WO2013124001A1

WO2013124001A1 - Self stabilizing halloysite aluminum metal matrix compound

Info

Publication number: WO2013124001A1
Application number: PCT/EP2012/053226
Authority: WO
Inventors: Horst Adams
Original assignee: Adamco Ag
Priority date: 2012-02-25
Filing date: 2012-02-25
Publication date: 2013-08-29

Abstract

By embedding a mechanically interlinked network of spot-pinned halloysite nanotubes in an aluminum matrix a ductile compound material is created with an internal super-elastic three dimensional halloysite fiber network and aluminum as the integrated high viscosity damping material. The halloysite-reinforced Al alloy composite material shows excellent specific strength, specific modulus of elasticity, fatigue, and ductility properties.

Description

SELF STABILIZING HALLOYSITE ALUMINUM METAL MATRIX COMPOUND

Technical Field

The present invention relates to a method for creating a reinforcing halloysite fiber network in an aluminum matrix resulting in a high strength metal matrix compound with improved ductility, elastic and fatigue properties. The compound material can be used for manufacturing low weight mechanical components requiring excellent ductility, elastic and/or fatigue properties, for instance, in aerospace, automotive, mass transportation, and construction applications. The present invention also relates to a method of manufacturing such an aluminum / halloysite compound material.

Background Art and Current Technical Problems

The use of fiber-reinforced materials with improved materials strength characteristics is an option to reduce weight of mechanical parts in aerospace, automotive, building, construction, and energy generating applications.

Recently, metal-based-reinforced composite materials have increased in importance. Reinforcing metal materials with fibers makes it possible to clearly increase the mechanical strength characteristics and also, to some extend, the damage tolerance characteristics of metal materials. However, usually, those metal-based-reinforced composite materials show poor ductility, creep and fatigue characteristics and are significantly higher in cost. This is mainly due to increased production costs.

An example for making such a fiber-reinforced material is the lamination of metal sheets with fibers containing adhesive foils. From EP 0 312 151, a laminate is known which consists of at least two metal sheets, wherein a glass filaments containing plastic layer is disposed between the sheets. Such metal laminates are in particular suitable for lightweight structures for aircraft-related applications because these structures have advantageous mechanical characteristics while being of low structural weight.

From EP 0 056 288 a further metal laminate is known, wherein plastic layers are used, containing polymer fibers from the group of aramides, polyaromatic hydracids and aromatic polyesters.

From EP 0 573 507 an alloy laminate material is known, wherein a plastic matrix, containing reinforcing fibers like carbon fibers, polyaromatic amide fibers, aluminum oxide fibers, silicon carbide fibers or mixtures thereof are used.

Laminated materials have a clearly higher damage tolerance characteristic compared to equivalent monolithic sheets. The crack propagation characteristics of fiber-reinforced metal laminates are 10 to 20 times better than those of monolithic sheets.

However, when compared to those of monolithic materials, beside high manufacturing cost the static characteristics of known laminated materials are inferior. Depending on the adhesive systems and fiber types used, the limit of elasticity during exposure to tensile, compressive or shearing stress of known laminated materials is 5 to 20% below that of equivalent monolithic materials and in addition show a strong directionality. It is therefor desirable to create metal matrix compounds which are easier to manufacture and, which do not have a layered structure and thus do not show any directionality.

In order to create a metal matrix compound without any directionality, the mechanical properties of a metal can be improved by infiltrating the metal into a reinforcing structure, whether spontaneous, pressure-assisted or vacuum-assisted. One method is to incorporate the reinforcing particles in a first step in a polymer matrix, then remove the polymer by using a variety of different methods, leaving behind a loosely formed preform scaffold (preform) consisting of the reinforcing particles, which is then infiltrated by a liquid metal. In this method the mechanical properties of the resulting MMC are mostly limited by the degree to which the polymer binder can be removed. In addition, MMCs manufactured in the above described way rely on a good mechanical interface between the reinforcing particles and the metal matrix since there is no direct interconnection between the reinforcing particles themselves. In most cases the interface has to be prepared in a separate step by functionalizing the surfaces of the reinforcing particles prior to embedding them in the polymer. Functionalization, polymer embedding, and polymer removal are additional manufacturing steps which make this type of MMCs rather expensive, limiting their industrial application. It is therefor desirable to develop a method for making an MMC, wherein these steps are not necessary.

