WO2014060047A1

WO2014060047A1 - Nano-dendrites reinforced metal

Info

Publication number: WO2014060047A1
Application number: PCT/EP2012/070799
Authority: WO
Inventors: Horst Adams
Original assignee: Adamco Ag
Priority date: 2012-10-19
Filing date: 2012-10-19
Publication date: 2014-04-24

Abstract

The present invention discloses a dendrite reinforced metal in which a coarse dendrite structure is entangled with a fine, preferably nano-scaled, dendrite structure, and a method for making such material. The method for making the entangled dendrite structure reinforced metal consists of the triggered and controlled growth of a first coarse dendrite structure of one or more alloying element(s) in a metal melt, and a subsequent rapid solidification processing of the coarse dendrites containing melt, in which a second, fine, preferably nano-scaled, three-dimensional dendrite structure is created in the solidified metal, which is entangled with the first coarse dendrite structure, so that the combined scaffold of coarse and fine, entangled dendrite structures acts as a mechanical reinforcement of the metal.

Description

NANO-DENDRITES REINFORCED METAL

Technical Field

The present invention discloses a dendrite reinforced metal in which a coarse dendrite structure is entangled with a fine, preferably nano-scale, dendrite structure, and a method for making such material.

Background Art

"A dendrite in metallurgy is a characteristic, tree-like structure of crystals growing as molten metal freezes, the shape produced by faster growth along energetically favourable crystallographic directions. This dendritic growth has large consequences in regards to material properties (Fig. 1).

Dendrites form in unary (one-component) systems as well as multi-component systems. The requirement is that the liquid (the molten material) be undercooled, aka supercooled, below the freezing point (...). Initially, a spherical solid nucleus grows in the undercooled melt. As the sphere grows, the spherical morphology becomes unstable and its shape becomes perturbed. The solid shape begins to express the preferred growth directions of the crystal. This growth direction may be due to anisotropy in the surface energy of the solid-liquid interface, or to the ease of attachment of atoms to the interface on different crystallographic planes, or both (...). In metallic systems, interface attachment kinetics is usually negligible (...). In metallic systems, the solid then attempts to minimize the area of those surfaces with the highest surface energy. The dendrite thus exhibits a sharper and sharper tip as it grows. If the anisotropy is large enough, the dendrite may present a faceted morphology. The microstructural length scale is determined by the interplay or balance between the surface energy and the temperature gradient (which drives the heat/solute diffusion) in the liquid at the interface.

As solidification proceeds, an increasing number of atoms lose their kinetic energy, making the process exothermic. For a pure material, latent heat is released at the solid-liquid interface so that the temperature remains constant until the melt has completely solidified. The growth rate of the resultant crystalline substance will depend on how fast this latent heat can be conducted away. A dendrite growing in an undercooled melt can be approximated as a parabolic needle-like crystal that grows in a shape-preserving manner at constant velocity. Nucleation and growth determine the grain size in equiaxed solidification while the competition between adjacent dendrites decides the primary spacing in columnar growth. Generally, if the melt is cooled slowly, nucleation of new crystals will be less than at large undercooling. The dendritic growth will result in dendrites of a large size. Conversely, a rapid cooling cycle with a large undercooling will increase the number of nuclei and thus reduce the size of the resulting dendrites (and often lead to small grains). Smaller dendrites generally lead to higher ductility of the product. (...).

As dendrites develop further into the liquid metal, they get hotter because they continue to extract heat. If they get too hot, they will remelt. This remelting of the dendrites is called recalescence." (source: http://en.wikipedia.org/wiki/Dendrite_(metal)).

Homogeneous nucleation occurs only in lab experiments where a pure metal melt is isolated from its environment. In industrial environments, solidification occurs by heterogeneous nucleation and dendrite formation, mainly on impurity or precipitation particles and wherever the liquid comes into contact with the container surface.

