EP1941953B1 - Method for impact extrusion of magnesium forgeable alloys and workpiece manufactured thereby - Google Patents

Abstract

In the first stage, a cast bar is extruded to make a blank. The deformation ratio exceeds 1 : 20. For this stage, the microstructure of the material transversely to the direction of deformation, is modified to a grain size less than 20 mu m. One of the elements Zn, Mn , Ca, Si, Sb, Ag is added to the magnesium to manufacture the cast bar and/or up to 2 wt% Zr is added to refine the grain microstructure of the alloy. In the second stage, extrusion billets are produced from the blank; they are taken out at right angles to the direction of extrusion. In the third stage, the billets are extruded transversely to the direction of blank deformation, at room temperature or a temperature no greater than 200[deg] C. No lubricant is used. The shape of the billets is matched with that of the extrusion die, especially at the edges. The equipment used, includes a matrix or die, and a pressing plunger moving relatively to it. The billet alloy has a hexagonal lattice structure. Alternatively, using a temperature below 200[deg] C the wall thickness range is 0.1-13.6 mm, preferably 0,5 mm and less. Independent claims are included for the following: (A) corresponding equipment; (B) and the object produced.

Description

The invention relates to a process for the production of articles made of magnesium, in particular magnesium wrought alloys.

Furthermore, the invention relates to articles of the aforementioned alloys.

The formability of a material is generally influenced by many factors: the material properties (crystal lattice, chemical composition, microstructure, anisotropy), the thermodynamic conditions (thermally activated processes, forming speed), the forming behavior (friction, geometry of the forming zone, tool geometry, forming history) and the Stress condition during forming.

Due to their hexagonal lattice structure, magnesium alloys can only be formed to a limited extent at room temperature. Metals with a hexagonal lattice structure have only one sliding system at room temperature (base plane slip), as well as twin formation for plastic deformation, unlike metals with cubic face-centered lattice structure with 5 independent sliding systems. Only at a temperature above 225 ° C are more pyramidal slip planes activated.

H.-W. Wagener ["Deep drawing and impact extrusion of magnesium alloys at room temperature", Advanced Engineering Materials 2003, 5, no. 4 and " Extrusion of magnesium wrought and cast alloys ", sheet metal, tubes, profiles, UTF Science I / 2002 ] attempted to compensate for low forming capacity by increasing the hydrostatic pressure component in extrusion press experiments with magnesium alloys. The compressive stress superposition is achieved by using a closed tool with non-back-turned dies or non-back-turned dies (forward extrusion). During the deformation, one or two force-loaded punches counteract the extrusion direction. ZK60A room temperature full-forward extrusion tests with back pressure do not provide a satisfactory result. But an increase in sample temperature 250 ° C - 400 ° C leads to defect-free workpieces even with non-heated tools. Due to the limited ductility of the investigated alloys (AE42HP, AM20HP, AM70HP, AZ91HP, AZ31B, AZ80, ZK60A), crack-free forward extrusion with backpressure is only conditionally possible. With full-forward extrusion with back pressure, flawless workpieces can only be achieved with increased back pressure or at higher forming temperatures.

When producing semifinished products from a magnesium alloy or from it or components produced therewith by hot extrusion, it is according to EP 1 295 957 A Alloying by zirconium or rare earth concentrations of 0.1 to 10 wt .-% possible to convey the material increased ductility and possibly also increased energy absorption, compressive and tensile strength and toughness.

It has also been reported (Kühlein R. O. Riemelmoser M et al "Improving the Formability of Magnesium Alloys" ASMET FORUM 2006) that new micro-alloyed single-phase magnesium alloys have a particularly fine grain and that this alloying technique enables cold extrusion.

The invention has for its object to provide a method by means of which objects made of magnesium, in particular magnesium wrought alloys can be pressed free of defects at low temperatures.

Another object of the invention is to provide an article prepared by the process of this invention by F / ießpressen of magnesium, in particular magnesium alloys, by means of a tool.

The goal is achieved in a method of the type mentioned by first in a cast bolt from a hot-forming process such as extruding with a forming or pressing ratio of greater than 1:20 a semi-finished product is produced, from which in a second step extrusion bars are taken and manufactured transversely to the forming of the semifinished product, and these in a third step with hexagonal lattice structure of the material, are extruded at a temperature of less than 200 ° C to objects, in particular to unilaterally open hollow bodies.