U.S. Pat. No. 5,020,584 discloses a method of infiltrating of a preform, formed from a mixture of a powdered matrix metal and a powder filler (ceramic), with a molten metal. It is disclosed that when the powdered matrix metal is aluminum and the filler aluminum oxide, the infiltrating atmosphere forms a skin (such as an oxide or nitrogen compound) on the metal that prevents particle separation. U.S. Pat. No. 5,020,584 is incorporated by reference in its entirety herein. This is a way to use aluminum oxide or aluminum nitride (depending on the gas used for the infiltration process) as the interface which is naturally formed on the surface of each aluminum powder particle.

PCT/EP2012/052360 discloses a method to use the aluminum oxide structure of the inner tubular walls of halloysite nanotubes (HNTs) as the interface to a surrounding aluminum matrix creating a so called "spot pinning" effect because of the differences in bonding strengths between the aluminum matrix on the one side, and the end segments and the remaining outer wall areas of the reinforcing HNTs on the other side. This leads to a so called internal "micro torque" effect in the material giving it excellent tensile strength properties.

Both U.S. Pat. No. 5,020,584 and PCT/EP2012/052360 do not require any functionalization or polymer embedding of the reinforcing particles. However, although MMCs made according to the U.S. Pat. No. 5,020,584 are relatively cost effective and have excellent tensile- and compression strengths their ductility and fatigue properties are limited. It is therefor desirable to develop a method for making an aluminum / HNT MMC based on the technology disclosed in PCT/EP2012/052360 but with improved ductility, elasticity and fatigue properties.

US 2003/0084970 A1 describes a titanium alloy having high ductility, fatigue strength, and rigidity, and a method of making the same. The specific an objective of the invention was to develop a titanium alloy which is capable of hot forging or hot rolling, and which has a tensile strength not less than 1100 MPa and a Young's modulus not less than 130 GPa, together with high ductility and fatigue strength. The Young's modulus of a titanium alloy was enhanced by dispersing particles having a high Young's modulus into the titanium matrix. The dispersed particles are titanium carbide or titanium boride particles, which are produced by crystallization and/or precipitation in the matrix. In addition to the relatively complex titanium alloy composition and precipitation hardening processes, it is also necessary to apply a special thermal treatment and a mechanical work hardening to the composite material in order to reach the targeted material properties Although the achieved tensile strength and modulus levels are good the use of the material is limited because of the relatively high specific weight in comparison to aluminum and the high materials and manufacturing cost.

Therefor it is desirable to

a) use aluminum instead of titanium because of the lower cost and specific weight,

b) reach comparable material properties without complex alloying,

c) reach comparable material properties without precipitation hardening,

d) reach comparable material properties without heat treatment,

e) reach comparable material properties without work hardening.

Biomorphic materials, as a new kind of carbon containing composite materials, are usually fabricated by carbonizing wood or wood like materials impregnated with phenolic resin under vacuum at an elevated temperature of 300∼2800°C. Biomorphic materials not only offer the potential of improved material properties of the bulk material but also maintain the micro-fine structure of the natural biological materials, thus transferring structural properties like flexibility to artificial materials. Biomorphic materials are currently used as self-lubricating materials, biomedical materials, heat insulating materials, and for electromagnetic shielding (Griel P 2001 J. Eur. Ceram. Soc. 21 105, Zhang Di, Sun Binghe and Fan Tongxiang 2004 Sci. China E47 470, Odeshi A G, Mucha H and Wielage B 2006 Carbon 44 1994).

