U.S. patent application number 10/204460 was filed with the patent office on 2003-03-27 for method for the manufacture of metal foams by electrolytic reduction of porous oxidic preforms.
Invention is credited to Godfrey, Alastair B, Ward Close, Charles M.
Application Number | 20030057101 10/204460 |
Document ID | / |
Family ID | 26243686 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030057101 |
Kind Code |
A1 |
Ward Close, Charles M ; et
al. |
March 27, 2003 |
Method for the manufacture of metal foams by electrolytic reduction
of porous oxidic preforms
Abstract
A method for the manufacture of a foamed metal or alloy article
including the steps of: A) selecting a particulate feedstock having
suitable proportions of a metal element or combination of metal
elements M.sub.1 contaminated by one or more contaminants X to form
an alloy suitable for the foamed article; B) mixing the feedstock
with a binder to form a slurry; C) preforming the slurry into a
near net shape of the desired article and drying the preform to
remove the binder; D) sintering the dried preform to provide a
bonded foamed article; E) introducing the sintered article into an
electrochemical cell, the cell containing a liquid electrolyte
comprising a fused salt or mixture of salts generally designated as
M.sub.2Y in which contaminant(s) X is soluble, and a relatively
inert anode; F) conducting electrolysis under conditions favourable
to the selective dissolution of the contaminant(s) X in preference
to the M.sub.2 cation; and G) following electrolysis reclaiming the
purified foam article from the cathode.
Inventors: |
Ward Close, Charles M;
(Farnborough Hants, GB) ; Godfrey, Alastair B;
(Farnborough Hants, GB) |
Correspondence
Address: |
Nixon & Vanderhye
8th Floor
1100 North Glebe Road
Arlington
VA
22201-4714
US
|
Family ID: |
26243686 |
Appl. No.: |
10/204460 |
Filed: |
September 10, 2002 |
PCT Filed: |
February 19, 2001 |
PCT NO: |
PCT/GB01/00661 |
Current U.S.
Class: |
205/363 ;
205/367 |
Current CPC
Class: |
C22B 34/1263 20130101;
C25C 3/28 20130101; C25C 5/04 20130101; C22B 4/06 20130101; B22F
9/20 20130101; B22F 2999/00 20130101; C25C 3/00 20130101; C22B 5/00
20130101; C22B 34/129 20130101; C22B 5/02 20130101; C22C 47/04
20130101; C22C 47/14 20130101; B22F 2999/00 20130101; B22F 9/20
20130101; C25C 3/28 20130101 |
Class at
Publication: |
205/363 ;
205/367 |
International
Class: |
C25C 003/36; C25C
003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 22, 2000 |
GB |
0003971.9 |
May 8, 2000 |
GB |
0010873.8 |
Claims
1. A method for the manufacture of a metal or alloy mass from a
metal oxide or mixed metal oxide feedstock, by electrolysis in a
fused salt of M.sub.2Y or a mixture of such salts under conditions
such that ionisation of oxygen rather than M.sub.2 deposition
occurs and that oxygen dissolves in the electrolyte M.sub.2Y,
characterised in that the feedstock consists of a foamed, dried and
sintered metal oxide preform and produces a foamed metal or alloy
mass.
2. A method according to claim 1 wherein the preform is foamed by
passing a gas through a slurry comprising a metal oxide
particulate, binder and water.
3. A method according to claim 1 wherein the preform is foamed by
gas evolved from a chemical reaction of organic foaming agents and
the foam is then fired to remove the organic material.
4. A method according to claim 1 wherein the preform is made by
infiltrating a natural or synthetic polymeric foam with a slurry
comprising a metal oxide particulate, binder and water and
subsequently burning off the polymeric foam.
5. A method according to claim 4 wherein the vaporisation point of
the foam template is lower than the melting point of the metal
oxide or mixed metal oxide to be foamed.
6. A method according to any preceding claim wherein the preform is
in the near net shape of the intended metal or alloy article.
7. A method as claimed in any preceding claim wherein the metal
oxide comprises TiO.sub.2.
8. A method as claimed in any preceding claim wherein Y is
chloride.