When the liquid metal immediately ahead of an advancing solidification front starts solidifying, the solidification energy released warms the liquid in front of it, which slows or stops further growth in that particular direction. Spikes develop as the dendrite grows in the directions in which the liquid is coolest. As these areas also warm up in turn, secondary, and then tertiary, spikes develop creating a tree-like dendrite structure. If the solidification speed is increased the dendrite arm spacing is decreased leading to finer dendrite structures. At cooling rates in the order of >10⁶ degrees/second a nano-scale dendrite structure can be created. As a thumb rule one can find that as finer the structure is, as stronger the resulting solidified metal will be, and as higher also the ductility of the resulting will be.

Cooling rates of >10⁶ degrees/second can be achieved by applying the so-called melt-spinning method, in which the metal melt is poured through a funnel onto a spinning wheel being kept a low temperature:

" Melt-spinning is a technique used for rapid cooling of liquids. A wheel is cooled internally, usually by water or liquid nitrogen, and rotated (Fig. 2). A thin stream of liquid is then dripped onto the wheel and cooled, causing rapid solidification. This technique is used to develop materials that require extremely high cooling rates in order to form, such as metallic glasses. The cooling rates achievable by melt-spinning are on the order of 10⁴ -10⁷ kelvins per second (K/s)." (source: http://en.wikipedia.org/wiki/Melt_spinning)

"Development of methods for controlling the dendritic microstructure currently serves as a powerful tool in the arsenal of manufacturers of modern metal alloys. Dendrites are the most commonly found solidification microstructures in alloys. The shapes and sizes of the dendrites that are first formed upon solidification of the liquid metal typically have a profound impact on the material's properties, even after downstream processing. Dendrites that grow slowly will be more blunt, and the branches will be few and large. However, if the metal is cooled rapidly, the dendrites will grow more quickly, and produce a finer branch structure. This branch structure has the effect of establishing the initial grain size scale of the microstructure, which is well-known to affect important properties such as the strength of the material (described by the Hall-Petch relationship)." (source: http://en.wikipedia.org/wiki/User:Lacomj/Dendrite)

The Hall-Petch relation basically says that the material strength S is inversely proportional to the squared grain size d:

S~1/d²

This means that, if an ultrafine dendrite structure can be produced in the metal, causing the grain size to be on the nano scale, the resulting bulk metal will have an increased strength level according to the Hall-Petch relation.

The present invention describes a method for producing such a nano-dendrite reinforced metal.

Technical Problem

The technical problem to be solved by means of the method described in this invention is to create a first coarse dendrite structure of alloying elements in a base metal melt, to freeze-in and create a second nano-scaled dendrite structure, which is entangled with the first coarse dendrite structure, by rapid solidification, to create a hardened metal powder in which each powder particle has an internal nano-scaled dendritic grain structure, to screen, remix, can and degas this metal powder at elevated temperatures, and to create a bulk metal by hot isostatic pressing (HIP) of the canned metal powder, and subsequent extrusion or rolling of the resulting HIP-block, so that the resulting bulk metal has an entangled, nano-scaled, nano-dendrite based internal grain structure which gives it an increased tensile strength with respect to the base metal and sufficient ductility (>3% maximum elongation, preferably >5%) for further standard industrial processing like forging, rolling, deep drawing etc..

Technical Solution

The following text describes the technical solution using aluminum as an example for the base metal. However, the technology is not limited to aluminum but can also be used for other metals like titanium, magnesium, zinc, copper, and intermetallics for instance of the Al₃X type (with X being Ti, Zr, Nb, or V) like Al₃Ti, and others.

In order to create a defined, nano-scaled dendrite structure in a metal melt it is advantageous to use an electromagnetic stirrer crucible or furnace (also called magneto-hydrodynamic (MHD) induction stirring system) for melting the base metal and for mixing-in the alloying elements into the melt, because it is possible to stir the melt at variable speeds without creating larger turbulences as it would be the case with a mechanical stirrer. In case aluminum is the base metal, the main alloying elements (0.1-30 weight-%) can be for instance Zn, Cu, Mg, Si, B, Fe, Sb, Be, Bi, Pb, Cd, Ca, Cr, Ga, In, Li, Mn, Ni, Zr, Va, and others, and smaller percentages (<2 weight-%) of rare earth elements (mainly from the Lanthanide series), like for instance Sc, Sm, Ce, Tb, Y and others. Nonmetallic ingredients can be carbon, carbides, and oxides, mainly acting as starting points for dendrite formation and having a total weight percentage of <6 weight-%, preferably <2 weight-%.

