CZ309485B6 - Compacting powders by volumetric moulding - Google Patents

Abstract

Compacting metal powders by volumetric moulding is that the powder intended for compaction consists of a metal matrix containing at least one type of hardening metal oxide with a volume fraction of 1 to 10%. The powder is stored in a deformable and airtight work container, from which the air is pumped out, and subsequently, at 700 to 1,300 °C, the volume forming of the work container is carried out mainly by rotary forging or rolling. After compaction, the compacted powder is annealed at 1000 to 1400 °C for 0.5 to 24 hours to ensure secondary re -crystallization.

Description

The method of compacting powders by volumetric molding

Field of technology

The invention deals with the method of densification of solidified powders with an oxide dispersion by volume forming, in particular by rotary forging, the conditions and method of preparation presented then produce highly mechanically resistant steel alloys at very high temperatures.

Current state of the art

Powder metallurgy is a field dealing with methods of compacting powder mixtures based on metals, non-metals, oxides and carbides, or their mixtures. The advantage of these methods is their relatively high production accuracy and the possibility of creating mixtures that would not be created by classical casting. Compared to other methods of metal processing and production, the resulting products usually have lower mechanical strength.

One of the widespread methods of powder metallurgy is the so-called die forging (swaging), used, for example, to reduce the diameter of metal cylinders, when it is forged with mechanical hammers around its entire circumference at the same time. In powder metallurgy, this method is applied, for example, by document GB 981065 A. The document describes the method of manufacturing metal pipes or rods based on zirconium, niobium, or their alloys. The alloy powder is placed in a metal container, heated to a temperature of approximately 1000 °C and subsequently subjected to die forging. The metal container is then removed and the final product can be used. However, the presented method is not universally applicable to all types of metal powders, as it is always necessary to choose a suitable set of parameters, which can also vary by adding a fraction of an additive to the powder mixture.

A similar method is rotary forging, described for example in document EP 0238186 B1. Powdered materials based on aluminum, aluminum compounds, copper, iron, steel or bronze with grains up to 50 µm in size are placed in the work vessel and tamped down to obtain a mixture with the highest possible density. Subsequently, the material is subjected to rotary forging, when it is acted upon by a hammer rotating on the one hand around its own axis and further performing a precessional movement around the axis of the container in which the compacted powder is stored. However, the method presented in this document does not allow the creation of long cylindrical products based on metal powders.

Another used method for forming powders based on carbides is described in document EP 1231014 B1. The method describes the creation of a powder mixture of carbide and metal additives, a binder and other additives affecting the properties of the resulting material. The powder is placed in a metal container, which is then subjected to isostatic pressure so that the resulting product acquires a significantly higher density than the starting powder.

Current research in the field of powder metallurgy is aimed at the development of advanced materials that show high resistance to oxidation and creep deformations, especially at very high temperatures. Nickel-based compounds can be used up to temperatures of the order of 1100 °C, ODS (Oxide dispersion strengthened) alloys up to temperatures of the order of 1300 °C and tungsten alloys up to 1500 °C. However, the preparation of these materials is quite demanding, as it is necessary to choose appropriate process parameters, e.g. a suitable proportion of aluminum in the powder mixture significantly affects the resistance of the resulting material to oxidation, on the contrary, its too high a proportion results in a change in the internal structure of the material, i.e. different mechanical properties and sometimes shorter lifespan. In ODS alloys, the iron-based crystal lattice enables sufficient aluminum enrichment, which guarantees excellent oxidation resistance of the material.

- 1 CZ 309485 B6

Alloys based on ODS are usually quite demanding for small-volume and large-volume production due to the relatively complex technological process of obtaining the resulting material, which leads to a high price of the final products.

The essence of the invention

The above-mentioned shortcomings are largely eliminated by the method of compacting powders by volumetric molding. The metal powder mixture consists of a metal matrix based on iron and aluminum and at least one type of hardening metal oxide. The powder is stored in a deformable and airtight work container, from which the air is subsequently pumped out and a vacuum is created. Volumetric forming takes place at a temperature of 700 to 1300 °C. The advantage of the mentioned method is the high proportion of curing oxide in the resulting material, which is 1 to 10%. The curing oxide is then yttrium oxide Y2O3.

Volumetric forming refers to rotary forging or rolling, especially biaxial rolling. Bulk forming then takes place preferably at a temperature of 700 to 1000 °C.

The compacted powder includes atoms of iron and aluminum or other metals, as well as a hardening oxide, which is preferably yttrium oxide. Advantageously, the metal powder also includes at least one alloy, e.g. chromium or manganese, or another additional material influencing the resulting properties of the obtained compacted body. The presence of hardening oxide ensures the high hardness of the obtained material, which is usually annealed for 0.5 to 24 hours at a temperature of 1000 to 1400 °C after volume forming. Annealing ensures secondary recrystallization of the obtained material, creating elongated grains along the axis of the rotary forging, which further improves the mechanical properties of the material at very high temperatures and removes local defects in the crystalline structure. To ensure optimal processing, volume forming is carried out in three phases.

Before processing itself, the powder is prepared by mechanical alloying.