Recently, a variety of biological materials have been used as bio-templates (bio-preforms) to prepare biomorphic materials, based on

a) wood ( Min Luo, Jiqiang Gao and Xiao Zhang 2006 Rare Metal Mater. Eng. 35 133; Ozao Riko, Nishimoto Yuko and Weiping Pan 2006 Therm. Acta 440 75; Kaul V S and Faber K T 2008 Scr. Mater. 58 886),

b) bamboo (Dong Liu 2009 Preparation and characterization of bamboo based SiC/C composite biological Mimesis Ceramics, M.Sc. thesis, Beijing Forestry University, Beijing),

c) paper (Yang Gangbin, Liu Yinjuan, Qiao Guanjun, Yang Jianfeng and Wang Hongjie 2008 Mater. Sci. Eng. A492 327), and

d) cotton (Amirthan G, Udayakumar A, BhanuPrasad V V and Balasubramanian M 2009 Ceram. Int. 35 967).

Various biomorphic materials also have been prepared based on

a) oxides (Dong Qun, Su Huilan, Xu Jiaqiang, Zhang Di and Wang Ruibing 2007 Mater. Lett. 61 2714),

b) carbides (Sun Binghe, Fan Tongxiang and Zhang Di 2004 Mater. Lett. 58 798; Kim Jae-Won, Myoung Sang-Won, Kim Hyeon-Cheol, Lee Je-Hyun, Jung Yeon-Gil and Jo Chang-Yong 2006 Sci. Eng. A434 171; Martinez-Escandell M, Narciso J and Rodriguez-Reinoso F 2009 Carbon 4 002), and

c) nitrides (Rambo C R, Sieber H and Genova L A 2008 J. Porous Mater. 15 419).

Other methods to produce biomorphic materials are:

a) Sol-gel and carbothermal reduction methods, used by Qian et al. (Qian Jun-Min and Jin Zhi-Hao 2006 J. Eur. Ceram. Soc. 26 1311), and

b) liquid metal infiltration, used by (Wang T C, Fan T X, Zhang D and Zhang G D 2006a Mater. Lett. 60 2695; Wang T C, Fan T X, Zhang D and Zhang G D 2006b Carbon 44 900; Mallick D, Chakrabarti O P, Majumdar R and Maiti H S 2007 Ceram. Int. 33 217).

All biomorphic structures based on natural materials have the disadvantage that also the defects of natural materials, such as poor homogeneity or imperfections of the natural structure are copied into the artificial biomorphic material. It is therefor desirable to create an artificial composite material with an efficient mechanical load supporting lattice topology, and which has an internal structure similar to a natural template, but without being limited by imperfections inherent to direct copies of natural templates.

Technical Solution and Preferred Embodiment

The present invention uses the so called "spot-pinning" effect of halloysite nanotubes (HNTs) in an aluminum matrix which was disclosed earlier in PCT/EP2012/052360. PCT/EP2012/052360 discloses that the ends of a HNT in an aluminum matrix are much stronger mechanically interconnected to the surrounding aluminum matrix than the other areas of the outer hull of the HNT because of the Al2O3 interface present at the open ends of the tubes. Figure 1 shows a HNT where the end regions of the tube having a high bonding strength to the surrounding aluminum matrix are marked with circles.

PCT/EP2012/052360 then discloses the so called "micro-torque" effect acting on individual HNTs which gives the compound material a superior mechanical strength in comparison to the pure matrix material, because the torque, induced by external stress and acting on each individual HNT, is transferred to the aluminum matrix and converted into heat. A maximum of tensile strength improvement was disclosed at a loading of about 10 weight percent (w%) of HNTs in the aluminum matrix.

In contrast to PCT/EP2012/052360 the present invention relies on a concentration of HNTs in the aluminum matrix which is high enough, so that the areas around the end parts of the HNTs overlap, where the mechanical interface to the surrounding aluminum matrix is strong (Figure 2). If the HNTs are randomly distributed in the aluminum matrix, several HNTs can dock on the same interfacing region under different angles (Figure 3). In a three dimensional situation this means that an irregular network, or scaffold, of randomly interconnected HNTs can develop in the aluminum matrix (Figure 4). The HNTs (grey bars) can be regarded as elastic beams, interconnecting the areas of strong mechanical attachment (marked as black circular shapes) around the ends of different HNTs.

