EP1680246B1 - Method for producing metal matrix composite materials - Google Patents

Abstract

The invention relates to a method for producing metal matrix composite materials, comprising at least one proportion of magnesium or one magnesium alloy and involving at least one production step in which a thixomolding ensues. According to the invention, an Mg2Si phase having a volume fraction of at least 2 % is incorporated in a metal matrix preferably comprised of magnesium or of a magnesium alloy. The inventive method uses the thixomolding method for the in-situ production of a metallic composite material and is advantageous in that a broad range of adjustable volume fractions of the Mg2Si phase in the composite material results whereby enabling the properties of the composite material to be individually modified. The inventive metal matrix composite material is particularly suited for producing thermally stressed parts of motor vehicles such as pistons or the like.

Description

The present invention relates to a method for the production of metal-matrix composite materials comprising at least one portion of magnesium or a magnesium alloy and at least one production step in which a thixomolding takes place.

Due to its low modulus of elasticity, the high coefficient of thermal expansion and the lack of wear resistance, the material magnesium can not readily be used for certain applications, such as pistons in motor vehicle engines or other engine components, in particular of engines. However, the properties mentioned can be positively influenced by the material is reinforced by means of a second, usually much firmer and harder phase-usually find ceramic or carbon-based short or long fibers or particles. These can be introduced in a melt-metallurgical production either in the form of a porous shaped body (so-called preform), which is infiltrated with liquid molten metal, or in the case of particles also by stirring into the metallic matrix. Another way to reinforce a metallic material by fibers or particles consists in the self-or "in situ" formation of the reinforcing component. In addition to the aforementioned melting metallurgical processes, metallic composite materials can also be produced by powder metallurgy.

When preforms are used as infiltratable moldings, squeeze casting has become established as the preferred casting method. In this case, the molten metal is pressed into the porous fiber or particle body at somewhat lower filling speeds, but somewhat higher pressures than in the classical die casting. This produces a virtually pore-free composite material with closed fiber-matrix bonds.

When stirring ceramic particles are usually supplied as a loose bed of moving molten metal by trickling or blowing. Composite material of this type can be cast directly in the form of castings or ingots. In the in-situ process, the composite is formed by a reaction between two or three a plurality of alloying elements of the metallic matrix or phases of the overall system, usually forming a new, usually intermetallic phase.

The preparation and characterization of the system Mg-Mg ₂ Si has been described several times. It is based, for example, on the revelation of DE 41 25 014 A1 directed. The emergence of the intermetallic phase in terms of amplification may have been assigned to the in situ process. This is usually done by infiltration of Si-particle-shaped fiber preforms or by precipitation of primary magnesium silicide from hypereutectic Mg-Si alloys. While the primary coarse withdrawal after falling below the liquidus line, block-shaped form Mg ₂ Si precipitates, the Mg ₂ Si formed at the reactive conversion of pure Si in a preform a globular. Eucleatively excreted Mg ₂ Si, in turn, usually shows the characteristic "Chinese script" structure.

The DE 101 35 198 A1 describes a process for the production of magnesium alloys by thixomolding, which may also contain a proportion of silicon among other elements.

In the thixomolding process, the metallic material is fed as granules to the thixomolding machine and moved within a heated cylinder by a transport screw in the direction of the spray nozzle. Under the effect of the shear forces and the temperature, which lies between the liquidus and solidus temperature of the metal, this partially liquefies, while the remaining solid content forms globular The behavior of the thixotropic material is pseudoplastic, that is, the viscosity decreases with increasing shear. The Thixomolding is particularly suitable for the production of very thin-walled components with high dimensional stability, since it hardly comes to shrinkage and distortion due to the favorable temperature level between liquidus and solidus

Disadvantages of the above-mentioned process routes for the production of metallic composite materials are in the case of preform infiltration in the complex system technology, the limited design capability, the fiber content of the preforms and their high cost level. Complex geometries are currently hardly or only with increased technical and financial effort feasible, so that a net-shape production of fiber or particle-reinforced components by infiltration is currently hardly possible. This usually results in a relatively high processing outlay, which is difficult and cost-intensive in the use of ceramic hard phases as reinforcements, since, for example, the machining of a reinforced with SiC or Al ₂ O ₃ - fibers Body is possible only by means of diamond-studded tools. Moreover, the infiltration ability of preforms with high fiber and particle contents in classical die casting is not readily given, preferably the method of squeeze casting is used for this purpose, for which in turn special casting plants are necessary. The difficulties that can arise in die-casting infiltration are mainly due to the high filling rate of the process and the low pressure which can be exerted via the melt due to the small bleed. However, this is needed to overcome the normally very low tendency to wetting between the metallic melt and the ceramic molding. In addition, the preform must be heated significantly above the melt temperature in order to prevent premature solidification of the melt on the fiber body.