9. A method as claimed in any preceding claim wherein M.sub.2 is
calcium.
10. A method as claimed in any preceding claim wherein the alloy is
a beta titanium alloy.
11. A method as claimed in any preceding claim further comprising
applying a metal working process to the metal or alloy mass after
it has been reclaimed.
12. Armour manufactured according to the method of any of claims 1
to 11.
13. Armour as claimed in claim 12 wherein the alloy is Ti-6Al-4V
alloy.
14. An orthopaedic implant for a human or animal body manufactured
according to the method of any of claims 1 to 11.
15. An orthopaedic implant according to claim 14 wherein said
implant comprises a fully dense core and an outer foam layer.
Description
[0001] This invention relates to methods for the manufacture of
metal foams and to novel applications for these technologies. The
invention is more particularly directed to, but not limited to
manufacture of titanium and titanium alloy foams.
[0002] WO99/64638 and the applicant's co-pending applications
British Patent applications nos. GB 0003971.9 and GB 0010873.8 (the
disclosures of which are incorporated herein by reference) describe
methods for the electrolytic reduction of metal compounds.
[0003] Certain embodiments of these methods involve the
electrolysis of metal oxides or other compounds (M.sub.1X) in a
cell containing a liquid (fused salt M.sub.2Y) electrolyte and an
anode, the metal oxide or other compound forming the cathode.
Conditions are controlled so as to bring about the selective
dissolution of the oxygen or other contaminant of the cathode in
preference to deposition of the metal cation. Improved efficiency
of this process can be achieved by various methods as described in
GB 0003971.9 and GB 0010873.8 some of which are summarised
below.
[0004] Feedstock Production by Addition of Binder to Rutile and
Amorphous Titania
[0005] The manufacture of titanium dioxide from the raw ore (sand
mined illemite) comprises a large number of steps in the production
of titanium.
[0006] During one of these stages titanium dioxide in the form of
an amorphous slurry undergoes calcining. The titanium dioxide
slurry can be used as the principle feedstock in the above
described electrolytic method. A small percentage of calcined
material is mixed with amorphous material and a binder to obtain
the most satisfactory results after sintering. The calcined
material should constitute at least about 5% by weight of the
mixture.
[0007] Production of Metal Foams
[0008] Metal foams, more typically titanium foams, are attractive
for a number of applications such as filters, medical implants and
structural fillers. The fabrication of a sponge-like sintered oxide
pre-form from the starting material M.sub.1X can be converted into
a solid metal/alloy foam via the electrolytic method previously
described. Various established methods may be used to make the foam
like material from the mixture of oxide powders. The foam preform
desirably has open porosity that is, porosity which is
interconnected and open to the exterior.
[0009] In a preferred embodiment of this method, a natural or
synthetic polymeric foam is infiltrated with a metal (eg titanium)
oxide slip, then dried and fired to remove the polymeric foam,
leaving an open "foam" which is an inversion of the original
polymeric foam. The sintered preform is then electrolytically
reduced in accordance with the previously described method to
convert it into a titanium/titanium alloy foam. The foam is then
washed or vacuum distilled to remove the salt.
[0010] Alternatively, the metal oxide powder may be mixed with
organic foaming agents. These materials are typically two liquids
which when mixed, react to evolve a foaming gas, and then cure to
give a solidified foam with either an open or closed structure. The
metal powder is mixed with one or both of the precursor liquids
prior to production of the foam. The foam is then fired to remove
the organic material, leaving the ceramic foam which is then
electrolytically reduced in accordance with the previously
described method.
[0011] Production of Metal or Metal Alloy Components
[0012] A near net shape component may be made using the previously
described electrolytic method by reducing a ceramic facsimile of
the component made from a mixture of a metal oxide or mixture of
metal oxide and the oxides of other alloying elements. Again this
method is particularly suited to the manufacture of titanium metal
and alloy components. The ceramic facsimile may be produced using
any of a variety of well known production methods for ceramic
articles which include; pressing, injection moulding, extrusion and
slip casting, followed by firing (sintering). Full density of the
metallic component can be achieved by sintering with or without the
application of pressure, either in the electrochemical cell, or in
a subsequent operation. Shrinkage of the component during the
conversion to metal or alloy should be allowed for by making the
ceramic facsimile proportionally larger than the desired
component.