The concentration (weight percentage) of the HNTs in the matrix influences the type of scaffold formed in the aluminum matrix. If the concentration is low, relatively simple structures are formed (Figure 05). However, if the concentration of HNTs is high, complex three dimensional networks are created. (Figure 06). In any case, the overlapping areas of strong mechanical interfacing represent areas where the compound has a higher strength than in the other areas. The result of a numerical simulation of this effect for a sample containing 60 w% HNTs is shown in Figure 07. The voids (black elliptical areas) represent areas along the side walls of the HNTs where the mechanical interface with the aluminum matrix is week.

There is an obvious similarity between the simulated strength picture in the aluminum / HNT compound material with 60 w% HNT and a micrograph taken from a natural Walnut wood sample (Figure 08). Figure 9 shows the result of a numerical simulation for a sample containing 45 w% HNTs in an aluminum matrix. Although there is a structural change in comparison to the high concentration simulation, again there is a clear similarity between Figure 9 and a micrograph taken from a natural Pine wood sample (Figure 10). Obviously it is possible to find different natural equivalents for aluminum compound materials with different HNT contents. Since the natural structures follow fractal geometries, it is concluded that the development of the HNT framework also follows fractal rules in the sense that there is a self assembling mechanism forming the different scaffold types. The general shape of the resulting simulated artificial network depends strongly on the HNT concentration while the mesh size (void diameter) is depending on the aspect ratio of the HNTs. Short fibers lead to a fine mesh while long fibers generate a coarse mesh. Figure 11 shows a micrograph taken from the etched fracture surface of a real aluminium / HNT compound material sample with a concentration of 60 w% HNT in the aluminum matrix and a wide length spectrum of the HNTs (Figure 12). The HNT network is clearly visible in Figure 11 and corresponds qualitatively to the corresponding simulation prediction. From the observation, that for the HNT length distribution shown in Figure 12 and concentrations below 30 w% of HNT in the aluminum matrix, no scaffold formation could be observed, it was concluded that for a given length distribution there is a minimum concentration necessary to initiate the interlinking effect and in turn the HNT lattice formation.

In a second simulation the elastic properties of a high concentration aluminum / HNT compound material were investigated using the above derived assumptions. Figure 12 shows that, if the simulated sample is exposed to a compression stress, the HNTs behave like elastic beams of constant length, interconnected by joints which correspond to the areas of high strength in the matrix. Since aluminum is much softer than the HNTs the aluminum matrix is deformed under the compression stress and slides along the HNTs while the HNTs keep their length and only swivel in the interconnection hinges. The aluminum matrix practically acts as a high viscosity damping medium in which a the scaffold of stiff HNTs, which are interconnected by three dimensional hinges, is embedded.

Preferred Embodiment and Advantageous Effects

Three different steps are necessary for the manufacturing of the self organized (stabilized) aluminum / HNT compound material as an embodiment of the present invention:

step 1) Powder metallurgical mixing of aluminum powder with HNTs. In this step the inner tubular space of the HNTs is filled (completely or in part) with aluminum particles. This process was disclosed earlier in PCT/EP2012/052360.

The filled HNTs are then separated from the residual aluminum powder and ready to be used in the following steps.

step 2) Stir casting of a blend of liquid aluminum and the aluminum powder filled HNTs prepared in step 1. In this step a blender crucible first melts the aluminum alloy prior to blending. Then the HNTs are added via a commercial feeder system. The HNTs are mixed into the liquid aluminum with a mixing impeller, that is submerged into the melt. The blend is kept in the the crucible blender long enough to allow the aluminum particles sitting the inner tubes of the HNTs to melt (Figure 13, picture source: Advanced Materials & Processes /July 2001).