The MHD system should comprise a sensor for measuring the rotation speed of the melt in the crucible and an electronic feed-back system which allows to keep the rotation of the melt at a constant speed while measuring the electrical energy needed to do so. The measured electrical energy is proportional to the viscosity of the melt, which means, if the viscosity of the melt rises due to partial solidification of the melt, the degree of solidification can be detected by monitoring the electrical power needed to keep the melt rotating at the same speed. This type of crucible with an integrated MHD stirrer also provides for the possibility to make controlled temperature changes of the melt wile maintaining chemical homogeneity throughout the melt.

Another advantageous property of the preferred crucible is a precisely controllable heating and cooling system, preferably a fast reacting inductive type heating system in combination with an effective water cooling, making it possible to keep the crucible precisely at a preselected temperature for a defined time, and then bringing it rapidly to a different, preselected temperature and hold it there again for a preselected time.

In order to prepare a suitable aluminum alloy for the creation of a reinforcing nano-dendrite structure, the crucible is filled with the predetermined amounts of base metal and alloying elements and heated up to a temperature T₀ which is higher than the melting point of the particular alloying ingredient which should solidify first when the melt is cooled down again. All ingredients having a melting point higher than T₀ or which are not soluble in the melt will remain as solid dispersoids or particulates in the melt. The melt is kept at this temperature T₀ long enough so that all ingredients having melting temperatures lower than T₀ are completely dissolved and a homogenous melt is generated by means of the above described MHD induction stirring system.

Then the temperature of the melt is lowered to a level T₁ slightly below the melting point of the alloying element having the highest melting temperature of the alloying elements in the melt which are intended to form dendrites. Simultaneously the stirring speed is reduced to the lowest possible value still ensuring a very slow rotation velocity of he melt with respect to the inner surface of the crucible. The ideal movement of the melt is a constant angular velocity v₀, with only small turbulences at the contact areas between the melt and the crucible walls. Since the temperature T₁ is kept constant and since there is no temperature gradient in the melt because of the stirring, the first alloying element will start solidifying in a random, however homogeneous nucleation pattern in the melt and will start forming randomly, and evenly distributed three-dimensional dendrites. The dendrite formation can be triggered by a defined content of non-soluble dispersoids or particulates in the melt because then the dendrites will start forming mainly on and around the particulates in the melt. The dendrite formation at the inner crucible surfaces is suppressed because of the slow movement of the melt along these walls, and the turbulences in these areas. As the dendrites grow, they will form a three dimensional structure of spikes (Fig. 1).

Although there is no way to monitor the dendrite formation directly, their development can be indirectly monitored by a temperature increase of the melt since the solidification process reduces the degree of freedom of the atoms/molecules in the melt or, with other words, its kinetic energy. The corresponding released energy manifests itself as an increase in temperature T₁+Τ_i and is compensated instantaneously by the cooling system of the crucible. The energy necessary to bring the temperature to the predetermined level T₁ is proportional to the amount of dendrites formed in the melt.

As soon as the dendrites have grown to a macroscopic size and start to entangle, they cause an increase of the viscosity of the melt, resulting in a decrease -v_i of the angular rotation velocity v₀. The onset of entangling can be determined by monitoring the energy necessary to bring the reduced angular rotation velocity v₀-v_i of the melt back to v₀. As soon as there is an increase of that necessary stirring energy the entangling has started.

When it is confirmed that the entangling has started, the temperature of the melt T₁ is manually adjusted to a slightly increased temperature T₁+Τ_ii by means of the inductive heating system. This stops any further dendrite growth and even causes some dendrites to convert back into the liquid phase. Simultaneously the viscosity of the melt will decrease again indicating a reduced entangling of the dendrites. By establishing a feedback loop between the viscosity signal and the temperature control of the crucible, it is possible to keep the dendrite concentration constant at any level above the onset of entangling. In this way it is possible to create well defined dendrite scaffolds in the melt with variable, yet precisely controllable density.