[0013] Electrolysis of a Preformed Sintered Mass
[0014] The electrolysis is performed on a preformed sintered mass
comprising a mixture of metal oxide made up of a proportion of
particles of size generally greater than 20 microns and a
proportion of finer particles of less than 7 microns. Preferably
the finer particles make up between 10 and 55% by weight of the
sintered block.
[0015] High density granules of approximately the size required for
the powder are manufactured and then are mixed with very fine
unsintered metal oxide (e.g., titanium dioxide), binder and water
in the appropriate ratios and formed into the required shape of
feedstock. This feedstock is then sintered to achieve the required
strength for the reduction process. The resulting feedstock, after
sintering but before reduction, consists of high density granules
in a lower density (porous) matrix.
[0016] The feedstock can be reduced in block form using the
previously described electrolytic method and the result is a
friable block which can easily be broken up into powder.
[0017] The calcine discharge used can be replaced by cheaper
amorphous TiO.sub.2. The key requirement for this "matrix" material
is that it sinters easily with significant shrinkage during the
sintering process. Any oxide or mixture of oxides which fulfil
these criteria would be usable. In the case of TiO.sub.2 this means
the particle size must be less than about 1 .mu.m. It is estimated
that at least 5% of the matrix material should be present in order
to give any significant strength to the sintered product.
[0018] The starting granules for this method need not be rutile
sand but could be manufactured by a sintering and crushing process,
and in principle there is no reason to suppose that alloy powders
could not be made by this route.
[0019] In any of the aforementioned methods X may be a metalloid
such as oxygen, sulphur, carbon or nitrogen, preferably, X is
oxygen. M.sub.1 may be a Group IVA element such as Ti, SI, Ge, Zr,
Hf, Sm, Nd, Mo, Cr, Nb or an alloy of any of the preceding metals,
preferably, M.sub.1 comprises titanium. A preferred electrolyte,
M.sub.2Y, is calcium chloride (CaCl.sub.2). Other suitable
electrolytes include but are not limited to the molten chlorides of
all common alkali and alkaline earth metals. Other preferred metals
for M.sub.2 are barium, caesium, lithium, strontium and yttrium.
The anode of the cell is preferably of a relatively inert material.
One suitable anode material is graphite.
[0020] Processing conditions suitable for the favourable
dissolution of the contaminant X require that the potential of the
cell preferably be maintained at a potential which is less than the
decomposition potential of the molten electrolyte M.sub.2Y during
the process. Allowing for polarisation and resistive losses in the
cell, it will be understood that the cell potential may be
maintained at a level equal to, or marginally higher than, the
decomposition potential of M.sub.2Y and still achieve the desired
result. Potentiostatic methods may be used to control the
potential.
[0021] It is also preferred that the temperature of the cell is
maintained at an elevated temperature which is significantly above
the melting point of M.sub.2Y but below the boiling point of
M.sub.2Y. Where M.sub.2Y is CaCl.sub.2, suitable processing
parameters include a potential of up to about 3.3V and a processing
temperature of between about 825 and 1050.degree. C.
[0022] The present invention provides a method for the manufacture
of a foamed metal or alloy article including the steps of:
[0023] A. selecting a particulate feedstock having suitable
proportions of a metal element or combination of metal elements
M.sub.1 contaminated by one or more contaminants X to form an alloy
suitable for the foamed article;
[0024] B. mixing the feedstock with a binder to form a slurry;
[0025] C. preforming the slurry into a near net shape of the
desired article and drying the preform to remove the binder;
[0026] D. sintering the dried preform to provide a bonded foamed
article;
[0027] E. introducing the sintered article into an electrochemical
cell, the cell containing a liquid electrolyte comprising a fused
salt or mixture of salts generally designated as M.sub.2Y in which
contaminant(s) X is soluble, and a relatively inert anode;
[0028] F. conducting electrolysis under conditions favourable to
the selective dissolution of the contaminant(s) X in preference to
the M.sub.2 cation; and
[0029] G. following electrolysis reclaiming the purified foam
article from the cathode.