step 3) After the blending step, the MMC material is transferred to a separate preheated holding furnace. The holding furnace has a built-in low-speed magnetic hydrodynamic (MHD) mixing system, producing a gentle circular agitation that keeps the HNTs from settling out without any mechanical stirrer or any other mechanical means disturbing the liquid aluminum / HNT blend. The temperature of the holding furnace is then slowly lowered while the mixing frequency of the MHD system is reduced simultaneously. The cooling rate has to be slow enough to give the HNTs enough time to self arrange and form a scaffold when the areas of higher bonding strength between the aluminum matrix and the HNT ends are interconnecting. The Al2O3 brought in on the surface of the powder particles filled into the inner tubes of the HNTs in step 1 now acts as an effective mechanical interface between the aluminum matrix and the Al2O3 like inner wall structure of the HNTs. Shortly before the temperature reaches the melting point of the aluminum alloy the MHD stirring system is switched off which causes the circular motion of the melt to slowly stop. As soon as the movement of the melt has stopped completely, the cooling rate is increased to a high value causing the blend to solidify while the HNT scaffold in the matrix is frozen in. The solidified ingot represents then the self stabilized halloysite / aluminum matrix compound as an embodiment of this invention.

The achieved material properties improvements in comparison to the pure matrix alloy AA 6061-T6 are summarized in Table 1.

Table1

	AA 6061-T6	AA 6061+40w%HNT
Ultimate Tensile Strength	310 MPa	720 MPa
Tensile Yield Strength	276 MPa	640 MPa
Elongation at Break	17%	12%
Modulus of Elasticity	68.9 GPa	96 GPa
Fatigue Strength	96.5 MPa	155 MPa
Fracture Toughness	29 MPa x m½	81 MPa x m½

Description of Drawings

Figure 01: Schematic picture of a halloysite nanotube wherein the areas of strong mechanical bonding with the aluminum matrix are marked with circles.

Figure 02: Two halloysite nanotubes interlinked in a joint area of strong mechanical bonding with the aluminum matrix.

Figure 03: Three halloysite nanotubes interlinked in a joint area of strong mechanical bonding with the aluminum matrix under various angles.

Figure 04: Three dimensional scaffold formed by interlinked halloysite nanotubes in the aluminum matrix.

Figure 05: Simple scaffold structure at low HNT concentrations in the aluminum matrix.

Figure 06: Complex scaffold structure at low HNT concentrations in the aluminum matrix.

Figure 07: Simulated strength structure for a concentration of 60 w% of HNTs in an aluminum matrix.

Figure 08: Micrograph of the internal structure of a Walnut wood sample.

Figure 09: Simulated strength structure for a concentration of 45 w% of HNTs in an aluminum matrix.

Figure 10: Micrograph of the internal structure of a Pine wood sample.

Figure 11: Micrograph of the etched fracture surface of an aluminium / HNT compound material sample containing 60 w% HNT in the aluminum matrix.

Figure 12: Length distribution of the halloysite nanotubes as received.

Figure 13: Schematic of a blender crucible used for mixing the HNTs homogeneously into the liquid aluminum (picture source: Advanced Materials & Processes /July 2001).

Claims

Matrix material containing a sufficient weight percentage of spot-pinned fibers in the matrix so that the fibers are mechanically interlinked and form a three dimensional network.
Matrix material according to claim 1 wherein the matrix material is a metal, or a polymer, or a ceramic.
Matrix material according to claim 1, wherein the internal material strength structure corresponds to the fractal structure of a natural material.
Matrix material according to claim 1, containing a sufficient weight percentage of halloysite nanotubes in an aluminum matrix so that the spot-pinned HNTs are mechanically interlinked and form a three dimensional network.
Metal matrix material according to claim 4 wherein aluminum acts as a damping material in an elastic HNT scaffold.
Metal matrix material according to claim 4 wherein the HNT content is between 30 and 80 weight percent.
Method for making a material as described in claim 4 by stirring a mixture of liquid aluminum and HNT, while continuously reducing the stirring speed and temperature until consolidation of the aluminum.
Method for making a material according to claim 7 wherein the HNTs are prefilled with aluminum powder particles.