The method of stirring is primarily reserved for the particulate reinforcements, as the use of fibers can lead to a large increase in viscosity of the melt, which makes a homogeneous distribution of the fibers very difficult or even impossible. In the case of particles, the stirring result depends on the particle size used, the stirrer speed and the temperature. Insufficient choice of parameters can lead to clumping, flooding of the particles into the slag or their sedimentation on the crucible bottom. If the particles and the melt are reactive systems, reaction reactions at the interfaces may result due to the long contact time between the two phases, resulting in particle damage. Example is the system magnesium - alumina, here is formed in the reaction between the two partners with decomposition of the particulate matter magnesium oxide and aluminum.

From the WO 03/027342 A a method of the type mentioned is known. This process is based on a magnesium powder or a powder of a magnesium alloy and a silicon powder, which are processed to produce a more compact product. In this case, a heating and a reaction takes place between the magnesium and the silicon, so that a metal-matrix composite material containing Mg ₂ Si as a reinforcing component is present. Furthermore, a hot forming process is provided.

The object of the present invention is to provide a method for the production of metal matrix composite materials of the type mentioned, which enables the production of light metal composite materials, in particular for use in temperature-loaded components, which is more variable and less expensive than the Hitherto known method and avoids the above-mentioned disadvantages associated with these.

The solution to this problem provides a method according to the invention for the production of metal matrix composite materials of the type mentioned above with the characterizing features of claim 1. According to the invention, the production of the light metal composite material in the thixomolding process, wherein in a metal matrix, an Mg ₂ Si Phase with a volume content of at least 2%.
The particular advantages of the method according to the present invention result from the combination of the thixomolding method with the method for the in-situ production of a metallic composite material. According to the invention Mg-Mg ₂ Si composites are to be prepared with a volume content of at least 2% Mg ₂ Si by a granule of silicon or a silicon alloy and granules of magnesium or a magnesium alloy are supplied together to the thixomolding process and there under shear a form at least partially liquid melt, which solidifies in the form of a magnesium body. Advantages of the method are the wide range of adjustable volume contents of Mg ₂ Si, the ability to dispense with fiber or Partikelpreforms and determine the size and amount of Si particles, the amount and size of the forming Mg ₂ Si crystals In turn, properties such as thermal expansion coefficient, modulus of elasticity, tensile and yield strength, and wear behavior can be changed individually. So Si contents are adjustable, which can not be produced by melt metallurgy. The thus cast material can be supplied to subsequent forming operations such as a forging process.

Preferably, in the thixomolding process according to the invention, a cast body is produced from the metal-matrix composite material which is subsequently processed further. In particular, the cast body is subsequently reshaped in at least one method step. Such a forming process may include, for example, at least one forging process.

The features mentioned in the dependent claims relate to preferred developments of the task solution according to the invention. Further advantages of the invention will become apparent from the following detailed description.

Hereinafter, the present invention will be described in detail with reference to embodiments.

For example, metal-matrix composites made by the process of this invention may be used to make pistons or other engine parts for diesel or gasoline fueled engines. The metal-matrix composites are also suitable, for example, for the production of liners for shafts, cylinders and other rotationally symmetrical parts, especially in engines. They are also suitable for the production of other on wear stressed automotive parts such as brake discs.

The volume content of the Mg ₂ Si phase in the metal matrix is preferably in the range between about 5 and about 40 volume percent. The metal-matrix composites of the present invention are obtainable, for example, from standard alloys such as AZ91, AM50, MR1230D, MR1253M or other Mg die cast alloys obtained an addition of Si. Essential here is the reaction 2 Mg + Si → Mg ₂ Si. In the context of the invention, an addition of at least about 2 percent by weight of Si and preferably at most about 15 percent by weight of Si is suitable. The resulting volume percentages of Mg ₂ Si are listed in Table 1 below, which represent exemplary levels of Mg ₂ Si phase in the metal matrix composite. Table 1 Addition amounts of Si in wt .-% and the resulting amounts in volume percent wt .-% Si Vol .-% Mg ₂ Si 2 5.08 3 7.63 4 10.19 5 12.77 6 15.35 7 17.95 8th 20.55 9 23.17 10 25,80 11 28.44 12 31.09 13 33.75 14 36.42 15 39.10

Mg ₂ Si is a comparatively high-melting phase with a melting point near 1100 ° C. Thus, this phase is suitable as a reinforcement for improving the high-temperature properties of the matrix material. This applies both to the creep behavior and characteristic values such as the thermal conductivity and also the thermal expansion coefficient. In addition to other physical and mechanical properties, these values can be adjusted specifically with regard to an application. The exact numerical values depend, inter alia, on the base alloy, the volume fraction of Mg ₂ Si, further precipitates in the matrix alloy, and also on the operating temperature or the operating temperature range. These data are to be determined experimentally for each application.

Another influencing factor is the manifestation of Mg ₂ Si precipitates. Usually they are found as so-called "Chinese script" excretions, ie as acicular precipitates, which are very similar in their shape to Chinese characters. By the addition of alloying elements such. B. Ca, however, produce primary polygonal precipitates that behave like a particle reinforcement. Both types of precipitation also have an effect on mechanical and physical properties.