The compacted material obtained in the above-mentioned manner shows a 30% higher creep strength than ODS-based materials obtained by similar methods, especially at application temperatures of 1100 to 1300 °C. The above-mentioned compacted material includes a metal matrix based on iron and aluminum and a curing oxide, which is, for example, yttrium oxide. Advantageously, the compacted material also includes at least one alloy, e.g. manganese or chromium. The obtained material has a creep strength of 70 to 120 MPa and an elongation of 1 to 10% at a temperature of 1100 °C. Advantageously, the proportion of curing oxide in the obtained material is 4 to 6%, i.e. approx. 10 times higher than in similarly obtained materials. The presence of a large proportion of aluminum in the material then provides high resistance to oxidation due to the presence of a thin surface layer of aluminum on the material during reaction with air.

Clarification of drawings

The essence of the invention is further clarified by examples of its implementation, which are described using the attached drawings, where on:

Fig. 1 schematically shows the presented method of compacting powders by bulk forming;

Fig. 2 is a schematic representation of a device for rotary forging;

Fig. 3 is a schematic illustration of a rolling device;

Fig. 4 is a picture of the microstructure of the sample obtained by the presented method during rolling, the picture was taken with a scanning electron microscope;

- 2 CZ 309485 B6 Fig. 5 is a picture of the microstructure of a sample obtained by the presented method during rotary forging, the picture was taken with a scanning electron microscope; and Fig. 6 shows a graphical comparison of the strength of samples obtained by the presented method in rolling, rotary forging and commercially produced ODS alloy MA 956.

Examples of implementation of the invention

The invention will be further explained on examples of implementation with reference to the relevant drawings, which, however, have no limiting effect in terms of the scope of protection.

The presented method of powder compaction, shown schematically in Fig. 1, uses powders with a metal matrix based on iron, aluminum and titanium and further including a curing oxide. For example, this powder can be Fe-10Al-3Y2O3-1Ti (in weight %). The powder intended for compaction is stored in a deformable and airtight work container and smothered. Subsequently, the pressure in the working vessel is reduced to a value of 1 to 10 Pa and the vessel is closed and sealed. Closure and sealing can take place, for example, by welding. In the next phase, the powder, which is closed and sealed in the work container, is subjected to volume forming, which takes place in the temperature range of 800 to 1300 °C, a more favorable temperature interval is limited to 800 to 1000 °C, while, for example, volume forming takes place at a temperature of 900 °C. Volumetric forming means mainly rotary forging or rolling, especially biaxial rolling. During rotary forging, the piston 2 exerts a force on the processed body 1 along its entire circumference at the same time, as shown in Fig. 2, at the same time, during rotary forging, either the device performing the rotary forging or the processed body 1, the workpiece, or the work container, which is in this exemplary embodiment of a cylindrical shape, while the rotary forging is carried out in a direction perpendicular to the central axis of the cylinder. After rotary forging, the volume of the processed material is reduced by the volume of the space between the compacted powder grains. As a result of rotary forging, the diameter of the processed roller, or the work container, decreases. The diameter of the work vessel after rotary forging is 1.5x to 5x smaller than before rotary forging. During rolling, the working container with the compacting powder is subjected to the pressure of the rollers acting from two opposite sides, viz. Fig. 3. Rolling corresponds to the forming of the processed body 1, which passes through the space between two rotating cylinders 3, where the gap between the cylinders 3 is smaller than the height of the input processed body 1. For example, the compacted material can be processed by biaxial rolling, when the axis of the forming pressure remains the same, but the orientation of the processed material changes, usually by 90°. By rolling, the diameter of the working container in the case of a cylindrical container, or the height of the parallelogram in the case of a different shape of the working container, decreases by 1.5x to 10x. Advantageously, the volumetric shaping of the compacted powder is carried out at least three times in the work vessel. After volume forming, the obtained material is annealed for 0.5 to 24 hours at a temperature of 1000 to 1400 °C. In an exemplary embodiment, annealing takes place for 4 hours at a temperature of 1200°C. This ensures secondary recrystallization of the grains forming the obtained material.

In an exemplary embodiment, the subject method of powder compaction by volume molding is preceded by a step of preparing the powder for compaction. A highly homogeneous powder comprising a metal matrix and yttrium oxide Y2O3 nanoparticles is created by mechanical alloying. The metal matrix can also include oxides of other metals, or other alloys, such as chromium and manganese. The next step in the preparation of powders can be, for example, thermomechanical processing in order to reduce the porosity of the materials and increase their resistance. Thermomechanical processing means, for example, biaxial rolling at elevated temperatures (Hot cross rolling). After initial processing, the powder shows a high degree of homogeneity, when oxygen originating from yttrium oxide and oxidized metal matrices is completely dissolved in the powder mixture and trapped in defects, disorders, the crystalline base of the powder, especially in dislocations and vacancies. The powder prepared in this way is then enclosed in a metal work container and subjected to consolidation at an elevated temperature. This ensures the removal of porosity and the initiation of dynamic recrystallization. Dynamic recrystallization then leads to the creation of ultra-fine grained microstructures. Highly fine-grained microstructures are stabilized by the presence of nanooxides with sizes of the order of 5 nm

- 3 CZ 309485 B6 up to temperatures in the range of 1000 to 1100 °C. By exceeding the critical temperature of 1200 °C for several hours, for example in the interval of 0.5 to 24 hours, secondary recrystallization occurs. As a result, the final microstructure includes coarse grains of the order of 100 pm reinforced by a homogeneous dispersion of yttrium oxide of approximately 20 nm in size.