Surprisingly, it has been found by a person skilled in the art that magnesium or a magnesium wrought alloy, if it is subjected to hot deformation in the first step, in particular hot extrusion by extrusion, with a forming or pressing ratio of greater than 1:20 the sequence transverse to the forming of the semifinished product by extrusion at a temperature of less than 200 ° C, ie in the range of a hexagonal atomic structure is deformable. The advantages achieved by the invention are a much simpler and more economical production of the articles and improved quality of the parts. If, as is provided according to the invention, in the second step of the method according to the invention the extrusion billets are taken transversely to the forming direction of the semifinished product, this results in an improved material flow during extrusion.

This aforementioned effect can be further enhanced if, in the third step of the method, the slugs are extruded perpendicularly to the deformation direction of the semifinished product.

It has been found to be particularly advantageous if in the first step of the method according to the invention, a microstructure of the material structure is set transversely to the forming direction with a particle size of less than 20 .mu.m. A person skilled in the art can provide by means of his knowledge a corresponding degree of deformation of the cast bolt and / or by a heat treatment the material structure according to the invention.

If, according to a variant of the invention, by adding at least one element from the group zinc (Zn), manganese (Mn), calcium (Ca), silicon (Si), antimony (Sb), silver (Ag) to the magnesium in the preparation of Gussbolzens created a magnesium alloy and / or by adding zirconium (Zr) up to 2.0 wt .-% grain refinement of the microstructure of the material is made, particularly favorable properties and / or an effective fine grain structure of the material can be achieved, which is highly advantageous can affect the extrusion conditions in the third step of the process and improve the quality even complicated shaped and thin-walled objects.

Processually, but also economically, it has proven to be particularly favorable if the slugs are extruded lubricant-free in the third step. This finding was particularly surprising for a person skilled in the field of extrusion of light metals.

If, according to the invention, the geometric shape of the extrusion billets is adjusted, in particular in the edge area of the mold shape, the conditions for flow of the material and in particular the edge areas of the articles can be improved in their quality.

Production advantages, but also high efficiency are given if according to the invention, the slits are introduced with room temperature (RT) in the mold and extruded.

For special formats of the articles to be produced and, if appropriate, for an increase in the toughness of the tools, provision may also be made for extruding the billet in the third step using tools which have an elevated temperature but not more than 200 ° C.

The further object of the invention is an article of a magnesium-Knetiegierung, prepared by the inventive method, with a wall thickness in the range of 0.1 to 13.6mm, preferably 0.5mm and less, prepared by the extrusion method from Pressbutzen with hexagonal lattice structure of the material or with a temperature of less than 200 ° C by means of a tool, consisting essentially of a die and a relatively movable to this punch, which has a coaxial rear rotation after the front shaping region solved.

By this tool geometry, the flow of the material in the gap can be advantageously improved.

Based on examples from the developments, the invention will be explained in more detail.

The sample material used had the following chemical composition: material Zn Mn Zr Mg and impurities ZK31 3.0 1.0 rest ZM21 2.0 1.0 rest

Continuous casting bolts of ZM21 (grain size approx. 1 - 2mm) and of ZK31 (grain size approx. 80μm) were extruded with different press ratios (1: 5, 1: 9, 1:14, 1:20, 1:26).

The metallographic examination of the extruded starting material showed that the pressing ratios of 1: 5 and 1: 9 are too low to achieve complete kneading.

Extrusion with a compression ratio of 1:20, in particular of 1:26, however, leads to a significant reduction in the grain size. The microstructures of the cast state and the extruded material (press ratio PV = 1:26) are in the Fig. 1a - 1d shown. The Fig. 1 a shows a micrograph of a cast sample of the alloy ZM21 with a dendritic microstructure with grain sizes of 1 - 2mm. Through extrusion with a pressing rate of 1:26 a reduction of the grain sizes to <200μm is achieved ( Fig. 1 b) , The Fig. 1c shows the micrograph of a cast sample of the alloy ZK31, which has an average grain size of 80 μm in the initial state. By extrusion at a press rate of 1:26, the grain size was reduced to <30 microns. ( Fig. 1d ).

ZM21 and ZK31 extruded profiles with a 1: 9 and 1: 14 press ratio were rolled or forged and heat treated.

From ingots with a diameter of 109mm, starting material of the alloys ZM21 and ZK312 was extruded with a compression ratio of 1: 9 and subsequently rolled into plates with a thickness of 22 mm. The slabs were then heat treated at 350 ° C for one hour. The slugs for the extrusion tests were taken from the slabs perpendicular to the rolling direction.

With a press ratio of 1: 9 and 1:14 extruded profiles of ZM21 and ZK31 were further formed by forging. The ZM21 profiles were heated to 390 ° C before forging and the ZK31 profiles to 450 ° C. The grain size distribution of the forged ZM21 profiles was inhomogeneous due to the low compression ratio.