[0030] Conveniently, the binder is water. Preferably, prior to
drying, the preform in step C is subjected to foaming by the
blowing of a gas through the slurry. As well as removing some of
the water from the preform and assisting in the drying process,
this step results in the formation of bubbles in the preform which
are retained as cells in the foam. Alternatively, foaming agents
may be introduced into the slurry to form gas bubbles within the
body of the preform. Optionally, the preform in step C may be
provided by packing the slurry into the open cells of a foam
article which is provided in the desired net shape of the preform.
This foam template should comprise a material with a vaporisation
point significantly lower than the melting point of the
contaminated metal or alloy to be foamed. The foam template can
then subsequently be burnt off leaving a network of open cells
within the resulting metal article.
[0031] In one embodiment of the method, a quantity of crushed
titanium oxide feedstock is mixed with around 300 ml of water per
kilo of the feedstock and placed in a mould of the desired foamed
article. The article has dimensions of the order of a few
centimetres. Air is blown through the mould to assist in foaming
the preform. The preform is then left to dry at room temperature
and pressure for about 5 days. Once dried, the article is sintered
in an oven at between about 1100.degree. C. to 1300.degree. C. for
around 2 hours.
[0032] The sintered article is then introduced to an
electrochemical cell comprising a molten calcium chloride bath and
carbon graphite anode and electrolysis performed in accordance with
methods previously described to remove the contaminant oxygen. Once
the desired quantity of oxygen has been removed by this method, the
purified foamed titanium article is reclaimed from the cell.
[0033] Various applications for metal foams produced by the method
of this invention may occur to the skilled addressee.
[0034] One application may include the manufacture of armour. A
foamed titanium alloy such as Ti-6Al-4V alloy may be preformed into
the net shape of the armour in accordance with the invention. The
foamed alloy is considerably lighter than full density armour for
similar high strength, high stiffness and high temperature
properties. The foaming provides the additional advantage that the
foamed structure begins to collapse on impact thereby absorbing
energy from the projectile penetrating the armour and considerably
reducing the risk or extent of injury to the protected persons.
[0035] Another application may be in the manufacture of orthopaedic
and other medical implants. Titanium alloys are widely recognised
as good bio-materials as they are relatively inert in the
environment provided by a human body. Recent developments on
orthopaedic research suggest that the life of an implant and the
health of tissue surrounding the implant can be greatly improved
where the implant is provided with a knurled or otherwise pitted
surface. Tissues, in particular bone tissue surrounding the pitted
surface of the implant, grow into the pits providing anchorage for
the implant and resulting in more even distribution of load from
the implant to the bone. It is widely accepted that bone strength
and health is compromised by prolonged periods of under loading,
hence bone health may be improved by the provision of pits or
channels within an orthopaedic implant.
[0036] By using the present invention, foamed titanium alloy
implants may be provided by forming the preform in the near net
shape of the implant. Since the foam structure provides channels
passing in varying directions through the implant, exceptional
anchorage and load transfer to the bone can be predicted. Where the
impact loads of the implant are particularly high, it may be
desirable to retain a fully dense alloy core to the implant with an
outer foamed layer. This can easily be accommodated by planting a
fully dense core at the centre of the preform and coating with the
slurry to be foamed. Previous attempts to obtain articles of this
sort have involved drilling of holes in the fully dense implant or
other complex or cumbersome machining operation, all of which add
significantly to the cost of the implant, risk damage to the
structural integrity of the implant and provide a far less random
and extensive network of channels through which the bone may grow.
Thus the present method may be used to provide a more cost
effective product with significantly improved clinical
performance.
[0037] Other applications for metal foams made in accordance with
the invention include, the manufacture of filters, sound proofing
applications, particularly in high temperature or highly corrosive
environments and any structural applications requiring high
strength and stiffness with low weight. Such structural
applications might include aircraft components, windmill propellers
and the like.
* * * * *