In an exemplary embodiment of the invention, the presented method uses Fe-10Al-3Y2O3-1Ti powder made from iron, aluminum, yttrium oxide and titanium powders, where the purity of the powders is at least 99.9%. The individual powders forming the resulting metal powder are sealed in an airtight ball mill, which contains, for example, 40 mm bearing balls made of low-alloy steel. In the ball mill, mechanical alloying of the powders is subsequently carried out by rotating it around a horizontal axis. For example, this process is carried out in a vacuum at a pressure of 1 to 10 Pa for a period of 2 to 3 weeks. After mechanical alloying, the properties of the powder are saturated, with the powder particles containing a significant amount of defects, such as vacancies and dislocations.

At a temperature of 1100 °C and a deformation rate of 10 ^-6 s ^{-1 ,} the material obtained in the above-mentioned manner using rolling and secondary recrystallization shows a strength of 75 MPa with a deviation of the order of 5% and an elasticity in the range of 1 to 10%, typically 1 to 2%. The microscopic structure of the rolled material shows a large number of ultrafine grains with an approximately homogeneous distribution, Fig. 4 shows an image of the resulting material obtained by backscattered electron analysis in a scanning electron microscope.

At a temperature of 1100 °C and a deformation rate of 10 ^-6 s ^-1 , the material obtained by the above-mentioned method using rotary forging and secondary recrystallization shows a strength of 115 MPa with a deviation of the order of 5% and an elasticity in the range of 1 to 10%, typically 1 to 3%. The microscopic structure of the material obtained in this way after rotary forging shows a larger grain size compared to rolling. Fig. 5 shows an image of the resulting material obtained by analyzing backscattered electrons in a scanning electron microscope.

Fig. 6 graphically compares the creep strength of the samples obtained by the above-mentioned method at a temperature of 1100 °C. Specimen 1 refers to the sample obtained by the above method using rolling, Specimen 2 refers to the sample obtained by the above method using rotary forging. MA 956 is a commercially available ODS alloy. On the horizontal axis are the values of the applied stress and on the vertical axis is the time until the specimen breaks. The sample obtained by spin forging then shows a much higher strength than the commercial ODS alloy or the rolled material. Creep strength is a property of the sample tested in the temperature range of use of the given material. For samples obtained by the above-mentioned method, a creep strength of 70 to 100 MPa is achieved at an applied temperature of 1100 °C and a required time to fracture of 1000.

Industrial applicability

The presented method of compacting powders by volumetric molding produces about 30% stronger materials than conventional methods of compacting powders based on ODS. These materials are becoming available for application at temperatures of 1100 to 1300 °C.

Claims (13)

PATENT CLAIMS 1. A method of compacting powders by volumetric molding, where the powder is formed by a metal matrix containing at least one type of hardening metal oxide and is stored in a deformable and airtight work container, where volume molding takes place in the temperature range of 700 to 1300 °C in the work container at a gas pressure lower , than atmospheric pressure, characterized in that the metal matrix is based on iron and aluminum, and that the proportion of curing oxide in the powder mixture is 1 to 10%, the curing oxide being yttrium oxide Y2O3. 2. A method of compacting powders by volume forming according to claim 1, characterized in that the volume forming is pressing, forging, rotary forging or rolling. 3. A method of compacting powders by volume molding according to any of the preceding claims, characterized in that the volume molding takes place at a temperature of 700 to 1000 °C. 4. A method of compacting powders by volume molding according to any one of the preceding claims, characterized in that the compacted powder further includes at least chromium or manganese. 5. A method of compacting powders by volume molding according to any one of the preceding claims, characterized in that after volume molding the compacted powder is annealed at a temperature of 1000 to 1400 °C for a period of 0.5 to 24 hours. 6. A method of compacting powders by bulk molding according to any of the preceding claims, characterized in that the bulk molding is multi-stage. 7. A method of compacting powders by bulk forming according to any one of the preceding claims, characterized in that the powder is prepared by mechanical alloying. 8. Compacted material consisting of a metal matrix based on an alloy of iron, aluminum and curing oxide, characterized in that the proportion of curing oxide in the compacted material is 1 to 10%. 9. Compacted material according to claim 8, characterized in that it has a creep strength of 70 to 100 MPa at 1000 hours to fracture in air at temperature. 10. Compacted material according to any one of claims 8 to 9, characterized in that it has an elasticity of 1 to 10%. 11. Compacted material according to any one of claims 8 to 10, characterized in that the curing oxide is yttrium oxide. 12. Compacted material according to any one of claims 8 to 11, characterized in that the proportion of curing oxide is 4 to 6%. 13. Compacted material according to any one of claims 8 to 12, characterized in that it further includes at least chromium or manganese.