PREPARATION OF A METAL MONOLITH BY A RAPID HOT PRESS METHOD

TECHNICAL FIELD

[0001]The invention relates to a high-pressure method for preparing a metal monolith either as dense or as highly porous form, by inductive heating hot pressing equipment. The method of the invention allows to obtain a metal monolith from metal powder without binder or any other additives.

[0002]By metal monolith, it is meant in the sense of the present invention a single-phase or multi-phase metal or metal-metal composite or metal matrix composite monolith (hereinafter designated by the acronym MMC).

[0003]By high pressure mould, it is meant in the sense of the present invention an assembly consisting of a cylinder (internal diameter 10 mm) and 2 well-adjusted mobile pistons.

BACKGROUND

[0004]It is known from the man skilled in the art methods and equipments for realizing metal monoliths. For instance, US patent application US 2012/0098162 relative to Rapid Hot Pressing Using Inductive Heater leads to rapid consolidation of powdered materials by induction hot pressing based on a graphite mould which applies the use of vacuum or inert gas to avoid destruction by oxidation. These graphite moulds cannot be used in air atmosphere and pollute the monolith mainly on the surface.

SUMMARY OF THE INVENTION

[0011]The method according to the invention allows to consolidate under air in extreme short duration (notably for a time range 1-60 minutes), avoiding oxidation and metal chemical reaction in the case of mixture of metal chemical elements.

[0012]The mould made of high strength material allows its usage in air atmosphere which is impossible for the graphite mold used in conventional inductive powder metallurgy process.

[0013]The method of the invention allows to realize dense or lowly/highly (more than 50%) porous metal monolith at temperatures below those involved in conventional inductive heating equipment applying the use of atmosphere protection, vacuum or inert gas to avoid oxidation of metal powder during the process. The method of the invention has a shorter duration, simplified process steps, easy handling, economical costs, easy adjustment of porosity (size, ratio) compared to currently known processes.

[0014]According to a first advantageous embodiment of this invention, the measurement of the temperature may be realized within the mould by means of a thermocouple placed within the mould and capable of being easily manipulated to and from the mould, the inductive coil partially encircling the mould so as to present an open configuration to allow the thermocouple to be inserted into the mould without heating it and to facilitate handling of the apparatus. For instance, the inductive coil may be in the form of a single loop forming several U-shaped folds, which are arranged on either side of a central loop-free part.

[0015]According to a second embodiment of this invention, the measurement of the temperature may be realized within the mould by means of an optical pyrometer, the inductive coil presenting a closed configuration with respect to each other.

[0016]Advantageously, if a dense monolith is to be made by the process according to the invention, then an uniaxial pressure of between 100 MPa and 1200 MPa, and preferably between 500 MPa and 1200 MPa, may be applied to the mould at temperatures T1 equal to or greater than 100° C.

[0020]For both variants, with a temperature T1 of at least 100° C., depending on the target porosity and also on the chemical elements of the metal powder.

[0021]The porosity may be measured by different methods known to the man skilled in the art: geometrical or using a porosimeter or a pycnometer.

[0022]With respect to the first variant of the method of the invention (first variant) mixing a space holder template which is crushed end sieved, the latter may be for example sugar, or any powdered material easily soluble in water (salt, sugar, bicarbonate, . . . ) or easily degradable in an oven (corn, . . . ): the space holder template can be made of a natural or synthetic organic material that is able to remain solid and that does not degrade (burn or oxidise) in air up to 550° C. Preferably, the space holder template may be made of NaCl and can be easily removed by washing with water from the metal monolith. This dissolving process may be made faster by washing with water in an ultrasonic tank at room temperature or by boiling the water.

[0023]Advantageously, the induction heating of the method of the invention may comprise a heating ramp from T₀ to T₁ carried out at a heating rate of 50° C./min, a plateau at T1 and a cooling ramp from T₀ to T₁ carried out at a cooling rate of 25° C./min, for instance by means of an air jet or by by heat exchange fluid circulation. Preferably, the duration of the plateau may be between 1 s and 24 hours, and preferably between 1 and 3 minutes.