Because the temperature of the melt is kept constant only slightly below the melting temperature of the first alloy determined to form dendrites, the size of the dendrites and also the dendrite arm spacing is relatively large, usually in the >10μm region. However, in order to increase strength and ductility of the final dendrite reinforced material it is necessary to create an additional nano-dendrite sub-structure wich has a much smaller dendrite size and finer dendrite arm spacing in the order of <100nm. Dendrites with these <100nm dimensions are called nano-dendrites in the present invention.

In order to create a nano-dendrite substructure, the melt prepared according to the above described procedure and containing the coarse dendrite scaffold is poured from the crucible through a preheated funnel onto the surface of a fast spinning wheel of a melt-spinning apparatus (Fig. 2). The wheel is kept at a defined low temperature, preferably (but not limited to) either room temperature (air cooling), about 0°C (fluid cooling) or -196°C (LN₂ cooling). As soon as the melt gets into contact with the wheel surface, it will solidify at high speed, causing the formation of a fine dendrite structure entangled with the already existing coarse dendrite structure, and instantaneously freezing-in this combined dendrite structure. Since the wheel is spinning at high speed, a very thin layer of solidified material will be formed, ensuring that there is almost no temperature gradient across the layer. The thickness of the layer is <1mm, preferably <0.25mm. In this way, homogeneous cooling rates in the order of > 10⁵K/sec, preferably >10⁶K/sec, and ideally >=10⁷K/sec can be realized across the thickness of the layer. The extreme cooling rates lead to the formation of new, very fine dendrite structures of also all other alloying elements in the melt which have a lower melting point than the first alloy which had already been solidified and had formed the existing coarse dendrite structure in the previous steps. The size of the new dendrite structures is in the nano-scale and the new super-fine dendrites fill the space between the already existing coarse dendrite branch structure. Together, these two (or more, depending of number of alloying elements), entangled dendrite structures form a reinforcing scaffold inside the surrounding aluminum matrix.

Because of the high rotation speed of the melt-spinning wheel, the solidified metal layer is quickly carried away from the solidifying area, forming a ribbon-like structure on the wheel. The speed of the spinning wheel is adjusted to such a value that the solidified ribbon detaches from the wheel and flies away from the wheel shortly after solidification because of centrifugal forces. It is very important to keep the rotating speed high enough so that the ribbon does not travel a full circle around the wheel because it would severely disturb the solidification process if it would reach the solidification zone again . If the speed is adjusted properly, the ribbon detaches from the wheel at a constant distance after the solidification spot and flies away tangentially from the wheel. The speed at which the ribbon is ejected from the melt spinning system is >10m/sec, preferably >40m/sec.

In order to enhance the heat transfer interface between the ribbon and the wheel, it is advantageous to apply a uniform electromagnetic force on the ribbon, pressing it onto the wheel after the liquid melt has solidified on the wheel surface. This force is preferably applied in an area immediately after the solidification zone, and can be realized by means of an Eddy-current inductor which is placed above the ribbon, and which induces an alternating electrical current in the ribbon, and electro- magnets on each side of the ribbon shortly after the solidification area. The phase of the electrical current driving the electromagnets is coupled to the Eddy current generator, so that the resulting electromagnetic Lorentz force acting on the ribbon always presses the ribbon onto the wheel, ensuring good mechanical contact and optimum heat transfer. In an advantageous setup the induced electrical current is adjusted so that the resulting Lorentz force is higher than the centrifugal force acting on the ribbon, ensuring that the ribbon stays firmly attached to the wheel as long as it inside the Lorentz force area. As soon as the ribbon leaves the Lorentz force area, the centrifugal force will become predominant, creating a well defined detaching point from the wheel for the solidified ribbon. By varying the Lorentz force the "detach point" can be moved closer to, or farther away from the solidification area making it possible to optimize the time the ribbon stays attached to the wheel for optimum nano-dendrite structure formation.