In the production of semi-finished products from the metal matrix composite materials according to the invention, the parameters selected during further processing have a significant effect on the property profile. If a forming takes place, for example, by extrusion, then the alignment of planes of the Mg crystallites parallel to the extrusion direction leads to anisotropy. The magnitude of the anisotropy depends on various factors, in particular on the forming ratio, the temperature in the tool, the preheating, heat transfer after pressing and thus the dynamic and static recrystallization. The alloy composition including the influence of impurities is also an influencing factor.

Parameters for the production:

The temperature control in the production of metal-matrix composite materials according to the inventive method is directly related to the selected alloy, the shot weight and the tool, in particular its component geometry, sprue, etc., the geometry of worm and cylinder in thixomolding, the feed rate and also the shooting speed. These parameters must be determined empirically for each component and are also dependent on the design of the machine and its data profile. Likewise, the properties also depend on the solid phase fraction. This affects the mechanical properties of the matrix alloy alone as well as the composite material, d. H. the combination of matrix and amplification.

With regard to the liquid phase content, the reaction means 2 Mg + Si → Mg ₂ Si, that although the alloys build up a high proportion of liquid phase more quickly, at the same time there is an increasing proportion of solid phase due to the formation of Mg ₂ Si. The reaction not only takes place in the area of the cylindrical worm of the thixomolding machine, but can also take place after the casting in the workpiece. Especially in areas with accumulations of material, this behavior can be expected. Under certain circumstances, a reprint is therefore more successfully applied, as is still a part of matrix alloy in the molten phase by the exothermic reaction. Related conclusions can be obtained by examining metallographic cuts.

With regard to the matrix alloy, the melting interval plays a major role. For example, the alloy AZ91 is listed, the melting interval in the range of 440 to 600 ° C. It is known from the literature that for this alloy a high proportion of liquid phase in the range of 95% leads to an improvement of the mechanical properties in the component. With such a liquid phase fraction, one can speak of a supercooled melt. After injection into the tool, a high nucleation rate is the result in the inventive method therefore at the same time a very high number of germs. This leads to the expression of a very fine structure, which has very good mechanical properties due to the Hall-Petsch relationship. Due to the supercooling of the melt, the shrinkage is also very low overall. It is the lower, the lower the proportion of liquid phase. This means at the same time that in comparison with die casting less internal stress and thus less distortion occurs.

In connection with the addition of Si, there is an exothermic reaction between Mg and the Si at the first occurrence of melt. This means that the heating power of the machine can be reduced. An order of magnitude for this depends on various parameters, in particular on the ambient temperature, the thermal insulation of the machine used in each case as well as the thermal conductivities of the various components involved (materials). Especially in the area of heat transfer coefficients at elevated temperatures in a closed system, as it is a thixomolding machine, the relationships are very complex.

The grain size of the granules is usually not a determinant size. Depending on the machine and selected component then each other a different screw geometry can be selected. The grain size and the grain shape must be matched to the screw geometry. This is completely independent of the alloy or the composite material. In the further consequence, the particle size ratio Mg-Si must be matched. However, this usually only makes sense for a previously defined screw geometry.

The addition of granules can, for example, by a simple conveyor at the same time or shortly after the granule application (both materials are still solid), which can be additionally attached to the machine. In principle, a machine of conventional design can be used, as is available on the market, for example, from the companies Thixomat or Japan Steel Works.

Claims (9)

1. Method for producing metal-matrix composite materials based on Mg-Mg₂Si with a metal matrix comprising magnesium or a magnesium alloy and with a content of an Mg₂Si phase incorporated into the metal matrix, characterized in that granules of silicon or a silicon alloy and granules of magnesium or a magnesium alloy are used as starting material, and these are processed together in a thixomoulding process, and in that an Mg₂Si phase with a volume content of at least 2% is incorporated into the metal matrix.
2. Method according to Claim 1, characterized in that the amount or size of the Mg₂Si crystals formed and/or the silicon content of the composite material is determined via the size and/or amount of the particles of silicon or silicon alloy.
3. Method according to Claim 1 or 2, characterized in that a cast body of the metal-matrix composite material is produced in the thixomoulding process, which is subsequently processed further.
4. Method according to Claim 3, characterized in that the cast body made of the metal-matrix composite material is subsequently shaped in at least one method step.
5. Method according to Claim 4, characterized in that the cast body made of the metal-matrix composite material is subsequently shaped in at least one forging method and/or extrusion method.
6. Method according to one of Claims 1 to 5, characterized in that at least 2 per cent by weight of Si and at most 15 per cent by weight of Si are added during the production of the composite material.
7. Method according to one of Claims 1 to 6, characterized in that an Mg₂Si phase with a volume content of from at least 5% to at most 40% is incorporated into a metal matrix.
8. Method according to one of Claims 1 to 7, characterized in that one of the standard magnesium alloys AZ91, AM50, MR1230D, MR1253M or an Mg die-cast alloy is used as starting material for the production of the metal-matrix composite material.
9. Method according to one of Claims 1 to 8, characterized in that after adding the Si, the heating power of the thixomoulding device is reduced when the melt first occurs.