[0024]Advantageously, the metal powder for use in the present invention may comprises one single metal element or at least two metal elements, said metal elements of the metal powder being preferably selected from Al, Ti, Cu and mixtures thereof (for example). Preferably, the metal powder presents a micrometric distribution.

[0025]Another object of the invention consists in a metal monolith obtainable by the method of the invention. This metal monolith may be single or multi-phase metal or a metal matrix composite, or a metal-metal composite.

[0026]The metal monolith according to the invention may be a dense monolith presenting a porosity lower than 2% or a porous monolith presenting a porosity comprised greater than 2%, and preferably greater than 50% and up to 80%.

[0030]Preferably, the mould may be made of a material selected from metals and ceramics or ceramic-metal composite such as cermet, and preferably made of steel or a nickel alloy.

BRIEF DESCRIPTION OF THE FIGURES

[0031]Other innovative features and advantages of the invention will emerge from a reading of the following description followed by way of indication and in no way imitatively, with reference to the accompanying drawings, in which the figures illustrate schematically examples of implementation of the chips according to the invention. The figures are presented below:

[0032]FIG. 1 is a picture showing the Rapid Hot Press Equipment of the invention;

[0033]FIG. 2 is a detailed view of the Rapid Hot Press Equipment shown in FIG. 1, notably showing the high pressure mould (internal diameter 10 mm) 2 for receiving the metal powder, the inductive coil 3 disposed around the mould 2 and a uniaxial press 4;

[0034]FIG. 3 is a curve showing the effect of low uniaxial pressure on the porosity of a copper monolith obtained from a copper powder of either dentritic or spherical Cu particles, when using the process according to the invention, with induction heating including a plateau at 400° C. (T1) for 10 minutes (see example 1);

[0035]FIG. 4 is a curve showing the effect of high uniaxial pressure on the relative density of a copper monolith obtained from a copper powder comprising dentritic Cu particles of 26 μm, when using the method according to the invention with induction heating including a plateau at 20° C. and 150° C. (T1) for 1 minute (see example 2);

[0036]FIG. 5 (see example 3) is a XRD diffractogram showing the non-oxidation under air of copper during the execution of the method according to the invention, using a copper powder comprising spherical Cu particles of 15 μm in diameter, when using the method according to the invention with induction heating including a plateau at 400° C. (T1) for 10 minutes and uniaxial pressing of 400 MPa (with and without the presence of a space holder template of NaCl);

[0037]FIG. 6 is a XRD diffractogram (see example 6) showing the non-oxidation under air of copper and carbon fibers, for elaborating a MMC, during the execution of the method according to the invention, using a powder comprising a mixture of dentritic Cu particles (60% in weight of the powder) and carbon fibres (40% in weight of the powder), when using the method according to the invention with induction heating including a plateau at 250° C. (T1) for 60 minutes and uniaxial pressing of 255 MPa (with and without the presence of a space holder template of NaCl);

[0038]FIG. 7 is a curve showing the effect of the temperature on the relative density of a titanium monolith from a titanium powder comprising spherical Ti particles of 50 μm in diameter, when using the method according to the invention with induction heating for 10 minutes and uniaxial pressing of 820 MPa or with induction heating for 1 minute and uniaxial pressing of 990 MPa;

[0039]FIG. 8 is a curve (see example 8) showing the effect of the pressure on the relative density of a titanium monolith obtained from a titanium powder comprising spherical Ti particles of 50 μm in diameter, when using the method according to the invention with induction heating including a plateau at 20° C. and 150° C. (T1) for 1 minute or at 20° C. for 10 minutes;

[0040]FIG. 9 is a XRD diffractogram (see example 9) showing the non-oxidation under air of titanium during the execution of the method according to the invention, obtained from a titanium powder comprising spherical Ti particles, when using the method according to the invention with induction heating including a plateau at 400° C. (T1) for 10 minutes and uniaxial pressing of 970 MPa with and/or without the presence of a space holder template of NaCl;