After detaching from the wheel the solidified metal ribbon is then captured by a ribbon catching and spooling device. The correct angle to hit the capturing device can be adjusted by varying the above described Lorentz force, which moves the detaching point back and forth, and in turn varies the ejection direction accordingly. The ribbon can be spooled on a drum and stored for later processing, or it can directly be guided into an chopping system. The chopping system converts the solid ribbon into metal granules. The size of the granules can be coarsely predetermined by choosing chopping wheels with different numbers of chopping blades and can be fine-tuned by varying the rotation speed of the chopping wheel. However, each metal granule, or each metal powder particle, produced in this way, contains the entangled, reinforcing nano-dendrite structures.

There are a number of standard industrial processes for metal powder (or granules) compacting. In this context "compacting" means baking or fusing metal particles together in order to form a solid, bulk and dense piece of metal. The most common compacting methods are:

1. direct sintering

2. direct powder extrusion

3. direct powder rolling

4. hot isostatic pressing

5. cold isostatic pressing

6. pulsed current sintering, etc.

Although each of the above listed processes can be used to compact the dendrite reinforced metal particles, it has been found to be advantageous to use hot isostatic pressing (HIP) as the preferred compacting method (Fig. 3).

In order to prepare the granules for hot isostatic pressing they need to be filled in a suitable pressure vessel or canister, a process called canning. It is important that the granules are filled into the vessel with a sufficient tap density in order to guarantee for a density of the compacted metal of >97%, preferably >99% after hot isostatic pressing (hipping). The HIP canister is preferably made from pure aluminum (or in general the base metal) and mechanically designed in such a way that after the mechanical deformation imposed to it by hot isostatic pressing, the shape of the canister is either a straight cylinder or a block with flat surfaces. This means for instance, that in case a subsequent extrusion is intended, the canister should be a straight cylinder after hipping. In case a subsequent rolling processing is intended, the vessel should be a block with flat surfaces . The hipping canister is preferably made from pure aluminum (i.e. the same non alloyed metal as the base metal of the reinforced granules). In this case there is no need to remove (mill) the canister walls after hipping and before extrusion or rolling. On the contrary, the soft (non-reinforced) and thin aluminum (or base metal) layer acts as a lubricant during extrusion or rolling, facilitating smooth processing and preventing surface cracking.

In order to ensure an optimum tap density of the metal powder (or granules) in the pressure canister, it is advantageous to screen the reinforced metal particles before canning, using a multiple stage screening device (sieving machine). Since the screens in each stage have different mesh sizes it is possible to collect different size classes of particles, depending on the number and respective mesh width of screens. At least three different particle size classes (fine = <50μm, medium = 50μm - 500μm, coarse = >500μm) should be prepared. However, it has been found that it is advantageous to prepare five different particle size classes (super fine = <25μm, fine = 25μm - 100μm, medium= 100μm - 250μm, coarse= 251μm - 500μm, big= >500μm). After preparing these particle size classes it is then possible to remix the individual classes in a well defined way in order to prepare the optimum particle size distribution for a maximum metal powder tap density in the canister.

Another important step in preparing the hipping process is the thoroughly degassing of the filled hipping canister. This is preferably accomplished by placing the metal powder filled hipping canister in an oven while connecting the canister through a tubular flange to a vacuum system (Fig. 3). The temperature of the degassing oven is kept for several hours, preferably 1-3 hours, at a level which ensures a complete evaporation of all volatile substances from the surfaces of the metal particles and the inner canister walls. The temperature range is usually between 350°C and 550°C, preferably between 400°C and 480°C. The evaporated volatile substances are then evacuated from the vessel through the tubular flange which is connected to the vacuum system. The degassing process is monitored by metering the vacuum with a pressure gauge connected to the tubular flange. The degassing process is completed after a sufficient vacuum in the vessel has been reached, preferably a pressure <10^-6bar. After the end pressure has been reached, the flange is thoroughly squeezed together (sealed-off) in order to avoid any air leaking into the canister.