For ZK31 with a press ratio of 1:14, the particle size distribution after forging is significantly more homogeneous than for the ZM21. The forged profiles were then heat treated at 375 ° C for one hour. The slugs for the extrusion direction were taken from the forging samples perpendicular to the forging direction.

For the time being, extrusion of cup-shaped parts with a wall thickness of 3.6 mm and a side length of 20 mm was carried out with a hydraulic 100t press and a 600t knee press in the temperature range 300 ° C - 120 ° C from the ZM21 and ZK31 starting material. Up to a temperature of 150 ° C, good parts could be produced throughout. Extrusion tests at 100 ° C were successful in this series of tests in any alloy.

The metallographic examination of the extruded parts showed that in the critical deformation area at the cup bottom at higher temperatures (T> 150 ° C) and high pressure ratio, the flow line course runs almost parallel to the extrusion direction. During extrusion tests at low temperatures or lower compression ratio of the starting material there is a fault in this zone and consequently also cracking.

By improving the microstructure, the extrusion tests performed with a wall thickness of 1.5 mm and 90 mm leg length when using the alloy ZK31 with a tool according to Fig. 2 consistently perfect parts. Occasionally small cracks appeared on the rim of the alloy ZM21.

In order to further improve the formability, based on the findings of the first experiments, the geometries of the extrusion tools were modified. The Fig. 3 shows such, optimized for the extrusion of magnesium tool with a background rotation in the area B.

The next experiments were carried out with this optimized tool, with samples with wall thicknesses of 1.5mm, 0.7mm, 0.5mm, 0.35mm and 0.25mm were pressed.

The tool was heated by special heating elements in the matrix, as well as in the stamp to a temperature of below 200 ° C. In these experiments, the starting material (magnesium) was preheated to a temperature of below 200 ° C.

Since the process of stable slug preheating at about 200 ° C in mass production poses a technical challenge and causes additional, considerable production costs, it was also an object of the present invention to transform magnesium such as aluminum at room temperature. For this purpose, the Butzenvorwärmtemperatur was continuously reduced in the experiments with 0.5mm wall thickness, until it was possible to press the alloy ZK31 even at room temperature with preheated tools.

In the last series of experiments, the production of cups with a wall thickness of 0.35 mm in a faultless forming at room temperature was carried out for individual parts.

Claims (9)

1. A method for manufacturing work-pieces of magnesium, particularly forgeable magnesium alloys, by impact extrusion, wherein in a first step a semi-finished material is produced from a cast bolt by a hot transformation procedure, such as extrusion, with a transformation or pressing ratio greater than 1:20, from which, in a second step, extrusion moulded bunches are taken transversely to the transformation direction of the semi-finished material and produced, and these, in a third step, are extrusion moulded with a hexagonal lattice structure of the material at a temperature of below 200°C into work-pieces, particularly into unilaterally open hollow bodies.
2. Method according to claim 1, wherein in the third step bunches are extrusion moulded perpendicularly to the deformation direction of the semi-finished material.
3. Method according to claim 1 or 2, wherein in the first step a micro-structure of the material is adjusted transversely to the transformation direction with a grain size of less than 20µm.
4. Method according to any of claims 1 to 3, wherein, by adding at least one element of the group of zinc (Zn), manganese (Mn), calcium (Ca), silicon (Si), antimony (Sb), silver (Ag) to the magnesium, when producing a cast bolt, a magnesium alloy is provided and/or by adding zirconium (Zr) up to 2.0 % by weight, grain refining of the micro-structure of the material is effected.
5. Method according to any of claims 1 to 4, wherein in the third step bunches are extrusion moulded free of any lubricant, in particular are backward extruded.
6. Method according to any of claims 1 to 5, wherein the geometric shape of the extrusion moulded bunches, particularly in the edge region, are adapted to the shape of the tool.
7. Method according to any of claims 1 to 6, wherein in the third step bunches are introduced into the tool at room temperature (RT) and are extrusion moulded.
8. Method according to any of claims 1 to 7, wherein in the third step extrusion moulding of the bunch is effected using tools, which have an elevated temperature, but of 200°C in maximum.
9. Work-piece of a forgeable magnesium alloy, produced by a method according to any of claims 1 to 8, having a wall thickness in the range of 0.1 - 13.6mm, preferably of 0.5mm or less, produced according to the extrusion moulding method from extrusion bunches having a hexagonal lattice structure of the material or at a temperature of below 200°C, by means of a tool consisting substantially of a female mould and a pressure ram movable relative to it, which after the forward shaping region comprises a coaxial backing-off.

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