[0041]FIG. 10 is a curve (see example 11) showing the effect of high uniaxial pressure on the relative density of a metal monolith obtained from a metal powder comprising dentritic Cu particles of 26 μm (50% in weight of the powder) and spherical Ti particles of 50 μm in diameter 50% in weight of the powder), when using the method according to the invention with induction heating including a plateau at 20° C. and 150° C. (T1) for 1 minute, or at 150° C. for 60 minutes, or at 400° C. for 1 minute and 60 minutes;

[0042]FIG. 11 is a curve (see example 13) showing the effect of the pressure on the relative density of an aluminium monolith obtained from an aluminium powder comprising Al particles of 40 μm in diameter, when using the method according to the invention with induction heating including a plateau at 20° C. or 150° C. (T1) for 1 minute or at 400° C. for 1 minute;

[0043]FIG. 12 is a curve (see example 15) showing the effect of the pressure on the relative density of a metal monolith obtained from a metal powder comprising spherical Cu particles (50% in weight of the powder) and Al particles of 50 μm in diameter (50% in weight of the powder), for elaborating a metal-metal composite, when using the method according to the invention with induction heating including a plateau at 150° C. or 400° C. (T1) for 1 minute or 60 minutes;

[0044]FIG. 13 is a XRD diffractogram (see example 17) showing the possibility for elaborating a multi-phase metal with the presence of interphase (alloy: Cu₉Al₄) or without interphase for a metal-metal composite between both metals and non-oxidation under air of metals, said metal monolith being obtained from a metal powder comprising spherical Al particles (2.7 g/cc) and Cu particles (8.92 g/cc), when using the method according to the invention with induction heating including a plateau at 150° C. or 400° C. (T1) for 60 minutes;

[0045]FIG. 14 comprises different SEM photographs showing of different monoliths obtained by the method according to the invention from different metal powders (a) spherical Cu particles, b) mixture of dentritic Cu particles and carbon fibers, c) Ti particles, d) dentritic Cu particles): the pictures show the cold welding between grains (consolidation effect).

[0046]For clarity, the identical or similar elements are marked by identical signs on all the figures.

DETAILED DESCRIPTION

[0047]The figures and the following description illustrate specific exemplary embodiments of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within the scope of the invention.

[0048]Furthermore, any examples described herein are intended to aid in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited examples and conditions. As a result, the invention is not limited to the specific embodiments or examples described below, but by the claims and their equivalents.

[0049]In the following description, well-known functions or constructions by the person skilled in the art are not described in detail since they would obscure the invention in unnecessary detail.

[0050]In the following description, the figures are commented on in detail in the examples and serve as a basis for them.

EXAMPLES

Equipment

[0051]FIGS. 1 and 2 show an example a Rapid Hot Press Equipment (or RHP) 1 suitable for use in the examples according to the invention. This Rapid Hot Press Equipment 1 comprises a high-pressure mould (internal diameter 10 mm) 2 for receiving a metal powder and an inductive coil 3 disposed around the mould 2 and a uniaxial press 4, the coil 3 being coupled to a RF electrical power supply 5. The mould 2 is realized in a non-oxidable and high strength material having a yield strength of at least 0.1 GPa, preferably at least 1.2 GPa, under air atmosphere (such as steel or a nickel alloy). The RHP equipment 1 of the invention further comprises a thermocouple 6 placed within the mould 2 and capable of being easily manipulated to and from the mould 2, while the inductive coil 3 partially surrounds the mould as a single loop forming a plurality of U-shaped folds 31 arranged around the mould symmetrically on either side of a central part 21 of the mould (2).