The temperature level at which the metal powder filled canister is kept during the degassing process has an influence on the tensile strength and the ductility of the bulk metal after hipping, because it affects the dendrite structure in the metal particles. By applying moderate temperatures and relatively short holding times it is possible to enhance the entangling of the dendrite structures which leads to a stronger final bulk metal. However, a high degassing temperature applied for extended holding times leads to a relaxation and partial disintegration of the dendrite structures, making the final product softer. By applying different degassing temperature levels with different holding times to the dendrite reinforced metal granules, it is possible to tune the tensile strength and ductility values of the final bulk metal in a wide range. Also, certain combinations of crucible temperatures during coare dendrite formation and subsequent degassing temperatures lead to particular high strength and, simultaneously, high ductility values of the final bulk metal.

"The HIP process subjects (...) `the metal granules filled canister to both elevated temperature and isostatic gas pressure in a high pressure containment chamber. The pressurizing gas most widely used is argon. An inert gas is used, so that the material does not chemically react. The chamber is heated, causing the pressure inside the vessel to increase. Many systems use associated gas pumping to achieve the necessary pressure level. Pressure is applied to the material from all directions (hence the term 'isostatic').

(...) Metal powders can (...) be turned to compact solids by this method. The inert gas is applied between 7,350 psi (50.7 MPa) and 45,000 psi (310 MPa), with 15,000 psi (100 MPa) being most common. Process soak temperatures range from 900 °F (482 °C) for aluminium (...) to 2,400 °F (1,320 °C) for nickel-based superalloys. (...) The simultaneous application of heat and pressure eliminates internal voids and microporosity through a combination of plastic deformation, creep, and diffusion bonding." ( http://en.wikipedia.org/wiki/Hot_isostatic_pressing)

The resulting product after hipping is a solid block of metal with a density preferably > 99%. Depending on the design of the pressure canister, this block can be cylindrical or rectangular. Cylindrically shaped blocks are preferably used for subsequent hot extrusion while rectangular blocks are preferably used for subsequent hot rolling. Since both, extrusion and rolling, apply severe plastic deformation (depending on the extrusion or rolling ratio) at elevated temperatures to the solid metal, strength- and ductility properties of the final product are also affected by this process. In any case, after extrusion or rolling the resulting material has a 100% density. However, it is the proper combination of three major processing parameter sets which ultimately determines the maximum achievable mechanical properties of the final product:

set 1: temperatures, holding times, and solidification speed while alloying, dendrite formation, and melt spinning,

set 2: degassing temperatures and hipping temperatures,

set 3: extrusion temperature (typically 350°C-450°C) and extrusion ratio (typically 12-36).

The basic internal dendrite structure created using set 1 predetermines the main strength and ductility levels of the final material, around which the material properties can be tuned by combining suitable parameter sets 2 and 3 in the subsequent industrial processing steps.

Advantageous Effects

The entangled nano-dendrite structures prepared using the above method provide for a reinforcing scaffold in the base metal matrix which have a strengthening effect relative to the base metal analogous to a steel reinforcement in concrete. Since this is not a particulate reinforcement but a metal scaffold in a metal matrix, the mechanical interface between the matrix and the scaffold is excellent, in contrast to often difficult interfacing situations with many non-metal fiber reinforcements. The combination of the well embedded coarse dendrite structure together with the entangled nano-dendrite structures provides for an increased tensile strength and good ductility of the resulting bulk metal at the same time.

Because of the high degree of entangling between the coarse and fine dendrite structures and the excellent mechanical interface between the combined dendrite structure and the base metal matrix, any external mechanical stress applied to a nano-dendrite reinforced specimen can be distributed very evenly in the matrix, which improves not only the tensile strength but also fracture toughness, fatigue properties and cracking properties, and helps in particular to control crack propagation.

