions of the edge and central region of the sample.

Figs. 4.1 and 4.2 are cross-sectional images of the edge and the center of the rod, Figs. 4, 3 and 4.4 show the corresponding longitudinal section images. An average particle size of 9 μm to 6 μm was measured.

FIG. 5 shows the globulitic cast structure of an alloy L2 according to the invention. With a largely homogeneous particle size distribution over the cast block, the average particle size was 40 μm.

6 shows the cast structure of FIG. 5 (L2) in its highly fine embodiment with scale values of 500 μm (FIG. 6.1), 200 μm (FIG. 6.2), 50 μm (FIG. 6.3) and 20 μm (FIG ). There are small fine precipitation phases at the grain boundaries.

FIG. 7 shows the structure of a compact of an alloy L2 of a cast block pressed at a temperature of 380 ° C. with a press ratio of 1:25 in the transverse direction at the edge (FIG. 7.1) and in the center region (FIG. 7.2) and in the longitudinal direction at the edge (Fig. 7.3) and in the central region of the rod (Fig. 7.4). The average grain size was about 2 microns.

The cast structure of a block of a comparative alloy AZ31 is shown in FIG. 8. A measurement of the microstructure resulted in a grain size of 360 μm with a substantially homogeneous distribution over the cross section. After extruding at 380 ⁰ C, the microstructure was partially coarsely recrystallized and inhomogeneous, whereby no secured grain size determination was possible.

As shown in Fig. 9, the cast structure (chill casting) in the block of comparative alloy ZK31 was globulitic and had a grain size of 80 μm with good homogeneity across the cross section.

After a hot pressing of the cast bolt, the extruded profile was partially inhomogeneously recrystallized. A crystal size determination with a certain significance was not possible at the press profile.

Tab. 1: Compositions of magnesium alloys according to the prior art (in% by weight)

essentially cast alloys

Tab. 2: Compositions of tested materials (in% by weight)

** no standard alloys

Claims

claims

1. A fine grain magnesium base alloy containing in wt .-% zinc (Zn) more than 0.8, but less than 6.2 zirconium (Zr) traces, but less than 1.0

Manganese (Mn) more than 0.04, but less than 0.6

Calcium (Ca) more than 0.04, but less than 2.0

Silicon (Si) traces, but less than 1.0

Antimony (Sb) traces, but less than 0.5 silver (Ag) more than 0.1, but less than 2.0, magnesium and manufacturing impurities as the remainder.

2. Magnesium base alloy according to claim 1, in which the micro-alloying elements Mn, Ca, Si have a sum concentration of greater than 0.1, but less than 0.65, preferably greater than 0.15, but less than 0.5.

The magnesium base alloy according to claim 1 or 2, wherein the concentration in wt% of one or more alloying elements Zn is more than 1.0, preferably more than 1.5, but less than 5.9, preferably less than 4 , 0

Zr less than 0.8, preferably less than 0.6

Mn more than 0.06, preferably more than 0.09 but less than 0.4, preferably less than 0.2 Ca more than 0.1, preferably more than 0.14 but less than 1, 0, preferably less than 0 , 6

Si is less than 0.5, preferably less than 0.2

Sb less than 0.25, preferably less than 0.1

Al is less than 0.1, preferably less than 0.08 Ag more than 0.2, preferably more than 0.38 but less than 1, 2, preferably less than 0.9.

4. A magnesium base alloy semi-finished product having a chemical composition according to any one of claims 1 to 3, deformed with a compression ratio of at least 1:20, which semi-finished product has a particle size of less than 10 microns and has extensive isotropy.