Products

Metal Powders:

Example 1: Effect of Low Force on Cu Porosity and Consolidation of the Metal Monolith

Experimental Conditions:

Results

[0065]FIG. 3 shows that it is possible, by applying a pressure as low as 0.1 kN, to realize a copper monolith with a relative porosity of 80% (from dentritic particles) while it is not possible to achieve a consolidation of the monolith (diameter 10 mm) under 1.2 kN when using spherical particles. By adjusting the force (for very low uniaxial forces), it is possible to make a very porous monolith, whose porosity can be adjusted. The use of dentritic copper particles allows very high porosities to be achieved without the use of templates.

Example 2: Effect of High Force on Cu Densification and Consolidation

Experimental Conditions:

Results:

[0068]FIG. 4 shows that a very dense monolith can be obtained at 20° C. and 150° C. if very high pressures are applied. With a pressure of 1 GPa, a completely dense monolith can be obtained with very little heating, using the RHP equipment according to the invention.

Example 3: Demonstration of Cu Non-Oxidation

Experimental Conditions:

Results:

[0071]FIG. 5 shows the non-oxidation of copper during the execution of the method according to the invention, without or in presence of NaCl (template) after H₂O washing.

Example 4: Effect of Addition of NaCl (Grinding+Sieving) as Space Holder (to Calibrate the Porosity Size) in Cu Dendritic

Experimental Conditions:

$\mathrm{Cu}\mathrm{dendritic}+\mathrm{NaCl}\left(100\mathrm{\mu m}<\varphi <200\mathrm{\mu m}\right)$

[0072]The other experimental conditions (temperature and pressure, and percentage of space holder) and the corresponding results of the experimentations are both in table 1 below:

[0073]Whatever the applied temperature (100° C., 200° C. or 400° C.), it is possible with the process according to the invention and using a space holder template to obtain a very porous monolith (from 69% up to 80% of porosity) by applying a rather low pressure (from 13 to 497 MPa).

Example 5: Effect of Addition of NaCl (Grinding+Sieving) as Space Holder (to Calibrate the Porosity Size) on Metal Matrix Composite (MMC)

Experimental Conditions:

$\mathrm{Cu}\mathrm{dendritic}+\mathrm{NaCl}\left(100\mathrm{\mu m}<\varphi <200\mathrm{\mu m}\right)+40\%\mathrm{volume}\mathrm{carbon}\mathrm{fibers}\left(\mathrm{CF}\right)$

[0074]The other experimental conditions (temperature and pressure, and percentage of space holder) and the corresponding results of the experimentations are both in table 2 below:

[0075]Whatever the applied temperature (200° C. or 400° C.), it is possible with the process according to the invention and using a space holder template to obtain a very porous Metal Matrix Composite (from 69% up to 80% of porosity) by applying a rather low pressure (from 2 to 433 MPa). At 250° C. and with a pressure as low as 250 MP, it is not possible to obtain a very porous MMC without a space holder (the porosity of the obtained MMC is 2%).

Example 6: Demonstration of Cu Non-Oxidation

Experimental Conditions:

Results:

[0077]FIG. 6 shows the non-oxidation of copper during the execution of the method according to the invention, without or in presence of NaCl (template) after H₂O washing.

Comparative Example 1

[0078]This comparative example shows the benefit of the RHP method of the invention over the conventional process taught in the publication “Effect of porosity on the thermal conductivity of copper processed by powder metallurgy” in Journal of Physics and Chemistry of Solids 73 (2012) 499-504^[1].

[0079]In the conventional process^[1] (see notably page 501), it is taught that Dendritic copper powder was hot pressed, under vacuum or a reducing atmosphere of Ar/H₂ at 650° C. 10 min under pressures ranging from 0 to 30 bars (i.e. 0.1 to 3 MPa), as shown in FIG. 2b. According to the diagram, the porosity decreases when the pressure applied increases. At the maximum applied pressure of 30 bars, the densification is close to 100%.