In comparison to non-metal fiber reinforcement techniques (like for instance reinforcing with CNTs) the here described method provides for a "metal only" solution or at least for a solution with only a small, non-metallic particulates content in the matrix. This has a substantial advantage when it comes to reproducible corrosion properties in particular with aluminum as the matrix metal. The excellent corrosion properties of aluminum are due to the natural formation of an aluminum oxide layer on the surface of an aluminum substrate which cannot be penetrated by oxygen molecules or other aggressive media, thus preventing any further oxidation or corrosion. In contrast, in case for instance of CNT reinforcements, the CNTs are embedded in in the matrix and also in the surface of the substrate in a random pattern. In those surface areas where CNTs lay parallel to the surface, exposing parts of it to the outside, or even stick out of the surface, the formation of the protective oxide layer is disturbed. Since the CNTs are arranged randomly on and in the surface this leads to a reduced and uncontrollable corrosion stability. The nano-dendrite reinforcement however, is a mostly "metal only" solution, and does not influence the protective oxide layer formation which leads to excellent and well controllable corrosion properties.

There are applications for reinforced metals, where the maximum allowed content of non metallic particulates in the metal matrix is limited to certain low levels. For instance, in Formula 1 applications, the maximum allowed content is 2 weight-% of non-metallic particulates in the metal matrix. In these cases, nano-dendrite reinforced metals provide the optimum solution since it is either a "metal only" solution or its non metallic particulate content can easily be kept below the maximum allowed limit without sacrificing the excellent mechanical properties.

Description of Drawings

Fig. 1 Principle shape of a metal dendrite and possible growth directions (source: http://www.ami.ac.uk/courses/topics/0131_mb/index.html)

Fig. 2 Principle setup of a melt-spinning system ( adopted from: http://dx.doi.org/10.1016/j.bbr.2011.03.031 )

Fig. 3 Principle setup of a hot isostatic pressing system (adopted from: http://www.pmt.usp.br/academic/antschip/hns2002book.htm)