[0080]In the process according to the method of the invention, there is no need for vacuum or reducing atmosphere and a higher pressure is used to consolidate and densify at lower temperatures to avoid oxidation under air. The method according to the invention makes it possible to obtain, under air, a porosity higher than 15% at lower temperature during a shorter duration and by applying a lower force. The method according to the invention also makes it possible to densify 100% under air, at lower temperature during a shorter duration and a higher force.

[0081]Furthermore, in the conventional process [ ], the passage page 501 (and also FIG. 2) teaches that “copper oxide remains in the core, which hinders the densification. The sample obtained is heterogeneous and not fully densified. It is fundamental to deoxidize the spherical Cu powder before doing the densification to minimize the residual porosities”. This article also teaches that “copper is thermodynamically stable at 650° C. until an oxygen partial pressure of 2.104 Pa, based on Ellingham diagram. The sintering is performed under a vacuum of about 7 Pa, which corresponds to a domain of stability for pure copper.».

[0082]The method of the invention avoids this oxidation problem existing on the surface of the grains, initially, because the applied lower temperature: there is no oxidation under air due to the protective effect of the electromagnetic field generated by the inductive heating.

Example 7: Effect of High Force on Ti Densification and Consolidation

Experimental Conditions:

Results:

[0085]FIG. 7 shows that a very dense monolith can be obtained even at very low temperatures if high pressures (820 MPa and 990 MPa) are applied. With a pressure of 990° C., a completely dense monolith can be obtained at 300° C., while it is necessary to heat up to 400° C. with a pressure of 820 MPa, using the RHP equipment according to the invention.

Example 8: Effect of Temperature on Ti Densification and Consolidation

Experimental Conditions:

Results:

[0088]FIG. 8 shows that a very dense monolith (90% of density) can be obtained even at very low temperatures (20° C.) if high pressures (1 GPa) are applied.

Example 9: Demonstration of Ti Non-Oxidation

Experimental Conditions:

Ti Dendritic

[0089]Induction heating including a plateau at 400° C. for 10 minutes and uniaxial pressing of 970 MPa (without the presence of a space holder template of NaCl).

Results:

[0090]FIG. 9 shows the non-oxidation of Ti during the execution of the method according to the invention.

Example 10: Effect of Addition of NaCl (Grinding+Sieving) as Space Holder (to Calibrate the Porosity Size) in Ti Spherical (50 μm)

Experimental Conditions:

Ti Spherical

$40\%\left(\mathrm{vol}\right)\mathrm{of}\mathrm{Nacl}\left(100\mathrm{\mu m}<\varphi <200\mathrm{\mu m}\right)$

[0091]The other experimental conditions (temperature and pressure, and percentage of space holder) and the corresponding results of the experimentations are both in table 3 below:

[0092]It is possible with the process according to the invention to obtain a porous monolith.

Comparative Example 2

[0093]This comparative example shows the benefit of the RHP method of the invention over the conventional process taught in the publication “Inductive hot-pressing of titanium and titanium alloy powders” in Materials Chemistry and Physics 131 (2012) 672-679^[2].

[0094]According to page 672 of this publication^[2], «Hot-pressing, the simplest of the hot consolidation techniques, consists of loading a loose powder into a graphite mold that is then placed between two punches and heated in an enclosed furnace. The most conventional heating method for hot-pressing is high-temperature resistance; however, other options are available, such as inductive hot-pressing where the heating of the powder is done by means of the surrounding graphite mold as susceptor».

[0095]According to page 673 of this publication^[2], «To carry out the consolidation, approximately 1.5 g of powder was loaded into a 10-mm diameter graphite mold. After hot pressing, discs about 3.8 mm in height were obtained. The inductive heating press used in this process is home-made. It is composed of a stainless-steel vacuum chamber, a servo-hydraulic test machine (LF 70S) capable of dynamic loads up to 70 kN»

[0096]According to page 674 of this publication^[2], «temperatures of 1100° C. and 1300° C., a pressure of 50 MPa, a heating rate of 50° C. min−1, a dwell time at maximum temperature of 15 min and a vacuum level of 10−3 mbar». If comparing the process according to the invention with FIG. 2, page 674 (density), with the process according to the invention, a higher pressure allows to consolidate and densify at a lower temperature.