Claims

Dendrite reinforced base metal in which a first coarse dendrite structure of alloying elements is entangled with at least a second, finer dendrite structure.
Material according to claim 1, in which the base metal is aluminum, or titanium, or magnesium, or zinc, or copper, or intermetallics.
Material according to claim 2, in which at least one nonmetallic ingredient consists of either carbon, carbides, or oxides, or a combination thereof, having a total weight percentage of <6 weight-%, preferably <2 weight-%.
Material according to claims 1 or 3, in which the main alloying element ingredient with a total content of >= 1weight-% is either Al, Zn, Cu, Mg, Si, B, Fe, Sb, Be, Bi, Pb, Cd, Ca, Cr, Ga, In, Li, Mn, Ni, Zr, Va, or a combination thereof.
Material according to claims 4 , in which a minor alloying element ingredient with a respective content of < 1 weight-% is a Lanthanide series element or a combination of Lanthanide series elements.
Material according to claim 1, in which the first coarse dendrite structure of alloying elements has a medium arm spacing of >100nm and at least a second, finer dendrite structure has a medium arm spacing <=100nm.
Method for making a material according to claim 4 or 5 characterized in that a base metal is mixed with one or more alloying elements in a crucible which is equipped with a magneto-hydrodynamic (MHD) induction stirring system, and which has a sensor for measuring the rotation speed of the melt and an electronic feed-back system which allows an electronic control circuit to keep the rotation of the melt at a constant speed while measuring the electrical energy needed to do so, and which is equipped with a precisely controllable heating and cooling system, making it possible to keep the crucible precisely at a preselected temperature for a defined time.
Method according to claim 7 characterized in that the temperature of the melt is kept long enough at a level T₀ so that all ingredients having melting temperatures lower than T₀ are completely dissolved and a homogenous melt is generated by means of the MHD induction stirring system which is moving the melt at such a low speed that turbulences occur only at the interface between the melt and the walls of the crucible.
Method according to claim 8 characterized in that the coarse dendrite structure formation is initiated by lowering the temperature of the melt to a level T₁ slightly below the melting point of the alloying element having the highest melting temperature of the alloying elements in the melt which are intended to form dendrites.
Method according to claim 9 characterized in that the degree of coarse dendrite structure formation is monitored by measuring the cooling energy needed to compensate for a corresponding temperature increase of the melt.
Method according to claim 10 characterized in that the onset of entangling of the coarse dendrite structure is determined by monitoring the viscosity of the melt by measuring the energy necessary to keep the angular rotation velocity of the melt constant.
Method according to claim 11 characterized in that the melt is poured from the crucible through a preheated funnel onto the surface of a fast spinning wheel of a melt-spinning apparatus as soon as the onset of entangling of the coarse dendrite structure has been observed.
Method according to claim 12 characterized in that an additional fine dendrite structure is created in the melt by rapid solidification on the cold surface of the melt spinning wheel, which is entangled with the already existing coarse dendrite structure and that the solidified material forms a thin ribbon on the surface of the wheel of the melt-spinning system which detaches from the spinning wheel shortly after the solidification zone because of centrifugal forces.
Method according to claim 13 in which an electromagnetic force is applied to the ribbon, pressing it onto the wheel directly after the solidification zone.
Method according to claim 14 in which the electromagnetic force is preferably applied in an area immediately after the solidification zone, and is realized by means of an alternating current inductor which is placed above the ribbon, and which induces an alternating electrical current in the solidified ribbon, and electro-magnets on each side of the ribbon shortly after the solidification area, while the phase of the electrical current driving the electromagnets is coupled to the alternating current generator above the ribbon, so that the resulting electromagnetic Lorentz force acting on the ribbon always presses the ribbon onto the wheel, ensuring good mechanical contact and optimum heat transfer.
Method according to claim 15 in which the induced electrical current is adjusted so that the resulting Lorentz force is higher than the centrifugal force acting on the ribbon, ensuring that the ribbon stays firmly attached to the wheel as long as it inside the Lorentz force area, while as soon as the ribbon leaves the Lorentz force area, the centrifugal force will become predominant, creating a well defined detaching point from the wheel for the solidified ribbon.
Method according to claim 16 in which by varying the Lorentz force acting at the solidified ribbon, the "detach point" can be moved closer to, or farther away from the solidification area making it possible to optimize the time the ribbon stays attached to the wheel for optimum nano-dendrite structure formation.
Method according to claim 16 in which by varying the Lorentz force acting at the solidified ribbon, the "detach point" can be moved closer to, or farther away from the solidification area making it possible to vary the ejection direction when the ribbon detaches from the wheel.
Method according to claim 13 in which the solidified, melt-spun ribbon detaching from the wheel is spooled on a drum and stored for later processing.
Method according to claim 13 in which the solidified, melt-spun ribbon detaching from the wheel is directly fed into a mechanical chopping system which converts the metal ribbon to metal granules.
Method for canning the granules prepared according to claim 20 in a hot isostatic pressing canister characterized in that the canister is made from the same non alloyed metal as the base metal of the reinforced granules and mechanically designed in such a way that after the mechanical deformation imposed to it by hot isostatic pressing, the shape of the canister is either a straight cylinder or a block with flat surfaces.
Method for extruding or rolling the hipped canister designed according to claim 21 characterized in that the soft (non-reinforced) base metal walls of the canister act as a lubricating medium during extrusion or rolling, facilitating smooth processing and preventing surface cracking.
Method for canning the granules prepared according to claim 20 in a hipping canister made according to claim 21 characterized in that the reinforced metal particles are screened before canning, using a multiple stage screening device (sieving machine), creating at least three different particle size classes which are then remixed in such a way that the optimum particle size distribution is created for maximum metal powder tap density in the canister.
Method for degassing the canister filled with the granules prepared according to claims 20 or 23 prior to isostatic pressing characterized in that the canister is put in an oven whose temperature kept for a sufficient time at a level which ensures a complete evaporation of all volatile substances from the surfaces of the granules and the iiner surfaces of the canister.
Method for using the degassing method according to claim 24 to tune the tensile strength and the ductility of the bulk metal after hipping by exposing the filled canister to different degassing temperature levels with different holding times, thereby affecting the dendrite structure in the metal particles in such a way that the entangling of the dendrite structures is enhanced at moderate temperatures and defined holding times or decreased at high temperatures and extended holding times.