[0097]According to page 675 of this publication^[2], «an inaccurate preparation of the graphite matrix, which led to the embedding of some graphite into the surface of the specimens».

[0098]If comparing the process according to the invention with FIG. 6, page 677 (oxidation) of this publication [2], with the process according to the invention, a lower temperature avoids oxidation under air.

[0099]In the process according to the method of the invention, there is no need for vacuum or reducing atmosphere. Furthermore, there is no pollution control problem with the graphite mould.

Example 11: Effect of High Force on the Densification and Consolidation of a Mixture of 2 Metals (Dentritic Cu and Spherical Ti)

Experimental Conditions:

Results:

[0102]FIG. 10 shows that it is possible to obtain dense monoliths (more than 80%) even at 20° C. by applying a pressure of 375 MPa. With a pressure of 1 GPa, a highly dense monolith (above 90%) can be obtained at 150° C., using the RHP equipment according to the invention.

Example 12: Effect of Addition of NaCl (Grinding+Sieving) as Space Holder (to Calibrate the Porosity Size) in a Mixture of Ti Spherical and Cu Dendritic

Experimental Conditions:

[0104]The other experimental conditions (temperature and pressure, and percentage of space holder) and the corresponding results of the experimentations are both in table 4 below:

[0105]It is possible with the process according to the invention to obtain a porous monolith.

Example 13: Effect of High Force on the Densification and Consolidation of Aluminium

Experimental Conditions:

Results:

[0108]FIG. 11 shows that it is possible to obtain dense monoliths (more than 85%) even at 20° C. by applying a pressure of 250 MPa. With a pressure of 1 GPa, a highly dense monolith (above 95%) can be obtained even at 20° C., using the RHP equipment according to the invention.

Example 14: Effect of Addition of NaCl (Grinding+Sieving) as Space Holder (to Calibrate the Porosity Size) in an Aluminium Powder (40 μm)

Experimental Conditions:

[0110]The other experimental conditions (temperature and pressure, and percentage of space holder) and the corresponding results of the experimentations are both in table 5 below:

[0111]It is possible with the process according to the invention to obtain a porous monolith.

Example 15: Effect of High Force on the Densification and Consolidation of a Mixture of 2 Metals

Experimental Conditions:

Results:

[0114]FIG. 12 shows that it is possible to obtain dense monoliths (more than 90%) even at 150° C. by applying a pressure of 250 MPa. With a pressure of 1 GPa, a highly dense monolith (above 95%) can be obtained even at 150° C., using the RHP equipment according to the invention.

Example 16: Effect of Addition of NaCl (Grinding+Sieving) as Space Holder (to Calibrate the Porosity Size) in a Mixture of Cu Spherical and Ti Spherical

Experimental Conditions:

[0116]The other experimental conditions (temperature and pressure, and percentage of space holder) and the corresponding results of the experimentations are both in table 6 below:

[0117]It is possible with the process according to the invention to obtain a porous monolith, whatever the percentage (50% vol or 70% vol).

Example 17: Demonstration on the Possibility for Elaborating a Multi-Phase Metal with the Presence of Interphase (Alloy) or without (Metal-Metal Composite) Between Both Metals and Non-Oxidation of Metals

Experimental Conditions:

Results:

[0120]FIG. 13 shows that long time duration (50 minutes) allows the allow formation (Cu₉Al₄)

Example 18: Demonstration of Cold Welding Between Grains: Consolidation Effect

[0121]FIG. 14 comprises different SEM photographs showing of different monoliths obtained by the method according to the invention from different metal powders (a) spherical Cu particles, b) mixture of dentritic Cu particles and carbon fibers, c) Ti particles, d) dentritic Cu particles): the pictures show the cold welding between grains (consolidation effect).

REFERENCES